Exploring the Concept of Time for Photons

[Cato]

TL;DR Summary

Since photons travel at the speed of light, is time stopped for them? If it is, how are they able to interact with anything?

Do photons, I'm not sure how to express this, "experience" time? Since they move at the speed of light does time not exist for them? If time does not exist, how is it possible for anything to happen to them. If interacting with matter requires a change in their condition, how can a change in condition happen if there is no time in which a change can happen?

 

[phinds]

Turns out that's a common but essentially meaningless question. You have to define "experience" and that's what turns out to be meaningless for a photon. As for how they can interact, that simply isn't a problem and the fact that your question seems to imply that it is a problem is just a reflection on one aspect of why it's a meaningless question.

 

[Cato]

Fair enough.

 

[PeterDonis]

If time does not exist, how is it possible for anything to happen to them.

"Time does not exist for photons" is not a correct statement. The correct statement is that photons are fundamentally different kinds of things (lightlike objects) from the kinds of things that we normally think of as "experiencing time" (timelike objects). For timelike objects, things happen to them as they experience time. But for lightlike objects, things happen to them even though the concept of "proper time" (the correct physics term for what you mean by "experienced time") does not apply to them. That's just one manifestation of the fundamental physical difference between lightlike objects and timelike objects.

 

[HallsofIvy]

Turns out that's a common but essentially meaningless question. You have to define "experience" and that's what turns out to be meaningless for a photon. As for how they can interact, that simply isn't a problem and the fact that your question seems to imply that it is a problem is just a reflection on one aspect of why it's a meaningless question.

I am trying to imagine photons wearing little teeny-tiny wristwatches!

 

[Cato]

"For timelike objects, things happen to them as they experience time. But for lightlike objects, things happen to them even though the concept of "proper time" (the correct physics term for what you mean by "experienced time") does not apply to them." -- Thanks for that. I had not heard of the timelike/lightlike distinction before. Even though I don't really understand what that means, it doesn't matter. There are more than a few things that I don't understand that nevertheless are true and produces predictable, measure and useful effects.

 

[PeroK]

"For timelike objects, things happen to them as they experience time. But for lightlike objects, things happen to them even though the concept of "proper time" (the correct physics term for what you mean by "experienced time") does not apply to them." -- Thanks for that. I had not heard of the timelike/lightlike distinction before. Even though I don't really understand what that means, it doesn't matter. There are more than a few things that I don't understand that nevertheless are true and produces predictable, measure and useful effects.

An important point is that physics is an empirical science where the theory predicts what we can observe, in nature directly and through experiment. This requires a framework of spacetime in which to study the theory and physical things like clocks and rulers to carry out measurements.

Physics can be done theorectically and practically by humans, and we can also do it on behalf of massive particles, whose properties allow a spacetime framework to be applied to them. In other words, a massive particle can be given a valid reference frame.

Physics cannot be done on behalf of photons, as no spacetime framework exists for a massless particle following a lightlike path. In other words, photons cannot be given a valid reference frame in which the laws of physics can be studied.

That doesn't stop photons being part of the physics, theoretically and experimentally, carried out by timelike objects, such as us!

 

[Cato]

Yes. Thanks for that take on it...

 

[PeterDonis]

Physics cannot be done on behalf of photons, as no spacetime framework exists for a massless particle following a lightlike path. In other words, photons cannot be given a valid reference frame in which the laws of physics can be studied.

I strongly object to this choice of words. The fact that there is no inertial frame in which a photon is at rest does not mean that physics cannot be done "on behalf of" photons, or that "no spacetime framework exists" for photons. Lightlike worldlines are perfectly valid curves in spacetime, and there certainly exist valid coordinate charts in which lightlike worldlines have all coordinates constant except one. There are no inertial frames corresponding to such charts, but that doesn't mean they don't exist or that they can't be used to do physics. You can write down all of the laws of physics in such charts.

 

[vanhees71]

I am trying to imagine photons wearing little teeny-tiny wristwatches!

Then there's no problem since teeny-tiny wristwatches are massive and thus show their well-defined proper time.

 

[Ibix]

Then there's no problem since teeny-tiny wristwatches are massive and thus show their well-defined proper time.

They should use light clocks!

(Yes, I am aware that light clocks do not, and cannot, move at the speed of light.)

 

[jbriggs444]

They should use light clocks!

(Yes, I am aware that light clocks do not, and cannot, move at the speed of light.)

What if they are very very light?

 

[timmdeeg]

What if they are very very light?

Would then exist frame of reference where they are at rest?

 

[phinds]

Would then exist frame of reference where they are at rest?

jbriggs' joke was funny. If your comment is intended as a joke, I'm not getting it.

 

[Ibix]

Would then exist frame of reference where they are at rest?

We're joking. You can't build a clock that moves at light speed. It's contradictory to consider a clock moving at light speed - it would have to follow a worldline that is both null (for it to move at light speed) and tinelike (for time to have a meaning). But a worldline that's null isn't timelike and vice versa.

 

[timmdeeg]

We're joking. You can't build a clock that moves at light speed.

Is there a theoretical claim that a photon can't have a very very tiny not yet detectable rest mass?

 

[bahamagreen]

Summary:: Since photons travel...

I've read around here that a photon has no position operator,
that in relativistic field theory photons can't be enumerated,
that only interactions may be measured...

maybe the concept of "travel" applied to a photon is highly problematic?

 

[phinds]

Is there a theoretical claim that a photon can't have a very very tiny not yet detectable rest mass?

No theory that I'm aware of but it's not impossible. If it did nothing in physics would change, we'd just have to find a new symbol for the speed of light since "c" is taken by the universal speed limit (which would not change). Also, of course, all the places where "the universal speed limit" is meant when "the speed of light" is stated, would need to be changed.

 

[Ibix]

Is there a theoretical claim that a photon can't have a very very tiny not yet detectable rest mass?

You can describe a massive photon and its consequences, yes. I believe the upper bound on mass given our failure to detect such consequences is something like 10^-50kg. But if its rest mass is non-zero then it doesn't travel at $c$ and could, in principle, be stopped. Also, more seriously, our jokes wouldn't make sense.

 

[timmdeeg]

You can describe a massive photon and its consequences, yes.

So if yes, does it have a frame of reference? My question in #13 wasn't a joke. And I think @jbriggs444 question in #12 wasn't a joke too.

 

[Ibix]

No theory that I'm aware of but it's not impossible.

I believe the physics of a spin-1 massive quantum field (i.e., an EM field with a massive photon) was worked out by Proca. One of its predictions is that the shell theorem wouldn't apply exactly to EM, so you can place an upper bound on photon mass by charging a hollow conducting sphere and checking for an electrostatic field inside.

 

[Ibix]

So if yes, does it have a frame of reference? My question in #13 wasn't a joke.

Yes, just like any other massive particle. To be pedantic, you should ask if there is an inertial frame in which it is at rest, but your meaning is clear here anyway.

 

[timmdeeg]

Yes, just like any other massive particle.

Thanks.

 

[timmdeeg]

To be pedantic, you should ask if there is an inertial frame in which it is at rest, but your meaning is clear here anyway.

If a particle in free fall is at rest in a frame of reference what else could it be except an inertial frame?

 

[Ibix]

If a particle in free fall is at rest in a frame of reference what else could it be except an inertial frame?

My point is that "does it have a frame of reference" isn't particularly well defined. Nobody is obliged to use their rest frame for anything. "Does it have a rest frame" would work, and is cleaner than what I wrote above.

 

[pervect]

It's possible to mark off regular intervals along a light-like object's worldline. These regular intervals can be assigned a number, an "affine parameterization".

If one considers light as a an example of something that has a lightlike worldline, one can imagine two interfering light beams as generating, under the right circumstances, a static "standing wave", interference pattern. The peaks (or valleys) of some particular interference pattern can be regarded as a physical example of an "affine parameterization", the first peak could be assigned the number 1, the second 2, etc.

Identifying these affine parameters with "time" is incorrect, though, and will lead to confusion.

 

[timmdeeg]

My point is that "does it have a frame of reference" isn't particularly well defined. Nobody is obliged to use their rest frame for anything. "Does it have a rest frame" would work, and is cleaner than what I wrote above.

Agreed. My point was I didn't understand @jbriggs444's post #12 as a joke.

 

[jbriggs444]

So if yes, does it have a frame of reference? My question in #13 wasn't a joke. And I think @jbriggs444 question in #12 wasn't a joke too.

My question in #12 was indeed intended as a joke. The whole light/light thing seemed like too good a pun to pass up. Apologies since it seems to have been taken seriously.

 

[PeroK]

My question in #12 was indeed intended as a joke. The whole light/light thing seemed like too good a pun to pass up. Apologies since it seems to have been taken seriously.

There's a whole array of things that photons might get up to: a bit of light music, light reading and they would manufacture their light clocks with a bit of light engineering!

 

[Ibix]

And they never have luggage, since they're traveling light.

 

[timmdeeg]

And they never have luggage, since they're traveling light.

Unless it turns out that they have a very very light luggage.

But seriously after all it seems Physicists don't take this into serious consideration. So ok no joke.

However did they take into consideration that neutrinos have a mass long time ago?

 

[zoki85]

You can describe a massive photon and its consequences, yes. I believe the upper bound on mass given our failure to detect such consequences is something like 10^-50kg. But if its rest mass is non-zero then it doesn't travel at $c$ and could, in principle, be stopped.

There are some speculations that the speed of propagation of photons in a vacuum might depend on the photon's energy. Personally I don't like such ideas but people like to experimentally test various ideas.
Here you can see some experimental results.

 

[Ibix]

But seriously after all it seems Physicists don't take this into serious consideration. So ok no joke.

However did they take into consideration that neutrinos have a mass long time ago?

People do measure the photon mass - that 10^-50kg upper bound didn't happen by accident. But my understanding is that a lot of things are simpler if its mass is zero - and given that our measurements are consistent with that, why make life more complicated than it has to be?

I must say I don't know much about neutrinos. I believe they were initially thought to be massless, but (I think) research into the solar neutrino problem (there were too few by a factor of three) led eventually to the idea that they had to have masses.

Edit: by the way, "long time ago" is in my lifetime.

 

[PeterDonis]

However did they take into consideration that neutrinos have a mass long time ago?

Because, as @Ibix said, the solar neutrino problem is evidence that neutrinos are not massless. Whereas we have no evidence at all that photons are not massless.

Briefly, the solar neutrino evidence is this: we can calculate the rate of nuclear reactions in the Sun's core from its energy output, which in turn gives us the rate at which neutrinos are produced in the Sun's core. All of those neutrinos are electron neutrinos. We have set up detectors on Earth that detect electron neutrinos, but those detectors only see about 1/3 as many electron neutrinos as expected.

The only way we know of to account for this is that the electron neutrinos from the sun have a nonzero amplitude to turn into muon or tau neutrinos as they travel--or, more generally, that neutrinos with a definite energy (such as neutrinos coming from particular nuclear reactions in the Sun) do not have a definite flavor ("electron", "muon" , or "tau"), but have amplitudes for being each of the three different species, and those amplitudes vary with time as the neutrinos travel. So calling the neutrinos produced by nuclear reactions in the Sun "electron neutrinos" is not quite correct: they are actually not in the "electron neutrino" flavor eigenstate. They are mixtures of all three flavors that just happen to be produced at the point in the time variation of the flavor amplitudes at which all of the amplitude is in the "electron" flavor. By the time they have traveled from the Sun to the Earth, the variation of the flavor amplitudes with time has turned them into a mixture of all three flavors, in roughly equal proportions, and our detectors only detect the "electron" flavor portion, so we only detect about 1/3 of them.

The point is that this variation of flavor amplitude with time can only happen if neutrinos are not massless. This is because if neutrinos are massless, the flavor eigenstates and the energy eigenstates have to be the same. (The reasons for this are fairly detailed technically.) So evidence that the flavor eigenstates and the energy eigenstates are not the same is evidence for neutrinos not being massless.

 

[timmdeeg]

Because, as @Ibix said, the solar neutrino problem is evidence that neutrinos are not massless. Whereas we have no evidence at all that photons are not massless.

So any comparison with regard to a hypothetical rest mass fails.
Thanks for your detailed explanation! I appreciate that very much.