Some weird experiments.

[Guv]

Pay careful attention to the following and then try to come up with an explanation. In the last 20 years or so, physicists have done similar experiments with photons, electrons, and helium atoms. These experiments indicate that quantum level entities behave in peculiar ways.

The photon experiments involved sending first beams of light and then individual photons toward a barrier which had two holes in it. Photographic film on the other side of the barrier recorded the results. Much of the light was absorbed by the barrier. Some got though the holes.

• First, the left hole was covered and a beam of light was directed at the barrier. The results were a circle of light behind the right hole. The light was most intense at the center of the circle and faded out farther away from the center. This was the result you would expect.
• Then the right hole was covered and the left uncovered. Again a beam of light was sent toward the barrier. The result was overlapping circles, one centered behind the left hole and the other centered behind the right hole. The brightest areas were behind each hole and in the area of overlap. Once again, this is what one would expect.
• Fresh film was put behind the barrier, and both holes were uncovered. A beam of light was sent toward the barrier. The result was alternating bright and dark areas. Classical physics explains this as being due to light being a "Wave phenomenon." The light going through one hole interfered with the light going though the other. The bright areas are due to constructive interference: The waves added. The dark areas are due to destructive interference: The waves canceled. With a careful experiment, you can get similar results by dropping pebbles in a still pond of water. Experiments like this caused physicists to believe that light is a wave rather than a particle phenomenon. All well and good so far.
• The first two experiments above were done again using individual photons instead of beams of light. The result was the two overlapping circles you would expect.
• With both holes open, individual photons were used. The result was the interference pattern: Alternating light and dark areas. Each individual photon seemed to have interfered with itself.

When individual photons were used and the film was observed during the experiment, the patterns could be seen being gradually built as individual photons arrived. The patterns were built in a random fashion.

If you believe in "photon intelligence" or "Photon Fairies," the explanation is easy. Each individual photon knows whether both holes are open or that only one hole is open. Therefore each photon knows what pattern should be built. If each photon also knows where previous photons hit the film, it can pick a spot which will contribute to the correct pattern. The earliest photons to arrive can land most anywhere, and that is what they seem to do. Perhaps there are photon fairies which tell the photons where to land.

Some brilliant physicists disagree about how to explain the above. What do you folks think?

[simonm]

I have also heard of the experiment you describe.

You did not mention that if you put a photon detector at either one of the holes, the interference pattern goes away.

This seems to imply that once you 'know' which hole a photon has travelled through, it can't have travelled through the other and therefore there can be no interference pattern.

When you don't know which hole the photon has travelled through, it could have travelled through either and the interference pattern remains.

I find this very interesting as, intuitively, we think that whether we observe the photon or not, it must have gone through one hole or the other.

The explanation that I believe is that it is incorrect to make assumptions about the 'underlying' reality of the photon before it is 'measured'. We can't assume it has gone through one hole or the other if we haven't observed to find out which.

[Fried Egg]

When the individual photons are passing through either of the holes with equal probability, two simultaneous universes exist in parellel.

In one universe, the photon goes through the first hole. And vise versa with the other.

These parellel universes existing side by side cause the interfernce effect of a wave function.

If you make an observation at one of the holes, you know that the photon either went through your hole or the other. The wave function collapses and only one reality remains.

[HarryW]

Well there are many proposed explanations, just pick one. None of them are particularly 'normal'.

There was an interesting article about this sort of thing that touched on this very subject in the New Scientist of 17th Feb. Take a look if you like: http://www.newscientist.co.uk/featur...jsp?id=ns22781