Quantum Gravity

In their early attempts to build upon Feynman’s theory, physicists were not able to reconcile the general theory of relativity with quantum field theory in a unified theory of quantum gravity that described gravity at the level of individual particles. Using computational techniques developed for quantum field theories, one finds that the probabilities of physical amplitudes involving quantum aspects of gravity–such as the scattering of two quanta of gravitational waves hitting one another (what is called two “gravitons”)–can yield infinitely large numbers. That’s a serious problem, because a probability greater than one–let alone infinitely big–is a meaningless concept.