# Interference between the particles

1. Sep 24, 2011

### jackychenp

Hi,

I am new to QM and really confused with some concepts.

Assume two alpha particles come out from two different sources and reach the same detector, so the probability to detect some particles should be |f+g|^2 (f, g are the amplitude of 1st and 2nd alpha particle, respectively). If they are one electron and one alpha particle, then probability should be |f|^2+|g|^2. Since the whole process is the wave behavior, why alpha particle "wave" can not interfere with electron "wave"?

Dirac once mentioned a photon state which associates with two or more beams. When we measure the energy of one beam, the photon will change suddenly from being partly in one beam to being entirely in one of the beams. How does that happen? Will it still happen if the two beams are very far away from each other? (that probably will break relativity law)

Last edited: Sep 24, 2011
2. Sep 24, 2011

### Ken G

The same f+g wave amplitude applies to both alpha particles because they are indistinguishable-- we have a law that says nature cannot "tag" them separately with an f for one and a g for the other. But an electron and an alpha particle are distinguishable, so the f applies to the electron, and the g to the alpha particle. So it's not even |f|2 + |g|2, it's just |f|2 and |g|2, separately for the two particles.

3. Sep 24, 2011

### jackychenp

Does that mean double slits diffraction will not work if sources on two slits are different, say electrons in one slit, alpha particles in the other? This is weird since both can be see as waves just with different wavelength, and the interference should occur.

4. Sep 24, 2011

### Ken G

It would be hard to restrict alpha particles to one slit and electrons to the other-- usually with the double slit you send each particle toward both slits. Then the electron would get interference from its own wave function, it wouldn't need to care about the alpha particle. But yes, if you made it so only electrons could go through one slit, and only alpha particles through the other, you would not get a two-slit pattern for either one.