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Why do we know the velocity of all photons are the same as the velocity of light?Can we deduce this or we must have experiment test?What is the experiment test?
Photons are light. They have the speed of their speed. Is that what you really meant to ask?Why do we know the velocity of all photons are the same as the velocity of light?Can we deduce this or we must have experiment test?What is the experiment test?
(Lecture 2, 2.2 Difficulties of Speculative Theories):
He asked, "Tell me, Professor Feynman, how sure are you that the photon has no rest mass?"
...[snip]...
My answer was that, if we agreed that the mass of the photon was related to the frequency as [itex] \omega=\sqrt{k^2+m^2} [/itex]
photons of different wavelengths would travel with different velocities. Then in observing an eclipsing double star, which was sufficiently far away, we would observe the eclipse in blue light and red light at different times. Since nothing like this is observed, we can put an upper limit on the mass, which, if you do the numbers, turns out to be of the order of [itex] 10^{-9} [/itex] electron masses.
Ch 3 Implications of a photon mass 83
3.1. The dispersion of light 83
3.2. The Yukawa potential in static fields 84
3.3. The longitudinal photon 84
3.4. Special relativity with nonzero photon mass 85
3.5. AB and AC effects with finite photon mass 85
3.6. Monopoles and the photon mass 87
3.7. The Casimir effect for massive photons 88
3.8. Photon mass and blackbody radiation 89
3.9. Other implications
So again: photons are light [and are em waves]. They have the speed of their speed.I am starting read Special Relativity,then I like to know why photon has same speed as of EM wave speed of light, without relying on Special Relativity theory.
I am not sure that this makes any sense. A photon is a concept of QED so it doesn’t make sense to ask about a photon outside of that context. QED has relativity built into it. So I don’t think there is a way to ask about a photon without relying on relativity.I am starting read Special Relativity,then I like to know why photon has same speed as of EM wave speed of light, without relying on Special Relativity theory.
Yes, everything follows from that. You can also derive it from the basic symmetry principles like homogeneity, isotropy, etc.Does it only depend on invariant of spacetime interval?
Photon(particle) has statistical manifestation
in what sense does photon have statistical manifestation?
Yes. You cannot say 'where it is' ever. Hence the fact that people say a photon has a presence at either of the two Young's Slits; it could be anywhere in space during the, (poorly described as) flight. There is no problem with the fact that it has to end up somewhere definite because it will experience no effect of time (SR) and so the initial worry about that information getting to all parts of the photon at the instant it's detected, is groundless.Then because speed of photon is definite, then the position of photon is very uncertainly?
We have a lot of experiments, which clearly show that photons do NOT all travel at the same speed.
Is that according to NMQFT (Newtonian-Maxwellian-Quantum Field Theory)?If we suppose position of photon is somewhere in EM wave packet, then how can we explain Vavilov experiment?But it is reasonable to suppose the position of photon is limited in wave packet of EM field.
Photons travel at the same speed(that of light) only in vacuum.
Can you name just one that measures c in a vacuum and gets a spread of values? Dispersion and observed gravitational bending of light do not disprove that c is constant - you just need to describe the phenomena in the right terms.We have a lot of experiments, which clearly show that photons do NOT all travel at the same speed.
What do you mean when saying NMQFT?
Well-defined position of a particle (without measurement) is a concept from Newtonian physics.If we suppose position of photon is somewhere in EM wave packet, then how can we explain Vavilov experiment?But it is reasonable to suppose the position of photon is limited in wave packet of EM field.
It's unfortunate that you start to learn about quantum mechanics on the example of photons. The problem is that photons were the first discovery of the fact that classical physics needs to be substituted by quantum mechanics due to Planck's discovery of the theoretical foundation of the black-body radiation spectrum. Now many textbook writers think they have to talk about "photons" in the introductory sections of quantum-mechanics textbooks and also in the popular-science literature.Then because speed of photon is definite, then the position of photon is very uncertainly?
No, and that´ s my point. We know what it looks like when photons have different speeds, and it always appears related to the effects of medium on photon, not intrinsic properties of photon. Whereas rest mass of photon would show up as low frequency radio waves having index of refraction that diverges at low frequencies independent of medium.Can you name just one that measures c in a vacuum and gets a spread of values?
Do we know? Are you sure? Because I have no idea.We know what it looks like when photons have different speeds
That doesn't make any sense to me. How would that suggest a rest mass of photons?Whereas rest mass of photon would show up as low frequency radio waves having index of refraction that diverges at low frequencies independent of medium.