[hexag1]

the explanation given by Feynaman in "The Character of Physical Law" sums it up best:

first we must keep in mind the following:
When we look at anything under a microscope, our smallest resolution is given by the frequency of the light we use too look. In other words, if we have two dots, we can only distinguish them from each other if the light we use to look has a shorter wavelength than the distance between the two dots. Anything shorter and they look like one dot.

Now, with that in mind:

If we let the electron pass through the two slits we will get a pattern that indicates that that electron behaves like a wave.

If the intensity of the electron beam is turned down low enough, then we can see that they come in lumps; individual units. The probability of us catching a lump at any point after the two slits is given by a sinusoidal wave function. When all the probilities are mapped, the pattern of electron arrival looks like a ripple.

If we try too 'look' at the electrons by shooting light at the electrons before they hit, it will seem like the electrons definitely pass through one slit or another. The pattern of arrival for the electrons on the other side of the slits will look like electrons obey ballistic mechanics.

We can try to look at the electrons before they pass through the slits. We do this by shooting light at them. But the light affects the path of the electrons. When we shine a light, the path of the electrons is drastically affected. They no longer behave like waves, but instead like ballistic particles.

We can try to turn down the energy of the light we use to look at them. If we turn it down low enough, the light no longer affects the electrons enough, and they behave like waves again. However, a new problem arises. Now that we have decreased the energy of the light (and therefore the wavelength of the light) we shine on the electrons, we can no longer distinguish between the two slits the electrons pass through. The wavelength that equals the distance between the two slits, equals the wavelength that no longer affects the path of the electrons.

Therefore it is IN PRINCIPLE impossible to tell through which slit the electron will pass. Nature is rigged in just such a way that we can never tell what happens. Some say that nature herself doesnt know through which slit the electron will pass.

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