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Quantum gravity is the field of theoretical
physics attempting to unify the subjects of Quantum mechanics
and General relativity.
Much of the difficulty in merging these theories comes from the radically different assumptions that these theories have on
how the universe works. Quantum mechanics depends on particle fields embedded in the flat space-time of either Newtonian mechanics or special relativity. Einstein's theory of general relativity models gravity as a curvature within space-time that changes as mass moves. The most obvious ways of combining the two (such
as treating gravity as simply another particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and adding together all of the
interactions results in many infinite values which can not easily be cancelled out. This is in contrast with quantum electrodynamics where the interactions sometimes
evaluate to infinite results, but those are few enough in number to be removable via renormalization.
Another difficulty comes from the success of both quantum
mechanics and general relativity. Both have been highly
successful and there are no known phenomenon that contradict the two. The energies and conditions at which quantum gravity are
likely to be important are inaccessible to laboratory experiments. The result of this is that there are no experimental
observations which would provide any hints as to how to combine the two.
The general approach taken in deriving a theory of quantum gravity is to assume that the underlying theory will be simple and
elegant and then to look at current theories for symmetries and hints for how to combine them elegantly into an overarching
theory. One problem with this approach is that it is not known if quantum gravity will be a simple and elegant theory.
Such a theory is required in order to understand those problems involving the combination of very large mass or energy and
very small dimensions of space, such as the behaviour of black holes, and the
origin of the universe.
Theories and Proto-theories
There are a number of proposed quantum gravity theories and proto-theories including:
Quantum gravity theorists
Related topics
External link
- Loop Quantum Gravity by Carlo Rovelli
The famous spoof of postmodernism by Alan Sokal (see Sokal Affair) was entitled "Transgressing the Boundaries: Toward a Transformative
Hermeneutics of Quantum Gravity".
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