STEPHEN HAWKING

 

STEPHEN HAWKING

       Stephen Hawking, the world-famous theoretical physicist, has died at the age of 76.

Hawking’s children, Lucy, Robert, and Tim said in a statement: “We are deeply saddened that our beloved father passed away today.

“He was a great scientist and an extraordinary man whose work and legacy will live on for many years. His courage and persistence with his brilliance and humor inspired people across the world.

“He once said: ‘It would not be much of a universe if it wasn’t home to the people you love.’ We will miss him forever.”

The most recognizable scientist of our age, Hawking holds an iconic status. His genre-defining book, A Brief History of Time, has sold more than 10 million copies since its publication in 1988 and has been translated into more than 35 languages. He appeared on Star Trek: The Next Generation, The Simpsons, and The Big Bang Theory. His early life was the subject of an Oscar-winning performance by Eddie Redmayne in the 2014 film The Theory of Everything. He was routinely consulted for oracular pronouncements on everything from time travel and alien life to Middle Eastern politics and nefarious robots. He had an endearing sense of humor and a daredevil attitude – relatable human traits that, combined with his seemingly superhuman mind, made Hawking eminently marketable.

But his cultural status – amplified by his disability and the media storm it invoked – often overshadowed his scientific legacy. That’s a shame for the man who discovered what might prove to be the key clue to the theory of everything, advanced our understanding of space and time, helped shape the course of physics for the last four decades, and whose insight continues to drive progress in fundamental physics today.

Beginning with the big bang:-

        Hawking’s research career began with disappointment. Arriving at the University of Cambridge in 1962 to begin his Ph.D., he was told that Fred Hoyle, his chosen supervisor, already had a full complement of students. The most famous British astrophysicist at the time, Hoyle was a magnet for the more ambitious students. Hawking didn’t cut. Instead, he was to work with Dennis Sciama, a physicist Hawking knew nothing about. In the same year, Hawking was diagnosed with amyotrophic lateral sclerosis, a degenerative motor neuron disease that quickly robs people of the ability to voluntarily move their muscles. He was told he had two years to live.

Although Hawking’s body may have weakened, his intellect stayed sharp. Two years into his Ph.D., he was having trouble walking and talking, but it was clear that the disease was progressing more slowly than the doctors had initially feared. Meanwhile, his engagement to Jane Wilde – with whom he later had three children, Robert, Lucy, and Tim – renewed his drive to make real progress in physics.

Working with Sciama had its advantages. Hoyle’s fame meant that he was seldom in the department, whereas Sciama was around and eager to talk. Those discussions stimulated the young Hawking to pursue his scientific vision. Hoyle was vehemently opposed to the big bang theory (in fact, he had coined the name “big bang” in mockery). Sciama, on the other hand, was happy for Hawking to investigate the beginning of time.

Time’s arrow:-

       Hawking was studying the work of Roger Penrose, which proved that if Einstein’s general theory of relativity is correct, at the heart of every black hole must be a point where space and time themselves break down – a singularity. Hawking realized that if time’s arrow were reversed, the same reasoning would hold for the universe as a whole. Under Sciama’s encouragement, he worked out the maths and was able to prove it: the universe according to general relativity began in a singularity.

Hawking was well aware, however, that Einstein didn’t have the last word. General relativity, which describes space and time on a large scale, doesn’t take into account quantum mechanics, which describes matter’s strange behavior at much smaller scales. Some unknown “theory of everything” was needed to unite the two. For Hawking, the singularity at the universe’s origin did not signal the breakdown of space and time; it signaled the need for quantum gravity.

Luckily, the link that he forged between Penrose’s singularity and the singularity at the big bang provided a key clue for finding such a theory. If physicists wanted to understand the origin of the universe, Hawking had just shown them exactly where to look: a black hole.

Black holes were a subject ripe for investigation in the early 1970s. Although Karl Schwarzschild had found such objects lurking in the equations of general relativity back in 1915, theoreticians viewed them as mere mathematical anomalies and were reluctant to believe they could exist.

Albeit frightening, their action is reasonably straightforward: black holes have such strong gravitational fields that nothing, not even light, can escape their grip. Any matter that falls into one is forever lost to the outside world. This, however, is a dagger in the heart of thermodynamics.

Thermodynamic threat:-

          The second law of thermodynamics is one of the most well-established laws of nature. It states that the entropy, or level of disorder in a system, always increases. The second law gives form to the observation that ice cubes will melt into a puddle, but a puddle of water will never spontaneously turn into a block of ice. All matter contains entropy, so what happens when it is dropped into a black hole? Is entropy lost along with it? If so, the total entropy of the universe goes down and black holes would violate the second law of thermodynamics.

Hawking thought that this was fine. He was happy to discard any concept that stood in the way of a deeper truth. And if that meant the second law, then so be it.

Bekenstein and breakthrough:-

         But Hawking met his match at a 1972 physics summer school in the French ski resort of Les Houches, France. Princeton University graduate student Jacob Bekenstein thought that the second law of thermodynamics should apply to black holes too. Bekenstein had been studying the entropy problem and had reached a possible solution thanks to an earlier insight of Hawking’s.

A black hole hides its singularity with a boundary known as the event horizon. Nothing that crosses the event horizon can ever return to the outside. Hawking’s work had shown that the area of a black hole’s event horizon never decreases over time. What’s more, when matter falls into a black hole, the area of its event horizon grows.

Bekenstein realized this was key to the entropy problem. Every time a black hole swallows matter, its entropy appears to be lost, and at the same time, its event horizon grows. So, Bekenstein suggested, what if – to preserve the second law – the area of the horizon is itself a measure of entropy?

Hawking immediately disliked the idea and was angry that his work had been used in support of a concept so flawed. With entropy comes heat, but the black hole couldn’t be radiating heat – nothing can escape its pull of gravity. During a break from the lectures, Hawking got together with colleagues Brandon Carter, who also studied under Sciama, and James Bardeen, of the University of Washington, and confronted Bekenstein.

The disagreement bothered Bekenstein. “These three were senior people. I was just out of my Ph.D. You worry whether you are just stupid and these guys know the truth,” he recalls.

Back in Cambridge, Hawking set out to prove Bekenstein wrong. Instead, he discovered the precise form of the mathematical relationship between entropy and the black hole’s horizon. Rather than destroying the idea, he had confirmed it. It was Hawking’s greatest breakthrough.

Hawking radiation:-

            Hawking now embraced the idea that thermodynamics played a part in black holes. Anything that has entropy, he reasoned, also has a temperature – and anything that has a temperature can radiate.

His original mistake, Hawking realized, was in only considering general relativity, which says that nothing – no particles, no heat – can escape the grip of a black hole. That changes when quantum mechanics comes into play. According to quantum mechanics, fleeting pairs of particles and antiparticles are constantly appearing out of space, only to annihilate and disappear in the blink of an eye. When this happens in the vicinity of an event horizon, a particle-antiparticle pair can be separated – one falls behind the horizon while one escapes, leaving them forever unable to meet and annihilate. The orphaned particles stream away from the black hole’s edge as radiation. The randomness of quantum creation becomes the randomness of heat.

“I think most physicists would agree that Hawking’s greatest contribution is the prediction that black holes emit radiation,” says Sean Carroll, a theoretical physicist at the California Institute of Technology. “While we still don’t have experimental confirmation that Hawking’s prediction is true, nearly every expert believes he was right.”

Experiments to test Hawking’s prediction are so difficult because the more massive a black hole is, the lower its temperature. For a large black hole – the kind astronomers can study with a telescope – the temperature of the radiation is too insignificant to measure. As Hawking himself often noted, it was for this reason that he was never awarded a Nobel Prize. Still, the prediction was enough to secure him a prime place in the annals of science, and the quantum particles that stream from the black hole’s edge would forever be known as Hawking radiation.

Some have suggested that they should more appropriately be called Bekenstein-Hawking radiation, but Bekenstein himself rejects this. “The entropy of a black hole is called Bekenstein-Hawking entropy, which I think is fine. I wrote it down first, Hawking found the numerical value of the constant, so together we found the formula as it is today. The radiation was Hawking’s work. I had no idea how a black hole could radiate. Hawking brought that out very clearly. So that should be called Hawking radiation.”