Quantum Mechanics
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To describe the progress of quantum theory can be seen as a succession of 'quantisation levels'.
The original 1'st quantisation was applied in the 30's to particles such as the electrons
in their orbitals around an atomic nucleus described by Schrödingers equation.
In the 50's new phenomena such as antiparticles and the Lamb shift caused the development
of 2'nd quantised theories that works on operators on (virtual) particles that fill up what
we ordinarily thinks of as just empty space.
But vacuum is really a very complex entity containing variuos fields that can emit particles
in pair production as is exploited in the accelerators such as CERN's LEP and Fermilabs Tevatron.
To combine general relativity with quantum theory seems to require a 3'rd quantised theory
that is formulated as functionals working on the particle operators.
What such a theory descibes is vacuum states such as fundamental quanta of
space-time curvature and the ground states of perturbative field theory.
These functionals are expressed by linking loops (strings) together, parallells to this can perhaps be
seen in crystallography or more poetically in the old nordic myth of the 'norns' that weaved the
fates of men.