If we do not really 100% know how light works as a oscillation we also do not know is its speed indeed constant no matter is there a space or space-time motion that can affect it. Could a device that is just a very long evacuated tube that at side A has a laser emitter and at side B a mirror that reflects the laser light back towards A help in that our uncertainity? So if we could somehow measure the timelapse between laser emition and reflected laser beam detection at A, is than possible find out if gravity affects its speed? Let say we position this tube vertically so that the possible space operator on the speed of that beam light can be mathematically solved by our assumption that the first part of light journey gravity as space or spacetime motion will slow down the beam speed but when the beam hits the mirror and reflects down back to A the space motion {even frame dragging may have something common with gravity as space or spacetime motion} will increase the beam speed. But as far as I know this number differs from t+t=2t and something is lost... In the case of the horizontal position of the tube the two times are equal so the total time is indeed t+t=2t...So if in the vertical case the total time between emition and detection is a number that slightly differs from 2t and in the horizontal case the total time is precisely t+t=2t than is it right think that we can grasp that the light beam has no same speed in both cases {V and H}? Of course my question takes in account that the MM experiment showed that the motion of Earth could not in any case affect the same beam as possibly gravity would...As most people can easly deduce a gravitational frequency shift can be obtained in two ways... 1)by inverse wavelenght shift or 2)by speed of radiated waves shift.. But for more details please read my comment in the first answer 'comment area' ... Thanks.I am sadly pretty sure that these kind of questions force most users for an easy downvote but I am asking this not because I have no any information about the problem but because although science can explain most facts about how our universe works there are problems that GR cannot explain like the counterintuitive effect of perihel event horizon stretching instead shrinking as it is in the case of a black holes binary system....
-
1$\begingroup$ The speed of light is constant. $\endgroup$– LolloBoldoCommented Jul 23, 2023 at 7:28
-
$\begingroup$ @LolloBoldo My question to You is are you 100% sure? Just think about frame dragging that affects Mercury orbit making it preccess... Why then the same should not happen with light? $\endgroup$– Krešimir BradvicaCommented Jul 23, 2023 at 7:54
-
1$\begingroup$ For the same reason why in special relativity if you shoot a laser standing still the light moves at $c$ and if you shoot a laser moving at $2c/3$ then the light still moves at $c$ $\endgroup$– LolloBoldoCommented Jul 23, 2023 at 9:41
-
$\begingroup$ @LolloBoldo It is more correct to say that the speed of light is experimentally constant to high precision. The real problem here is that the constancy of the speed of light is a postulate of general relativity. You cannot use mathematics to reason about a violation of its postulates. $\endgroup$– John DotyCommented Jul 23, 2023 at 13:21
-
$\begingroup$ @JohnDoty yes, i mean experimentally is constant, we even know from the Planck data that Lorentz invariance, ie a constant speed of light for all inertial observers, holds down to the Planck scale, so i feel pretty safe saying that $c$ is actually a constant and not a function of the metric $\endgroup$– LolloBoldoCommented Jul 23, 2023 at 16:13
2 Answers
You are basically talking about the "Shapiro delay". The fact that gravity affects the apparent light speed is measured routinely when bouncing signals from reflectors on the Moon, bouncing radar signals from Solar System bodies (Shapiro 1971) or when communicating with spacecraft in the Solar System (sending signals to a distant satellite and receiving a ping back; Bertotti et al. 2003), where General Relativistic effects must be accounted for.
The speed of light measured locally is always $c$, and that's all that GR ever claims.
If we do not really 100% know how light works as a oscillation we also do not know is its speed indeed constant no matter is there a space or space-time motion that can affect it.
We do really 100% know how light works. By this I mean, we have theories of how light works that correctly explain 100% of the observed behavior of electromagnetic radiation.
As mentioned by @ProfRob, what you describe is called the Shapiro delay. It is a prediction of general relativity and has been observed as predicted. Being a prediction of general relativity it is based on the fact that in any inertial frame the speed of light is $c$.
In the presence of tidal gravity there are no global inertial frames and so the speed of light being $c$ becomes a local principle. In curved spacetime the extra time can be attributed to curvature, which makes the distance different.
Similarly with your proposal. No material can be infinitely rigid. So the length of the tube can change. A variation in the time would thus be correctly understood as a change in the length of the tube.
-
$\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Physics Meta, or in Physics Chat. Comments continuing discussion may be removed. $\endgroup$– Buzz ♦Commented Jul 24, 2023 at 2:21