If electrons have no size, how can two electrons collide?

That is an interesting question!

The answer is no, electrons do not collide. But not in the traditional sense.

This means that electrons don’t interact directly with electrons. Or, in quantum field theory’s language, the electron field is “linear”, without self-interactions.

But electrons can interact with the electromagnetic fields: they absorb and emit photons. They also act as sources for the electromagnetic field and alter its properties, due to their electric charge.

Two electrons can become close together and each one begins to react to changes in the electromagnetic fields caused by the other electron. They exchange virtual photons at an increasing rate, if that is what you want to use. This exchange becomes more intense the closer they get to each other, and manifests itself as a repelling force between electrons. The electrons “bounce off” one another, even though they never touch each other, a phenomenon we call a collision.

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