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

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  • #1
jumpjack
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"If a photon had a mass, time travels would be possible"

Could it be true?
Why?

How does mass relate to time travel and/or time warp in relativity?
 

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  • #2
Fredrik
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"if a photon had a mass" is like saying "if a weekend was in the middle of the week". If you're asking about a massive spin-1 particle, then no, the existence of such a particle wouldn't make time travel possible. It wouldn't have anything to do with time travel.

There's no short answer to your last question. I suggest the book "Black holes and time warps: Einstein's outrageous legacy", by Kip Thorne.
 
  • #3
jumpjack
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"if a photon had a mass" is like saying "if a weekend was in the middle of the week".
Forget reality :biggrin:, I was just wondering about formulas.

If you're asking about a massive spin-1 particle, then no, the existence of such a particle wouldn't make time travel possible. It wouldn't have anything to do with time travel.
Particles with mass can't reach light speed due to needed amount of energy to bring them to that speed;
photons have no mass but already have light speed;
the more is the speed, the slower is the time.
Is then time not running for photons?
I remember this kind of connection between time and mass, raised during a lecture I heard time ago...

There's no short answer to your last question.
I know.... :rolleyes:
 
  • #4
Fredrik
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Forget reality :biggrin:, I was just wondering about formulas.
I'm fine with unrealistic assumptions, like "if I just ate a million hamburgers", but not self-contradictory assumptions like "if I just ate myself". (That example works better in a language where "ate myself" doesn't sound autoerotic).

Is then time not running for photons?
This question comes up almost every week. See this quote and the thread I linked to in there:

Your concern about time at the speed of light is answered by the following, which I originally posted in another forum:

The reason why we associate a specific inertial coordinate system with the motion of an inertial observer is that there's a clock synchronization procedure that makes that the natural choice. All the statements about Lorentz contracton, time dilation, etc., are consequences of that choice. The claim that massless particles experience no time comes from applying the usual time dilation formula for speed v and taking the limit v→c, but there's no reason why we should think of the result of that procedure as "a photon's point of view". There is however a good reason not to: The clock synchronization procedure doesn't work for massless particles. See my posts in this thread (at Physics Forums) for more about this.
 
  • #5
jumpjack
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  • #6
hamster143
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The EM field is just a field. You can tweak properties of the EM field within reasonable limits without changing the nature of spacetime (and, therefore, without enabling time travel). Many thought experiments in special relativity presume that photons are massless, but that is merely out of convenience.
 
  • #7
bcrowell
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"if a photon had a mass" is like saying "if a weekend was in the middle of the week".

No, it's like saying, "what if we do such-and-such an experiment tomorrow, and the result is such-and-such." It's perfectly possible that the photon has a nonvanishing rest mass.

Baez has a nice summary of this:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

(The relevant part is at the end.)

Here are some experimental papers on this topic:

Goldhaber and Nieto, "Terrestrial and Extraterrestrial Limits on The Photon Mass," Rev. Mod. Phys. 43 (1971) 277–296

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html

Luo et al., “New Experimental Limit on the Photon Rest Mass with a Rotating Torsion Balance”, Phys. Rev. Lett, 90, no. 8, 081801 (2003)

The Luo paper is controversial.
 
  • #8
Fredrik
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It depends on if we consider a concept like "photon" to be defined by a specific theory or by a class of similar theories that are consistent with experiments.
 
  • #9
yuiop
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In futuristic Startrek like scenarios you could in principle transmit all the information about the DNA and atoms that make up your body to a distance location and effectively a copy of you could be transmitted at the speed of light (ignoring the time to reconstruct your body) but you would not end up with time travel in the sense of travelling backwards in time.

How does mass relate to time travel and/or time warp in relativity?

Having rest mass prevents you from travelling exactly at the speed of light. Even if there was some hypthetical method such as the one I mentioned above that allows you to travel at the speed of light you would stillnot be able to travel backwards in time. To travel backwards in time you have to travel faster than the speed of light and particles with rest mass and light with zero rest mass can not exceed the speed of light. In theory a particle with imaginary rest mass can travel faster than light (eg the hypothetical tachyons) but these particles are unable to slow down to the speed of light or less.

What does it mean to have imaginary mass? A particle with imaginary mass can not interact with normal matter and it is very likely that it imposssible to detect tachyons using equipment made of normal matter. In theory there could be a whole universe of tachyonic matter superimposed on our universe but the tachyons would be unable to detect us (just as we can not detect them) and the tachyons would theorise that in principle there is matter that travels slower than light but those slow particles are undetectable and only theoretical (although we know we exist!). In other words to a tachyon we are made of imaginary mass.
 
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  • #10
bcrowell
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It depends on if we consider a concept like "photon" to be defined by a specific theory or by a class of similar theories that are consistent with experiments.

No, it's purely an experimental question. We used to think neutrinos were massless, but experiments ended up showing that they had nonvanishing rest mass. It's not a matter of theory or definition, it's simply an empirical observation. The same holds for photons.
 
  • #11
Fredrik
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No, it's purely an experimental question.
No, it's not. They are massless by definition in QED. So if your "photons" have mass, you didn't use the QED definition of that term.
 
  • #12
matheinste
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No, it's purely an experimental question. We used to think neutrinos were massless, but experiments ended up showing that they had nonvanishing rest mass. It's not a matter of theory or definition, it's simply an empirical observation. The same holds for photons.

So our assumptions about the neutrino were incorrect. And of course the possiblity is always present that any of our assumptions about any presently accepted physical facts or theories may in future be proven to be wrong by experiment. But would it not be harmless and lead to less confusion, for beginners such as myself to continue, for the purposes of this forum, to assume that photons have zero rest mass.

If we assume the possibility that the rest mass of the photon is non zero, not a totally unreasonable possibility, some of the questions asked in this forum and often rendered meaningless by the photon not having an inertial rest frame due to its zero rest mass, cease to be meaningless, unfortunately causing complications and at worst, misunderstandings.

Can we please agree, for the sake of simplicity, that until proven otherwise the photon having zero rest mass is a fact.

Matheinste.
 
  • #13
Frame Dragger
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I'm sorry, but this is a bit astray from "if a photon has mass, time travels would be possible". The simple answer is: "No"
The better answer is: "What?!"
The best answer is the one Frederick gave several times.

@Matheinste: Anyone confused that the photon has a non-zero rest mass isn't confused, they're deluded.
 
  • #14
bcrowell
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No, it's not. They are massless by definition in QED. So if your "photons" have mass, you didn't use the QED definition of that term.

If experimentalists find out tomorrow that the photon has nonzero rest mast, do you really think everybody will start using a different word for to describe it? Of course not. They'll do the same thing they did with the neutrino. They'll keep on using the same word, but they'll attribute different properties to it.

Anyone confused that the photon has a non-zero rest mass isn't confused, they're deluded.
Statement #1: "The photon has non-zero rest mass."
Statement #2: "The rest mass of the photon must be empirically determined, and future experiments may prove it to be nonzero."

You're setting up a straw man by making it sound as though someone has made statement #1. Nobody has.

Statement #2 is correct. If it were incorrect, then I doubt that Phys Rev Letters would have accepted Lakes (1998).

I suspect that a lot of people replying to the OP are suffering from the same confusion that the OP was suffering from. They may believe that light somehow plays a fundamental role in relativity. If that were the case, then a change in the experimental status of light's properties would affect the foundations of relativity. If one is firmly wedded to relativity, then one may be inclined to reject the possibility that the experimental status of light's properties would ever change.

Light does not play a fundamental role in relativity. That's why the answer to "If a photon had a mass, time travels would be possible" is no.

I suggest that folks participating in this thread read at least the abstract of the Lakes paper, and then ask, "If I'd been the referee that this paper was sent to, would I have rejected it based on the views I've expressed in this thread?" If the answer is yes, then maybe the views you've expressed in this thread are wrong; either that or PRL messed up by accepting a totally pointless paper.

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html
 
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  • #15
Fredrik
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If experimentalists find out tomorrow that the photon has nonzero rest mast, do you really think everybody will start using a different word for to describe it?
What you're saying here doesn't make sense. You can't find out that something that's massless by definition is massive. (And if photons are defined by QED, they are massless by definition). What you can find out is that there's a theory that makes better predictions than QED. If that theory is a QFT that's identical to QED except that the spin-1 field is massive, then it would make sense to call the particles corresponding to that field "photons". But it only makes sense to say that you have "measured the mass of the photon to be non-zero" if you defined the word "photon" using that class of theories instead of a single theory. I'm surprised you keep objecting to that. It's not exactly a radical claim.

I suggest that folks participating in this thread read at least the abstract of the Lakes paper, and then ask, "If I'd been the referee that this paper was sent to, would I have rejected it based on the views I've expressed in this thread?" If the answer is yes, then maybe the views you've expressed in this thread are wrong; either that or PRL messed up by accepting a totally pointless paper.
You seem to be replying to something very different from what's been posted in this thread.
 
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  • #16
Frame Dragger
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What you're saying here doesn't make sense. You can't find out that something that's massless by definition is massive. (And if photons are defined by QED, they are massless by definition). What you can find out is that there's a theory that makes better predictions than QED. If that theory is a QFT that's identical to QED except that the spin-1 field is massive, then it would make sense to call the particles corresponding to that field "photons". But it only makes sense to say that you have "measured the mass of the photon to be non-zero" if you defined the word "photon" using that class of theories instead of a single theory. I'm surprised you keep objecting to that. It's not exactly a radical claim.

You continually teach me (along with others here), better ways to teach and handle misconceptions instead of scoffing or becoming annoyed. Thank you.
 
  • #17
yuiop
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@Matheinste: Anyone confused that the photon has a non-zero rest mass isn't confused, they're deluded.
This is what the physics FAQ http://www.desy.de/user/projects/Physics/ParticleAndNuclear/photon_mass.html has to say:
If the rest mass of the photon were non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalisable; also, charge conservation would no longer be absolutely guaranteed, as it is if photons have zero rest mass.

but it then follows up with

But regardless of what any theory might predict, it is still necessary to check this prediction by doing an experiment.

The FAQ then describes two experiments that put a limit on the mass of a photon:
The Charge Composition Explorer spacecraft was used to derive an upper limit of 6 × 10-16 eV with high certainty.
This was slightly improved in 1998 by Roderic Lakes in a laboratory experiment that looked for anomalous forces on a Cavendish balance.
and this other FAQ http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SR/experiments.html#photon_mass describes other research that puts a limit on the mass of a photon:
Goldhaber and Nieto, “New Geomagnetic Limit on the Mass of the Photon”, Phys. Rev. Lett. 21 no. 8 (1968), pg 567.

A limit of 2.3×10−15 eV/c2.

Goldhaber and Nieto, “Terrestrial and Extraterrestrial limits on the Photon Mass”, Rev. Mod. Phys. 43 no. 3 (1971), pg 277.

A review article discussion about various experimental limits.

Davis et al., “Limit on the Photon Mass Deduced from Pioneer-10 Observations of Jupiter's magnetic Fields”, Phys. Rev. Lett. 35 no. 21 (1975), pg 1402.

A limit of 6×10−16 eV/c2.

Lakes, “Experimental limits on the Photon Mass and Cosmic Magnetic Vector Potential”, Phys. Rev. Lett. 80 no. 9 (1998), pg 1826.

An experimental approach using a toroid Cavendish balance.

Luo et al., “New Experimental Limit on the Photon Rest Mass with a Rotating Torsion Balance”, Phys. Rev. Lett, 90, no. 8, 081801 (2003).

A limit of 1.2×10−51 g (6×10−19 eV/c2).

See also the Particle Data Group's summary on “Gauge and Higgs Bosons”. As of July 2007, their reported limit on the photon mass is 6×10−17 eV/c2.
That seams a lot of money wasted on research when if they only read physicsforums they would realise that the mass of a photon has been defined as zero.
 
  • #18
Frame Dragger
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This is what the physics FAQ http://www.desy.de/user/projects/Physics/ParticleAndNuclear/photon_mass.html has to say:


but it then follows up with



The FAQ then describes two experiments that put a limit on the mass of a photon:

and this other FAQ http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SR/experiments.html#photon_mass describes other research that puts a limit on the mass of a photon:

That seams a lot of money wasted on research when if they only read physicsforums they would realise that the mass of a photon has been defined as zero.

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?!
 
  • #19
Parlyne
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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?!

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.
 
  • #20
Frame Dragger
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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.

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).
 
  • #21
Parlyne
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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.

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.

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".

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.

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.

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.

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).

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.
 
  • #22
Frame Dragger
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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.
 
  • #23
physics.alex
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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
 
  • #24
Matterwave
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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
Naty1
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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.

That's because there IS no "exact" value....
 
  • #26
Frame Dragger
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that's because there is no "exact" value....

thank you.
 
  • #27
Parlyne
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Your last point would be why the Higgs is believed to be the result of SSB, would it not?

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.

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.

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.
 
  • #28
Dale
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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
Frame Dragger
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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.

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.
 
  • #30
Dale
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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.
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.
 
  • #31
Frame Dragger
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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.

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).
 
  • #32
Dale
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I find that digging through the thin veneer of speculation rarely unearths anything of value (on the net at least).
:rofl: yes, too true.
 
  • #33
Frame Dragger
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:rofl: yes, too true.

:wink:

That said, I'm a little curious as to the OP's thinking myself. Like you, the title is so... baffling.
 
  • #34
yuiop
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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.

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 0.9999999999999999999999999999999999999999999999999999999999999999999999999999999999c 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.
 
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  • #35
Flatland
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No, it's purely an experimental question. We used to think neutrinos were massless, but experiments ended up showing that they had nonvanishing rest mass. It's not a matter of theory or definition, it's simply an empirical observation. The same holds for photons.

As I understand it neutrinos don't travel at c.
 

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