We've talked about individual particles each having a wave, but quantum theory includes the possibility that the waves of similar particles combine to form multi-particle waves. In some ways the idea is straightforward, as we've already talked about the fact that overlapping waves simply add-up to make another hybrid wave of a more complicated shape. However, in quantum theory the idea of particle waves combining leads to yet more strange results.

You will remember from earlier posts that a particle's wave can suddenly change its shape when a property of the particle is measured. Imagine, then, a wave that is the combination of the individual waves of two particles. According to quantum theory, if you make a measurement on one of the two particles, the measurement can cause the combined wave to suddenly change shape- that means the measurement affects both the component of the wave belonging to the particle you are measuring and the other component belonging to the particle you are not measuring. So, making a measurement on one particle can affect the wave belonging to another particle. To see why this is weird, imagine the two particles start close together but then spread apart. The particles could be millions of miles apart, but, according to quantum theory, if their waves are entangled then making a measurement on one of the particles can have an effect on the other, and the effect is immediate.

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