Suppose in the course of experimental physics research, scientists discover incontrovertible evidence of a particle that can accelerate from speeds less than c to speeds greater than c. This would, of course, turn decades, maybe centuries of research on its ear. How far back would physicists have to back would scientists have to go to start over? That is, what would be the last “known good” theory in which we are certain (or reasonably so) that it accurately describes the Universe and can incorporate this new phenomenon?
Well it wouldn’t be centuries since before the Special Theory of Relativity (1905) there was no physics theory which prohibited this. There was some evidence like Maxwell’s equation which had this strange velocity constant c which seemed to be frame independent and there was the Michelson-Morely experiment that could not find a variation in the speed of light that depended on the Earth’s movements.
But I don’t think your question is framed right. We couldn’t go back to Newtonian mechanics either as it doesn’t explain why muons, for example, last much longer without decaying when moving near the speed of light or why particles in accelerators get more massive just as predicted by relativity.
So even with the violation you propose, relativity would still explain somethings much better than old theories. Theorists would probably start by trying to adjust relativity in various ways realizing that it was a good approximation in many circumstances just as Newtonian physics and gravity is a very good predictor under most circumstances except for very high speeds or in very large gravity fields.
Physics doesn’t really work that way. If a phenomenon is discovered that relativity gets wrong, it doesn’t make the thousands of other things relativity got right suddenly bunk. It just shows that relativity needs to be modified to include this new situation.
According to current physics, we shouldn’t even be able to detect faster than light particles, should they exist. But suppose that we do, that means we have to step forward, not backward.
Yes, it would be more like relativity superseding Newtonian mechanics. Relativity works; there’s too much consistent evidence for it to simply be thrown out.
According to relativity FTL travel it the same as time travel; you’ll see physicists re-evaluating and tweaking it this time using the assumption that FTL/time travel is possible.
There’s actually a model that allows for different particles to have different “speed limits”, called the Standard Model Extension. A handful of physicists (disclaimer: myself included) have been working on it for some years now. (“Lorentz symmetry”, referred to in the linked article, is the principle that the speed of light is the same for all observers.) For the most part, it allows us to say things like “the maximum speed of an electron differs from the speed of a photon by no more than one part in 10[sup]17[/sup].” However, it could certainly be extended to include newly discovered particles that have a “speed limit” significantly larger than the speed of light.
A new particle that could travel “faster than light” wouldn’t, for the most part, upend all of physics — so long as the new particle itself had a “speed limit”. In this event, the maximum speed of the new particle would just become the new “speed limit of the universe”, and light would be demoted to just some particle that happened to travel less quickly than this.
What would really give physicists trouble would be if this new particle appeared to have no speed limit at all. A particle that could travel arbitrarily fast would violate something called “well-posedness”, which basically means that if I set up an experiment in some region of space, there’s some amount of time for which I can accurately predict what will happen in that region of space. Well-posedness is intimately related to determinism, which has been part of the bedrock of physics for hundreds of years now. (Determinism even survives, to a certain extent, in quantum mechanics.) A particle with no “speed limit” would require us to throw out determinism entirely, which would be a huge disruption to physics as it stands.
The first thing that physicists would do would be to see if the particle would be fitted into the idea of a tachyon, which has been considered within the theory of relativity:
They don’t. In the Standard Model Extension, everything is (in general) well-posed, and everything has a finite “speed limit”.
In the SME, it does not (in general) hold true that physics in all moving frames is equivalent. This symmetry between observers moving at different rates is the “Lorentz symmetry” that the SME is designed to violate. So in particular, the speed limits will be different for different observers.
I feel honor-bound to interject here that as of this date, there is no experimental evidence that Lorentz symmetry is anything but an exact symmetry of nature. The idea that moving observers all observe the same laws of physics is a compelling & beautiful assumption, but it’s still an assumption and needs to be experimentally tested. The SME is primarily designed to help us figure out where to look for evidence that this assumption might not be true.
But couldn’t one say that tachyons do have a “speed limit” – it just happens to bounds their speed from below? Does it matter at all whether we’re discussing tachyons or the OP’s suggested trans-luminal particle?
Well, you’d have this small problem with this definition, being that you’ll need a guaranteed infinite number of distinct particles for it to be meaningful…
No offense, MikeS, but “Standard model extension” is probably the least imaginative name I’ve seen for a physical model, and one of the least informative, too. Couldn’t you have gone with “Non-Lorentzian dynamics” or something, instead?
Don’t blame me, it’s been that way for a good 10-15 years now. I think the guy who came up with the name of the SME wishes he’d named it more snappily, because nowadays he seems to intentionally try to come up with names that are much more interesting (and sometimes a little silly.) When I was working with him, I did some work on the “bumblebee model”, for example.
