# Can someone please tell me the problem with this method to measure one-way speed of light? It's driving me crazy!

I am having a hard time grasping the idea that one-way speed of light is immeasurable. I have watched several videos (including the Veritasium one), read the Wikipedia article, and read some questions on Stack Exchange and no one seems to proposed this exact setup that seems the most obvious way to do it to me. Can someone please point out what I'm missing here?

If I am equidistant from the source light as from the mirror light, then I KNOW how far away each light is, correct? Therefore, there seems to be two possibilities:

• The light takes the SAME time to travel both equidistant directions

OR

• The light takes DIFFERENT times to travel each equidistant direction

To test this first question, I can simply change the orientation of the source, mirror, and detector (the distances remaining the same between them each time) and run the experiment multiple times. If the time between the detector receiving photons from both light sources is THE SAME for every orientation in a complete sphere then I can conclude:

• The light travels the same speed when traveling the same distance regardless of direction (This only indicates light travels the same speed TOWARDS me, but it may be a different speed AWAY from me.)

OR

• The distortion between the two distances is coupled somehow (i.e. one shortens exactly as the other lengthens) If THIS was true, then changing the angles between the vectors (i.e. changing the height of the isosceles triangle) should produce different results because it is changing two directions but not the direction between source and mirror.

If the latter case is true then we have proven light travels different speeds in different directions and that opens a whole can of worms. (Probably not the case.)

However, if the first case is true then we have a way to measure the one-way speed of light because we can now simply subtract both travel times from the light traveling from the source as from the mirror and get a one-way speed of light between the source and mirror. (It doesn't matter that the towards and away times might still be different.)

Basic equation:

[(time observer sees reflection from mirror) - (travel time from mirror to observer)] - [(time observer sees source) - (travel time from source to observer ] = (travel time from source to mirror)

(t_om - t_mo) - (t_os - t_so) = t_om - t_os = t_sm

When  t_so = t_mo

s = source,     m = mirror,     o = observer/detector


Once you have the travel time from the source to the mirror, simply measure the distance from the source to the mirror and you have your one-way speed!

What am I missing?

(See diagram of setup below.)

• Not sure, thus making it a comment not an answer. SO and MO have components (considering them to be vectors) along the direction of SM. These components have opposite sign and thus possibly different velocities and you are back at the initial problem. Jun 16, 2021 at 8:51
• If you travel from Paris to London then the local time on arrival is only a little more than the local time where you set out. If you travel in the other direction then there is a bigger time difference. It is no use measuring speeds to detect this effect. It is merely a result of the convention of setting clocks in different countries. The "one way speed of light" idea is the same; it has been mis-named. Jun 16, 2021 at 9:10