# One Way Speed of Light a Convention?

To synchronize two clocks that are stationary relative to each other, we send a time stamped light (or radio) signal from the master clock to the secondary clock. Knowing the distance between the two clocks and assuming that light travels at exactly c between the two clocks, the propagation delay is calculated. This delay is then added to the time stamp at the moment that the signal is received and the secondary clock is set accordingly.

As an example, say we have two spaceships at rest relative to each other, separated by exactly one light second in free space. At time 12:00:00 the mother ships sends a light signal time stamped 12:00:00 to the sister ship. The sister ship receives the signal, adds 1 second to the time stamp and sets its clock to 12:00:01.

If we now want to measure the one way speed of light by sending a signal between the two ships, we must obviously get c, what else? My question: is there a way to measure the one-way speed of light without needing two clocks that were synchronized by assuming the speed of light to be c?

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### 41 thoughts on “One Way Speed of Light a Convention?”

1. Con:

There’s only one problem with your approach (especially as one attempts greater accuracy and/or goes to higher velocities): The measurement is model dependent.

Now, that’s not, necessarily, a fatal flaw. However, since the most likely reason for wanting to determine a “one way” speed of light is in order to test our model of what’s going on, this probably greatly diminishes the usefulness of this approach.

David

2. Dr. Halliday

Thanks, that was just a quickie to show feasibility based on red shift measurements. It could use some improvements. Fixing the detector and moving the source would eliminate small v/c problems in detection. Source movement would require great precision and the source would need to be of very high Q and long term stability in order to obtain reasonable results. Also an adequate number of samples would be required to determine statistics.
On the other hand as within the observers frame of reference the arrow of time always points away, rulers does not change due to change of direction and direction in Maxwellâ€™s equations is the direction of propagation one would need to invoke absolute motion to see a difference. Now if rotation is added, like in a laser ring gyro, all bets are off.

Aside
I have been reading your posts with Jim Arnold. You are indeed a man of great patience and persistence. Seems like he wants space to be deformed around a mass but rigid at the same time. Good luckâ€¦..Con

3. Actually one way measurement is possible in the observers frame of reference. At low velocities, say 300 meters/s or less, the doppler shift for light is approximated (to better than 0.01%) by z=v/c. For relativistic velocities the Lorentz correction “gamma” must be used. A fixed light source and a moving detector is required. Then c = v/z. Keeping v and z in meters then c is in meters.

c=velocity of light
v=velocity of detector or source
z=doppler velocity

Con Morton

4. David, thank you for this. I think our other strand is progressing, and I prefer it for now. Christopher