Eminent physicist and cosmologist Stephen Hawking
Eminent physicist and cosmologist Stephen Hawking

Stephen Hawking’s ideas on the nature of black holes and the big bang might seem far removed from earthly life, but they stem from the kinds of questions that children and curious adults contemplate: why can’t we move around in time the way we do in space? Why do we seem to be stuck on a relentless one-way conveyor belt into the future? Was there a beginning of the universe, and if so, what happened before?

One of the great achievements of Hawking’s bestselling book A Brief History of Time was to connect seemingly arcane areas of theoretical physics to questions that tap into the curiosity of ordinary people. Scientists are still hashing out the nature of time today, but Hawking, who died on March 14, at the age of 76, was a leader in the development of new ideas on the nature of time, space, matter, order, information and how the world came to be.

Physicists don’t generally set out to be experts on time, said Sean M. Carroll, a physicist at the California Institute of Technology. But they want to understand the fundamental laws of the universe, and time and space are a part of that.

Did the universe have a beginning? What happened before that? Stephen Hawking, who died March 14, used theoretical physics to find answers to impossible questions

Hawking proposed a view of time that, like an everlasting tapestry, stretches from past to present to future, with a definite beginning at the big bang. But how could time itself not have existed? Time, in that way of thinking, is a quantity, like pressure, said Carroll, which can be measured in a room full of air, but which doesn’t exist if there’s just one or two molecules. Time might in that sense be what physicists call an “emergent property,” as opposed to something that’s built into the fabric of reality. 

In that picture of time, the notion of past, present and future represent a subjective idea, like the concepts of here and there, said Lee Smolin, a physicist at the Perimeter Institute for Theoretical Physics near Toronto.

Other physicists embrace an alternative scenario in which time has always existed, the flow of time making up the most fundamental entity in the universe. In that picture, our experience of time as an ever-moving moment of present tense is not an illusion but the way the world works. The laws of physics can change over time, while the only aspect of the universe that’s timeless is time itself.

These questions do have a connection to real life, Smolin said. The physics of time tell us whether the future is unformed and open to change, as it appears to most of us, or whether it’s predetermined, with our lives unfolding like a movie with a set plotline.

The general agreement among physicists is that Hawking’s biggest contribution to science was in the study of black holes — objects thought to be so dense that their powerful gravity won’t let even light escape. Hawking showed that according to the laws of physics, and contrary to intuition, black holes should radiate away energy and slowly evaporate.

Black holes can’t be seen directly, but scientists have observed dozens of places where an invisible force is causing stars to whirl around as if in orbit around an object too massive to be anything but a black hole. Such observations of our own galaxy show that at its heart is a black hole 4.6 million times the mass of the sun. So-called “Hawking radiation” has yet to be directly observed emanating from any known black holes, said Carroll, but scientists quickly accepted the idea, because Hawking so clearly laid out a mathematical argument for why it should exist. Hawking didn’t come up with the notion out of the blue, but extrapolated from the well-established theories of quantum mechanics and thermodynamics, Carroll said: “People realised it fit together with other things we think are true.”

The idea had big implications, leading to one of the hottest areas of theoretical physics today — a question about whether black holes permanently erase information from the universe. That is, if you throw a book into a black hole, the information would appear to go away forever, according to Hawking’s idea. But this creates a paradox because the known laws of physics — in particular, quantum mechanics — don’t allow information to be destroyed.

Black holes are of such fascination to scientists because they should be ruled by the same physics as the big bang — the event from which our universe sprang. In both cases, the conditions are extreme in a way that force the laws of gravity, which rules big objects, to mesh with the laws of quantum physics, which rules the world at the scale of subatomic particles. “Black holes really changed our understanding of the relationship between gravity and quantum theory,” said MIT physicist Alan Guth. And without that understanding, he said, it’s impossible to talk intelligently about the origin of the universe.

Hawking proposed that time itself began at the big bang, implying that time is self-contained and that our universe could come into being with no help from outside forces. Scientists need more work understanding the physics of the big bang before they will know for sure if this is the case, said Carroll. MIT’s Guth said that some physicists now favour the idea that our universe is but one expanding bubble in a much larger pre-existing area of space-time, sometimes called the ‘multiverse’. But that only puts off the question of how the multiverse originated.

These are hard questions, and it may take generations of scientists to reach an answer. That’s why Hawking never won a Nobel Prize. Physics Nobels are generally only awarded for ideas that have been subject to testing against experiments or observations before the theorist proposing them dies.

But Hawking understood the value of longer-term, riskier areas of exploration. His ideas may be tested far in the future, but Nobel or not, he was still both a renowned scientist and a celebrated populariser, who allowed millions of people to share the insights that science has brought to address questions that have tantalised humanity for centuries.

By arrangement with Bloomberg View

Published in Dawn, EOS, April 1st, 2018