Here's Stephen Hawking's Incredible Solution to His Black Hole Information Paradox
And if it has a temperature, then it must radiate energy, yet the whole point of a black hole is that nothing gets out. For this reason, most physicists — including Hawking — thought Bekenstein's proposal made no sense. Even Bekenstein himself said that the black hole's apparent temperature couldn't be "real" since it leads to a paradox. But when Hawking set out to prove Bekenstein wrong, he found that the young student was, as he later admitted, "basically correct". In order to show this, he had to bring together two areas of physics that nobody else had managed to unify: general relativity and quantum theory.
Quantum theory is used to describe invisibly small things, like atoms and their component particles, while general relativity is used to describe matter on the cosmic scale of stars and galaxies. The two theories seem fundamentally incompatible. General relativity assumes that space is smooth and continuous like a sheet, whereas quantum theory insists that the world and everything in it is grainy at the smallest scales, parcelled into discrete lumps.
Physicists have struggled for decades to unify the two theories — which might then point to a "theory of everything". In his early career Hawking expressed a yearning for such a theory, but his analysis of black holes did not pretend to offer one. Instead, his quantum analysis of black holes used a sort of patchwork of the two existing theories. According to quantum theory, allegedly empty space is in fact far from a void, because space cannot be smoothly, absolutely empty at all scales. Instead it is alive with activity.
Pairs of particles are constantly fizzing spontaneously into existence, one made of matter and the other antimatter. One of the particles has positive energy and the other negative, so overall no new energy is being created. The two then annihilate one another so quickly that they cannot be directly detected. As a result, they are called "virtual particles".
Hawking suggested that these pairs of particles could be upgraded from virtual to real, but only if they are created right next to a black hole. There is a chance that one of the pair will be sucked inside the event horizon, leaving its partner stranded. This severed twin may then shoot out into space. If the negative-energy particle is absorbed by the black hole, the total energy of the black hole decreases, and therefore so does its mass.
The other particle then carries away positive energy. The end result is that the black hole radiates energy, now known as Hawking radiation, while gradually getting smaller. In other words, Hawking had proved himself wrong: black holes can get smaller after all. This is tantamount to saying that the black hole will slowly evaporate, and that it is not truly black at all.
In Hawking conceived a radical new vision of black holes. During the Big Bang, he suggested, some lumps of matter could have collapsed into miniature black holes.
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Each lump would weigh billions of tons, which sounds a lot but is far smaller than the Earth, and the resulting black hole would be smaller than an atom. Because a black hole's temperature increases as its event horizon's surface area gets smaller, black holes this tiny would be hot: Hawking described them as "white hot". They would fizz with Hawking radiation, shedding mass until they eventually disappeared.
And they would not go quietly. A mini-black hole would get hotter as it got smaller, until eventually it would explode with the energy of a million one-megaton hydrogen bombs. View image of Is all matter really made of tiny strings? Hawking outlined his theory of Hawking radiation and exploding primordial mini-black holes in a paper in Nature in It was a shocking, controversial idea. Yet nowadays most physicists believe that Hawking radiation really will be generated by black holes.
So far nobody has managed to detect this radiation. That's not surprising, though: an ordinary black hole's temperature would barely be above absolute zero, so the energy it emits as Hawking radiation would be extremely tiny. Seven years later, Hawking announced another disturbing implication of disappearing black holes. They destroy information, he said.
View image of Is information lost in black holes?
When particles or light rays pass inside a black hole's event horizon, they never return to the rest of the universe. Any such entity can be considered to carry information: for example, information about a particle's mass and position. This information is also locked away inside the black hole. However, what happens to that information if the black hole evaporates?
There are two possibilities: either it is somehow encoded in the Hawking radiation emitted by the black hole, or it is gone for good. Hawking claimed that it vanished. When Hawking suggested that black holes destroy information, Susskind argued that he was plain wrong.
When Hawking spoke in San Francisco in about the paradox of vanishing information in black-hole physics, the American physicist Leonard Susskind disagreed. He was one of the few who appreciated just how disturbing it would be if information were lost from the universe. We like to imagine that causes come before their effects, not the other way around.
In principle, although generally not in practice, that means we could trace and reconstruct the history of any particle in the universe based on the information about its current state. But that reconstruction from effects to cause would become impossible if information is being destroyed in black holes. If information is truly being lost, the whole notion of cause and effect starts to look shaky.
So when Hawking suggested that black holes destroy information, Susskind argued that he was plain wrong. The debate raged, in a fairly collegial manner, for decades. In it took on the form of a wager, something Hawking loves to indulge. Hawking bet John Preskill of the California Institute of Technology an encyclopaedia that information was indeed lost in black holes, while Preskill bet that it was not. At a conference in Dublin in , Hawking finally conceded that Susskind was right — and that Preskill should get his encyclopaedia.
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But in typically stubborn fashion, he qualified that statement by claiming that the information was only returned to the universe in a corrupted form that was virtually impossible to read, and that he had proved that this was so. Hawking spelt out his argument in a short paper the following year.
It did not convince everyone that his argument was better than Susskind's. The episode was characteristic of Hawking's style. He is bold and brilliant, but not always rigorous enough to fully persuade, and sometimes seemingly driven by an intuition that can turn out to be quite wrong — as when he bet against experimental detection of the Higgs particle. The melange of general relativity, quantum theory, thermodynamics and information theory in Hawking's work on black holes is innovative and remarkable.
Nothing else he has done has equalled it. In the s he tried to describe the Big Bang in quantum mechanical terms. Working with James Hartle, he developed a simple quantum equation that supposedly describes the entire universe in its early stages. But it does so in such general terms that, for many physicists, it doesn't say anything very meaningful. The one thing the equation does suggest, however, is that it is futile to ask about the ultimate origin of the universe.
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When the universe was still extremely tiny, less than a billionth of a yoctometre across, quantum theory implies that the distinction between space and time was extremely fuzzy. That means the early universe did not have meaningful boundaries in time or space, even though it was still self-contained.
The very concept of an "origin" in time vanishes into the quantum foam. This is the model explained in Hawking's best-selling A Brief History of Time , which secured his status as a global celebrity.
The idea is still debated. There is now a sense that Hawking is tinkering, inventively but somewhat marginally, at the end of his career, taking thoughtful excursions into ideas largely conceived by others. He has more than earned the right to do that. It is far less clear that he has earned the right to pronounce on artificial intelligence , genetic engineering or alien civilizations , let alone to perpetuate the gender stereotypes of a s undergraduate.
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