Theories that describe the nature of today, are presented at the most fundamental level of the general theory of relativity and the Standard Model of elementary particle physics. General relativity - a classical theory, while the standard model - a quantum field theory . The first should not be the Heisenberg uncertainty principle, the second - should be. Both theories together can describe all the observations that we have at the moment, although some aspects of this description are unsatisfactory, such as missing the microscopic structure of dark matter. The combination of these two theories consistent with the observation, but the big problem is that it is self-contradictory.

This is most clearly demonstrated by the problem of mismatch loss of information black hole. Association of general relativity with quantum field theory leads to what is known as quantum field theory in curved space. She partly classical, partly quantized theory, "semiclassical gravity." In theory, this combination can be calculated that black holes emit radiation - Hawking radiation, named after its discoverer.

Hawking radiation - a range of blackbody radiation with a single parameter: temperature, which depends on the initial mass of the black hole. This means that all the black holes one initial mass to evaporate a final state, regardless of which formed. The formation of a black hole and subsequent evaporation, thus irreversible: even if we fully know the final state, we can not determine the original state. Information is lost. The problem is that this essentially irreversible process is incompatible with quantum field theory, which we use for the process: it is an internal contradiction, inconsistency, so there can not be, it should not be. Mathematics brings us to this conclusion.

Semiclassical combination of general relativity and the standard model leads to other problems. We do not know, for example, what happens with the gravitational field of an electron passing through a double slit. We know that the electron wave function in the superposition and passes through both slits, creating the statistical distribution of the screen during measurement. We also know that the electron energy transfer. And we know that the energy creates a gravitational field. But because gravity field - a classic, it may not be in superposition and to pass through both slits, like the electron. What happens with the gravitational field of the electron? No one knows, because it is too weak to measure. So simple and so difficult.

The third reason that persuades physicists that a combination of general relativity and the Standard Model is an incomplete description of nature, leads to the formation of singularities under fairly general conditions. Singularity - a cluster of infinite energy density and infinite curvature. They are nonphysical and should not appear in sound theory. Singularity also appear in hydrodynamics, for example, when a drop of water is compressed. In this case, however, we know that a singularity - an artifact of using approximations - hydrodynamics - which will not work on subatomic distances. In the shorter distances, we must use a more fundamental theory (the theory of quantized, discrete particles) to describe a drop of water, and is expected to disappear singularity.

It is believed that the quantization of gravity solve these three problems, revealing the structure of space-time for very short distances. Unfortunately, gravity is not quantized, like other interactions in the Standard Model. Applying the same methods to gravity, we come to the theory of "effective quantum gravity", which can not solve these problems: it is destroyed when a strong curvature. This naive ("perturbatively") quantized gravity does not solve the problem with singularities and evaporation of black holes, because it works only with low gravity. In short, it makes no sense. What physicists call "quantum gravity", to be a theory that will be good, regardless of how much stronger the gravity.

Currently, there are several theoretical approaches to quantum gravity. The most famous are asymptotically safe gravitation theory, loop quantum gravity, string theory and causal dynamical triangulation, as well as the idea that a serious approach to the hydrodynamic analogy and consider gravity as resulting phenomenon. While we can not say which approach would be the best description of nature.

In the wake of polarization measurements of the cosmic microwave background BICEP, it was stated that such a measure will provide evidence of quantum gravity. It's not really there. Firstly, note that it is once again affects the weak gravitational field and is not a fundamental theory of quantum gravity. In addition, someone just did not bother to formulate a normal argument. Yes, quantum gravitational fluctuations in the young universe may have left an imprint on the microwave background, which is potentially seeing. But much more difficult to demonstrate that quantum gravity - the only way to produce the observed fluctuations. There must be something else, such as proof of Bell's theorem that the classical theory can not produce such, but they are missing.

Quantum gravity - not a very large field of research, when compared with the physics of condensed matter research or cancer, for example. The community of scientists is not very big, but it attracts a lot of public interest. And deservedly so. Without quantum gravity, we do not know how to actually behave in space and time, and can not understand why our universe began. We need a quantum gravity, which tells us that holds the space together.

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