the word is “normalcy”

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.

But I guess it’s not going to happen.

Do people not remember back in the 2010s when bit.ly was the main link shortener used everywhere on the internet, and then Ghadafi, the then dictator of Libya, declared the site to be incompatible with Muslin decency norms because it was used for porn? And then all bit.ly links were just dead links?

How many times do we have to learn this lesson? Domain name hacks are fun but just not worth it. And in 2023, now we have all the new TLDs. This was a dumb decision

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

complex numbers are just numbers in a plane instead of a line. saying you don’t believe in i is like saying you don’t believe in “up”

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.

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.

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.

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.

There is no tree level photon-photon interaction. Photons scatter off electrons (or any other charged particle), not off neutral photons.

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.

The fact that light cannot change speed is one of the core axioms of relativity

Photons cannot accelerate

Oh it’s Trotsky

Stalin on top, Lenin on the right, Marx in the middle. Who is the guy on the left?

Even if dems win a majority in the house, they may not have a majority of state delegations. the house vote is by state delegation.

Lemme see the Goldilocks one too, for comparison

I mean, do you think Islamophobia is rooted in theological disputes? The Christian nationalists who find common ground between Catholics, evangelicals, Eastern Orthodox, and Jews hate Muslims because of their almost indistinguishable tenets of faith?

The order “requires the investigation and arrest of people planting, cultivating, purchasing, transporting, distributing, and engaging in practices with, or possessing entheogenic Plants or plant compounds to be the lowest priority for law enforcement in Minneapolis,

Mushrooms are not plants