Since their conceptions, the theories made to explain quantum theory have been at odds with those used to explain relativistic physics. Einstein himself questioned the validity of quantum theory when it predicted that the spin of a particle was absolute regardless of frame of reference, and the idea of entanglement contradicts the relativistic concept of simultaneous events. Even today, many years after the conception of both theories, Quantum Gravity remains as an official unsolved problem in theoretical physics.
Every theory that has been developed to explain gravity has had some sort of ‘effective range’, or a cut-off point after which the theory’s predictions stop working. Classical Newtonian physics has cut-off points at very high velocities and masses, as well as very low ones. Einstein’s relativity theories removed the upper cut-offs, and quantum mechanics attempted to remove the lower ones. The problem that arose from these theories was that it seemed impossible to combine relativistic mechanics and quantum mechanics in to a single explanation for physical phenomena.
Relativity and Quantum Theory
Two key points of contention between quantum mechanics and general relativity are the fact that they provide different results when charting the behavior of matter near black holes, and that the uncertainty in velocity and location has yet to be adequately explained by any gravitational field theory, since the theory of relativistic gravity assumes that fields are of a constant nature. Combining this with quantum mechanics would lead to uncertainties in space-time that could lead to potential contradictions.
Several theories have been proposed with the intent of solving the quantum gravity problem, and today, there are two main contenders in the explanation of quantum gravity, string theory and loop quantum gravity.
String theory proposes a solution by expanding the definition of a particle to have the properties of a small, vibrating string, which effectively adds another dimension to the material world. In normal circumstances, particles would vibrate so fast that they would be indistinguishable from points, which is what other definitions of particles use. For some extreme cases of electric charge and energy, string particles would act differently. This offers a potential explanation for some of the problems faced in explaining quantum gravity, and it has been shown that full string theory produces an eleven-dimensional universe that maintains consistency with experimental data. The problem is that these theories only begin to come in to effect for cases that have yet to be successfully verified by experiment, and there is some question as to whether it will ever be possible to verify the predictions of string theory in any meaningful way.
Loop Quantum Gravity
Loop quantum gravity makes some predictions that are directly in opposition to those of string theory, which has caused physicists to become split over which theory to subscribe to. Loop quantum gravity attempts to explain gravity by considering structures of particles that act on one another in a distinctive looping manner. If space was composed of these loops, then gravity and other interactions could be explained by the entanglement of these loops, which allows for a unification of general relativity and quantum mechanics without requiring the extra dimensions proposed by string theory. Like string theory, loop quantum gravity has yet to make a prediction that differs from the standard model of general relativity in a way meaningful enough to allow for a test to be performed.
In essence, quantum gravity is still a truly unsolved problem.
Quantum Gravity in the Future
While string theory and loop quantum gravity maintain strong positions in the scientific community, only time and breakthroughs in experimentation will be able to verify one theory or another, and it is quite possible that neither theory proves to give accurate predictions.