Centre for Quantum Information and Spacetime

The visible universe adheres faithfully to the laws of quantum theory, but Einstein’s theory of gravity is in contradiction with this. Modern physics is built atop a fundamental inconsistency, which generations of scientists have failed to reconcile. The holy grail of physics – a quantum theory of gravity -- has remained elusive for more than a century. Einstein’s theory of gravity, general relativity, posits that matter curves spacetime. If matter has a quantum nature, then the assumption has been that spacetime must also be quantum. However, gravity is different to the quantum forces, in fact, as Einstein showed us, gravity is not even a force! Rather, gravity is the fact that objects freely fall in curved spacetime. There are reasons to believe that quantising spacetime may be a flawed approach. Without a satisfactory theory of quantum gravity for our universe, the nature of spacetime remains a mystery .

At the end of the day, it is irrelevant what physicists believe, and we must turn back to experiment to guide us. The Centre for Quantum Information and Spacetime at University College London stands at the forefront of an ambitious program to test whether spacetime has a quantum nature. While probing the quantum nature of spacetime was once thought to require energies that put such experiments out of reach, we now understand that ideas from quantum information theory, combined with the rapid  development of quantum technologies, now make testing the nature of spacetime feasible.  We have proposed experiments which can directly verify whether spacetime is quantum by detecting gravitationally mediated entanglement, as well as experiments that aim to place bounds on the diffusion of spacetime. Other tabletop experiments, including short distance precision tests of gravity, combined with astrophysical observation also place constraints on theories of gravity.

We are also pursuing a theoretical alternative to quantum gravity. Instead of modifying spacetime, the theory – dubbed a “postquantum theory of classical gravity” – modifies quantum theory and predicts an intrinsic breakdown in predictability that is mediated by spacetime itself. This results in random fluctuations in spacetime that are larger than envisaged under quantum theory. The program aims to develop the theory and explore its consequences, both on the astrophysical scale, and in the setting of cosmology. You can find out more about the theory here.

While we do not know the fundamental nature of spacetime, from an effective point of view, we are often interested in the regime where we can treat spacetime classically. The techniques we are developing also serve as effective theories of quantum gravity, which are applicable to scenarios where matter is highly nonclassical so that mean field approaches are no longer valid.