If you ask most people, most physicists and most LLM's (Large Language Models) if the one way speed of light is constant they all will say it is and that it is part of Special Relativity (SR). If you ask most, "how can that be", they will answer the contraction of space and dilation of time, but if you drill down deeper you learn that actually it isn't, it is a postulate of the 1905 paper on Special Relativity and postulate is a fancy word for an assumption that is made but not typically or explained within.
But if you drill down deeper, you find it isn't even that! The constancy of the speed of light (in each direction) is neither explained by, nor necessary for nor a postulate of the 1905 paper! What the 1905 paper DOES say is essentially two key things, both postulates (again, postulates = assumptions typically not covered in the theory being presented, the foundation of it). The first is that the speed of light is not affected by the velocity of the emitter. <Doesn't mention observers motion, The next is that the laws of physics are the same in all inertial frames. <Doesn't require the one way speed of light to be C, just the 2 way speed of light to be C in all inertial frames for that. I thought Einstein supported the idea that the one way speed of light (the speed of light in each direction) is C, however he claims no such thing in any of his writings according to chat GPT and Claude 2. The 2 way speed of light being C is most assuredly believed to be C, but the one way, if he believed in it he never seemingly mentioned it. And while I will concede that the one way (single direction) speed of light is impossible to measure if SR is correct, if LET, (Lorentz Ether Theory) is correct (which many physicists and LLM's can tell you is compatible with every experiment that is considered to support SR) then it becomes possible to measure the one way speed of light. If Einsteins model is taken as a cheat, an untrue but simplifying mechanism that makes it easier to use Lorenzian transformations without needing to worry how we are moving relative to the aether it is a success! But if we take it as the truth and even make it more extreme by believing the one way speed of light is C it becomes a comical nonsense! And we will see just how badly below. But let's see how we got here! Light, big shock, moves at a speed. And speeds are or can be seen as relative to our own inertial frame. And so the velocity of any real moving thing, even a photon is relative to your motion. And it's motion, which is also affected by the medium of either... The velocity of the thing that emitted it (seems not to be the case, and SR assets it can't be). OR the medium, the medium of that magnetizability and polarizability (The permeability and permittivity) AKA The Ether or Aether. Since we have established that Einstein never claimed the one way speed of light isn't C and didn't try to explain how it could be either as I will show soon how impossible that is, we can't have a relativistic aether that offers no preferred frame. Yes, that is essentially what he tried to create, but failed. Even if you can't know what the one way speed of light is, you can know as I will show that it can't be equal. Also: https://www.youtube.com/watch?v=pTn6Ewhb27k Why No One Has Measured The Speed Of Light - Veritasium So if we go back to the Micelson Morley experiment we see that an interferometer was used to try and find evidence of earth motion through the Aether, and this produced a generally negative result. Now as I tried to write the rest of this email I have come to a problem, I was going to explain why the Michelson Morley experiment which used an interferometer with two paths, one perpendicular and one along the earth presumed direction of motion. However in trying to explain why the number of wavelengths that fit in the two path should vary based on the axis of movement of the aetheric medium, I have found a problem, it seems that the number of wavelengths would not change even if light was speed wasn't constant! It is worth noting that the Michelson Morley experiment didn't measure light speed at all, nor would time dilation have any effect on interference fringes, only wavelength matters. It seems that the Doppler shift from super and sub-luminal light would lead to the same number of wavelengths in the round trip back to the angled plate that initially splits and then recombines the light. So while the number of wavelengths that fit in the path change for each direction it sums to the same number on the round trip! I would note that I had some weird variable answers from LLM's sometimes using the wrong Doppler shift equation so it works best if you have it manually calculate the number of waves that would fit in based on the distance and the speed of light (presuming of course a variable speed) which gives you the travel time and the frequency of light gives you the number of wavelengths. The point is that you get a null result with an interferometer even if we don't use Lorentz transformations and assume light isn't C, not even the 2 way speed of light! So while the SPEED of light of the round trip might or might or might not be constant based on motion though the aether, the Michelson Morley experiment tells us NOTHING about the movement of the Aether or the speed of light! Now, EVEN IF it did have potential to (and it DOESN'T) Lorentz contraction could be used for the null result but the Lorentz's Ether Theory is compatible with the speed of light not being constant in each direction, indeed it requires it! It only makes the 2 way speed of light constant. And so how does Lorentz contraction and time dilation work and why doesn't it make the one way speed of light C? Because if you are moving through the Aether, light that is coming towards you and hence presumed to have added velocity above that of C only becomes faster when your watch ticks a few times while it passes, and if your ruler is shorter it has less distance to go. And if somehow the speed of light were magically C in the one way sense (again, Einstein never made this claim apparently) , then the addition of Lorentz transformations only make it all superluminal again! In other words Lorentz transformations are only needed if things aren't already C, but their effect is to push things further from C with respect to the one way speed of light. Lorentz contraction makes no sense when you drill down to it. Ok, you say, so the one way speed of light isn't C in all frames, so what, Einstein / Special Relativity didn't insist it was. No, I suppose not, but if we admit that the speed of light, even just the one way speed of light isn't C then it means there IS a prefered frame, THERE IS AN AETER! And if there is a prefered frame (and if Lorentz contractions even exists which BTW the Michelson Morley experiment does NOTHING to indicate unless I and several LLM's are very mistaken) then time Dilation and Length contraction presuming they truly exist (they seem to but I'm doubting everything now) they are obviously manifested relative to the Prefered frame which MUST exist as shown, and if the one way speed of light isn't impossibly and automagically C which even Einstein and SR (originally) didn't claim and can't explain and is incompatible with Lorentz contraction and time dilation then these transformations must be based on your absolute motion through that prefered frame! And if that is the case then twin paradoxes are solved, there is no paradox in the slightest, this is good news as it is easy to create examples where the twin paradox can't be resolved with no preferred frame. But if there is a prefered frame which is responsible for the speed of light and time dilation being affected by your motion then it IS possible even if not entirely easy to measure the one way speed of light or find the frame where time dilation is zero and lengths are longest. This finds SR in a collapsed state, it's failed at everything but being a handy tool with close enough results for most things. And again, there isn't an iota of experimental evidence that favors SR over LET! So there you have it, there is an Aether, there might be Lorentz transformations but the Michelson Morley type interferometer experiments only tell us how easily Scientists can be bamboozled! I hope I have made this easy to understand and conclusive, feedback appreciated Jonathan Berry