The Interface of Quantum Mechanics and Gravity

Abstract

Quantum theories of gravity have been discussed for more than half a century, and the progress has been slow. Apart from theoretical technical reasons, lack of guidance by experimental or observational clues has been a major problem. While this situation will not be changed very much if the scale of quantum gravity is the Planck scale, there are two areas of study one should pursue where there is hope of arriving at some helpful indications. One is obviously cosmology, and astrophysical observations on the cosmological scale where Planck scale signatures could be visible in principle. Small quantum effects can get accumulated over cosmological scales of propagation and become observable. Another avenue is the interface of gravity and quantum mechanics as studied through the dynamics of quantum systems and fundamental particles in gravitational fields. Even classical experiments, like test of Universality of Free-Fall (UFF) using classical mass elements come under this category, and certain limited questions concerning gravity-quantum mechanics interface can be addressed and possibly answered. In this article I review some of these possibilities. Some of the issues we address are, ‘Is the equivalence principle valid for a general quantum system?’, ‘Do Casimir energy (energy of the quantum vacuum) obey the equivalence principle?’ , ‘Can observations on cosmological scales give clues about quantum gravity?’, ‘What is the influence of our specific Universe on local quantum systems like atoms?’, and ‘Are there signatures of quantum processes in the original of the Universe observable today?’ Some of the answers have to be speculative since it is often difficult to have independent confirmations since even arriving at one clue is difficult. Yet, the hope is that some of this discussion will initiate new thoughts that can make progress regarding the issue of a general theory of gravity, including aspects of quantum gravity.