Photon Energy Change: Speed of Light

[danielhaish]

photos are in the speed of light which means that the fasts change in photons energy would take infinite time for the outside observer so does it means that the photons can't spin or interact with each other or have any kind of change

 

[vanhees71]

You have a wrong conception about photons, I guess from the plethora of bad popular-science books providing this totally wrong picture of photons as if they were like little massless particles. That doesn't make any sense from a scientific point of view.

A much better picture is a plane electromagnetic wave. You can just think of it as the weakest possible electromagnetic wave of a given frequency. That's also not an entirely correct picture, because we deal with a quantized electromagnetic field rather than a classical one, but it's imho a much better analogy than the conception of massless particles provided by the usual popular-science literature.

That said, it's really no problem to understand that of course "photons" can change when interacting with matter consisting of charged particles as any electromagnetic wave does and as you well know from everyday life: Light is also an electromagnetic wave field, and indeed it is changed when interacting with matter in manifold ways. E.g., going through transparent material (physically spoken a dielectric) it is refracted and reflected, i.e., it changes its direction of propagation (in the correct quantumfieldtheoretical picture of a photon the wave vector is analogous to the momentum of the photon, related by the famous de Broglie relation $\vec{p}=\hbar \vec{k}$).

Another quite fascinating feature of the correct quantum theory of light, i.e., quantum electrodynamics, is that as a higher-order effect in perturbation theory indeed also light scatters on light (Delbrück scattering), but that's another story.

It is also clear from this picture that it doesn't make sense to think about a "rest frame of a photon". In fact that was Einstein very early thought experiment concerning the problems of classical electrodynamics with the Galilean spacetime structure of Newtonian mechanics: If you could run along the propgation direction of a light wave at the speed of light, you'd have to see some static periodic electromagnetic field. On the other hand the special principle of relativity tells you that the Maxwell equations should be valid also in the rest frame of Einstein running along the light wave with the speed of light, but there are no oscillatory solutions of the static, i.e., time-independent Maxwell equations.

The resolution of this paradox of course is Einstein's discovery of the correct interpretation of this issue: The special principle of relativity is still true, but one has to use another space-time description, the socalled Minkowski space rather than the Galilei-Newtonian spacetime, and correspondingly the rules, how to transform the physical quantities from one inertial reference frame to another one changes. Particularly it turns out that two inertial reference frames can only move with a (of course constant) velocity relative to each other whose magnitude is smaller than the speed of light, and a electromagnetic wave (in a vacuum) always propagates with the speed of light in any such inertial frame, i.e., the speed of light in vacuo is independent on the motion of the source relative to any inertial observer always $c$ (which since 2019 is just a conversion factor to define the unit of length, metre, in terms of the unit of time, second, as is natural in relativistic physics).

 

[danielhaish]

You have a wrong conception about photons, I guess from the plethora of bad popular-science books providing this totally wrong picture of photons as if they were like little massless particles. That doesn't make any sense from a scientific point of view.

A much better picture is a plane electromagnetic wave. You can just think of it as the weakest possible electromagnetic wave of a given frequency. That's also not an entirely correct picture, because we deal with a quantized electromagnetic field rather than a classical one, but it's imho a much better analogy than the conception of massless particles provided by the usual popular-science literature.

That said, it's really no problem to understand that of course "photons" can change when interacting with matter consisting of charged particles as any electromagnetic wave does and as you well know from everyday life: Light is also an electromagnetic wave field, and indeed it is changed when interacting with matter in manifold ways. E.g., going through transparent material (physically spoken a dielectric) it is refracted and reflected, i.e., it changes its direction of propagation (in the correct quantumfieldtheoretical picture of a photon the wave vector is analogous to the momentum of the photon, related by the famous de Broglie relation p→=ℏk→).

Another quite fascinating feature of the correct quantum theory of light, i.e., quantum electrodynamics, is that as a higher-order effect in perturbation theory indeed also light scatters on light (Delbrück scattering), but that's another story.

It is also clear from this picture that it doesn't make sense to think about a "rest frame of a photon". In fact that was Einstein very early thought experiment concerning the problems of classical electrodynamics with the Galilean spacetime structure of Newtonian mechanics: If you could run along the propgation direction of a light wave at the speed of light, you'd have to see some static periodic electromagnetic field. On the other hand the special principle of relativity tells you that the Maxwell equations should be valid also in the rest frame of Einstein running along the light wave with the speed of light, but there are no oscillatory solutions of the static, i.e., time-independent Maxwell equations.

The resolution of this paradox of course is Einstein's discovery of the correct interpretation of this issue: The special principle of relativity is still true, but one has to use another space-time description, the socalled Minkowski space rather than the Galilei-Newtonian spacetime, and correspondingly the rules, how to transform the physical quantities from one inertial reference frame to another one changes. Particularly it turns out that two inertial reference frames can only move with a (of course constan

n's discovery of the correct i

t) velocity relative to each other whose magnitude is smaller than the speed of light, and a electromagnetic wave (in a vacuum) always propagates with the speed of light in any such inertial frame, i.e., the speed of light in vacuo is independent on the motion of the source relative to any inertial observer always c (which since 2019 is just a conversion factor to define the unit of length, metre, in terms of the unit of time, second, as is natural in relativistic physics).

thanks got it ,so why we relate photon momentum (spin) if it just and change in the electromagnetic filed that going up and down and what make this change spread ?

 

[etotheipi]

photon momentum (spin)

The photon momentum $\hbar \vec{k}$, and its spin angular momentum, are two different things.

 

[Nugatory]

The entire idea that photons "don't experience time" is a misunderstanding, so the question doesn't have any clear answer. It's a common enough misunderstanding that we even have a FAQ for it: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

You can sort of get to that bogus idea through a sequence of missteps:
1) Think of a photon as something that moves through space (but it's not)
2) Assume that there is an inertial frame whose origin is moving at the speed of light relative to another inertial frame, one in which you are at rest (but such a frame cannot exist)
3) Assume that the photon is at rest in that frame (but we know that the speed of light is $c$ in all frames)
4) Apply the time dilation formula between the two frames (but the time dilation formula is derived using assumptions equivalent to the assumption that the relative speed of the origins of the frames is less than $c$, so it can't be used here).
5) Apply the result of #4 to to conclude that "time stops" for the photon (but we could just as incorrectly use this calculation to "prove" that time stops for us instead)

Pretty clearly every step here is wrong, and the statement that photons don't experience time is somewhere between meaningless and incorrect.

As this thread is based on a misunderstanding, it is now closed.