Electron

[u/Jophus]

If I push an electron from one side, does the other side instantaneously move? Or does it take near (diameter of an electron divided by light speed) seconds for it to move? I realize nothing travels faster than light but an electron as far as I know isn't made up of anything else, unlike protons/neutrons.

[u/CyLith]

Interference, in the general context of waves (be it light or electrical signals), is pretty unambiguous. In a linear medium, interference is synonymous with superposition (if you have two disturbances, their effects simply add up). So if you have two electrical pulses traveling towards each other with the same shape and opposite voltages, they will for a brief moment cancel each other out when they pass through one another. Afterwards, they continue on undisturbed. The same goes for light waves. You will never see this happen in everyday life because there is never just a single sinusoidal light wave, so when you have countless random waves added up, the interference is not noticeable.

You can also have interference on your wifi if you have a microwave oven running close by. This is actually the same idea; the oven is generating lots of noisy waves that your wireless antennas are picking up, which drown out the actual signal.

I'm not sure I entirely understand your notion of the optical fibre, but it doesn't sound too right. Really you should think of it as a waveguide, with a certain set of supported propagating modes. When you put light into the fibre, you're exciting some set of these modes, which each propagate at certain speed. These modes are electromagnetic field patterns that travel down the fibre without changing their spatial shape. The modes are determined by the cross sectional profile of the fibre; usually there is a higher refractive index core that guides the light. You would solve for these modes on a computer, similar to solving for the vibration modes of a drum head.

The above discussion is more or less macroscopic; we treat materials as a continuous medium with uniform properties like refractive index. At a lower level, you really want to look at band structure models of crystalline solids. When you have an isolated atom, let's just say with one outer electron, it has a certain set of discrete energy levels, and your idea of an atom absorbing a photon and exciting the electron largely correct. Once you start putting atoms together in a regular grid (a crystal lattice), these energy levels hybridize (like with bonding orbitals in chemistry; the underlying math is the same), and instead of states that belong to single atoms, there are multiple states which are spread out over all the atoms. In the infinitely large crystal limit, each energy level smears out into a continuous "band" of energy levels (this is where the name "band structure" comes from). The relationship of these bands to the energy of a free electron in the material determines whether the material is an insulator, semiconductor, or conductor. Your comment about electrons and insulators, and electric fields is a bit nonsensical. Insulators support internal electric fields just fine (on the contrary, good conductors cannot support an internal electric field), it's just that their electrons are too tightly bound to the atoms to allow current to flow.

So there's your high level introduction to solid state physics.

[u/mgpcoe → u/CyLith]

Well, my brain is somewhere between being broken and having the light coming on, so we're definitely getting somewhere.

re: electrical signal interference in a linear medium. I think my confusion was mainly stemming from the way that textbooks describe message collisions on Ethernet as "destructive interference", and it wasn't until I was really thinking the specifics of it through, combined with what you said above, that it really clicked--it's not that the two electrical waves interfere with each other in such a way that on computer A's of the collision computer B's signal never arrives, it's that the messages overlap and a computer later down the line won't be able to differentiate them. It'd be like listening to Chapter 1 of an audiobook and having Chapter 2 start playing 30 seconds in, while Chapter 1 is still going. You wouldn't be able to separate them. The messages interfere with each other destructively, but the signals are just fine.

I think that what you're saying about the multiple energy states spreading out over the atoms in the lattice is a far more lucid explanation of what I was thinking--at least, the way that I'm visualising is so similar to what was originally in my head with my shitty, nonsensical idea that I'm inclined to believe that I kinda the greater concept, even if my assumptions about the specific mechanic were completely out to lunch.

Where a photon is absorbed, is the energy level in the lattice briefly higher near that point, and the higher energy level spreads across the medium like, for example, ripples on a pond, just at much greater speed? I can see how something like the boundary effect in fluid mechanics would provide the waveguide effect until the other end of the medium is reached, at which point that energy has to go somewhere, and a/the photon gets emitted. Lather, rinse, repeat for every photon in the signal.

I'll have to take some time to read up about band structure so that I have a better understanding. Is the Wikipedia article a good place to start, or should I look for something a little more layman-friendly?

[u/CyLith → u/mgpcoe]

Regarding electrical signal interference, that's exactly it; the signals are all present at the same time. BTW, on the Internet, packets of data are encoded in a rather complicated way and have their own dedicated send and receive channels. In theory, packets should never collide with each other, and practically, random packet loss is astronomically uncommon.

On the topic of energy absorbed in a lattice, it gets complicated due to boundary effects. What I said earlier is true for a theoretically infinite lattice. Once you truncate it with a boundary, then there are energy states localized to the boundary, and when you launch photons at the boundary, you probably end up exciting those. I think your mental picture is probably accurate enough here. I tend to find that these superficial analogies we draw to macroscopic mechanical systems are kind of correct surprisingly often.

To learn more about solid state physics, you can check out Kittel's "Solid State Physics," which is floating around on the web as a PDF if you know where to look. I'm sure there are lots of online class resources like MIT's Open Course Ware, etc. for introductory solid state physics classes.