Why is it assumed that elementary particles are identical?

In a two-slit experiment with individual particles, they do not end up in the same place. Similar conditions, different behaviour.

Why then is it assumed that the particles are identical?

Asker: Alex, 47 years old

Answer

Dear Alex,

When scientists say that identical particles (for example 2 electrons, an electron and a photon are distinguishable) are indistinguishable, they are talking about something very subtle in the context of quantum mechanics.

First of all about your example with the slit experiment. This is a difference in semantics. We only know the position of the particles after we have measured their position. By doing this we have changed their state and forced the particles to take a position. The particles are in an indistinguishable state for the measurement. For the measurement, the particles are described by a wave function. The particles interact with each other so that we get one large wave function. For example, we cannot tell which particle is where, even across from each other.

That the same particles are indistinguishable is a kind of postulate that arises from experiments (formally it is not a postulate, but is respected by quantum mechanics). This is especially important in experiments described by statistical mechanics (this theory derives thermodynamics from quantum mechanics) where many particles interact with each other. Until now, if an experiment had a theoretical outcome A for distinguishable and an outcome B for indistinguishable, B has always been the outcome.

Hopefully this is an answer to your question.

Answered by

Master Robbe Vansintjan

Royal Observatory of Belgium
Boulevard Boechout 10 1020 Brussels
http://www.astro.oma.be/

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