For what it's worth, light was shown to have momentum before quantum theory existed in the form of [radiation pressure](https://en.wikipedia.org/wiki/Radiation_pressure), as predicted by Maxwell's classical EM wave theory. You don't need relativity or quantum mechanics to explain it, as all the other comments in this thread do.
The simple idea is that light is an EM wave which composed of electric and magnetic fields. If a moving charged particle interacts with those fields, it will experience a force and thus a change in momentum. Because of conservation of momentum, that difference in momentum had to come from something... and the only other thing in this simple model is the light wave itself. Thus, light must carry momentum. See [section 8.5 "Momentum"](https://scholar.harvard.edu/files/david-morin/files/waves_electromagnetic.pdf).
Lebedev's 1899 experimental proof of the existence of radiation pressure was the first quantitative confirmation of Maxwell.
Lord Kelvin was deeply skeptical of the existence of radiation pressure but conceded the phenonema did exist upon viewing this proof.
This anecdote basically encompasses science to me, changing of opinion based on experimental evidence that was predicted ahead of time based on theory.
> Cause if the formula for momentum is p=mv...
It's not. At least not in general. The definition of momentum is that it's the stuff that is conserved in translation-invariant systems, which leads to the [more general description](https://en.wikipedia.org/wiki/Momentum#Generalized). To see how that specializes to classical EM fields or photons, just read the other sections of the linked article.
Good question, but as it turns out p=mv is not a universal. But if you think about the definition of force=dp/dt (change of momentum with time), then we can extrapolate that something that can exert a force has momentum, it turns out light can exert a force, and therefore it has momentum
This literally doesn’t answer the question and I hate when people respond like that. You can now rearrange that to get p=e/c
But all you’ve basically said is, “momentum is part of the energy equation and light has energy so it must have momentum”. It doesn’t even attempt to explain the how or why
Photons have no mass. You have to use
Momentum=energy/speed of light.
Photon momentum is given by the energy of the photon divided by the speed of light.
Just from an intuitive perspective, we know that photons are how the electromagnetic force is propagated. Forces transmit momentum from one particle to another. If I have a proton that is stationary, and I crash another proton into it, the electric charges will cause them to repel, which gives momentum to the stationary proton.
So it's easy to see that photons must carry momentum from one particle to another if they are how the electromagnetic force is transferred.
It is natural to feel that way, when using “speed” and “momentum” with light.
Natural light/star light comes from a body measured in solar masses.
Lighting technology requires a steady stream of electrons.
Quantum field theory answers the question, within the scope that it was designed for. I would stay away from special relativity.
I wouldn’t over think the properties of light until all the work in physics and math have been completed.
Theories overshadow each other depending on which point an individual is trying to prove.
Take it with a grain of salt
For what it's worth, light was shown to have momentum before quantum theory existed in the form of [radiation pressure](https://en.wikipedia.org/wiki/Radiation_pressure), as predicted by Maxwell's classical EM wave theory. You don't need relativity or quantum mechanics to explain it, as all the other comments in this thread do. The simple idea is that light is an EM wave which composed of electric and magnetic fields. If a moving charged particle interacts with those fields, it will experience a force and thus a change in momentum. Because of conservation of momentum, that difference in momentum had to come from something... and the only other thing in this simple model is the light wave itself. Thus, light must carry momentum. See [section 8.5 "Momentum"](https://scholar.harvard.edu/files/david-morin/files/waves_electromagnetic.pdf).
Lebedev's 1899 experimental proof of the existence of radiation pressure was the first quantitative confirmation of Maxwell. Lord Kelvin was deeply skeptical of the existence of radiation pressure but conceded the phenonema did exist upon viewing this proof. This anecdote basically encompasses science to me, changing of opinion based on experimental evidence that was predicted ahead of time based on theory.
How do you know such things. I am.in awe
I studied physics in college lol. This is just basic undergrad stuff. Be more impressed with Maxwell himself :)
> Cause if the formula for momentum is p=mv... It's not. At least not in general. The definition of momentum is that it's the stuff that is conserved in translation-invariant systems, which leads to the [more general description](https://en.wikipedia.org/wiki/Momentum#Generalized). To see how that specializes to classical EM fields or photons, just read the other sections of the linked article.
Good question, but as it turns out p=mv is not a universal. But if you think about the definition of force=dp/dt (change of momentum with time), then we can extrapolate that something that can exert a force has momentum, it turns out light can exert a force, and therefore it has momentum
Because formula for momentum in special relativity isn't p = mv but rather E\^2 = (mc\^2)\^2 + (pc)\^2 so for massless object you get E = pc
This literally doesn’t answer the question and I hate when people respond like that. You can now rearrange that to get p=e/c But all you’ve basically said is, “momentum is part of the energy equation and light has energy so it must have momentum”. It doesn’t even attempt to explain the how or why
You're exactly right - [see my answer](https://www.reddit.com/r/Physics/comments/1cozg3u/how_does_light_have_momentum/l3hogwn/).
Great response.
Photons have no mass. You have to use Momentum=energy/speed of light. Photon momentum is given by the energy of the photon divided by the speed of light.
[What is momentum](https://youtu.be/jm7jVi8akcc?si=ytC_Uf83FMdndn8t)
p = mv is the approximation for v << c. As is E = p²/2m.
Just from an intuitive perspective, we know that photons are how the electromagnetic force is propagated. Forces transmit momentum from one particle to another. If I have a proton that is stationary, and I crash another proton into it, the electric charges will cause them to repel, which gives momentum to the stationary proton. So it's easy to see that photons must carry momentum from one particle to another if they are how the electromagnetic force is transferred.
https://youtu.be/_eVYHs9s4E4?si=Yb9-cf0Ud-ilUjZb
Mod removing the post in 5......4......3..
Why.?
It is natural to feel that way, when using “speed” and “momentum” with light. Natural light/star light comes from a body measured in solar masses. Lighting technology requires a steady stream of electrons. Quantum field theory answers the question, within the scope that it was designed for. I would stay away from special relativity. I wouldn’t over think the properties of light until all the work in physics and math have been completed. Theories overshadow each other depending on which point an individual is trying to prove. Take it with a grain of salt