Schrodinger’s tape measure

Posted on June 28, 2010
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From The New Scientist

Take the simple process of measuring a photon’s spin. Thanks to the strange nature of the quantum world, it can actually be spinning in two directions at once, a phenomenon known as superposition. When we use a detector to measure the spin, however, the superposition disappears and we register a spin occurring in one direction or the other.

Quantum theory does not explain why this happens. “We don’t really understand the measurement process,” admits Stephen Adler at the Institute for Advanced Study in Princeton, New Jersey.

If you want to know how little we know, ask a roomful of physicists what goes on when we measure a particle’s properties. All will be able to calculate the result of the measurement, but the explanation they give will differ wildly. Some will tell you that new parallel universes necessarily sprang into being. Others will say that, before a measurement is performed, talk of particles having real properties is meaningless. Still others will say that hidden properties come into play.

Another group will tell you that they deal with physics, not philosophy, and dismiss the question without giving you an answer. It has been thus for more than 80 years. “These conceptual challenges are still not understood at all,” says Markus Aspelmeyer at the University of Vienna in Austria. “We’re still right at the beginning.”

Experiments investigating the quantum world have traditionally focused on what are known as interferometers. Researchers fire a single quantum particle, such as a photon, towards two apertures in a screen. Common sense says the photon has to go through one aperture or the other. However, as long as you don’t measure which aperture it went through, something remarkable happens.

At a screen on the far side of the twin slits, an interference pattern forms. This can only occur if the photon goes through both slits at the same time and interferes with itself. In other words, as long as nobody is watching, the photon exists in two different places at once.

A measurement changes everything, however. If you set up the experiment so you can see which slit the photon goes through, the interference pattern disappears; the photon will have gone through one slit or the other, but not both.

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