# Electromagnetic field in a box vs. boundary conditions

I understand that a commonly applied step (see Wikipedia for an example) in quantizing the electromagnetic field is "enclosing the field in a cubic box" and later taking a limit of that box to infinity. This allows working in a discrete countable basis of eigenvectors of momentum, finite energy per mode, number states, etc., and is used to to derive partial results an easier way than in a full-blown continuous field theory, assuming that one can defer taking the limit to generalizing these.

I'm confused by this step. It's said that in the "box", the wave vectors can only take particular discrete values. However, this would be a result of boundary conditions. (A box without boundary conditions is just an imaginary window into the entire space and doesn't pose any other restrictions than the the latter would.) It's important to specify properly what these are, which is what all the sources I've seen so far neglect silently.

If the walls of the box were, for example, reflective, this would be a major problem because this does not permit running waves at all. They always come in linear combinations resulting in a standing wave. Mathematically, their amplitudes are not independent and the canonical commutator wouldn't be true.

It seems to me that what is indeed assumed are periodic boundary conditions. This would mean the system is not enclosed in a box, but rather lives in a 3-torus! Such "box" couldn't even be constructed in our space! What's the motivation to consider something so unphysical for a model and expect the results to generalize correctly? Or, if this is not the case, what am I missing?

• Possible duplicates: physics.stackexchange.com/q/323776/2451 , physics.stackexchange.com/q/87132/2451 and links therein. May 18, 2018 at 15:45
• @Qmechanic Thank you. I know nothing of solid state physics but I've heard there were many connections. "Links therein" really help, most of them this one. But Born-von Karman (which is the duplication target thereof) won't likely help possible future readers. How to proceed? May 18, 2018 at 16:16
• I'm thinking of a self-answer that would cite and link to physics.stackexchange.com/a/250342/10712, instead of marking this a duplicate. Is that OK from a moderator's perspective? May 18, 2018 at 16:20
• Well, I leave for now that judgement up to you. Keep in mind that the Phys.SE community may later choose to close/reopen. May 18, 2018 at 16:31

Similar questions have been asked on this site in different scenarios, the closest match asking about phonons.

In the accepted answer there, it is argued that

In the [linear size of the system L to infinity], boundary conditions don't really matter, and most physical observables will be the same for all boundary conditions.

Indeed, in a reformulation, there's no difference between $\mathbb{R}^3$ and a sufficiently large $T^3$ that we could distinguish using local measurement of limited reach in a limited time. So if we want to treat the former as a limit of some space of a finite volume, a torus (or, assuming periodic boundary conditions) is the most advantageous choice as we get planar waves with linearly independent (in Coulomb gauge) amplitudes that naturally generalize to continuous-$\vec k$ planar waves for the same reason Fourier series has Fourier transform for its limit.