# How does one fermion change the energy of another fermion

1. Aug 18, 2013

### susskind99

My understanding of how one fermion changes the energy of another is something like: fermion x approaches fermion y and x emits bosons which are absorbed by y. But why does one billiard ball transfer a lot of its energy to another billiard ball on contact? Say billiard ball x approaches billiard ball y, x hits y, x stops and y moves. I realize the Pauli Exclusion Principle forbids x from occupying the same place as y. But why does y absorb more bosons from x than vice versa? Why would the fact that x is moving y to absorb more bosons from x than x absorbs from y?

2. Aug 18, 2013

### tiny-tim

hi susskind99!
sorry, but that's just a myth told by mathematicians to frighten physicists!

those boson lines you see in feynman diagrams "carrying the force" are just a mathematical trick that help in the calculations

after all, when two electrons repel each other, do you think that can be explained by by emitting (presumably negative-mass) bosons?

(and btw, it's really the momentum that's affected, rather than the energy)

3. Aug 18, 2013

### susskind99

Then how does a fermion's energy change?

4. Aug 19, 2013

### mpv_plate

Like kinetic energy? You can accelerate a charged fermion via electric field.

Or you mean increase of electron energy in atomic orbital? Electron absorbs a real photon.

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