A few days ago, March 14, the world left is one of the most eminent physicists of our time, Stephen Hawking. Despite his severe illness, Hawking has managed to write a lot of scientific books, many of which became real bestsellers, and all his life maintained the optimism was an active popularizer of science and has even become a kind of pop icon, thanks to his participation in various entertainment shows. Unfortunately, at the time of his death, he never found the answer to one of the most interesting questions, the solution of which he was very active in recent years: 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 papers: “Black Hole Explosions?” (Explosions black holes?). In it a young scientist decided to depart from the usual at that time, the understanding of these mysterious objects, and thus actually made the first important step in addressing this issue.
“In the classic sense black holes were “perfectly cold” objects, absorbing everything and emitting nothing. It is this understanding of them had science in the beginning of 70-ies,” commented Robert Maknis, a physicist at the University of Chicago Loyola.
Such black holes do not radiate any energy. Of them can not escape no matter. Science saw them as objects that simply exist. Cold, silent, eternal. But the work of Hawking quickened them, made them mortal.
“In the mid 70-ies, when Stephen studied the effects of quantum mechanics, he found that black holes should in principle generate radiation, as if they were heat objects with temperatures,” continues Maknis.
“But in this case they could radiate energy, and their weight would be reduced. And because it will decrease their weight, they should decrease in size, which in turn should lead to higher temperatures and more intense radiation”.
The end result of these processes should be either complete disappearance (evaporation) of a black hole, or reducing its size to a tiny kernel. However, in the absence of a suitable tool that would merge the ideas of General relativity and quantum mechanics in the theory of “quantum gravity” to build the so-called “theory of everything” that describes all the known fundamental interactions, understanding the final stages of this “evaporation” of a black hole remains a complete mystery to science to this day.
“The problem is that, according to the calculations of Hawking’s radiation must be the ideal heat. It should not contain information about the state of matter from which the black hole formed, and this, in turn, contradicts one of the fundamental laws of quantum mechanics,” says Macnish.
One of the requirements of quantum physics is the ability to isolate a particle condition (past, present and future), and to determine the relationship of these States through a series chain of causal and probabilistic events. But if the black hole is capable of releasing an undifferentiated soup of particles, each of which carries its information (its history) is permanently removed (after a dive), it follows that this situation would be contrary to the fundamental postulates of quantum mechanics.
“Physicists call it the information paradox of black holes. The resolution of this conflict is a necessary step towards quantum gravity”, says Maxis.
A biographical publications claim that Hawking has been held 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 in the past. However, the scientist all this time continued to publish very provocative and controversial scientific work, trying to overcome the paradox with which he has faced several decades earlier.
Perhaps the most dramatic, if I may say so, the work in the late career Hawking became a research article in which he argued that black holes in their classic sense does not exist.
In “information Preservation and weather forecasting for black holes”, published in 2014, he suggested that “event horizon” — the point, overcoming which the object (even light) cannot escape becoming entangled in the black hole – in fact, does not exist. According to Hawking, there is an “apparent horizon” — the surface on which the radiation going from the center of a black hole, only delayed. In contrast to the classical event horizon, “seeming” may at some point disappear, and what was in that black hole to come to light.
“No event horizon means no black holes as objects, where the radiation can’t leave ever,” wrote Hawking.
His idea, he tried to explain through the earlier concept of the so-called “firewall” of black holes, proposed in 2012, the American physicist Joseph Polchinski. According to this idea, is the event horizon black holes have a kind of “wall” boundary consisting of high energy particles and radiation flux, blocking everything behind it gets. This border is like a prison, only temporary. The scientist did not describe the reasons for the apparent horizon could disappear, but physicist don page of Canada’s University of Alberta then suggested that this can occur 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 Hawking in resolving the issue. In 2016 in collaboration with a physicist at the University of Cambridge Malcolm Perry, as well as a physicist at Harvard University Andrew Strominger he published a work entitled “Soft hair of black holes” (Soft Hair on Black Holes).
In this work, the team of scientists wrote about the fact 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, which will be recorded data about some properties of particles “eaten” by a black hole. To extract information from them, even if you manage to find a way to interact with these photons would be extremely difficult – the authors compared this with the task of finding out what was burned in the fire, through the smoke and flames.
A consequence of the existence of these photons is that instead of having clear lines of event horizon of the black hole will be a unique set of “hair” from “soft photons”, which, as a holographic plate will be recorded part of the information about the absorbed particles. This kind of “screen” will update its contents every time a black hole will emit another batch of Hawking radiation, which further complicate its study. However, Hawking and his colleagues concluded that this idea would resolve the information paradox without resorting to fantastic and unlikely assumptions and deviations from modern physical theories.
“A full description of the holographic plate and the resolution of the information paradox remains an open question, for which we have provided a new and more specific tools” — wrote in the conclusions of the researchers.
Even at the end of life Hawking was very efficient and hard-working scientists, offers a number of ideas that were taken and rejected by his colleagues.
“It seems to me that work 2014 not won great popularity. At the same time, his work, written in 2016 in collaboration with Perry and Strominger is the direction in which people are still actively working,” says Macnish.
“The information paradox of black holes is one of the defining problems in need of solution for those who are working on a theory of quantum gravity. It is still not resolved, but remains, in my opinion, the most interesting question raised by Hawking,” added Maknis.