In physics, the graviton is a hypothetical elementary particle that mediates the force
of gravitation in the framework of quantum field theory.
If it exists, the graviton is expected to be massless and must be a spin-2 boson.
The spin follows from the fact that the source of gravitation is the stress–energy tensor,
a second-rank tensor.
Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable
from gravitation, because a massless spin-2 field must couple to the stress–energy tensor
in the same way that the gravitational field does.
This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton,
so that the only experimental verification needed for the graviton may simply be the
discovery of a massless spin-2 particle.
Theory The three other known forces of nature are
mediated by elementary particles: electromagnetism by the photon, the strong interaction by the
gluons, and the weak interaction by the W and Z bosons.
The hypothesis is that the gravitational interaction is likewise mediated by an – as yet undiscovered
– elementary particle, dubbed as the graviton.
In the classical limit, the theory would reduce to general relativity and conform to Newton's
law of gravitation in the weak-field limit.
Gravitons and renormalization When describing graviton interactions, the
classical theory and semiclassical corrections behave normally, but Feynman diagrams with
two loops lead to ultraviolet divergences; that is, infinite results that cannot be removed
because the quantized general relativity is not renormalizable, unlike quantum electrodynamics.
That is, the usual ways physicists calculate the probability that a particle will emit
or absorb a graviton give nonsensical answers and the theory loses its predictive power.
These problems, together with some conceptual puzzles, led many physicists to believe that
a theory more complete than quantized general relativity must describe the behavior near
the Planck scale.
Comparison with other forces Unlike the force carriers of the other forces,
gravitation plays a special role in general relativity in defining the spacetime in which
events take place.
In some descriptions, matter modifies the 'shape' of spacetime itself, and gravity is
a result of this shape, an idea which at first glance may appear hard to match with the idea
of a force acting between particles.
Because the diffeomorphism invariance of the theory does not allow any particular space-time
background to be singled out as the "true" space-time background, general relativity
is said to be background independent.
In contrast, the Standard Model is not background independent, with Minkowski space enjoying
a special status as the fixed background space-time.
A theory of quantum gravity is needed in order to reconcile these differences.
Whether this theory should be background independent is an open question.
The answer to this question will determine our understanding of what specific role gravitation
plays in the fate of the universe.
Gravitons in speculative theories String theory predicts the existence of gravitons
and their well-defined interactions.
A graviton in perturbative string theory is a closed string in a very particular low-energy
vibrational state.
The scattering of gravitons in string theory can also be computed from the correlation
functions in conformal field theory, as dictated by the AdS/CFT correspondence, or from matrix
theory.
An interesting feature of gravitons in string theory is that, as closed strings without
endpoints, they would not be bound to branes and could move freely between them.
If we live on a brane this "leakage" of gravitons from the brane into higher-dimensional space
could explain why gravitation is such a weak force, and gravitons from other branes adjacent
to our own could provide a potential explanation for dark matter.
See brane cosmology.
Experimental observation Unambiguous detection of individual gravitons,
though not prohibited by any fundamental law, is impossible with any physically reasonable
detector.
The reason is the extremely low cross section for the interaction of gravitons with matter.
For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit
around a neutron star, would only be expected to observe one graviton every 10 years, even
under the most favorable conditions.
It would be impossible to discriminate these events from the background of neutrinos, since
the dimensions of the required neutrino shield would ensure collapse into a black hole.
However, experiments to detect gravitational waves, which may be viewed as coherent states
of many gravitons, are underway.
Although these experiments cannot detect individual gravitons, they might provide information
about certain properties of the graviton.
For example, if gravitational waves were observed to propagate slower than c, that would imply
that the graviton has mass.
Astronomical observations of the kinematics of galaxies, especially the galaxy rotation
problem and modified Newtonian dynamics, might point toward gravitons having non-zero mass.
Difficulties and outstanding issues Most theories containing gravitons suffer
from severe problems.
Attempts to extend the Standard Model or other quantum field theories by adding gravitons
run into serious theoretical difficulties at high energies because of infinities arising
due to quantum effects.
Since classical general relativity and quantum mechanics seem to be incompatible at such
energies, from a theoretical point of view, this situation is not tenable.
One possible solution is to replace particles with strings.
String theories are quantum theories of gravity in the sense that they reduce to classical
general relativity plus field theory at low energies, but are fully quantum mechanical,
contain a graviton, and are believed to be mathematically consistent.
See also Gravitomagnetism
Gravitational wave Planck mass
Gravitation Static forces and virtual-particle exchange
Multiverse Gravitino
References
External links Graviton on In Our Time at the BBC.