# LOG#106. Basic Cosmology (I).

The next thread is devoted to Cosmology. I will intend to be clear and simple about equations and principles of current Cosmology with a General Relativity background.

First of all…I will review the basic concepts of natural units I am going to use here. We will be using the following natural units:

$\hbar=c=k_B=1$

We will take the Planck mass to be given by

$M_P=\sqrt{8\pi G_N}\approx 1\mbox{.}2\cdot 10^{19}GeV$

The solar mass is $M_\odot=2\cdot 10^{30}kg$ and the parsec is given by the value

$1pc=3\mbox{.}26lyr=3\mbox{.}1\cdot 10^{16}m$

Well, current Cosmology is based on General Relativity. Even if I have not reviewed this theory with detail in this blog, the nice thing is that most of Cosmology can be learned with only a very little knowledge of this fenomenal theory. The most important ideas are: metric field, geodesics, Einstein equations and no much more…

In fact, newtonian gravity is a good approximation in some particular cases! And we do know that even in this pre-relativistic theory

$\mbox{Gravitational force}=\mbox{Matter/Mass density}$

via the Poisson’s equation

$\nabla^2\phi =4\pi G_N\rho$

This idea, due to the equivalence principle, is generalized a little bit in the general relativistic framework

$\mbox{Spacetime geometry}=\mbox{Matter content/Energy-momentum}$

The spacetime geometry is determined by the metric tensor $g_{\mu\nu}(x)$. The matter content is given by the stress-energy-momentum tensor $T_{\mu\nu}$. As we know one of these two elements, we can know, via Eisntein’s field equations the another. That is, given a metric tensor, we can tell how energy-momentum “moves” in space-time. Given the energy-momentum tensor, we can know what is the metric tensor in spacetime and we can guess how the spacetime bends… This is the origin of the famous motto: “Spacetime says matter how to move, energy-momentum says spacetime how to curve”! Remember that we have “deduced” the Einstein’s field equations in the previous post. Without a cosmological constant term, we get

$G_{\mu\nu}=\kappa^2T_{\mu\nu}=8\pi G_NT_{\mu\nu}$

Given a spacetime metric $g_{\mu\nu}$, we can calculate the (affine/Levi-Civita) connection

$\Gamma^\sigma_{\;\;\mu\nu}=\dfrac{1}{2}g^{\sigma\rho}\left(\partial_\mu g_{\nu\rho}+\partial_\nu g_{\rho\mu}-\partial_\rho g_{\mu\nu}\right)$

The Riemann tensor that measures the spacetime curvature is provided by the equation

$R^\rho_{\;\; \sigma\mu\nu}=\partial_\mu \Gamma^\rho_{\;\;\mu\sigma}-\partial_\mu \Gamma^\rho_{\;\; \mu \sigma}+\Gamma^\rho_{\;\;\mu\lambda}\Gamma^\lambda_{\;\;\nu\sigma}-\Gamma^\rho_{\;\;\nu\lambda}\Gamma^\lambda_{\;\;\mu\sigma}$

The Ricci tensor is defined to be the following “trace” of the Riemann tensor

$R_{\mu\nu}=R^\lambda_{\;\;\mu\lambda \nu}$

The Einstein tensor is related to the above tensors in the well-known manner

$G_{\mu\nu}=R_{\mu\nu}-\dfrac{1}{2}Rg_{\mu\nu}$

The Einstein’s equations can be derived from the Einstein-Hilbert action we learned in the previous post, using the action principle and the integral

$\boxed{S_{EH}=\int d^4x \sqrt{-g} \left(\kappa^{-2}R+\mathcal{L}_M\right)}$

The geodesic equation is the path of a freely falling particle. It gives a “condensation” of the Einstein’s equivalence principle too and it is also a generalization of Newton’s law of “no force”. That is, the geodesic equation is the feynmanity

$\boxed{\dfrac{d^2 x^\mu}{d\tau^2}+\Gamma^\mu _{\rho\sigma}\dfrac{dx^\rho}{d\tau}\dfrac{dx^\sigma}{d\tau}=0}$

Finally, an important concept in General Relativity is that of isometry. The symmetry of the “spacetime manifold” is provided by a Killing vector that preserves transformations (isometries) of that manifold. Mathematically speaking, the Killing vector fields satisfy certain equation called the Killing equation

$\boxed{\xi_{\mu ; \nu}+\xi_{\nu ; \mu}=0}$

Maximally symmetric spaces have $n(n+1)/2$ Killing vectors in n-dimensional (nD) spacetime. There are 3 main classes or types of 2D maximally symmetric that can be generalized to higher dimensions:

1. The euclidean plane $E^2$.

2. The pseudo-sphere $H^2$. This is a certain “hyperbolic” space.

3. The spehre $S^2$. This is a certain “elliptic” space.

The Friedmann-Robertson-Walker Cosmology

Current cosmological models are based in General Relativity AND  a simplification of the possible metrics due to the so-called Copernican (or cosmological) principle: the Universe is pretty much the same “everywhere” you are in the whole Universe! Remarkbly, the old “perfect” Copernican (cosmological) principle that states that the Universe is the same “everywhere” and “every time” is wrong. Phenomenologically, we have found that the Universe has evolved and it evolves, so the Universe was “different” when it was “young”. Therefore, the perfect cosmological principle is flawed. In fact, this experimental fact allows us to neglect some old theories like the “stationary state” and many other “crazy theories”.

What are the observational facts to keep the Copernican principle? It seems that:

1st. The distribution of matter (mainly galaxies, clusters,…) and radiation (the cosmic microwave background/CMB) in the observable Universe is homogenous and isotropic.

2nd. The Universe is NOT static. From Hubble’s pioneer works/observations, we do know that galaxies are receeding from us!

Therefore, these observations imply that our “local” Hubble volume during the Hubble time is similar to some spacetime with homogenous and isotropic spatial sections, i.e., it is a spacetime manifold $M=\mathbb{R}\times \Sigma$. Here, $\mathbb{R}$ denotes the time “slice” and $\Sigma$ represents a 3D maximally symmetric space.

The geometry of a locally isotropic and homogeneous Universe is represented by the so-called Friedmann-Robertson-Walker metric

$\boxed{ds^2_{FRW}=-dt^2+a^2(t)\left[\dfrac{dr^2}{1-kr^2}+r^2\left(d\theta^2+\sin\theta^2d\phi^2\right)\right]}$

Here, $a(t)$ is the called the scale factor.  The parameter $k$ determines the geometry type (plane, hyperbolic or elliptical/spherical):

1) If $k=0$, then the Universe is “flat”. The manifold is $E^3$.

2) If $k=-1$, then the Universe is “open”/hyperbolic. The manifold would be $H^3$.

3) If $k=+1$, then the Universe is “closed”/spherical or elliptical. The manifold is then $S^3$.

Remark: The ansatz of local homogeneity and istoropy only implies that the spatial metric is locally one of the above three spaces, i.e., $E^3,H^3,S^3$. It could be possible that these 3 spaces had different global (likely topological) properties beyond these two properties.

Kinematical features of a FRW Universe

The first property we are interested in Cosmology/Astrophysics is “distance”. Measuring distance in a expanding Universe like a FRW metric is “tricky”. There are several notions of “useful” distances. They can be measured by different methods/approaches and they provide something called sometimes “the cosmologidal distance ladder”:

1st. Comoving distance. It is a measure in which the distance is “taken” by a fixed coordinate.

2nd. Physical distance. It is essentially the comoving distance times the scale factor.

3rd. Luminosity distance. It uses the light emitted by some object to calculate its distance (provided the speed of light is taken constant, i.e., special relativity holds and we have a constant speed of light)

4th. Angular diameter distance. Another measure of distance using the notion of parallax and the “extension” of the physical object we measure somehow.

There is an important (complementary) idea in FRW Cosmology: the particle horizon. Consider a light-like particle with $ds^2=0$. Then,

$dt=a(t)\dfrac{1}{\sqrt{1-kr^2}}$

or

$\dfrac{dr}{\sqrt{1-kr^2}}=\dfrac{dt}{a(t)}$

The total comoving distance that light have traveled since a time $t=0$ is equal to

$\boxed{\eta=\int_0^{r_H}\dfrac{dr}{\sqrt{1-kr^2}}=\int_0^t\dfrac{dt'}{a(t')}}$

It shows that NO information could have propagated further and thus, there is a “comoving horizon” with every light-like particle! Here, this time is generally used as a “conformal time” as a convenient tiem variable for the particle. The physical distance to the particle horizon can be calculated

$\boxed{d_H(t)=\int_0^{r_H}\sqrt{g_{rr}}dr=a(t)\int_0^t\dfrac{dt'}{a(t')}=a(t)\eta}$

There are some important kinematical equations to be known

A) For the geodesic equation, the free falling particle, we have

$\Gamma^0_{ij}=\dfrac{\dot{a}}{a}\overline{g}_{ij}$

$\Gamma^i_{0j}=\Gamma^i_{j0}=\dfrac{\dot{a}}{a}\delta_{ij}$

$\Gamma^i_{jk}=\overline{\Gamma}^i_{jk}$

for the FRW metric and, moreover, the energy-momentum vector $P^\mu=(E,\mathbf{p})$ is defined by the usual invariant equation

$P^\mu=\dfrac{dx^\mu}{d\lambda}$

This definition defines, in fact, the proper “time” $\lambda$ implicitely, since

$\dfrac{d}{d\lambda}=\dfrac{dx^0}{d\lambda}\dfrac{d}{dx^0}=E\dfrac{d}{dt}$

and the 0th component of the geodesic equation becomes

$E\dfrac{dE}{dt}=-\Gamma^0_{ij}p^ip^j=-\delta_{ij}a\dot{a}p^ip^j$

$g_{\mu\nu}p^\mu p^\nu=-E^2+a^2\delta_{ij}p^ip^j=-m^2$

$EdE=a^2\vert \mathbf{p}\vert d\vert \mathbf{p}\vert$

$a^2 p\dfrac{dp}{dt}=-a\dot{a} p^2$

$\dfrac{1}{\vert \mathbf{p}\vert }\dfrac{d\vert \mathbf{p}\vert}{dt}+\dfrac{\dot{a}}{a}=0$

Therefore we have deduced that $\vert \mathbf{p}\vert \propto a^{-1}$. This is, in fact, the socalled “redshift”.  The cosmological  redshift parameter is more generally defined through the equation

$\boxed{\dfrac{a(t_0)}{a(t)}=1+z=\dfrac{\lambda_0}{\lambda}}$

B) The Hubble’s law.

The luminosity distance measures the flux of light from a distant object of known luminosity (if it is not expanding). The flux and luminosity distance are bound into a single equation

$\boxed{F=\dfrac{L}{4\pi d^2_L}}$

If we use the comoving distance between a distant emitter and us, we get

$\chi (a)=\int_t^{t_0}\dfrac{dt'}{a(t')}=\int_a^1\dfrac{da'}{a'^2 H(a')}$

for a expanding Universe! That is, we have used the fact that luminosity itself goes through a comoving spherical shell of radius $\chi (a)$. Moreover, it shows that

$F=\dfrac{L (\chi)}{4 \pi \chi (a)^2 a_0^2}=\dfrac{L}{4\pi (\chi (a)/a)^2}$

The luminosity distance in the expanding shell is

$d_L=\dfrac{\chi (a)}{a}=\left(\dfrac{L}{4\pi F}\right)^{1/2}$

and this is what we MEASURE in Astrophysics/Cosmology. Knowing $a(t)$, we can express the luminosity distance in terms of the redshift. Taylor expansion provides something like this:

$H_0d_L=z+\dfrac{1}{2}(1-q_0)z^2+\ldots$

where higher order terms are sometimes referred as “statefinder parameters/variables”. In particular, we have

$\boxed{H_0=\dfrac{\dot{a}_0}{a_0}}$

and

$\boxed{q_0=-\dfrac{a_0\ddot{a}_0}{\dot{a}_0^2}}$

C) Angular diameter distance.

If we know that some object has a known length $l$, and it gives some angular “aperture” or separation $\theta$, the angular diameter distance is given by

$\boxed{d_A=\dfrac{l}{\theta}}$

The comoving size is defined as $l/a$, and the coming distance is again $\chi (a)$. For “flat” space, we obtain that

$\theta=\dfrac{l/a}{\chi (a)}$

that is

$d_A=a\chi (a)=\dfrac{\chi}{1+z}$

In the case of “curved” spaces, we get

$d_A=\dfrac{a}{H_0\sqrt{\vert \omega_k\vert}}\cdot\begin{cases}\sinh \left( \sqrt{\Omega_k}H_0\chi\right),\Omega_k>0\\ \sin \left( \sqrt{-\Omega_k}H_0\chi\right),\Omega_k<0\end{cases}$

FRW dynamics

Gravity in General Relativity, a misnomer for the (locally) relativistic theory of gravitation, is described by a metric field, i.e., by a second range tensor (covariant tensor if we are purist with the nature of components). The metric field is related to the matter-energy-momentum content through the Einstein’s equations

$G_{\mu\nu}=-\kappa^2 T_{\mu\ nu}$

The left-handed side can be calculated for a FRW Universe as follows

$R_{00}=-3\dfrac{\ddot{a}}{a}$

$R_{ij}=(a\ddot{a}+2\dot{a}^2+2k)\overline{g}_{ij}$

$R=6\left(\dfrac{\ddot{a}}{a}+\dfrac{\dot{a}^2}{a^2}+\dfrac{k}{a^2}\right)$

The right-handed side is the energy-momentum of the Universe. In order to be fully consistent with the symmetries of the metric, the energy-momentum tensor MUST be diagonal and $T_{11}=T_{22}=T_{33}=T$. In fact, this type of tensor describes a perfect fluid with

$T_{\mu\nu}=(\rho+p)U_\mu U_\nu+pg_{\mu\nu}$

Here, $\rho, p$ are functions of $t$ (cosmological time) only. They are “state variables” somehow. Moreover, we have

$U_\mu =(1,0,0,0)$

for the fluid at rest in the comoving frame. The Friedmann equations are indeed the EFE for a FRW metric Universe

$3\left(\dfrac{\dot{a}^2}{a^2}+\dfrac{k}{a^2}\right)=\kappa^2\rho$ for the 00th compoent as “constraint equation.

$2\dfrac{\ddot{a}}{a}+\dfrac{\dot{a}^2}{a^2}+\dfrac{k}{a^2}=-\kappa^2p$ for the iith components.

Moreover, we also have

$G_{\mu\nu}^{;\nu}=T_{\mu\nu}^{;\nu}=0$

and this conservation law implies that

$\dot{\rho}+3\dfrac{\dot{a}}{a}(\rho+p)=0$

Therefore, we have got two independent equations for three unknowns $(a, \rho, p)$. We need an additional equation. In fact, the equation of state for $p=p(\rho)$ provides such an additional equation. It gives the “dynamics of matter”!

In summary, the basic equations for Cosmology in a FRW metric, via EFE, are the Friedmann’s equations (they are secretly the EFE for the FRW metric) supplemented with the energy-momentum conservations law and the equation of state for the pressure $p=p(\rho)$:

1) $\boxed{\dfrac{\dot{a}^2}{a^2}+\dfrac{k^2}{a^2}=\dfrac{\kappa^2}{3}\rho}$

2) $\boxed{\dot{\rho}+3\dfrac{\dot{a}}{a}(\rho+p)=0}$

3) $\boxed{p=p(\rho)}$

There are many kinds of “matter-energy” content of our interest in Cosmology. Some of them can be described by a simple equation of state:

$\boxed{p=\omega \rho}$

Energy-momentum conservation implies that $\rho\propto a^{-3(\omega +1)}$. 3 special cases are used often:

1st. Radiation (relativistic “matter”). $\omega=1/3$ and thus, $p=1/3\rho$ and $\rho\propto a^{-4}$

2nd. Dust (non-relativistic matter). $\omega=0$. Then, $p=0$ and $\rho\propto a^{-3}$

3rd. Vacuum energy (cosmological constant). $\omega=-1$. Then, $p=-\rho$ and $\rho=\mbox{constant}$

Remark (I): Particle physics enters Cosmology here! Matter dynamics or matter fields ARE the matter content of the Universe.

Remark (II): Existence of a Big Bang (and a spacetime singularity). Using the Friedmann’s equation

$\dfrac{\ddot{a}}{a}=-\dfrac{\kappa^2}{6}(\rho+3p)$

if we have that $(\rho+3p)>0$, the so-called weak energy condition, then $a=0$ should have been reached at some finite time in the past! That is the “Big Bang” and EFE are “singular” there. There is no scape in the framework of GR. Thus, we need a quantum theory of gravity to solve this problem OR give up the FRW metric at the very early Universe by some other type of metric or structure.

Particles and the chemical equilibrium of the early Universe

Today, we have DIRECT evidence for the existence of a “thermal” equilibrium in the early Universe: the cosmic microwave background (CMB). The CMB is an isotropic, accurate and non-homogeneous (over certain scales) blackbody spectrum about $T\approx 3K$!

Then, we know that the early Universe was filled with a hot dieal gas in thermal equilibrium (a temperature $T_e$ can be defined there) such as the energy density and pressure can be written in terms of this temperature. This temperature generates a distribution $f(\mathbf{x},\mathbf{p})$. The number of phase space elements in $d^3xd^3p$ is

$d^3xd^3p=\dfrac{d^3\mathbf{x}d^3\mathbf{p}}{(2\pi\hbar)^3}$

and where the RHS is due to the uncertainty principle. Using homogeneity, we get that, indeed, $f(x,p)=f(p)$, and where we can write the volume $d^3x=dV$. The energy density and the pressure are given by (natural units are used)

$\rho_i=g_i\int \dfrac{d^3p}{(2\pi)^3}f_i(p)E(p)$

$p_i=g_i\int \dfrac{d^3p}{(2\pi)^3}f_i (p)\dfrac{p^2}{3E(p)}$

When we are in the thermal equilibrium at temperature T, we have the Bose-Einstein/Fermi-Dirac distribution

$f(p)=\dfrac{1}{e^{(E-\mu)/T}\pm 1}$

and where the $+$ is for the Fermi-Dirac distribution (particles) and the $-$ is for the Bose-Einstein distribution (particles). The number density, the energy density and the pressure are the following integrals

$\boxed{\mbox{Number density}=n=\dfrac{N}{V}=\dfrac{g}{2\pi^2}\int_m^\infty \dfrac{(E^2-m^2)^{1/2}}{e^{(E-\mu)/T}\pm 1}dE}$

$\boxed{\mbox{Density energy}=\rho=\dfrac{E}{V}=\dfrac{g}{2\pi^2}\int_m^\infty \dfrac{(E^2-m^2)^{1/2}E^2}{e^{(E-\mu)/T}\pm 1}dE}$

$\boxed{\mbox{Pressure}=p=\dfrac{g}{6\pi^2}\int_m^\infty \dfrac{(E^2-m^2)^{3/2}}{e^{(E-\mu)/T}\pm 1}dE}$

And now, we find some special cases of matter-energy for the above variables:

1st. Relativistic, non-degenerate matter (e.g. the known neutrino species). It means that $T>>m$ and $T>>\mu$. Thus,

$n=\left(\dfrac{3}{4}\right)\dfrac{\zeta (3)}{\pi^2}gT^3$

$\rho=\left(\dfrac{7}{8}\right)\dfrac{\pi^2}{30}gT^4$

$p=\dfrac{1}{3}\rho$

2nd. Non-relativistic matter with $m>>T$ only. Then,

$n=g\left(\dfrac{mT}{2\pi}\right)^{3/2}e^{-(m-\mu)/T}$

$\rho= mn+\dfrac{3}{2}p$, and $p=nT<<\rho$

The total energy density is a very important quantity. In the thermal equilibrium, the energy density of non-relativistic species is exponentially smaller (suppressed) than that of the relativistic particles! In fact,

$\rho_R=\dfrac{\pi^2}{30}g_\star T^4$ for radiation with $p_R=\dfrac{1}{3}\rho_R$

and the effective degrees of freedom are

$\displaystyle{\boxed{g_\star=\sum_{bosons}g_b+\dfrac{7}{8}\sum_{fermions}g_f}}$

Remark: The factor $7/8$ in the DOF and the variables above is due to the relation between the Bose-Einstein and the Fermi-Dirac integral in d=3 space dimensions. In general d, the factor would be

$(1-\dfrac{1}{2^d})=\dfrac{2^d-1}{2^d}$

Entropy conservation and the early Universe

The entropy in a comoving volume IS a conserved quantity IN THE THERMAL EQUILIBRIUM. Therefore, we have that

$\dfrac{\partial p_i}{\partial T}=g_i\int \dfrac{d^3p}{(2\pi)^3}\dfrac{df}{dT}\dfrac{p^2}{3E(p)}=g_i\int \dfrac{4\pi pE dE}{(2\pi)^3}\dfrac{df}{dE}\left(-\dfrac{E}{T}\right)\dfrac{p^2}{3E}$

and then

$\dfrac{\partial p_i}{\partial T}=\dfrac{g_i}{2\pi^2}\int \left(-\dfrac{d}{dE}\left(f\dfrac{p^3E}{3T}\right)+f\dfrac{d}{dE}\left(\dfrac{p^3E}{3T}\right)\right)dE$

or

$\dfrac{\partial p_i}{\partial T}=\dfrac{1}{T}(\rho_i+p_i)$

Now, since

$\dfrac{\partial \rho}{\partial t}+3\dfrac{\dot{a}}{a}(\rho+p)=0$

then

$\dfrac{\partial}{\partial t}\left(a^3(\rho+p)\right)-a^3\dfrac{\partial p}{\partial t}=0$

$\dfrac{1}{a^3}\dfrac{\partial (a^3(\rho +p))}{\partial t}-\dfrac{\partial \rho}{\partial t}=0$

if we multiply by $T$ and use the chain rule for $\rho$, we obtain

$\dfrac{1}{a^3}\dfrac{\partial}{\partial t}\left(\dfrac{a^3(\rho+p)}{T}\right)=0$

but it means that $a^3s=\mbox{constant}$, where $s$ is the entropy density defined by

$\boxed{s\equiv \dfrac{\rho+p}{T}}$

Well, the fact is that we know that the entropy or more precisely the entropy density is the early Universe is dominated by relativistic particles ( this is “common knowledge” in the Stantard Cosmological Model, also called $\Lambda CDM$). Thus,

$\boxed{s=\dfrac{2\pi^2}{45}g_\star T^3}$

It implies the evolution of temperature with the redshift in the following way:

$T\propto g_\star^{-1/3}a^{-1}$

Indeed, since we have that $n\propto a^{-3}$, $s\propto a^{-3}$, the yield variable

$Y_i\equiv \dfrac{n_i}{s}$

is a convenient quantity that represents the “abundance” of decoupled particles.

See you in my next cosmological post!

# LOG#074. Dual units.

The last of my three posts tonight is a mysterious dual system of units developed my rival blog (in Spanish language mainly):

His author launched an interesting but speculative dual system of units in which every physical quantity seems to have a lower and maximal bound. I have never seen such an idea published before, so I translated to English the table he posted here in Spanish language

Is duality the principle/symmetry  behind this table? I don’t know for sure, but I came to some similar ideas in my own thoughts about “enhanced relativities”, so I found this table as mysterious as the $\kappa_0$ constant from Pavšič units.

What do you think?

See you soon in a new blog post!

# LOG#073. The G2 system.

The second paper I am going to discuss today is this one:

http://inspirehep.net/record/844954?ln=en

In Note on the natural system of units, Sudarshan, Boya and Rivera introduce a new kind of “fundamental system of units”, that we could call G2 system  or the Boya-Rivera-Sudarshan system (BRS system for short). After a summary of the Gamov-Ivanenko-Landau-Okun cube (GILO cube) and the Planck natural units, they make the following question:

Can we change the gravitational constant $G_N$ for something else?

They ask this question due to the fact the $G_N$ seems to be a little different from $h, c$. Indeed, many researchers in quantum gravity use to change $G_N$ with the Planck length as fundamental unit! The G2 system proposal is based in some kind of twodimensional world. Sudarshan, Boya and Rivera search for a “new constant” $G_2$ such as $G_2/r$ substitutes $G_N/r^2$ in the Newton’s gravitational law. $\left[G_2\right]=L$ in this new “partial” fundamental system. Therefore, we have

$F_N=G_2Mm/r$

and the physical dimensions of time, length and mass are expressed in terms of $G_2$ as follows (we could use $\hbar$ instead of h, that is not essential here as we do know from previous discussions) :

$T=c^{-4}hG_2$

$L=c^{-3}hG_2$

$M=c^2/G_2$

In fact, they remark that since $G_2$ derives from a 2+1 dimensional world and Einstein Field equations are generally “trivial” in 2+1 spacetime, $G_2$, surprisingly, is not related to gravitation at all! We are almost “free” to fix $G_2$ with some alternative procedure. As we wish to base the G2 system in well known physics, the election they do for $G_2$ is the trivial one ( however I am yet thinking about what we could obtain with some non-trivial alternative definition of $lates G_2$):

$\boxed{G_2=\dfrac{c^2}{M_P}=G_N/L_P \approx 4.1\cdot 10^{24}MKS=4.1\cdot 10^{25}CGS}$

and any other equivalent expression to it. Please, note that if we fix the Planck length to unit, we get $G_N=G_2$, so it is equivalent to speak about $G_2$ or $G_N$ in a system of units where Planck length is set to the unit. However, the proposal is independent of this fact, since, as we said above, we could choose some other non-trivial definition for $G_2$, although I don’t know what kind of guide we could follow in those alternative and non-trivial definition.

The final remark I would like to make here is that, whatever we choose instead of $G_N$, it is ESSENTIAL to a quantum theory of gravity, provided it exists, it works and it is “clear” from its foundational principles.

See you in my next blog post!

# LOG#071. Pavšič Units.

Following the thread I began in the previous blog post, today we are going to learn about a non common but very general and universal system of natural units. They were officiallly introduced without name in the literature, until a physicist realized its importance, to my knowledge, by first time.

In his book, The Landscape of Theoretical Physics: A Global View, available here and here for free,  the slovenian physicist Matej Pavšič introduced a dilationally invariant system of units, where “all” the fundamental “constants” are set to the unit. More precisely, he defines the D-system or  Pavšič units M system characterized by

$\boxed{G_N=\hbar=c=k_B=k_C^{-1}=1}$

and where the Coulomb constant $k_C=1/4\pi\varepsilon_0$ so $k_C^{-1}=4\pi\varepsilon_0=1$. This sistem of units, let me call it M (for Matej) or D-system (we will see later the meaning of D), is very useful but it has not been discussed in the literature deeper, unlike the case of natural HEP units, Stoney units or the Planck units.

We have seen in the previous post that addapted system of units do exist where the physical quantities lack any spurious conversion constant (like c or any power or it, the Planck constant and any power of it, and so on). Indeed, we could say that these “fundamental” constants are yet there, but they are “invisible” due to the election of some clever system of units ( the so-called natural units).

The M system of units or D-system ( following M. Pavšič) defines a dilationally invariant system of units, and it is the fact that this system has some special behaviour under “dilatations” why it receives such a name. In the MKSA system, the quantities of this system are written as follows:

We can compare MKSA units and D-system units. Firstly, in MKSA, we have:

$L_P=\dfrac{\hbar}{M_Pc}$

However, in D units, we also have:

$L_P=\dfrac{1}{E_P}=\dfrac{1}{M_P}$

Of course, this result is the same than the Planck system of units. In addition to these equations, we can also define:

$M=M_P\sqrt{\alpha}$ and $L=\sqrt{\alpha}L$

Furthermore, we can also set the electric charge to “mass” as follows (in the D-system):

$e=M=L=\sqrt{\alpha}$

Length is defined as the radius a classical particle with mass M and a charge elementary e would have. In MKSA, we easily derive:

$L=\dfrac{e^2}{4\pi\varepsilon_0Mc^2}$

By the other hand, in terms of units D, it yields

$L_D=\dfrac{e^2}{M}$

Thus, length seems to be the ratio between “squared charge” and “mass” somehow from D units, i.e., if we further impose that $L_D=1$, we would “deduce” that $M=e^2$ or more generally the relationship $M=Q^2$ (recall the striking convergence to the charge-mass equivalence from these dimensional arguments as it does also happen in black hole physics!). The classical radius of the electron is, as every physicist knows:

$r_c=\dfrac{e^2}{4\pi\varepsilon_0m_ec^2}$

in MKSA units, but it is

$r_{c,D}=\dfrac{e^2}{m_e}$

in D units. Taking the ratio $r_c/L$, it yields:

$\dfrac{r_c}{L}=\dfrac{M}{m_e}=\dfrac{e}{m_e}\sqrt{\dfrac{1}{4\pi\varepsilon_0G_N}}\equiv \dfrac{1}{\kappa_0}=\kappa_0^{-1}$

and thus, we can write in D units

$\boxed{m_e=\kappa_0M=\kappa_0\sqrt{\alpha}=\kappa_0e}$

This equation says that mass is equivalent to charge up to a tiny constant, denoted by $\kappa_0$. This dimensionless constant is very mysterious and it has not been discussed in the literature as far as I know excepting in that M. Pavšič book! Its reciprocal value is also a curious number:

$\boxed{\kappa_0^{-1}=2.042\cdot 10^{21}}$

This number is roughly 1/295 times the Avogrado constant $N_A=6.022\cdot 10^{23}$. Matej  Pavšič also mentions that the ratio $r_c/L$ or its reciprocal represents the “scale of length” of the classical electron radius relative to the length L, and we have

$r_c\kappa_0=e$

The next table provides the conversion factors between D-system units and MKSA units:

Furthermore, the D units $\hbar, c,G_N$ and also $k_C=1/4\pi\varepsilon_0$, plus the conventional and commonly known as $M_P, T_P, L_P, L, M, e$ are also invariant under dilatations! It is quite a claim, so let me review the arguments that  Pavšič gives in his book. If we make a dilatation transformation of position, linear momentum, mass, force and acceleration (after a  “scale transformation”), we write:

$X^{\mu'}=\rho X^\mu$

$P^{\mu'}=\rho^{-1}P^\mu$

$M'=\rho^{-1}M$

$F^{\mu'}=\rho^{-2}F^\mu$

$a^{\mu'}=\rho^{-1}a^\mu$

Instead of the inhomogeneous coordinates $X^\mu$ one can introduce the homogeneous coordinates $\overline{X}^\mu=\kappa X^\mu$ where $\overline{X}^\mu=\kappa X^\mu$  are invariant under dilatations provided that the quantity κ transforms as follows:

$\kappa'=\kappa\rho^{-1}$

For instance, if initially $X^0=1s$, then after applying a dilatation, say by the factor ρ = 3, we have $X^0 =3s$. Setting $\rho=\kappa^{-1}=3$ fixes the scale transformation relative to some homogeneous coordinate.

If we write a given equation we can check its consistency by comparing the dimension of its left hand and right hand side. In the MKSA system the dimensional control is in checking the powers of meters, kilograms, seconds and ampères on both sides of the equation. Moreover, even the Einstein Field Equations of gravity

$G_{\mu\nu}=8\pi G_NT_{\mu\nu}$

can be expressed in units where $\kappa$ is “visible” as

$G_{\mu\nu}=8\pi G_N\kappa^{-2}T_{\mu\nu}$

If we choose κ = 1 then the equations for this particular choice correspond to the usual Einstein Field  Equations, and we may use either the MKSA system or the D system.

In the D-system, some equations simplify further, even more than in Planck units! Let’s see some interesting uncommon equations in this new system of units:

The final part of this nice post corresponds to a nice exercise included in the  Pavšič book. Suppose you are given an equation in the D system, e.g.,

$a_0=\dfrac{1}{m_ee^2}=\kappa_0^{-1}e^{-3}=\kappa_0^{-1}\sqrt{\alpha^{-3}}=\kappa_0^{-1}\alpha^{-3/2}D$

How can we turn it to usual MKSA units? Using conversion constants, we can rewrite the previous equation in the next way:

$\boxed{a_0=\dfrac{e}{m_e\sqrt{4\pi\varepsilon_0G_N}}\left(\dfrac{e^2}{4\pi\varepsilon_0\hbar c}\right)^{-3/2}D}$

If we put 1D = 1 then the right hand side of the latter equation is a pure dimensionless quantity. If we wish to obtain a quantity of the dimension of length we have to insert the factor:

$1D=L_P=\sqrt{\dfrac{\hbar G_N}{c^3}}$

and we finally obtain the Böhr radius:

$a_0=\dfrac{4\pi\varepsilon_0\hbar^2}{m_ee^2}=\dfrac{\hbar}{m_ec\alpha}=\dfrac{M_PL_P}{m_e\alpha}$

Instead of rewriting equations from the D system in the MKSA system, we can keep all the equations in the D system and perform all the algebraic and numerical calculations in the D units. If we wish to know the numerical results in terms of the MKSA units, we can use the numbers of Table 2 above. For instance, in the previous example, going backwards, we have

$a_0=\kappa_0^{-1}e^{-3}=2.04136\cdot 10^{21}\cdot 137.03604^{3/2}D$

Taking D units of length, from Table 1, we read off that $1D=1.615989\cdot 10^{-35}m=L_P$ and we obtain the classical result

$a_0=0.529177\cdot 10^{-10}m$

Let me quote M. Pavšič:

“(…)However, I do not propose to replace the international MKSA system with the D system. I only wish to recall that most modern theoretical works do not use the MKSA system and that it is often very tedious to obtain the results in meters, seconds, kilograms and ampères. (…)besides the Planck length, Planck time and the Planck mass, which are composed of the fundamental constants, $\hbar, c, G_N$, we have also introduced (see Table 2) the corresponding electromagnetic quantity, namely the charge

$Q_P=\sqrt{4\pi\varepsilon_0\hbar c}$

(or, equivalently, the current and the potential difference), by bringing into play the fundamental constant $\varepsilon_0$. We have then extended the Planck system of units in which $c=G_N=\hbar$ to a more complete system where $G_N=\hbar=c=4\pi\varepsilon_0=1$ in order to incorporate all known sorts of physical quantities(…).”

Remarkably, at the end, M. Pavšič points out a classical paper by J.M. Levy-Leblond, see here:http://inspirehep.net/record/128108 , in which, Matej says, is clearly stated that our progress in understanding the unity of nature follows the direction of eliminating from theories various (inessential) numerical constants with the improper name of “fundamental” constants. In fact, those constants are merely the constants which result from our unnatural choice of units, the choice due to our incomplete understanding of the unified theory behind.

Matej Pavšič has become a blogger recently (you can find his site in my links on the right or here) and he has also a nice webpage about his research and lots of interesting topics. He has also studied tachyons, extended objects, geometric algebra, Clifford spaces (C-spaces), brane worlds, relativistic dynamics with an invariant evolution parameter (the so-called Stuckelberg formalism), Kaluza-Klein theories and many other interesting topics. With Carlos Castro Perelman, he is one of the proponents of the theory of relativity in Clifford spaces (C-spaces), an extension of special relativity/general relativity/QFT including grassmann valued quantities and arbitrary signatures on equal footing, but living NOT in spacetime but in some kind of nontrivial and purely algebraic space. You can visit his page and have a look to his works here.

Have you liked this post? Let me know. And be right back for my next communication! Physics is cool, Physmatics is cool and Mathematics is an incredible language for Physmatics!

# LOG#070. Natural Units.

Happy New Year 2013 to everyone and everywhere!

Let me apologize, first of all, by my absence… I have been busy, trying to find my path and way in my field, and I am busy yet, but finally I could not resist without a new blog boost… After all, you should know the fact I have enough materials to write many new things.

So, what’s next? I will dedicate some blog posts to discuss a nice topic I began before, talking about a classic paper on the subject here:

https://thespectrumofriemannium.wordpress.com/2012/11/18/log054-barrow-units/

The topic is going to be pretty simple: natural units in Physics.

First of all, let me point out that the election of any system of units is, a priori, totally conventional. You are free to choose any kind of units for physical magnitudes. Of course, that is not very clever if you have to report data, so everyone can realize what you do and report. Scientists have some definitions and popular systems of units that make the process pretty simpler than in the daily life. Then, we need some general conventions about “units”. Indeed, the traditional wisdom is to use the international system of units, or S (Iabbreviated SI from French language: Le Système international d’unités). There, you can find seven fundamental magnitudes and seven fundamental (or “natural”) units:

1) Space: $\left[ L\right]=\mbox{meter}=m$

2) Time: $\left[ T\right]=\mbox{second}=s$

3) Mass: $\left[ M\right]=\mbox{kilogram}=kg$

4) Temperature: $\left[ t\right]=\mbox{Kelvin degree}= K$

5) Electric intensity: $\left[ I\right]=\mbox{ampere}=A$

6) Luminous intensity: $\left[ I_L\right]=\mbox{candela}=cd$

7) Amount of substance: $\left[ n\right]=\mbox{mole}=mol(e)$

The dependence between these 7 great units and even their definitions can be found here http://en.wikipedia.org/wiki/International_System_of_Units and references therein. I can not resist to show you the beautiful graph of the 7 wonderful units that this wikipedia article shows you about their “interdependence”:

In Physics, when you build a radical new theory, generally it has the power to introduce a relevant scale or system of units. Specially, the Special Theory of Relativity, and the Quantum Mechanics are such theories. General Relativity and Statistical Physics (Statistical Mechanics) have also intrinsic “universal constants”, or, likely, to be more precise, they allow the introduction of some “more convenient” system of units than those you have ever heard ( metric system, SI, MKS, cgs, …). When I spoke about Barrow units (see previous comment above) in this blog, we realized that dimensionality (both mathematical and “physical”), and fundamental theories are bound to the election of some “simpler” units. Those “simpler” units are what we usually call “natural units”. I am not a big fan of such terminology. It is confusing a little bit. Maybe, it would be more interesting and appropiate to call them “addapted X units” or “scaled X units”, where X denotes “relativistic, quantum,…”. Anyway, the name “natural” is popular and it is likely impossible to change the habits.

In fact, we have to distinguish several “kinds” of natural units. First of all, let me list “fundamental and universal” constants in different theories accepted at current time:

1. Boltzmann constant: $k_B$.

Essential in Statistical Mechanics, both classical and quantum. It measures “entropy”/”information”. The fundamental equation is:

$\boxed{S=k_B\ln \Omega}$

It provides a link between the microphysics and the macrophysics ( it is the code behind the equation above). It can be understood somehow as a measure of the “energetic content” of an individual particle or state at a given temperature. Common values for this constant are:

$k_B=1.3806488(13)\times 10^{-23}J/K = 8.6173324(78)\times 10^{-5}eV/K$

$k_B=1.3806488(13)\times 10^{-16}erg/K$

Statistical Physics states that there is a minimum unit of entropy or a minimal value of energy at any given temperature. Physical dimensions of this constant are thus entropy, or since $E=TS$, $\left[ k_B\right] =E/t=J/K$, where t denotes here dimension of temperature.

2. Speed of light.  $c$.

From classical electromagnetism:

$\boxed{c^2=\dfrac{1}{\sqrt{\varepsilon_0\mu_0}}}$

The speed of light, according to the postulates of special relativity, is a universal constant. It is frame INDEPENDENT. This fact is at the root of many of the surprising results of special relativity, and it took time to be understood. Moreover, it also connects space and time in a powerful unified formalism, so space and time merge into spacetime, as we do know and we have studied long ago in this blog. The spacetime interval in a D=3+1 dimensional space and two arbitrary events reads:

$\Delta s^2=\Delta x^2+\Delta y^2+\Delta z^2-c^2\Delta t^2$

In fact, you can observe that “c” is the conversion factor between time-like and space-like coordinates.  How big the speed of light is? Well, it is a relatively large number from our common and ordinary perception. It is exactly:

$\boxed{c=299,792,458m/s}$

although you often take it as $c\approx 3\cdot 10^{8}m/s=3\cdot 10^{10}cm/s$.  However, it is the speed of electromagnetic waves in vacuum, no matter where you are in this Universe/Polyverse. At least, experiments are consistent with such an statement. Moreover, it shows that $c$ is also the conversion factor between energy and momentum, since

$\mathbf{P}^2c^2-E^2=-m^2c^4$

and $c^2$ is the conversion factor between rest mass and pure energy, because, as everybody knows,  $E=mc^2$! According to the special theory of relativity, normal matter can never exceed the speed of light. Therefore, the speed of light is the maximum velocity in Nature, at least if specially relativity holds. Physical dimensions of c are $\left[c\right]=LT^{-1}$, where L denotes length dimension and T denotes time dimension (please, don’t confuse it with temperature despite the capital same letter for both symbols).

3. Planck’s constant. $h$ or generally rationalized $\hbar=h/2\pi$.

Planck’s constant (or its rationalized version), is the fundamental universal constant in Quantum Physics (Quantum Mechanics, Quantum Field Theory). It gives

$\boxed{E=h\nu=\hbar \omega}$

Indeed, quanta are the minimal units of energy. That is, you can not divide further a quantum of light, since it is indivisible by definition! Furthermore, the de Broglie relationship relates momentum and wavelength for any particle, and it emerges from the combination of special relativity and the quantum hypothesis:

$\lambda=\dfrac{h}{p}\leftrightarrow \bar{\lambda}=\dfrac{\hbar}{p}$

In the case of massive particles, it yields

$\lambda=\dfrac{h}{Mv}\leftrightarrow \bar{\lambda}=\dfrac{\hbar}{Mv}$

In the case of massless particles (photons, gluons, gravitons,…)

$\lambda=\dfrac{hc}{E}$ or $\bar{\lambda}=\dfrac{\hbar c}{E}$

Planck’s constant also appears to be essential to the uncertainty principle of Heisenberg:

$\boxed{\Delta x \Delta p\geq \hbar/2}$

$\boxed{\Delta E \Delta t\geq \hbar/2}$

$\boxed{\Delta A\Delta B\geq \hbar/2}$

Some particularly important values of this constant are:

$h=6.62606957(29)\times 10^{-34} J\cdot s$
$h=4.135667516(91)\times 10^{-15}eV\cdot s$
$h=6.62606957(29)\times 10^{-27} erg\cdot s$
$\hbar =1.054571726(47)\times 10^{-34} J\cdot s$
$\hbar =6.58211928(15)\times 10^{-16} eV\cdot s$
$\hbar= 1.054571726(47)\times 10^{-27}erg\cdot s$

It is also useful to know that
$hc=1.98644568\times 10^{-25}J\cdot m$
$hc=1.23984193 eV\cdot \mu m$

or

$\hbar c=0.1591549hc$ or $\hbar c=197.327 eV\cdot nm$

Planck constant has dimension of $\mbox{Energy}\times \mbox{Time}=\mbox{position}\times \mbox{momentum}=ML^2T^{-1}$. Physical dimensions of this constant coincide also with angular momentum (spin), i.e., with $L=mvr$.

4. Gravitational constant. $G_N$.

Apparently, it is not like the others but it can also define some particular scale when combined with Special Relativity. Without entering into further details (since I have not discussed General Relativity yet in this blog), we can calculate the escape velocity of a body moving at the speed of light

$\dfrac{1}{2}mv^2-G_N\dfrac{Mm}{R}=0$ with $v=c$ implies a new length scale where gravitational relativistic effects do appear, the so-called Schwarzschild radius $R_S$:

$\boxed{R_S=\dfrac{2G_NM}{c^2}=\dfrac{2G_NM_{\odot}}{c^2}\left(\dfrac{M}{M_{\odot}}\right)\approx 2.95\left(\dfrac{M}{M_{\odot}}\right)km}$

5. Electric fundamental charge. $e$.

It is generally chosen as fundamental charge the electric charge of the positron (positive charged “electron”). Its value is:

$e=1.602176565(35)\times 10^{-19}C$

where C denotes Coulomb. Of course, if you know about quarks with a fraction of this charge, you could ask why we prefer this one. Really, it is only a question of hystory of Science, since electrons were discovered first (and positrons). Quarks, with one third or two thirds of this amount of elementary charge, were discovered later, but you could define the fundamental unit of charge as multiple or entire fraction of this charge. Moreover, as far as we know, electrons are “elementary”/”fundamental” entities, so, we can use this charge as unit and we can define quark charges in terms of it too. Electric charge is not a fundamental unit in the SI system of units. Charge flow, or electric current, is.

An amazing property of the above 5 constants is that they are “universal”. And, for instance, energy is related with other magnitudes in theories where the above constants are present in a really wonderful and unified manner:

$\boxed{E=N\dfrac{k_BT}{2}=Mc^2=TS=Pc=N\dfrac{h\nu}{2}=N\dfrac{\hbar \omega}{2}=\dfrac{R_Sc^4}{2G_N}=\hbar c k=\dfrac{hc}{\lambda}}$

Caution: k is not the Boltzmann constant but the wave number.

There is a sixth “fundamental” constant related to electromagnetism, but it is also related to the speed of light, the electric charge and the Planck’s constant in a very sutble way. Let me introduce you it too…

6. Coulomb constant. $k_C$.

This is a second constant related to classical electromagnetism, like the speed of light in vacuum. Coulomb’s constant, the electric force constant, or the electrostatic constant (denoted $k_C$) is a proportionality factor that takes part in equations relating electric force between  point charges, and indirectly it also appears (depending on your system of units) in expressions for electric fields of charge distributions. Coulomb’s law reads

$F_C=k_C\dfrac{Qq}{r^2}$

Its experimental value is

$k_C=\dfrac{1}{4\pi \varepsilon_0}=\dfrac{c^2\mu_0}{4\pi}=c^2\cdot 10^{-7}H\cdot m^{-1}= 8.9875517873681764\cdot 10^9 Nm^2/C^2$

Generally, the Coulomb constant is dropped out and it is usually preferred to express everything using the electric permitivity of vacuum $\varepsilon_0$ and/or numerical factors depending on the pi number $\pi$ if you choose the gaussian system of units  (read this wikipedia article http://en.wikipedia.org/wiki/Gaussian_system_of_units ), the CGS system, or some hybrid units based on them.

## H.E.P. units

High Energy Physicists use to employ units in which the velocity is measured in fractions of the speed of light in vacuum, and the action/angular momentum is some multiple of the Planck’s constant. These conditions are equivalent to set

$\boxed{c=1_c=1}$ $\boxed{\hbar=1_\hbar=1}$

Complementarily, or not, depending on your tastes and preferences, you can also set the Boltzmann’s constant to the unit as well

$k_B=1_{k_B}=1$

and thus the complete HEP system is defined if you set

$\boxed{c=\hbar=k_B=1}$

This “natural” system of units is lacking yet a scale of energy. Then, it is generally added the electron-volt $eV$ as auxiliary quantity defining the reference energy scale. Despite the fact that this is not a “natural unit” in the proper sense because it is defined by a natural property, the electric charge,  and the anthropogenic unit of electric potential, the volt. The SI prefixes multiples of eV are used as well: keV, MeV, GeV, etc. Here, the eV is used as reference energy quantity, and with the above election of “elementary/natural units” (or any other auxiliary unit of energy), any quantity can be expressed. For example, a distance of 1 m can be expressed in terms of eV, in natural units, as

$1m=\dfrac{1m}{\hbar c}\approx 510eV^{-1}$

This system of units have remarkable conversion factors

A) $1 eV^{-1}$ of length is equal to $1.97\cdot 10^{-7}m =(1\text{eV}^{-1})\hbar c$

B) $1 eV$ of mass is equal to $1.78\cdot 10^{-36}kg=1\times \dfrac{eV}{c^2}$

C) $1 eV^{-1}$ of time is equal to $6.58\cdot 10^{-16}s=(1\text{eV}^{-1})\hbar$

D) $1 eV$ of temperature is equal to $1.16\cdot 10^4K=1eV/k_B$

E) $1 unit$ of electric charge in the Lorentz-Heaviside system of units is equal to $5.29\cdot 10^{-19}C=e/\sqrt{4\pi\alpha}$

F) $1 unit$ of electric charge in the Gaussian system of units is equal to $1.88\cdot 10^{-18}C=e/\sqrt{\alpha}$

This system of units, therefore, leaves free only the energy scale (generally it is chosen the electron-volt) and the electric measure of fundamentl charge. Every other unit can be related to energy/charge. It is truly remarkable than doing this (turning invisible the above three constants) you can “unify” different magnitudes due to the fact these conventions make them equivalent. For instance, with natural units:

1) Length=Time=1/Energy=1/Mass.

It is due to $x=ct$, $E=Mc^2$ and $E=hc/\lambda$ equations. Setting $c$ and $h$ or $\hbar$ provides

$x=t$, $E=M$ and $E=1/\lambda$.

Note that natural units turn invisible the units we set to the unit! That is the key of the procedure. It simplifies equations and expressions. Of course, you must be careful when you reintroduce constants!

2) Energy=Mass=Momemntum=Temperature.

It is due to $E=k_BT$, $E=Pc$ and $E=Mc^2$ again.

One extra bonus for theoretical physicists is that natural units allow to build and write proper lagrangians and hamiltonians (certain mathematical operators containing the dynamics of the system enconded in them), or equivalently the action functional, with only the energy or “mass” dimension as “free parameter”. Let me show how it works.

Natural units in HEP identify length and time dimensions. Thus $\left[L\right]=\left[T\right]$. Planck’s constant allows us to identify those 2 dimensions with 1/Energy (reciprocals of energy) physical dimensions. Therefore, in HEP units, we have

$\boxed{\left[L\right]=\left[T\right]=\left[E\right]^{-1}}$

The speed of light identifies energy and mass, and thus, we can often heard about “mass-dimension” of a lagrangian in the following sense. HEP units can be thought as defining “everything” in terms of energy, from the pure dimensional ground. That is, every physical dimension is (in HEP units) defined by a power of energy:

$\boxed{\left[E\right]^n}$

Thus, we can refer to any magnitude simply saying the power of such physical dimension (or you can think logarithmically to understand it easier if you wish). With this convention, and recalling that energy dimension is mass dimension, we have that

$\left[L\right]=\left[T\right]=-1$ and $\left[E\right]=\left[M\right]=1$

Using these arguments, the action functional is a pure dimensionless quantity, and thus, in D=4 spacetime dimensions, lagrangian densities must have dimension 4 ( or dimension D is a general spacetime).

$\displaystyle{S=\int d^4x \mathcal{L}\rightarrow \left[\mathcal{L}\right]=4}$

$\displaystyle{S=\int d^Dx \mathcal{L}\rightarrow \left[\mathcal{L}\right]=D}$

In D=4 spacetime dimensions, it can be easily showed that

$\left[\partial_\mu\right]=\left[\Phi\right]=\left[A^\mu\right]=1$

$\left[\Psi_D\right]=\left[\Psi_M\right]=\left[\chi\right]=\left[\eta\right]=\dfrac{3}{2}$

where $\Phi$ is a scalar field, $A^\mu$ is a vector field (like the electromagnetic or non-abelian vector gauge fields), and $\Psi_D, \Psi_M, \chi, \eta$ are a Dirac spinor, a Majorana spinor, and $\chi, \eta$ are Weyl spinors (of different chiralities). Supersymmetry (or SUSY) allows for anticommuting c-numbers (or Grassmann numbers) and it forces to introduce auxiliary parameters with mass dimension $-1/2$. They are the so-called SUSY transformation parameters $\zeta_{SUSY}=\epsilon$. There are some speculative spinors called ELKO fields that could be non-standandard spinor fields with mass dimension one! But it is an advanced topic I am not going to discuss here today. In general D spacetime dimensions a scalar (or vector) field would have mass dimension $(D-2)/2$, and a spinor/fermionic field in D dimensions has generally $(D-1)/2$ mass dimension (excepting the auxiliary SUSY grassmanian fields and the exotic idea of ELKO fields).  This dimensional analysis is very useful when theoretical physicists build up interacting lagrangians, since we can guess the structure of interaction looking at purely dimensional arguments every possible operator entering into the action/lagrangian density! In summary, therefore, for any D:

$\boxed{\left[\Phi\right]=\left[A_\mu\right]=\dfrac{D-2}{2}\equiv E^{\frac{D-2}{2}}=M^{\frac{D-2}{2}}}$

$\boxed{\left[\Psi\right]=\dfrac{D-1}{2}\equiv E^{\frac{D-1}{2}}=M^{\frac{D-1}{2}}}$

Remark (for QFT experts only): Don’t confuse mass dimension with the final transverse polarization degrees or “degrees of freedom” of a particular field, i.e., “components” minus “gauge constraints”. E.g.: a gauge vector field has $D-2$ degrees of freedom in D dimensions. They are different concepts (although both closely related to the spacetime dimension where the field “lives”).

In summary:

i) HEP units are based on QM (Quantum Mechanics), SR (Special Relativity) and Statistical Mechanics (Entropy and Thermodynamics).

ii) HEP units need to introduce a free energy scale, and it generally drives us to use the eV or electron-volt as auxiliary energy scale.

iii) HEP units are useful to dimensional analysis of lagrangians (and hamiltonians) up to “mass dimension”.

## Stoney Units

In Physics, the Stoney units form a alternative set of natural units named after the Irish physicist George Johnstone Stoney, who first introduced them as we know it today in 1881. However, he presented the idea in a lecture entitled “On the Physical Units of Nature” delivered to the British Association before that date, in 1874. They are the first historical example of natural units and “unification scale” somehow. Stoney units are rarely used in modern physics for calculations, but they are of historical interest but some people like Wilczek has written about them (see, e.g., http://arxiv.org/abs/0708.4361). These units of measurement were designed so that certain fundamental physical constants are taken as reference basis without the Planck scale being explicit, quite a remarkable fact! The set of constants that Stoney used as base units is the following:

A) Electric charge, $e=1_e$.

B) Speed of light in vacuum, $c=1_c$.

C) Gravitational constant, $G_N=1_{G_N}$.

D) The Reciprocal of Coulomb constant, $1/k_C=4\pi \varepsilon_0=1_{k_C^{-1}}=1_{4\pi \varepsilon_0}$.

Stony units are built when you set these four constants to the unit, i.e., equivalently, the Stoney System of Units (S) is determined by the assignments:

$\boxed{e=c=G_N=4\pi\varepsilon_0=1}$

Interestingly, in this system of units, the Planck constant is not equal to the unit and it is not “fundamental” (Wilczek remarked this fact here ) but:

$\hbar=\dfrac{1}{\alpha}\approx 137.035999679$

Today, Planck units are more popular Planck than Stoney units in modern physics, and even there are many physicists who don’t know about the Stoney Units! In fact, Stoney was one of the first scientists to understand that electric charge was quantized!; from this quantization he deduced the units that are now named after him.

The Stoney length and the Stoney energy are collectively called the Stoney scale, and they are not far from the Planck length and the Planck energy, the Planck scale. The Stoney scale and the Planck scale are the length and energy scales at which quantum processes and gravity occur together. At these scales, a unified theory of physics is thus likely required. The only notable attempt to construct such a theory from the Stoney scale was that of H. Weyl, who associated a gravitational unit of charge with the Stoney length and who appears to have inspired Dirac’s fascination with the large number hypothesis. Since then, the Stoney scale has been largely neglected in the development of modern physics, although it is occasionally discussed to this day. Wilczek likes to point out that, in Stoney Units, QM would be an emergent phenomenon/theory, since the Planck constant wouldn’t be present directly but as a combination of different constants. By the other hand, the Planck scale is valid for all known interactions, and does not give prominence to the electromagnetic interaction, as the Stoney scale does. That is, in Stoney Units, both gravitation and electromagnetism are on equal footing, unlike the Planck units, where only the speed of light is used and there is no more connections to electromagnetism, at least, in a clean way like the Stoney Units do. Be aware, sometimes, rarely though, Planck units are referred to as Planck-Stoney units.

What are the most interesting Stoney system values? Here you are the most remarkable results:

1) Stoney Length, $L_S$.

$\boxed{L_S=\sqrt{\dfrac{G_Ne^2}{(4\pi\varepsilon)c^4}}\approx 1.38\cdot 10^{-36}m}$

2) Stoney Mass, $M_S$.

$\boxed{M_S=\sqrt{\dfrac{e^2}{G_N(4\pi\varepsilon_0)}}\approx 1.86\cdot 10^{-9}kg}$

3) Stoney Energy, $E_S$.

$\boxed{E_S=M_Sc^2=\sqrt{\dfrac{e^2c^4}{G_N(4\pi\varepsilon_0)}}\approx 1.67\cdot 10^8 J=1.04\cdot 10^{18}GeV}$

4) Stoney Time, $t_S$.

$\boxed{t_S=\sqrt{\dfrac{G_Ne^2}{c^6(4\pi\varepsilon_0)}}\approx 4.61\cdot 10^{-45}s}$

5) Stoney Charge, $Q_S$.

$\boxed{Q_S=e\approx 1.60\cdot 10^{-19}C}$

6) Stoney Temperature, $T_S$.

$\boxed{T_S=E_S/k_B=\sqrt{\dfrac{e^2c^4}{G_Nk_B^2(4\pi\varepsilon_0)}}\approx 1.21\cdot 10^{31}K}$

## Planck Units

The reference constants to this natural system of units (generally denoted by P) are the following 4 constants:

1) Gravitational constant. $G_N$

2) Speed of light. $c$.

3) Planck constant or rationalized Planck constant. $\hbar$.

4) Boltzmann constant. $k_B$.

The Planck units are got when you set these 4 constants to the unit, i.e.,

$\boxed{G_N=c=\hbar=k_B=1}$

It is often said that Planck units are a system of natural units that is not defined in terms of properties of any prototype, physical object, or even features of any fundamental particle. They only refer to the basic structure of the laws of physics: c and G are part of the structure of classical spacetime in the relativistic theory of gravitation, also known as general relativity, and ℏ captures the relationship between energy and frequency which is at the foundation of elementary quantum mechanics. This is the reason why Planck units particularly useful and common in theories of quantum gravity, including string theory or loop quantum gravity.

This system defines some limit magnitudes, as follows:

1) Planck Length, $L_P$.

$\boxed{L_P=\sqrt{\dfrac{G_N\hbar}{c^3}}\approx 1.616\cdot 10^{-35}s}$

2) Planck Time, $t_P$.

$\boxed{t_P=L_P/c=\sqrt{\dfrac{G_N\hbar}{c^5}}\approx 5.391\cdot 10^{-44}s}$

3) Planck Mass, $M_P$.

$\boxed{M_P=\sqrt{\dfrac{\hbar c}{G_N}}\approx 2.176\cdot 10^{-8}kg}$

4) Planck Energy, $E_P$.

$\boxed{E_P=M_Pc^2=\sqrt{\dfrac{\hbar c^5}{G_N}}\approx 1.96\cdot 10^9J=1.22\cdot 10^{19}GeV}$

5) Planck charge, $Q_P$.

In Lorentz-Heaviside electromagnetic units

$\boxed{Q_P=\sqrt{\hbar c \varepsilon_0}=\dfrac{e}{\sqrt{4\pi\alpha}}\approx 5.291\cdot 10^{-19}C}$

In Gaussian electromagnetic units

$\boxed{Q_P=\sqrt{\hbar c (4\pi\varepsilon_0)}=\dfrac{e}{\sqrt{\alpha}}\approx 1.876\cdot 10^{-18}C}$

6) Planck temperature, $T_P$.

$\boxed{T_P=E_P/k_B=\sqrt{\dfrac{\hbar c^5}{G_Nk_B^2}}\approx 1.417\cdot 10^{32}K}$

From these “fundamental” magnitudes we can build many derived quantities in the Planck System:

1) Planck area.

$A_P=L_P^2=\dfrac{\hbar G_N}{c^3}\approx 2.612\cdot 10^{-70}m^2$

2) Planck volume.

$V_P=L_P^3=\left(\dfrac{\hbar G_N}{c^3}\right)^{3/2}\approx 4.22\cdot 10^{-105}m^3$

3) Planck momentum.

$P_P=M_Pc=\sqrt{\dfrac{\hbar c^3}{G_N}}\approx 6.52485 kgm/s$

A relatively “small” momentum!

4) Planck force.

$F_P=E_P/L_P=\dfrac{c^4}{G_N }\approx 1.21\cdot 10^{44}N$

It is independent from Planck constant! Moreover, the Planck acceleration is

$a_P=F_P/M_P=\sqrt{\dfrac{c^7}{G_N\hbar}}\approx 5.561\cdot 10^{51}m/s^2$

5) Planck Power.

$\mathcal{P}_P=\dfrac{c^5}{G_N}\approx 3.628\cdot 10^{52}W$

6) Planck density.

$\rho_P=\dfrac{c^5}{\hbar G_N^2}\approx 5.155\cdot 10^{96}kg/m^3$

Planck density energy would be equal to

$\rho_P c^2=\dfrac{c^7}{\hbar G_N^2}\approx 4.6331\cdot 10^{113}J/m^3$

7) Planck angular frequency.

$\omega_P=\sqrt{\dfrac{c^5}{\hbar G_N}}\approx 1.85487\cdot 10^{43}Hz$

8) Planck pressure.

$p_P=\dfrac{F_P}{A_P}=\dfrac{c^7}{G_N^2\hbar}=\rho_P c^2\approx 4.6331\cdot 10^{113}Pa$

Note that Planck pressure IS the Planck density energy!

9) Planck current.

$I_P=Q_P/t_P=\sqrt{\dfrac{4\pi\varepsilon_0 c^6}{G_N}}\approx 3.4789\cdot 10^{25}A$

10) Planck voltage.

$v_P=E_P/Q_P=\sqrt{\dfrac{c^4}{4\pi\varepsilon_0 G_N}}\approx 1.04295\cdot 10^{27}V$

11) Planck impedance.

$Z_P=v_P/I_P=\dfrac{\hbar^2}{Q_P}=\dfrac{1}{4\pi \varepsilon_0 c}\approx 29.979\Omega$

A relatively small impedance!

12) Planck capacitor.

$C_P=Q_P/v_P=4\pi\varepsilon_0\sqrt{\dfrac{\hbar G_N}{ c^3}} \approx 1.798\cdot 10^{-45}F$

Interestingly, it depends on the gravitational constant!

Some Planck units are suitable for measuring quantities that are familiar from daily experience. In particular:

1 Planck mass is about 22 micrograms.

1 Planck momentum is about 6.5 kg m/s

1 Planck energy is about 500kWh.

1 Planck charge is about 11 elementary (electronic) charges.

1 Planck impendance is almost 30 ohms.

Moreover:

i) A speed of 1 Planck length per Planck time is the speed of light, the maximum possible speed in special relativity.

ii) To understand the Planck Era and “before” (if it has sense), supposing QM holds yet there, we need a quantum theory of gravity to be available there. There is no such a theory though, right now. Therefore, we have to wait if these ideas are right or not.

iii) It is believed that at Planck temperature, the whole symmetry of the Universe was “perfect” in the sense the four fundamental foces were “unified” somehow. We have only some vague notios about how that theory of everything (TOE) would be.

The physical dimensions of the known Universe in terms of Planck units are “dramatic”:

i) Age of the Universe is about $t_U=8.0\cdot 10^{60} t_P$.

ii) Diameter of the observable Universe is about $d_U=5.4\cdot 10^{61}L_P$

iii) Current temperature of the Universe is about $1.9 \cdot 10^{-32}T_P$

iv) The observed cosmological constant is about $5.6\cdot 10^{-122}t_P^{-2}$

v) The mass of the Universe is about $10^{60}m_p$.

vi) The Hubble constant is $71km/s/Mpc\approx 1.23\cdot 10^{-61}t_P^{-1}$

## Schrödinger Units

The Schrödinger Units do not obviously contain the term c, the speed of light in a vacuum. However, within the term of the Permittivity of Free Space [i.e., electric constant or vacuum permittivity], and the speed of light plays a part in that particular computation. The vacuum permittivity results from the reciprocal of the speed of light squared times the magnetic constant. So, even though the speed of light is not apparent in the Schrödinger equations it does exist buried within its terms and therefore influences the decimal placement issue within square roots. The essence of Schrödinger units are the following constants:

A) Gravitational constant $G_N$.

B) Planck constant $\hbar$.

C) Boltzmann constant $k_B$.

D) Coulomb constant or equivalently the electric permitivity of free space/vacuum $k_C=1/4\pi\varepsilon_0$.

E) The electric charge of the positron $e$.

In this sistem $\psi$ we have

$\boxed{G_N=\hbar =k_B =k_C =1}$

1) Schrödinger Length $L_{Sch}$.

$L_\psi=\sqrt{\dfrac{\hbar^4 G_N(4\pi\varepsilon_0)^3}{e^6}}\approx 2.593\cdot 10^{-32}m$

2) Schrödinger time $t_{Sch}$.

$t_\psi=\sqrt{\dfrac{\hbar^6 G_N(4\pi\varepsilon_0)^5}{e^{10}}}\approx 1.185\cdot 10^{-38}s$

3) Schrödinger mass $M_{Sch}$.

$M_\psi=\sqrt{\dfrac{e^2}{G_N(4\pi\varepsilon_0)}}\approx 1.859\cdot 10^{-9}kg$

4) Schrödinger energy $E_{Sch}$.

$E_\psi=\sqrt{\dfrac{e^{10}}{\hbar^4(4\pi\varepsilon_0)^5G_N}}\approx 8890 J=5.55\cdot 10^{13}GeV$

5) Schrödinger charge $Q_{Sch}$.

$Q_\psi =e=1.602\cdot 10^{-19}C$

6) Schrödinger temperature $T_{Sch}$.

$T_\psi=E_\psi/k_B=\sqrt{\dfrac{e^{10}}{\hbar^4(4\pi\varepsilon_0)^5G_Nk_B^2}}\approx 6.445\cdot 10^{26}K$

## Atomic Units

There are two alternative systems of atomic units, closely related:

1) Hartree atomic units:

$\boxed{e=m_e=\hbar=k_B=1}$ and $\boxed{c=\alpha^{-1}}$

2) Rydberg atomic units:

$\boxed{\dfrac{e}{\sqrt{2}}=2m_e=\hbar=k_B=1}$ and $\boxed{c=2\alpha^{-1}}$

There, $m_e$ is the electron mass and $\alpha$ is the electromagnetic fine structure constant. These units are designed to simplify atomic and molecular physics and chemistry, especially the quantities related to the hydrogen atom, and they are widely used in these fields. The Hartree units were first proposed by Doublas Hartree, and they are more common than the Rydberg units.

The units are adapted to characterize the behavior of an electron in the ground state of a hydrogen atom. For example, using the Hartree convention, in the Böhr model of the hydrogen atom, an electron in the ground state has orbital velocity = 1, orbital radius = 1, angular momentum = 1, ionization energy equal to 1/2, and so on.

Some quantities in the Hartree system of units are:

1) Atomic Length (also called Böhr radius):

$L_A=a_0=\dfrac{\hbar^2 (4\pi\varepsilon_0)}{m_ee^2}\approx 5.292\cdot 10^{-11}m=0.5292\AA$

2) Atomic Time:

$t_A=\dfrac{\hbar^3(4\pi\varepsilon_0)^2}{m_ee^4}\approx 2.419\cdot 10^{-17}s$

3) Atomic Mass:

$M_A=m_e\approx 9.109\cdot 10^{-31}kg$

4) Atomic Energy:

$E_A=m_ec^2=\dfrac{m_ee^4}{\hbar^2(4\pi\varepsilon_0)^2} \approx 4.36\cdot 10^{ -18}J=27.2eV=2\times(13.6)eV=2Ry$

5) Atomic electric Charge:

$Q_A=q_e=e\approx 1.602\cdot 10^{-19}C$

6) Atomic temperature:

$T_A=E_A/k_B=\dfrac{m_ee^4}{\hbar^2(4\pi\varepsilon_0)^2k_B}\approx 3.158\cdot 10^5K$

The fundamental unit of energy is called the Hartree energy in the Hartree system and the Rydberg energy in the Rydberg system. They differ by a factor of 2. The speed of light is relatively large in atomic units (137 in Hartree or 274 in Rydberg), which comes from the fact that an electron in hydrogen tends to move much slower than the speed of light. The gravitational constant  is extremely small in atomic units (about 10−45), which comes from the fact that the gravitational force between two electrons is far weaker than the Coulomb force . The unit length, LA, is the so-called and well known Böhr radius, a0.

The values of c and e shown above imply that $e=\sqrt{\alpha \hbar c}$, as in Gaussian units, not Lorentz-Heaviside units. However, hybrids of the Gaussian and Lorentz–Heaviside units are sometimes used, leading to inconsistent conventions for magnetism-related units. Be aware of these issues!

## QCD Units

In the framework of Quantum Chromodynamics, a quantum field theory (QFT) we know as QCD, we can define the QCD system of units based on:

1) QCD Length $L_{QCD}$.

$L_{QCD}=\dfrac{\hbar}{m_pc}\approx 2.103\cdot 10^{-16}m$

and where $m_p$ is the proton mass (please, don’t confuse it with the Planck mass $M_P$).

2) QCD Time $t_{QCD}$.

$t_{QCD}=\dfrac{\hbar}{m_pc^2}\approx 7.015\cdot 10^{-25}s$

3) QCD Mass $M_{QCD}$.

$M_{QCD}=m_p\approx 1.673\cdot 10^{-27}kg$

4) QCD Energy $E_{QCD}$.

$E_{QCD}=M_{QCD}c^2=m_pc^2\approx 1.504\cdot 10^{-10}J=938.6MeV=0.9386GeV$

Thus, QCD energy is about 1 GeV!

5) QCD Temperature $T_{QCD}$.

$T_{QCD}=E_{QCD}/k_B=\dfrac{m_pc^2}{k_B}\approx 1.089\cdot 10^{13}K$

6) QCD Charge $Q_{QCD}$.

In Heaviside-Lorent units:

$Q_{QCD}=\dfrac{1}{\sqrt{4\pi\alpha}}e\approx 5.292\cdot 10^{-19}C$

In Gaussian units:

$Q_{QCD}=\dfrac{1}{\sqrt{\alpha}}e\approx 1.876\cdot 10^{-18}C$

## Geometrized Units

The geometrized unit system, used in general relativity, is not a completely defined system. In this system, the base physical units are chosen so that the speed of light and the gravitational constant are set equal to unity. Other units may be treated however desired. By normalizing appropriate other units, geometrized units become identical to Planck units. That is, we set:

$\boxed{G_N=c=1}$

and the remaining constants are set to the unit according to your needs and tastes.

## Conversion Factors

This table from wikipedia is very useful:

where:

i) $\alpha$ is the fine-structure constant, approximately 0.007297.

ii) $\alpha_G=\dfrac{m_e^2}{M_P^2}\approx 1.752\cdot 10^{-45}$ is the gravitational fine-structure constant.

Some conversion factors for geometrized units are also available:

Conversion from kg, s, C, K into m:

$G_N/c^2$  [m/kg]

$c$ [m/s]

$\sqrt{G_N/(4\pi\varepsilon_0)}/c^2$ [m/C]

$G_Nk_B/c^2$ [m/K]

Conversion from m, s, C, K into kg:

$c^2/G_N$ [kg/m]

$c^3/G_N$ [kg/s]

$1/\sqrt{G_N4\pi\varepsilon_0}$ [kg/C]

$k_B/c^2$[kg/K]

Conversion from m, kg, C, K into s

$1/c$ [s/m]

$G_N/c^3$[s/kg]

$\sqrt{\dfrac{G_N}{4\pi\varepsilon_0}}/c^3$ [s/C]

$G_Nk_B/c^5$ [s/K]

Conversion from m, kg, s, K into C

$c^2/\sqrt{\dfrac{G_N}{4\pi\varepsilon_0}}$[C/m]

$(G_N4\pi\varepsilon_0)^{1/2}$ [C/kg]

$c^3/(G_N/(4\pi\varepsilon_0))^{1/2}$[C/s]

$k_B\sqrt{G_N4\pi\varepsilon_0}/c^2$   [C/K]

Conversion from m, kg, s, C into K

$c^4/(G_Nk_B)$[K/m]

$c^2/k_B$ [K/kg]

$c^5/(G_Nk_B)$ [K/s]

$c^2/(k_B\sqrt{G_N4\pi\varepsilon_0})$ [K/C]

Or you can read off factors from this table as well:

and

Natural units have some advantages (“Pro”):

1) Equations and mathematical expressions are simpler in Natural Units.

2) Natural units allow for the match between apparently different physical magnitudes.

3) Some natural units are independent from “prototypes” or “external patterns” beyond some clever and trivial conventions.

4) They can help to unify different physical concetps.

However, natural units have also some disadvantages (“Cons”):

1) They generally provide less precise measurements or quantities.

2) They can be ill-defined/redundant and own some ambiguity. It is also caused by the fact that some natural units differ by numerical factors of pi and/or pure numbers, so they can not help us to understand the origin of some pure numbers (adimensional prefactors) in general.

Moreover, you must not forget that natural units are “human” in the sense you can addapt them to your own needs, and indeed,you can create your own particular system of natural units! However, said this, you can understand the main key point: fundamental theories are who finally hint what “numbers”/”magnitudes” determine a system of “natural units”.

Remark: the smart designer of a system of natural unit systems must choose a few of these constants to normalize (set equal to 1). It is not possible to normalize just any set of constants. For example, the mass of a proton and the mass of an electron cannot both be normalized: if the mass of an electron is defined to be 1, then the mass of a proton has to be $\approx 6\pi^5\approx 1936$. In a less trivial example, the fine-structure constant, α≈1/137, cannot be set to 1, because it is a dimensionless number. The fine-structure constant is related to other fundamental constants through a very known equation:

$\alpha=\dfrac{k_Ce^2}{\hbar c}$

where $k_C$ is the Coulomb constant, e is the positron electric charge (elementary charge), ℏ is the reduced Planck constant, and c is the again the speed of light in vaccuum. It is believed that in a normal theory is not possible to simultaneously normalize all four of the constants c, ℏ, e, and kC.

## Fritzsch-Xing  plot

Fritzsch and Xing have developed a very beautiful plot of the fundamental constants in Nature (those coming from gravitation and the Standard Model). I can not avoid to include it here in the 2 versions I have seen it. The first one is “serious”, with 29 “fundamental constants”:

However, I prefer the “fun version” of this plot. This second version is very cool and it includes 28 “fundamental constants”:

## The Okun Cube

Long ago, L.B. Okun provided a very interesting way to think about the Planck units and their meaning, at least from current knowledge of physics! He imagined a cube in 3d in which we have 3 different axis. Planck units are defined as we have seen above by 3 constants $c, \hbar, G_N$ plus the Boltzmann constant. Imagine we arrange one axis for c-Units, one axis for $\hbar$-units and one more for $G_N$-units. The result is a wonderful cube:

Or equivalently, sometimes it is seen as an equivalent sketch ( note the Planck constant is NOT rationalized in the next cube, but it does not matter for this graphical representation):

Classical physics (CP) corresponds to the vanishing of the 3 constants, i.e., to the origin $(0,0,0)$.

Newtonian mechanics (NM) , or more precisely newtonian gravity plus classical mechanics, corresponds to the “point” $(0,0,G_N)$.

Special relativity (SR) corresponds to the point $(0,1/c,0)$, i.e., to “points” where relativistic effects are important due to velocities close to the speed of light.

Quantum mechanics (QM) corresponds to the point $(h,0,0)$, i.e., to “points” where the action/angular momentum fundamental unit is important, like the photoelectric effect or the blackbody radiation.

Quantum Field Theory (QFT) corresponds to the point $(h,1/c,0)$, i.e, to “points” where both, SR and QM are important, that is, to situations where you can create/annihilate pairs, the “particle” number is not conserved (but the particle-antiparticle number IS), and subatomic particles manifest theirselves simultaneously with quantum and relativistic features.

Quantum Gravity (QG) would correspond to the point $(h,0,G_N)$ where gravity is quantum itself. We have no theory of quantum gravity yet, but some speculative trials are effective versions of (super)-string theory/M-theory, loop quantum gravity (LQG) and some others.

Finally, the Theory Of Everything (TOE) would be the theory in the last free corner, that arising in the vertex $(h,1/c,G_N)$. Superstring theories/M-theory are the only serious canditate to TOE so far. LQG does not generally introduce matter fields (some recent trials are pushing into that direction, though) so it is not a TOE candidate right now.

## Some final remarks and questions

1) Are fundamental “constants” really constant? Do they vary with energy or time?

2) How many fundamental constants are there? This questions has provided lots of discussions. One of the most famous was this one:

http://arxiv.org/abs/physics/0110060

The trialogue (or dialogue if you are precise with words) above discussed the opinions by 3 eminent physicists about the number of fundamental constants: Michael Duff suggested zero, Gabriel Veneziano argued that there are only 2 fundamental constants while L.B. Okun defended there are 3 fundamental constants

3) Should the cosmological constant be included as a new fundamental constant? The cosmological constant behaves as a constant from current cosmological measurements and cosmological data fits, but is it truly constant? It seems to be…But we are not sure. Quintessence models (some of them related to inflationary Universes) suggest that it could vary on cosmological scales very slowly. However, the data strongly suggest that

$P_\Lambda=-\rho c^2$

It is simple, but it is not understood the ultimate nature of such a “fluid” because we don’t know what kind of “stuff” (either particles or fields) can make the cosmological constant be so tiny and so abundant (about the 72% of the Universe is “dark energy”/cosmological constant) as it seems to be. We do know it can not be “known particles”. Dark energy behaves as a repulsive force, some kind of pressure/antigravitation on cosmological scales. We suspect it could be some kind of scalar field but there are many other alternatives that “mimic” a cosmological constant. If we identify the cosmological constant with the vacuum energy we obtain about 122 orders of magnitude of mismatch between theory and observations. A really bad “prediction”, one of the worst predictions in the history of physics!

Be natural and stay tuned!