Understanding the speed of light

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Discussion Overview

The discussion centers around the nature of photons, specifically addressing concepts of time and distance from a photon's perspective, the existence of a wave function for photons, and implications of these ideas on the measurement of the speed of light. The scope includes theoretical considerations and conceptual clarifications related to quantum mechanics and relativity.

Discussion Character

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

Main Points Raised

  • Some participants propose that for a photon, time and distance do not exist, leading to questions about the implications of this on the wave function and its interaction with the universe.
  • Others argue against the notion of a photon wave function in the traditional sense, stating that photons cannot be treated as particles in the usual way and can be confined in cavities.
  • A participant mentions that time dilation and length contraction apply only to massive objects, suggesting that photons, being massless, do not experience these effects.
  • There is a contention regarding whether photons have a wave function, with some asserting that they do, while others maintain that the properties of photons do not necessitate a wave function in the same way as other particles.
  • Some participants discuss the concept of localization of photons, referencing specific studies that explore the challenges of defining a wave function for photons.
  • There is a debate on whether a photon experiences the passage of time during its travel, with conflicting views on the implications of relativistic effects on massless particles.

Areas of Agreement / Disagreement

Participants express multiple competing views on the existence and nature of a wave function for photons, as well as the implications of masslessness on time and distance. The discussion remains unresolved with no consensus reached on these topics.

Contextual Notes

Limitations include varying definitions of wave functions and the implications of quantum mechanics on the behavior of photons, which are not universally agreed upon among participants.

jnorman
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1. is it appropriate to infer that, for a photon, time and distance do not exist?

2. if so, is it therefore appropriate to infer that once a photon is emitted, it's wave function permeates the entire universe immediately?

3. and if so, does our measurement of the "speed of light" at a fixed rate of C imply something peculiar about our own reference frame, ie that our measurement of C may be more reflective of the time required for the wave function to collapse, or some odd aspect of our ability to "measure", rather than the time required for a photon to travel a given distance?
 
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jnorman said:
1. is it appropriate to infer that, for a photon, time and distance do not exist?

Yes, I guess you could say that.

2. if so, is it therefore appropriate to infer that once a photon is emitted, it's wave function permeates the entire universe immediately?

No, for many different reasons. The main reason being that there is no such as a photon wavefunction (at least not in the usual sense); photons are not "particles" as such. It is perfectly possible to confine a generated photon using e.g. a cavity.
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f95toli said:
It is perfectly possible to confine a generated photon using e.g. a cavity.
Well… I would moderate that given the beautiful experiments of S. Haroche where he does just that : trap a photon in a cavity :smile:
 
I think it might be useful to point out that time dilation and length contraction only apply to things which have mass. since a photon is massless, it doesn't experience either time dilation or length contraction.
 
f95toli said:
jnorman said:
1. is it appropriate to infer that, for a photon, time and distance do not exist?
Yes, I guess you could say that.
jnorman: Do note that f95toli begrudgingly said "Yes". A better answer is that your question is nonsensical. That is not meant to belittle you; you are trying to come to grip with some weird concepts.

By way of analogy, I suspect you have seen various "proofs" that 0=1, 1=2, etc. There is almost always a division by zero hidden somewhere in these proofs. Dividing by zero is a nonsensical concept in the sense that division by zero leads to nonsense results. Because of this dividing by zero is not allowed. All that it takes to shoot down a proof as invalid is to show that some step involves a division by zero.

Back to the problem at hand: Asking about questions about how things look from the perspective of a photon is nonsensical is precisely because of division by zero.
 
No, for many different reasons. The main reason being that there is no such as a photon wavefunction (at least not in the usual sense); photons are not "particles" as such.
Photon has a wavefunction. It also is a particle, no less than anything else. It carries momentum, it interferes, it can hit an electron and can do virtually everything wavefunction or particle can do.
 
haael said:
Photon has a wavefunction. It also is a particle, no less than anything else. It carries momentum, it interferes, it can hit an electron and can do virtually everything wavefunction or particle can do.

Not exactly… It is rather similar to a quasiparticle, like a phonon… :smile:
 
haael said:
Photon has a wavefunction. It also is a particle, no less than anything else. It carries momentum, it interferes, it can hit an electron and can do virtually everything wavefunction or particle can do.

No it doesn't. The properties you list have nothing to do with whether or not you can write down a wavefunction for a photon.
 
No it doesn't. The properties you list have nothing to do with whether or not you can write down a wavefunction for a photon.
What is necessary to have a wavefunction then? For me if it quacks like a duck, then it is a duck.

Not exactly… It is rather similar to a quasiparticle, like a phonon…
I don't see a strict line between quasiparticles and "real" particles. On the other hand: all particles currently known are just quasiparticles if Higghs mechanism is correct. Except photon :), it's unaffected by Higgs.
 
  • #10
jnorman said:
1. is it appropriate to infer that, for a photon, time and distance do not exist?

The "point of view of a photon" is an ever-popular topic next door in the relativity forum. There's a thread about it going on right now, in fact:

If I was light...

You might like to check out that thread and similar ones that we have had in the past.
 
  • #11
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  • #12
haael said:
What is necessary to have a wavefunction then? For me if it quacks like a duck, then it is a duck.

It depends what you mean by “wavefunction”.

If you mean “something which obeys a Schrödinger equation”, you can define such a thing for a photon.
If you mean “something which gives the probability density of finding the photon at a certain point in space”, you cannot define such a thing for the photon.

:smile:
 
  • #13
If you mean “something which gives the probability density of finding the photon at a certain point in space”, you cannot define such a thing for the photon.
Photon wavefunction in a Schrödinger sense gives me interference pattern in double-slit experiment. So I can just claim that the intensity of light is the probability of finding photon particle at some point.

I don't see much difference to any other particle's wavefunction, neither mathematical nor physical.
 
  • #14
Nope, that's not correct :smile:
Have a look at this reference : Iwo and Zofia Bialynicki-Birula “Why photons cannot be sharly localized”, PRA 79, pp. 032112 (2009).
As one say : the title says it all :biggrin: They tried, and succeed in a sense, to construct a wavefunction for the photon which can be used to spatially localize it. Two problems : 1) you can either localize the electric or magnetic character of the photon, not both. 2) upon time evolution, this localization is lost at the speed of light… Talk about localization… :biggrin:
 
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  • #15
i think the original question was, 'does a photon experience the passage of time - since it travels at the speed of light.' i think the answer is yes, a photon experiences the entire year of a 1 lightyear trip. the reason being, that relativistic time dilation doesn't apply to a massless object.
 
  • #16
billbray said:
i think the original question was, 'does a photon experience the passage of time - since it travels at the speed of light.' i think the answer is yes, a photon experiences the entire year of a 1 lightyear trip. the reason being, that relativistic time dilation doesn't apply to a massless object.
That's not the correct conclusion. See the thread that jtbell linked to.
 
  • #17
haael said:
Photon has a wavefunction. It also is a particle, no less than anything else. It carries momentum, it interferes, it can hit an electron and can do virtually everything wavefunction or particle can do.

A photon can't have an Anti-Symmetric wave function. It requires an Anti-Symmetric wave function to have point particle like nature from what I understand.
 
  • #18
guerom00 said:
Nope, that's not correct :smile:
Have a look at this reference : Iwo and Zofia Bialynicki-Birula “Why photons cannot be sharly localized”, PRA 79, pp. 032112 (2009).

This is available here on arXiv as well:

http://arxiv.org/abs/0903.3712
 

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