The meaning of measurement, and decoherence

Quantum theory says that a particle has an associated wave, and that the wave can change shape suddenly when a measurement takes place. For example, when you measure the position of a particle that has a widely spread-out wave, its wave suddenly changes to a single tall spike where the particle has appeared.

There has been a lot of debate about why these changes happen when a 'measurement' occurs. Some quite famous physicists have even speculated that measurement is somehow linked to observation by a sentient being, and that somehow consciousness makes the fundamental particles jump from vague states into certain ones.

Nowadays it is hard to find any mainstream physicist who supports such mystical interpretations. My view is that by 'measurement' we just mean an interaction between the particle and whatever lumps of matter constitute the measuring device. It seems that the particle is most likely to have a wavelike behaviour when it is moving in free space or interacting with another quantum particle, but behaves in a more pointlike way when it interacts something much larger (like the screen behind the two slits).

A related question is why big objects don't behave in wavy quantum ways. Humans are made out of protons and electrons which behave in weird quantum ways individually, so why does the weirdness disappear when large numbers of particles are brought together to make big objects?

One train of thought that is being investigated is an idea called 'decoherence'. When you have a couple of waves, they can interfere with each other to give noticeable interference effects. In an earlier post we talked about the example of two separate circular waves on an otherwise glassy pond. Where the waves cross you will see very clear patterns. But as more and more waves get added to the surface of the pond you end up with a random choppy mess with no discernible pattern to it. Crudely, that's the idea behind decoherence. When lots of quantum particles are brought together their waves overlap is such a variety of different ways that any detectable quantum patterns are overwritten by noise.