Lemdro.id

1 point

1 year ago

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.

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Ook the Librarian@lemmy.world

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3 points

1 year ago

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.

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1 point

1 year ago

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.

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Ook the Librarian@lemmy.world

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2 points

1 year ago

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.

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1 point

1 year ago

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

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Ook the Librarian@lemmy.world

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1 point

1 year ago

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.

But I do love knowing that there is a connection even if I don’t see all the details. Like I don’t think I’ll ever understand sentences like “One way to incorporate the standard model of particle physics into heterotic string theory is the symmetry breaking of E8 to its maximal subalgebra SU(3)×E6.”. I need to know about Lie symmetries, but I’m not in physics or algebra. So I don’t think I’ll flesh out this connection, but it really makes me ponder The Unreasonable Effectiveness of Mathematics in the Natural Sciences.

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.

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