The profound impact of Stephen Hawking
Stephen Hawking died yesterday. I had dinner with him once in Paris. We were at the same table, but not seated particularly close together. He was accompanied by a phalanx of nurses and attendants. We didn’t get a chance to converse. Hawking entered my consciousness with A Brief History of Time, which I read when I was in high school. His life story is an inspiration, and he is celebrated in culture and pop culture as a public intellectual with an impish sense of humour. He remains a luminary for those of us who study gravity for a living for his deep physical insights. I want to share some of his science and explore how the questions he asked challenge and invigorate our conception of how the universe is structured.
The story begins with Albert Einstein. General relativity teaches us that geometry responds dynamically to the presence of matter or energy by curving, and conversely, that matter moves according to the curvature of spacetime. This is the force of gravity. Among the first solutions to Einstein’s equations is the black hole. Black holes are a laboratory for fundamental physics. Hawking’s scientific contributions consisted of thought experiments performed in this unusual, wonderful laboratory.
To define what a black hole is, we must first notice that nature has a speed limit set by light. Unlike traffic on South Africa’s roads, we cannot go any faster. A black hole is the geometry associated to a mass so heavy that not even light can escape the attractive tug of gravity. At the centre of a black hole there resides a singularity. This is a point where spacetime becomes so strongly curved that the assumption that went into the formulation of general relativity - that geometry is smooth - breaks down. Hawking and Roger Penrose showed that the Big Bang, whence the universe began, corresponds to a similar singularity. Hawking suggested that a quantum theory of gravity may explain physics at the singularity. In order to do this, he and his collaborator James Hartle proposed a wave function of the universe.
Using quantum mechanics, which describes physics at subatomic scales, Hawking observed in the mid-1970s that black holes aren’t black. They radiate. This happens because the vacuum in a quantum theory isn’t empty nothingness. It is dynamical. Particle and antiparticle pairs pop in and out of existence constantly due to vacuum fluctuations. When this process transpires in the vicinity of a black hole, one of the pair may cross through the event horizon, and once it does, it cannot get out again. Consistent with symmetry, its opposite number travels in the other direction, away from the black hole. An observer far away sees the black hole spitting out particles of Hawking radiation. The black hole loses mass, and eventually it evaporates. Hawking’s insight, that the densest objects in the universe glow like a lump of coal, invented black hole thermodynamics.
Black holes are unique solutions to general relativity. This means that any two objects with identical mass and identical rotational properties collapse gravitationally to form the same black hole. One of these objects may have been a star undergoing supernova. The other may have been a cloud of dust. Looking at the black hole in general relativity, it is impossible to tell what it was before the collapse. This fact is in conflict with quantum mechanics, which has as one of its central tenets the principle that information can be neither created nor destroyed. Hawking refined this tension into a sharp paradox. He initially believed that the resolution to the paradox is that information must be lost and the foundations of quantum mechanics need revision.
Like every great scientist, Hawking had the ability to change his mind. In the end, he argued that quantum mechanics prevailed and information is subtly preserved.
Hawking’s ultimate ambition was to explain the structure of spacetime both at the microscopic level and at large scales, to understand how the universe is organised, and why it even exists. These are modern echoes of the first scientific questions that our early human forbears must have asked when they looked up into the night, bellies full after a successful hunt, and contemplated the beauty and the grandeur of the Milky Way. Despite his physical frailties, Hawking had an intuitive grasp of the right questions to ask in order to advance his aim.
Our understanding of what happened at the Big Bang, the quantum underpinnings of black hole thermodynamics, and how information is processed in a black hole spacetime remains incomplete. My own research and the work of much of the international theoretical physics community focuses on these issues. Hawking’s ideas have had a profound and lasting impact on what we know today and how we think about such questions. The ideas also shape the next set of questions we must ask. As we continue our quest, we are extremely fortunate to use as a stepping stone a lifetime of Hawking’s insights.
Vishnu Jejjala is the South Africa Research Chair in Theoretical Particle Cosmology. He is an associate professor in the School of Physics at the University of the Witwatersrand.