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Quantum Physics
Quantum fluctuations & 'virtual particles'
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[QUOTE="vanhees71, post: 6825675, member: 260864"] Fluctuations are of course nothing else than uncertainties in random experiments, i.e., the statistical fluctuations around the expectation value of some observable. There is no vacuum energy or, rather, its value within special relativistic or Newtonian physics is entirely arbitrary. Nothing in the physics changes when you add arbitrary values to the total energy. The only state in relativistic QFT which does not fluctuate is the ground state aka the vacuum. What's often discussed as "virtual particles" are in fact fluctuations of fields. E.g., having an atom in an excited state is quantum-field theoretically a state with the atom in this state and no photons. Since this is not an energy eigenstate and since photon number is not conserved there is some probability for the atom to emit photons and go to a lower state. This is a random process and in this sense a "fluctuation". This spontaneous emission is one of the simplest processes which are only describable in quantum field theory, i.e., it's showing the necessity to quantize the electromagnetic field in addition to the (charged) particles and that the semiclassical approximation, where the electromagnetic field is treated as a classical field and only the particles are quantized. Another argument for "vacuum fluctuations" is usually the Casimir force between uncharged conductors. The usual calculation in introductory chapters of QFT books (e.g., Itzykson+Zuber) is, however, an approximation in the limit of infinite charge of the conduction electrons. A more detailed treatment of the Casimir force shows that it's in fact due to fluctations of the charges and the em. field and has nothing to do with "vacuum fluctuations". Ironically, the full vacuum of the interacting theory doesn't fluctuate at all. [/QUOTE]
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