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Photons cannot accelerate
Acceleration and Velocity are vectors. Changes in a velocity vector are an acceleration. Therefore when photons change direction technically it’s a form of acceleration.
I thought photons are always moving in straight lines from their perspective, and it's space that's bent. Unless it's through a medium, then they just get absorbed and re-emitted, sort of.
Space bending is a general relativity thing, which isn’t really related much to how mirrors work.
Regarding the medium bit, photons being absorbed and remitted can’t explain how light moves slower in glass. This is just an extremely popular myth. Photons are only absorbed by atoms at very specific frequencies. Also, the entire reason glass is transparent to begin with is that it’s not absorbing the photons (requires too much energy to bump the electron’s energy level so the photon isn’t absorbed and it keeps on trucking). Also photon absorption and remission is stochastic so there’s no way to control the direction it happens in or how quickly it happens. Random directions of remitted light would make glass translucent, not transparent. So for a few reasons, that’s not how it works.
Ok but photons don’t change direction either. Treating photon scattering as an individual particle accelerating due to an applied force, well that’s just not a correct description of how perturbative QED models photon interactions.
They do at 0m/s^2.
Not with that attitude.
What about diffraction?
Since photons are indistinguishable, it's hard to say too much concretely, but it some sense a diffracted photon is different photon. In order for a photon to interact with say, a diffraction grating, the interaction is done with "virtual photons".
So for a photon to change course, aka accelerate, it does it by absorbing a virtual photon and emitting another. Whether that is the "same photon" after the interaction is kinda more philosophy than physics, at least to me.
Feynman diagrams are surprisingly accessible for how much information they contain. It's one way to think about photon (and other particle) reactions.
There is no tree level photon-photon interaction. Photons scatter off electrons (or any other charged particle), not off neutral photons.
Are you claiming this is done without a force carrier? If you are working outside the standard model, I guess that's fine, but I don't want to spend time arguing with you.
The electromagnetic field does have a force carrier. It is the photon.
The photon mediates the force between electrically charged particles. It cannot mediate any reaction between two neutral photons.
Ah, I see. Sorry for the snark. I was thinking more in line with the Compton effect, and thought you were talking about something like that too. (Even though it's clear that you were explicitly not. I thought you were denying photon-virtual photon interaction because I was talking about it in a funny way.)
I would still say it's still philosophical whether photons experience acceleration, but I concede that photon-photon interaction is not done by virtual photon exchange.
I am indeed denying the existence of photons interacting with virtual photons. I am also saying there is no tree level photon-photon interaction of on shell photons. Neither Compton scattering nor Bhabha nor pair production nor pair annihilation involves a photon-photon interaction. There is no photon-photon vertex in QED. There is no tree level Feynman diagram that you can look at and say “this is, at least philosophically, a photon changing its momentum”.
There is a 1 loop diagram that represents photon-photon scattering. But even that doesn’t have any photon-photon vertices, instead it is mediated by electron-positron pair.
Non-abelian gauge bosons (gluons) couple to themselves. So does gravity (gravitons). Abelian ones (photons) do not.
Photons don’t accelerate. They are emitted or absorbed. That’s their only interaction.
Someone asked if diffracted light accelerated. I said no. A diffracted photon is a different photon.
I gave some lip service to the Feynman "there is but one electron" idea, and you seemed to take that personally.
If someone asks you if diffracted light accelerates, answer them how you want. I just thought it'd be cool to show them Feyman diagrams.
Look bro. Your top level comment that I replied to was generally correct, and also very helpful. I liked it. I liked the suggestion for people to look at the Feynman diagrams. I agreed with it. I upvoted it.
I hope I'm not giving you the impression that I'm taking a personal issue with you. I'm not. I like you and I hope we'll still be friends when this is all over. I promise to read Discworld soon.
The only quibble I had with what you wrote was this one sentence:
So for a photon to change course, aka accelerate, it does it by absorbing a virtual photon and emitting another.
Photons do not absorb virtual photons. And real on-shell photons do not interact. In Compton scattering and 1 loop photon-photon scattering, you can think of photons emitting e+e- pairs. But never do they emit or absorb other photons.
Maybe that's not what you meant with that sentence, and I misunderstood. If that's the case, my bad. Maybe you didn't need the explanation. If someone else made the same misunderstanding reading your comment that I did, then maybe my comments will help them even if you don't need them. Or if not, if it's just me being dumb, well c'est la vie.
Cheers bro.
You're right. And I'm the one being less than friendly. It's nothing personal. It's just something I've noticed about myself. It's that I hate talking about physics on the internet.
I'm high on lemmy, not in my office. I read a terrible meme. So I open the comments, and see your comment. It was exactly what I was thinking. "Photons don't accelerate." Which I took to mean "your meme is bad and you should feel bad". And again, I agree, it is horrible, this meme.
I like to shoot the shit about, say, quantum loop gravity (i'm honestly clueless about it) with people at the office, but on lemmy, academics piss me off. I don't know why.
So from your reply, natural question arises: What about diffraction?
You went academic. I'm high. So I just steer them to a right answer while bringing up less academic (but valid (maybe)) ideas about philosophy. I did that because I hate when academics try to seriously discuss that "there is only one electron idea" and similarly unfalsifiable crap. That shit belongs on dumb internet forums with bad memes. And man did I find a bad meme. So was angling for a stupid debate about whether any particle can ever accelerate. You can't trace them from idenitical copies. Are they the same particle after an interaction knowing that force carriers exist in the standard model? Not an actual quantum field theory debate.
But to give you some closure. I do see that I clearly did imply a tree-level interaction in my initial reply. It is wrong to say a photon emits anything. You were also very direct in your correction. I read it along with other comments and must have confused myself. So in all the back-peddling I was doing, I was avoiding defining "an interaction". I was just trying to say any influence is an interaction. Not two photons touching on a diagram.
Also, I have a vague memory in grad school. Two people smarter than me were debating whether in a universe consisting only of 3 photons, would they be able to interact? I couldn't focus on what was said. I was having an existantial crisis. So I had that clacking around in the back of my head. So I'm just going to stop writing now, because as I mentioned, I'm high. So I should just stop.