Does a photon experience time when slowed down by gravity?

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Discussion Overview

The discussion revolves around whether a photon experiences time, particularly in the context of its behavior under the influence of gravity and in different mediums. Participants explore concepts related to the nature of light, time, and the implications of photons being massless.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that light does not experience time because it travels at the speed of light, leading to the idea that its internal clock is "frozen."
  • Others argue that while light does not experience proper time, it still defines the causal structure of spacetime, allowing for the comparison of time between different objects.
  • A participant suggests that when light slows down due to gravity, it might experience more time, raising questions about the nature of time in such scenarios.
  • Some contributions clarify that light always moves at speed c locally, and any apparent slowing is due to spacetime curvature or the medium it travels through.
  • There is a debate about whether a photon "experiences" time when absorbed by an electron, with some suggesting that it ceases to exist as a photon during absorption.
  • Participants discuss the implications of length contraction and whether it applies to photons, with some asserting that the concept is not valid for objects traveling at the speed of light.
  • Disagreements arise regarding the interpretation of wave-particle duality and the properties of photons, including their frequency and mass.

Areas of Agreement / Disagreement

Participants express multiple competing views on the nature of time as it relates to photons, with no consensus reached on whether photons experience time or how their properties change under various conditions.

Contextual Notes

Some statements rely on specific interpretations of physics concepts, such as wave-particle duality and the nature of spacetime, which may not be universally accepted. The discussion includes unresolved mathematical and conceptual challenges regarding the behavior of light and time.

  • #31
PeterDonis said:
[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.
 
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  • #32
DrGreg said:
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.
 
  • #33
Simon Bridge said:
... it is more proper to consider the long wavelength limit isn't it?

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

Simon Bridge said:
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.
 
  • #34
Phy_Man said:
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.
 
  • #35
PeterDonis said:
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.
 
  • #36
PeterDonis said:
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.
 
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  • #37
PeterDonis said:
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!
 
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  • #38
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.
 
  • #39
naturale said:
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 ;)

naturale said:
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/
 
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  • #40
naturale said:
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.
 
  • #41
DaleSpam said:
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.
 
  • #42
naturale said:
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.

naturale said:
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.

naturale said:
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.

naturale said:
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.

naturale said:
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?
 
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  • #43
Simon Bridge said:
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.

Simon Bridge said:
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.
 
  • #44
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.
 
  • #45
jtbell said:
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.
 

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