The energy of their creation is “borrowed” from vacuum fluctuations, so they must give it back again in accordance with the equation e = h / t, where e is the total energy of the particles, h is the Planck constant, and t is time. The Planck constant is extremely small, so the time must be incredibly short in order for the total energy of the particles to be available.

This means that, not only is the probability of particle type A proportional to the number of states in which it can attain, it’s also dependent on the rest mass of the particle. The lower its mass, the more likely it is to come into existence. That’s why virtual electrons are relatively plentiful, but virtual protons, neutrons, and other even more exotic heavy particles are very rare.

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