Quantum Physics, Double Slit experiment & observer

[extropalopakettle]

The basic scenario is outlined in the following goofy google video:

http://video.google.com/videoplay?docid=-4237751840526284618

Some will already be quite familiar with it.

Questions:
1) Is the video fairly accurate?

2) In the final part of the experiment, where we somehow "observe" which slit the electron (or photon) passes through, how is this observation done? Can we observe which slit the electron passes through and still let it proceed to the screen?

3) What, exactly, constitutes an "observer"? ... the type that will cause the pattern on the screen to change from an interference pattern, to two bands? Can a machine be an observer, if what it observes can't possibly, in turn, be observed by something living?

4) What is the explanation for it all?

[extro...*]

What I really want to know:

The way it is often presented, supposedly, the act of making an observation of which slit the "particle" passes through causes it to lose its wave-like characteristics (as I understand it). This creates a definite and somewhat profound change in the outcome of the experiment, though supposedly without there being any well understood (I don't understand it) manner of interfering with it. Physics is physics - a precise science. Precisely, what is "making an observation"?

And how (by what means) does it affect the outcome?

Oh - and another far-fetched challenge for the clever and inventive: Come up with a try-it-at-home version of the experiment. What's the minimum budget required to demonstrate the effect of seeing that the act of observation "causes the wave function to collapse"?

[Chuck]

If the path of the electron could be seen if anyone were watching but no one is watching would there be an interference pattern or is the potential to be seen good enough to collapse the wave? If there must actually be an observer then what if he's observing an experiment on the moon using a really good telescope in low earth orbit, 1.3 light seconds away? When the observer starts watching he'd see the interference pattern stop. The experimenter on the moon would see the interference pattern disappear 1.3 seconds before the observation was made. This would allow faster than light communication. If that's not possible then consciousness isn't necessary to collapse the wave.

[Lepton*]

There are a number of alternate explanations to the (Copenhagen) "observation collapses the wave function" explanation. Here [wikipedia.org] is a summary of a few of them. This has largely become a philosophical debate; physicists hope that a deeper understanding of quantum mechanics (such as that which might arise from understanding string theory) will provide some insight, but for now the question is in the "to do" pile.

The video is cartoonish but accurate.
We can observe which slit the electron passes through by putting electron detectors (ie: TV screens) beyond the slits.
If we observe the electrons as they pass through, they will not make an interference pattern.
The observer question is metaphysical: a computer could observe an event, but it would be equally okay to let the computer become part of the quantum system... we can do this with humans, too: for example, you can represent all other humans except yourself by their wavefunctions and they will be uncollapsed.
I don't know the explanation.

For the challenge, remember that your light source will need to be monochromatic. The sodium lamp is a favourite.

[Chuck]

If I let all observers except myself be part of an uncollapsed quantum system then I should observe an interference pattern while they won't. That would be strange since we're all looking at the same thing.

[worm]

i wish they had cartoons like that for more scientific experiments...NMR comes to mind.

the questions about observation are interesting (to me, anyway). obviously, we can't simply put a camera at the slit similar to the eye in the cartoon to observe the electrons. sure this works for marbles...we can see macroscopic objects because light bounces off them, but the wavelengths of visible light are too large for stuff like electrons and photons.

it's been too long since i looked at my instrumental analysis book, but how do they simultaneously detect the electrons and allow them to hit the screen where it makes the pattern? doesn't the electron/photon/whatever have to "hit" the detector somehow? doesn't that interfere with its path?

[Chuck]

Doesn't every particle in the universe interact with every other particle at least to some small degree? Can an electron make it to the slits without affecting anything along its path? If it affects anything then its path might be detected by observing the particles that it affects.

If I don't observe any affected particles but someone else does then will I see an interference pattern? That can't be right since the other observer and I will both see the same hits on the screen beyond the slits. How about if the other observer is far away with really good equipment? Will the electrons know that someone will later know which paths they take and not form an interference pattern even though I don't know which paths they're taking? That can't be right either since that would allow the other observer to send me a faster than light signal by choosing to observe or not observe.