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.
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.
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.
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.
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.
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.
I’ll level with you. I know how to use QED to compute the cross section of a scattering reaction. But I do not remember, or perhaps never knew, what the QED theoretic description of classical wave mechanical phenomena like diffraction, reflection, refraction, and dispersion look like.
Well… actually of those phenomena, I think diffraction is fine. A single waveform will exhibit diffraction. It doesn’t entail any interactions. A single photon can still exhibit a diffraction pattern. It doesn’t mean that the photon has changed directions or circled around or in any way accelerated. The only reason you might think so is that you’re thinking of photons as billiard ball type classical particles, but of course they are not, they are quantum particles with spread out wavefunctions.
Dispersion I guess is just scattering combined with absorption re-emission (and as we discussed, even scattering is itself a form of absorption & re-emission). But as for reflection and refraction? Those are the phenomena that Entropius was pointing to elsewhere in this thread. I remember how those look in terms of solutions to Maxwell’s equations and boundary conditions, but that’s classical wave mechanics. I do not remember how to translate that into the language of QED.
QED is a fundamental theory, so I assume that a description exists, and of course because I know what QED looks like, so I am certain that it will still be true that in this description, photons will be absorbed & emitted by charged particles, but photons will not interact with photons. However beyond that I cannot say much. How do we describe reflection of light in a mirror as photons scattering off electrons? I don’t know exactly.
One thing I can say is that generally classical states are modeled in quantum mechanics as coherent states, which are eigenstates of the annihilation operator. They look something like exp(N)|0> where N is the number operator, which means that they are states with a superposition of 0 photons, 1 photons, 2 photons, etc. They don’t have a well defined number of particles. So maybe if you want a QED theoretic description of reflection, you can have it, but you won’t be able to talk about specific numbers of photons. But again, I don’t know the details of this.
I wonder whether this concept of classical waveforms as coherent states with a superposition of all numbers of particles will help at all with this philosophical debate about whether two photons are the same particle or not, or about whether you can have a universe with only 3 photons
I’ll level with you. I only called it philosophical so I could hide behind that as a shield against an actual physics debate. But then I so showed my ass and mentioned the standard model. Thus leaving philosophy. I can’t hide behind unfalsifiable bullshit.
So I hope someone read this and went down some wikipedia rabbit holes. I’ll happily be “Cunningham’s fool”. I’ll give you, weird reader, some more wiki nuggets below.
I don’t think you should let some rando make you doubt anything. I don’t have a Ph.D. in physics. I only have a mild intro this stuff. I was on my way to getting a phd in physics (nuclear at that, not particle) and got distracted by math.
I don’t want to be super specific so as to not dox myself with a research fingerprint, but my research has crossed paths with things like Agmon metrics. Which although feels like I’m doing physics, it doesn’t change the fact that physicists don’t read my papers.
So I do find myself saying “apparently these graded algebras show up in quantum mechanics” and stuff like that. Maybe some day I’ll go back and learn it deeper, but I doubt it.
So online, I’d rather play the role of a street preacher spouting things like “nature can’t take a derivative. there is no continuum.” and hoping people read the links when I claim nature solves differential equations by means of weak solutions thereby only integrates. Integration is what nature does. I know that the phrase “nature solves differential equations” is nonsense. But it’s fun. So going deeper, nature can’t take a derivative because the idea of point particles destroys continuity. This is what saves the natural world from pathologies like the Banach–Tarski paradox. Those ideas are kinda basic, but I’m shooting for 1 in 10,000 read to whom the topic is both new and interesting for.
Sorry that you engaged with an internet crazy person. I hope it wasn’t too infuriating.
Lie groups are my favorite thing in all of mathematics, and gauge theory is my favorite thing in physics. E8 and all its connections to other subjects is one example of how amazing this subject can be.
It would be a coup de grace of the highest order, just the crowning intellectual achievement of mankind, if we could stumble upon a theory of everything explaining the entire Standard Model, just by fiddling around with how to fit SU(3)xSU(2)xU(1) fits inside E8 or whatever.
Light doesn’t travel the same speed in water or glass as in a vacuum.
In a medium, light usually does not propagate at a speed equal to c; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. A physical signal with a finite extent (a pulse of light) travels at a different speed. The overall envelope of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.
But doesn’t relativity explicitly state that c is the speed of light in a vacuum, and travelling through other mediums explicitly changes and is explained by relativity?
Not really no. Special relativity explains the relationship between space and time. General relativity expands on this to account for gravitation.
One of the postulates (i.e. assumptions) of relativity is that the speed of light in vacuum is the same for all observers. But the theory doesn’t actually require any particular value for c, it only needs it to be constant. And it doesn’t explain the behavior of light in a medium at all.
In fact, relativity doesn’t explain the mechanism by which light interacts at all, that is the domain of Quantum Electro Dynamics.
Wow that is so interesting. So am I understanding that relativity explains space, time and gravity’s interactions with one another, while quantum science explains interactions with much smaller objects like matter?
the speed of light expressed in units of distance per time, is a dimensionful quantity so it probably doesn’t mean anything to say some theory does or does not predict a value for it. The value is entirely determined by how big you choose your yardsticks and sundials to be, which is arbitrary convention.
It is only meaningful to talk about theoretical predictions of the values of constants if they are dimensionless, like the fine structure constant.
However relativity does suggest as a natural point of view that space and time are just orthogonal directions in a unified spacetime. In this point of view, relativity gives you the option of measuring your timelike and spacelike coordinates with the same yardstick (which you may still choose arbitrarily). And then relativity does predict its value. It’s 1. No units.
That's light as an aggregate wave. Photons, actual light, always travel at c. What's happening in a medium is the rapid absorption and readmission of photons. The probability of admission is based on structure of material causing things like lens or mirrors to work.
You can think of it as the photons having to jump between platforms before the can continue running at c.
What’s happening in a medium is the rapid absorption and readmission of photons. […]
You can think of it as the photons having to jump between platforms before the can continue running at c.
That’s an intuitive model, but unfortunately it doesn’t have the advantage of actually being correct. Photons are not being absorbed and reemitted. See here for why: lemmy.world/comment/5444224
That is wrong. Stochastic yes. Photons emission is probabilistic. Destructive interference causes emission to overwhelming follow classical wave theory. Here's a better explanation with a neat graphic.
It sounds like you’re conflating different concepts. A stochastic process like absorption/reemission would blur the light, so that’s not it. And the linked explanation is basically correct (in classical physics at least), but it doesn’t corroborate what you originally claimed as that’s not necessarily requiring absorbing anything. Photons can jiggle the charged particles in glass and get them to make new phase shifted light despite not being absorbed.
Don't think about individual photons. Think about billions of them with destructive and constructive interference. The probabilities of all the sitting l additive waves of light.
well, if it get reflected and change direction it going to be at light speed, so it can be interpreted (probably incorrectly lol) that it “accelerated instantly to the other direction after the reflection”?
This is an interesting question. Instant acceleration is mathematically implausible, but I don’t know if there’s a better physical interpretation for what happens to a bouncing photon. I’m guessing this is one of those “less particle, more wave” situations where the instantaneous velocity of the photon is undefined.
According to some random internet sources, reflection is the not-quite-instantaneous process of the photon being absorbed and then emitted by the electrons in the mirror.
As a rule, it’s probably best to avoid “random” internet sources on matters of how light works because there’s so much confidently parroted misinformation out there. For example, this is completely wrong: youtu.be/FAivtXJOsiI See here for correct answers to that issue: youtu.be/CiHN0ZWE5bk
The speed of light is different depending on the medium though isn’t it? So to change speed I would have thought some acceleration would have to be involved.
Add comment