Arcus wrote:Two electrons are sitting on a couch, watching Star wars. The original trilogy of course. Then a third electron comes, and asks:"hey guys, can I sit there too?" To which the other electrons answer in unison:" well of course not! Do we look like bosons to you?!"

Explanation time

Is everyone exited yet? (probably not, but here we go anyways)

So all particles have a certain spin. this property sadly has no classical equivalent, so i cant explain it very well. Anyway, these spins can have the value of integers (0, 1, 2, 3, ...) or half-integers (0.5, 1.5, 2.5, ..). Bosons are particles with integer spin, Fermions are particles with half-integer spin. They actually differ quite a bit.

So now we get to the core of the joke. Electrons are Fermions, they have spin values +1/2 OR -1/2. Electrons (or other particles) confined in a potential (for example in an atom, or square well) arent at random energy levels within the confined area. Due to quantum mechanics they can only have specific energies. This energy level is the first quantum number (n). The second one is Angular Momentum (l), the third is the magnetic orbital quantum number (m) and the fourth is spin (s). These four numbers make up a particles quantum state. According to the Pauli Exclusion Principle only two fermions cannot be in the same quantum state. This means they can have the same n, l and m numbers but have to differ in spin.

That means for example in a carbon atom (8 electrons) that in the lowest energy state there can only two electrons (one with spin +1/2 and one with spin -1/2). This is because at the lowest energy state there is also only one possible l number and consequentially one possible m number. For higher energy states it gets a bit more complex (you also get l and m numbers), but the basic rule stands. No two electrons can occupy the exact same quantum state (in one potential system).

So the bench in the joke is a quantum state for the electron, and the third one isnt allowed to join, because quantum physics. The reason they say "do we look like bosons to you?!" is because the pauli exclusion principle only goes for fermions. Bosons can be in a Bose-Einstein condensate, which means they all have the same quantum state. So photons (s=1, therefore a boson) would welcome a third photon wanting to join! yay Photons!

So... To conclude: Fermions are asocial pricks. Bosons are nice social guys.

(and before anyone asks, yes, there is also a "two Photons are sitting on a bench watching Star Wars..." version)

Also, enjoy a picture of two electrons sitting on a bench.