For a basic treatment of the Michelson-Morley experiment please see 1. It's not important to know the technical details of the experiment to answer your questions though. The only relevant thing is the result, let me put it in basic terms since you seem to struggle with the "physics slang":

*While the total velocity of a ball thrown from a truck is the sum of the velocity of the ball relative to the truck and the velocity of the truck relative to the observer, the velocity of a light beam emitted from the truck is not. Much more the velocity of the light beam seems completely independent of the velocity of the truck.*

Michelson and Morely didn't have a truck, they had the earth orbiting the sun.

Please make it clear to yourself that this experimental fact can be explained by **stating** that the speed of light is constant. If I say to you the speed of light is constant in every frame of reference, then the above result isn't surprising at all to you.

But you want more. You want me to prove to you that the speed of light is universally constant. I cannot. There will never be an experiment that shows that this **axiom** is universally true. How should one ever construct such an experiment, how should one, for example, test the theory in the Andromeda galaxy? It's impossible, but it doesn't matter: Why not just stick with the axiom, as long as we can explain everything we see around us with it?

As you already said there's an interesting connection between the
invariance of the speed of light and Maxwell's equations. One can indeed prove that the speed of light has to be constant, otherwise, Maxwell's theory can't be true for all inertial frames. But this is no proof that can convince you either, since accepting Maxwells equations is no different to accepting the invariance of the speed of light. Furthermore, the basis of Einstein's theory is not the invariance of the speed of light, but the invariance of the speed of action. Which cannot be concluded from Maxwell's theory, even though it's a reasonable guess.

**Physical theories are not provable. But as long as they comply with reality, we accept them as truths.**

Addendum: I recommend this short lecture for layman by R. Feynman on the topic. Feynman and I present a very similar line of reasoning.

The reason is that the light is the fastest possible way of transmission of information. Yes, before measuring one-way speed of light we have to synchronize spatially separated clocks and to synchronize clocks we need to know velocity of the one way speed of light.

But, we can measure two-way speed of light with a single clock.

To use the sun as a source of synchronization is the same so as to use an equidistant from clocks source of light. But, the moment they will see the sun can is very “subjective” and depends on one-way speed of light from the sun towards them, which can be different.

Let’s this laboratory moves relatively to this source of light and one-way speed of light is different leftward and rightward. When the beam comes to these clocks, you still have to assume what was the one-way velocity so as to set hands of these clocks. You still don’t know this one-way velocity and are free to assign it certain value. If you will assume, that one-way velocity was c, these clocks will be Einstein – synchronized, and measured one-way velocity will be exactly c, even if this synchronization is “wrong”.

It is well-known, that Einstein – synchronization is a special case of broader Reichenbach – synchronization, which is also self–consistent. Reichenbach synchronization allows anisotropic one-way velocities until two-way speed of light is c. For example, if in one direction it will be infinitely large in the other direction it will be very close to c/2.

Lorentz Ether theory (which is empirically equivalent to SR assumes, that the Earth moves in “preferred” frame, or Ether. Yes, according to this theory, as you say “*light is NOT constant since we on earth are moving though space. & since we are moving though space we should observe speed of light as speed of light +- our speed of travel in the opposite direction.*“ In Lorentz theory one way velocities of light are the same only in the Ether, but we cannot measure it because of well – known reasons. What we can measure, is two-way velocity, which is isotropic indeed, see Michelson Morley experiment. But, this experiment, as well as any other is not able to say something about one-way speed of light.

It is often said that one-way speed of light is conventional. So, time dilation in certain sense is also conventional, because it depends on synchronization procedure. If each observer synchronizes clocks by Einstein, they will see dilation of each other clock.

Let’s there are two relatively moving reference frames, S and S’. Let’s S’ moves very close to c. If clocks in these frames synchronized by Einstein, they will be “slower” each other.

If in the frame S the observer synchronizes clocks by Einstein and the observer in S’ synchronizes clocks by Reichenbach, assuming that he himself is moving in the frame S and one- way speed of light is anisotropic in his frame (since it is isotropic in S), this observer S’ will see, that clock S is ticking $\gamma$ times faster.

There are a couple of free sources that compare “isotropic one-way ” relativity and “anisotropic one” :

On the simplest examples of floating in water ships this article or this book simulates all kinematic effects of Special Relativity, Lorentz transformations, anisotripic one-way speed of light and isotropic two-way, time dilation, length contraction, relativistic velocities addition, Relativistic Doppler effect, reciprocity of Lorentz transformations, Twin paradox, Bell’s spaceship paradox etc.

Another was: Janssen, Michel (1995), A Comparison between Lorentz's Ether Theory and Special Relativity in the Light of the Experiments of Trouton and Noble, but I was not able to find it right now.

Another well-known book: Max Jammer; Concepts of simultaneity: from Antiquity to Einstein and beyond - https://muse.jhu.edu/book/3280

Good to note, that the one way speed of light is anisotropic on rotating ring.

Does Sagnac effect imply anisotropy of speed of light in this inertial frame of reference?

## Best Answer

The bottom line is that this is the wrong question to ask. You don't ever prove an axiom in physics.

You're not quite right about the ether: while the

first ordereffect cancels out in "there and back again" experiments, the second order effect doesn't, which is why the Michelson-Morley experiment stood a chance of testing for the ether. But Michelson-Morley wasn't the end of ether theory, because you could always add fixes to account for the results. For example, the famous Lorentz contraction in special relativity was originally invented for ether theory; the idea was that flowing ether physically squeezed objects smaller. Einstein just took effects like these more seriously.Ether theory limped on for another 40 years, getting progressively more complicated as more results came in. Ether was not abandoned because it was disproven by experiments or because special relativity was proven by experiments, because this never happens. It was abandoned by the 1930s it could only explain experiments using tons of epicycles, while special relativity just worked perfectly out of the box.