# LOG#056. Gravitational alpha(s).

The topic today is to review a beautiful paper and to discuss its relevance for theoretical physics. The paper is: Comment on the cosmological constant and a gravitational alpha by R.J.Adler. You can read it here: http://arxiv.org/abs/1110.3358

One of the most intriguing and mysterious numbers in Physics is the electromagnetic fine structure constant $\alpha_{EM}$. Its value is given by

$\alpha_{EM}=7.30\cdot 10^{-3}$

or equivalenty

$\alpha_{EM}^{-1}=\dfrac{1}{\alpha_{EM}}=137$

Of course, I am assuming that the coupling constant is measured at ordinary energies, since we know that the coupling constants are not really constant but they vary slowly with energy. However, I am not going to talk about the renormalization (semi)group in this post.

Why is the fine structure constant important? Well, we can undertand it if we insert the values of the constants that made the electromagnetic alpha constant:

$\alpha_{EM}=\dfrac{e^2}{\hbar c}$

with $e$ being the electron elemental charge, $\hbar$ the Planck’s constant divided by two pi, c is the speed of light and where we are using units with $K_C=\dfrac{1}{4\pi \varepsilon_0}=1$. Here $K_C$ is the Coulomb constant, generally with a value $9\cdot 10^9Nm^2/C^2$, but we rescale units in order it has a value equal to the unit. We will discuss more about frequently used system of units soon.

As the electromagnetic alpha constant depends on the electric charge, the Coulomb’s electromagnetic constant ( rescaled to one in some “clever” units), the Planck’s constant ( rationalized by $2\pi$ since $\hbar=h/2\pi$) and the speed of light, it codes some deep information of the Universe inside of it. The electromagnetic alpha $\alpha_{EM}$ is quantum and relativistic itself, and it also is related to elemental charges. Why alpha has the value it has is a complete mystery. Many people has tried to elucidate why it has the value it has today, but there is no reason of why it should have the value it has. Of course, it happens as well with some other constants but this one is particularly important since it is involved in some important numbers in atomic physics and the most elemental atom, the hydrogen atom.

In atomic physics, there are two common and “natural” scales of length. The first scale of length is given by the Compton’s wavelength of electrons. Usint the de Broglie equation, we get that the Compton’s wavelength is the wavelength of a photon whose energy is the same as the rest mass of the particle, or mathematically speaking:

$\boxed{\lambda=\dfrac{h}{p}=\dfrac{h}{mc}}$

Usually, physicists employ the “reduced” or “rationalized” Compton’s wavelength. Plugging the electron mass, we get the electron reduced Compton’s wavelength:

$\boxed{\lambda_C=\dfrac{\lambda}{2\pi}=\dfrac{\hbar}{m_ec}=\dfrac{\hbar}{m_ec}=3.86\cdot 10^{-13}m}$

The second natural scale of length in atomic physics is the so-called Böhr radius. It is given by the formula:

$\boxed{a_B=\dfrac{\hbar^2}{m_e e^2}=5.29\cdot 10^{-11}m}$

Therefore, there is a natural mass ratio between those two length scales, and it shows that it is precisely the electromagnetic fine structure constant alpha $\alpha_{EM}$:

$\boxed{R_\alpha=\dfrac{\mbox{Reduced Compton's wavelength}}{\mbox{B\"{o}hr radius}}=\dfrac{\lambda_C}{a_B}=\dfrac{\left(\hbar/m_e c\right)}{\left(\hbar^2/m_ee^2\right)}=\dfrac{e^2}{\hbar c}=\alpha_{EM}=7.30\cdot 10^{-3}}$

Furthermore, we can show that the electromagnetic alpha also is related to the mass ration between the electron energy in the fundamental orbit of the hydrogen atom and the electron rest energy. These two scales of energy are given by:

1) Rydberg’s energy ( electron ground minimal energy in the fundamental orbit/orbital for the hydrogen atom):

$\boxed{E_H=\dfrac{m_ee^4}{2\hbar^2}=13.6eV}$

2) Electron rest energy:

$\boxed{E_0=m_ec^2}$

Then, the ratio of those two “natural” energies in atomic physics reads:

$\boxed{R'_E=\dfrac{\mbox{Rydberg's energy}}{\mbox{Electron rest energy}}=\dfrac{m_ee^4/2\hbar^2}{m_ec^2}=\dfrac{1}{2}\left(\dfrac{e^2}{\hbar c}\right)^2=\dfrac{\alpha_{EM}^2}{2}=2.66\cdot 10^{-5}}$

or equivalently

$\boxed{\dfrac{1}{R'_E}=37600=3.76\cdot 10^4}$

R.J.Adler’s paper remarks that there is a cosmological/microscopic analogue of the above two ratios, and they involve the infamous Einstein’s cosmological constant. In Cosmology, we have two natural (ultimate?) length scales:

1st. The (ultra)microscopic and ultrahigh energy (“ultraviolet” UV regulator) relevant Planck’s length $L_P$, or equivalently the squared value $L_P^2$. Its value is given by:

$\boxed{L_P^2=\dfrac{G\hbar}{c^3}\leftrightarrow L_P=\sqrt{\dfrac{G\hbar}{c^3}}=1.62\cdot 10^{35}m}$

This natural length can NOT be related to any “classical” theory of gravity since it involves and uses the Planck’s constant $\hbar$.

2nd. The (ultra)macroscopic and ultra-low-energy (“infrared” IR regulator) relevant cosmological constant/deSitter radius. They are usualy represented/denoted by $\Lambda$ and $R_{dS}$ respectively, and they are related to each other in a simple way. The dimensions of the cosmological constant are given by

$\boxed{\left[\Lambda \right]=\left[ L^{-2}\right]=(\mbox{Length})^{-2}}$

The de Sitter radius and the cosmological constant are related through a simple equation:

$\boxed{R_{dS}=\sqrt{\dfrac{3}{\Lambda}}\leftrightarrow R^2_{dS}=\dfrac{3}{\Lambda}\leftrightarrow \Lambda =\dfrac{3}{R^2_{dS}}}$

The de Sitter radius is obtained from cosmological measurements thanks to the so called Hubble’s parameter ( or Hubble’s “constant”, although we do know that Hubble’s “constant” is not such a “constant”, but sometimes it is heard as a language abuse) H. From cosmological data we obtain ( we use the paper’s value without loss of generality):

$H=\dfrac{73km/s}{Mpc}$

This measured value allows us to derive the Hubble’s length paremeter

$L_H=\dfrac{c}{H}=1.27\cdot 10^{26}m$

Moreover, the data also imply some density energy associated to the cosmological “constant”, and it is generally called Dark Energy. This density energy from data is written as:

$\Omega_\Lambda =\Omega^{data}_{\Lambda}$

and from this, it can be also proved that

$R_{dS}=\dfrac{L_H}{\sqrt{\Omega_\Lambda}}=1.46\cdot 10^{26}m$

where we have introduced the experimentally deduced value $\Omega_\Lambda\approx 0.76$ from the cosmological parameter global fits. In fact, the cosmological constant helps us to define the beautiful and elegant formula that we can call the gravitational alpha/gravitational cosmological fine structure constant $\alpha_G$:

$\boxed{\alpha_G\equiv \dfrac{\mbox{Planck's length}}{\mbox{normalized de Sitter radius}}=\dfrac{L_P}{\dfrac{R_{dS}}{\sqrt{3}}}=\dfrac{\sqrt{\dfrac{G\hbar}{c^3}}}{\sqrt{\dfrac{1}{\Lambda}}}=\sqrt{\dfrac{G\hbar\Lambda}{c^3}}}$

or equivalently, defining the cosmological length associated to the cosmological constant as

$L^2_\Lambda=\dfrac{1}{\Lambda}=\dfrac{R^2_{dS}}{3}\leftrightarrow L_\Lambda=\sqrt{\dfrac{1}{\Lambda}}=\dfrac{R_{dS}}{\sqrt{3}}$

$\boxed{\alpha_G\equiv \dfrac{\mbox{Planck's length}}{\mbox{Cosmological length}}=\dfrac{L_P}{L_\Lambda}=\dfrac{\sqrt{\dfrac{G\hbar}{c^3}}}{\sqrt{\dfrac{1}{\Lambda}}}=\sqrt{\dfrac{G\hbar\Lambda}{c^3}}=L_P\sqrt{\Lambda}=L_P\dfrac{R_{dS}}{\sqrt{3}}}$

If we introduce the numbers of the constants, we easily obtaint the gravitational cosmological alpha value and its inverse:

$\boxed{\alpha_G=1.91\cdot 10^{-61}\leftrightarrow \alpha_G^{-1}=\dfrac{1}{\alpha_G}=5.24\cdot 10^{60}}$

They are really small and large numbers! Following the the atomic analogy, we can also create a ratio between two cosmologically relevant density energies:

1st. The Planck’s density energy.

Planck’s energy is defined as

$\boxed{E_P=\dfrac{\hbar c}{L_P}=\sqrt{\dfrac{\hbar c^5}{G}}=1.22\cdot 10^{19}GeV=1.22\cdot 10^{16}TeV}$

The Planck energy density $\rho_P$ is defined as the energy density of Planck’s energy inside a Planck’s cube or side $L_P$, i.e., it is the energy density of Planck’s energy concentrated inside a cube with volume $V=L_P^3$. Mathematically speaking, it is

$\boxed{\rho_P=\dfrac{E_P}{L_P^3}=\dfrac{c^7}{\hbar G^2}=2.89\cdot 10^{123}\dfrac{GeV}{m^3}}$

It is an huge density energy!

Remark: Energy density is equivalent to pressure in special relativity hydrodynamics. That is,

$\mathcal{P}_P=\rho_P=\tilde{\rho}_P c^2=4.63\cdot 10^{113}Pa$

wiht Pa denoting pascals ($1Pa=1N/m^2$) and where $\tilde{\rho}_P$ represents here matter (not energy) density ( with units in $kg/m^3$). Of course, turning matter density into energy density requires a multiplication by $c^2$. This equivalence between vacuum pressure and energy density is one of the reasons because some astrophysicists, cosmologists and theoretical physicists call “vacuum pressure” to the “dark energy/cosmological constant” term in the study of the cosmic components derived from the total energy density $\Omega$.

2nd. The cosmological constant density energy.

Using the Einstein’s field equations, it can be shown that the cosmological constant gives a contribution to the stress-energy-momentum tensor. The component $T^{0}_{\;\; 0}$ is related to the dark energy ( a.k.a. the cosmological constant) and allow us to define the energy density

$\boxed{\rho_\Lambda =T^{0}_{\;\; 0}=\dfrac{\Lambda c^4}{8\pi G}}$

Using the previous equations for G as a function of Planck’s length, the Planck’s constant and the speed of light, and the definitions of Planck’s energy and de Sitter radius, we can rewrite the above energy density as follows:

$\boxed{\rho_\Lambda=\dfrac{3}{8\pi}\left(\dfrac{E_P}{L_PR^2_{dS}}\right)=4.21 \dfrac{GeV}{m^3}}$

Thus, we can evaluate the ration between these two energy densities! It provides

$\boxed{R_\rho =\dfrac{\mbox{Planck's energy density}}{\mbox{CC energy density}}=\dfrac{\rho_P}{\rho_\Lambda}=\left( \dfrac{3}{8\pi}\right)\left(\dfrac{L_P}{R_{dS}}\right)^2=\left(\dfrac{1}{8\pi}\right)\alpha_G^2=1.45\cdot 10^{-123}}$

and the inverse ratio will be

$\boxed{\dfrac{1}{R_\rho}=6.90\cdot 10^{122}}$

So, we have obtained two additional really tiny and huge values for $R_\rho$ and its inverse, respectively. Note that the power appearing in the ratios of cosmological lengths and cosmological energy densities match the same scaling property that the atomic case with the electromagnetic alpha! In the electromagnetic case, we obtained $R\sim \alpha_{EM}$ and $R_E\sim \alpha_{EM}^2$. The gravitational/cosmological analogue ratios follow the same rule $R\sim \alpha_G$ and $R_\rho\sim \alpha_G^2$ but the surprise comes from the values of the gravitational alpha values and ratios. Some comments are straightforward:

1) Understanding atomic physics involved the discovery of Planck’s constant and the quantities associated to it at fundamental quantum level ( Böhr radius, the Rydberg’s constant,…). Understanding the Cosmological Constant value and the mismatch or stunning ratios between the equivalent relevant quantities, likely, require that $\Lambda$ can be viewed as a new “fundamental constant” or/and it can play a dynamical role somehow ( e.g., varying in some unknown way with energy or local position).

2) Currently, the cosmological parameters and fits suggest that $\Lambda$ is “constant”, but we can not be totally sure it has not varied slowly with time. And there is a related idea called quintessence, in which the cosmological “constant” is related to some dynamical field and/or to inflation. However, present data say that the cosmological constant IS truly constant. How can it be so? We are not sure, since our physical theories can hardly explain the cosmological constant, its value, and why it is current density energy is radically different from the vacuum energy estimates coming from Quantum Field Theories.

3) The mysterious value

$\boxed{\alpha_G=\sqrt{\dfrac{G\hbar\Lambda}{c^3}}=1.91\cdot 10^{-61}}$

is an equivalent way to express the biggest issue in theoretical physics. A naturalness problem called the cosmological constant problem.

In the literature, there have been alternative definitions of “gravitational fine structure constants”, unrelated with the above gravitational (cosmological) fine structure constant or gravitational alpha. Let me write some of these alternative gravitational alphas:

1) Gravitational alpha prime. It is defined as the ratio between the electron rest mass and the Planck’s mass squared:

$\boxed{\alpha'_G=\dfrac{Gm_e^2}{\hbar c}=\left(\dfrac{m_e}{m_P}\right)^2=1.75\cdot 10^{-45}}$

$\boxed{\alpha_G^{'-1}=\dfrac{1}{\alpha_G^{'}}=5.71\cdot 10^{44}}$

Note that $m_e=0.511MeV$. Since $m_{proton}=1836m_e$, we can also use the proton rest mass instead of the electron mass to get a new gravitational alpha.

2) Gravitational alpha double prime. It is defined as the ratio between the proton rest mass and the Planck’s mass squared:

$\boxed{\alpha''_G=\dfrac{Gm_{prot}^2}{\hbar c}=\left(\dfrac{m_{prot}}{m_P}\right)^2=5.90\cdot 10^{-39}}$

and the inverse value

$\boxed{\alpha_G^{''-1}=\dfrac{1}{\alpha_G^{''}}=1.69\cdot 10^{38}}$

Finally, we could guess an intermediate gravitational alpha, mixing the electron and proton mass.

3) Gravitational alpha triple prime. It is defined as the ration between the product of the electron and proton rest masses with the Planck’s mass squared:

$\boxed{\alpha'''_G=\dfrac{Gm_{prot}m_e}{\hbar c}=\dfrac{m_{prot}m_e}{m_P^2}=3.22\cdot 10^{-42}}$

and the inverse value

$\boxed{\alpha_G^{'''-1}=\dfrac{1}{\alpha^{'''}_G}=3.11\cdot 10^{41}}$

We can compare the 4 gravitational alphas and their inverse values, and additionally compare them with $\alpha_{EM}$. We get

$\alpha_G <\alpha_G^{'} <\alpha_G^{'''} < \alpha_G^{''}<\alpha_{EM}$

$\alpha_{EM}^{-1}<\alpha^{''-1}_G <\alpha^{'''-1}_G <\alpha^{'-1}_G < \alpha^{-1}_G$

These inequations mean that the electromagnetic fine structure constant $\alpha_{EM}$ is (at ordinary energies) 42 orders of magnitude bigger than $\alpha_G^{'}$, 39 orders of magnitude bigger than $\alpha_G^{'''}$, 36 orders of magnitude bigger than $\alpha_G^{''}$ and, of course, 58 orders of magnitude bigger than $\alpha_G$. Indeed, we could extend this analysis to include the “fine structure constant” of Quantum Chromodynamics (QCD) as well. It would be given by:

$\boxed{\alpha_s=\dfrac{g_s^2}{\hbar c}=1}$

since generally we define $g_s=1$. We note that $\alpha_s >\alpha_{EM}$ by 3 orders of magnitude. However, as strong nuclear forces are short range interactions, they only matter in the atomic nuclei, where confinement, and color forces dominate on every other fundamental interaction. Interestingly, at high energies, QCD coupling constant has a property called asymptotic freedom. But it is another story not to be discussed here! If we take the alpha strong coupling into account the full hierarchy of alphas is given by:

$\alpha_G <\alpha_G^{'} <\alpha_G^{'''} < \alpha_G^{''}<\alpha_{EM}<\alpha_s$

$\alpha_s^{-1}<\alpha_{EM}^{-1}<\alpha^{''-1}_G <\alpha^{'''-1}_G <\alpha^{'-1}_G < \alpha^{-1}_G$

Fascinating! Isn’t it? Stay tuned!!!

ADDENDUM: After I finished this post, I discovered a striking (and interesting itself) connection between $\alpha_{EM}$ and $\alpha_{G}$. The relation or coincidence is the following relationship

$\dfrac{1}{\alpha_{EM}}\approx \ln \left( \dfrac {1}{16\alpha_G}\right)$

Is this relationship fundamental or accidental? The answer is unknown. However, since the electric charge (via electromagnetic alpha) is not related a priori with the gravitational constant or Planck mass ( or the cosmological constant via the above gravitational alpha) in any known way I find particularly stunning such a coincidence up to 5 significant digits! Any way, there are many unexplained numerical coincidences that are completely accidental and meaningless, and then, it is not clear why this numeral result should be relevant for the connection between electromagnetism and gravity/cosmology, but it is interesting at least as a curiosity and “joke” of Nature.

Some quotes about the electromagnetic alpha from wikipedia http://en.wikipedia.org/wiki/Fine-structure_constant

“(…)There is a most profound and beautiful question associated with the observed coupling constant, e – the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won’t recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It’s one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the “hand of God” wrote that number, and “we don’t know how He pushed his pencil.” We know what kind of a dance to do experimentally to measure this number very accurately, but we don’t know what kind of dance to do on the computer to make this number come out, without putting it in secretly! (…)”. R.P.Feynman, QED: The Strange Theory of Light and Matter, Princeton University Press, p.129.

“(…) If alpha [the fine-structure constant] were bigger than it really is, we should not be able to distinguish matter from ether [the vacuum, nothingness], and our task to disentangle the natural laws would be hopelessly difficult. The fact however that alpha has just its value 1/137 is certainly no chance but itself a law of nature. It is clear that the explanation of this number must be the central problem of natural philosophy.(…)” Max Born, in A.I. Miller’s book Deciphering the Cosmic Number: The Strange Friendship of Wolfgang Pauli and Carl Jung. p. 253. Publisher W.W. Norton & Co.(2009).

“(…)The mystery about α is actually a double mystery. The first mystery – the origin of its numerical value α ≈ 1/137 has been recognized and discussed for decades. The second mystery – the range of its domain – is generally unrecognized.(…)” Malcolm H. Mac Gregor, M.H. MacGregor (2007). The Power of Alpha.

# LOG#050. Why riemannium?

## DEDICATORY

This special 50th log-entry is dedicated to 2 special people and scientists who inspired (and guided) me in the hard task of starting and writing this blog.

These two people are

1st. John C. Baez, a mathematical physicist. Author of the old but always fresh/brand new This Week Finds in Mathematical Physics, and now involved in the Azimuth blog. You can visit him here

http://johncarlosbaez.wordpress.com/

and here

http://math.ucr.edu/home/baez/

I was a mere undergraduate in the early years of the internet in my country when I began to read his TWF. If you have never done it, I urge to do it. Read him. He is a wonderful teacher and an excellent lecturer. John is now worried about global warming and related stuff, but he keeps his mathematical interests and pedagogical gifts untouched. I miss some topics about he used to discuss often before in his hew blog, but his insights about virtually everything he is involved into are really impressive. He also manages to share his entusiastic vision of Mathematics and Science. From pure mathematics to physics. He is a great blogger and scientist!

2nd. The professor Francis Villatoro. I am really grateful to him. He tries to divulge Science in Spain with his excellent blog ( written in Spanish language)

http://francisthemulenews.wordpress.com/

Finally, let me express and show my deepest gratitude to John and Francis. Two great and extraordinary people and professionals in their respective fields who inspired (and yet they do) me in spirit and insight in my early and difficult steps of writing this blog. I am just convinced that Science is made of little, ordinary and small contributions like mine, and not only the greatest contributions like those making John and Francis to the whole world. I wish they continue making their contributions in the future for many, many years yet to come.

Now, let me answer the question Francis asked me to explain here with further details. My special post/log-entry number 50…It will be devoted to tell you why this blog is called The Spectrum of Riemannium, and what is behind the greatest unsolved problem in Number Theory, Mathematics and likely Physics/Physmatics as well…Enjoy it!

## 1. THE RIEMANN ZETA FUNCTION ζ(s)

The Riemann zeta function is a device/object/function related to prime numbers.

In general, it is a function of complex variable $s=\sigma+i\tau$ defined by the next equation:

$\boxed{\displaystyle{\zeta (s)=\sum_{n=1}^{\infty}n^{-s}=\sum_{n=1}^{\infty}\dfrac{1}{n^s}=\prod_{p=2}^{\infty}\dfrac{1}{1-p^{-s}}=\prod_{p,\; prime}\dfrac{1}{1-p^{-s}}}}$

or

$\boxed{\displaystyle{\zeta (s)=\dfrac{1}{1-2^{-s}}\dfrac{1}{1-3^{-s}}\ldots\dfrac{1}{1-137^{-s}}\ldots}}$

Generally speaking, the Riemann zeta function extended by analytical continuation to the whole complex plane is “more” than the classical Riemann zeta function that Euler found much before the work of Riemann in the XIX century. The Riemann zeta function for real and entire positive values is a very well known (and admired) series by the mathematicians. $\zeta (1)=\infty$ due to the divergence of the harmonic series. Zeta values at even positive numbers are related to the Bernoulli numbers, and it is still lacking an analytic expression for the zeta values at odd positive numbers.

The Riemann zeta function over the whole complex plane satisfy the following functional equation:

$\boxed{\pi^{-\frac{s}{2}}\Gamma \left(\dfrac{s}{2}\right)\zeta (s)=\pi^{-\frac{(1-s)}{2}}\Gamma \left(\dfrac{1-s}{2}\right)\zeta (1-s)}$

Equivalently, it can be also written in a very simple way:

$\boxed{\xi (s)=\xi (1-s)}$

where we have defined

$\xi (s)=\pi^{-\frac{s}{2}}\Gamma \left(\dfrac{s}{2}\right)\zeta (s)$

Riemann zeta values are an example of beautiful Mathematics. From $\displaystyle{\zeta (s)=\sum_{n=1}^{\infty}n^{-s}}$, then we have:

1) $\zeta (0)=1+1+\ldots=-\dfrac{1}{2}$.

2) $\zeta (1)=1+\dfrac{1}{2}+\dfrac{1}{3}+\ldots =\infty$. The harmonic series is divergent.

3) $\zeta (2)=1+\dfrac{1}{2^2}+\dfrac{1}{3^2}+\ldots =\dfrac{\pi^2}{6}\approx 1.645$. The famous Euler result.

4) $\zeta (3)=1+\dfrac{1}{2^3}+\dfrac{1}{3^3}+\ldots \approx 1.202$. And odd zeta value called Apery’s constant that we do not know yet how to express in terms of irrational numbers.

5) $\zeta (4)=\dfrac{\pi^4}{90}\approx 1.0823$.

6) $\zeta (-2n)=-\dfrac{\pi^{-n}}{2\Gamma (-n+1)}=0,\;\;\forall n=1,2,\ldots ,\infty$. Trivial zeroes of zeta.

7) $\zeta (2n)=\dfrac{(-1)^{n+1}(2\pi)^{2n}B_{2n}}{2(2n)!}\;\;\forall n=1,2,\ldots ,\infty$, where $B_{2n}$ are the Bernoulli numbers. The first 13 Bernoulli numbers are:

$B_0=1, B_1=-\dfrac{1}{2}, B_2=\dfrac{1}{6}, B_3=0, B_4=-\dfrac{1}{30}, B_5=0, B_6=\dfrac{1}{42}$

$B_7=0, B_8=-\dfrac{1}{30}, B_9=0, B_{10}=\dfrac{5}{66}, B_{11}=0, B_{12}=-\dfrac{691}{2730}, B_{13}=0$

8) We note that $B_{2n+1}=0,\;\; \forall n\geq 1$.

9) $\zeta (-2n+1)=-\dfrac{B_{2n}}{2n}, \;\; \forall n=1,2,\ldots ,\infty$.

For instance, $\zeta (-1)=-\dfrac{1}{12}=1+2+3+\ldots$, $\zeta (-3)=\dfrac{1}{120}$, and $\zeta (-5)=-\dfrac{1}{252}$. Indeed, $\zeta (-1)$ arises in string theory trying to renormalize the vacuum energy of an infinite number of harmonic oscillators. The result in the bosonic string is $\dfrac{2}{2-D}$. In order to match with Riemann zeta function regularization of the above series, the bosonic string is asked to live in an ambient spacetime of D=26 dimensions. We also have that

$\sum \vert n\vert^3=-\dfrac{1}{60}$

10) $\zeta (\infty)=1$. The Riemann zeta value at the infinity is equal to the unit.

11) The derivative of the zeta function is $\displaystyle{\zeta '(s)=-\sum_{n=1}^{\infty}\dfrac{\log n}{n^s}}$. Particularly important of this derivative are:

$\displaystyle{\zeta '(0)=-\sum_{n=1}^\infty \log n=-\log \prod_{n=1}^\infty n=\zeta (0)\log (2\pi)=-\dfrac{1}{2}\log (2\pi)=-\log \sqrt{2\pi}=\log \dfrac{1}{\sqrt{2\pi}}}$

or $\zeta '(0)=\log \sqrt{\dfrac{1}{2\pi}}$

This allow us to define the factorial of the infinity as

$\displaystyle{\infty !=\prod_{n=1}^{\infty}n=1\cdot 2\cdots \infty=e^{-\zeta '(0)}=\sqrt{2\pi}}$

and the renormalized infinite dimensional determinant of certain operator A as:

$\det _\zeta (A)=a_1\cdot a_2\cdots=\exp \left(-\zeta_A '(0)\right)$, with $\displaystyle{\zeta _A (s)=\sum_{n=1}^\infty \dfrac{1}{a_n^s}}$

12) $\zeta (1+\varepsilon )=\dfrac{1}{\varepsilon}+\gamma_E +\mathcal{O} (\varepsilon )$. This is a result used by theoretical physicists in dimensional renormalization/regularization. $\gamma_E\approx 0.577$ is the so-called Euler-Mascheroni constant.

The alternating zeta function, called Dirichlet eta function, provides interesting values as well. Dirichlet eta function is defined and related to the Riemann zeta fucntion as follows:

$\boxed{\displaystyle{\eta (s)=\sum_{n=1}^\infty \dfrac{(-1)^{n+1}}{n^s}=\left(1-2^{1-s}\right)\zeta (s)}}$

This can be thought as “bosons made of fermions” or “fermions made of bosons” somehow. Special values of Dirichlet eta function are given by:

$\eta (0)=-\zeta (0)=\dfrac{1}{2}$ $\eta (1)=\log 2$ $\eta (2)=\dfrac{1}{2}\zeta (2)=\dfrac{\pi^2}{12}$

$\eta (3)=\dfrac{3}{4}\zeta (3)\approx \dfrac{3}{4}(1.202)$ $\eta (4)=\dfrac{7}{8}\zeta (4)=\dfrac{7}{8}\left(\dfrac{\pi^4}{90}\right)$

Remark(I): $\zeta(2)$ is important in the physics realm, since the spectrum of the hydrogen atom has the following aspect

$E(n)=-\dfrac{K}{n^2}$

and the Balmer formula is, as every physicist knows

$\Delta E(n,m)=K\left(\dfrac{1}{n^2}-\dfrac{1}{m^2}\right)$

Remark (II): The fact that $\zeta (2)$ is finite implies that the energy level separation of the hydrogen atom in the Böhr level tends to zero AND that the sum of ALL the possible energy levels in the hydrogen atom is finite since $\zeta (2)$ is finite.

Remark(III): What about an “atom”/system with spectrum $E(n)=\kappa n^{-s}$? If $s=2$, we do know that is the case of the Kepler problem. Moreover, it is easy to observe that $s=-1$ corresponds to tha harmonic oscillator, i.e., $E(n)=\hbar \omega n$. We also know that $s=-2$ is the infinite potential well. So the question is, what about a $n^{-3}$ spectrum and so on?

In summary, does the following spectrum

$\boxed{E=\mathbb{K}\dfrac{1}{n^{s}}}$

with energy separation/splitting

$\boxed{\Delta E(n,m;s)=\mathbb{K}\left(\dfrac{1}{n^{s}}-\dfrac{1}{m^{s}}\right)}$

exist in Nature for some physical system beyond the infinite potential well, the harmonic oscillator or the hydrogen atom, where $s=-2$, $s=-1$ and $s=2$ respectively?

It is amazing how Riemann zeta function gets involved with a common origin of such a different systems and spectra like the Kepler problem, the harmonic oscillator and the infinite potential well!

## 2. THE RIEMANN HYPOTHESIS

The Riemann Hypothesis (RH) is the greatest unsolved problem in pure Mathematics, and likely, in Physics too. It is the statement that the only non-trivial zeroes of the Riemann zeta function, beyond the trivial zeroes at $s=-2n,\;\forall n=1,2,\ldots,\infty$ have real part equal to 1/2. In other words, the equation or feynmanity has only the next solutions:

$\boxed{\mbox{RH:}\;\;\zeta (s)=0\leftrightarrow \begin{cases} s_n=-2n,\;\forall n=1,\ldots,\infty\;\;\mbox{Trivial zeroes}\\ s_n=\dfrac{1}{2}\pm i\lambda_n, \;\;\forall n=1,\ldots,\infty \;\;\mbox{Non-trivial zeroes}\end{cases}}$

I generally prefer the following projective-like version of the RH (PRH):

$\boxed{\mbox{PRH:}\;\;\zeta (s)=0\leftrightarrow \begin{cases} s_n=-2n,\;\forall n=1,\ldots,\infty\;\;\mbox{Trivial zeroes}\\ s_n=\dfrac{1\pm i\overline{\lambda}_n}{2}, \;\;\forall n=1,\ldots,\infty \;\;\mbox{Non-trivial zeroes}\end{cases}}$

The Riemann zeta function can be sketched on the whole complex plane, in order to obtain a radiography about the RH and what it means. The mathematicians have studied the critical strip with ingenious tools an frameworks. The now terminated ZetaGrid project proved that there are billions of zeroes IN the critical line. No counterexample has been found of a non-trivial zeta zero outside the critical line (and there are some arguments that make it very unlikely). The RH says that primes “have music/order/pattern” in their interior, but nobody has managed to prove the RH. The next picture shows you what the RH “say” graphically:

If you want to know how the Riemann zeroes sound, M. Watkins has done a nice audio file to see their music.

You can learn how to make “music” from Riemann zeroes here http://empslocal.ex.ac.uk/people/staff/mrwatkin/zeta/munafo-zetasound.htm

And you can listen their sound here

http://empslocal.ex.ac.uk/people/staff/mrwatkin/zeta/zeta.mp3

Riemann zeroes are connected with prime numbers through a complicated formula called “the explicit formula”. The next equation holds  $\forall x\geq 2$ integer numbers, and non-trivial Riemann zeroes in the complex (upper) half-plane with $\tau>0$:

$\boxed{\displaystyle{\pi (x)+\sum_{n=2}^\infty \dfrac{\pi \left( x^{1/n}\right)}{n}=\text{Li} (x)-\sum_{\lambda =\sigma+i\tau }\left(\text{Li}(x^\lambda)+\text{Li}\left( x^{1-\lambda}\right)\right)+\int_x^\infty\dfrac{du}{u(u^2-1)\ln u}-\ln 2}}$

and where $\pi (x)$ is the celebrated Gauss prime number counting function, i.e., $\pi (x)$ represents the prime numbers that are equal than x or below. This explicit formula was proved by Hadamard. The explicit formula follows from both product representations of $\zeta (s)$, the Euler product on one side and the Hadamard product on the other side.

The function $\text{Li} (x)$, sometimes written as $\text{li} (x)$, is the logarithmic integral

$\displaystyle{\text{Li} (x) =\text{li} (x)= \int_2^x\dfrac{du}{\ln x}}$

The explicit formula comes in some cool variants too. For instance, we can write

$\pi (x)=\pi_0 (x)+\pi_1 (x)=\pi_{\mbox{smooth}}+\pi_{\mbox{osc-chaotic}}$

where

$\displaystyle{\pi_0 (x)=\sum_{n=1}^\infty\dfrac{\mu (n)}{n}\left[\mbox{Li}(x^{1/n})-\sum_{k=1}^\infty\mbox{Li}(x^{-2k/n})\right]}$

and

$\displaystyle{\pi_1 (x)=-2\mbox{Re}\sum_{n=1}^\infty\dfrac{\mu (n)}{n}\sum_{\alpha=1}^\infty\mbox{Li}(x^{(\sigma_\alpha+i\tau_\alpha)/n})}$

For large values of x, we have the asymptotics

$\pi_0 (x)\approx \mbox{Li} (x)$

and

$\displaystyle{\pi_1 (x)\approx -\dfrac{2}{\ln x}\sum_{\alpha=1}^\infty\dfrac{x^{\sigma_\alpha}}{\sigma_\alpha^2+\tau_\alpha^2}\left(\sigma_\alpha\cos (\tau_\alpha \ln x)+\tau_\alpha \sin (\tau_\alpha \ln x)\right)}$

Remark: Please, don’t confuse the logarithmic integral with the polylogarithm function $\text{Li}_x (s)$.

Gauss also conjectured that

$\pi (x)\sim \text{Li} (x)$

## 3. THE HILBERT-POLYA CONJECTURE

Date: January 3, 1982. Andrew Odlyzko wrote a letter to George Pólya about the physical ground/basis of the Riemann Hypothesis and the conjecture associated to Polya himself and David Hilbert. Polya answered and told Odlyzko that while he was in Göttingen around 1912 to 1914 he was asked by Edmund Landau for a physical reason that the Riemann Hypothesis should be true, and suggested that this would be the case if the imaginary parts, say $T$ of the non-trivial zeros

$\dfrac{1}{2}+iT$

of the Riemann zeta function corresponded to eigenvalues of an unbounded and unknown self adjoint operator $\hat{T}$. That statement was never published formally, but  it was remembered after all, and it was transmitted from one generation to another. At the time of Pólya’s conversation with Landau, there was little basis for such speculation. However, Selberg, in the early 1950s, proved a duality between the length spectrum of a Riemann surface and the eigenvalues of its Laplacian. This so-called Selberg trace formula shared a striking resemblance to the explicit formula of certain L-function, which gave credibility to the speculation of Hilbert and Pólya.

## 4. RANDOM MATRIX THEORY

Dialogue(circa 1970). “(…)Dyson: So tell me, Montgomery, what have you been up to? Montgomery: Well, lately I’ve been looking into the distribution of the zeros of the Riemann zeta function.  Dyson: Yes? And?  Montgomery: It seems the two-point correlations go as….(…) Dyson: Extraordinary! Do you realize that’s the pair-correlation function for the eigenvalues of a random Hermitian matrix? It’s also a model of the energy levels in a heavy nucleus, say U-238.(…)”

A step further was given in the 1970s, by the mathematician Hugh Montgomery. He investigated and found that the statistical distribution of the zeros on the critical line has a certain property, now called Montgomery’s pair correlation conjecture. The Riemann zeros tend not to cluster too closely together, but to repel. During a visit to the Institute for Advanced Study (IAS) in 1972, he showed this result to Freeman Dyson, one of the founders of the theory of random matrices. Dyson realized that the statistical distribution found by Montgomery appeared to be the same as the pair correlation distribution for the eigenvalues of a random and “very big/large” Hermitian matrix with size NxN. These distributions are of importance in physics and mathematics. Why? It is simple. The eigenstates of a Hamiltonian, for example the energy levels of an atomic nucleus, satisfy such statistics. Subsequent work has strongly borne out the connection between the distribution of the zeros of the Riemann zeta function and the eigenvalues of a random Hermitian matrix drawn from the theoyr of the so-calle Gaussian unitary ensemble, and both are now believed to obey the same statistics. Thus the conjecture of Pólya and Hilbert now has a more solid fundamental link to QM, though it has not yet led to a proof of the Riemann hypothesis. The pair-correlation function of the zeros is given by the function:

$R_2(x)=1-\left(\dfrac{\sin \pi x}{\pi x}\right)^2$

In a posterior development that has given substantive force to this approach to the Riemann hypothesis through functional analysis and operator theory, the mathematician Alain Connes has formulated a “trace formula” using his non-commutative geometry framework that is actually equivalent to certain generalized Riemann hypothesis. This fact has therefore strengthened the analogy with the Selberg trace formula to the point where it gives precise statements. However, the mysterious operator believed to provide the Riemann zeta zeroes remain hidden yet. Even worst, we don’t even know on which space the Riemann operator is acting on.

However, some trials to guess the Riemann operator has been given from a semiclassical physical environtment as well. Michael Berry  and Jon Keating have speculated that the Hamiltonian/Riemann operator $H$ is actually some kind of quantization of the classical Hamiltonian $XP$ where $P$ is the canonical momentum associated with the position operator $X$. If that Berry-Keating conjecture is true. The simplest Hermitian operator corresponding to $XP$ is

$H = \dfrac1{2} (xp+px) = - i \left( x \dfrac{\mathrm{d}}{\mathrm{d} x} + \dfrac{1}{2} \right)$

At current time, it is still quite inconcrete, as it is not clear on which space this operator should act in order to get the correct dynamics, nor how to regularize it in order to get the expected logarithmic corrections. Berry and Germán Sierra, the latter in collaboration with P.K.Townsed, have conjectured that since this operator is invariant under dilatations perhaps the boundary condition $f(nx)=f(x)$ for integer $n$ may help to get the correct asymptotic results valid for big $n$. That it, in the large $n$ we should obtain

$s_n=\dfrac{1}{2} + i \dfrac{ 2\pi n}{\log n}$

## 5. QUANTUM CHAOS AND RIEMANN DYNAMICS

Indeed, the Berry-Keating conjecture opened another striking attack to prove the RH. A topic that was popular in the 80’s and 90’s in the 20th century. The weird subject of “quantum chaos”. Quantum chaos is the subject devoted to the study of quantum systems corresponding to classically chaotic systems. The Berry-Keating conjecture shed light further into the Riemann dynamics, sketching some of the properties of the dynamical system behind the Riemann Hypothesis.

In summary, the dynamics of the Riemann operator should provide:

1st. The quantum hamiltonian operator behind the Riemann zeroes, in addition to the classical counterpart, the classical hamiltonian $H$, has a dynamics containing the scaling symmetry. As a consequence, the trajectories are the same at all energy scale.
2nd. The classical system corresponding to the Riemann dynamics is chaotic and unstable.
3rd. The dynamics lacks time-reversal symmetry.
4th. The dynamics is quasi one-dimensional.

A full dictionary translating the whole correspondence between the chaotic system corresponding to the Riemann zeta function and its main features is presented in the next table:

## 6. THE SPECTRUM OF RIEMANNIUM

In 2001, the following paper emerged, http://arxiv.org/abs/nlin/0101014. The Riemannium arxiv paper was published later (here: Reg. Chaot. Dyn. 6 (2001) 205-210). After that, Brian Hayes  wrote a really beautiful, wonderful and short paper titled The Spectrum of Riemannium in 2003 (American Scientist, Volume 91, Number 4 July–August, 2003,pages 296–300). I remember myself reading the manuscript and being totally surprised. I was shocked during several weeks. I decided that I would try to understand that stuff better and better, and, maybe, make some contribution to it. The Spectrum of Riemannium was an amazing name, an incredible concept. So, I have been studying related stuff during all these years. And I have my own suspitions about what the riemannium and the zeta function are, but this is not a good place to explain all of them!

The riemannium is the mysterious physical system behind the RH. Its spectrum, the spectrum of riemannium, are given by the RH and its generalizations.

Moreover, the following sketch from Hayes’ paper is also very illustrative:

What do you think? Isn’t it suggestive? Is it amazing?

## 7. ζ(s) AND RENORMALIZATION

Riemann zeta function also arises in the renormalization of the Standard Model and the regularization of determinants with “infinite size” (i.e., determinants of differential operators and/or pseudodifferential operators). For instance, the $\infty$-dimensional regularized determinant is defined through the Riemann zeta function as follows:

$\displaystyle{\det _\zeta \mathcal{P}=e^{-\zeta_{\mathcal{P}}^{'}(0)}}$

The dimensional renormalization/regularization of the SM makes use of the Riemann zeta function as well. It is ubiquitous in that approach, but, as far as I know, nobody has asked why is that issue important, as I have suspected from long time ago.

## 8. ζ(s) AND QUANTUM STATISTICS

Riemann zeta function is also used in the theory of Quantum Statistics. Quantum Statistics are important in Cosmology and Condensed Matter, so it is really striking that Riemann zeta values are related to phenomena like Bose-Einstein condensation or the Cosmic Microwave Background and also the yet to be found Cosmic Neutrino Background!

Let me begin with the easiest quantum (indeed classical) statistics, the Maxwell-Boltzmann (MB) statistics. In 3 spatial dimensions (3d) the MB distribution arises ( we will work with units in which $\hbar =1$):

$f(p)_{MB}=\dfrac{1}{(2\pi)^3}e^{\frac{\mu -E}{k_BT}}$

Usually, there are 3 thermodynamical quantities that physicists wish to compute with statistical distributions: 1) the number density of particles $n=N/V$, 2) the energy density $\varepsilon=U/V$ and 3) the pressure $P$. In the case of a MB distribution, we have the following definitions:

$\displaystyle{n=\dfrac{1}{(2\pi)^3}\int d^3p e^{\frac{\mu -E}{k_BT}}}$

$\displaystyle{\varepsilon =\dfrac{1}{(2\pi)^3}\int d^3p Ee^{\frac{\mu -E}{k_BT}}}$

$\displaystyle{\varepsilon =\dfrac{1}{(2\pi)^3}\int d^3p \dfrac{1}{3}\dfrac{\vert\mathbf{p}\vert^2}{E}e^{\frac{\mu -E}{k_BT}}}$

We can introduce the dimensionless variables $late z=\dfrac{mc^2}{k_BT}$, $\tau =\dfrac{E}{k_BT}=\dfrac{\sqrt{p^2+m^2c^4}}{k_BT}$. In this way,

$\vert p\vert=\dfrac{k_BT}{c}\sqrt{\tau^2-z^2}$

$c^2\vert\mathbf{p}\vert d\vert \mathbf{p}\vert=k_B^2T^2\tau d\tau$

$c^3\vert\mathbf{p}\vert^2d\vert\mathbf{p}\vert=k_B^3T^3\tau\sqrt{\tau^2-z^2}d\tau$

With these definitions, the particle density becomes

$\displaystyle{n=\dfrac{4\pi k_B^3T^3}{(2\pi)^3}e^{\frac{\mu}{k_BT}}\int_z^\infty d\tau (\tau^2-z^2)^{1/2}\tau e^{-\tau}}$

This integral can be calculated in closed form with the aid of modified Bessel functions of the 2th kind:

$K_n (z)=\dfrac{2^nn!}{(2n)!z^n}\int_z^\infty d\tau (\tau^2-z^2)^{n-1/2}e^{-\tau}$ or equivalently

$K_n (z)=\dfrac{2^{n-1}(n-1)!}{(2n-2)!z^n}\int_z^\infty d\tau (\tau^2-z^2)^{n-3/2}\tau e^{-\tau}$

$K_{n+1} (z)=\dfrac{2nK_n (z)}{z}+K_{n-1} (z)$

$\displaystyle{K_2 (x)=\dfrac{1}{z^2}\int_z^\infty (\tau^2-z^2)^{1/2}\tau e^{-\tau}d\tau}$

And thus, we have the next results (setting $c=1$ for simplicity):

$\mbox{Particle number density}\equiv n=\dfrac{N}{V}=\dfrac{k_B^3T^3}{2\pi^2}z^2K_2 (z)=\dfrac{k_B^3T^3}{2\pi^2}\left(\dfrac{m}{k_BT}\right)^2K_2\left(\dfrac{m}{k_BT}\right)e^{\frac{\mu}{k_BT}}$

$\mbox{Energy density}\equiv\varepsilon=\dfrac{k_B^4T^4}{2\pi^2}\left[ 3\left(\dfrac{m}{k_BT}\right)^2K_2\left(\dfrac{m}{k_BT}\right)+\left(\dfrac{m}{k_BT}\right)^3K_1\left(\dfrac{m}{k_BT}\right)\right]e^{\frac{\mu}{k_BT}}$

$\mbox{Pressure}\equiv P=\dfrac{k_B^4T^4}{2\pi^2}\left(\dfrac{m}{k_BT}\right)^2K_2\left(\dfrac{m}{k_BT}\right)e^{\frac{\mu}{k_BT}}$

Even entropy density is easiy to compute:

$\mbox{Entropy density}\equiv s=\dfrac{m^3}{2\pi^2}e^{\frac{\mu}{k_BT}}\left[ K_1\left(\dfrac{m}{k_BT}\right)+\dfrac{4k_BT-\mu}{m}K_2\left(\dfrac{m}{k_BT}\right)\right]$

These results can be simplified in some limit cases. For instance, in the massless limit $z=m/k_BT\rightarrow 0$. Moreover, we also know that $\displaystyle{\lim_{z\rightarrow 0}z^nK_n (z)=2^{n-1}(n-1)!}$. In such a case, we obtain:

$n\approx \dfrac{k_B^3T^3}{\pi^2}e^{\frac{\mu}{k_BT}}$

$\varepsilon \approx \dfrac{3k_B^4T^4}{\pi^2}e^{\frac{\mu}{k_BT}}$

$P\approx \dfrac{k_B^4T^4}{\pi^2}e^{\frac{\mu}{k_BT}}$

We note that $\varepsilon=3P$ in this massless limit.

Remark (I): In the massless limit, and whenever there is no degeneracy, $\varepsilon =3P$ holds.

Remark (II): If there is a quantum degeneracy in the energy levels, i.e., if $g\neq 1$, we must include an extra factor of $g_j=2j+1$ for massive particles of spin j. For massless photons with helicity, there is a $g=2$ degeneracy.

Remark (III): In the D-dimensional (D=d+1) Bose gas with dispersion relationship $\varepsilon_p=cp^{s}$, it can be shown that the pressure is related with the energy density in the following way

$\mbox{Pressure}\equiv P=\dfrac{s}{d}\dfrac{U}{V}=\dfrac{s}{d}\varepsilon$

Remark (IV): Let us define $p^s (n)$ as the number of ways an integer number can be expressed as a sum of the sth powers of integers. For instance,

$p^1 (5)=7$ because $5=4+1=3+2=3+1+1=2+2+1=2+1+1+1=1+1+1+1+1$

$p^2 (5)=2$ because $5=2^2+1^2=1^2+1^2+1^2+1^2+1^2$

If $E_n=n^s$ with $n\geq 1$ and $s>0$, then $x=e^{-\beta}$ and the partition function is

$\displaystyle{Z=\prod_{k}\left( 1+e^{\frac{\mu-E}{k_BT}}\right)}$

We will see later that $\displaystyle{\sum_{N=0}^\infty x^N=\begin{cases}1+x, FD \\ \dfrac{1}{1-x}, BE\end{cases}}$

with $\mu =0$ is nothing but the generatin function of the partitions $p^s (n)$

$\displaystyle{Z(x=e^{-\beta})=\prod_{n=1}^\infty \dfrac{1}{1-x^{n^s}}=\sum_{n=1}^\infty p^s (n) x^n\approx \int_1^\infty dn p^s (n) e^{-\beta n}}$

The Hardy-Ramanujan inversion formula reads (for the case s=1 only):

$p(n) \approx \dfrac{1}{4\sqrt{3}N}e^{\pi\sqrt{2N/3}}$

Remark (V): There are some useful integrals in quantum statistics. They are the so-called Bose-Einstein/Fermi-Dirac integrals

$\displaystyle{\int_0^\infty dx \dfrac{x^{n-1}}{e^x\mp 1}=\begin{cases}\Gamma (n) \zeta (n), \;\; BE\\ \Gamma (n)\eta (n)=\Gamma (n) (1-2^{1-n})\zeta (n),\;\; FD\end{cases}}$

The BE-FD quantum distributions in 3d are defined as follows:

$\displaystyle{f(p)=\dfrac{1}{(2\pi)^3}\sum_{n=1}^{\infty}(\mp)^{n+1}e^{-n\frac{(E-\mu)}{k_BT}}}$

where the minus sign corresponds to FD and the plus sign to BE.

We will firstly study the BE distribution in 3d. We have:

$\displaystyle{n=\dfrac{1}{(2\pi)^3}\int d^3p \left(e^{\frac{\mu-E}{k_BT}}-1\right)^{-1}=\dfrac{1}{(2\pi)^3}\int d^3p \sum_{n=1}^{\infty}(+1)^{n+1}e^{\frac{n\mu-nE}{k_BT}}}$

Introducing a scaled temperature $T'=T/n$, we get

$\displaystyle{n=\sum_{n=1}^{\infty}\left[\dfrac{1}{(2\pi)^3}\int d^3p e^{\frac{n\mu-nE}{k_BT'}}\right]=\sum_{n=1}^{\infty}\dfrac{k_B^3T^3}{2\pi^2}\dfrac{1}{n^3}\left(\dfrac{nm}{k_BT}\right)^2K_2\left(\dfrac{nm}{k_BT}\right)e^{\frac{n\mu}{k_BT}}}$

$\displaystyle{\varepsilon=\sum_{n=1}^{\infty}\dfrac{k_B^4T^4}{n^4(2\pi^2)}\left[3\left(\dfrac{nm}{k_BT}\right)^2K_2\left(\dfrac{nm}{k_BT}\right)+\left(\dfrac{nm}{k_BT}\right)^3K_1\left(\dfrac{nm}{k_BT}\right)\right]e^{\frac{n\mu}{k_BT}}}$

$\displaystyle{P=\sum_{n=1}^{\infty}\dfrac{k_B^4T^4}{n^4(2\pi^2)}\left(\dfrac{nm}{k_BT}\right)^2K_2\left(\dfrac{nm}{k_BT}\right)e^{\frac{n\mu}{k_BT}}}$

Again, we can study a particularly simple case: the massless limit $m\rightarrow 0$ with $\mu\rightarrow 0$. In this case, we get:

$\displaystyle{n=\dfrac{k_B^3T^3}{\pi^2}\sum_{n=1}^\infty \dfrac{1}{n^3}=\dfrac{k_B^3T^3}{\pi^2}\zeta (3)\approx 1.202\dfrac{k_B^3T^3}{\pi^2}}$

$\displaystyle{\varepsilon=\sum_{n=1}^\infty\dfrac{3(k_BT)^4}{\pi^2}\dfrac{1}{n^4}=\dfrac{3(k_BT)^4\zeta (4)}{\pi^2}=\dfrac{\pi^2}{30}(k_BT)^4}$

$\displaystyle{P=\sum_{n=1}^\infty\dfrac{(k_BT)^4}{\pi^2}\dfrac{1}{n^4}=\dfrac{(k_BT)^4\zeta (4)}{\pi^2}=\dfrac{\pi^2(k_BT)^4}{90}}$

The FD distribution in 3d can be studied in a similar way. Following the same approach as the BE distribution, we deduce that:

$\displaystyle{n=\sum_{n=1}^\infty (-1)^{n+1}\dfrac{(k_BT)^3}{2\pi^2n^3}\left(\dfrac{nm}{k_BT}\right)^2K_2\left(\dfrac{nm}{k_BT}\right)e^{\frac{\mu n}{k_BT}}}$

$\displaystyle{\varepsilon= \sum_{n=1}^\infty (-1)^{n+1}\dfrac{(k_BT)^4}{2\pi^2}\left[3\left(\dfrac{nm}{k_BT}\right)^2K_2\left(\dfrac{nm}{k_BT}\right)+\left(\dfrac{nm}{k_BT}\right)^3K_1\left(\dfrac{nm}{k_BT}\right)\right]e^{\frac{\mu n}{k_BT}}}$

$\displaystyle{P=\sum_{n=1}^\infty (-1)^{n+1}\dfrac{(k_BT)^4}{2\pi^2}\dfrac{1}{n^4}\left(\dfrac{nm}{k_BT}\right)^2K_2\left(\dfrac{nm}{k_BT}\right)e^{\frac{n\mu}{k_BT}}}$

and again the massless limit $m=0$ and $\mu\rightarrow 0$ provide

$\displaystyle{n\approx \dfrac{(k_BT)^3}{\pi^2}\sum_{n=1}^\infty (-1)^{n+1}\dfrac{1}{n^3}=\dfrac{(k_BT)^3}{\pi^2}\eta (3)=\dfrac{(k_BT)^3}{\pi^2}\left(\dfrac{3}{4}\right)\zeta (3)}$

$\displaystyle{\varepsilon\approx \dfrac{3(k_BT)^4}{\pi^2}\sum_{n=1}^\infty (-1)^{n+1}\dfrac{1}{n^4}=3(k_BT)^4\eta (4)=3(k_BT)^4\dfrac{7}{8}\zeta (4)=\dfrac{\pi^2(k_BT)^4}{30}\left(\dfrac{7}{8}\right)}$

$\displaystyle{P\approx \dfrac{(k_BT)^4}{\pi^2}\sum_{n=1}^\infty (-1)^{n+1}\dfrac{1}{n^4}=\left(\dfrac{7}{8}\right)\dfrac{\pi^2(k_BT)^4}{90}}$

Remark (I): For photons $\gamma$ with degeneracy $g=2$ we obtain

$n_\gamma =\dfrac{2\zeta (3) (k_BT)^3}{\pi^2}$

$\varepsilon_\gamma= 3P_\gamma =\dfrac{\pi^2 (k_BT)^4}{15}$

$s_\gamma =P'(T)=\dfrac{4}{3}\left(\dfrac{\pi^2}{15}\right)(k_BT)^3=\dfrac{2\pi^4}{45\zeta (3)}n$

Remark (II): In Cosmology, Astrophysics and also in High Energy Physics, the following units are used

$1eV=1.602\cdot 10^{-19}J$

$\hbar=1=6.58\cdot 10^{-22}MeVs=7.64\cdot 10^{-12}Ks$

$\hbar c=1=0.19733GeV\cdot fm=0.2290 K\cdot cm$

$1 K=0.1532\cdot 10^{-36}g\cdot c^2$

The Cosmic Microwave Background is the relic photon radiation of the Big Bang, and thus it has a temperature due to photons in the microwave band of the electromagnetic spectrum. Its value is:

$T_\gamma \approx 2.725K$

Indeed, it also implies that the relic photon density is about $n_\gamma =410\dfrac{1}{cm^3}$

It is also speculated that there has to be a Cosmic Neutrino Background relic from the Big Bang. From theoretical Cosmology, it is related to the photon CMB temperature in the following way:

$T_\nu =\left(\dfrac{4}{11}\right)^{1/3}2.7K$ or equivalently

$T_\nu\approx 1.9K$

This temperature implies a relic neutrino density (per species, i.e., with $g_\nu=1$) about

$n_\nu=54\dfrac{1}{cm^3}$

The cosmological density entropy due to these particles is

$s_0=\dfrac{S_0}{V}=\dfrac{4\pi^2}{45}\left[1+\dfrac{2\cdot 3}{2}\left(\dfrac{7}{8}\right)\left(\dfrac{4}{11}\right)\right]T_{0\gamma}^3=2810\dfrac{1}{cm^3}\left( \dfrac{T_{0\gamma}}{2.7K}\right)^3$

and then we get

$s_0\approx 7.03n_{0\gamma}$

Remark (III): In Cosmology, for fermions in 3d ( note that BE implies $\varepsilon=3P$, and that we must drop the factors $\left( 7/8\right), \left( 3/4\right), \left( 7/6\right)$ in the next numerical values) we can compute

$n=\begin{cases}\left(\dfrac{g}{2}\right)\left(\dfrac{3}{4}\right)\dfrac{2\zeta (3)}{\pi^2}(k_BT)^3\\ \left(\dfrac{g}{2}\right)\left(\dfrac{3}{4}\right)31.700\left(\dfrac{k_BT}{GeV}\right)^3\dfrac{1}{fm^3}\\ \left(\dfrac{g}{2}\right)\left(\dfrac{3}{4}\right)20.288\left(\dfrac{T}{K}\right)^3\dfrac{1}{cm^3}\end{cases}$

$\varepsilon=3P=\begin{cases}\left(\dfrac{g}{2}\right)\left(\dfrac{7}{8}\right)\left(\dfrac{\pi^2}{15}\right)(k_BT)^4\\ \left(\dfrac{g}{2}\right)\left(\dfrac{7}{8}\right)(85.633)\left(\dfrac{k_BT}{GeV}\right)\dfrac{GeV}{fm^3}\\ \left(\dfrac{g}{2}\right)\left(\dfrac{7}{8}\right)\left(0.841\cdot 10^{-36}\right)\left(\dfrac{T}{K}\right)^4\dfrac{g}{cm^3}\end{cases}$

$s=\dfrac{S}{V}=\left(\dfrac{g}{2}\right)\left(\dfrac{7}{8}\right)\left(\dfrac{4\pi^2}{45}\right)(k_BT)^3=\dfrac{7}{6}\left[\dfrac{2\pi^4}{45\zeta (3)}\right] n$

Remark (IV): An example of the computation of degeneracy factor is the quark-gluon plasma degeneracy $g_{QGP}$. Firstly we compute the gluon and quark degeneracies

$g_g=(\mbox{color})(\mbox{spin})=2^3\cdot 2=8\cdot 2=16$

$g_q=(p\overline{p})(\mbox{spin})(\mbox{color})(\mbox{flavor})=2\cdot 2\cdot 3\cdot N_f=12N_f$

Then, the QG plasma degeneracy factor is

$g_{QGP}=g_g+\dfrac{7}{8}g_q=16+\dfrac{7}{8}12N_f=16+\dfrac{21}{2}N_f \leftrightarrow \boxed{g_{QGP}=16+\dfrac{21}{2}N_f}$

In general, for charged leptons and nucleons $g=2$, $g=1$ for neutrinos (per species, of course), and $g=2$ for gluons and photons. Remember that massive particles with spin j will have $g_j=2j+1$.

Remark (V): For the Planck distribution, we also get the known result for the thermal distribution of the blackbody radiation

$\displaystyle{I(T)=\int_0^\infty f(\nu ,T)d\nu=\dfrac{8\pi h}{c^3}\int_0^\infty \dfrac{\nu^3d\nu}{e^{\frac{h\nu}{k_BT}}-1}=\dfrac{8\pi^5k_B^4T^4}{15c^3h^3}}$

Remark (VI): Sometimes the following nomenclature is used

i) Extremely degenerated gas if $\mu>>k_BT$

ii) Non-degenerated gas if $\mu <<-k_BT$

iii) Extremely relativistic gas ( or ultra-relativistic gas) if $p>> mc$

iv) Non-relativistic gas if $p<

## 9. ζ(s) AND GROUP ENTROPIES

Let us define the following shift operator $\hat{T}$:

$\hat{T}f(x)=f(x+\sigma)$

where $\sigma\in \mathbb{R}$. Moreover, there is certain isomorphism  between the shift operator space and the space of functions through the map $\hat{T}\leftrightarrow x^\sigma$.

We define the generalized logarithm as the image under the previous map of $\hat{T}$. That is:

$\displaystyle{\mbox{Log}_G(x)\equiv \dfrac{1}{\sigma}\sum_{n=l}^{m}k_n x^{\sigma n}}$

where $l,m\in \mathbb{Z}$, with $l, $m-l=r$ and $x>0$. Furthermore, the next contraints are also given for every generalized logarithm:

1st. $\displaystyle{\sum_{n=1}^m k_n=0}$.

2nd. $\displaystyle{\sum_{n=l}^m nk_n=c}$, $k_m\neq 0$, and $k_l\neq 0$.

3rd. $\displaystyle{\sum_{n=l}^m\vert n\vert^l k_n=K_l}$, $\forall l=2,3,\ldots ,m-l$ and where $K_l \in \mathbb{R}$.

With these definitions we also have that

A) $\mbox{Log}_G(x)=\ln (x)$

B) $\mbox{Log}_G(1)=0$

Examples of generalized logarithms are:

1) The Tsallis logarithm.

$\mbox{Log}_T(x)=\dfrac{x^{1-q}-1}{1-q}$

$\mbox{Log}_K(x)=\dfrac{x^\kappa-x^{-\kappa}}{2\kappa}$

3) The Abe logarithm.

$\mbox{Log}_A(x)=\dfrac{x^{\sigma -1}-x^{\sigma^{-1}-1}}{\sigma-\sigma^{-1}}$

4) The biparametric logarithm.

$\mbox{Log}_B(x)=\dfrac{x^a-x^b}{a-b}$

with $a=\sigma-1$ and $b=\sigma^{-1}-1$ in the case of the Abe logarithm.

Group entropies are defined through the use of generalized logarithms. Define some discrete probability distribution $\left[ p_i\right]_{i=1,\ldots,W}$ with normalization $\displaystyle{\sum_{i=1}^Wp_i=1}$. Therefore, the group entropy is the following functional sum:

$\boxed{\displaystyle{S_G=-k_B\sum_{i=1}^{W}p_i \mbox{Log}_G \left(\dfrac{1}{p_i}\right)}}$

where we have used the previous definition of generalized logarithm and the Boltzmann’s constant $k_B$ is a real number. It is called group entropy due to the fact that $S_G$ is connected to some universal formal group. This formal group will determine some correlations for the class of physical systems under study and its invariant properties. In fact, the Tsallis logarithm itself is related to the Riemann zeta function through a beautiful equation! Under the Tsallis group exponential, the isomorphism $x\leftrightarrow e^t$ is defined to be $e_G^t=\dfrac{e^{(1-q)t}-1}{1-q}$, and thus we easily get:

$\displaystyle{\dfrac{1}{\Gamma (s)}=\int_0^\infty\dfrac{1}{\dfrac{e^{(1-q)t}-1}{1-q}}t^{s-1}dt=\dfrac{\zeta (s)}{(1-q)^{s-1}}}$

$\forall s$ such as $Re (s)>1$ and $q<1$.

## 10. ζ(s) AND THE PRIMON GAS

The primon gas/free Riemann gas is a statistical mechanics toy model illustrating in a simple way some correspondences between number theory and concepts in statistical physics, quantum mechanics, quantum field theory and dynamical systems.

The primon gas IS  a quantum field theory (QFT) of a set of non-interacting particles, called the “primons”. It is also named a gas or a free model because the particles are non-interacting. There is no potential. The idea of the primon gas was independently discovered  by Donald Spector (D. Spector, Supersymmetry and the Möbius Inversion Function, Communications in Mathemtical Physics 127 (1990) pp. 239-252) and Bernard Julia (Bernard L. Julia, Statistical theory of numbers, in Number Theory and Physics, eds. J. M. Luck, P. Moussa, and M. Waldschmidt, Springer Proceedings in Physics, Vol. 47, Springer-Verlag, Berlin, 1990, pp. 276-293). There have been later works by Bakas and Bowick (I. Bakas and M.J. Bowick, Curiosities of Arithmetic Gases, J. Math. Phys. 32 (1991) p. 1881) and Spector (D. Spector, Duality, Partial Supersymmetry, and Arithmetic Number Theory, J. Math. Phys. 39 (1998) pp.1919-1927) in which it was explored the connection of such systems to string theory.

This model is based on some simple hypothesis:

1st. Consider a simple quantum Hamiltonian, $H$, having eigenstates $\vert p\rangle$ labelled by the prime numbers “p”.

2nd. The eigenenergies or spectrum are given by $E_p$ and they have energies proportional to $\log p$. Mathematically speaking,

$H\vert p\rangle = E_p \vert p\rangle$ with $E_p=E_0 \log p$

Please, note the natural emergence of a “free” scale of energy $E_0$. What is this scale of energy? We do not know!

3rd. The second quantization/second-quantized version of this Hamiltonian converts states into particles, the “primons”. Multi-particle states are defined in terms of the numbers $k_p$ of primons in the single-particle states $p$:

$|N\rangle = |k_2, k_3, k_5, k_7, k_{11}, \ldots, k_{137},\ldots, k_p \ldots\rangle$

This corresponds to the factorization of $N$ into primes:

$N = 2^{k_2} \cdot 3^{k_3} \cdot 5^{k_5} \cdot 7^{k_7} \cdot 11^{k_{11}} \cdots 137^{k_{137}}\cdots p^{k_p} \cdots$

The labelling by the integer “N” is unique, since every number has a unique factorization into primes.

The energy of such a multi-particle state is clearly

$\displaystyle{E(N) = \sum_p k_p E_p = E_0 \cdot \sum_p k_p \log p = E_0 \log N}$

4th. The statistical mechanics partition function $Z$ IS, for the (bosonic) primon gas, the Riemann zeta function!

$\displaystyle{Z_B(T) \equiv\sum_{N=1}^\infty \exp \left(-\dfrac{E(N)}{k_B T}\right) = \sum_{N=1}^\infty \exp \left(-\dfrac{E_0 \log N}{k_B T}\right) = \sum_{N=1}^\infty \dfrac{1}{N^s} = \zeta (s)}$

with $s=E_0/k_BT=\beta E_0$, and where $k_B$ is the Boltzmann’s constant and T is the absolute temperature. The divergence of the zeta function at the value $s=1$ (corresponding to the harmonic sum) is due to the divergence of the partition function at certain temperature, usually called Hagedorn temperature. The Hagedorn temperature is defined by:

$T_H=\dfrac{E_0}{k_B}$

This temperature represents a limit beyond the system of (bosonic) primons can not be heated up. To understand why, we can calculate the energy

$E=-\dfrac{\partial}{\partial \beta}\ln Z_B=-\dfrac{E_0}{\zeta (\beta E_0)}\dfrac{\partial \zeta (\beta E_0)}{\partial \beta}\approx \dfrac{E_0}{s-1}$

A similar treatment can be built up for fermions rather than bosons, but here the Pauli exclusion principle has to be taken into account, i.e. two primons cannot occupy the same single particle state. Therefore $m_i$ can be 0 or 1 for all single particle state. As a consequence, the many-body states are labeled not by the natural numbers, but by the square-free numbers. These numbers are sieved from the natural numbers by the Möbius function. The calculation is a bit more complex, but the partition function for a non-interacting fermion primon gas reduces to the relatively simple form

$Z_F(T)=\dfrac{\zeta (s)}{\zeta (2s)}$

The canonical ensemble is of course not the only ensemble used in statistical physics. Julia extended the Riemann gas approach to the grand canonical ensemble by introducing a chemical potential $\mu$ (Julia, B. L., 1994, Physica A 203(3-4), 425), and thus, he replaced the primes p with new primes $pe^{-\mu}$. This generalisation of the Riemann gas is called the Beurling gas, after the Swedish mathematician Beurling who had generalised the notion of prime numbers. Examining a boson primon gas with fugacity $-1$, it shows that its partition function becomes

$\overline{Z}_B=\dfrac{\zeta (2s)}{\zeta (s)}$

Remarkable interpretation: pick a system, formed by two sub-systems not interacting with each other, the overall partition function is simply the product of the individual partition functions of the subsystems. From the previous equation of the free fermionic riemann gas we get exactly this structure, and so there are two decoupled systems. Firstly, a fermionic “ghost” Riemann gas at zero chemical potential and, secondly, a boson Riemann gas with energy-levels given by $E(N)=2E_0\ln p_N$. Julia also calculated the appropriate Hagedorn temperatures and analysed how the partition functions of two different number theoretical gases, the Riemann gas and the “log-gas” behave around the Hagedorn temperature. Although the divergence of the partition function hints the breakdown of the canonical ensemble, Julia also claims that the continuation across or around this critical temperature can help understand certain phase transitions in string theory or in the study of quark confinement. The Riemann gas, as a mathematically tractable model, has been followed with much attention because the asymptotic density of states grows exponentially, $\rho (E)\sim e^E$, just as in string theory. Moreover, using arithmetic functions it is not extremely hard to define a transition between bosons and fermions by introducing an extra parameter, called kappa $\kappa$, which defines an imaginary particle, the non-interacting parafermions of order $\kappa$. This order parameter counts how many parafermions can occupy the same state, i.e. the occupation number of any state falls into the interval $\left[0,\kappa-1\right]$, and therefore $\kappa=2$ belongs to normal fermions, while $\kappa\rightarrow\infty$ are the usual bosons. Furthermore, the partition function of a free, non-interacting κ-parafermion gas can be defined to be (Bakas and Bowick,1991, in the paper Bakas, I., and M. J. Bowick, 1991, J. Math. Phys. 32(7), 1881):

$Z_\kappa=\dfrac{\zeta (s)}{\zeta (\kappa s)}$

Indeed, Bakas et al. proved, using the Dirichlet convolution $\star$, how one can introduce free mixing of parafermions with different orders which do not interact with each other

$\displaystyle{f\star g=\sum_{d\vert n}f(d)g\left(\dfrac{n}{d}\right)}$

where the symbol $d\vert n$ means d is a divisor of n. This operation preserves the multiplicative property of the classically defined partition functions, i.e., $Z_{\kappa_1\star \kappa_2}=Z_{\kappa_1}\star Z_{\kappa_2}$. It is even more intriguing how interaction can be incorporated into the mixing by modifying the Dirichlet convolution with a kernel function or twisting factor

$\displaystyle{f\odot g=\sum_{d\vert n}f(d)g\left( \dfrac{n}{d}\right) K(n,d)}$

Using the unitary convolution Bakas establishes a pedagogically illuminating case, the mixing of two identical boson Riemann gases. He shows that

$Z_\infty\star Z_\infty=\zeta ^2(s)\zeta(2s)=\dfrac{\zeta (s)}{\zeta(2s)}\zeta (s)=Z_2Z_\infty=Z_FZ_B$

This result has an amazing meaning. Two identical boson Riemann gases interacting with each other through the unitary twisting, are equivalent to mixing a fermion Riemann gas with a boson Riemann gas which do not interact with each other. Therefore, one of the original boson components suffers a transmutation/mutation into a fermion gas!

Remark (I): the Möbius function, which is the identity function with respect to the  $\star$ operation (i.e. free mixing), reappears in supersymmetric quantum field theories as a possible representation of the $(-1)^F$ operator, where F is the fermion number operator!  In this context, the fact that $\mu (n)=0$ for square-free numbers is the manifestation of the Pauli exclusion principle itself! In any QFT with fermions, $(-1)^F$ is a unitary, hermitian, involutive operator where $F$ is the fermion number operator and is equal to the sum of the lepton number plus the baryon number, i.e., $F=B+L$, for all particles in the Standard Model and some (most of) SUSY QFT.  The action of this operator is to multiply bosonic states by 1 and fermionic states by -1. This is always a global internal symmetry of any QFT with fermions and corresponds to a rotation by an angle $2\pi$. This splits the Hilbert space into two superselection sectors. Bosonic operators commute with $(-1)^F$ whereas fermionic operators anticommute with it. This operator really is, therefore, more useful in supersymmetric field theories.

Remark (II): potential attacks on the Riemann Hypothesis  may lead to advances in physics and/or mathematics, i.e., progress in Physmatics!

Remark (III): the energy of the ground state is taken to be zero and the energy spectrum of the excited state is $E(n)=E_0\ln (p_n)$, where $p_n$, $n=2,3,5,\ldots$, runs over the prime numbers. Let N and E denote now the number of particles in the ground state and the total energy of the system, respectively. The fundamental theorem of arithmetic allows only one excited state configuration for a given energy

$E=\ln (n) \;\; mod E_0$

where n is an integer. It immediately means that this gas preserves its quantum nature at any temperature, since only one quantum state is permitted to be occupied. The number fluctuation of any state (even the ground state) is therefore zero. In contrast, the changes in the number of particles in the ground state $\delta n_0$ predicted by the canonical ensemble is a smooth non-vanishing function of the temperature, while the grand-canonical ensemble still exhibits a divergence. This discrepancy between the microcanonical (combinatorial) and the other two ensembles remains even in the thermodynamic limit.

One could argue that the Riemann gas is fictitious/unreal and its spectrum is unrealisable/unphysical. However, we, physicists, think otherwise, since the spectrum $E_N=\ln (N)$ does not increase with N more rapidly than $n^2$, therefore the existence of a quantum mechanical potential supporting this spectrum is possible (e.g., via inverse scattering transform or supplementary tools). And of course the question is: what kind of system has such an spectrum?

Some temptative ideas for the potential based on elementary Quantum Mechanics will be given in the next section.

## 11. LOG-OSCILLATORS

Instead of considering the free Riemann gas, we could ask to Quantum Mechanics if there is some potential providing the logarithmic spectrum of the previous section. Indeed, there exists such a potential. Let us factorize any natural number in terms of its prime “atoms”:

$N=p_1^{n_1}p_2^{n_2}\cdots p_m^{n_m}$

Take the logarithm

$\log N=\log \left(p_1^{n_1}p_2^{n_2}\cdots p_m^{n_m}\right)=n_1\log p_1+n_2\log p_2+\ldots+n_m\log p_m$

$\displaystyle{\log N=\sum_{i=1}^{m}n_i\log p_i}$

where $p_i$ are prime numbers (note that if we include “1” as a prime number it gives a zero contribution to the sum).

Now, suppose a logarithmic oscillator spectrum, i.e.,

$\varepsilon_i=\log p_i$ with $p_i=(1),2,3,5,7,11,13,\ldots,137,\ldots,\infty$

with $i=0,1,2,3,4,\ldots,\infty$. In order to have a “riemann gas”/riemannium, we impose an spectrum labelled in the following fashion

$\varepsilon_s =\log (2s+1)$ $\forall s=0,1,2,3,\ldots,\infty$

Equivalently, we could also define the spectrum of interacting riemannium gas as

$\varepsilon_s=\log (s)$ $\forall s=1,2,3,\ldots,\infty$

In addition to this, suppose the next quantum postulates:

1st. Logarithmic potential:

$V(x)=V_0\ln\dfrac{\vert x\vert}{L}$ with positive constants $V_0, L>0$

From the physical viewpoint, the positive constant $V_0$ means repulsive interaction (force).

2nd. Bohr-Sommerfeld quantization rule:

a) $\displaystyle{I=\dfrac{1}{2\pi}\oint pdx=\hbar \left(s+\dfrac{1}{2}\right)}\; \forall s=0,1,\ldots,\infty$

or equivalently we could also get

b) $\displaystyle{I=\dfrac{1}{2\pi}\oint pdx=\hbar s}\; \forall s=1,2,\ldots,\infty$

3rd. Turning point condition:

$x_s=L\exp \left(\dfrac{\varepsilon_s}{V_0}\right)$

In the case of 2a) we would deduce that

$\displaystyle{\dfrac{\hbar \pi}{2}\left(s+\dfrac{1}{2}\right)=\int_0^{x_s}dx\sqrt{2m\left(\varepsilon_s-V_0\ln \dfrac{x}{L}\right)}}$

so

$\displaystyle{\dfrac{\hbar \pi}{2}\left(s+\dfrac{1}{2}\right)=\int_0^{x_x}dx\sqrt{-\ln \left(\dfrac{x}{x_s}\right)}=\sqrt{2mV_0}x_s\Gamma \left(\dfrac{3}{2}\right)}$

and then

$x_s=\sqrt{\dfrac{\pi}{2mV_0}}\hbar \left( s+\dfrac{1}{2}\right)$

Then, using the turning point condition in this equation, we finally obtain

$\boxed{\dfrac{\varepsilon_s}{V_0}=\ln (2s+1)+\ln \left(\dfrac{\hbar}{2L}\sqrt{\dfrac{\pi}{2mV_0}}\right)}$ $\forall s=0,1,\ldots,\infty$

In the case of 2b) we would obtain

$\boxed{\dfrac{\varepsilon_s}{V_0}=\ln (s)+\ln \left(\dfrac{\hbar}{L}\sqrt{\dfrac{\pi}{2mV_0}}\right)}$ $\forall s=1,2,\ldots,\infty$

In summary, the logarithmic potential provides a model for the interacting Riemann gas!

## 12. LOG-POTENTIAL AND CONFINEMENT

Massive elementary particles (with mass m) can be understood as composite particles made of confined particles moving with some energy $pc$ inside a sphere of radius R. We note that we do not define futher properties of the constituent particles (e.g., if they are rotating strings, particles, extended objects like branes, or some other exotic structure moving in circular orbits or any other pattern as trajectory inside the composite particle).

Let us make the hypothesis that there is some force $F$ needed to counteract the centrifugal force $F_c=\dfrac{\kappa c^2}{R}$. The centrifugal force is equal to $pc/R$, i.e., the balancing force F is $F=pc/R$. Then, assuming the two forces are equal in magnitude, we get

$F=F_c=\dfrac{A_1}{R}$

where $A_1$ is some constant, and that equation holds regardless the origin of the interaction. The potentail energy $U$ necessary to confine a constituent particle will be, in that case,

$\displaystyle{U=\int \dfrac{A_1}{R}dR=A_1\int \dfrac{1}{R}dR=A_1\ln \dfrac{R}{R_\star}}$

with $R_\star$ some integration constant to be determined later. The center of mass of the “elementary particle”, truly a composite particle, from the external observer and the mass assinged to the composited system is:

$m=\dfrac{\hbar}{cR}$

The logarithmic potential energy is postulated to be proportional to $m/R$, and it provides

$U=\dfrac{A_2 m}{R}$

with $A_2$ is another constant. In fact, $A_1, A_2$ are parameters that don’t depend, a priori, on the radius R but on the constitutent particle properties and coupling constants, respectively. Indeed, for instance, we could set and fix the ratio $A_2/A_1$ to the constant $c^2/G_N$, where $G_N$ is the gravitational constant. However, such a constraint is not required from first principles or from a clear physical reason. From the following equations:

$m=\dfrac{\hbar}{cR}$ and $U=\dfrac{A_2 m}{R}$

we get $\boxed{U=\dfrac{A_2 \hbar}{cR^2}}$

Quantum Mechanics implies that the angular momentum should be quantized, so we can make the following generalization

$U=\dfrac{A_2 m}{cR^2}\rightarrow U_n=\dfrac{A_2 \hbar}{cR_n^2}=\dfrac{A_2 (n+1)\hbar}{cR_0^2}$

$\forall n=0,1,2,\ldots,\infty$

so $R_n^2=\dfrac{R_0^2}{n+1}\leftrightarrow R_n=\dfrac{R_0}{\sqrt{n+1}}$

Using the previous integral and this last result, we obtain

$\ln \left(\dfrac{R_\star}{R_0}\right)=-(n+1)\dfrac{R_\star^2}{R_0^2}$

This is due to the fact that $U_n=A_2\dfrac{\hbar}{cR_n^2}=\dfrac{A_2\hbar (n+1)}{cR_0^2}$ and $U=A_2\ln \dfrac{R}{R_\star}$

Combining these equations, we deduce the value of $R_\star$ as a function of the parameters $A_1,A_2$

$\boxed{R_\star=\sqrt{\dfrac{A_2\hbar}{A_1 c}}}$

The ratio $R_\star/R_0$ can be calculated from the above equations as well, since

$\ln \left(\dfrac{R_\star}{R_0}\right)=-(n+1)\dfrac{R_\star^2}{R_0^2}$ for the case n=0 implies that

$\ln \left(\dfrac{R_\star}{R_0}\right)=-\dfrac{R_\star^2}{R_0^2}$, and after exponentiation, it yields

$\dfrac{R_\star}{R_0}=e^{-\frac{R_\star^2}{R_0^2}}$

Introducing the variable $x=\dfrac{R_\star}{R_0}$ we have to solve the equation $x=e^{-x^2}$

The solution is $\phi=\dfrac{1}{x}=1.53158$ from which the relationship between $R_\star$ and $R_0$ can be easily obtained. Indeed, we can make more deductions from this result. From $\ln \phi=1/\phi^2$, then

$R_n=R_\star e^{(n+1)\ln\phi}$

If we take $R_\star=\alpha R_0$, with $R_0=\hbar/mc$, then

$\alpha=m_0\sqrt{\dfrac{A_2 c}{A_1\hbar}}$ so

$R_n=R_0e^{K\varphi_n}$ with $K=\dfrac{1}{2\pi}\ln \phi$ and $\varphi_n=2\pi (n+1)+\varphi_s$ $\varphi_s=2\pi \left(\dfrac{\ln \alpha}{\ln \phi}\right)$

Equivalently, the masses would be dynamically generated from the above equations, since

$m_n=\dfrac{\hbar}{R_nc}$ and $m_0=\dfrac{\hbar}{R_0c}$

so we would deduce a particle spectrum given by a logarithmic spiral, through the equation

$\boxed{m_n=m_0e^{K\varphi_n}}$

Remark: The shift $K\rightarrow -K$ implies that the spiral would begin with $m_0$ as the lowest mass and not the biggest mass, turning the spiral from inside to the outside region and vice versa.

In summary, the logarithmic oscillator is also related to some kind of confined particles and it provides a toy model of confinement!

## 13. HARMONIC  OSCILLATOR AND TSALLIS GAS

Is the link between classical statistical mechanics and Riemann zeta function unique or is it something more general? C. Tsallis explained long ago the connection of non-extensive Tsallis entropies an the Riemann zeta function, given supplementary arguments to support the idea of a physical link between Physics, Statistical Mechanics and the Riemann hypothesis. His idea is the following.

A) Consider the harmonic oscillator with spectrum

$E_n=\hbar\omega n$

$E(n),\;\forall n=0,1,2,\ldots,\infty$, are the H.O. eigenenergies.

B) Consider the Tsallis partition function

$\displaystyle{Z_q (\beta )=\sum_{n=0}^{\infty}e_q^{-\beta E_n}=\sum_{n=0}^{\infty}e_q^{-\beta\hbar\omega n}}$

where $q>1$ and the deformed q-exponential is defined as

$e_q^z\equiv \left[1+(q-1)z\right]_+^{\frac{1}{1-q}}$

and $\left[\alpha\right]=\begin{cases}\alpha, \alpha>0\\ 0,otherwise\end{cases}$

and the inverse of the deformed exponential is the q-logarithm

$\ln_q z=\dfrac{z^{1-q}-1}{1-q}$

It implies that

$\boxed{\displaystyle{Z_q=\sum_{n=0}^{\infty}\dfrac{1}{\left[1+(q-1)\beta\hbar\omega n\right]^{\frac{1}{q-1}}}=\dfrac{1}{\left[(q-1)\beta\hbar \omega\right]^{\frac{1}{q-1}}}\sum_{n=0}^{\infty}\dfrac{1}{\left[\left(\dfrac{1}{(q-1)\beta\hbar\omega}\right)+n\right]^{\frac{1}{q-1}}}}}$

Now, defining the Hurwitz zeta function as:

$\displaystyle{\zeta (s,Q)=\sum_{n=0}^{\infty}\dfrac{1}{\left(Q+n\right)^{s}}=\dfrac{1}{Q^s}+\sum_{n=1}^{\infty}\dfrac{1}{\left(Q+n\right)^{s}}}$

the last equation can be rewritten in a simple and elegant way:

$\boxed{\displaystyle{Z_q=\dfrac{1}{\left[(q-1)\beta\hbar\omega\right]^{\frac{1}{q-1}}}\zeta \left(\dfrac{1}{q-1},\dfrac{1}{(q-1)\beta\hbar\omega}\right)}}$

This system can be called the Tsallis gas or the Tsallisium. It is a q-deformed version (non-extensive) of the free Riemann gas. And it is related to the harmonic oscillator! The issue, of course, is the problematic limit $q\rightarrow 1$.

In the limit $Q\rightarrow 1$ we get the Riemann zeta function from the Hurwitz zeta function:

$\displaystyle{\zeta (s,1)\equiv \zeta (s)=\sum_{n=1}^{\infty}n^{-s}=\sum_{n=1}^{\infty}\dfrac{1}{n^s}=\prod_{p=2}^{\infty}\dfrac{1}{1-p^{-s}}=\prod_{p}\dfrac{1}{1-p^{-s}}}$

or

$\displaystyle{\zeta (s)=\dfrac{1}{1-2^{-s}}\dfrac{1}{1-3^{-s}}\ldots\dfrac{1}{1-137^{-s}}\ldots}$

The above equation, the partition function of the Tsallis gas/Tsallisium, connects directly the Riemann zeta function with Physics and non-extensive Statistical Mechanics. Indeed, C.Tsallis himself dedicated a nice slide with this theme to M.Berry:

Remark (I): The link between Riemann zeta function and the free Riemann gas/the interacting Riemann gas goes beyond classical statistical mechanics and it also appears in non-extensive statistical mechanics!

Remark (II): In general, the Riemann hypothesis is entangled to the theory of harmonic oscillators with non-extensive statistical mechanics!

## 14. TSALLIS ENTROPIES IN A NUTSHELL

For readers not familiarized with Tsallis generalized entropies, I would like to expose you the main definitions of such a generalization of classical statistical entropy (Boltzmann-Gibbs-Shannon), in a nutshell! I have to discuss more about this kind of statistical mechanics in the future, but today, I will only anticipate some bits of it.

Tsallis entropy (and its Statistical Mechanics/Thermodynamics) is based on the following entropy functionals:

1st. Discrete case.

$\boxed{\displaystyle{S_q=k_B\dfrac{1-\displaystyle{\sum_{i=1}^W p_i^q}}{q-1}=-k_B\sum_{i=1}^Wp_i^q\ln_q p_i=k_B\sum_{i=1}^Wp_i\ln_q \left(\dfrac{1}{p_i}\right)}}$

plus the normalization condition $\boxed{\displaystyle{\sum_{i=1}^Wp_i=1}}$

2nd. Continuous case.

$\boxed{\displaystyle{S_q=-k_B\int dX\left[p(X)\right]^q\ln_q p(X)=k_B\int dX p(X)\ln_q\dfrac{1}{p(X)}}}$

plus the normalization condition $\boxed{\displaystyle{\int dX p(X)=1}}$

3rd. Quantum case. Tsallis matrix density.

$\boxed{\displaystyle{S_q=-k_BTr\rho^q\ln _q\rho\equiv k_BTr\rho \ln_q\dfrac{1}{\rho}}}$

plus the normatlization condition $\boxed{Tr\rho=1}$

In all the three cases above, we have defined the q-logarithm as $\ln_q z\equiv\dfrac{z^{1-q}-1}{q-1}$, $\ln_1 z\equiv \ln z$, and the 3 Tsallis entropies satisfy the non-additive property:

$\boxed{\dfrac{S_q(A+B)}{k_B}=\dfrac{S_q (A)}{k_B}+\dfrac{S_q (B)}{k_B}+(1-q)\dfrac{S_q (A)}{k_B}\dfrac{S_q (B)}{k_B}}$

## 15. BEYOND QM/QFT: ADELIC WORLDS

Theoretical physicsts suspect that Physics of the spacetime at the Planck scale or beyond will change or will be meaningless. There, the spacetime notion we are familiarized to loose its meaning. Even more, we could find those changes in the fundamental structure of the Polyverse to occur a higher scales of length. Really, we don’t know yet where the spacetime “emerges” as an effective theory of something deeper, but it is a natural consequence from our current limited knowledge of fundamental physics.  Indeed, it is thought that the experimental device making measurements and the experimenter can not be distinguished at Planck scale. At Planck scale, we can not know at this moment how the framework of cosmology and the Hilbert space tool of Quantum Mechanics could be obtained with some unified formalism. It is one of the challenges of Quantum Gravity.

Many people and scientists think that geometry and topology of sub-Planckian lengths should not have any relation with our current geometry or topology. We say and believe that geometry, topology, fields and the main features of macroscopic bodies “emerge” from the ultra-Planckian and “subquantum” realm. It is an analogue to the colours of the rainbow emerging from the atoms or how Thermodynamics emerge from Statistical Mechanics.

There are many proposed frameworks to go beyond the usual notions of space and time, but the p-adic analysis approach is a quite remarkable candidate, having several achievements in its favor.

Motivations for a p-adic and adelic approaches as the ultimate substructure of the microscopic world arise from:

1) Divergences of QFT are believed to be absent with such number structures. Renormalization can be found to be unnecessary.

2) In an adelic approach, where there is no prime with special status in p-adic analysis, it might be more natural and instructive to work with adeles instead a pure p-adic approach.

3) There are two paths for a p-adic/adelic QM/QFT theory. The first path considers particles in a p-adic potential well, and the goal is to find solutions with smoothly varying complex-valued wavefunctions. There, the solutions share  certain kind of familiarity from ordinary life and ordinary QM. The second path allows particles in p-adic potential wells, and the goal is to find p-adic valued wavefunctions. In this case, the physical interpretation is harder. Yet the math often exhibits surprising features and properties, and some people are trying to explores those novel and striking aspects.

Ordinary real (or even complex as well) numbers are familiar to everyone. Ostroswski’s theorem states that there are essentially only two possible completions of the rational numbers ( “fractions” you do know very well). The two options depend on the metric we consider:

1) The real numbers. One completes the rationals by adding the limit of all Cauchy sequences to the set. Cauchy sequences are series of numbers whose elements can be arbitrarily close to each other as the sequence of numbers progresses. Mathematically speaking, given any small positive distance, all but a finite number of elements of the sequence are less than that given distance from each other. Real numbers satisfy the triangle inequality $\vert x+y\vert \leq \vert x\vert +\vert y\vert$.

2) The p-adic numbers. The completions are different because of the two different ways of measuring distance. P-adic numbers satisfy an stronger version of the triangle inequality, called ultrametricity. For any p-adic number is shows

$\vert x+y\vert _p\leq \mbox{max}\{\vert x\vert_p ,\vert y \vert_p\}$

Spaces where the above enhanced triangle inequality/ultrametricity arises are called ultrametric spaces.

In summary, there exist two different types of algebraic number systems. There is no other posible norm beyond the real (absolute) norm or the p-adic norm. It is the power of Mathematics in action.

Then, a question follows inmediately. How can we unify such two different notions of norm, distance and type of numbers. After all, they behave in a very different way. Tryingo to answer this questions is how the concept adele emerges. The ring of adeles is a framework where we consider all those different patterns to happen at equal footing, in a same mathematical language. In fact, it is analogue to the way in which we unify space and time in relativistic theories!

Adele numbers are an array consisting of both real (complex) and p-adic numbers! That is,

$\mathbb{A}=\left( x_\infty, x_2,x_3,x_5,\ldots,x_p,\ldots\right)$

where $x_\infty$ is a real number and the $x_p$ are p-adic numbers living in the p-adic field $\mathbb{Q}_p$. Indeed, the infinity symbol is just a consequence of the fact that real numbers can be thought as “the prime at infinity”. Moreover, it is required that all but finitely many of the p-adic numbers $x_p$ lie in the entire p-adic set $\mathbb{Z}_p$. The adele ring is therefore a restricted direct (cartesian) product. The idele group is defined as the essentially invertible elements of the adelic ring:

$\mathbb{I}=\mathbb{A}^\star =\{ x\in \mathbb{A}, \mbox{where}\;\; x_\infty \in \mathbb{R}^{\star} \;\; \mbox{and} \;\; \vert x_p\vert _p=1,\; \mbox{for all but finitely many primes p.}\}$

We can define the calculus over the adelic ring in a very similar way to the real or complex case. For instance, we define trigonometric functions, $e^X$, logarithms $\log (x)$ and special functions like the Riemann zeta function. We can also perform integral transforms like the Mellin of the Fourier transformation over this ring. However, this ring has many interesting properties. For example, quadratic polynomials obey the Hasse local-global principle: a rational number is the solution of a quadratic polynomial equation if and only if it has a solution in $\mathbb{R}$ and $\mathbb{Q}_p$ for all primes p. Furthermore, the real and p-adic norms are related to each other by the remarkable adelic product formula/identity:

$\displaystyle{\vert x\vert_\infty \prod_p\vert x\vert_p=1}$

and where $x$ is a nonzero rational number.

Beyond complex QM, where we can study the particle in a box or in a ring array of atoms, p-adic QM can be used to handle fractal potential wells as well. Indeed, the analogue Schrödinger equation can be solved and it has been useful, for instance, in the design of microchips and self-similar structures. It has been conjectured by Wu and Sprung, Hutchinson and van Zyl,here http://arXiv.org/abs/nlin/0304038v1 , that the potential constructed from the non-trivial Riemann zeroes and prime number sequences has fractal properties. They have suggested that $D=1.5$ for the Riemann zeroes and $D=1.8$ for the prime numbers. Therefore,  p-adic numbers are an excellent method for constructing fractal potential wells.

By the other hand, following Feynman, we do know that path integrals for quantum particles/entities manifest fractal properties. Indeed we can use path integrals in the absence of a p-adic Schrödinger equation. Thus, defining the adelic version of Feynman’s path integral is a necessary a fundamental object for a general quantum theory beyond the common textbook version. However, we need to be very precise with certain details. In particular, we have to be careful with the definition of derivatives and differentials in order to do proper calculations. Indeed we can do it since both, the adelic and idelic rings have a well defined translation-invariant Haar measure

$Dx=dx_\infty dx_2dx_3\cdots dx_p\cdots$ and $Dx^\star=dx_\infty^\star dx_2^\star dx_3^\star\cdots dx_p^\star\cdots$

These measures provide a way to compute Feynman path integrals over adelic/idelic spaces.  It turns out that Gaussian integrals satisfy a generalization of the adelic product formula introduced before, namely:

$\displaystyle{\int_{\mathbb{Q}_p}\chi_\infty (ax_\infty^2+bx_\infty)dx_\infty \prod_p \int_{\mathbb{Q}_p}\chi_p (ax_p^2+bx_p)dx_p=1}$

where $\chi$ is an additive character from the adeles to complex numbers $\mathbb{C}$ given by the map:

$\displaystyle{\chi (x)=\chi_\infty (x_\infty)\prod_p \chi_p (x_p)\rightarrow e^{-2\pi ix_\infty}\prod_p e^{2\pi i\{p\}_p}}$

and  $\{x_p\}_p$ is the fractional part of $x_p$ in the ordinary p-adic expression for x. This can be thought of as a strong generalization of the homomorphism $\mathbb{Z}/\mathbb{Z}_n\rightarrow e^{2\pi i/n}$.Then, the adelic path integral, with input parameters in the adelic ring $\mathbb{A}$  and generating complex-valued wavefunctions follows up:

$\displaystyle{K_{\mathbb{A}} (x'',t'';x',t') =\prod_\alpha \int_{(x' _\alpha ,t' _\alpha)}^{(x'' _\alpha ,t'' _\alpha)}\chi_\alpha \left(-\dfrac{1}{h}\int_{t' _\alpha}^{t''_\alpha}L(\dot{q} _\alpha ,q_\alpha ,t_\alpha )dt_\alpha \right) Dq_\alpha}$

The eigenvalue problem over the adelic ring is given by:

$U(t) \psi_\alpha (x)=\chi (E_\alpha (t))\psi_\alpha (x)$

where U is the time-development operator, $\psi_\alpha$ are adelic eigenfunctions, and $E_\alpha$ is the adelic energy. Here the notation has been simplified by using the subscript $\alpha$, which stands for all primes including the prime at infinity. One notices the additive character $\chi$ which allows these to be complex-valued integrals. The path integral can be generalized to p-adic time as well, i.e., to paths with fractal behaviour!

How is this p-adic/adelic stuff connected to the Riemannium an the Riemann zeta function? It can be shown that ground state of adelic quantum harmonic oscillator is

$\displaystyle{\vert 0\rangle =\Psi_0 (x)=2^{1/4}e^{-\pi x_\infty^2}\prod_p \Omega (\vert x_p\vert_p)}$

where $\Omega \left(\vert x_p \vert _p\right)$  is 1 if $\vert x_p\vert_p$ is a p-adic integer and 0 otherwise. This result is strikingly similar to the ordinary complex-valued ground state. Applying the adelic Mellin transform, we can deduce that

$\Phi (\alpha)=\sqrt{2}\Gamma \left(\dfrac{\alpha}{2}\right)\pi^{-\alpha/2}\zeta (\alpha)$

where $\Gamma, \zeta$ are, respectively, the gamma function and the Riemann zeta function. Due to the Tate formula, we get that

$\Phi (\alpha)=\Phi (1-\alpha)$.

and from this the functional equation for the Riemann zeta function naturally emerges.

In conclusion: it is fascinating that such simple physical system as the (adelic) harmonic oscillator is related to so significant mathematical object as the Riemann zeta function.

## 16. STRINGS, FIELDS AND VACUUM

The Veneziano amplitude is also related to the Riemann zeta function and string theory. A nice application of the previous adelic formalism involves the adelic product formula in a different way. In string theory, one computes crossing symmetric Veneziano amplitudes$A(a,b)$ describing the scattering of four tachyons in the 26d open bosonic string. Indeed, the Veneziano amplitude can be written in terms of Riemann zeta function in this way:

$A_\infty (a,b)=g_\infty^2 \dfrac{\zeta (1-a)}{\zeta (a)}\dfrac{\zeta (1-b)}{\zeta (b)}\dfrac{\zeta (1-c)}{\zeta (c)}$

These amplitudes are not easy to calculate. However, in 1987, an amazingly simple adelic product formula for this tachyonic scattering was found to be:

$\displaystyle{A_\infty (a,b)\prod_p A_p (a,b)=1}$

Using this formula, we can compute and calculate the four-point amplitudes/interacting vertices at the tree level exactly, as the inverse of the much simpler p-adic amplitudes. This discovery has generated a quite a bit of activity in string theory, somewhat unknown, although it is not very popular as far as I know. Moreover, the whole landscape of the p-adic/adelic framework is not as easy for the closed bosonic string as the open bosonic strings (note that in a p-adic world, there is no “closure” but “clopen” segments instead of naive closed intervals). It has also been a source of controversy what is the role of the p-adic/adelic stuff at the level of the string worldsheet. However, there is some reasearch along these lines at current time.

Another nice topic is the vacuum energy and its physical manifestations. There are some very interesting physical effects involving the vacuum energy in both classical and quantum physics. The most important effects are the Casimir effect (vacuum repulsion between “plates”) , the Schwinger effect ( particle creation in strong fields) , the Unruh effect ( thermal effects seen by an uniformly accelerated observer/frame) , the Hawking effect (particle creation by Black Holes, due to Black Hole Thermodynamcis in the corresponding gravitational/accelerated environtment) , and the cosmological constant effect (or vacuum energy expanding the Universe at increasing rate on large scales. Itself, does it gravitate?). Riemann zeta function and its generalizations do appear in these 4 effects. It is not a mere coincidence. It is telling us something deeper we can not understand yet. As an example of why zeta function matters in, e.g., the Casimir effect, let me say that zeta function regularizes the following general sum:

$\boxed{\displaystyle{\sum_{n\in \mathbb{Z}}\vert n\vert^d =2\zeta (-d)}}$

Remark: I do know that I should have likely said “the cosmological constant problem”. But as it should be solved in the future, we can see the cosmological constant we observe ( very, very smaller than our current QFT calculations say) as “an effect” or “anomaly” to be explained. We know that the cosmological constant drives the current positive acceleration of the Universe, but it is really tiny. What makes it so small? We don’ t know for sure.

Remark(II): What are the p-adic strings/branes? I. Arefeva, I. Volovich and B. Dravogich, between other physicists from Russia and Eastern Europe, have worked about non-local field theories and cosmologies using the Riemann zeta function as a model. It is a relatively unknown approach but it is remarkable, very interesting and uncommon.  I have to tell you about these works but not here, not today. I went too far, far away in this log. I apologize…

## 17. SUMMARY AND OUTLOOK

I have explained why I chose The Spectrum of Riemannium as my blog name here and I used the (partial) answer to explain you some of the multiple connections and links of the Riemann zeta function (and its generalizations) with Mathematics and Physics. I am sure that solving the Riemann Hypothesis will require to answer the question of what is the vibrating system behind the spectral properties of Riemann zeroes. It is important for Physmatics! I would say more, it is capital to theoretical physics as well.

Let me review what and where are the main links of the Riemann zeta function and zeroes to Physmatics:

1) Riemann zeta values appear in atomic Physics and Statistical Physics.

2) The Riemannium has spectral properties similar to those of Random Matrix Theory.

3) The Hilbert-Polya conjecture states that there is some mysterious hamiltonian providing the zeroes. The Berry-Keating conjecture states that the “quantum” hamiltonian corresponding to the Riemann hypothesis is the corresponding or dual hamiltonian to a (semi)classical hamiltonian providing a classically chaotic dynamics.

4) The logarithmic potential provides a realization of certain kind of spectrum asymptotically similar to that of the free Riemann gas. It is also related to the issue of confinement of “fundamental” constituents inside “elementary” particles.

5) The primon gas is the Riemann gas associated to the prime numbers in a (Quantum) Statistical Mechanics approach. There are bosonic, fermionic and parafermionic/parabosonic versions of the free Riemann gas and some other generalizations using the Beurling gas and other tools from number theory.

6) The non-extensive Statistical Mechanics studied by C. Tsallis (and other people) provides a link between the harmonic oscillator and the Riemann hypothesis as well. The Tsallisium is the physical system obtained when we study the harmonic oscillator with a non-extensive Tsallis approach.

7) An adelic approach to QM and the harmonic oscillator produces the Riemann’s zeta function functional equation via the Tate formula. The link with p-adic numbers and p-adic zeta functions reveals certain fractal patterns in the Riemann zeroes, the prime numbers and the theory behind it. The periodicity or quasiperiodicity also relates it with some kind of (quasi)crystal and maybe it could be used to explain some behaviour or the prime numbers, such as the one behind the Goldbach’s conjecture.

8) A link between entropy, information theory and Riemann zeta function is done through the use of the notion of group entropy.  Connections between the Veneziano amplitudes, tachyons, p-adic numbers and string theory arise after the Veneziano amplitude in a natural way.

9) Riemann zeta function also is used in the regularization/definition of infinite determinants arising in the theory of differential operators and similar maps. Even the generalization of this framework is important in number theory through the uses of generalizations of the Riemann zeta function and other arithmetical functions similar to it. Riemann zeta function is, thus, one of the simplest examples of arithmetical functions.

10) There are further links of the Riemann zeta function and “vacuum effects” like the Schwinger effect ( pair creating in strong fields) or the Casimir effect ( repulsive/atractive forces between close objects with “nothing” between them). Riemann zeta function is also related to SUSY somehow, either by the striking similarity between the Dirichlet eta function used in Fermi-Dirac statistics or directly with the explicit relationship between the Möbius function and the $(-1)^F$ operator appearing in supersymmetric field theories.

In summary, Riemann zeta function is ubiquitious and it appears alone or with its generalizations in very different fields: number theory, quantum physics, (semi)classical physics/dynamics, (quantum) chaos theory, information theory, QFT, string theory, statistical physics, fractals, quasicrystals, operator theory, renormalization and many other places. Is it an accident or is it telling us something more important? I think so. Zeta functions are fundamental objects for the future of Physmatics and the solution of Riemann Hypothesis, perhaps, would provide such a guide into the ultimate quest of both Physics and Mathematics (Physmatics) likely providing a complete and consistent description of the whole Polyverse.

A) What is the Riemann zeta function? What is the riemannium/tsallisium and what kind of physical system do they represent really? What is the physical system behind the Riemann non-trivial zeroes? What does it mean for the Riemann zeroes arising from the Riemann zeta function  generalizations in form of L-functions?

B) What is the Riemann-Hilbert-Polya operator? What is the space over the Riemann operator is acting?

C) Are Riemann zeta function and its generalization everywhere as they seem to be inside the deepest structures of the microscopic/macroscopic entities of the Polyverse?

I suppose you will now understand better why I decided to name my blog as The Spectrum of Riemannium…And there are many other reasons I will not write you here since I could reveal my current research.

However, stay tuned!

Physmatics is out there and everywhere, like fractals, zeta functions and it is full of lots of wonderful mathematical structures and simple principles!

# LOG#046. The Cherenkov effect.

The Cherenkov effect/Cherenkov radiation, sometimes also called Vavilov-Cherenkov radiation, is our topic here in this post.

In 1934, P.A. Cherenkov was a post graduate student of S.I.Vavilov. He was investigating the luminescence of uranyl salts under the incidence of gamma rays from radium and he discovered a new type of luminiscence which could not be explained by the ordinary theory of fluorescence. It is well known that fluorescence arises as the result of transitions between excited states of atoms or molecules. The average duration of fluorescent emissions is about $\tau>10^{-9}s$ and the transition probability is altered by the addition of “quenching agents” or by some purification process of the material, some change in the ambient temperature, etc. It shows that none of these methods is able to quench the fluorescent emission totally, specifically the new radiation discovered by Cherenkov. A subsequent investigation of the new radiation ( named Cherenkov radiation by other scientists after the Cherenkov discovery of such a radiation) revealed some interesting features of its characteristics:

1st. The polarization of luminiscence changes sharply when we apply a magnetic field. Cherenkov radiation luminescence is then causes by charged particles rather than by photons, the $\gamma$-ray quanta! Cherenkov’s experiment showed that these particles could be electrons produced by the interaction of $\gamma$-photons with the medium due to the photoelectric effect or the Compton effect itself.

2nd. The intensity of the Cherenkov’s radiation is independent of the charge Z of the medium. Therefore, it can not be of radiative origin.

3rd. The radiation is observed at certain angle (specifically forming a cone) to the direction of motion of charged particles.

The Cherenkov radiation was explained in 1937 by Frank and Tamm based on the foundations of classical electrodynamics. For the discovery and explanation of Cherenkov effect, Cherenkov, Frank and Tamm were awarded the Nobel Prize in 1958. We will discuss the Frank-Tamm formula later, but let me first explain how the classical electrodynamics handle the Vavilov-Cherenkov radiation.

The main conclusion that Frank and Tamm obtained comes from the following observation. They observed that the statement of classical electrodynamics concerning the impossibility of energy loss by radiation for a charged particle moving uniformly and following a straight line in vacuum is no longer valid if we go over from the vacuum to a medium with certain refractive index $n>1$. They went further with the aid of an easy argument based on the laws of conservation of momentum and energy, a principle that rests in the core of Physics as everybody knows. Imagine a charged partice moving uniformly in a straight line, and suppose it can loose energy and momentum through radiation. In that case, the next equation holds:

$\left(\dfrac{dE}{dp}\right)_{particle}=\left(\dfrac{dE}{dp}\right)_{radiation}$

This equation can not be satisfied for the vacuum but it MAY be valid for a medium with a refractive index gretear than one $n>1$. We will simplify our discussion if we consider that the refractive index is constant (but similar conclusions would be obtained if the refractive index is some function of the frequency).

By the other hand, the total energy E of a particle having a non-null mass $m\neq 0$ and moving freely in vacuum with some momentum p and velocity v will be:

$E=\sqrt{p^2c^2+m^2c^4}$

and then

$\left(\dfrac{dE}{dp}\right)_{particle}=\dfrac{pc^2}{E}=\beta c=v$

Moreover, the electromagnetic radiation in vaccum is given by the relativistic relationship

$E_{rad}=pc$

From this equation, we easily get that

$\left(\dfrac{dE}{dp}\right)_{radiation}=c$

Since the particle velocity is $v, we obtain that

$\left(\dfrac{dE}{dp}\right)_{particle}<\left(\dfrac{dE}{dp}\right)_{radiation}$

In conclusion: the laws of conservation of energy and momentum prevent that a charged particle moving with a rectilinear and uniform motion in vacuum from giving away its energy and momentum in the form of electromagnetic radiation! The electromagnetic radiation can not accept the entire momentum given away by the charged particle.

Anyway, we realize that this restriction and constraint is removed and given up when the aprticle moves in a medium with a refractive index $n>1$. In this case, the velocity of light in the medium would be

$c'=c/n

and the velocity v of the particle may not only become equal to the velocity of light $c'$ in the medium, but even exceed it when the following phenomenological condition is satisfied:

$\boxed{v\geq c'=c/n}$

It is obvious that, when $v=c'$ the condition

$\left(\dfrac{dE}{dp}\right)_{particle}=\left(\dfrac{dE}{dp}\right)_{radiation}$

will be satisfied for electromagnetic radiation emitted strictly in the direction of motion of the particle, i.e., in the direction of the angle $\theta=0\textdegree$. If $v>c'$, this equation is verified for some direction $\theta$ along with $v=c'$, where

$v'=v\cos\theta$

is the projection of the particle velocity v on the observation direction. Then, in a medium with $n>1$, the conservation laws of energy and momentum say that it is allowed that a charged particle with rectilinear and uniform motion, $v\geq c'=c/n$ can loose fractions of energy and momentum $dE$ and $dp$, whenever those lost energy and momentum is carried away by an electromagnetic radiation propagating in the medium at an angle/cone given by:

$\boxed{\theta=arccos\left(\dfrac{1}{n\beta}\right)=\cos^{-1}\left(\dfrac{1}{n\beta}\right)}$

with respect to the observation direction of the particle motion.

These arguments, based on the conservation laws of momenergy, do not provide any ide about the real mechanism of the energy and momentum which are lost during the Cherenkov radiation. However, this mechanism must be associated with processes happening in the medium since the losses can not occur ( apparently) in vacuum under normal circumstances ( we will also discuss later the vacuum Cherenkov effect, and what it means in terms of Physics and symmetry breaking).

We have learned that Cherenkov radiation is of the same nature as certain other processes we do know and observer, for instance, in various media when bodies move in these media at a velocity exceeding that of the wave propagation. This is a remarkable result! Have you ever seen a V-shaped wave in the wake of a ship? Have you ever seen a conical wave caused by a supersonic boom of a plane or missile? In these examples, the wave field of the superfast object if found to be strongly perturbed in comparison with the field of a “slow” object ( in terms of the “velocity of sound” of the medium). It begins to decelerate the object!

Question: What is then the mechanism behind the superfast  motion of a charged particle in a medium wiht a refractive index $n>1$ producing the Cherenkov effect/radiation?

Answer:  The mechanism under the Cherenkov effect/radiation is the coherent emission by the dipoles formed due to the polarization of the medium atoms by the charged moving particle!

The idea is as follows. Dipoles are formed under the action of the electric field of the particle, which displaces the electrons of the sorrounding atoms relative to their nuclei. The return of the dipoles to the normal state (after the particle has left the given region) is accompanied by the emission of an electromagnetic signal or beam. If a particle moves slowly, the resulting polarization will be distribute symmetrically with respect to the particle position, since the electric field of the particle manages to polarize all the atoms in the near neighbourhood, including those lying ahead in its path. In that case, the resultant field of all dipoles away from the particle are equal to zero and their radiations neutralize one to one.

Then, if the particle move in a medium with a velocity exceeding the velocity or propagation of the electromagnetic field in that medium, i.e., whenever $v>c'=c/n$, a delayed polarization of the medium is observed, and consequently the resulting dipoles will be preferably oriented along the direction of motion of the particle. See the next figure:

It is evident that, if it occurs, there must be a direction along which a coherent radiation form dipoles emerges, since the waves emitted by the dipoles at different points along the path of the particle may turn our to be in the same phase. This direction can be easiy found experimentally and it can be easily obtained theoretically too. Let us imagine that a charged particle move from the left to the right with some velocity $v$ in a medium with a $n>1$ refractive index, with $c'=c/n$. We can apply the Huygens principle to build the wave front for the emitted particle. If, at instant $t$, the aprticle is at the point $x=vt$, the surface enveloping the spherical waves emitted by the same particle on its own path from the origin at $x=0$ to the arbitrary point $x$. The radius of the wave at the point $x=0$ at such an instant t is equal to $R_0=c't$. At the same moment, the wave radius at th epint x is equal to $R_x=c'(t-(x/v))=0$. At any intermediate point x’, the wave radius at instant t will be $R_{x'}=c'(t-(x'/v))$. Then, the radius decreases linearly with increasing $x'$. Thus, the enveloping surface is a cone with angle $2\varphi$, where the angle satisfies in addition

$\sin\varphi=\dfrac{R_0}{x}=\dfrac{c't}{vt}=\dfrac{c'}{v}=\dfrac{c}{vn}=\dfrac{1}{\beta n}$

The normal to the enveloping surface fixes the direction of propagation of the Cherenkov radiation. The angle $\theta$ between the normal and the $x$-axis is equal to $\pi/2-\varphi$, and it is defined by the condition

$\boxed{\cos\theta=\dfrac{1}{\beta n}}$

or equivalently

$\boxed{\tan\theta=\sqrt{\beta^2n^2-1}}$

This is the result we anticipated before. Indeed, it is completely general and Quantum Mechanics instroudces only a light and subtle correction to this classical result. From this last equation, we observer that the Cherenkov radiation propagates along the generators of a cone whose axis coincides with the direction of motion of the particle an the cone angle is equal to $2\theta$. This radiation can be registered on a colour film place perpendicularly to the direction of motion of the particle. Radiation flowing from a radiator of this type leaves a blue ring on the photographic film. These blue rings are the archetypical fingerprints of Vavilov-Cherenkov radiation!

The sharp directivity of the Cherenkov radiation makes it possible to determine the particle velocity $\beta$ from the value of the Cherenkov’s angle $\theta$. From the Cherenkov’s formula above, it follows that the range of measurement of $\beta$ is equal to

$1/n\leq\beta<1$

For $\beta=1/n$, the radiation is observed at an angle $\theta=0\textdegree$, while for the extreme with $\beta=1$, the angle $\theta$ reaches a maximum value

$\theta_{max}=\cos^{-1}\left(\dfrac{1}{n}\right)=arccos \left(\dfrac{1}{n}\right)$

For instance, in the case of water, $n=1.33$ and $\beta_{min}=1/1.33=0.75$. Therefore, the Cherenkov radiation is observed in water whenever $\beta\geq 0.75$. For electrons being the charged particles passing through the water, this condition is satisfied if

$T_e=m_ec^2\left(\dfrac{1}{\sqrt{1-\beta^2}}-1\right)=0.5\left( \dfrac{1}{\sqrt{1-0.75^2}}-1\right)=0.26MeV$

As a consequence of this, the Cherenkov effect should be observed in water even for low-energy electrons ( for isntance, in the case of electrons produced by beta decay, or Compton electrons, or photoelectroncs resulting from the interaction between water and gamma rays from radioactive products, the above energy can be easily obtained and surpassed!). The maximum angle at which the Cherenkov effec can be observed in water can be calculated from the condition previously seen:

$\cos\theta_{max}=1/n=0.75$

This angle (for water) shows to be equal to about $\theta\approx 41.5\textdegree=41\textdegree 30'$. In agreement with the so-called Frank-Tamm formula ( please, see below what that formula is and means), the number of photons in the frequency interval $\nu$ and $\nu+d\nu$ emitted by some particle with charge Z moving with a velocity $\beta$ in a medium with a refractive indez n is provided by the next equation:

$\boxed{N(\nu) d\nu=4\pi^2\dfrac{(Zq)^2}{hc^2}\left(1-\dfrac{1}{n^2\beta^2}\right) d\nu}$

This formula has some striking features:

1st. The spectrum is identical for particles with $Z=constant$, i.e., the spectrum is exactly the same, irespectively the nature of the particle. For instance, it could be produced both by protons, electrons, pions, muons or their antiparticles!

2nd. As Z increases, the number of emitted photons increases as $Z^2$.

3rd. $N(\nu)$ increases with $\beta$, the particle velocity, from zero ( with $\beta=1/n$) to

$N=4\pi^2\left(\dfrac{q^2Z^2}{hc^2}\right)\left(1-\dfrac{1}{n^2}\right)$

with $\beta\approx 1$.

4th. $N(\nu)$ is approximately independent of $\nu$. We observe that $dN(\nu)\propto d\nu$.

5th. As the spectrum is uniform in frequency, and $E=h\nu$, this means that the main energy of radiation is concentrated in the extreme short-wave region of the spectrum, i.e.,

$\boxed{dE_{Cherenkov}\propto \nu d\nu}$

And then, this feature explains the bluish-violet-like colour of the Cherenkov radiation!

Indeed, this feature also indicates the necessity of choosing materials for practical applications that are “transparent” up to the highest frequencies ( even the ultraviolet region). As a rule, it is known that $n<1$ in the X-ray region and hence the Cherenkov condition can not be satisfied! However, it was also shown by clever experimentalists that in some narrow regions of the X-ray spectrum the refractive index is $n>1$ ( the refractive index depends on the frequency in any reasonable materials. Practical Cherenkov materials are, thus, dispersive! ) and the Cherenkov radiation is effectively observed in apparently forbidden regions.

The Cherenkov effect is currently widely used in diverse applications. For instance, it is useful to determine the velocity of fast charged particles ( e.g, neutrino detectors can not obviously detect neutrinos but they can detect muons and other secondaries particles produced in the interaction with some polarizable medium, even when they are produced by (electro)weak intereactions like those happening in the presence of chargeless neutrinos). The selection of the medium fo generating the Cherenkov radiation depends on the range of velocities $\beta$ over which measurements have to be produced with the aid of such a “Cherenkov counter”. Cherenkov detectors/counters are filled with liquids and gases and they are found, e.g., in Kamiokande, Superkamiokande and many other neutrino detectors and “telescopes”. It is worth mentioning that velocities of ultrarelativistic particles are measured with Cherenkov detectors whenever they are filled with some special gasesous medium with a refractive indes just slightly higher than the unity. This value of the refractive index can be changed by realating the gas pressure in the counter! So, Cherenkov detectors and counters are very flexible tools for particle physicists!

Remark: As I mentioned before, it is important to remember that (the most of) the practical Cherenkov radiators/materials ARE dispersive. It means that if $\omega$ is the photon frequency, and $k=2\pi/\lambda$ is the wavenumber, then the photons propagate with some group velocity $v_g=d\omega/dk$, i.e.,

$\boxed{v_g=\dfrac{d\omega}{dk}=\dfrac{c}{\left[n(\omega)+\omega \frac{dn}{d\omega}\right]}}$

Note that if the medium is non-dispersive, this formula simplifies to the well known formula $v_g=c/n$. As it should be for vacuum.

Accodingly, following the PDG, Tamm showed in a classical paper that for dispersive media the Cherenkov radiation is concentrated in a thin  conical shell region whose vertex is at the moving charge and whose opening half-angle $\eta$ is given by the expression

$\boxed{cotan \theta_c=\left[\dfrac{d}{d\omega}\left(\omega\tan\theta_c\right)\right]_{\omega_0}=\left(\tan\theta_c+\beta^2\omega n(\omega) \dfrac{dn}{d\omega} cotan (\theta_c)\right)\bigg|_{\omega_0}}$

where $\theta_c$ is the critical Cherenkov angle seen before, $\omega_0$ is the central value of the small frequency range under consideration under the Cherenkov condition. This cone has an opening half-angle $\eta$ (please, compare with the previous convention with $\varphi$ for consistency), and unless the medium is non-dispersive (i.e. $dn/d\omega=0$, $n=constant$), we get $\theta_c+\eta\neq 90\textdegree$. Typical Cherenkov radiation imaging produces blue rings.

THE CHERENKOV EFFECT: QUANTUM FORMULAE

When we considered the Cherenkov effect in the framework of QM, in particular the quantum theory of radiation, we can deduce the following formula for the Cherenkov effect that includes the quantum corrections due to the backreaction of the particle to the radiation:

$\boxed{\cos\theta=\dfrac{1}{\beta n}+\dfrac{\Lambda}{2\lambda}\left(1-\dfrac{1}{n^2}\right)}$

where, like before, $\beta=v/c$, n is the refraction index, $\Lambda=\dfrac{h}{p}=\dfrac{h}{mv}$ is the De Broglie wavelength of the moving particle and $\lambda$ is the wavelength of the emitted radiation.

Cherenkov radiation is observed whenever $\beta_n>1$ (i.e. if $v>c/n$), and the limit of the emission is on the short wave bands (explaining the typical blue radiation of this effect). Moreover, $\lambda_{min}$ corresponds to $\cos\theta\approx 1$.

By the other hand, the radiated energy per particle per unit of time is equal to:

$\boxed{-\dfrac{dE}{dt}=\dfrac{e^2V}{c^2}\int_0^{\omega_{max}}\omega\left[1-\dfrac{1}{n^2\beta^2}-\dfrac{\Lambda}{n\beta\lambda}\left(1-\dfrac{1}{n^2}\right)-\dfrac{\Lambda^2}{4\lambda^2}\left(1-\dfrac{1}{n^2}\right)\right]d\omega}$

where $\omega=2\pi c/n\lambda$ is the angular frequency of the radiation, with a maximum value of $\omega_{max}=2\pi c/n\lambda_{min}$.
Remark: In the non-relativistic case, $v<, and the condition $\beta n>1$ implies that $n>>1$. Therefore, neglecting the quantum corrections (the charged particle self-interaction/backreaction to radiation), we can insert the limit $\Lambda/\lambda\rightarrow 0$ and the above previous equations will simplify into:

$\boxed{\cos\theta=\dfrac{1}{n\beta}-\dfrac{c}{nv}}$

$\boxed{-\dfrac{dE}{dt}=\dfrac{e^2 v}{c^2}\int_0^{\omega_{max}}\omega\left(1-\dfrac{c^2}{n^2v^2}\right)d\omega}$

Remember: $\omega_{max}$ is determined with the condition $\beta n(\omega_{max})=1$, where $n(\omega_{max})$ represents the dispersive effect of the material/medium through the refraction index.

THE FRANK-TAMM FORMULA

The number of photons produced per unit path length and per unit of energy of a charged particle (charge equals to $Zq$) is given by the celebrated Frank-Tamm formula:

$\boxed{\dfrac{d^2N}{dEdx}=\dfrac{\alpha Z^2}{\hbar c}\sin^2\theta_c=\dfrac{\alpha^2 Z^2}{r_em_ec^2}\left(1-\dfrac{1}{\beta^2n^2(E)}\right)}$

In terms of common values of fundamental constants, it takes the value:

$\boxed{\dfrac{d^2N}{dEdx}\approx 370Z^2\sin^2\theta_c(E)eV^{-1}\cdot cm^{-1}}$

or equivalently it can be written as follows

$\boxed{\dfrac{d^2N}{dEdx}=\dfrac{2\pi \alpha Z^2}{\lambda^2}\left(1-\dfrac{1}{\beta^2n^2(\lambda)}\right)}$

The refraction index is a function of photon energy $E=\hbar \omega$, and it is also the sensitivity of the transducer used to detect the light with the Cherenkov effect! Therefore, for practical uses, the Frank-Tamm formula must be multiplied by the transducer response function and integrated over the region for which we have $\beta n(\omega)>1$.

Remark: When two particles are close toghether ( to be close here means to be separated a distance $d<1$ wavelength), the electromagnetic fields form the particles may add coherently and affect the Cherenkov radiation. The Cherenkov radiation for a electron-positron pair at close separation is suppressed compared to two independent leptons!

Remark (II): Coherent radio Cherenkov radiation from electromagnetic showers is significant and it has been applied to the study of cosmic ray air showers. In addition to this, it has been used to search for electron neutrinos induced showers by cosmic rays.

CHERENKOV DETECTOR: MAIN FORMULA AND USES

The applications of Cherenkov detectors for particle identification (generally labelled as PID Cherenkov detectors) are well beyond the own range of high-energy Physics. Its uses includes: A) Fast particle counters. B) Hadronic particle indentifications. C) Tracking detectors performing complete event reconstruction. The PDG gives some examples of each category: a) Polarization detector of SLD, b) the hadronic PID detectors at B factories like BABAR or the aerogel threshold Cherenkov in Belle, c) large water Cherenkov counters liket those in Superkamiokande and other neutrino detector facilities.

Cherenkov detectors contain two main elements: 1) A radiator/material through which the particle passes, and 2) a photodetector. As Cherenkov radiation is a weak source of photons, light collection and detection must be as efficient as possible. The presence of a refractive material specifically designed to detect some special particles is almost vindicated in general.

The number of photoelectrons detected in a given Cherenkov radiation detector device is provided by the following formula (derived from the Tamm-Frank formula simply taking into account the efficiency in a straightforward manner):

$\boxed{N=L\dfrac{\alpha^2 Z^2}{r_em_ec^2}\int \epsilon (E)\sin^2\theta_c(E)dE}$

where $L$ is the path length of the particle in the radiator/material, $\epsilon (E)$ is the efficiency for the collector of Cherenkov light and transducing it in photoelectrons, and

$\boxed{\dfrac{\alpha^2}{r_em_ec^2}=370eV^{-1}cm^{-1}}$

Remark: The efficiencies and the Cherenkov critical angle are functions of the photon energy, generally speaking. However, since the typical energy dependen variation of the refraction index is modest, a quantity sometimes called Cherenkov detector quality fact $N_0$ can be defined as follows

$\boxed{N_0=\dfrac{\alpha^2Z^2}{r_em_ec^2}\int \epsilon dE}$

In this case, we can write

$\boxed{N\approx LN_0<\sin^2\theta_c>}$

Remark(II): Cherenkov detectors are classified into imaging or threshold types, depending on its ability to make use of Cherenkov angle information. Imaging counters may be used to track particles as well as identify particles.

Other main uses/applications of the Vavilov-Cherenkov effect are:

1st. Detection of labeled biomolecules. Cherenkov radiation is widely used to facilitate the detection of small amounts and low concentrations of biomolecules. For instance, radioactive atoms such as phosphorus-32 are readily introduced into biomolecules by enzymatic and synthetic means and subsequently may be easily detected in small quantities for the purpose of elucidating biological pathways and in characterizing the interaction of biological molecules such as affinity constants and dissociation rates.

2nd. Nuclear reactors. Cherenkov radiation is used to detect high-energy charged particles. In pool-type nuclear reactors, the intensity of Cherenkov radiation is related to the frequency of the fission events that produce high-energy electrons, and hence is a measure of the intensity of the reaction. Similarly, Cherenkov radiation is used to characterize the remaining radioactivityof spent fuel rods.

3rd. Astrophysical experiments. The Cherenkov radiation from these charged particles is used to determine the source and intensity of the cosmic ray,s which is used for example in the different classes of cosmic ray detection experiments. For instance, Ice-Cube, Pierre-Auger, VERITAS, HESS, MAGIC, SNO, and many others. Cherenkov radiation can also be used to determine properties of high-energy astronomical objects that emit gamma rays, such as supernova remnants and blazars. In this last class of experiments we place STACEE, in new Mexico.

4th. High-energy experiments. We have quoted already this, and there many examples in the actual LHC, for instance, in the ALICE experiment.

Vacuum Cherenkov radiation (VCR) is the alledged and  conjectured phenomenon which refers to the Cherenkov radiation/effect of a charged particle propagating in the physical vacuum. You can ask: why should it be possible? It is quite straightforward to understand the answer.

The classical (non-quantum) theory of relativity (both special and general)  clearly forbids any superluminal phenomena/propagating degrees of freedom for material particles, including this one (the vacuum case) because a particle with non-zero rest mass can reach speed of light only at infinite energy (besides, the nontrivial vacuum itself would create a preferred frame of reference, in violation of one of the relativistic postulates).

However, according to modern views coming from the quantum theory, specially our knowledge of Quantum Field Theory, physical vacuum IS a nontrivial medium which affects the particles propagating through, and the magnitude of the effect increases with the energies of the particles!

Then, a natural consequence follows: an actual speed of a photon becomes energy-dependent and thus can be less than the fundamental constant $c=299792458m/s$ of  speed of light, such that sufficiently fast particles can overcome it and start emitting Cherenkov radiation. In summary, any charged particle surpassing the speed of light in the physical vacuum should emit (Vacuum) Cherenkov radiation. Note that it is an inevitable consequence of the non-trivial nature of the physical vacuum in Quantum Field Theory. Indeed, some crazy people saying that superluminal particles arise in jets from supernovae, or in colliders like the LHC fail to explain why those particles don’t emit Cherenkov radiation. It is not true that real particles become superluminal in space or collider rings. It is also wrong in the case of neutrino propagation because in spite of being chargeless, neutrinos should experiment an analogue effect to the Cherenkov radiation called the Askaryan effect. Other (alternative) possibility or scenario arises in some Lorentz-violating theories ( or even CPT violating theories that can be equivalent or not to such Lorentz violations) when a speed of a propagating particle becomes higher than c which turns this particle into the tachyon.  The tachyon with an electric charge would lose energy as Cherenkov radiation just as ordinary charged particles do when they exceed the local speed of light in a medium. A charged tachyon traveling in a vacuum therefore undergoes a constant proper-time acceleration and, by necessity, its worldline would form an hyperbola in space-time. These last type of vacuum Cherenkov effect can arise in theories like the Standard Model Extension, where Lorentz-violating terms do appear.

One of the simplest kinematic frameworks for Lorentz Violating theories is to postulate some modified dispersion relations (MODRE) for particles , while keeping the usual energy-momentum conservation laws. In this way, we can provide and work out an effective field theory for breaking the Lorentz invariance. There are several alternative definitions of MODRE, since there is no general guide yet to discriminate from the different theoretical models. Thus, we could consider a general expansion  in integer powers of the momentum, in the next manner (we set units in which $c=1$):

$\boxed{E^2=f(p,m,c_n)=p^2+m^2+\sum_{n=-\infty}^{\infty}c_n p^n}$

However, it is generally used a more soft expansion depending only on positive powers of the momentum in the MODRE. In such a case,

$\boxed{E^2=f(p,m,a_n)=p^2+m^2+\sum_{n=1}^{\infty}a_n p^n}$

and where $p=\vert \mathbf{p}\vert$. If Lorentz violations are associated to the yet undiscovered quantum theory of gravity, we would get that ordinary deviations of the dispersion relations in the special theory of relativity should appear at the natural scale of the quantum gravity, say the Planck mass/energy. In units where $c=1$ we obtain that Planck mass/energy is:

$\boxed{M_P=\sqrt{\hbar^5/G_N}=1.22\cdot 10^{19}GeV=1.22\cdot 10^{16}TeV}$

Lets write and parametrize the Lorentz violations induced by the fundamental scale of quantum gravity (naively this Planck mass scale) by:

$\boxed{a_n=\dfrac{\Xi_n}{M_P^{n-2}}}$

Here, $\Xi_n$ is a dimensionless quantity that can differ from one particle (type) to another (type). Considering, for instance $n=3,4$, since the $n<3$ seems to be ruled out by previous terrestrial experiments, at higer energies the lowest non-null term will dominate the expansion with $n\geq 3$. The MODRE reads:

$E^2=p^2+m^2+\dfrac{\Xi_a p^n}{M_P^{n-2}}$

and where the label $a$ in the term $\Xi_a$ is specific of the particle type. Such corrections might only become important at the Planck scale, but there are two exclusions:

1st. Particles that propagate over cosmological distances can show differences in their propagation speed.
2nd. Energy thresholds for particle reactions can be shifted or even forbidden processes can be allowed. If the $p^n$-term is comparable to the $m^2$-term in the MODRE. Thus, threshold reactions can be significantly altered or shifted, because they are determined by the particle masses. So a threshold shift should appear at scales where:

$\boxed{p_{dev}\approx\left(\dfrac{m^2M_P^{n-2}}{\Xi}\right)^{1/n}}$

Imposing/postulating that $\Xi\approx 1$, the typical scales for the thresholds for some diffent kind of particles can be calculated. Their values for some species are given in the next table:

We can even study some different sources of modified dispersion relationships:

1. Measurements of time of flight.

2. Thresholds creation for: A) Vacuum Cherenkov effect, B) Photon decay in vacuum.

3. Shift in the so-called GZK cut-off.

4. Modified dispersion relationships induced by non-commutative theories of spacetime. Specially, there are time shifts/delays of photon signals induced by non-commutative spacetime theories.

We will analyse this four cases separately, in a very short and clear fashion. I wish!

Case 1. Time of flight. This is similar to the recently controversial OPERA experiment results. The OPERA experiment, and other similar set-ups, measure the neutrino time of flight. I dedicated a post to it early in this blog

https://thespectrumofriemannium.wordpress.com/2012/06/08/

In fact, we can measure the time of flight of any particle, even photons. A modified dispersion relation, like the one we introduced here above, would lead to an energy dependent speed of light. The idea of the time of flight (TOF) approach is to detect a shift in the arrival time of photons (or any other massless/ultra-relativistic particle like neutrinos) with different energies, produced simultaneous in a distant object, where the distance gains the usually Planck suppressed effect. In the following we use the dispersion relation for $n=3$ only, as modifications in higher orders are far below the sensitivity of current or planned experiments. The modified group velocity becomes:

$v=\dfrac{\partial E}{\partial p}$

and then, for photons,

$v\approx 1-\Xi_\gamma\dfrac{p}{M}$

The time difference in the photon shift detection time will be:

$\Delta t=\Xi_\gamma \dfrac{p}{M}D$

where D is the distance multiplied (if it were the case) by the redshift $(1+z)$ to correct the energy with the redshift. In recent years, several measurements on different objects in various energy bands leading to constraints up to the order of 100 for $\Xi$. They can be summarized in the next table ( note that the best constraint comes from a short flare of the Active Galactic Nucleus (AGN) Mrk 421, detected in the TeV band by the Whipple Imaging Air Cherenkov telescope):

There is still room for improvements with current or planned experiments, although the distance for TeV-observations is limited by absorption of TeV photons in low energy metagalactic radiation fields. Depending on the energy density of the target photon field one gets an energy dependent mean free path length, leading to an energy and redshift dependent cut off energy (the cut off energy is defined as the energy where the optical depth is one).

2. Thresholds creation for: A) Vacuum Cherenkov effect, B) Photon decay in vacuum. By the other hand, the interaction vertex in quantum electrodynamics (QED) couples one photon with two leptons. When we assume for photons and leptons the following dispersion relations (for simplicity we adopt all units with M=1). Then:

$\omega_k^2=k^2+\xi k^n$                $E^2_p=p^2+m^2+\Xi p^n$

Let us write the photon tetramomentum like $\mathbb{P}=(\omega_k,\mathbf{k})$ and the lepton tetramomentum $\mathbb{P}=(E_p,\mathbf{p})$ and $\mathbb{Q}=(E_q,\mathbf{q})$. It can be shown that the transferred tetramomentum will be

$\xi k^n+\Xi p^n-\Xi q^n=2(E_p\omega_k-\mathbf{p}\cdot\mathbf{k})$

where the r.h.s. is always positive. In the Lorentz invariant case the parameters $\xi, \Xi$  are zero, so that this equation can’t be solved and all processes of the single vertex are forbidden. If these parameters are non-zero, there can exist a solution and so these processes can be allowed. We now consider two of these interactions to derive constraints on the parameters $\Xi, \xi$. The vacuum
Cherenkov effect $e^-\rightarrow \gamma e^-$ and the spontaneous photon-decay $\gamma\rightarrow e^+e^-$.

A) As we have studied here, the vacuum Cherenkov effect is a spontaneous emission of a photon by a charged particle $0.  These effect occurs if the particle moves faster than the slowest possible radiated photon in vacuum!
In the case of $\Xi>0$, the maximal attainable speed for the particle $c_{max}$ is faster than c. This means, that the particle can always be faster than a zero energy photon with

$\displaystyle{c_{\gamma_0}=c\lim_{k\rightarrow 0}\dfrac{\partial \omega}{\partial k}=c\lim_{k\rightarrow 0}\dfrac{2k+n\xi k^{n-1}}{2\sqrt{k^2+\xi k^n}}=c}$

and it is independent of $\xi$. In the case of $\Xi<0$, i.e., $c_{par}$ decreases with energy, you need a photon with $c_\gamma. This is only possible if $\xi<\Xi$.

Therefore, due to the radiation of photons such an electron loose energy. The observation of high energetic electrons allows to derive constraints on $\Xi$ and $\xi$.  In the case of $\Xi<0$, in the case with n=3, we have the bound

$\Xi<\dfrac{m^2}{2p^3_{max}}$

Moreover, from the observation of 50 TeV photons in the Crab Nebula (and its pulsar) one can conclude the existens of 50 TeV electrons due to the inverse Compton scattering of these electrons with those photons. This leads to a constraint on $\Xi$ of about

$\Xi<1.2\times 10^{-2}$

where we have used $\Xi>0$ in this case.

B) The decay of photons into positrons and electrons $\gamma\rightarrow e^+e^-$ should be a very rapid spontaneous decay process. Due to the observation of Gamma rays from the Crab Nebula on earth with an energy up to $E\sim 50TeV$. Thus, we can reason that these rapid decay doesn’t occur on energies below 50 TeV. For the constraints on $\Xi$ and $\xi$ these condition means (again we impose n=3):

$\xi<\dfrac{\Xi}{2}+0.08, \mbox{for}\; \xi\geq 0$

$\xi<\Xi+\sqrt{-0.16\Xi}, \mbox{for}\;\Xi<\xi<0$.

3. Shift in the GZK cut-off. As the energy of a proton increases,the pion production reaction can happen with low energy photons of the Cosmic Microwave Background (CMB).

This leads to an energy dependent mean free path length of the particles, resulting in a cutoff at energies around $E_{GZK}\approx 10^{20}eV$. This is the the celebrated Greisen-Kuzmin-Zatsepin (GZK) cut off. The resonance for the GZK pion photoproduction with the CMB backgroud can be read from the next condition (I will derive this condition in a future post):

$\boxed{E_{GZK}\approx\dfrac{m_p m_\pi}{2E_\gamma}=3\times 10^{20}eV\left(\dfrac{2.7K}{E_\gamma}\right)}$

Thus in Lorentz invariant world, the mean free path length of a particle of energy 5.1019 eV is 50 Mpc i.e. particle over this energy are readily absorbed due to pion photoproduction reaction. But most of the sources of particle of ultra high energy are outside 50 Mpc. So, one expects no trace of particles of energy above $10^{20}eV$ on Earth. From the experimental point of view AGASA has found
a few particles having energy higher than the constraint given by GZK cutoff limit and claimed to be disproving the presence of GZK cutoff or at least for different threshold for GZK cutoff, whereas HiRes is consistent with the GZK effect. So, there are two main questions, not yet completely unsolved:

i) How one can get definite proof of non-existence GZK cut off?
ii) If GZK cutoff doesn’t exist, then find out the reason?

The first question could by answered by observation of a large sample of events at these energies, which is necessary for a final conclusion, since the GZK cutoff is a statistical phenomena. The current AUGER experiment, still under construction, may clarify if the GZK cutoff exists or not. The existence of the GZK cutoff would also yield new limits on Lorentz or CPT violation. For the second question, one explanation can be derived from Lorentz violation. If we do the calculation for GZK cutoff in Lorentz violated world we would get the modified proton dispersion relation as described in our previous equations with MODRE.

4. Modified dispersion relationships induced by non-commutative theories of spacetime. As we said above, there are time shifts/delays of photon signals induced by non-commutative spacetime theories. Noncommutative spacetime theories introduce a new source of MODRE: the fuzzy nature of the discreteness of the fundamental quantum spacetime. Then, the general ansatz of these type of theories comes from:

$\boxed{\left[\hat{x}^\mu,\hat{x}^\nu\right]=i\dfrac{\theta^{\mu\nu}}{\Lambda_{NC}^2}}$

where $\theta^{\mu\nu}$ are the components of an antisymmetric Lorentz-like tensor which components are the order one. The fundamental scale of non-commutativity $\Lambda^2_{NC}$ is supposed to be of the Planck length. However, there are models with large extra dimensions that induce non-commutative spacetime models with scale near the TeV scale! This is interesting from the phenomenological aside as well, not only from the theoretical viewpoint. Indeed, we can investigate in the following whether astrophysical observations are able to constrain certain class of models with noncommutative spacetimes which are broken at the TeV scale or higher. However, there due to the antisymmetric character of the noncommutative tensor, we need a magnetic and electric background field in order to study these kind of models (generally speaking, we need some kind of field inducing/producing antisymmetric field backgrounds), and then the dispersion relation for photons remains the same as in a commutative spacetime. Furthermore, there is no photon energy dependence of the dispersion relation. Consequently, the time-of-flight experiments are inappopriate because of their energy-dependent dispersion. Therefore, we suggest the next alternative scenario: suppose, there exists a strong magnetic field  (for instance, from a star or a cluster of stars) on the path photons emitted at a light source (e.g. gamma-ray bursts). Then, analogous to gravitational lensing, the photons experience deflection and/or change in time-of-arrival, compared to the same path without a magnetic background field. We can make some estimations for several known objects/examples are shown in this final table:

In summary:

1st. Vacuum Cherenkov and related effects modifying the dispersion relations of special relativity are natural in many scenarios beyond the Standard Relativity (BSR) and beyond the Standard Model (BSM).

2nd. Any theory allowing for superluminal propagation has to explain the null-results from the observation of the vacuum Cherenkov effect. Otherwise, they are doomed.

3rd. There are strong bounds coming from astrophysical processes and even neutrino oscillation experiments that severely imposes and kill many models. However, it is true that current MODRE bound are far from being the most general bounds. We expect to improve these bounds with the next generation of experiments.

4th. Theories that can not pass these tests (SR obviously does) have to be banned.

5th. Superluminality has observable consequences, both in classical and quantum physics, both in standard theories and theories beyond standard theories. So, it you buid a theory allowing superluminal stuff, you must be very careful with what kind of predictions can and can not do. Otherwise, your theory is complentely nonsense.

As a final closing, let me include some nice Cherenkov rings from Superkamiokande and MiniBoone experiments. True experimental physics in action. And a final challenge…

FINAL CHALLENGE: Are you able to identify the kind of particles producing those beautiful figures? Let me know your guesses ( I do know the answer, of course).

Figure 1. Typical SuperKamiokande Ring.  I dedicate this picture to my admired Japanase scientists there. I really, really admire that country and their people, specially after disasters like the 2011 Earthquake and the Fukushima accident. If you are a japanase reader/follower, you must know we support your from abroad. You were not, you are not and you shall not be alone.

Figure 2. Typical MiniBooNe ring. History: I used this nice picture in my Master Thesis first page, as the cover/title page main picture!