Does a photon experience time when slowed down by gravity?

[Simon Bridge]

[zero momentum] is impossible if a photon can't be at rest.

... it is more proper to consider the long wavelength limit isn't it?

Similarly, though the reference frame of a photon makes no sense, we can ask what things look like at ultra-relativistic speeds.

 

[Phy_Man]

That doesn't mean "experiences a time equal to zero". It means the concept of time relative to a photon doesn't make sense, it's undefined.

Myself, I question what "a photon experiences time" means.

 

[PeterDonis]

... it is more proper to consider the long wavelength limit isn't it?

A long finite wavelength is fine. An infinite wavelength is not.

Similarly, though the reference frame of a photon makes no sense, we can ask what things look like at ultra-relativistic speeds.

Sure, but that's not the same thing.

 

[PeterDonis]

The 4-momentum of a particle is defined as either the product of the particle's 4-velocity and the particles proper mass.

This is only true for particles with nonzero rest mass. It doesn't work for photons, but photons still have a perfectly well-defined 4-momentum.

 

[Simon Bridge]

A long finite wavelength is fine. An infinite wavelength is not.

Is there a limit to how long the wavelength can be?
Of course an infinite wavelength is a horizontal line ... you don't have a wave (and other issues).

It can be sensible to talk about a limit to a value even though the value itself is not attainable and does not make sense in the context.

Sure, but that's not the same thing.

And I'm not saying that - it's just a way forward. Now people get to say whether this is the sort of thing intended and will remember to be more careful with language in future. It's a common-enough mistake.

 

[naturale]

No, you find that the concept of "reference frame of the photon" doesn't make sense. We have a forum FAQ on this:

https://www.physicsforums.com/showthread.php?t=511170
Do you have an actual reference for this claim? It looks to me like you are misinterpreting something.

The preference frame of the photon is the reference frame traveling at the speed of light (I did not said rest frame of the photon). If you travel at c like light the gamma terms of the Lorentz transformation IS infinite. The Compton length of the photon is infinite.

I have more important things to do than teach special relativity in a forum. If you learn textbooks by heart you will have a brilliant academic career but you will give poor contribution to physics.

 

[naturale]

Do you have an actual reference for this claim? It looks to me like you are misinterpreting something.

I' am referring to the Einstein's mirror, that he thought when he was 16:

http://www.321books.co.uk/biography/einstein/mirror.htm

"Einstein's mirror is a hand held mirror. To perform the experiment, hold the mirror and look at your reflection. While retaining hold of the mirror, imagine what would happen if you were traveling at the speed of light. (Hint: Would you see your reflection?)"

With this I leave this obtuse discussion!

 

[WannabeNewton]

A lorentz frame cannot have a velocity of $c$ with respect to another lorentz frame; this is basic SR. You cannot apply a lorentz boost to something traveling at $c$. You should go learn SR first and note that your contentious behavior is nothing more than comical.

 

[Simon Bridge]

I have more important things to do than teach special relativity in a forum. If you learn textbooks by heart you will have a brilliant academic career but you will give poor contribution to physics.

Quite - OTOH: people who want to go around making startling comments should expect to defend those comments. This is central to how good science gets done and has the bonus of clearing up possible misunderstandings arising from imperfect communication.

If you are not prepared to defend your statements - don't make them ;)

I' am referring to the Einstein's mirror, that he thought when he was 16:

http://www.321books.co.uk/biography/einstein/mirror.htm

"Einstein's mirror is a hand held mirror. To perform the experiment, hold the mirror and look at your reflection. While retaining hold of the mirror, imagine what would happen if you were traveling at the speed of light. (Hint: Would you see your reflection?)"

The passage in the link does not support the claim naturale was challenged to support.

Einsteins Mirror is a thought experiment that uses the "luminiferous ether" model for light and was used by him as a way to illustrate problems with that model. In that model, traveling at the speed of light would appear to make sense ... however, the thought experiment (in a nutshell) showed that it did not make sense.

Also see:
http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/

 

[Dale]

The preference frame of the photon is the reference frame traveling at the speed of light

There is no such thing. A reference frame is orthonormal, and to move at the speed of light one vector of the tetrad would be null.

 

[nitsuj]

A reference frame is orthonormal, and to move at the speed of light one vector of the tetrad would be null.

It couldn't be said more simply, without being too vague.

 

[PeterDonis]

The preference frame of the photon is the reference frame traveling at the speed of light

Which is mathematically invalid; there is no such thing.

If you travel at c like light the gamma terms of the Lorentz transformation IS infinite.

No, the Lorentz transformation is mathematically invalid if v = c.

The Compton length of the photon is infinite.

No, it isn't. A photon doesn't have a Compton wavelength; the concept does not apply to photons.

I have more important things to do than teach special relativity in a forum.

But apparently you are able to make multiple misstatements about special relativity in a forum.

If you learn textbooks by heart you will have a brilliant academic career but you will give poor contribution to physics.

First of all, I'm not an academic; I post on PF for fun, it has nothing to do with my day job. Second, yes, I know some relativity textbooks well but what I'm saying doesn't come from parroting textbooks, it comes from my own understanding. Third, if we're going to talk about contributions to physics, what are yours?

 

[PeterDonis]

Is there a limit to how long the wavelength can be?

Theoretically, no, as long as it's finite. There may be in a practical sense, since there will be some minimum energy/momentum, and therefore some maximum wavelength, that can be detected.

It can be sensible to talk about a limit to a value even though the value itself is not attainable and does not make sense in the context.

If the limit makes sense physically, yes. I don't know of any limit as wavelength goes to infinity that makes sense physically.

 

[jtbell]

According to the uncertainty principle, you can't have an object with exactly and purely zero momentum. The best you can do is a wave packet whose expectation value for momentum is zero. Such a packet (for one-dimensional motion) is a superposition of waves going both to the left and to the right. It seems to me the result would be a standing wave, at least if each leftward-moving component is balanced by a rightward-moving component with equal amplitude.

 

[PeterDonis]

According to the uncertainty principle, you can't have an object with exactly and purely zero momentum. The best you can do is a wave packet whose expectation value for momentum is zero.

This is a good point, and brings up a question: is such a wave packet possible for a photon? More precisely, is such a wave packet possible for a photon that travels over a significant macroscopic distance--long enough that the only wave modes that contribute to the amplitude are the transverse ones? I think the answer is no, but I don't know that I've seen this precise question addressed.