# Why can't we measure the one-way speed of light?

I believe the problem is because we need to synchronize clocks. But ok let's assume we can't do that.

Can't we just use some external reference - Eg The Sun & Solar Noon?

For example given the Solar Noon happens at the same moment along a Meridian.If we have:

------Sender A--------------Middle Receiver--------------Sender B------

--- = The Maridian.

If we were able to accuratly determine the solar noon on any given day & both senders & the Receiver had the ability to determine this. It would give us a time of reference with which to measure the one-way speed of light would it not?

In theory that would work right as-long as we could accurately get the exact moment of solar noon.

Second part of my question, if the results are the same in multiple directions (which assumes we were able to get multiple sun like inputs at 90 degrees to the experiment) am I correct in thinking that would prove 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.

I think my question is different to the one linked, because I in theory wouldn't need the clock.

• Possible duplicate of Is there any way to measure the one-way speed of light? Aug 14, 2019 at 6:51
• Comments are not for extended discussion; this conversation has been moved to chat. Aug 14, 2019 at 17:25
• Romer measured the speed of light in one direction, en.wikipedia.org/wiki/… Admittedly it wasn't very accurate, but you can measure the speed of light in one direction.
– jim
Apr 26, 2021 at 19:07

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?

• "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." This is true to some degree the angle with be slightly different, so in theory they may not trigger at exactyl the same time. However since the sun is so far away the difference in angle between the light from sun -> point A & sun -> point B will be minuscule. It's essentially the same direction. Aug 14, 2019 at 12:00
• I would assume that the one way speed of light, be the same if it's travelling in very roughly the same direction though the same material. So the fact it may change shouldn't matter. Thanks for the rest of your response, it may take me a few days to absorb it. Aug 14, 2019 at 12:04
• Distance does not play any role, it can be one light year, one mile or one yard, that doesn't matter. It still takes some time for the light to reach you, so you cannot know how exactly old the image you see if you assume that one way speed of light may be different from c. Jammer's book considers many, many ways to measure one - way speed of light. There are much more interesting ones, for example Double Fizeau toothed wheel, but even these methods do not allow to measure one way speed of light. I believe that "Simulation of SR by the methods of classical physics" gives some good info. Aug 14, 2019 at 12:13
• The point is that I don't need to know exactly how old the image of the sun is. I just need to know that it's the same image at the same time. If the two points are the same distance from the sun in the same direction. Then if the 2 points fire the laser at a point directly in-between the two points at time where they both see the sun at 45 degrees then if the lasers don't each the middle point at exactly the same time we know that the speed of light was different in the different directions. Essentially this uses the earth as the clock. The light from the sun is the clocks hand. Aug 14, 2019 at 12:19
• Also if we enable the middle "reciever" point to also see when the sun is at 45 degrees then we could measure the speed not just if one direction was faster than the other. Other then the problem of detecting when this sun is at exactly at 45 degress is there any other issue with this? Aug 14, 2019 at 12:23