"If a photon had a mass, time travels would be possible"

Frame Dragger

Well, to adress your first quote, why not ask people more qualified than me if they believe QED is so terribly flawed that the photon has rest mass? As others have said, first you'd need a QFT to simply "accept" the 'new photon' or the new theory would describe them differently.

As for your last point, if there were an experiment to determine if the photon had rest mass, can you even IMAGINE a test using the tech of the next saaaaay... 20 years? I can't, so it's probably going to be a matter of either a replacement theory that doesn't say "photon", or some very similar QFT.

EDIT: By the way, what theory doesn't require experimentation?!

Parlyne

We already know that QED isn't a complete theory of particle physics. It arises from the spontaneous breaking of the electroweak SU(2)XU(1) symmetry. So, there's no a priori theoretical reason to assume that the U(1) symmetry that's left over is somehow completely sacrosanct. (And, to be clear, the statement that the photon is massless is isomorphic to the statement that the U(1) symmetry of QED is exactly obeyed by nature. Were this symmetry broken, the photon would either get a mass directly in the process of the symmetry breaking, or it would be generated by loop corrections.)

The fact that the U(1) gauge symmetry appears to be a good symmetry of nature is an empirical fact, not a mathematical one; so, it's subject to experimental testing. And, since experiments always have finite precision, our statement that the photon is massless (or, equivalently that the U(1) symmetry holds) is only good to a certain precision. Beyond that precision, we have no right to assume anything.

Frame Dragger

True... but then we'll never be able to CONFIRM that the photon is massless. We can keep setting bounds on its mass, but so what? You can continue to make your statement as long as QED exists, U(1) symmetry HOLDS and doesn't devolve into some kind of Higgs Mechanism for photons, I get it.

Now, refer that back to basics; if U(1) symmetry IS broken, I don't think we'd just say "whoopsie, now the photon has rest mass", it would probably be as the result of an emergence of a QFT which replaces QED. In that scenario, it's hard to imagine a "photon" still being useful as a description, anymore than you'd go around calling EDM "Heavy Tau Neutrionos".

Yes, all experiments have finite precision, and all theories are wrong... that's also basic. As for testing U(1) gauge symmetry, by all means how would you do so with any confidence? Like the folks at SETI, your argument is valid, but your search would be a loooooong negative with an unknown (possibly 0) probability that you'll find anything, but more negatives.

Just as it's been obvious that the SU(2) gauge group exhibits spontaneus symmetry breaking as we have MASS, it seems obvious that the U(1) gauge does NOT. That, to be fair, is my opinion, and nothing more (as if that needed to be said).

Parlyne

This is, of course, true; but, it's not really relevant. We'll also never know exact values for any other particle masses or, for that matter, any other physical parameters.

Of course it will still be called a "photon." Even if photons turn out to have mass, it won't change either that their exchange is responsible for EM forces or that on-shell photons make up light and other EM radiation.

So, since we've been directly detecting the darn things for centuries and have been doing single photon detections for decades, I doubt you'll suddenly find too many people who want to start referring to the particles they've been detecting by a different name just because it turns out that they have a tiny bit of mass.

I don't think I've said anything to the effect that I expect anyone to find a non-zero photon mass. However, it's not terribly hard to include in practical calculations; so, we can do so and then look for effects in experiments which are best understood by the photon mass being non-zero.

I'll also point out that there are a good number of QED calculations in which it is necessary to use a non-zero photon mass in intermediate steps to avoid infrared singularities; so, people already do these sorts of calculations by necessity.

In fact EWSB requires breaking both SU(2) and U(1). What it leaves as an unbroken U(1) symmetry generator is a linear combination of the generator of the original U(1) and one of the generators of SU(2). If EWSB only broke SU(2), the three heavy particles left would all have the same mass. But, in reality, the Z is heavier than the Ws.

Frame Dragger

Your last point would be why the Higgs is believed to be the result of SSB, would it not?

As for the various intermediary stages of calculation in ALL of QM, I pay attention in the same way I do Virtual Photons; they are tools to make up for the flaws in the model, but I don't see how mass on the photon would be more than incidental to all of that. You can't leave the calculations at that intermediary step and have a meaningful answer, and that step is non-physical.

physics.alex

Hi,
Sorry if I posted in a wrong place. Since I have a simple question about Time-travel.
EFE allows time travel to the future. And some peopel say time travel is impossible due to whatever reason.

So I am thinking if time travel is impossible, does it mean EFE has problem??? or if EFE is correct, does it mean time travel is really possible????

Either result will cause me feel surprise.

Alex

Matterwave

We are always time traveling to the future...and even SR allows us to "time travel" into the future by going real fast.

It's time travel to the past that is the problem.

Naty1

That's because there IS no "exact" value....

Frame Dragger

thank you.

Parlyne

I don't know that I would say that the Higgs is the result of SSB, but rather that it's a left over degree of freedom after SSB which is necessarily present for the gauge invariance of the unbroken theory. However, the fact that there is a scalar degree of freedom left over doesn't depend on the exact structure of the symmetry breaking. If EWSB were just the breaking of SU(2), there would still have to be a complex scalar doublet to break it; and, complex scalar doublets have four degrees of freedom, whereas there are only three gauge bosons acquiring mass and, therefore, needing to absorb a degree of freedom to act as the extra polarization state. The difference here would be that the doublet had no U(1) charge.

My point here was a much less technical. It was simply that we already know how to do calculations as if the photon had mass because it turns out to be a necessary fiction in certain types of calculations subject to IR divergences. I wasn't trying to argue that in those calculations the photon mass has any a priori reality. The point was simply that, were we to do such calculations taking the photon mass more seriously, the photon mass need not ultimately be taken to zero but can be left as a free parameter to be fit to data.

Again, I'm making absolutely no claim about whether the photon is truly massless or not. I'm simply arguing that there is no fundamental theoretical barrier to either case being true and pointing out that a tiny photon mass would only lead to very small changes in the ordinary QED predictions and that we know how to characterize those changes.

DaleSpam

I actually would like to go back to the OP and find out how the idea of a massive photon is connected with the idea of time travel in any way. I don't understand that point at all.

Frame Dragger

As far as I can see, it was dismissed as frivolous in the first page, and then we all kind of ran with another semi-related topic. It turns out there was a "?" not a "." at the end of the title of the thread, so... ah well.

@Parlyne: I take your points as meant, and I don't believe you're arguing for non-zero photon mass. I don't know that I agree, but I don't believe I have any new arguments for my view. I'll think about what you've said, and you've injected some doubt where it will do the most good, which is a good thing in this case.

@OP: From what I understand, the photon plays no special role in time travel (to the past), only the geometry of spacetime.

DaleSpam

Yes, I agree that it is frivolous, but I don't understand the connection. Usually, with common mistakes (e.g. twin paradox or fast-moving things turning into black holes) I at least understand the train of thought that gets them to the erroneous conclusion. Here, I just don't even have a guess as to what the reasoning would be that would lead from massive photons to time travel.

Frame Dragger

Hmmm... if I were to guess, maybe the OP thinks that a photon with mass would avoid some forms of chronology protection because they couldn't reach arbitrarily high energy densities through multiple passage. Another idea might be that the OP ran for the hills as soon as they were shot down and the thread, um... "stepped sideways from the sun".

I find that digging through the thin veneer of speculation rarely unearths anything of value (on the net at least).

DaleSpam

yes, too true.

Frame Dragger

That said, I'm a little curious as to the OP's thinking myself. Like you, the title is so... baffling.

yuiop

I think the OP was suffering from a misconception that if we (made of massive particles) could travel at the speed of light then we could time travel and then reasoned that if photons had some minuscule rest mass then in priciple massive particles could travel at the speed of light and then in principle time travel would be possible.

However, if photons had a vanishingly small rest mass they would be travelling slightly slower than c and so even if photons had miniscule rest mass it would still be true that nothing with rest mass can travel at exactly c (including photons in this case). However this raises the disturbing problem that if the speed of photons was 0.999999999999999999999999999999999999999999999999999999999999999999999 9999999999999c then ordinary massive particles would in principle be able to exceed the speed of photons (without exceeding c). In fact a vanishingly small mass for photons raises the possibility that photons could be slowed in a vacuum, to a relative velocity of much less than c which has never been observed to occur. Another problem with photons having vanishingly small rest mass and velocity slightly less than c is that the speed of photons would not be invariant.

I know people do not like to consider what happens IF we could travel at the speed of light of light (or photons) because it is physically impossible but I think it is reasonable to conjecture that even IF we could then it does not open the door to time travel, in the sense that photons are not considered to time travel because they take finite amount of coordinate time to traverse space and never go backwards in time.