So the lighter one goes faster and the heavier one goes slower. Now, for a time, you might think that the gun keeps track of the momentum of the bullet. But we're going to let the bullet fall into a supernova star which scatters the momentum all over the galaxy. It still keeps track of the other half of its momentum which used to be packaged in the bullet but is now packaged in the Universe at large. So it keeps track of the Universe at large.
Momentum is always half of something.
Because there is nothing with respect to which it could have some momentum. Momentum, angular momentum and electric charge are always half of something, and the other half, the opposite half, is packaged in the Universe at large. And if you can watch the great Foucault pendulum swinging in the hall at the science center or the planetarium and not get goose bumps, it may just be that you're asleep. Now in 1965, the mathematician John Bell came up with an interesting theorem. And his theorem says that if matter behaves as quantum mechanics says it should behave, then one of two things must fail.