vacuum fluctuations
Vacuum fluctuations are the restless, irreducible quantum jitter of fields even when no particles are present. In quantum field theory each field behaves like a quantum oscillator, and the uncertainty principle forbids any oscillator from being perfectly still: if its value were exactly fixed, its rate of change would be wildly uncertain, and vice versa. The compromise is a permanent, low-level trembling that never switches off, not even in the emptiest region of space.
These fluctuations are sometimes described picturesquely as 'virtual particles' flickering in and out of existence. That phrase is a useful image but should be handled with care: virtual particles are bookkeeping devices in calculations, not little objects you could ever catch in a net. What is physically real is the fluctuating field itself, and the measurable consequences it produces.
And those consequences are genuinely measurable, which is the honest test that vacuum fluctuations are more than a story. They shift atomic energy levels by a tiny, precisely predicted amount, they drive atoms to emit light spontaneously, and they generate the attractive Casimir force between metal plates. Far from being a quirk of theory, the trembling of the vacuum is one of the best-confirmed features of the quantum world.
The uncertainty principle applied to a field forbids absolute stillness, so empty space quietly trembles.
Virtual particles are a vivid figure of speech, not literal particles popping up and disappearing. They cannot be detected directly; only the real effects of the fluctuating field, such as the Lamb shift and the Casimir force, can be.