The Nobel Prize in Physics 2005

Quantum mechanics – what is it?

Quantum mechanics describes the laws and phenomena in the microcosmos, among tiny particles and incredibly weak light, photons. These laws do not always agree with our common sense, nor do they fit in with our everyday experience. For example, particles can move right through matter and be in two places at the same time. They are also affected when they are measured. Nevertheless, the laws of quantum mechanics have every time up to now been in agreement with experiments. If anyone could demonstrate that quantum mechanics does not work, it would be a truly revolutionary discovery.


An important part of Glauber's theory is that the laws of quantum mechanics apply when measuring light. This means that all measurement will always affect what is being measured.

Let us suppose that we want to measure where a photon is with a number of light detectors alongside each other. If we get a click in a detector, we know that at that very moment the photon is just there. The problem is that before the measurement the photon probably extended over a much larger area than just the detector that registered a hit. The measurement affects the photon so that it is observed at a particular place.

One way to deal with this problem is to take many measurements. In the end a pattern of how probable a particular result is will emerge. This is what the diagram of the wave pattern shows. Every detection of a photon is a measurement, resulting in the photon being observed just there. A few measurements alone seem to be random, but if we collect many such measurements, a wave pattern emerges – a picture of the properties of the photon before it was measured.


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