Quantum gravity is a theory that has been the target of decades of study by physicists worldwide. If this idea is proven, it would tie together the General Theory of Relativity (which governs gravitational fields) with quantum mechanics, and the bizarro-world of subatomic particles.
Gravitational waves, produced by accelerating objects, ripple through space-time, according to most interpretations of the General Theory of Relativity penned by famed physicist Albert Einstein. Researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) have announced they detected these disturbances in the fabric of time and space for the first time.
Certain aspects of subatomic behavior is quantized - they can only move or exist in particular whole number states. This characteristic may be thought of like steps to walk up into an apartment. Many physicists believe gravitational waves are similarly quantized, made up of individual quantum particles of gravity - gravitons.
Although it is not certain, many physicists believe that these particles join together, forming the gravitational waves that travel through space. Like photons of light, these gravitons, would have no rest mass, and move at the speed of light.
The effects of quantum gravity are predicted to be quite pronounced in the region immediately surrounding the center of black holes. However, it is impossible to collect data from events near a singularity. The events witnessed by astronomers are LIGO-recorded activity from just outside the event horizons of a pair of black holes as they collided.
The LIGO detector cannot detect single gravitons, and cannot, by itself, test the theory of quantum gravity. However, there is reason to believe that either LIGO, or a future gravitational wave detector, could be used to find evidence of quantum gravity by examining the emission spectrum of energy seen surrounding the event horizons of black holes.
According to some theories, even at the event horizon, the effects of gravitons could cause gravitational waves to be more powerful, and less regular, than they would be without their influence.
"Certain scenarios with strong quantum modifications in a region extending well outside the horizon are expected to modify classical evolution, and distort the near-peak gravitational wave signal, suggesting a search for anomalies such as decreased regularity of the signal and increased power," Steven Giddings of the University of California, Santa Barbara, said.
Any variation seen between observations and graviton-free theories of gravitational waves, such as Einstein's, could assist physicists seeking to understand the ultimate units of gravity.
As astronomers use LIGO and other detectors to search for elusive ripples in space-time, they may also come across evidence of other strange features of space, including cosmic strings, theoretical one-dimensional strings of energy, which may have been created long ago, when the universe was young.
Moreover, as additional findings of gravitational waves are recorded, physicists will search the data for behavior of the ripples which might suggest the presence of gravitons. If they are found, the discovery could herald a new age of understanding how gravity works. Such a finding could suggest that other notions of gravity, such as string theory, could prove to be the basis of future work on the nature of gravity.
But until such variations are seen, the existence of gravitons remains strictly theoretical.
