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I'm trying to grasp Einstein's special relativity theory and can't seem to find a clear answer as to why Einstein concluded that the speed of light is constant to observers in different inertial frames.

  1. Sometimes I read it's because of the Michelson-Morley experiment. But that only proves there's no aether/medium for light to travel through.

  2. Sometimes I read that he derived it from Maxwell's equations. I still need to study those, so I can't judge about that.

  3. And sometimes I read that it's just an axiom that we need to accept.

So what's the right answer?

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    $\begingroup$ I tried to grasp SR using this paper. I think it's the original paper which introduced many of the concepts. $\endgroup$ – user191954 Jul 1 '18 at 8:16
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    $\begingroup$ I deleted some comments which looked like they were answering the question. Please keep in mind that answers should be posted as answers, not comments. Incidentally, I'm torn about whether this should be considered a duplicate. It sounds like this question is asking about why the speed of light is invariant within SR-as-an-axiomatic-system, while the proposed duplicate target is asking why it's invariant in the real world. I'm not sure if the distinction between the two is enough that this shouldn't be marked as a duplicate of the other. $\endgroup$ – David Z Jul 1 '18 at 8:25
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    $\begingroup$ Related: physics.stackexchange.com/q/134356/2451 , physics.stackexchange.com/q/245346/2451 and links therein. $\endgroup$ – Qmechanic Jul 1 '18 at 8:40
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    $\begingroup$ Possible duplicate of Do we know why there is a speed limit in our universe? $\endgroup$ – sammy gerbil Jul 1 '18 at 9:34
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    $\begingroup$ I don't think it's a duplicate, for basically the reason David Z gave. (To me the distinction seems plenty.) $\endgroup$ – Nathaniel Jul 1 '18 at 12:24
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None of the other answers seem to directly address the historical nature of the question, which asks why Einstein concluded that the speed of light is constant to observers in different inertial frames. Note this is clarified in the comments, where the asker states

"It's more like: hey, I'm Einstein, living in 1905 and I just came up with my second postulate. Here's why.... that's the answer I'm looking for."

So, to answer...

Among your three options, it is my understanding that the historical evidence suggests a mixture of the second and third options: Einstein had certain (axiomatic, if you like) convictions about absolute motion, and from the interplay of these convictions with (results derived from) Maxwell's equations, he was motivated to create his 1905 theory. Contrary to what some of the other answers suggest, Einstein was motivated much less by the Michelson-Morley result.

I think this quote from Einstein sums all this up fairly well:

In my own development Michelson’s result has not had a considerable influence. I even do not remember if I knew of it at all when I wrote my first paper on the subject (1905). The explanation is that I was, for general reasons, firmly convinced that there does not exist absolute motion and my problem was only how this could be reconciled with our knowledge of electro-dynamics. One can therefore understand why in my personal struggle Michelson’s experiment played no role or at least no decisive role.

(Emphasis added.)

However, the role that the Michelson-Morley experiment played in constributing to the ideas in Einstein's 1905 paper is a subject of debate. This paper discusses some of the conflicting lines of evidence as to how much Einstein knew about Michelson-Morley and how much it influenced his initial theory (note that the above quote is contained within the linked paper, as well as additional ones which may better expose some of the nuances of what contributed to Einstein's thinking).

Hope that helps!

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All of the above. Let's examine them in turn:

  1. Sometimes I read it's because of the Michelson-Morley experiment. But that only proves there's no aether/medium for light to travel through.

This seems a slightly back-to-front way of looking at it. What the Michelson-Morley experiment shows is that the round-trip speed of light is constant in every direction and doesn't change as the Earth moves around the Sun. From this we conclude that the luminiferous ether theory is incorrect, since it predicts that the speed of light would be different depending on the Earth's motion relative to the ether. But the actual measurement essentially just says the speed of light doesn't depend on the reference frame, at least for observers fixed to the surface of the Earth.

(In fact one can't even really conclude that there is no ether, since the measurements could also be explained by the Earth dragging the ether around with it somehow. I believe this was seriously considered at the time. The ether theory is ruled out by the development of special and general relativity and the much better predictions they make, not by the Michelson-Morley experiment alone.)

It should be noted that Einstein himself said that the Michelson-Morley experiment was not a big part of his own personal motivation, as quoted in Grayscale's answer. Instead, his main motivation was to do with Maxwell's equations as described below. But clearly the idea of frame invariance was 'in the air' at the time, and both Michelson and Einstein were influenced by common ideas. (In fact, Lorentz had already worked out the mathematics of length contraction and time dilation, but interpreted them as physical phenomena rather than observer-dependent ones. So there were plenty of other people working on these kinds of ideas at that time as well.)

  1. Sometimes I read that it's derived from Maxwell's equations. I still need to study those, so I can't judge about that.

This is true too. The point is that if you assume Galilean invariance (i.e. if you don't assume relativity but you do assume that the laws of physics are the same for all observers) then it's possible to have an observer moving at the speed of light. If such an observer were to travel parallel to a light wave then they would see the electric and magnetic fields varying sinusoidally in space but not changing over time. But such a frozen light wave is not a solution to Maxwell's equations, so there is a paradox.

One of Einstein's main motivations was to resolve this, i.e. to express Maxwell's equations in such a way that they would be valid for all inertial observers, instead of only being valid for an observer "at rest".

  1. And sometimes I read that it's just an axiom that we need to accept.

This is true too. The theory of special relativity is very mathematically elegant in that it requires very few assumptions. If you make this assumption (plus a few other quite minimal ones), then the rest of the theory follows, much as if you assume Euclid's axioms then you can derive Pythagoras' theorem.

If you're a pure mathematician then that's all you need to know - you don't care about why you should make the assumptions you make, so you call them axioms. But in physics we generally don't just make assumptions for no reason - they are made because of measurements or because of holes in existing theories. Einstein made the assumption of a constant speed of light for several reasons, but the Michelson-Morley experiment and the observer-dependence of Maxwell's equations were probably the main ones.

So all three are true. You can treat it as an axiom if you want, and that's probably the best approach if your main goal is to understand the maths. But the axiom also has several physical justifications, and that's what the other points you listed are.

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  • $\begingroup$ MM experiment says absolutely nothing about one way speed of light while SR claims that one way speed of light is c in all frames. Moreower, observer in the center of the circumfernce will alway see, that mesuring rod of rotating observer id contracted and rotating obsrerver will always see, that clock in the center of the circumference is ticking faster and measuring rod in the center of circumference is gamms times longer. That gives him a hint that his interferometer is contracted and his own clock tuns slower. $\endgroup$ – Albert Jul 1 '18 at 11:08
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    $\begingroup$ @Albert your comment makes no sense. The constancy of the speed of light across several different reference frames is the main result of the Michelson-Morley experiment. $\endgroup$ – Nathaniel Jul 1 '18 at 12:25
  • $\begingroup$ en.m.wikipedia.org/wiki/One-way_speed_of_light there is no experiment even in principle that confirms that one way speed of light is the same in all inertial frames. Einstein synchronization is softly speaking is a convention, or even a fantasy. MM experiment shows that roundtrip speed of light is c. Actually they have meaured phase shifts. Lorentz explained absence of phase shifts by contraction of one of the shoulders (Lorentz - FitzGerald contraction). $\endgroup$ – Albert Jul 1 '18 at 13:01
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    $\begingroup$ Oh, I see, you took "constant in every direction" to mean one-way instead of round trip. That's not what I meant of course. I'll clarify it. $\endgroup$ – Nathaniel Jul 1 '18 at 13:20
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    $\begingroup$ Downvoted for three reasons. (1) The MM experiment had little, if any, impact on Einstein's thinking. (2) The MM experiment only proved that the luminiferous aether as conceived of at the time did not exist. (3) There are alternative axiomatizations that maintain the concept of an ether and that are indistinguishable from special relativity in terms of predictions of experimental outcomes. $\endgroup$ – David Hammen Jul 4 '18 at 17:26
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I'll answer the third questions posed in the question in reverse order, and finally return to the third.

  1. And sometimes I read that it's just an axiom that we need to accept.

What's wrong with axioms? Einstein himself listed the constancy of the speed of light as a postulate (aka axiom; they're synonyms). Newton's laws of motion are axioms. Hilbert's sixth problem, announced in 1900, addressed the need to axiomatize physics. There's nothing wrong with axioms.

To the contrary; axioms in physics are a very good thing. They form the mathematical basis from which physicists predict the outcomes of experiments. One or more of the underlying assumptions (another name for axioms) are falsified if the outcome disagrees with the prediction.

  1. Sometimes I read that he derived it from Maxwell's equations. I still need to study those, so I can't judge about that.

At the time Einstein developed special relativity, Maxwell's equations were consistent with experiments of an electromagnetic nature while Newtonian mechanics appeared to be inconsistent with such experiments. This was one of the key conundrums of late 19th century physics. One of those axiomatizations appears to be incorrect.

One of the many face-value consequences of Maxwell's equation is that the one-way speed of electromagnetic radiation (which includes light) is the same to all inertial observers. In particular, the expression for the speed of a wave of electromagnetic wave in vacuum depends on neither the speed of the source nor the receiver. It's simply $c = 1/\sqrt{\mu_0 \varepsilon_0}$, where $\mu_0$ and $\varepsilon_0$ are the magnetic permeability and electrical permittivity of space. Einstein simply took Maxwell's equations at face value, that the speed of light in vacuum is the same to all observers.

  1. Sometimes I read it's because of the Michelson-Morley experiment. But that only proves there's no aether/medium for light to travel through.

The Michelson-Morley experiment disproves the existence of a luminiferous aether as conceived of at the time of that experiment. It does not disprove the existence of an ether. A competing axiomatization, Lorenz Ether Theory, developed at the same time Einstein developed his special theory of relativity, maintained the concept of a preferred frame of reference and of an ether. More on this later.

As Grayscale noted in his answer, the Michelson-Morley experiment had little if any influence on Einstein's thinking. Einstein was much more influenced by the Fizeau experiment, which looked at the speed of light in a moving medium such as flowing water. If Newtonian mechanics was true, such a medium should have a linear affect on the speed of electromagnetic waves, just as blowing air linearly effects the speed of sound waves. This was not what the Fizeau experiment found. It instead found a relationship that, after the fact, was found to be consistent with special relativity (and also Lorenz Ether Theory).

  1. And sometimes I read that it's just an axiom that we need to accept.

You don't need to accept the constancy of the one-way speed of light as axiomatic. You could accept Lorenz Ether Theory instead. It is indistinguishable from special relativity in terms of predicted experimental outcomes. The problem with Lorenz Ether Theory is that it is chock full of "where did that come from" (WTF) assumptions. It makes length contraction, time dilation, a preferred frame of reference, and an ether that acts as a medium for electromagnetic radiation in empty space axiomatic. Length contraction and time dilation nicely hide the preferred frame of reference and the ether from any experimental test designed to detect them. Nice. (That was a sarcastic "nice".)

Special relativity only has one "where did that come from" assumption, the constancy of the one-way speed of light, and that is only a ""where did that come from" assumption because we are too preconditioned by Newtonian thinking. The one-way constancy of the speed of light is a beautiful thing that speaks directly about the geometry of space-time. In contrast, Lorenz Ether Theory is a rather ugly thing that fails Occam's razor that does not speak directly to the mathematics that underlies space and time.

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  • $\begingroup$ I think that you miss one thing. SR is the LET. In SR observer simply cannot say, that he is in motion, nothing else. It is simply illogical - to think, that you cannot move, i.e. change spatial position in someone's frame. Please find a problem here: spiff.rit.edu/classes/phys200/lectures/doppler/doppler.html. The problem is about Joe and Aliens. Joe is "at rest" and Aliens fly relatively to him. What do you think, will the Aliens see dilation of Joe's clock (redshift of frequency?). Hint: aberration of light. $\endgroup$ – Albert Jul 4 '18 at 21:42
  • $\begingroup$ In SR ANY observer MUST think that he is at rest. Even in SR, as soon, as an observer thinks, that he is in motion, or ASCRIBES himself state of motion, he will see, that clock "at rest" is running faster and measuring rod "at rest" is longer, than his own. Look at relativistic aberration formula:en.wikipedia.org/wiki/Aberration_of_light. In the denominator you see the distance along the x axis. Why these distances $\gamma$ times increase ? Because measuring rod of the moving observer contracts. Look for relativistic aberration here: feynmanlectures.caltech.edu/I_34.html $\endgroup$ – Albert Jul 4 '18 at 22:13
  • $\begingroup$ @Albert - While the predictions of SR are indistinguishable from the predictions of LET, SR is anything but LET. LET is an ugly beast with ad hoc and unneeded metaphysical axioms. SR is a beautiful thing in comparison. $\endgroup$ – David Hammen Jul 4 '18 at 22:59
  • $\begingroup$ Does SR predicts, that due to dilation of his own clock, moving observer sees, that clock "at rest" is ticking $\gamma$ times faster? How should you synchronize clocks in your frame, so as to get this result? One - way speed of light will not be c, if you understand what role plays synchronization of clock in Special Relativity. Do you understand, that reciprocity is achieved only due to the same synchronization procedure? $\endgroup$ – Albert Jul 4 '18 at 23:02
  • $\begingroup$ Please look here: mathpages.com/home/kmath587/kmath587.htm (Since both emitter and receiver have the speed v relative to this system of reference, there is no differential time dilation.). How should you synchronize clocks, so as not to see any time dilation? I will tell you. You have to synchronize them with that assumption, that one way velocity of light is not c. One frame moves and another frame also moves. In a frame that is moving, one - way velocity of light is not c, it is obvious. Look at transverse Doppler shifts, they tell everything, $\endgroup$ – Albert Jul 4 '18 at 23:15
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Nathaniel's answer accurately addresses your question. I would like to address your specific doubts.

  1. Sometimes I read it's because of the Michelson-Morley experiment. But that only proves there's no aether/medium for light to travel through.

True, and this in turn implies that the velocity of light must not be measured with respect to the aether. On the contrary, if $c$ has to have an absolute meaning in the Maxwell's equations, then it has to have an absolute value, i.e. it must be the same in all inertial frames.

  1. Sometimes I read that it's derived from Maxwell's equations. I still need to study those, so I can't judge about that.

The important point here is that it can be derived from Maxwell's equations provided that you know the correct transformation laws between inertial frames. In turn, Einstein derived these laws by postulating the constancy of $c$. But you could still postulate the transformation laws, and derive the constancy of $c$ through these laws or through Maxwell's equations. Or you could postulate (as Lorentz did) that Maxwell's equations should be the same in all inertial systems and use the postulate to derive the transformation laws.

  1. And sometimes I read that it's just an axiom that we need to accept.

This is usually what you do.

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You need to be very careful as to what you mean by "the speed of light". There are indeed one-way speeds and two-way speeds and they have completely different a status.

  • One-way. A light signal departs from $A$ at time $t_A$ and arrives at $B$ at time $t_B$, as marked by clocks respectively located at $A$ and $B$. The one-way speed is then $AB/(t_B - t_A)$.
  • Two-way. A light signal departs from $A$ at time $t_A$, hits a mirror at $B$ and is reflected back to $A$ where it arrives at time $t'_A$, where those times are recorded by a clock sitting at $A$. The two-way speed is then $2AB/(t'_A-t_A)$.

The one-way speed of light is a convention as it depends on how the clocks at $A$ and $B$ are synchronised. The two-way speed of light, on the contrary, is a genuine physical observable as it can be measured. In his 1905 paper, Einstein chose a synchronisation procedure for the clocks such that the time of flight of a light signal from $A$ to $B$ is equal to the time of flight from $B$ to $A$. As a result the one-way speed from $A$ to $B$ is equal to the one-way speed from $B$ to $A$, and therefore also to the two-way speed.

Your question is therefore ill-posed. If you meant the one-way speed, then being a convention, we can't say more without choosing one such convention. If we go for Einstein's method, then it boils down to discuss the two-way speed. But we could choose anything else.

If you meant the two-way speed, then Michelson-Morley experiment shows that it is isotropic at the surface of Earth. Now consider the most popular Ether theory of that time past: the Ether is at rest in the geocentric frame, or even in the heliocentric frame. Then the M&M apparatus, thanks to the Earth spinning and revolving about the Sun moves with a great variety of speeds with respect to the Ether. So we can say that the isotropy of the two-way speed of light is observed in the corresponding great variety of inertial frames. Thus assuming that it is isotropic in any inertial frame is a legit assumption to make, to see where that leads when one tries to find space and time transformations between frames which would realise that. That's precisely what Einstein did in his 1905 paper.

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  • $\begingroup$ @StéphaneRollandin yes, indeed, stupid me! Fixed. $\endgroup$ – frapadingue Jul 1 '18 at 15:40
  • $\begingroup$ Let’s take two toothed wheels, like the one Fizeau used in his experiment. Then we attach these wheels to the ends of a long, long metallic rod. Then we start rotating the rod from one end. Rod will be a bit twisted in the beginning, but after some time elastic force will straighten it out and the toothed wheel will rotate synchoronously and in the same phase. Then we send a beam of light through the wheels and measure one way speed of light like Fizeau did. Is this synchronization conventional? Or elastic force does not work within certain limits (according to Reichenbach synchronization)? $\endgroup$ – Albert Jul 4 '18 at 16:07
  • $\begingroup$ @Albert Not going into the details of your proposal, you have just described one synchronisation procedure among an infinity of possibilities. On what ground should it be preferred? Going into details, I actually think the bar will not straighten up and furthermore that the wheel at the other end will oscillate. $\endgroup$ – frapadingue Jul 5 '18 at 5:56
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Everything you have mentioned had played role developing Special Relativity.

Besides the fact, that measured by many experiments velocity of round - trip of light appears to be always c, other details must be considered, because this phenomena according to different theories has different explanation.

A. Einstein once noted: “Concerning the experiment of Michelson and Morley, H. A. Lorentz showed that the result obtained at least does not contradict the theory of an ether at rest.” http://www.relativitybook.com/resources/Einstein_space.html

I will express my opinion, I believe that some people may not agree.

I believe that this theory in Einstein's incarnation is a bit muddle (again, this is my opinion), though that absolutely does not mean, that formalism of this theory is wrong.

It should be noted, that Poincare and Lorentz before Einstein had mainly developed formalism of the theory. However, you will not find any reference in Einstein's 1905 paper.

Einstein in his second postulate simply says, that "light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body"

However, speed of waves on the surface of the pond does not depend on speed of your boat too, if you drop a stone into water. Though speed of waves relatively to the boat depend on the speed of the boat. That doesn’t seem very curious.

“Real” Einstein relativity with all its wonders comes when he speaks about clock synchronization. In relativity we conduct measurements by means of rods and synchronized clocks. Synchronization procedure is the essence of the theory, because outcome of your measurements would depend on synchronization procedure. Einstein proposes to synchronize two spatially separated clocks, that are at rest relatively to each other in a chosen reference frame by the same method, assuming that one way speed of light between these clock is c.

It is not correct to say, that experiments completely ruled out Ether theory. The first person, who explained null – result of MM experiment was H. A. Lorentz. According to Lorentz, measuring rod of moving in the Ether observer’s contracts in the direction of motion and his dilated clock shows not real “absolute” time, but so – called ”local time”. Lorentz’s theory predicts null result of any experiment which intends to detect absolute motion. In Lorentz theory, one way speed of light is isotropic only in absolute frame, or Ether.

Many researchers went through hundreds of different setups of experiments.

However, if you bear in mind that you clock dilated and measuring rod contracts you can predict, that these experiments would not be able to detect your motion through the Ether.

According to Lorentz, one – way speed of light in moving frame is different in different directions, we have to take into account velocity of laboratory in the Ether. However, since an observer cannot detect his own motion through the Ether, he doesn't know how to synchronize our clocks to measure one - way speed of light in our Earth. For example, he may consider himself as being "at rest" and synchronize his clocks assuming, that one way speed of light is isotropic in his frame.

Let's consider the following example: you have two clocks side by side. You and your laboratory on Earth move in Ether. The clocks are synchronized and their hands show 0. You stay are in the middle of the room and throw these clocks in opposite directions by identical cannons. They travel equal distances and stop (for example hit walls). Then you send a beam of light from one clock to the other and measure one - way speed of light. However, one clock moves a bit faster in the Ether and another moves a bit slower. Since they dilate at different magnitude, due to this effect clocks will be out of sync after they had stopped and measured by means of these clocks one- way speed of light will be exactly c.

Lorentz and Poincare deduced transformations from stationary frame into moving frame. These transformations had been named as Lorentz transformations. Good to note, that in Lorentz theory any observer can erroneously consider himself as being at rest and synchronize clocks in his frame assuming that one - way speed of light is isotropic.

Moving observer is actually "slower and shorter", than that one, who is at rest in the Ether. However, if each observer synchronizes clocks in his frame assuming that it is he who is "at rest" (one way speed of light is c, Einstein synchronization), each of them will see, that it is the "other guy” who is $\gamma$ times slower and shorter.

Lorentz theory is empirically equivalent to Special Relativity, it predicts the same outcome of all experiments and the same paradoxes and symmetry (reciprocity of Lorentz transformations). However, in Lorentz theory this symmetry is due to that reason, that all observers rule out their own motion and consider themselves being “at rest”.

Again, in Lorentz theory you clock dilate and rod contracts due to your own absolute motion, thus you cannot detect your own motion and anisotropy of light.

In Lorentz theory velocity of light as measured by any observer with a single clock “back and forth” is c in any inertial frame. In regard of Michelson - Morley experiment we have to bear in mind, that times of round - trip of light along different shoulders of interferometer in moving frame appear to be the same, since longitudinal shoulder $\gamma$ times contracts.

https://en.wikipedia.org/wiki/Lorentz_ether_theory

Einstein found a simple answer. He assumed, that absolute motion does not exist and one – way speed of light is c in all frames (every observer is at rest in his own frame) and thus, since synchronization procedure of spatially separated clocks is the same in all frames, that leads to so called relativity of simultaneity. Due to relativity of simultaneity you will always see, that it is not you, but the “the other relatively moving guy" is shorter and slower.

Some people say, that Lorentz contraction hypothesis is ad hoc. It is diffucul to understand this claim. Even in Special Relativity measuring rod that rotates around the sun contracts in the frame of the Sun. We can imagine a pinhole camera, that rotates around Sun. Due to Lorentz - contraction of the film of the camera, the Sun will appear stretched on the film. If it appeared stretched, that means the film had actually been contracted.

Einsteins hypothesis seemed to be a remedy from any headache (though creates new ones, for example in rotating frames). You needn’t think about dynamical explanations for Lorentz contraction or time dilation. You don't need to think about properties of medium. Null – result of MM experiment? Because you are at rest. All frames are "the same" and one way velocity of light in all frames is c!

So, when Einstein speaks about clock synchronization, he assumes, that one – way speed of light is c an all frames (Einstein synchronization).

Supporters of this theory rule out absolute motion. They consider Minkowski space – time as a “source” of relativistic phenomena. When it comes to explain twin paradox, they draw diagrams on a piece of paper, though in Lorentz theory resolution of the paradox is very simple. Even kids in elementary school can calculate it. Minkowski space - time, (in some sense) in turn is a consequence of relativity of simultaneity.

You can find some info about Einstein synchronization and one - way speed of light here:

https://en.wikipedia.org/wiki/One-way_speed_of_light

https://en.wikipedia.org/wiki/Einstein_synchronisation

Very good paper (though it doesn’t reveal certain details) is by M. Janssen: A Comparison between Lorentz's Ether Theory and Special Relativity in the Light of the Experiments of Trouton and Noble.

https://sites.google.com/a/umn.edu/micheljanssen/home/papers

To make conscious choice whether to accept or not to accept axiom about equality of speed of one way speed of light in all frames (Einstein synchronization) it is good to look through different phenomena through the prism of different theories.

This article – Simulation of Kinematics of Special Relativity by means of classical mechanics simulates all effects and paradoxes at turtle’s speed in a water – time dilation, length contraction, Lorentz transformation, relativistic velocity addition, twin paradox, Bell’s spaceship paradox, symmetry (barn and ladder paradox), Minkowski space time. In this model exists medium, absolute time and anisotropic velocity of information carriers in different inertial frames.

It doesn't introduce any ad-hoc hypotheses. It shows that "space - time" of moving barges is elementary mathematical construct that pertains more to the formalization of the measurement errors caused by the failure to take the presence of an aquatic environment into account than to the behavioral features of the barges on the water surface.

https://arxiv.org/abs/1201.1828

Note: there are some thought experiments (with transverse Doppler shifts due to aberration of light in relatively moving frames) that clearly show, that speed of light in different frames cannot be c, but it is another story.

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Einstein "borrowed" his constant-speed-of-light postulate from the ether theory:

Albert Einstein: "...I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz's theory of the stationary luminiferous ether..." https://en.wikipedia.org/wiki/Lorentz_ether_theory

In 1887 the Michelson-Morley experiment UNEQUIVOCALLY confirmed the variable speed of light posited by Newton's emission theory of light and refuted the constant (independent of the speed of the light source) speed of light posited by the ether theory and later adopted by Einstein as his 1905 second postulate:

John Norton: "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE." http://philsci-archive.pitt.edu/1743/2/Norton.pdf

Banesh Hoffmann, Relativity and Its Roots, p.92: "Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous." https://www.amazon.com/Relativity-Its-Roots-Banesh-Hoffmann/dp/0486406768

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    $\begingroup$ Revisionist history of science! $\endgroup$ – frapadingue Jul 1 '18 at 14:43

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