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The speed of light, c, is the same for all inertial observers but no one has ever actually measured it in one direction. So taking an normal emitter, reflector, detector setup, can we do the following? Make the first measurement, and log the time. Put a medium that allows light through but "slows" it down slightly, put in front of the emitter side, take another measurement. Then take that same medium and put it in front of the detector side, take the last measurement, and compare. Test a should give us the same we always get. If light moves the same velocity in every direction, test b and c should give the same result, if not then they will be different, meaning light changes velocity depending on the direction it is going.

If I am wrong, please tell me what is wrong and why. What are your thoughts on trying to measure the one-way speed of light, and do you think it is possible at all?

benrg
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Mephistophilus
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If the one way speed of light in vacuum varies in such a way that the 2 way speed of light is c, then the one way speed of light in a medium will necessarily vary in such a way that this measurement will get a null result.

The one way speed of light is not a physical fact. It is a convention. We choose the one way speed of light by choosing our synchronization convention. Choosing that convention sets both the one way speed of light in vacuum and in media.

Dale
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To expand the explanation of Dale's answer a little: To measure a one-directional speed of anything you need to be able to define what 'at the same time' means between two locations because you need to agree what the start time was in order to measure a duration. Unfortunately, in a relativistic universe, there is no such thing as 'at the same time' between two locations.

Of course you could synchronise two atomic clocks and move one of them to a second location, however as soon as it is moved any concept of simultaneity is gone. A difference in velocity causes a difference in time dilation and the difference in the observed time dilation between these two clocks will depend on the observers frame of reference. It would not even be possible to agree if one clock were ahead or behind the other.

It is not an easy concept to accept that the one thing we take as a fixed constant, the speed of light in any given direction, is not actually directly measurable, however that is because all other frames of reference are relative so there is nothing to measure it against.

Euan Smith
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There is no way to measure one way speed of light using clock/ruler method, relativity don't allow this, so we need search for solutions in area where relativity does not apply - quantum universe ...

Imagine an hypothetical experiment, where we build hadron collider, in universe space, that is able to turn and hold whatever stable orientation of collision against observable universe. We setup system to have a same constant energy for each collision in one system direction, for some longer period ... Then we change direction of collision for same period and maybe we see some differences.

For example, if we count how many Higs boson we saw each direction/period, or we deeper study collisions where time symmetry breaks...

All our hadron colliders are bounded to Earth, and Earth spins, moves ... But they already produced tons of data, so maybe there are some markings or dependencies that points to at least direction of velocity against universe...

Michal Steller
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