xkcd #3027: Exclusion Principle
xkcd #3027: Exclusion Principle
Fermions are weird about each other in a standoffish way. Integer-spin particles are weird about each other in a 'stand uncomfortably close while talking' kind of way.
You're viewing a single thread.
Not that I know much about this, but the Pauli principal always struck me as an observation rather than anything like an explanation. As an explanation it's extremely unsatisfying. "Why won't electrons occupy the same space?" "Well.. because of the Pauli principal". No.. that's not the reason, that's just restating what we observe. Glad to see physicists find it weird too (as is suggested in the comic)
It's a lot more complicated than that even.
Pauli Exclusion Principal is that two or more identical particles of half integer spin cannot occupy the same quantum state. So two electrons in an orbital must be made of a +1/2 and -1/2 spin. This is evidenced by observation, but the prediction was made long before that.
This is because the total wave function for fermions is antisymmetric (bosons, like the photon, are symmetric). It's sort of hard to describe how this works without paper and pen, but essentially there is different formula of solving a wave function. A symmetric wave function is a sum, and an antisymmetric wave function is a difference. The issue arises when you have two identical particles - symmetric functions can be any state as it results in a solution >0. If you have an assymetric function of two identical particles, the result is 0, which isn't a valid state.
The very uncomfortable part of physics is here: when we ask "why" the answer based on the math and the observation is quite literally "because that is the way math works." It's fundamental - just like x * 0 = 0.
But it's an outlier right? I guess that's why it's joked as an additional force in the comic, because even by "maths just describes reality" standards, it's weird. Like, it feels like a particle "knows" if the state is already occupied, and that's why it can't occupy it. But that implies some communication - the conveying of some force - else how does it "know"?
It’s not really communication. They ‘know’ because they become part of the same wave function. The wave function of the system is
|psi1 psi2> +- |psi2 psi1>
Note that if the +- is a plus, then exchanging psi1 and psi2 yields the exact same equation. If it’s a minus, you get a negative sign out front. Electron systems have a negative sign because of the spin statistics theorem (I don’t understand that part, so you can look it up if you want—it involves field theory iirc) Now, if electrons are exactly the same (indistinguishable), then exchanging them will yield the exact same wave function, leading to
|psi1 psi2> - |psi2 psi1> = |psi2 psi1> - |psi1 psi2>
The only solution here is |psi1 psi2> - |psi2 psi1> = 0
But recall that |psi1 psi2> - |psi2 psi1> describes the system as a whole. So this system is prohibited by quantum mechanics, and there’s no way for two electrons to have indistinguishable states (be in the same place at the same time).
So the way I "understood" the spin-statistics theorem is that it's basically this:
A given particle with a given intrinsic spin has a direct relationship to a collection of the same particles as a consequence of quantum math. Yeah. Just "it's related."
Proving that math is really freaking difficult and you need to use relativistic quantum field theory. I think it was Richard Feynman who said "We apologize for the fact that we cannot give you an elementary explanation."
Actually when I graduated there was another professor (can't remember his name) who was discussing his frustration with how they still can't explain it without all of QFT steps.
Basically, this is where the shared attitude of "the more you know about quantum physics, the more confusing it becomes."
The math is describing reality - but that's why I highlighted that the math predicted it long before there was experimental evidence.
From what we know about the quantum realm (my physics professor liked using that description, as if it's a whole different existence), it appears that it's actually the opposite: reality is obeying the math. Consider how wild that is - particle interactions are doing what they do because of how mathematics works. Something that we humans came up with to describe observations.
I gotta ask, what's with the three spaces between sentences?
Google keyboard weirdness. Maybe because I'm of an age where you put two spaces after a full stop. But then the keyboard is automatically adding one to be "helpful" because maybe people don't do that themselves anymore?