# Double Slit Experiment

1. Aug 4, 2006

### Geraint

Hi,

I've done a few terms worth of quantum mechanics, which was good on explaining the mathematics, but not good at explaining it's "derivation". I've been reading the Feynman lectures, and I've learnt a hell of a lot. It does seem though, the more I learn, the less I understand :grumpy: . But it's all good.

Anyway; the double slit experiment. Forgive me if this is a simple question, but I'm very confused (in general).

Feynman's lectures state that if there is an experiment done which can "in principle" be used to determine which of the two slits an electron passes through, then the interference pattern disappears. He uses photons to illustrate this effect.

What I don't understand: Why does the double slit its self not count as observing the electron? For example, when the electron passes through the slit, it could rebound off an atom in the side of the slit, providing information which could be used in principle to discover which path it's taken.

My thoughts: Maybe these are electrons which don't contribute to the interference effect? If this is the case, where does the y-momentum of the diffracted electrons come from? Where is the force?

(Electrons travelling in x-direction, slits in y-direction)

Any help would be appreciated, thanks,
Geraint.

2. Aug 4, 2006

### Schrodinger's Dog

http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html

I really am no expert but this is the best web site I've found to explain what it is your getting at in non mathematical terms.

The electron is not effected directly by the atoms in the y directional slits, but it is "defracted" by a superpostion of itself(look at the wave like properties of the electron) So that even a single electron or photon can under it's own influence interfere with itself. I'm posting this to see if I got the general understanding of the theory right, so don't take my word for it.

What's really interesting about this is the experimental result confirms that interference of any kind causes the wave to collapse and it behaves like a particle, striking the back of the screen with no peturbation. but if the interaction is minimised the decoherence doesn't occur and it behaves as we would expect a wave to behave if it had interfered with itself in a super position of all possible states. This I believe is confirmation of the http://plato.stanford.edu/entries/qm-copenhagen/" [Broken]

The very act of interfering with the particle destroys or dechores it's behaviour. Again this is my limited understanding, and it's been a while since I studied this in any depth. I await the real scientists. we can extend this uncertainty principle to this state of experimental affairs.

Caveat: again restate this passage:-

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