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

jumpjack

Could it be true?
Why?

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

Fredrik

"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.

jumpjack

Forget reality , I was just wondering about formulas.

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...

I know....

Fredrik

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).

This question comes up almost every week. See this quote and the thread I linked to in there:

jumpjack

hamster143

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.

bcrowell

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/physic...oton_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.

Fredrik

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.

yuiop

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.

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.

bcrowell

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.

Fredrik

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.

matheinste

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.

Frame Dragger

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.

bcrowell

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.

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

Fredrik

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 seem to be replying to something very different from what's been posted in this thread.

Frame Dragger

You continually teach me (along with others here), better ways to teach and handle misconceptions instead of scoffing or becoming annoyed. Thank you.

yuiop

This is what the physics FAQ http://www.desy.de/user/projects/Phy...oton_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/...ml#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.