The telltale dance of the black hole

Information that enters black holes is not lost. Most physicists already understood this a long time ago. Now even Stephen Hawking admits it

By Yuval Dror Ha'Artez, voila

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John Parskill, a physicist at the Cal-Tech Institute in California, was a bit taken aback. "I always hoped that when Steven admitted his mistake, there would be some witnesses around. I didn't imagine there would be so many witnesses," he said as he looked at the dozens of television and press crews packed into a hall in Dublin on Wednesday, July 21, 2004, where renowned British physicist Stephen Hawking admitted he had lost to Priscilla in the intervention. The decades-old high-profile intervention touched on one of the most popular topics in physics, in part because of his identification with Hawking: black holes.

Black holes are the cosmic killers that were born in the hearts of giant stars that exploded and collapsed inward, until all their matter was compressed into a small microscopic point known as a "Singularity". Therefore, black holes are the most extreme expression of gravity: nothing can escape them, not even the fastest thing in the universe - light. Black holes also represent the most important point of contact between the two central theories in physics: Einstein's general theory of relativity and quantum theory. Although both theories have been verified in countless experiments, they do not fit well with each other, and in the case of black holes it is a real contradiction that, according to Hawking's version, has so far presented the quantum theory as wrong.

In the 60s, Hawking was at the beginning of his scientific career. He devoted his doctoral thesis at the University of Cambridge to the study of black holes. Together with the physicist Roger Penrose, who was his senior, he published a series of articles and studies, which for the first time established the fact of the existence of the "black hole". In 1974, Hawking stunned the world of science once again and established his status as a first-rate scientist: he proved that black holes are not as black as they are thought to be, as they emit radiation (which was immediately named "Hawking radiation"). This radiation, he explained, is random and does not "tell" anything about the objects swallowed by the black holes.

This revolutionary theory had two main meanings: one, black holes do not live forever, since the emission of radiation leads to a loss of energy, which should cause their evaporation after tens or perhaps hundreds of billions of years. The second meaning is that Hawking created a paradox: if "Hawking radiation" is random and does not tell anything about the swallowed objects, where did the information about them disappear?

Hawking used quantum theory, which explains the behavior of particles at the subatomic level, to show that black holes "leak" - but he refused to accept another claim of quantum theory, which states that information is not lost but is preserved in one way or another forever. Thus, for example, quantum theory states that if an elephant fell into a black hole, the information about it is stored somewhere, while Hawking's theory contradicted this assumption.

For Hawking it was an opportunity to clash with quantum theory; For his opponents, this was an opportunity to argue that it is not quantum theory that is wrong, but rather the celebrated physicist. The debate heated up until Preskill intervened with Hawking that in the future his claims about the loss of information in the black hole would be proven wrong. The two agreed that the winner of the intervention would win the encyclopedia: from it, they both agreed, information can be retrieved whenever one wants. More than 30 years have passed since then and last week the intervention came to an end when Hawking presented an encyclopedia to Priscilla. He explained that he wanted to give Priscilla an encyclopedia about cricket, but the American Priscilla refused and asked for an encyclopedia about baseball, which Hawking had specially flown in from the US.

When the storm around Hawking's lecture subsided a little, it became clear that the uproar was mainly media. "Until his announcement, Hawking was actually the only person who still believed his theory was correct," said Dr. Leonard Susskind of Stanford University in an interview with The New York Times. Prof. Zvi Piren from the Rakah Institute of Physics at the Hebrew University agrees with him and adds that the main innovation in Hawking's current work is the complicated system of calculations, which indicates the way in which the information is stored inside the black hole. Why, then, does Hawking get all the glory? Many physicists refuse to go out of their way. "A lot of public relations", they say dryly.

So what is Hawking actually claiming now? It's hard to say. Even Parskill said at the end of the event that "I have to be honest and say that I didn't really understand his lecture". He wasn't the only one. Most of those present in the hall agreed that we should wait until Hawking publishes his article next month to understand exactly what he is saying.

From the few details that could still be gleaned from the event, it seems that Hawking returns to quantum theory to solve the paradox. "In quantum theory, there is no meaning to an exact location or an exact time," explains Dr. Tal Alexander from the Weizmann Institute. "Quantum theory does not see particles like electrons or atoms as fixed in one place. Instead, it refers to their probability of being at a certain point at a certain time."

Hawking also claims that the "Horizon Event" of the black hole, which is the limit of the area from which it is impossible to escape from the black hole and everything is sucked in, vibrates in space. And why does this help solve the problem? Because Hawking assumes that this dance of the horizon is not random but is influenced by the matter he has ingested.

"The non-random fluctuations of the black hole mean that the Hawking radiation emitted from it will also be non-random," explains Piren. "In the beginning there was an object that consisted of information. Then it was swallowed by the black hole and finally the information was ejected. Where is the information stored all the time, Hawking asks and now answers: in fluctuations". Hence, the oscillation of a black hole after swallowing an elephant will be different from the oscillation of a black hole that has swallowed a chair, and hence the "Hawking radiation" will also be different in both cases.

If this sounds too theoretical, it is because it is an ultra-theoretical issue. "Quantum theory states that if you take an encyclopedia and throw it into a fire, you will be able to connect the 'information' that was in it from the radiation that is emitted from the fire when it burns," explains Alexander, "now we will see you do it." Piren claims that if the example of the bonfire sounds complicated and theoretical, then in the case of black holes it is even more complicated, and therefore the whole discussion about the emission of "information" may confuse those who think that the black hole dictates to the scientist the list of items he swallowed during his life.

Another meaning of the fact that information is nevertheless preserved and exits from black holes, is that black holes cannot be used to move between parallel universes, also contrary to what Hawking believed in the past. The reason for this is that a person cannot be in two places - and in this case in two universes - at the same time, and if the information about a person continues to exist also in a black hole - he cannot also be in a parallel universe. "I'm sorry to disappoint science fiction fans," said Hawking.

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