Physicists from the Max Planck Institute and the Perimeter Institute in Canada have developed a new approach to the unification of the general theory of relativity and quantum theory.
Present-day physics cannot describe what happened in the Big Bang. Quantum theory and the theory of relativity fail in this almost infinitely dense and hot primal state of the universe. Only an all-encompassing theory of quantum gravity which unifies these two fundamental pillars of physics could provide an insight into how the universe began. Scientists from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm/Potsdam and the Perimeter Institute in Canada have made an important discovery along this route. According to their theory, space consists of tiny “building blocks”. Taking this as their starting point, the scientists arrive at one of the most fundamental equations of cosmology, the Friedmann equation, which describes the universe. This shows that quantum mechanics and the theory of relativity really can be unified.
For almost a century, the two major theories of physics have coexisted but have been irreconcilable: while Einstein’s General Theory of Relativity describes gravity and thus the world at large, quantum physics describes the world of atoms and elementary particles. Both theories work extremely well within their own boundaries; however, they break down, as currently formulated, in certain extreme regions, at extremely tiny distances, the so-called Planck scale, for example. Space and time thus have no meaning in black holes or, most notably, during the Big Bang.
Daniele Oriti from the Albert Einstein Institute uses a fluid to illustrate this situation: “We can describe the behavior of flowing water with the long-known classical theory of hydrodynamics. But if we advance to smaller and smaller scales and eventually come across individual atoms, it no longer applies. Then we need quantum physics.” Just as a liquid consists of atoms, Oriti imagines space to be made up of tiny cells or “atoms of space”, and a new theory is required to describe them: quantum gravity.
Continuous space is broken down into elementary cells
In Einstein’s relativity theory, space is a continuum. Oriti now breaks down this space into tiny elementary cells and applies the principles of quantum physics to them, thus to space itself and to the theory of relativity describing it. This is the unification idea.
A fundamental problem of all approaches to quantum gravity consists in bridging the huge dimensional scales from the space atoms to the dimensions of the universe. This is where Oriti, his colleague Lorenzo Sindoni and Steffen Gielen, a former postdoc at the AEI who is now a researcher at the Perimeter Institute in Canada, have succeeded. Their approach is based on so-called group field theory. This is closely related to loop quantum gravity, which the AEI has been developing for some time.