Quantum gravity is an overall term for theories that attempt to unify gravity with the other fundamental forces of physics (which are already unified together). It generally posits a theoretical entity, a graviton, which is a virtual particle that mediates the gravitational force. This is what distinguishes quantum gravity from certain other unified field theories -- although, in fairness, some theories that are typically classified as quantum gravity don't necessarily require a graviton.
What's a Graviton?
The standard model of quantum mechanics (developed between 1970 and 1973) postulates that the other three fundamental forces of physics are mediated by virtual bosons. Photons mediate the electromagnetic force, W and Z bosons mediate the weak nuclear force, and gluons (such as quarks) mediate the strong nuclear force.
The graviton, therefore, would mediate the gravitational force. If found, the graviton is expected to be massless (because it acts instantaneously at long distances) and have spin 2 (because gravity is a second-rank tensor field).
Is Quantum Gravity Proven?
The major problem in experimentally testing any theory of quantum gravity is that the energy levels required to observe the conjectures are unattainable in current laboratory experiments.
Even theoretically, quantum gravity runs into serious problems. Gravitation is currently explained through the theory of general relativity, which makes very different assumptions about the universe at the macroscopic scale than those made by quantum mechanics at the microscopic scale.
Attempts to combine them generally run into the "renormalization problem," in which the sum of all of the forces do not cancel out and result in an infinite value. In quantum electrodynamics, this happened occasionally, but one could renormalize the mathematics to remove these issues. Such renormalization does not work in a quantum interpretation of gravity.
The assumptions of quantum gravity are generally that such a theory will prove to be both simple and elegant, so many physicists attempt to work backward, predicting a theory that they feel might account for the symmetries observed in current physics and then seeing if those theories work.
Some unified field theories that are classified as quantum gravity theories include:
- String theory / Superstring theory / M-theory
- Loop quantum gravity
- Twistor theory
- Noncommutative geometry
- Euclidean quantum gravity
- Wheeler-deWitt equation
Of course, it's fully possible that if quantum gravity does exist, it will be neither simple nor elegant, in which case these attempts are being approached with faulty assumptions and, likely, would be inaccurate. Only time and experimentation will tell for sure.
It is also possible, as some of the above theories predict, that an understanding of quantum gravity will not merely consolidate the theories, but will rather introduce a fundamentally new understanding of space and time.
Edited by Anne Marie Helmenstine, Ph.D.