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**Interesting fact of the day**: The speed of light has only been proven to be a constant speed **round trip**. Not the instantaneous speed of light nor the speed of light while heading in a single direction has ever been able to be proven as constant.
In other words, it has never been experimentally disproved that light doesn't, for example, preferentially travel at half the speed of light in one orientation, but instantaneous in another. In fact it may very well be impossible to test the one-way speed of light due to the very consequences of relativity itself.
@freemo @Science This is not correct according to my understanding of physics. Specifically addressing the claim that it is not testable: you could have distance separated sensors send light to each other and see how long it takes. If it takes longer in one direction than the other than the theory that the speed of light is constant would be falsified. Maybe I'm not understanding something, but I believe your claim is false. If I am missing something I'd love to know what.
Yes you are misunderstanding the fundemental problem.. if you do this experiment as you propose it would require both sensors to have synchronized clocks with very high accuracy correct? Synchronizing the clocks would be impossible if the speed of light is asymmetrical without knowing what that asymmetry is ahead of time. Worse yet, the error in synchronizing those clocks due to the asymmetry would be exactly enough to cause the experiment to incorrectly show that the speed of light is the same in both directions.
Ergo, your experiment would not be able to detect asymmetry in the speed of light if it existed.
@freemo @Science No, the clocks are synchronized first then they are separated. For example one stays on Earth and one is sent to Mars. I understand that relativity will result in time dilation for whichever sensor accelerates to create the distance, but that can be accounted for with a Lorentz transformation. You still end up with two distance separated sensors that know what the other sensor's clock reads. Is there something I'm missing? Thanks for the response!
Yup you are still missing something.
When you synchronize the clocks and then separate them the act of separating them causes time dilation to occur on the clocks and thus desynchronizes them. Time dilation itself is dependent on the speed of light so if the speed of light is different in different directions then so too will be the effects of time dilation.
So if you synchronize the clocks first and separate them they will be out of sync and you will be unable to compensate for how much out sync they are at any one moment without knowing the extent of the speed of light asymmetry int he first place..
So with your proposed solution of synching the clocks first you haven't, as you can see, avoided the problem at all since the act of moving them breaks synchronization. And your lorentz transformation depends and requires symmetric values of C or to know the specific asymmetry up front to do the synchronization. So you are still shit out of luck :)
I am not mistaken, this is well established cannon in the scientific community regarding the speed of light, this isnt something I made up. There is a whole wikipedia article on it. To quote the wikipedia article linked below:
When using the term 'the speed of light' it is sometimes necessary to make the distinction between its one-way speed and its two-way speed. The "one-way" speed of light, from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again. Albert Einstein chose a synchronization convention (see Einstein synchronization) that made the one-way speed equal to the two-way speed. The constancy of the one-way speed in any given inertial frame is the basis of his special theory of relativity, although all experimentally verifiable predictions of this theory do not depend on that convention.[1][2]
https://en.wikipedia.org/wiki/One-way_speed_of_light
@3ammo By the way the above quote from wikipedia reinforces/agrees with what ive been saying, that the one-way speed of light as a constant is a mere convention and not a postulate or theory about any reality, as one-way speed of light is not measurable.
@freemo @3ammo @Science Oh, thanks so much for linking that wiki page! It's super interesting and I think at this point I'm revising my understanding considerably. I think your OP was accurate. However, I also think that in the wiki article "clock synchronization scheme" is doing a lot of work. Yes, you can come up with a synchronization scheme like ε-synchronization that makes it so c is dependent on direction and all observations are identical to the traditional scheme where c is independent.
Now your hitting on the crux of the problem. The problem comes from the inehrent fact that clock synchronization is not about actually synchronizing clocks in any reality... it is about creating a convention which when applied to clocks will give you consistent experimental results. Thats the key in all this. When we synchronize clocks in the typical einstein convention for it the result is such that experiments will give consistent results regardless of if c is symmetric or asymmetric, so the problem becomes moot.
Also someone recommended this amazing video shortly after my original post.. It says a lot of the same things I have said (though I hadnt seen the video before, its just these are common thought experiments reused in the scientific community anyway)... It explains everything i have been trying to explain with some visualizations:
@freemo If I'm understanding it right (not likely since I didn't understand your OP right) then it's just a matter of which scheme you choose and there's no reason to choose one or the other since they make the same predictions. I guess that's what you meant by "In fact it may very well be impossible to test the one-way speed of light due to the very consequences of relativity itself"
@seroom There are many schemes that would all result in consistent clock synchronization.. in fact as long as your scheme preserves two-way light speed and breaks one-way light speed (or doesn't break it) you will have a consistent and equivalent clock synchronization scheme that works.. and thats the key here. Now if there is some reality and one-way c has some asymmetry, or symmetry, then only one of the many schemes may result in clocks truly being synchronized (though this is impossible to know or ever do).. but the point is any of the infinite schemes that picks arbitrary asymmetries for c will result in clocks that will give meaningful consistent experimental results, so we really don't have to care about "true synchronization" which is unknowable anyway.
Did you see my earlier example where i showed two scenarios, one with symmetric c and the other asymmetric c and both using a time synchronization approach that assumes symmetric c, and how both scenarios in both universes give the apparent same results? Thats an important one.
@freemo yes, I think I've got a grasp on it now. I really do appreciate your responses. I do want to point out that ε-synchronization indicates that slow separated clocks time dilate. I think the counterintuitiveness of this is a legitimate reason to prefer constant c.
@seroom to me slow moving clocks dilating makes a lot of sense.. the slower you move a clock the less it dilates, but the longer it needs to stay in motion. So it will in the end dilate the same regardless of how fast you move it for the same distance.
That said if we are just talking about preference in terms of what we assume with our equations, then constant c should be prefered for no other reason than it produces the simplest model and gives the same results as more complex models where c is not one-way constant. The simplest model that gives correct results should always be preferred. But just because its preferred doesnt mean it dictates reality :)
@freemo @Science From that quote, it sounds like this has been experimented and it's at least very very close to the same. Also the Lorentz transformation results in minuscule time dilation on Martian rovers so even if you say the clocks are off by the full factor of the Lorentz transform, the uncertainty that introduces is not very much. Thanks for responding!
See my other link where it explicitly states one-way speed of light has not been tested:
@freemo @Science I continue to believe you are mistaken. From Wikipedia "the speed of light is isotropic, meaning that it has the same value regardless of the direction in which it is measured. Observations of the emissions from nuclear energy levels as a function of the orientation of the emitting nuclei in a magnetic field (see Hughes–Drever experiment), and of rotating optical resonators (see Resonator experiments) have put stringent limits on the possible two-way anisotropy."