Hawking's Breakthrough Is Still an Enigma

[David Morris]

http://www.nytimes.com/2002/01/22/science/space/22BLAC.html

Hawking's Breakthrough Is Still an Enigma

By DENNIS OVERBYE

CAMBRIDGE, England — In the fall of 1973 Dr. Stephen W. Hawking, who has 
spent his entire professional career at the University of Cambridge, found 
himself ensnared in a horrendous and embarrassing calculation. Attempting to 
investigate the microscopic properties of black holes, the gravitational 
traps from which not even light can escape, Dr. Hawking discovered to his 
disbelief that they could leak energy and particles into space, and even 
explode in a fountain of high-energy sparks.

[...]

Dr. Hawking's celebrated breakthrough resulted partly from a fight. He was 
hoping to disprove the contention of Jacob Bekenstein, then a graduate 
student at Princeton and now a professor at the Hebrew University in 
Jerusalem, that the area of a black hole's boundary, the point of no return 
in space, was a measure of the entropy of a black hole. In thermodynamics, 
the study of heat and gases, entropy is a measure of wasted energy or 
disorder, which might seem like a funny concept to crop up in black holes. 
But in physics and computer science, entropy is also a measure of the 
information capacity of a system — the number of bits that it would take to 
describe its internal state. In effect, a black hole or any other system was 
like a box of Scrabble letters — the more letters in the box the more words 
you could make, and the more chances of gibberish.

According to the second law of thermodynamics, the entropy of a closed 
system always stays the same or increases, and Dr. Hawking's own work had 
shown that the hole's surface area always increased, a process that seemed 
to ape that law.

But Dr. Hawking, citing classical physics, argued that an object with 
entropy had to have a temperature, and anything with a temperature — from a 
fevered brow to a star — must radiate heat and light with a characteristic 
spectrum. If a black hole could not radiate, it could have no temperature 
and thus no entropy. But that was before gravity, which shapes the cosmos, 
met quantum theory, the paradoxical rules that describe the behavior of 
matter and forces within it. When Dr. Hawking added a touch of quantum 
uncertainty to the standard Einsteinian black hole model, particles started 
emerging. At first he was annoyed, but when he realized this "Hawking 
radiation" would have the thermal spectrum predicted by thermodynamic 
theory, he concluded his calculation was right.

But there was a problem. The radiation was random, Dr. Hawking's theory 
said. As a result, all the details about whatever had fallen into the black 
hole could be completely erased — a violation of a hallowed tenet of quantum 
theory, which holds that it should always be possible to run the film 
backwards and find out the details of how something started — whether an 
elephant or a Volkswagen had been tossed into the black hole, for example. 
If he was right, Dr. Hawking suggested, quantum theory might have to be 
modified. Black holes, he said in his papers and talks in the late 1970's, 
were ravagers of information, spewing indeterminacy and undermining law and 
order in the universe.

"God not only plays dice with the universe," Dr. Hawking said, inverting the 
phrase by which Einstein had famously rejected quantum uncertainty, "but 
sometimes throws them where we can't see them." Such statements aroused the 
attention of particle physicists. Weird as it may be, quantum theory is 
nonetheless the foundation on which much of the modern world is built, 
everything from transistors to CD's, and it is the language in which all of 
the fundamental laws of physics, save gravity, are expressed. "This cannot 
be," Dr. Leonard Susskind, a theorist at Stanford, recalled saying to 
himself.

It was the beginning of what Dr. Susskind calls an adversarial relationship. 
"Stephen Hawking is one of the most obstinate people in the world; no, he is 
the most infuriating person in the universe," Dr. Susskind told the birthday 
workshop, as Dr. Hawking grinned in the back row.

In the ensuing 20 years, opinions have split mostly along party lines. 
Particle physicists like Dr. Susskind and Dr. Gerard 't Hooft, a physicist 
at the University of Utrecht and the 1999 Nobel Prize winner, defend quantum 
theory and say that the information must get out somehow, perhaps subtly 
encoded in the radiation. Another possibility — that the information was 
left behind in some new kind of elementary particle when the black hole 
evaporated — seems to have fallen from favor.



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