Physics Help
Graviton
Graviton
In
physics, the graviton is a hypothetical
elementary particle that transmits the force of
gravity in most
quantum gravity systems. In order to do this gravitons have to be
always-attractive (gravity never pushes), work over any distance (gravity is
universal) and come in unlimited numbers (to provide high strengths near stars).
In a quantum theory this defines an even-spin
(spin 2 in this case)
boson with a
rest mass of zero.
Gravitons are postulated simply because quantum has been so
successful in other fields. For instance,
electrodynamics can be very well explained by the application of
quantization to
photons. In this case photons are being continually created and destroyed by
all
charged particles, and the interactions between these photons produce the
macroscopic forces we are familiar with, like
magnetism.
Given the widespread success of quantum in describing the
vast majority of basic forces in the
universe, it seemed only natural that the same methods would work well on
gravity as well. Many attempts were made to introduce a so-far unseen graviton,
which would work in a fashion somewhat similar to the photon. It was hoped that
this would quickly lead to a quantum gravity theory, although the math was a bit
difficult.
It has not worked out that way. Any such theory would require
a graviton to operate in a fashion similar to a photon, but unlike
electrodynamics where the photons act directly on each other and their charged
particles, gravity just doesn't work so simply. Well-observed behaviours show
that gravity is created by any form of
energy (mass
simply being a particularily condensed form), which is difficult to describe in
a fashion similar to "charge". To date all attempts to create a consistent
simple quantum gravity theory have failed.
Detecting a graviton, if it exists, would prove rather
problematic. The particles carry very little energy1 so detecting
them would be very difficult. The only way to detect them would be to look for
cases where the overall motion or energy of an object changes in a way that is
different than predicted by
general relativity, but one of the basic principles of quantum gravity would
be that it matches those predictions as closely as possible.
It should be noted that a quantum gravity theory does not
require a graviton; for instance,
loop quantum gravity has no analogous particle.
Notes:
1) Many people are surprised to learn that gravity is the
weakest force. A simple experiment will demonstrate this, however: lift your
arm. You have now lifted several kilograms of mass against the gravity generated
by the entire planet.
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