# Could we measure the one-way speed of light with two connected rotating apertures? [duplicate]

Can we measure the one-way speed of light by passing light through angularly shifted apertures in two disks solidly connected and spinning at a known angular velocity?

Here is the apparatus I would make to measure the speed of light in one direction. (I will use real numbers and units for dimensions to make the important parts easy to visualize.)

I would machine a solid 1 meter-long shaft, let's say with a 10 mm radius from, let’s say, steel. At each end, I would widen the shaft to a radius of 20 mm for a length of 1 mm forming a solid 1-m long steel spool with the 10 mm smaller radius solid rod connecting the two 20-mm radius 1-mm thick disks at each end of this spool-like object. This being one solid piece, when the spool rotates around its axis, the solidly connected disks at both ends rotate perfectly synchronized at the same angular velocity.

Then I would drill a 1 mm hole through one of the disks in the axial direction (parallel with the shaft axis), let's call it disc A, at a 15 mm radius of the disc (halfway between the radius of the solid connecting shaft and the edge of the disk radius).

Then I would suspend the spool/shaft horizontally on some sort of bearings on top of a bench. I would affix a laser to the bench and point it so that its collimated light beam would shine from the outer side of the spool disk A through the 1-mm hole parallel along the axis of the 10-mm shaft until it hit the inner side of the disk at the other end of the shaft, let’s call that one disk B.

Looking in the direction of the laser light (from disk A to B), I would align the laser so that it would hit the inner side of disk B at the same 15-mm radius (the laser beam parallel with the shaft's axis). Now I would drill a 1-mm hole through disk B at a radius of 15 mm, but at an angle of 45 degrees counter-clockwise from the point where the laser light was hitting disk B. I would place a white surface, for example, a piece of paper, behind the outer side of disk B perpendicular to the disk axis.

Now I would darken the room and use some super high-speed motor to spin up the shaft in the clockwise direction. I would increase the speed until the laser beam light (or tiny pulses of it passing through the hole in disk A, which would be completely blocked by disk B at zero or low speed) would reach the white screen behind disc B through the hole in disk B.

Once the shaft with the discs at both ends would attain such speed that some of the laser beam light entering the path along the axis of the disks through the hole in disk A and exiting the path through the hole in disc B would end up on the white screen behind disk B, I would use the angular velocity of the rotating disks and the distance the light traversed between the disks to calculate the speed of the light in direction A to B.

I could then rotate the entire apparatus including the laser 180 degrees to reverse the direction of the laser beam and repeat the measurement and the speed of light in the reverse direction. Or just to prove that the speed of light is the same in both directions, I could employ two laser beams, one pointed through disk A to disk B, and the other pointed in the opposite direction, through disk B to disk A. The B-to-A laser would have a set of its two holes in the disks drilled in such a way that the hole in disk A would be 45 degrees lagging (counterclockwise, still looking from A to B) from the hole in disk B.

Then if the speed of light is equal in both directions, the light from the laser at disk A would appear behind disk B at the same time (same angular velocity) as the light from the laser at disk B would appear behind disk A.

Let me know where this idea breaks down because I am sure others would have thought of it if it was at least theoretically workable.

• There is no possible experiment that can determine the one way speed of light because this indeterminism is central to how the Special Theory of Relativity works, or rather, how Minkowski spacetime works. Lorentz transformations are exactly a slowing down of speed of light in one direction and speeding up in the opposite direction. Commented Mar 4 at 9:19
• Does this answer your question? Can One-Way Speed of Light be Instantaneous?
– Dale
Commented Mar 4 at 12:00
• Your device is sometimes called a "Marinov Bar" after Stefan Marinov: en.wikipedia.org/wiki/Stefan_Marinov. Commented Mar 4 at 13:43