[bcbrown]

> For another example, see speed of light in glass is slower than vacuum.

I'd love for you to explain what you mean by this. Either you're mistaken or I'm about to learn something really interesting. Here's a link a quick search brought me, for reference. Are you talking about something other than refractive index, ratios of c, and prisms? Like a latency metric of some kind? I'm very curious.

https://en.wikipedia.org/wiki/Speed_of_light#:~:text=For%20e....

[red75prime]

It's probably that photons always propagate at c and that the apparent change of speed of light in glass is caused by photon absorption and delayed reemission by atoms (at least it's how perturbation theory of quantum electrodynamics describes the process).

[eternauta3k]

Do you really need photons and quantum stuff though? "Just" learn about how dielectric materials work + how EM waves work from Griffiths and then everything will make sense.

[bcbrown]

I'm looking at Griffiths right now, and 9.3 Electromagnetic Waves in Matter states (emphasis in the original): "Since [the dielectric constant] is almost always greater than 1, light travels more slowly through matter - a fact that is well known from optics."

I'm still confused. How is it an inaccurate simplification to state that the speed of light in glass is lower than in a vacuum?

[eternauta3k]

I wouldn't call it inaccurate at all. I think it's a perfectly good model. You can explain the phenomena otherwise using a more detailed model, but you don't need to.

[red75prime]

Depends on what you want. A useful approximation, or the latest approximation that is closer to how it really is.

[eternauta3k]

What insight is being gained by going with the more complex model? You can work out a lot using classical EM and predict a bunch of cool optical/diffraction phenomena without a lot of math, and the math you do is crystal clear. QFT or even just QM+time-dependent perturbations are a pain in the ass in comparison.

[red75prime]

If you have engineering mindset (how to do something), then, sure, you are better off with the most useful approximation. If you have scientific mindset (how things are), then practical usefulness of the model of reality is secondary.

[Jun8]

Oh boy, it seems you're one of today's lucky 10000 (https://xkcd.com/1053/ :-). The concept of EM waves slowing down in a medium is a curious one: it sounds obvious (you can hike a shorter distance in harder train even though exerting same effort) but generates interesting questions, e.g. what is the mechanism that "slows them down", how do photons speed up after they exit a the medium (https://physics.stackexchange.com/questions/330994/how-does-...). I think the problem here is similar to the concept of "relativistic mass", a simple way to think about things and work problems but at the same time creates another set of problems and confusions.

red75prime's short answer in this thread is correct, for a longer one see this Physics SE questions and answers (https://physics.stackexchange.com/questions/11820/what-reall...). In ELI5 terms: photons always move at speed c, but in a medium now and then (depending on properties of the medium, density, etc.) they interact with its atoms. In this case the atom absorbs the photon and after a short time generates another photon. So the speed of each photon is always c but the "overall light wave" becomes less than c.

If you're a non physicist like me who would like to delve further into this, I suggest Feynman's excellent and very readable book QED: The Strange Theory of Light and Matter (https://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/06...)

[bcbrown]

I love QED! That was recommended to me by the professor who caused me to change majors from EE to physics. I just looked for it on my bookshelves but I must have misplaced it somewhere in the intervening two decades.

I understand what you're saying, and don't disagree that the quantum electrodynamics treatment is a theoretical model that can describe optical phenomena, but I guess I'm a little nonplussed when you compare tidal theory to optical theory.

Newton's theory of tides was incorrect; it does not accurately predict tidal phenomena[0]. Using classical EM field theory with refractive indices and ratios of c is not incorrect; it gives accurate predictions of optical phenomena at the macroscopic scale. It's certainly incomplete, but not inaccurate.

I think the disconnect might be that when studying physics, it gets drilled into you over and over that no model is right or wrong, just applicable or inapplicable to a given situation. For example, here's a passage in Griffiths' Introduction to Electrodynamics:

"In fact, when you stop to think about it, the electric field inside matter must be fantastically complicated, on the microscopic level. If you happen to be very near an electron, the field is gigantic, whereas a short distance away it may be small or point in a totally different direction. Moreover, an instant later, as the atoms move about, the field will have altered entirely. This true microscopic field would be utterly impossible to calculate, nor would it be of much interest if you could. Just as, for macroscopic purposes, we regard water as a continuous field, ignoring its molecular structure, so also we can ignore the microscopic bumps and wrinkles in the electric field inside matter, and concentrate on the macroscopic field. This is defined as the average field over regions large enough to contain many thousands of atoms (so that the uninteresting microscopic fluctuations are smoothed over), and yet small enough to ensure that we do not wash out any significant large-scale variations in the field. Ordinarily, the macroscopic field is what people mean when they speak of "the" field inside matter. (In case the introduction of the macroscopic field sounds suspicious to you, let me point out that you do exactly the same averaging whenever you speak of "the" field inside matter.)"

So I guess I'm saying that the physical intuition and mental models used to work with classical EM field theory are different fom those used to work with quantum theories is just part of what it means to be a physicist.

Anyway, I'm glad you're so excited about physics! If you liked QED, you might be interested in Schroedinger's "What is Life?", where he takes his experience from quantum physics to speculate on how the information-theoretical requirements of genetic inheritance in reproduction implies the existence of some sort of "aperiodic crystal" that encodes genetic information, decades before Crick & Watson discovered DNA. It's written for the layman, although it's not as readable as Feynman (although he's in a class of his own when it comes to scientific explanations).

Also, thanks for your original comment, because it gave me an opportunity to pull out some of my old textbooks and leaf through them for a while.

[0] My mental model of tidal phenomena corresponded to Newton's theory, so I found it fascinating how incomplete and inaccurate it is, prompting a long diversion down a Wikipedia rabbit-hole, to the point that I'm now considering giving a short lecture on tides at my birthday party in a couple weeks.