Stephen Hawking’s Masterpiece

Here is an appreciationn I wrote for the Weekly Standard, Washington, March 26, 2018

 

Stephen Hawking, 1942–2018

John Gribbin

Much as the name Tiger Woods is familiar to people who do not follow golf, so the name Stephen Hawking will be familiar even to people who care little about physics. His death on March 14 provoked an outpouring of eulogies of the kind usually reserved for rock stars and former presidents. His scientific work fully justifies such acclaim, quite apart from the inspirational impact his fame had in encouraging young people to become scientists themselves.

The story begins half a century ago, when Hawking was a doctoral student at the University of Cambridge. He was working on what then seemed a rather esoteric area of mathematical physics, applying the equations of Einstein’s general theory of relativity to the way massive objects collapse under their own weight. This was before such objects were dubbed “black holes,” a name popularized by the physicist John Wheeler in 1969, and a decade before astronomical observations proved that black holes exist by measuring their gravitational influence on companion stars. Nobody except a few theorists took the idea seriously in the mid-1960s, and it was just the kind of tricky but possibly pointless exercise to give a doctoral candidate. A little earlier, another young English physicist, Roger Penrose, had proved that such objects must, if Einstein’s theory was correct, collapse all the way down to a point of infinite density—what is called a singularity, a breakdown in the geometry of time and space. //JOHN: you’ll want to correct and improve. ‘Singularities’ seem so essential to understanding the material that I wanted to double-down on the definition.// Nobody worried too much about this as such spacetime singularities would be hidden inside black holes. But Hawking took Penrose’s work and extended it to a description of the whole Universe.

A black hole is an object collapsing to a singularity. But if you reverse the equations, you get a mathematical description of a Universe expanding away from a singularity. Hawking and Penrose together proved that our expanding Universe had been born in a singularity at the beginning of time if Einstein’s general theory of relativity is the correct description of the Universe. While they were completing their work, observational astronomers discovered the background radiation that fills all of space and is explained as the leftover energy from the super-dense fireball at the beginning of time. So what started out as an esoteric piece of mathematical research became a major contribution to one of the hottest topics in science in the 1970s. It is this work, updated with more observations, which makes it possible to say that the Universe was born when time began 13.8 billion years ago—that the Big Bang really happened. And, of course, Hawking got his Ph.D.

Hawking moved from explaining the birth of the Universe to explaining the death of black holes. These got their name because the gravitational pull of a black hole is so strong that nothing not even light, can escape from it. In 1970, everyone thought that this meant a black hole was forever. The unobservable singularity is surrounded by a spherical surface known as an event horizon, which lets anything in, but nothing—not even light—out. What Hawking realized was that the surface area of this event horizon must always increase, as more things are swallowed by the hole—or at the very least stay the same if it never swallows anything. He showed that this is linked with the concept of entropy in thermodynamics (the study of heat and motion). Entropy is a measure of the amount of disorder in some set of things. For example, an ice cube floating in a glass of water is a more ordered arrangement than the liquid water left in the glass when the ice cube melts, so the entropy in the glass increases as the ice melts.

Entropy always increases (or at best stays the same), like the area of a black hole. This means that information is being lost as things get simpler—there is more complexity, and so more information in a mixture of ice and water than in water alone. Hawking showed, following a suggestion from the physicist Jacob Bekenstein, that the area of a black hole is a measure of its entropy. This means that anything that falls into a black hole is scrambled up and lost, like the melting ice cube. There is no record left—no information—about what it was that went in. He had found a link between one of the greatest theories of 19th-century physics—thermodynamics—and one of the greatest theories of 20th-century physics—the general theory of relativity.

Hawking didn’t stop there.

Entropy is related to temperature. If the area of a black hole is a measure of entropy, then each black hole should have a temperature. But hot things radiate energy—and nothing can escape from a black hole. Hawking tried to find a flaw in the paradox, a mathematical loophole, but failed. Having set out to prove that black holes did not have temperature, he ended up proving the opposite. Like any good scientist, confronted by the evidence Hawking changed his mind. As the title of one of his lectures stated boldly: “Black Holes Ain’t as Black as They Are Painted.” The curious thing is that in order to explain how black holes could have temperature and radiate energy he had to bring in a third great theory of physics—quantum theory.

Hawking picked up on the prediction of quantum physics that in any tiny volume of space pairs of particles, known as virtual pairs, can pop into existence out of nothing at all, provided they disappear almost immediately—they have to come in pairs to balance the quantum books. His insight was that in the region of space just outside a black hole, when a virtual pair appears one of the particles can be captured by the black hole, while the other one steals energy from the gravity of the hole ad escapes into space. From outside, it looks as if particles are boiling away from the event horizon, stealing energy, which makes the black hole shrink. When the math confirmed that the idea was right, this became known as “Hawking radiation” and provided a way to measure the temperature of a black hole. Hawking had shown that quantum physics and relativity theory could be fruitfully combined to give new insights into the working of the Universe. And the link with thermodynamics is still there. If the area of a black hole shrinks, entropy is, it seems, running in reverse. Physicists think this means that information that is seemingly lost when objects fall into a black hole is in principle recoverable from the radiation when it evaporates.

In 2013, 40 years after he “discovered” the radiation that bears his name, Hawking was awarded the Breakthrough Prize, worth $3 million, for this theoretical work. It is not disparaging of his later life to say that he never came up with anything as profound again. This is akin to saying that after the general theory of relativity Einstein never came up with anything as profound again. In later life, Hawking made contributions to the theory of inflation, which explains how the Universe expanded away from a primordial seed, and studied the way in which that initial seed might have had its origin in a quantum fluctuation like the ones producing the virtual pairs outside black holes. And he espoused the idea of the multiverse, that our Universe is just one bubble in spacetime, with many other bubbles existing in dimensions beyond our own. But these are all areas of science where other researchers have made equally significant contributions. Just as Einstein’s place in the scientific pantheon is always linked with relativity theory, Hawking’s place in the scientific pantheon will always be linked with his namesake radiation. It is an assured and honored place.

Readers will have noticed that I have not mentioned that for most of his life Hawking suffered from a debilitating illness, Amyotrophic lateral sclerosis or what is known as Lou Gehrig’s disease. He suffered greatly and he suffered bravely. But this, like the color of his eyes or his favourite rock band, is completely irrelevant to his achievements as a scientist.

 

John Gribbin is a visiting fellow in astronomy at the University of Sussex and co-author (with Michael White) of Stephen Hawking: A Life in Science.

 

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4 comments on “Stephen Hawking’s Masterpiece

  1. A naive question: if nothing can escape from within the event horizon, and if the gravitational field of a black hole is carried by gravitons to which the same should apply, how can anything outside the event horizon observe that field?

    Or am I merely pointing out the incompatibility of general relativity and quantum theory as currently formulated?

    • The field, or warped spacetime, exists outside the event horizon. Gravitons, or gravitational waves, that we detect associated with the object are disturbances in that part of the Universe.

      • Thanks, an excellent reply to a question I’ve asked many others to no avail. A distantly related question: why is the predicted bending of staright under the general theory twice that expected under the special theory? Thinking of a light pulse entering a box in a gravitational field (or, equivalently, one being subjected to an acceleration) would lead me to expect that there would be no difference. Why should a photon behave differently from a tennis ball tossed in?

  2. asmodelle says:

    Nice piece John. I’ve always liked you lucid style of writing. You pretty much explained simply the main work of Stephen Hawking and pointed out the focal points: Bekenstein-Hawking entropy and Hawking Radiation. I have copies of his papers and many of his books, and I too was inspired or rather reaffirmed, by his popular book, “A Brief History of Time,” as a child. He is a legend, yet his level of productivity was probably not as large as many other researchers, such as Sir Roger Penrose, whose name is not widely known in the general community. Oddly, Penrose’s output has exceeded Hawking over the decades, yet the man in the street does not know his name. Even though ALS did not define who Stephen was, it was probably what made him a familiar name in a world that is obsessed by celebritydom. Had Stephen not had ALS, would he have been as famous as he had become? In any case, it is his scientific work that is relevant.

    I am currently working on my PhD at the moment, while my research focus, up till now, has been the special theory of relativity and the general theory of relativity, but now I am working on quantum mechanics and it is easy to see that some of Stephen’s ideas are so much a part of science that his legacy will be enshrined in theoretical physics as crucial building blocks.

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