Quantum gravity is associate degree overall term for theories that arrange to unify gravity with the opposite elementary forces of physics (which ar already unified together). It typically posits a theoretical entity, a graviton, that could be a virtual particle that mediates the gravitational attraction. this can be what distinguishes quantum gravity from sure different unified field theories -- though, in fairness, some theories that ar usually classified as quantum gravity do not essentially need a graviton.
What's a Graviton?
The standard model of quantum physics (developed between 1970 and 1973) postulates that the opposite 3 elementary forces of physics ar mediate by virtual bosons. Photons mediate the magnetism force, W and Z bosons mediate the weak nuclear force, and gluons (such as quarks) mediate the sturdy nuclear force.
The graviton, therefore, would mediate the gravitational attraction. If found, the graviton is anticipated to be massless (because it acts instantly at long distances) and have spin a pair of (because gravity could be a second-rank tensor field).
Is Quantum Gravity Proven?
The major downside in by experimentation testing any theory of quantum gravity is that the energy levels needed to watch the conjectures ar impossible in current laboratory experiments.
Even on paper, quantum gravity runs into serious issues. Gravitation is presently explained through the speculation of Einstein's theory of relativity, that makes terribly completely different assumptions concerning the universe at the megascopic scale than those created by quantum physics at the microscopic scale.
Attempts to mix them typically run into the "renormalization downside," during which the total of all of the forces don't wipe out associate degreed lead to an infinite worth. In QED, this happened sometimes, however one might normalise the arithmetic to get rid of these problems. Such renormalization doesn't add a quantum interpretation of gravity.
The assumptions of quantum gravity are typically that such a theory can encourage be each easy and chic, such a big amount of physicists arrange to work backward, predicting a theory that they feel may account for the symmetries discovered in current physics so seeing if those theories work.
Some unified field theories that ar classified as quantum gravity theories include:
String theory / particle theory / M-theory
Loop quantum gravity
Noncommutative pure mathematics
Euclidean quantum gravity
Of course, it's totally doable that if quantum gravity will exist, it'll be neither easy nor elegant, during which case these tries ar being approached with faulty assumptions and, likely, would be inaccurate. solely time and experimentation can tell evidently.
It is conjointly doable, as a number of the higher than theories predict, that associate degree understanding of quantum gravity won't just consolidate the theories, however can rather introduce a basically new understanding of area and time.