Friday, 04 March, 2005

It's recently been shown, by experiment, that Young's famous double slit experiment applies not only in space but in time. The experiment starts by going in to a darkened room and take a coherent light source and shining it through a thin slit on to a screen. In this single slit configuration the pattern on the screen is a single bright fringe located directly in front of the slit with the light spreading out to either side of the slit.

Next, we repeat the experiment as above but this time instead of shining the light source through a single slit we shine it through two slits. Now intuitively, you'd expect the pattern on the screen to be a bright fringe in front of each slit and then for the light to dim symmetrically on either side of the bright point in front of the double slits.

However, what you actually see is an interference pattern. You get a sequence of bright and dark fringes that are symmetric about a central bright fringe immediately in front of double slits. As you move further to the left or to the right from the central bright fringe the repeating pattern of friges gets dimmer and dimmer.

In Young's day they saw this pattern as strong evidence that light behaved as a wave. Indeed, if you do the double slit experiment with water waves you get the same interference pattern so this is a fairly natural conclusion to draw. Natural, that is, until Einstien showed that light also had particle like behaviour in his famous explanation of the photo-electric effect.

So what is light, a particle or a wave? Actually, it is neither, however, loosely speaking it behaves behaves like a particle when we know something concrete about it's history and it behaves like a wave when we lack information about it's history.

The reason we get an interference pattern when we shine a light through the double slits is because when a photon lands on the screen we have no way of objectively determining which slit that photon went through in order to arrive on the screen. Moreover, as soon as we try to measure which photon went through which slit the interference pattern disappears. This is precisely because by measuring which slit the photon went through we now know something definite about the photon's history.

Given this logic it is not at all surprising the effect also occurs in time as well as in space. If we have two very close events that produce photons with identical properties and there is no physical way to tell which of the two events produced which photon then we'd expect some kind of interference simply because we don't know the history of the photon. This is because it could have come from either event and there's no way to know which of the two events for sure. When the scientists actually performed this experiment they got exactly what the theory predicted: an interference pattern. At any rate, it's a pretty beautiful experiment and a lovely demonstration of the underlying subtleties of quantum mechanics.

22:30:50 GMT | #Randomness | Permalink
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