Stephen Hawking left us in mid-March 2018 at the age of 76. Many articles have already been written about him, not excluding my recent works:
Starting to write these articles, I came across the following material, written by me several years ago, which describes the scientific heritage of Stephen Hawking. One magazine requested it from me when Hawking fell ill and everyone thought that he would die - this was not the first time, and every time everyone was wrong. I am sure that this article was never released, so here it is!
Stephen Hawking's Scientific Heritage
Stephen Hawking is a rare example of a scientist who is both a celebrity and a cultural phenomenon. However, it is also a rare example of a cultural phenomenon with well-deserved fame. His contributions can be characterized very simply: Hawking has made more contributions to our understanding of gravity than any physicist since Albert Einstein.
And the word "gravity" is very important here. Most of Hawking's career, theoretical physicists were generally more interested in particle physics and other forces of nature — electromagnetism and strong and weak nuclear interactions. "Classical" gravity, ignoring the complexity of quantum mechanics, was fully described by Einstein in his general theory of relativity, and "quantum" gravity (the quantum version of general theory of relativity) seemed too complicated. By applying his astounding intelligence to the most well-known force of nature, Hawking was able to produce several results, which greatly surprised the entire community.
Without a doubt, the most important result of Hawking's work was the understanding that black holes are not completely black — they radiate like ordinary objects. Prior to this work, he proved important theorems about BH and singularities, and then he studied the universe as a whole. In each phase of his career, he made certain key contributions to science.
Classic period
While working on his doctoral thesis at Cambridge in the mid-1960s, Hawking became interested in questions of the origin and final fate of the universe. A suitable tool for investigating this problem was GR, Einstein's theory, which describes space, time, and gravity. According to GR, what we perceive as gravity is a reflection of the curvature of space-time. Understanding how curvature is created by matter and energy, we can predict the evolution of the Universe. This can be referred to as the “classical” Hawking period, in order to contrast classical GR and his later studies in the field of quantum field theory and quantum gravity.
At about the same time, Roger Penrose of Oxford held a remarkable proof: according to GR, with a very wide range of conditions, space and time would fall inward and form a singularity. If gravity is the curvature of space-time, then the singularity is such a moment in time at which this curvature becomes infinitely large. The theorem showed that the singularities were not just some curiosities; they are an important feature of GR.
The result of Penrose was applied to black holes - areas of space-time in which the gravitational field is so strong that even light cannot escape from there. Inside the black hole in the future lies a singularity. Hawking took Penrose's idea and turned it inside out, directing the past of the Universe. He showed that under the same general conditions, the space should have emerged from the singularity: the Big Bang. Modern cosmologists speak (and confuse everyone) both about the Big Bang model, which is a very successful theory describing the evolution of an expanding Universe over billions of years, and about the Big Bang singularity, an understanding of which we cannot yet boast.
Then Hawking turned his attention to black holes. Another interesting result of Penrose's calculations was that energy can be extracted from a rotating black hole, in fact, extracting energy from its rotation until it stops. Hawking was able to show that, although it is possible to extract energy, the region of the event horizon surrounding the black hole will increase in any physical process. This “area theorem” was important both in itself and in relation to a completely different field of physics: thermodynamics, which studies heat transfer.
Thermodynamics is subject to a set of famous laws. For example, the first law says that energy is conserved, and the second that entropy, the measure of the disorder of the Universe, never decreases with a closed system. Working with
James Bardeen and
Brandon Carter , Hawking proposed a set of laws for “black hole mechanics,” similar to thermodynamics. As in thermodynamics, the first law of BH mechanics ensures energy conservation. The second law, the Hawking Square Theorem, says that the area of the event horizon never decreases. In other words, the area of the BH event horizon is very similar to the entropy of the thermodynamic system — they increase with time.
Black hole evaporation
Hawking and his colleagues were rightfully proud of the laws of BH mechanics, but they considered them to be just a formal analogy, and not a literal connection between gravity and thermodynamics. In 1972, a graduate of Princeton University,
Yaakov Beckenstein , suggested that there was more to it. On the basis of brilliant thought experiments, he suggested that the behavior of the BH is not just similar to thermodynamics, it is thermodynamics. In particular, BH has entropy.
Like many bold ideas, this idea met with the resistance of experts - and at that moment, the world BH expert was Stephen Hawking. Hawking skeptical of her, and for good reason. If the BH mechanics turned out to be a form of thermodynamics, this would mean that the BH has a temperature. And objects with temperature emit - the famous “black body radiation”, which played a central role in the development of quantum mechanics. So, if Bekenstein was right, it would mean that the BH is not really black (although Bekenstein himself did not go so far in his statements).
To seriously approach this problem, it is necessary to expand attention beyond the limits of general relativity itself, since Einstein’s theory is purely “classical” —it does not include the ideas of quantum mechanics. Hawking knew that Russian physicists
Alexey Starobinsky and
Jacob Zeldovich were studying quantum effects near black holes and
predicted such an effect as “superradiance”. Just as Penrose showed that energy can be extracted from a rotating black hole, Starobinsky and Zel'dovich showed that rotating black holes can emit radiation spontaneously due to the effects of quantum mechanics. Hawking was not an expert in quantum field theory techniques, since at that time specialists in particle physics, and not GRT, were involved in this field. But he quickly learned, and pounced on the difficult task of understanding the quantum aspects of BH, in order to find an error with Bekenstein.
Instead, he surprised himself and in the process turned theoretical physics upside down. He discovered that Bekenstein was right - the BH has entropy - and that the incredible consequences of this idea were also true - the black holes are not completely black. Today we call this BH property “Beckenstein-Hawking entropy”, and they emit “Hawking radiation” at their “Hawking temperature”.
"On the fingers" can be understood Hawking radiation as follows. Quantum mechanics says (among other things) that the system cannot be forcibly brought to a certain classical state; There is always internal uncertainty in what you see when you look at it. This is true even for empty space - if you look closely enough, what seemed to be empty space will turn out to be filled with “virtual particles” that constantly appear and disappear. Hawking showed that near BH a pair of virtual particles can be separated, and one of them will fall into BH, and the other will run away as radiation. It is surprising that, from the point of view of an external observer, a particle falling inward will have negative energy. As a result, the radiation gradually takes the mass from the BH - and it evaporates.
The result of Hawking had an obvious and outstanding influence on our understanding of BH. Instead of becoming a cosmic dead end in which matter and energy disappear forever, they turned out to be dynamic objects that sooner or later completely disappear. More importantly for theoretical physics, this discovery raised a question to which we still do not have an answer: when matter falls in a black hole, and then the black hole disappears completely, where does the information go?
If you take the encyclopedia and throw it into the fire, you can find that the information contained in it has disappeared forever. But according to the laws of quantum mechanics, it did not disappear anywhere; if you could catch all the particles of light and ash that came from the fire, in principle you could exactly recreate everything that fell into the fire - even the pages of the book. But BH, if you accept the result of Hawking, as it is, completely destroy the information - at least from the point of view of the outside world. This riddle is called the “information paradox”, and it has been torturing physicists for several decades.
In recent years, progress in understanding quantum gravity (at the level of thought experiments) has been convincing an increasing number of people that information is being preserved. In 1997, Hawking argued with American physicists Kip Thorne and John Presquil; Hawking and Thorn said that information is being destroyed, Presquil said that information is preserved. In 2007, Hawking conceded, and acknowledged that BHs do not really destroy information. However, Thorn did not give up, and Presquil himself believes that this conclusion was premature. Radiation and entropy of BH remain the central directions of searches in the way of improving the understanding of quantum gravity.
Quantum cosmology
Hawking's work on BH radiation was based on a mixture of quantum and classical ideas. In his model, BH is estimated from the classical point of view, according to the rules of GR. In this case, virtual particles near the BH are estimated according to the rules of quantum mechanics. The ultimate goal of many theoretical physicists is to construct a true theory of quantum gravity, in which space-time itself would be part of a quantum system.
And if there is a place in which quantum mechanics and gravity play a crucial role, then this is the beginning of the universe. And it is precisely this question, which is not surprising, Hawking highlighted the last part of his career. And with this he approved a plan of work on an ambitious physical project of understanding the origins of the universe.
In quantum mechanics, the system has no location or speed; its state is described by the “wave function”, which tells us the probability that when measuring the system we get a certain location or speed. In 1983, Hawking and James Hartl published a paper under the simple title: "The Wave Function of the Universe." They proposed a simple procedure on the basis of which - in principle! - it would be possible to calculate the state of the entire universe. We do not know whether the Hartl-Hawking wave function is actually the correct description of the Universe. Since we do not have a complete theory of quantum gravity, we do not even know whether such a procedure is meaningful. But their work has shown that we can talk about the very beginning of the Universe in scientific terms.
The study of the origins of the universe offers the possibility of combining quantum gravity with the observed features of the universe. Cosmologists believe that the tiny changes in the density of matter from their earliest times gradually increased in the distribution of stars and galaxies that we observe today. A complete theory of the origin of the universe could predict these changes, and the implementation of this program is one of the main occupations of modern physicists. Hawking made several contributions to this program, both from the side of his wave function of the Universe and in the context of the “inflationary Universe” model proposed by Alan Guth.
Just talk about the origin of the universe - a provocative action. It implies the hope that science will be able to provide a complete and self-sufficient description of reality - and such hope goes beyond science and turns out to be in the field of philosophy and theology. Hawking, who always loved provocations, never felt shy of such consequences. He liked to recall the cosmology conference held in the Vatican, at which Pope John Paul II allegedly asked the assembled scientists not to delve into the origin of the universe, "because it was the moment of creation, and therefore the work of God." But such warnings never stopped Hawking; he lived his life in a relentless search for answers to the most fundamental of scientific questions.