# The cause of particle going through one slit of the other

1. Jun 2, 2013

### susskind99

In the double slit experiment wavicles go through one slit of the other in roughly a 50/50 ratio. How does this not violate conservation of momentum? They would need momentum to push them to the right or the left.

2. Jun 2, 2013

### Staff: Mentor

If you have interference, they do not go through the slits in any ratio - there is no way to say "this particle went through slit 1", otherwise you would not get interference.
The momentum and the position of your particle are not single, fixed numbers, they can vary a bit.

3. Jun 2, 2013

In experiments, the incident beam, whether laser or electron or other particle, always has a finite cross-section. The intrinsic spread in volume and momentum/energy of particle beams is ultimately a consequence of the uncertainty principle, which imposes limits on the (r, p) precision with which a beam can be formed. The restriction applies even if you do the experimental measurement one particle at a time, or with a hugely collimated beam.

In the lab, you would move your beam about on the diffraction slits (or vice versa) until you see the strongest signal (meaning that the spot is centred near the slits).

4. Jun 2, 2013

### VantagePoint72

The particles that go through one of the slits go through one or the other in a 50/50 ratio. First, as mfb said, you need which-way information for it to be sensible to talk about particles going through either slit. Second, plenty of the incident particles do not go through either slit; they're scattered off or absorbed by the barrier and never detected. The wavefunction of each individual particle is a wavepacket that is spread out along all three spatial axes (and the packet spreads out more as it propagates). When the wavefunction reaches the barrier, some of it is scattered back, some of it is absorbed, and some of it diffracts through the two slits. If the beam is properly centred on the slits, then the proportion that "leaks through" either slit will be the same and a which-way modification to the experiment would show a 50/50 split between the two slits.

5. Jun 3, 2013

### bahamagreen

What reaches the barrier, the waveform, and/or the wave function?

6. Jun 4, 2013