I was blown away when I heard about this. I was wondering if the theory of relativity took into account something like this? What would have been the scientific reaction if we could go back in the Wayback machine and give that era of scientists a demo?
Relativity talks about the maximum speed of light - it doesn’t say anything about the minimum speed. It’s perfectly well known that light only travels at exactly C in a perfect vacuum.
Einsteinian relativity concerns itself with a speed “c”, which is also the speed light travels at through empty space. But other things travel at c through empty space also, including all other kinds of electromagnetic radiation, and gravity waves. To take a step backwards into the deeper principles, c is the limiting speed for information to travel, or equivalently the limiting speed for causation. A cause and its effect that are in different locations must be linked by a transport of information at a speed less than c, and perhaps there is no more insightful statement of the meaning of c. It is unfortunate that the history and explanations of his relativity are so tied up with the phrase “speed of light”, because when “c” and “speed of light” are different, only “c” is of special importance in Einstein’s theories.
The OP might also be interested in reading about Cherenkov radiation.
Nitpick: Electromagnetic radiation is light.
I have no idea if it’s related but my brother has been, for the last few years working on a project similar to that at UW-Madison. I have no idea about the details of his project other then that they were trying to slow down/stop light and then start it again. His project is being funded by the telecommunications industry. Something about being able to cram even more information into a fiber optic line if they can control the flow of light.
Double nitpick: Light is EMR, not all EMR is light.
It’s been well-known since at least the 17th century that light waves travel at different speeds in different media — witness Fermat’s “principle of least time” derivation of Snell’s Law, which can’t really be taken seriously if you don’t think that light travels more slowly in glass than in air.
Triple nitpick: relative to light, “all other kinds of electromagnetic radiation” does not include light. That’s what the “other” was there for.
Anybody want to go for a home run?
Home Run: I was nitpicking the nitpick, not you.
Don’t think that counts - I knew you were, and I was nitpicking you, and the chain was up to three. Now I don’t know if it’s five, or still three, or broken, or what. That is, if we start trading “Naw aw” and “Yaw haw”, the count isn’t still climbing.
Moerover, I don’t know enough about baseball to provide further relevant metaphors. Do you get to go back to third base for any reason?
We could start a thread about relevance instead of relativity, I suppose.
I’ll try.
Home run nitpick: We predict that gravity waves travel at c, but they’ve never yet been measured. Or am I wrong?
Correct - we’ve never measured them directly (we’ve never measured gravity waves at all, directly) . However, there was an observation (of a transit of Jupiter as I recall) that measured the speed of propagation of a change in position of a gravitating body, and the error bounds for that test were consistent with the speed of gravity waves being c. Also, decade long observations of binary pulsars show orbital decay also consistent with gravity waves travelling at c.
Here are references: Jupiter transit Gravity Speed Measured
Binary pulsar Binary pulsar - Wikipedia
That measurement was consistent with a whole lot of things, actually. The derivation of the speed of gravity from that observation was dependent on general relativity, but then, general relativity all by itself is enough to predict the speed of gravity without observations. In other words, that and a quarter will get you a phone call, as my grandpap used to say.
Fair enough - but the binary pulsar stuff still stands as evidence for gravity waves at c.
Ah! Yes, that sure sounds like a home run to me!
The light itself isn’t actually traveling slower though is it? It’s just being absorbed and reemitted or bounced around on a less than straight path. That’s like saying a car traveling 60 mph that takes a curvy road is slower than a car traveling 60mph that takes a straight road to the same place.
A car that stops transfers it energy somewhere. It just can’t get it back.