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The Casimir force arises from one of those unlikely sounding real world manifestations of quantum mechanics.
It begins with considerations of what exactly is a vacuum. In the classical everyday
sense we think of a vacuum as what is left after we have removed all of the stuff,
molecules atoms etc. But that still leaves photons, so if we remove those as well
– including all the thermal energy then surely we should now have an absolute vacuum
which contains precisely nothing. Therein lies the problem. Heisenberg’s uncertainty
principle, describes the limitation on the knowledge of pairs of parameters in terms of
Planck’s constant; most well known being position and momentum. An equally important pairing
is energy and time, and quantum mechanics forbids the precise independent knowledge of these
two parameters. The absolute energy of a system is thus unknowable as a single parameter,
including the unique value of zero. So we cannot have a vacuum of absolute zero energy because
it violates the uncertainty principle.
The theoretical physics resolution of this paradox is to assume the existence of virtual particles which pop out of the vacuum and wander around for an undefined time and then pop back – thus giving the vacuum an average zero point energy, but without disturbing the real world too much.
One of the most remarkable results of Quantum Field Theory is the existence of vacuum fields, particles and zero-point energy. Vacuum is not a tranquil void but a quantum state made up of matter fields and energy fields.