Gravity holds people on Earth and planets in their orbits around the Sun. It is also the first fundamental interaction described by a mathematical formula. However, it is gravity that physicists know the least about.
The reason is the discrepancy between the two main theories of modern physics. The general theory of relativity describes gravity for planets and larger cosmic bodies, while quantum theory explains the behavior of matter and interactions in the microscopic world. No complete theory has yet been created that combines the two descriptions.
Four interactions, but only three quantum theories
There are four fundamental interactions known in physics: gravity, electromagnetic, strong nuclear, and weak nuclear interactions. For the last three, there are quantum theories that allow us to calculate how they act at the particle level.
Gravity, however, remains different. The general theory of relativity describes it as a property of space-time, rather than as an interaction that has already been explained by quantum particles. It is this difference that underlies the ongoing search for a quantum theory of gravity.
Oppenheim suggests another way
British physicist Jonathan Oppenheim, a professor at University College London, believes that the problem may not be the lack of a good enough quantum theory. According to him, the very idea of turning gravity into a quantum interaction may be wrong.
“Such a theory makes no sense”, Oppenheim claims.His proposal is to keep gravity classical, while other physical systems are described by quantum theory. This approach is known as a post-quantum theory of classical gravity. It differs from the leading trends in fundamental physics, which assume that all interactions must ultimately have a quantum description.
The idea is controversial because it calls into question the direction followed by physicists for decades. It does not mean that the mystery of gravity has been solved, but that it is possible to seek an answer to the wrong question.
Experiment will have the final say
Oppenheim's theory has not been confirmed experimentally. Scientists are developing proposals for experiments to test whether gravity has quantum properties or exhibits different, classical behavior. Such tests can take years because they require extremely sensitive measurements.
So far, it has been established that general relativity and quantum theory work extremely well in their respective fields, but they do not provide a unified description of nature. Whether gravity turns out to be the ultimate quantum interaction or an exception to the rule will be decided by experiments, not by a single theory.
Sources: scienceillustrated.com