Can Photons See the Future? (Or Is it Just That Humans Can Change the Past?!)

A Report on "The Delayed Choice Experiment" by Jenny L. Nielsen

Not so long ago, scientists were asking themselves the question: Do atoms know when we're looking at them? Even before this question has been satisfactorily answered, a new question has surfaced: Do atoms know that we're going to look at them before the event actually occurs? A documented experiment conducted by two prestigious universities actually implies the affirmative.

It's called the "delayed choice experiment," and it was originally a thought experiment dreamt up by the great theoretical physicist John Wheeler. It's a variation on the usual "double-slit" experiment, which proves that when a photon (or electron or photon or any sub-atomic particle) is fire through a sheet with two holes, it creates

an interference pattern on a screen set on the other side as if it had gone through both holes at once and interfered with itself. However, this behavior only seems to occur when the particle is not being watched when it hits the sheet. When a detector is put up to monitor the holes and what comes through, the particle is observed to be going through only one hole--and the interference pattern does not materialize on the screen at the opposite end.

The double-slit experiment is hard enough to understand on its own, even if John Wheeler hadn't come up with the idea of moving the detector. He had the interesting idea of monitering the particle after it had already made its "chosen" move through the holes, but still before it hits the screen which records the move. According to "common sense" (if one can use common sense in a case like this), since the scientists don't monitor the particle at the exact moment it is "choosing" whether or not to go through both holes at once, the particle is supposed to go through both the holes at once and cause the interference.

But it doesn't--not according to the independent experiments carried out by the University of Maryland and the University of Munich. These experiments confirm that the particle actually goes through only a single hole--just as if it had known that it was going to be observed. It makes only a solitary dot on the screen. The little scoundrel anticipates that a detector will be watching him later, and refuses to perform his startling bi-location behavior!

And of course, when the detector is removed, the particle goes through both holes, interferes with itself, and the screen shows the pattern to prove it.      The question now is "do the particles actually know that the scientists will be watching them later?" Or--to look at it a different way--"Do the scientists actually change what the particles did in the past by watching them in the present?!"

Of course this is altogether maddening to scientists, who have had enough trouble coping with subatomic antics even without having to explain the mysterious prophesying particles. The experiments need to be investigated further before any satisfying conclusions can be drawn. Right there's talk of using light from quasars bent by "gravitational lenses" to conduct a galactic scale version of the experiment, which would hypothetically provide the most dramatic proof that the experiment always works (or, less likely, that it's just some sort of international scientific anomaly).

Until then, I'll chalk it up as just another weird phenomenon of the mysterious quantum world around us.

Further Reading: Q is For Quantum: An Encyclopedia of Particle Physics, by John Gribbin