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I've read that attraction and repulsion between particles is caused by the exchange of virtual photons, and that virtual photons carry information.

I don't understand how a virtual photon actually causes any attraction or repulsion, and how does it carry information anyway if it's "virtual"?

Aren't photons an excitation of the electromagnetic field?

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Virtual particles, whether photons or electrons or... are, in the context of QFT, particles that are off-shell, i.e., their associated energy and momentum are not related by the relativistic energy-momentum relation.

Please read this to get an idea of how virtual particle exchange can create attractive or repulsive forces.

Photons are quanta of the modes of the quantized electromagnetic field. This isn't easy to explain without some background in the quantum harmonic oscillator and, even then...

However, imagine that a guitar string can vibrate with only discrete amplitudes and these amplitudes have an energy that is a multiple of a fundamental energy.

Then, if the guitar string were vibrating with the 1 unit of this fundamental energy, we would say that 1 "photon" of guitar string vibration were present. If the string were vibrating with n units of this fundamental energy, we would say that n "photons" were present.

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