What is needed for quantum gravity is more experiments.

Mathematics will not solve the problems of quantum gravity; only experiments can do this.

In the mid-1990s, I studied math. I was not completely sure about what I want to do in life, but I was amazed at the ability of mathematics to describe the natural world. After the lessons on differential geometry and Lie algebras, I attended a series of seminars from the mathematical department at which the greatest problem of fundamental physics was discussed: the quantification of gravity and the unification of all the forces of nature under one theoretical umbrella. Workshops were conducted around a new approach developed by Abay Ashtekar of the University of Pennsylvania. I did not come across this research before, and I left there with the full impression that the problem was solved, and nobody just knew about it.

All this seemed to be a pure victory for an open mind. The requirements of mathematical connectivity led, for example, to the discovery of the Higgs boson. Without it, the Standard Model for particles colliding with energies above 1 TeV would stop working - and such energies are available at the Large Hadron Collider. Probabilities would not give a total of 100% and would lose their mathematical meaning. Consequently, upon the transition of this energy boundary, something new should have appeared. Higgs was the simplest of possibilities physicists could come up with, and they naturally found it.


The fast-rotating neutron star PSR B1509-58 lives in this nebula. Neutron stars give out regular pulses in the radio band, and they can be used to search for quantum effects of gravity.
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In the 1920s and 1930s, a mathematical mismatch between Einstein’s special theory of relativity and the original version of quantum mechanics led to the appearance of quantum field theory on which the Standard Model was then based. The mathematical discrepancy between the special theory of relativity and Newtonian gravity led to the emergence of the general theory of relativity - our most modern theory of gravity. Now, physicists still have a discrepancy between the Standard Model and GR. We, of course, expect that the resolution of this problem in the form of a quantum theory of gravity will become the same cover failure as the previous cases.

But over time, I learned about other researchers who used other methods and were convinced that they, too, came close to solving the problem. String theory, loop quantum gravity, causal dynamic triangulation, asymptotically safe gravity, causal kits ... Scientists practicing these approaches were also sure that they could decipher nature using only mathematics. They differed not because one of them made a mistake in the mathematical conclusions, but because they started from different premises. Mathematics is needed to conduct a series of logical conclusions, but no mathematical conclusion will be better than its premises. Logic is not enough to choose between physical theories. The only way to find out which theory describes nature is to conduct an experimental test.

But the physicists who worked on different approaches rarely communicated with each other, and if they did, they never agreed. And why? In the absence of experimental evidence, they had no reason for agreement. Mathematics was accumulating, tens of thousands of articles were created, hundreds of conferences were held. And no approach gave an unequivocal solution. And as the decades went unsuccessfully, doubts hung over the search for quantum gravity.



Strange, but in the 90s almost no one tried to find observable evidence of quantum gravity; it was thought to be impossible. The effects of quantum gravity are extremely weak. Physicists have estimated the probability of detecting the alleged particles of gravity - gravitons - and found that the chances of this are small even when using detectors the size of Jupiter, orbiting a neutron star. [Rothman, T. & Boughn, S., Can gravitons be detected? Foundations of Physics 36, 1801-1825 (2006)]

But is it really necessary to detect gravitons directly in order to find evidence of quantum gravity? This question did not let me go. By the end of the 90s, I switched to the study of physics. Most physicists working with quantum gravity still believe that their mathematics will open the way for success to them. I do not believe in this. But I do not have pessimism about the experimental inaccessibility of quantum gravity. On the contrary, I carefully hope that even during my life we ​​will successfully demonstrate the quantification of gravity in the experiment.

Those of us who are seeking experimental evidence for quantum gravity are faced with a unique research challenge: we have neither theory nor data! But even in the absence of the generally accepted theory of quantum gravity, we can explore the basic properties expected from it, and found in various candidate theories.

For example, some theories point to the discreteness of space-time. In this case, he may have defects, like crystals, that can knock light off the path and blur the images of distant quasars. Some theories believe that space-time is some kind of basis or liquid, in which case even with a vacuum one could find the properties of materials, such as viscosity or dispersion. Some theories predict a violation of symmetries respected in GR; others believe that quantum fluctuations in space-time can disturb sensitive quantum systems. All this can be found.

You already know that we have not found anything - otherwise you would have heard about it. But even the lack of results helps to develop theories. Such cases teach us that some ideas — for example, that space-time may be a periodic lattice — are simply incompatible with observations.

Of course, it would be much better to get a real confirmation. In recent years, we have been able to find several new opportunities to get closer to the goal. Take the primary gravitational waves. These small fluctuations of space-time in the early universe should have left a distinct imprint on the background radiation. In 2014, the joint group BICEP2 announced the measurement of this footprint, and although they were mistaken, this does not mean that the waves do not exist. Just to find them will require more effort. And if we find them, their quantum properties will help us develop our model. Lawrence Kraus of the University of Arizona and Frank Wilcek of MIT argue that the detection of primary gravitational waves will show that gravity must be quantized [Krauss, L. & Wilczek, F. Using quantization of gravity. Physical Review D 89, 047501 (2014)]. Their argument is oversimplified, but we can’t show that cmb anisotropies are quantum-mechanical origin? Physical Review D 93, 023505 (2016)] and Eugene Bianchi [Bianchi, E., Hackl, L., & Yokomizo, N. Entanglement time in the primordial universe. International Journal of Modern Physics D 24, 1544006 (2015)] independently engaged in the analysis of relic radiation data that can distinguish quantum fluctuations from non-quantum fluctuations.


BICEP2 telescope at the South Pole

There are still black holes. The physics of black holes is one of the main topics in the study of quantum gravity. For a long time it was believed that quantum-gravitational effects would be noticeable only closer to the center of black holes, hidden behind the horizon, denoting its boundary, and therefore immeasurable outside. But in recent years, this faith has been shaken. For example, according to one theoretical assumption, black holes are surrounded by firewalls — material surfaces that destroy matter falling into them. Although I and some other scientists questioned this argument [Hossenfelder, S. Disentangling the black hole vacuum. Physical Review D 91, 044015 (2015)], it is not the only reason to assume that quantum gravity effects may appear on the horizon.

And if they manifest themselves, then the study of black holes can reveal to us information on quantum gravity. Michael Kavic of the University of Long Island suggested searching for binary systems consisting of a neutron star orbiting a black hole. The neutron star radiates radio waves, and if this beam hits the horizon of the black hole, then the observed pulse will be changed by the structure of this hole [Estes, J., Kavic, M., Lippert, M., & Simonetti, JH, Shining light -black hole binaries. arXiv: 1607.00018 (2016)]. Another approach from the Niimeesh Afshordi [Perimeter Institute] is studying the gravitational waves created by the confluence of black holes. Quantum effects can manifest themselves in those moments when the newly formed black hole takes its final form [Abedi, J., Dykaar, H., & Afshordi, N. Echoes from the Abyss: arXiv: 1612.00266 (2016)].

But the most promising idea came from an unexpected side. If the gravitational field can be quantized, it must have certain quantum characteristics, such as superposition, in which the system is simultaneously in different states.

Take the basic example of quantum behavior: an experiment with two slits. If you direct the electron beam to the screen in which two slits are cut, the electrons form a certain wave pattern. For its appearance, each electron must pass through both slits simultaneously - this is a superposition of paths. But the electron has a mass, and it affects the gravitational field. If the electron is in quantum superposition, then its field must also be in quantum superposition. This is a very strange idea. If the same thing happens to the whole Earth, then an apple fallen from a tree will experience two different gravitational fields and fall in two different directions at the same time. Such features are incompatible with quantum mechanics and GR; superposition of fields must be inherent in quantum gravity.

So far, no one has observed such effects, since the gravitational field of one electron is too weak to be measured. In recent years, several experimental groups have created superpositions for much more massive objects. Today's advanced edge of science is work with the mass in nanograms. With his Viennese group, Markus Aspelmeyer engaged in an ambitious project to measure the gravitational attraction of masses of 1 milligram [Schmöle, J., Dragosits, M., Hepach, H., & Aspelmeyer, M. of milligram masses. Classical and Quantum Gravity 33, 125031 (2016)]. The day is not far off when we can measure the gravitational field of quantum objects.

A similar approach is attempted by Mauro Paternostro and colleagues from Queens University in Belfast to determine exactly which signs should distinguish between a quantized gravitational field and a nonquanting one [Krisnanda, T., Zuppardo, M., Paternostro, M., Tomasz Paterek, T. Revealing non -classicality of unmeasured objects. arXiv: 1607.01140 (2016)]. Their approach is tied to a typical quantum property, entanglement, in which there is a correlation between the properties of different objects. Imagine two objects interacting through gravity. Correlations between them will depend on whether this field is quantized or not. In theory, one can measure correlations and determine field quantibility.


If we had glasses to observe the gravity waves, then the fusion of black holes would look brighter than a supernova explosion.

The fact that science requires experimental confirmation of ideas cannot be called news, but the dream of the ancient philosophers that reasoning alone can unravel the secrets of nature unfortunately lives among theorists working on quantum gravity. As a result, mental exercises, be they arbitrarily complex, are reduced to aesthetic or philosophical preferences when choosing prerequisites. A huge amount of literature on quantum gravity is engaged in the burial of these premises under the mathematical mountains.

Twenty years after I first heard about quantum gravity, this area is still dominated by scientists who rely on mathematical sequence. But the number of those who, like me, is exploring the possibilities of experimental verification of quantum gravity is increasing. And the more visible the failure of the mathematical method becomes, the clearer to us that the only way forward is to search for experimental evidence, regardless of its complexity. The first step is to demonstrate the quantization of gravity. And then you can proceed to the entire spectrum of gravitational phenomena. This is how we transfer quantum gravity from mathematics to physics.

And what has become physics can become engineering. Unlike many of my colleagues, I believe that understanding quantization of gravity can help us practically. Such a theory will not only improve our understanding of space and time, but also quantum systems in general. It will be a long way. But it took us 2,000 years to go from the four elements of Aristotle to the four forces of physics. So the journey will be long.