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I don't accept the claim that a photon has no mass.
You can't localize a photon, so you can't do that.
Maybe I am unclear?Are you telling me that gravity is observer dependent?
Quote from: lightarrow You can't localize a photon, so you can't do that.Everything I’ve seen in this thread speaks mostly about classical physics, e.g. relativity. In relativity one uses classical photons, which is basically a point particle having a classical trajectory but zero proper mass. Such a thing can be localized.
Classical photons? Which movie is it? 
Quote from: lightarrow linkClassical photons? Which movie is it? I don't understand what you mean, "Which movie is it?"lightarrow - Have you ever heard of the terms "classical photon" and "classical electron"?
Quote from: Pmb on 27/12/2012 06:25:21Quote from: lightarrow linkClassical photons? Which movie is it? I don't understand what you mean, "Which movie is it?"lightarrow - Have you ever heard of the terms "classical photon" and "classical electron"? "classical electron": yes"classical photon": no. The reason is because of qm history: a classical electron was a starting point for Bohr and Sommerfeld when they described the atom. But a classical photon couldn't have any meaning, because m = 0 in this case.
I believe a "classical photon" would be a ray. You get ray optics from light waves using the same procedure that you can use to get particle-like electrons from a more thorough quantum wave theory. But it sounds like Pmb's classical photons are like little bullets, not rays. I'm not sure how to get to those from the wave theory.
A ray would be approximated as a stream of classical photons. Picture a laser beam as an approximation of a ray.
But a beam isn't a point particle.
I understand that you can express a beam roughly as a density of classical point particle photons moving along raytrajectories at the speed of light, but the case of two classical photons ...
Quote from: Pmb on 29/12/2012 01:00:36A ray would be approximated as a stream of classical photons. Picture a laser beam as an approximation of a ray.It's the same mistake that one makes stating that an electron's track in a bubble chamber means that elementary particles have a precise trajectory. QM teaches us they actually don't have.
Heh I can see you and Pete gearing up to a argument Lightarrow
Quote from: lightarrow on 29/12/2012 18:29:47It's the same mistake that one makes stating that an electron's track in a bubble chamber means that elementary particles have a precise trajectory. QM teaches us they actually don't have.Hence the term "approximation". In classical mechanics we're most often concerned with physics only down to, perhaps, the micron level.
It's the same mistake that one makes stating that an electron's track in a bubble chamber means that elementary particles have a precise trajectory. QM teaches us they actually don't have.
No. When interactions among elementary particles are involved, ...
Quote from: lightarrowNo. When interactions among elementary particles are involved, ...Then you have to use QM. You did understand, didn't you, that I was speaking about classical relativity?
I wasn't speaking of quantum relativity since it doesn't exist yet.
Clearly, when one is speaking of a particle moving on a null geodesics one is thinking about classical luxons. And that's what I've been explaining here.
So you can't speak of photons...
AbstractWe follow through the different variants of Einstein's intuitive photon-in-a-box derivation of the inertia of the inertia of energy, then end with a very simple "radiating atom" derivation
Would you folks be interested if I started and/or split off a thread with a title along the lines of "Is there such a thing as a "classical" photon?" or are you OK with this thread being used for discussion?
JP, as a "hitch-hiker" where science is concerned, I got a bit bogged down in this thread. In general, I find your explanations reasonably easy to follow, as long as you don't wax too technical, so if you are going to say a bit more about the idea of "classical photons" I'll follow with interest.
It does not happen.
JP - When the wave nature of light is important I can't see how photons can be used. In fact that's what led to the wave-particle duality.
Conclusion: if you use the word "photon" you are talking of quantum mechanical description by definition.
What you spoke about above also holds true for electrons and when you use the term “electron” it doesn’t mean that you’re talking about a quantum mechanical description. We use the term “electron” in classical electrodynamics which by definition is a classical, not a quantum, branch of physics.
But in those cases you neglet the wave aspect of the particle.
Quote from: lightarrowBut in those cases you neglet the wave aspect of the particle. Of course. That’s entirely the point. Maybe that’s what has you confused. You must have thought that somewhere someone was making the claim that under all conceivable scenarios the photon can always be treated as a classical particle.
Not all experiments and observations pertain to the wave aspects of photons.
When one is, say, analyzing photons in Young's double slit experiment then one is going outside the scope of classical relativity since a classical trajectory cannot be used to describe the motion of the photon.However in other circumstances, say, when one is tracing a photon as a particle bouncing off mirrors which are moving in a vacuum or are being emitted and detected by photon emitters and detectors then one can use classical relativity and classical trajectories.
I have never thought it. The point is that the photon can never be treated as a classicle particle 
I'm wondering, now, if I should consider the center of the two-photons to be the center of energy; equivalent to center of mass.
Quote from: ][I have never thought it. The point is that the photon can never be treated as a classicle particle Your point is wrong. And its often treated as such.Please post a proof demonstrating that a photon can never be treated as a classical particle.
It's very simple, and I let you do it: define "photon".
... "Is there such a thing as a "classical" photon?" …
Quote from: JP on 01/01/2013 22:22:58... "Is there such a thing as a "classical" photon?" …The answer is “No.” Does that mean that the concept is not useful or that it’s not being used in the physics/relativity literature? The answer to that is “No” as well. Then again there is no such thing as a “classical electron” since no particle exists that has the same properties of a “quantum electron” and also does not move on a well-defined trajectory. Also no “classical electron” satisfies the uncertainty principle. That also doesn’t mean that the concept is not useful or that it’s not being used in the physics/relativity literature.
But there is a difference because you can treat electrons as classical particles in some cases, ...
Furthermore, considering the electron as a classical particle, you can describe the most important features of the hydrogen atom; in which case you can do something similar with the photon?
http://en.wikipedia.org/wiki/Photon<<A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation,>>just the term "quantum" removes every "classical" description *by definition*. A photon is the quantum of the electromagnetic field, that is a quantum object that comes from the QED "quantum electrodynamics", it doesn't have a classical origin as the electron, it borns quantum....
If you look at the Standard Model of particle physics, photons are a quantum of the electromagnetic field, and electrons are a quantum of the electron field: this symmetry between matter and fields is a major part of the Standard Model. We know that we can take a classical limit of the quantum description for electrons and arrive at classical electrons. I've never seen it done explicitly, but I suspect you could do the same for photons and arrive at a classical limit for photons--i.e. the classical photons Pmb brings up. Maybe there's a reason why you can't, but based on my (admittedly limited) understanding of the Standard Model, I don't see why this would be.