Black holes, these extreme cosmic entities, might well hold the missing clues to solve one of physics' greatest mysteries: the unification of general relativity and quantum mechanics.
(Left) the relatively calm black hole at the center of the Milky Way
(Right) the turbulent supermassive black hole of M87
Credit: EHT Collaboration
For over a century, Einstein's general relativity has brilliantly described gravity on large scales. Yet it remains incompatible with quantum mechanics, which governs phenomena at the particle scale. This conflict becomes unavoidable when trying to understand what happens at the heart of black holes, where these two theoretical frameworks must coexist.
Black holes, emerging directly from Einstein's equations, form a natural laboratory for testing the limits of our knowledge. Their center harbors a singularity, a zone of infinite density where known laws cease to function. This singularity suggests that general relativity is incomplete, and that a theory of quantum gravity is needed to go further.
It is in response to this problem that physicist Xavier Calmet (University of Sussex) and his colleagues proposed a new approach. By integrating quantum corrections into Einstein's equations, they obtained novel solutions to describe black holes. Their method doesn't rely on a complete theory, but offers a first concrete step toward a quantized description of gravity.
These solutions predict the existence of black holes slightly different from those described by general relativity. However, it remains currently very difficult to distinguish them with our observational means, particularly due to their distance.
Their work, published in
A Letters Journal Exploring the Frontiers of Physics, opens a promising path to reconcile the two great pillars of modern physics. Even if black holes still keep their secrets, they might well be the gateway to a unified theory.
What is quantum gravity?
Quantum gravity is a developing theory that seeks to unify general relativity and quantum mechanics. Each works in its own domain, but they become incompatible when applied together, for example in regions of extremely strong gravity concentrated in a tiny space.
Candidate theories are numerous, like string theory or loop quantum gravity, but none has yet been verified by experiment. The challenge is to understand extreme phenomena, like those inside black holes or during the Big Bang.
Why are black holes so important for quantum gravity?
Black holes are the only known objects where relativistic and quantum effects simultaneously become dominant. They offer a unique testing ground for hypotheses about quantum gravity.
Their theoretical center – the singularity – represents a failure of relativity. Incorporating quantum effects there might perhaps avoid this singularity, and better understand the intimate structure of spacetime.
Thanks to instruments like the Event Horizon Telescope, it's becoming possible to observe subtle effects around black holes. This data could one day discriminate between predictions from classical relativity and those arising from quantum gravity.