A new study suggests that gravity actually emerges from the quantum world in the same way that the flow of a liquid emerges from the chaotic motions of individual droplets.
One of the biggest challenges in modern physics is to find a single or ‘unified’ theory that is capable of describing all the laws of nature in a single framework. One that connects the two great (and irreconcilable) theories that, today, scientists use to understand reality: Einstein’s General Relativity, which describes the Universe on a large scale; and Quantum Mechanics, which describes our world at the atomic level. Why these two successful theories fit together is one of the biggest mysteries facing science.
If achieved, this theory of ‘quantum gravity’ would include both a macroscopic and microscopic description of reality, and would also give us a deep insight into phenomena that are inaccessible today, such as black holes or the instant in which the Universe was created.
But how to get it? For nearly a century, generations of physicists have tried unsuccessfully to figure out why the laws that hold in the realm of the very small don’t ‘work’ in the macroscopic world around us, and vice versa. Now, a team of researchers from the Chalmers University of Technology in Sweden, together with the American MIT, have published an article in ‘ Nature Communications ‘ in which they suggest that gravity, the force that dominates the Universe on a large scale, emerges actually from the quantum world. To reach this extraordinary conclusion, the researchers have resorted to advanced mathematics and the so-called ‘ holographic principle ‘.
“We strive to understand the laws of nature,” explains Daniel Persson, co-author of the study, “and the language in which those laws are written in mathematics. When we search for answers to questions in physics, they often lead us to new discoveries in mathematics as well. This interaction is particularly prominent in the quest for quantum gravity, where it is extremely difficult to perform experiments.”
An example of a phenomenon that requires this kind of unified description is black holes. A black hole forms when a sufficiently heavy star collapses under its own gravitational pull so that all of its mass is concentrated into an extremely small volume. The quantum mechanical description of black holes is still in its infancy, but it involves spectacularly advanced mathematics.
In the case of the unified theory, explains Robert Berman, the first author of the article, «the challenge is to describe how gravity arises as an ’emergent’ phenomenon. Just as everyday phenomena, such as the flow of a liquid, emerge from the chaotic motions of individual droplets, we want to describe how gravity emerges from the quantum-mechanical system at the microscopic level.”
In this way, the researchers showed how gravity emerges from a special Quantum Mechanics system, in a simplified model for quantum gravity called the ‘ holographic principle ‘.
“Using mathematical techniques that I had already investigated before,” Berman continues, “we managed to formulate an explanation of how gravity arises by the holographic principle, in a more precise way than before.”
The new item may also offer a new way to deal with the mysterious dark energy. In Einstein’s General Theory of Relativity, gravity is described as a geometric phenomenon. Just as a newly made bed bends under the weight of a person, heavy objects can bend spacetime, the ‘fabric’ that makes up the Universe.
But according to Einstein’s theory, even empty space, the ‘vacuum state ‘ of the Universe, has a rich geometric structure. If we could zoom in and look at this void with a microscope, we would see tiny quantum mechanical fluctuations or waves, known as dark energy, the mysterious form of energy believed to be responsible for the accelerating expansion of the Universe.
The study may lead to new insights into how and why these microscopic quantum-mechanical waves arise, as well as the relationship between Einstein’s theory of gravity and Quantum Mechanics, something scientists have been pursuing for decades.
«These results -concludes Persson- open the possibility of testing other aspects of the holographic principle, such as the microscopic description of black holes. We also hope to be able to use these new connections in the future to break new ground in mathematics.”
