Monday, July 25, 2011

So Can Electrons be at Two Places at Once?

One of the strangest things that you can hear about quantum mechanics is that microscopic particles in some instances can be in more than one place at a time. But how can this be true?

To understand this let's look at the most important experiment in quantum mechanics - the double slit experiment.  The idea of this experiment is simple. You send particles and waves through a double-slit device and observe the results on the screen. Naturally waves and particles, which are not microscopic behave differently. However, if you send a bunch of microscopic particles, like electrons through the double-slit device you get a very surprising result - electrons behave like waves. Even if you send them one at a time, they still create an interference pattern, which is typical for waves, which,of course, pass both slits at a time.

The result of the double-slit experiment using simple particles:


One of the explanations for this is that electrons somehow exist in two places at a time, that is the same electron passes both slits at the same time. Of course there are scientists who don't believe in this idea. But guess what, you can't check which slit the electron passes, as if you  try to detect it, the interference pattern on the screen disappears, that is the electron then simply passes through one of this slits.

The result of the double-slit experiment using microscopic particles:


In quantum mechanics, the electron is said to be in a superposition, that is it is at many places at the same time and until you observe the electron, you can't tell where it is exactly. However, this is just the mathematical way of explaining what we can observe experimentally. Nobody really knows in the electron actually is at more than one place at the same time.

Naturally scientists are wondering if it is possible to put a non-microscopic object in a superposition. One of the most ambitions experiments by scientists at the Max Planck institute in Germany, will try to put a glass sphere, which is 40 nanometres in diameter, into a superposition. By using a sensitive laser to "bounce" photons of the sphere, they will try to put it in a quantum superposition, that is it will be in more than place at once. If the experiment is successful it will be the most sensitive test of quantum theory yet.

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