The Apollo moon missions all had extended radio conversations which took place at the speed of light. Those conversations were recorded in multiple places: the transmissions from Earth & the received broadcasts from the Moon were recorded on Earth, while the transmissions & crew cabin conversations were recorded onboard the command & lunar modules (in addition to received broadcasts from Earth. As such, the delays between the receipt of a message requiring a response are recorded from both perspectives. Since the progress of time was recorded from both perspectives, if the recordings are played against one another on Earth (as they have been), does this not show that there is a predictable, consistent lag due specifically to the one-way travel time of light?

Imagine the astronauts receive a transmission from Earth requesting a particular data point from a particular piece of equipment. The message is received, understood, but requires clarification. After a certain delay the astronauts transmit the request for clarification. [INSERT UNKNOWN LIGHT DELAY HERE] The folks on the ground receive the clarification request, then after a certain delay during which the question is understood & a response formulated, the clarification is transmitted back to the Moon. [INSERT UNKNOWN LIGHT DELAY HERE] The astronauts receive the clarification, and after a brief delay in understanding the clarification, followed by another minor delay in seeking the requested data point, they transmit the requested data to Earth. [INSERT UNKNOWN LIGHT DELAY HERE] The people on Earth hear the message, there’s a delay in understanding & processing the message, then a response is sent confirming the receipt of the message. [INSERT UNKNOWN LIGHT DELAY HERE] Astronauts receive the confirmation of the receipt of data (end scene).

Normally, we’d be talking about how to synchronize the clocks of Earth & Moon personnel from a relativistic standpoint, and conceding that this couldn’t be done objectively. However, we have both sides of the conversation, recorded from each sides’ respective frame of reference & thus frozen in that respective frame. Further we know that both reference frames were at least nominally synchronized at launch. Thus, if we play them both back continuously from that point we should be able to tell how far the two recorded timelines diverged over the course of the mission by how the time lags at each end change… shouldn’t we? In other words, each recording done from its own respective reference frame will show how each delay appeared to the people in that same frame; if there were any measurable difference in either the length of each delay (from either perspective), would it not show up? And once we see how these two timelines match up, should we not then be able to see whether the speed of light was measurably different going from Earth-Moon vs from Moon-Earth?

My other question is about light which has no return path. For example, photons from a star which hit a detector on Earth (your eye) would not be reflected back, thus they only traveled in one direction. Isn’t it necessary at that point to assume this light’s speed to have been c, since there’s no return path and no possibility to average the forward and return speeds to arrive at the required c?
Or is this again an application of the anisotropic principle and the light could travel at different speeds at various points along the path? But even if it travels at different speeds through different parts of spacetime in between the source and destination, wouldn’t the average speed in one direction still need to equal c?

Again thanks for taking the time to field my silly questions.

Best,
Ryan

Having always assumed that the speed of light is isotropic, I have just discovered and begun to learn about the anisotropic synchrony convention; thus I found your article and others. It is stated repeatedly that the measurement of one-directional light is impossible due to relativity, but my question is this: is it possible show with some confidence that light never travels at infinite speed?

As an example: at the end of Cassini’s mission, NASA listed the one-way signal time as 85 minutes. I am assuming this meant 170 minutes were required for a full round-trip communication (send/acknowledge) sequence to complete.

It would seem that if the speed of light is infinite in one direction (to the “observer”), then the Cassini probe would have observed the signal from Earth instantly (from its perspective), and immediately responded with a return signal. Then on Earth, being the “observer” of the signal originating from Cassini, we would have instantly seen the response (i.e. no time delay from Send to Acknowledge on Earth).

However, this was not the case, as 170+ minutes delay was imposed after sending a message for a response to be received from the probe.

Could this time delay be used to confirm that neither light traveling toward or away from an observer is traveling at infinite speed? (It would not confirm a uniform speed, but would rule out infinite speed?) Or does relativity still render this type of measurement impossible? And in such an example, does it make any difference that the reply is NOT reflected light, but rather a generated response from the probe itself? (Bouncing light off a mirror would be reflected; however, sending a radio signal from an antenna would be generated.)

I’m sure I’m misunderstanding or missing several points, but I’d really appreciate your thoughts on this to help me understand more clearly.

Thank you!

Technically you didn’t want me to vet your work, you wanted to play the game of having me explain science to you while you continually shift the goalpost. We can play that game, but I have an hourly rate.

Regarding my second point on special relativity, you state, “we don’t need to assume isotropic light speed.” I am contending that it is necessary to assume isotropic light speed since the Lorentz transformations predict light speed isotropy (this is easily shown by differentiation) and therefore would be inconsistent.if light speed were anisotropic. Therefore any finding of one-way light speed anisotropy would be problematic for special relativity.

I would welcome your further comments.

Keep in mind this post is not arguing that relativity is somehow wrong, or that the speed of light isn’t isotropic. The point is that experiments have limitations, and its useful to know what are assumptions of a model and what are confirmed results. Just in terms of special relativity, we don’t need to assume isotropic light speed, though it is useful to do so. There is also no experimental or theoretical reason to presume light isn’t isotropic.