According to quantum theory, the fundamental particles from which everything is made combine the properties of a particle and of a wave. This idea is called the wave-particle duality, and is one of the confusing aspects of the theory.
More specifically, the theory says that every particle has a wave associated with it. Quite what is waving we don't know, but the idea leads to extremely accurate predictions of the properties of atoms (as we will see in the next post).
The idea is confusing because although the waves can be quite spread-out, whenever a particle interacts with a large number of other particles it always seems to do so at a very localised point, a point much smaller than the spread of its wave.
Imagine the following idea. Suppose there was a completely flat lake, and you dropped a stone into it. You would make a wave which would ripple out in a circle from where you had dropped the stone. If you had people in boats on the lake measuring the height of the surface of the water they would all be able to record when the wave went past them. Quantum theory says that the waves associated with fundamental particles behave in a different way. They seem to spread out like the wave on the water does, but the first time they are detected they seem to contract to a point at the detector. It is almost as if the wave on the water suddenly contracted to a spike at the first boat it made contact with.
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