Theoretical physicist Joseph Polchinsky of the University of California at Santa Barbara predicted the year of completion of the creation of the quantum theory of gravity. According to the scientist, this will happen in 2131, and it will be based on string theory, which the overwhelming majority of modern physicists and mathematicians recognize as the only candidate for the role of "theory of everything." Polchinsky, a laureate of the Fundamental Physics Prize established by Russian entrepreneur Yuri Milner, outlined his considerations in a preprint on the arXiv.org website.
In the process of development, physics investigated ever smaller scales of distances and ever larger scales of energies. At the beginning of the 20th century, scientists received their first ideas about phenomena occurring on an atomic scale. By now, physicists have access to scales of ten to the minus seventeenth power of centimeters, corresponding to experiments at the Large Hadron Collider, which made it possible to discover the Higgs boson. Comparing the stages and rates of development of physics in the XX and early XXI centuries, Polchinsky predicted that by 2131 the quantum theory of gravity would be finally formulated. For this, the scientist examined the evolution of physics over the past hundred-odd years and compared the achievements of mankind of certain energy scales with the time of this event.
In 1899, the German physicist Max Planck introduced into consideration the length named after him, composed of fundamental constants (Planck's constant, gravitational constant and the speed of light in a vacuum) and equal to ten to minus thirty-third powers of centimeters. At present, this value is considered to be an unattainable scale for modern experiments on which string theory operates. The scale of ten centimeters to the minus seventeenth power of centimeters on a logarithmic scale corresponds to the middle of the distance. Accordingly, the same amount of time remains before the creation of the "theory of everything" as 116 years have passed since the introduction of the Planck length into science.
Length scale.
The smallness of the Planck length allows, according to Polchinsky, to provide the necessary "smearing" of interactions, explaining the non-renormalizability (impossibility of eliminating divergences) of the theory of gravity. Thus, the SM and the three fundamental interactions described by it (electromagnetic, weak and strong) are renormalizable, while the version of quantum gravity obtained by naive quantization (that is, according to the same recipe as classical field theory), already in the second order of perturbation theory turns out to be divergent.
According to Polchinsky, on the Planck scale, space-time fluctuations become significant. They form the so-called space-time foam and provide the observed divergence of the naive version of quantum gravity. As a historical example, the scientist cites the theory of Enrico Fermi, which qualitatively well described the weak interaction, but was not renormalizable.
It was only after Steven Weinberg, Sheldon Glashow, and Abdus Salam created a renormalizable electroweak theory that combines electromagnetic and weak interactions and introduces intermediate electroweak bosons that it became clear that Fermi's theory is a low-energy approximation of another, more general model (in this case, the electroweak) … Polchinsky believes that the same will happen with quantum gravity.
Joseph Polchinsky.
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Polchinsky connects the uniqueness of the dynamics of string theory with the presence of only one parameter necessary to describe nature - the so-called string constant. Meanwhile, according to the scientist, at present, the "theory of everything" does not have any uniform principle (first principle) that allows it to be deduced in a deductive way. For general relativity, there is such an element: the principle of local equivalence between the gravitational field and motion with acceleration. The classic example of this beginning is in the elevator. With its uniformly accelerated upward motion relative to the Earth, the observer in it is not able to determine whether it is in a stronger gravitational field or moves in a man-made object.
In his article, Polchinsky mentions the importance of quantum fluctuations for solving string theory equations. Despite the fact that the modern equations of quantum field theory and general relativity describe the observed world well on the available experimental scales, they can be modified that does not contradict the first principles of these theories. Meanwhile, this leads to effects that are not observed to date, which are significant on the Planck scale.
Polchinsky refers to such modifications as the introduction of terms with higher derivatives into the equations of quantum field theory (at present, there are only quadratic terms with the first derivatives of the fields) and the addition of terms quadratic in the curvature of space-time to the Einstein equations in GR. These additions lead to the need to take into account the fluctuations of the space-time foam, which exists, according to the predictions of string theory, on the Planck scale.
Quantum foam.
Polchinsky explains the role of space for string theory using the example of mirror symmetry, which allows the existence of different manifolds of Eugenio Calabi and Shintana Yau, which, being compactified (folded into extremely small additional spatial dimensions) from different spaces, can lead to the same properties of elementary particles … This (together with the potential for the existence of additional spatial dimensions) suggests that the observed physics is a manifestation of the multidimensional geometry of space-time and its structure on the Planck scale.
The duality of gauge theories and quantum gravity, understood as holography, will allow, according to Polchinsky, to describe particle physics and gravity in a uniform way. The holographic principle, proposed in 1993 by the Dutch physicist Gerard t'Hooft, asserts that the information contained on its outer boundary (beam) is sufficient for a mathematical description of a world: in this case, an idea of an object of higher dimension can be obtained from holograms, having a lower dimension.
As applied to string theory, the principle was embodied in the idea of the AdS / CFT correspondence, which was pointed out in 1998 by the American theoretical physicist of Argentine origin Juan Maldacena. In this hypothesis, the equivalence of the description of physics in special spaces leads to the existence of unique connections between their parameters - dualities. Mathematically, this manifests itself in the presence of a relation that allows one to calculate the parameters of interactions of particles (or strings) of one of the theories, if such are known for the other.
The holographic universe.
Polchinsky connects progress in understanding the physics of black holes with the fact that in 1996, within the framework of string theory, Andrew Strominger and Kumrun Wafa demonstrated the derivation of the expression for the entropy of black holes, first obtained thermodynamically by the Israeli physicist Jacob Bekenstein in 1973. Their conclusion indicates that the evaporation of black holes preserves the unitarity of quantum mechanics (associated with a consistent interpretation of probability), which was previously questioned by British scientist Stephen Hawking.
Arbitrariness in the values of the observed fundamental constants, according to Polchinsky, although it is a serious difficulty in the "theory of everything", nevertheless can clarify some universal features of nature (in particular, the existence of the Multiverse). The scientist named the nonzero value of the cosmological constant (the lambda term in Einstein's equations) as the main feature that theoretically indicates the existence of parallel worlds. According to the scientist, the vast majority of string theories involve the Multiverse. These models also contain a nonzero cosmological constant. That is, according to Polchinsky, one cannot exist without the other. Moreover, applying Bayesian inference, the physicist estimated the probability of the existence of the Multiverse at 94 percent (this corresponds to a statistical significance of two standard deviations).
"You may disagree with my 94 percent estimate, but there is no rational argument that the multiverse does not exist, or that it is unlikely," Polchinsky writes. The scientist is optimistic about the prospects for the formulation of quantum gravity (within the framework of string theory), continues to work in this direction and does not exclude that the construction of the "theory of everything" will be completed ahead of schedule - earlier than the year 2131 predicted by him.
Andrey Borisov