How Stephen Hawking changed our understanding of black holes

An artistic illustration of a supermassive black hole ever discovered.

(Robin Dienel, courtesy of the Carnegie Institution for Science)

There is a lot that we still don’t know about black holes, but these light-gobbling giants would be even more mysterious if Stephen Hawking had not solicited their inky depth.

For starters, the famous cosmologist, who died yesterday (14 March) at the age of 76, has a more solid mathematical support for the concept of black holes, whose existence was predicted by Albert Einstein in his 1915 theory of general relativity.

“Hawking actually proved a number of rigorous mathematical theorems on the equations of Einstein for gravity that showed that, under fairly general conditions, there were places where the equations broke — the so-called singularity,” said Tom Banks, a professor in the physics and astronomy at Rutgers University in New Brunswick, New Jersey. “And, in particular, the region inside of a black hole is a singularity.” [Stephen Hawking: A Physics Icon, Remember, Photos]

But it was Hawking research on black holes ” of the nature that would prove to be revolutionary. In the first instance his work suggests that a black hole can never be smaller, namely that the surface area of the spherical event horizon, the point beyond which nothing can escape, can never fall.

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Also, the second law of thermodynamics states that “entropy” or disorder of a closed system can never go down. And, in the early 1970s, physicist Jacob Bekenstein explicitly connected with the concepts, imagine a black hole the entropy is linked to the surface of the event horizon.

Hawkins was initially skeptical of this idea, Banks said. After all, the entropy, and black holes do not seem to go together: Black holes would radiate no energy of any kind — hence the name — and you can’t have entropy without radiation.

But then Hawking crunched the numbers, in a way that nobody had ever done before.

“He then showed that, if the quantum mechanics of the game, you could show that, in fact, black holes were not really black,” said Banks “They actually emitted radiation.”

This radiation comes from “virtual particles” that are constantly popping in and out of existence in the bizarre quantum realm. They do that in matter-antimatter pairs, one of which is positive energy and negative energy.

Normally, these pairs immediately annihilate each other. But if this pair popping has occurred on the edge of a black hole with the event horizon, one particle can, in theory, get swallowed up, while the other rocket in the outer space. If the negative-energy particles were consumed, the black hole mass would shrink by a small amount, and the object would emit a tiny bit of radiation.

Hawking worked with this idea in 1974, which is the reason why this hypothesis of black hole light is known as Hawking radiation, or Hawking-Bekenstein radiation. No one has spotted these emissions, but most physicists believe that the emissions exist. Therefore, they posit, all black holes will shrink away to nothing in the end, if there is no matter more for them to scarf down. (This will happen at almost unimaginably long time-limits for the large black holes; some calculations suggest that the last of the extreme samples in the cores of galaxies, will not die for another 10^100 years or so.)

Although undoubtedly a genius, He was not always right, and one of his high-profile errors regarding black holes. The cosmologist famous posited that the data provided by each particle, information about the spin and the mass of, for example, hoovered by a black hole would be lost if the black hole evaporates. [Stephen Hawking is the Most Far-Out Ideas About Black Holes]

Most of the other physicists disagree, and for good reason, Banks said.

“It leads to equations that are in a huge contradiction to known experimental facts,” he said. “There are certain types of idealized black holes you can create in the string-theory, models, and there, it is very clear that there is no loss of information.”

Instead, this information must seep back out into the universe through Hawking radiation before the black hole disappears, most physicists think. Hawking eventually came around to this position, Banks said.

Hawking’s black hole work has also encouraged physicists to rethink their understanding of the universe on a more general level, Banks said. Previously, physicists had assumed that the entropy scales with system volume, so the entropy-area link that Hawking and Bekenstein established came as a big surprise.

“In a way, Hawking’s observation led to a potential revolution in the way we use the model of nature, period,” Banks said. “ of that is not yet realized, not yet. Not really a theory that everyone agrees is correct, but it is a kind of the big challenge that Hawking’s work.”

Hawking inspired deep thought and reflection more than his fellow-physicists and cosmologists, of course. For decades, the laity all over the world have marveled at the way He was by his debilitating motor neuron disease to make groundbreaking discoveries and bring exciting research to the masses in his best-selling books.

“It was remarkable how resilient he was, and how determined he was,” said Banks, who He knew personally. “That was one of the most inspiring parts of the to him.”

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