Quantizing Gravity

Theoretical physicist Xavier Bekaert on quantum interaction, coupling constant, and properties of gravity

videos | October 9, 2015

How is gravitational interaction different from nuclear and electromagnetic interactions? What is the behavior of gravitons at high and low energies? Are there any possible solutions accounting for quantum gravity? These and other questions are answered by Associate professor of Physics and researcher at the Laboratory of Mathematics and Theoretical Physics at the University of Tours Xavier Bekaert.

In classical physics, we usually describe objects as particles. For instant, when an object moves, like a ball, we describe it as a particle with a centre of mass. But if you go to very small objects, with very small mass or very small energy, then you should take into account that they are not really like particles ‒ they are more like waves. So, elementary particles should also be described as waves. That is one of the main lessons of modern particle physics. This means, when particles interact, they interact like waves, so they merge, interfere and then continue their way but still remember that they have interfered. This is what we call ‘quantum interaction’, in a sense.

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If you probe gravity at higher and higher energies, it seems that gravitons interact more and more strongly. That’s a nightmare. In fact, their interactions are so strong that they produce infinities in the equations and you don’t know how to give meaning to it. Strictly speaking, some infinities were appearing also for the strong interactions but in that case we could cure them. There were some techniques that were working but those do not work at all for gravity. The good news is if you go the other way around and look at very low energies, that is very large distances, then gravity is very weakly interacting. So it’s strange to see that gravity is actually the weakest fundamental interaction of all.

When we, theoretical physicists, try to quantize gravity, we observe that interaction becomes stronger and stronger. This means that we can only partly quantize gravity but it produces problems, it is too difficult. Essentially, this is one of the major open problems for the 21st century, if not the 22nd, but like in case of any scientific problem, we should go ahead and see what we can do. Trying to quantize gravity would mean trying to go beyond this problem, so various ideas have been suggested to do that. There are many theories people may have heard about from popular science. The most well-known is string theory, which shows some promising aspects. Another one, relatively popular, is loop quantum gravity. There is even a more conservative option, proposed by Weinberg, which is called ‘asymptotic safety scenario’ essentially admitting that, unsurprisingly, gravity indeed interacts very strongly when you go to high energies. So far, so good, we should just leave it at that and try to go ahead.

Associate professor of Physics; researcher at the Laboratory of Mathematics and Theoretical Physics; University of Tours
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