A few days ago, on March 14, this world was left by one of the most outstanding physicists of our time, Stephen Hawking. Despite his serious illness, Hawking managed to write many scientific books, many of which became real bestsellers, all his life was optimistic, was an active popularizer of science and even became a kind of pop icon, thanks to his participation in various entertaining shows. Unfortunately, at the time of his death, he never found an answer to one of the most interesting questions, the decision of which he had been very active in recent years: is it possible for a complete loss of information for the universe?
At the dawn of his scientific career, in 1974, 44 years ago, Hawking published, perhaps, one of his most famous scientific articles: “Black Hole Explosions?” (Explosions of black holes?). In it, the young scientist decided to move away from the familiar understanding of these mysterious objects at that time and thus actually took the first important step in solving this problem.
“In the classical sense, black holes were considered” ideally cold “objects, absorbing everything around and nothing radiating. This was the idea of science in the early 1970s, “commented Robert McNees, a physicist at the University of Chicago, Loyola.
Such black holes do not radiate any energy. Of them, no matter can break out. Science saw in them objects that simply exist. Cold, silent, eternal. But Hawking's work revived them, made them mortal.
“In the mid-70s, when Stephen studied the effects of quantum mechanics, he discovered that black holes in principle should produce radiation, as if they were thermal objects with a temperature “- continues Maknis.
” But in this case, if they could radiate energy, then their mass would have to decrease. And since their mass will decrease, they must also decrease in size, which in turn should lead to an increase in temperature and even more intense radiation. “
The end result of these processes should be either the complete disappearance (evaporation) of the black hole, or reduce its size to a tiny seed. However, in the absence of a suitable tool that would make it possible to combine the ideas of the general theory of relativity and quantum mechanics within the theory of “quantum gravity” to construct the so-called “theory of everything”, describing all known fundamental interactions, the understanding of the final stage of this “evaporation” of the black hole remains a complete mystery for science to this day.
“The problem is that, according to Hawking's calculations, this radiation should be perfectly thermal. It should not contain information about the state of matter from which this black hole was formed, and this in turn contradicts one of the fundamental laws of quantum mechanics, “says McNees.
One of the requirements of quantum physics is the possibility of isolating each state of a particle (past, present and future), as well as to determine the relationship of these states through a series of chain, causal and probabilistic events. But if a black hole is able to release an undifferentiated soup of particles, each of which carries its own information (its history), irrevocably erased (after immersion), then, therefore, this situation will contradict the fundamental postulates of quantum mechanics.
“Physicists call this the information paradox of black holes. The resolution of this contradiction is a necessary step in the way of building quantum gravity, “says Maxis.
Many biographical publications claim that Hawking took place as a very experienced physicist already by 1974 and after the publication of his most popular book A Brief History of Time in 1988 his most important scientific works were left in the past. However, the scientist continued to publish very bold and controversial scientific works all this time, trying to overcome the paradox that he faced several decades earlier.
Perhaps the most dramatic, if I may say so, work in the late career of Hawking was a scientific article in which he argued that there are no black holes in their “classical” understanding at all.
In his work “Preserving Information and Predicting the Weather for Black Holes”, published in 2014, he suggested that the “event horizon” preod lion that an object (even light) can not escape a tightening of a black hole – in fact, does not exist. According to Hawking, there is an “apparent horizon” – a surface on which radiation leaving the center of a black hole is only delayed. Unlike the classical event horizon, “seeming” can disappear at some point, and what was in the black hole can come out.
“The absence of an event horizon means that there are no black holes as objects, from which the radiation can never go away, “Hawking wrote at the time.
He tried to present his idea through the earlier concept of the so-called” firewalls “of black holes, proposed in 2012 by the American physicist Joseph Polchinsky. According to this idea, instead of the event horizon, black holes have a certain “wall”, a boundary consisting of high-energy particles and radiation fluxes that block everything that gets into it. This border is like a prison, only temporary. The scientist did not describe the reasons for the apparent horizon to disappear, but the physicist Don Page of the Canadian University of Alberta then suggested that this could happen when the black hole due to Hawking radiation becomes so small that the gravitational and quantum effects become indistinguishable.
But this was not the last word of Hawking's decision. In 2016, co-authored with the physicist at Cambridge University Malcolm Perry, as well as physicist at Harvard University Andrew Strominger, he published a paper titled “Soft Hair on Black Holes.”
In this work, a team of scientists wrote that ” that black holes are surrounded by so-called “soft hair” – photons with almost zero energy. Adding such particles to the equations describing the behavior of black holes, Hawking and his colleagues found that these photons will act as carriers of information on which data on some properties of particles “eaten” by a black hole will be recorded. Extract information from them, even if it will be possible to find a way to interact with these photons, it will be extremely difficult – the authors of the article compared this with the task of finding out what was burned in the fire, looking at the smoke and flames.
The consequence of the existence of these photons is that instead of a clear horizon line of events, a black hole will have a peculiar set of “hairs” of “soft photons” on which, as on a holographic plate, some information about absorbed particles will be recorded. This kind of “screen” will update its contents every time a black hole emits another portion of Hawking radiation, which further complicates its study. However, Hawking and his colleagues concluded that such an idea would allow the information paradox to be resolved without resorting to fantastic and unlikely assumptions and deviations from modern physical theories.
“A complete description of the holographic plate and the resolution of the information paradox remains an open question, for the solution of which we have provided new and more specific tools, “the researchers wrote in the conclusions.
Even at the end of his life, Hawking remained a very hardworking and hardworking scientist, The ideas that were both accepted and rejected by his colleagues
“It seems to me that the work of 2014 has not gained much popularity. At the same time, his work, written in co-authorship with Perry and Strominger in 2016, is a direction in which people are still working actively, “says McNees.
” The information paradox of black holes is one of the defining problems, who need a solution for those who are working on the theory of quantum gravity. It is still not resolved, but it remains, in my opinion, the most interesting issue raised by Hawking, “- added Maknis.