Why is there nothing faster than light?

[Thalita Luna]

Hi guys! Is there a simple explanation to describe why light owns the faster speed at the universe ?

 

[Dale]

Hi guys! Is there a simple explanation to describe why light owns the faster speed at the universe ?

Yes. It has 0 mass. Nothing is faster because nothing has less than 0 mass.

 

[DaveC426913]

Most of the universe's toys travel at the speed of light: all EMR, gravitational waves and other fundamental forces, for example.

A more insightful question might be: why do things with mass travel slower than the speed of light?

 

[Ibix]

Hi guys! Is there a simple explanation to describe why light owns the faster speed at the universe ?

I would say yes or no, depending on exactly what question you want answering.

@Dale's answer is correct: within relativity, anything without mass travels at $c$; anything with mass travels slower. If you try to describe something travelling faster than light it turns out to have an imaginary mass, but I gather that more careful modelling of imaginary mass fields shows they couldn't propagate faster than light either. So on one level, that's it: light is the fastest thing, travelling at $c$, because it's massless.

But you can ask why that should be. Why are the laws of physics such that there is a maximum speed? I don't think there's an answer to that yet. It's a direct consequence of the postulates of relativity that you cannot exceed light speed, which is why you cannot do it in our mathematical models. But why those postulates lead to accurate models of reality, we do not know.

 

[DaveE]

You may like this popular science video:

 

[Vanadium 50]

I think we are overstating the case.

Let's separate theoretical from observational objection.

Observational: we have never seen a tachyon. Therefore they must be rare, interact only feebly with ordinary matter, or most likely both. "Do not exist" is a limiting case of "rare".

Theoretical:
(1) SR, which was deveoped entirely by observations with v <= c, struggles with tachyons.
(2) QM, which was deveoped entirely by observations with v <= c, struggles with tachyons.
(3) It is often said that tachyons have imaginary mass. This is a common choice, but a choice. One could also have real mass and imaginary energy and/or momentum. In many ways that's worse - it's even less predictive, But this all assumes plugging in v > c into equations derived for v < c, and we know that procedure doesn't even work for v = c.
(4) In any theory involving spacetime, like SR, tachyons permit signalling into the past under certain circumstances. This is usually regarded as a fatal problem, although GR suffers from the same problem (Tipler cylinders) and people are OK with that.

So they are not needed to explain any observation, and have multiple theoretical problems. I think the theoretical problems are all bad, but any one of them can be potentially worked around, All four? Seems unlikely, but these are theoretical objections.

Should unambiguous and unequivocal evidence for tachyons be discovered, you can bet the next day there would be fifty papers on the arXiv saying "I knew it all along!"

 

[Nugatory]

This is usually regarded as a fatal problem, although GR suffers from the same problem (Tipler cylinders) and people are OK with that.

I expect that most, possibly all, of the people who are OK with that are taking the position that any solution of the EFE that allows for signalling into the past is unphysical.

 

[Vanadium 50]

And techyons signaling into the past might require a set of unphysical initial conditions. We don;t know, and with no observation and no real theory we are unlikely to.

I am working on a paper now. I don't want to discuss the details before it is published,, but I can tell you it is possible to cook up a model without CTCs and that these tachyons have a signature that has not really been looked for - a dedicated experiment can push the limits way, way down. Stay tuned.

 

[PAllen]

And techyons signaling into the past might require a set of unphysical initial conditions. We don;t know, and with no observation and no real theory we are unlikely to.

I am working on a paper now. I don't want to discuss the details before it is published,, but I can tell you it is possible to cook up a model without CTCs and that these tachyons have a signature that has not really been looked for - a dedicated experiment can push the limits way, way down. Stay tuned.

Well, it is easy to have tachyons that never allow CTCs or tachyon anti-telephone, in SR. Just posit that there exists a preferred inertial frame in which all tachyon trajectories must move forward in (coordinate) time. Many people (me included) reject this on the grounds that a preferred frame is at odds with the principle of relativity.

I suspect that a tachyon theory without CTC must have a preferred frame with the above features, but I'll await your paper.

 

[PeterDonis]

tachyons that never allow CTCs

Actually, the causality violations in tachyon models do not involve CTCs, because tachyon worldlines are spacelike. The causality violations come from allowing causal connections to occur along spacelike curves as well as timelike or null curves. That is really the root problem.

 

[PAllen]

Actually, the causality violations in tachyon models do not involve CTCs, because tachyon worldlines are spacelike. The causality violations come from allowing causal connections to occur along spacelike curves as well as timelike or null curves. That is really the root problem.

Ah, yes. What you have are closed causal curves (e.g. two spacelike sides and a timelike side forming a causal loop). And the preferred frame restriction prevents such a construction from occurring.

 

[Demystifier]

I am working on a paper now.

Have in mind that tachyon fields (as opposed to tachyon point particles) do not propagate faster than light. See the paper https://journals.aps.org/prd/abstract/10.1103/PhysRevD.18.3610
and also posts
https://www.physicsforums.com/threads/do-scientists-believe-in-tachyons.827961/post-5215601
https://www.physicsforums.com/threads/do-scientists-believe-in-tachyons.827961/post-5216458
https://www.physicsforums.com/threads/do-scientists-believe-in-tachyons.827961/post-5217560

 

[vanhees71]

This seems to be for "free tachyon" fields only. It's well known, that with free tachyons there are no problems with causality etc. The trouble starts when trying to make them interacting to be observable in our world of the "Standard Model". Then you rather get massive particles as excitations around a minimum of the effective (quantum) action with a non-vanishing VEV of the field.

 

[Demystifier]

Then you rather get massive particles as excitations around a minimum of the effective (quantum) action with a non-vanishing VEV of the field.

Yes, that's called tachyon condensation. https://en.wikipedia.org/wiki/Tachyon_condensation

 

[vanhees71]

This is not Tachyon condensation but just the usual case of a massive particle, described by a field with a non-vanishing vacuum expectation value.

 

[Demystifier]

This is not Tachyon condensation but just the usual case of a massive particle, described by a field with a non-vanishing vacuum expectation value.

Well, some physicists call it tachyon condensation.

 

[Vanadium 50]

These are all theoretical problems, and I think (hope?) we all agree that if tachyons were discovered some theoretical revisions are likely. This is also why I think it's unlikely that tachyons are real - they dont just break one thing, they break lots and lots. Doesn't mean we shouldn't look, but it probably means that booking your tickets to Stockholm the day your experiment gets approved is a little premature.

What does a conserved charge even mean for a spacelike worldline?

Yes, there are loopholes in the literature, and as I said I am working on a paper describing how to close one of the biggies. But don't want to discuss it here, especially while it is still half-baked. I think the situation is as I described it: there are lots of theoretical problems, none of which are air-tight, but the combination seems to be pointing us in a certain direction: there aren't any, or at least if there are they are sterile. Experiment is pointing in more or less the same direction: there aren't many, maybe not any, and if they exist, they interact with matter weakly and in a highly constrained way - and may be sterile.

But "there seem not to be any:" and "there can't be any" are not the same thing. Nor is "we don't have a clear idea how they would behave in a consistent manner" the same as "they can't exist".