# What is the complete proof that the speed of light in vacuum is constant in relativistic mechanics?

I study maths in uni and we have a course about relativity.

In the main principles I've read that the speed of light is invariant since we can calculate it from the Maxwell equations.

My problem is that the Maxwell equations I know are not relativistic. What is the clear way to formulate the Maxwell equations with respect to the relativistic spacetime? Using that formulation do we get the same value for $c$? How do we do that?

Edit: It is clear now what was my problem after the answers.

The wrong concept I had was that: From the classical maxwell equations we can calculate the speed of light, and with that information we can build up the relativistic spacetime where the maxwell equations might look different. And it was weird for me.

From the answers it became clear that the invariant speed of light is an observation not a result.

Ps: I find it interesting that my maths like approach did not consider the possibility of something just being an observation, not a result.

• You can derive the vacuum Maxwell's equations from the Lagrangian $L = -\frac{1}{4}F_{\mu\nu}F^{\mu\nu}$ where $F_{\mu\nu} = \partial_\mu A_\nu - \partial_\nu A_\mu$. This Lagrangian is Lorentz invariant. – John Apr 12 '17 at 12:42
• You should say invariant (unchanged under coordinate transformations) rather than constant (time-independent). – J.G. Apr 12 '17 at 12:59
• @J.G. - Interesting! What does it mean to say that it's not constant - that it varies over time, or something else? – Don Branson Apr 12 '17 at 17:02
• To be fair, the constancy of the speed of light is more of an observation than a computation – Jim Apr 12 '17 at 17:18
• @DonBranson The speed is constant too, and uniform, and isotropic, but your question was really about its invariance. These all have subtly different meanings. – J.G. Apr 12 '17 at 18:51

In the main principles I've read that the speed of light is constant since we can calculate it from the Maxwell equations.

The fact that the speed of light could be deduced from Maxwell's equations does not, in and of itself, imply that the speed of light is constant in all reference frames. Certainly the equations don't make an obvious reference to a reference frame; but once you've made the connection between electric and magnetic fields and light, it seems pretty obvious what the "natural" rest frame is (bolding mine):

We can scarcely avoid the inference that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.

– James Clerk Maxwell, On the Physical Lines of Force

In other words, one could easily imagine a world in which Maxwell's equations are only valid in the rest frame of the luminiferous aether — and from about 1860–1905 or so, this is precisely the universe that physicists thought we lived in. In such a universe, Maxwell's equations would in fact look different in different reference frames; a "full" version of these equations would include terms that depended on an observer's velocity $$\vec{v}$$ with respect to the aether. There is nothing mathematically inconsistent about the equations describing such a Universe.

What these equations are inconsistent with, however, is two things: (1) experimental evidence, and (2) our sense of symmetry. The Michelson-Morley experiment was designed to detect Earth's motion relative to the aether — in other words, to indirectly verify the presence of these $$\vec{v}$$-dependent terms in the hypothetical Maxwell's equations. Of course, they famously came up short.

The other problem is that there seem to be a lot of convenient coincidences between what seem to be the same phenomena described in different reference frames:

It is known that Maxwell's electrodynamics—as usually understood at the present time—when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise—assuming equality of relative motion in the two cases discussed—to electric currents of the same path and intensity as those produced by the electric forces in the former case.

Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the “light medium,” suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.

— Albert Einstein, On the Electrodynamics of Moving Bodies

Or, to summarize: if I move a coil near a magnet, the magnetic field causes the charges to flow. If I move a magnet near a coil, the changing magnetic field causes an electric field, which causes the charges to flow. These two descriptions seem very different, and yet somehow they give rise to exactly the same amount of current in the coil. Einstein's contention was that this couldn't be a coincidence, and that only relative velocity should matter.

If you buy that, then you find (as Einstein did) that when you go into another reference frame, the electric and magnetic fields intermingle with each other. If you look at the above link to Einstein's original paper, §6 describes how the electric and magnetic fields transform into each other. His notation is a little antiquated — what he calls $$(X, Y, Z)$$ we would nowadays usually call $$(E_x, E_y, E_z)$$, and what he calls $$(L, M, N)$$ we would usually call $$(B_x, B_y, B_z)$$. In different reference frames moving relative to each other in the $$x$$-direction, all of these components change, and the components $$E_y$$, $$E_z$$, $$B_y$$, and $$B_z$$ get mixed up with each other. In other words, the electric and magnetic field strengths observed by Observer A and Observer B are not necessarily the same.

These transformations between the fields are a necessary consequence of the postulate that the laws of physics are the same in all reference frames. But Maxwell's equations don't necessarily imply that the laws of physics are all the same in such reference frames; they are agnostic on the subject. Historically, physicists originally believed that there was in fact a privileged frame in which Maxwell's equations held exactly, and it was only after careful experimentation and careful thought that we figured out that Maxwell's equations were also consistent with the principle of relativity.

• Anecdote: when I was a graduate student, I took an electrodynamics course with the mathematical physicist Robert Geroch. At one point he actually went through the derivation of Maxwell's equations would look like if Galilean relativity still held. They look strange & intractable, but there's nothing wrong with them per se — they just don't describe the Universe we live in. – Michael Seifert Apr 12 '17 at 15:57
• Thank you for the answer! It is really what I wanted. My problem was that in maths we build things to each other. When I heard that they deduced the invariant speed of light from maxwells equations and then the relativity from the invariant c, it seemed weird that those equations might not be relativistic and those constants might not be the same in other reference frames. Even if they prove that the values are equal, the oder is strange. – Lewwwer Apr 13 '17 at 6:24
• Typo: should be "Michelson-Morley" (no "a") :) – psmears Apr 13 '17 at 10:23
• @psmears: I keep forgetting that he spelled his last name the wrong way. :-) – Michael Seifert Apr 13 '17 at 13:55
• That anecdote really does deserve to be an addendum to your answer. It's interesting in its own right and it does invite further exploration: it is indeed quite an eye opener (or an eye-sore) to work out Galilean-boosted Maxwell equations. – Selene Routley Apr 19 '17 at 0:31

It's an axiom based on observation. As far as I know, that means it cannot be proved.

Also, Maxwell's equations are relativistically covariant. The thing you have to keep in mind is that the electric and magnetic field get mixed by Lorentz transformations. Think of the force law on a moving charge:

$$\mathbf{F} = q \mathbf{E} + q\mathbf{v}\times \mathbf{B}.$$

Now, imagine there's a charge moving at speed $\mathbf{v}$ through some magnetic field, and there's no electric field. You'll see the charge accelerate under the influence of the magnetic force, curving it's path. Now, imagine what an observer also moving at $\mathbf{v}$, instantaneously, sees. That observer sees an instantaneously stationary charge, so it can't experience a magnetic force. That observer has to see the charge accelerate in some fashion, though, because you saw it accelerate. That can only be the case if that observer thinks that there are both electric and magnetic fields present.

• By "maxwell equations are relativistically covariant" you mean that if you make experiments in a moving inertial frame, you can measure the same constants (permittivity and permeability), and then get the same c? – Lewwwer Apr 12 '17 at 14:51
• @Lewwer That's part of it. The other part is that if you know how the electric and magnetic fields transform between reference frames the equations themselves are invariant. – Sean E. Lake Apr 12 '17 at 16:21
• @Lewwwer: That's not an "if" we have made such an observation at a time when people did not believe that constant c was true and found the results went against that belief - we had to accept the equations due to observation that the universe works that way, not the other way around. – slebetman Apr 12 '17 at 22:51
• – slebetman Apr 12 '17 at 22:52

As mentioned in other answers, Maxwell's equations are indeed invariant under Lorentz transformations. Easiest way to see this is to write them in a manifestly covariant form. This form is: \begin{align} \partial_\mu F^{\mu\nu} &= j^\nu \\ \partial^\tau F^{\mu\nu}+\partial^\mu F^{\nu\tau} + \partial^\nu F^{\tau\mu}&=0 \end{align} where $F^{\mu\nu}$ is electromagnetic tensor (which contains electric and magnetic field in a 4x4 matrix), $j^\mu$ is the four-current (which contains the charge density and the current density in a four-vector) and natural units $\epsilon_0 = \mu_0 = c=1$ are used for the sake of simplicity.

Electromagnetic tensor $F^{\mu\nu}$ is given in terms of four-potential $A^\mu$ (which contains electric (scalar) and magnetic (vector) potential) is related to $F^{\mu\nu}$ via \begin{equation} F^{\mu\nu}=\partial^\mu A^\nu - \partial^\nu A^\mu \end{equation} and when you plug it in $\partial_\mu F^{\mu\nu} = j^\nu$ without sources ($j^\mu=0$), you get \begin{equation} \Box A^\mu-\partial^\mu(\partial\cdot A)=0. \end{equation} However, there are multiple choices of $A^\mu$ which give the same $F^{\mu\nu}$. Transformation $A^\mu \to A^\mu+\partial^\mu \xi$, where $\xi$ is a scalar function, leaves $F^{\mu\nu}$ unchanged and it is called "gauge transformation". If you take the original $A^\mu$ and make a gauge transformation with $\xi=\partial\cdot A$, you get something called "Lorentz gauge", characterized by $\partial\cdot A=0$. In the Lorentz gauge, the above equation simplifies to \begin{equation} \Box A^\mu=0 \end{equation} which is a wave equation for waves moving at the speed $c$. Therefore, from Maxwell's equations follows that electromagnetic radiation, including light, travels at the speed $c$ in vacuum.

• Note that your final equation only holds in Lorenz gauge. – gj255 Apr 12 '17 at 15:12
• @gj255 Thank you for noticing that, I've updated my answer to include it. – Danijel Apr 12 '17 at 20:03
• I've always argued that this does not matter much. If experiments found that c was not invariant in different reference frames we'd simply introduce the concept of ether into physics and found a set of equations that worked. It's because the universe works this way that we accept this as true - that's science. The other way round, that because this is what the equations say therefore the universe must work this way is philosophy and religion. – slebetman Apr 12 '17 at 22:56

Maxwell equations are invariant under Lorentz transformations, which is the same as saying that they follow special relativity.

You can try to convince yourself by transforming to another inertial reference frame $S'$ and deriving the wave-equation of the fields, you should find that $c' = c$

One can deduce the form of the Lorentz transformation, without being able to fix a value for $c$, as the only experimentally-consistent member of a small handful of possible, needfully linear transformations that follow from basic spacetime homogeneity, isotropy and continuity postulates together with Galileo's principle that only relative motion can be detected between different inertial observers.

Once you know the Lorentz transformation, then it must follow that anything moving at the speed $c$ must be measured to be moving at that same speed by all inertial observers.

This is not a proof of course; it's simply showing that the constancy of $c$ postulate can be replaced by other axioms. But it is nonetheless interesting that this replacement can be done, as the other axioms are much more everyday and intuitively obvious. This is Ignatowski's approach. See, for example, my answer here or the papers:

Jean-Marc Lévy-Leblond, "One more derivation of the Lorentz transformation", Am. J. Phs. 44

Palash B. Pal, "Nothing but Relativity," Eur.J.Phys.24:315-319,2003

The implication of Einstein's argument in his 1905 paper, clarified by Bondi in Relativity and Common Sense, is that the definition of coordinates is determined by the speed of light. In particular, the modern definition of the metre depends on the speed of light. Then it follows directly from the general principle of relativity, that the laws of physics are the same in all reference frames, that the (local) speed of light is the same in all reference frames.

Bondi uses the radar method, which makes this explicit, but he also points out that the electromagnetic force is described in QED by photon exchange. Thus the structure of a ruler depends on essentially the same process.

“… the length of a rigid rod is determined by the electrical interactions of the atoms and, therefore, in fact, by a superposition of radar methods.” — Hermann Bondi, Relativity and Common Sense.

From this point of view

“So, with our modern outlook and modern technology the Michelson-Morley experiment is a mere tautology”. Hermann Bondi, Assumption and Myth in Physical Theory.