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If a photon had a mass, time travels would be possibleby jumpjack
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#19
Mar1710, 07:33 AM

P: 546

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. 


#20
Mar1710, 08:23 AM

P: 1,540

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). 


#21
Mar1710, 11:59 AM

P: 546

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


#22
Mar1710, 01:11 PM

P: 1,540

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 nonphysical. 


#23
Mar1710, 05:05 PM

P: 28

Hi,
Sorry if I posted in a wrong place. Since I have a simple question about Timetravel. 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 


#24
Mar1710, 05:09 PM

Sci Advisor
P: 2,851

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. 


#25
Mar1710, 05:27 PM

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#26
Mar1710, 05:42 PM

P: 1,540




#27
Mar1810, 12:22 AM

P: 546

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. 


#28
Mar1810, 07:13 AM

Mentor
P: 17,338

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.



#29
Mar1810, 09:48 AM

P: 1,540

@Parlyne: I take your points as meant, and I don't believe you're arguing for nonzero 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. 


#30
Mar1810, 12:06 PM

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#31
Mar1810, 12:32 PM

P: 1,540

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


#32
Mar1810, 12:35 PM

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#33
Mar1810, 12:43 PM

P: 1,540

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


#34
Mar1810, 12:47 PM

P: 3,967

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. 


#35
Mar2310, 02:43 AM

P: 153




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