# Understanding the speed of light

1. May 4, 2010

### jnorman

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?

2. May 4, 2010

### f95toli

Yes, I guess you could say that.

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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3. May 4, 2010

### guerom00

Well… I would moderate that given the beautiful experiments of S. Haroche where he does just that : trap a photon in a cavity

4. May 4, 2010

### billbray

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.

5. May 4, 2010

### D H

Staff Emeritus
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.

6. May 4, 2010

### haael

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.

7. May 4, 2010

### guerom00

Not exactly… It is rather similar to a quasiparticle, like a phonon…

8. May 4, 2010

### f95toli

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.

9. May 4, 2010

### haael

What is necessary to have a wavefunction then? For me if it quacks like a duck, then it is a duck.

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. May 4, 2010

### Staff: Mentor

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. May 4, 2010

### JDługosz

Last edited by a moderator: Apr 25, 2017
12. May 4, 2010

### guerom00

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.

13. May 4, 2010

### haael

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. May 5, 2010

### guerom00

Nope, that's not correct
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 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…

Last edited: May 5, 2010
15. May 5, 2010

### billbray

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. May 5, 2010

### Fredrik

Staff Emeritus
That's not the correct conclusion. See the thread that jtbell linked to.

17. May 5, 2010

### LostConjugate

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. May 6, 2010

### inflector

This is available here on arXiv as well:

http://arxiv.org/abs/0903.3712