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Stephen Hawking (1942–2018)

Hawking was a theoretical physicist whose work centred on the large-scale structure of spacetime — singularities, black holes, and the origin of the universe. His most influential results are the singularity theorems (with Penrose), which showed that singularities are unavoidable in general relativity; the discovery that black holes radiate thermally (Hawking radiation), which connected general relativity, quantum field theory, and thermodynamics for the first time; and the no-boundary proposal (with James Hartle), which attempts to specify the initial condition of the universe without a boundary or singularity. His framework for the arrow of time — three arrows (thermodynamic, cosmological, psychological) that must align in a universe containing observers — organised a question that had been discussed piecemeal since Boltzmann and Eddington.

Life

Born 8 January 1942 in Oxford, England. His father Frank Hawking was a medical researcher specialising in tropical diseases; the family was in Oxford because London was being bombed. Grew up in Highgate and St Albans. Undergraduate at University College, Oxford (BA in physics, 1962). PhD at Trinity Hall, Cambridge (1966), under Dennis Sciama, with a thesis on the properties of expanding universes. Sciama’s research group at Cambridge was a centre for general relativity and cosmology; other members included George Ellis, Brandon Carter, and Martin Rees.

Diagnosed with amyotrophic lateral sclerosis (motor neurone disease) in 1963, at twenty-one, and given approximately two years to live. The disease progressed slowly. By the late 1960s he needed a wheelchair; by the mid-1980s he could no longer speak and communicated through a speech-generating device. He continued to produce original research for five decades after diagnosis.

Research fellow at Gonville and Caius College, Cambridge (1965). Lucasian Professor of Mathematics at Cambridge (1979–2009) — Newton’s chair. A Brief History of Time (1988) sold more than ten million copies and made Hawking the most publicly recognised physicist since Einstein. The book’s success was partly the subject — the origin of the universe, the nature of time, the fate of black holes — and partly the figure: a physicist working at the frontier of theoretical physics while confined to a wheelchair and communicating through a synthesiser. The public image and the physics were entangled throughout his later career.

Died 14 March 2018 in Cambridge, at seventy-six — fifty-five years after a diagnosis that was expected to kill him within two.

Black holes

Hawking’s early work extended Penrose’s singularity theorem from gravitational collapse to cosmology. Penrose (1965) had shown that a collapsing massive body inevitably produces a singularity. Hawking reversed the time direction: an expanding universe, traced backward, implies a singularity at the beginning — the Big Bang singularity. The Penrose-Hawking singularity theorems (1966–70) established that singularities are generic in general relativity, not artefacts of simplified models.

The area theorem (1971). Hawking proved that the total area of black hole event horizons can never decrease in any classical process. The theorem is the gravitational analogue of the second law of thermodynamics — it implies that black holes have an entropy proportional to their horizon area. Jacob Bekenstein made the entropy interpretation explicit: a black hole’s entropy is proportional to its area in Planck units (the Bekenstein-Hawking entropy). This identification connected gravitation to thermodynamics for the first time.

Hawking radiation (1974). The most consequential result. Hawking showed that when quantum field theory is applied in the curved spacetime near a black hole’s event horizon, the black hole emits thermal radiation at a temperature inversely proportional to its mass. The radiation is not emitted from inside the black hole; it arises from quantum effects at the horizon — the vacuum state near the horizon differs from the vacuum state far away, and the difference manifests as a flux of particles. A black hole radiates, loses mass, and eventually evaporates completely.

The information paradox. Hawking radiation is thermal — it carries no information about what fell into the black hole. If a black hole forms from a pure quantum state and then evaporates completely into thermal radiation, the initial information is destroyed. This violates unitarity — the principle that quantum-mechanical evolution preserves information. Hawking argued in “Breakdown of Predictability in Gravitational Collapse” (1976) that information is genuinely lost, and that quantum mechanics must be modified in the presence of gravity. The claim was contested; John Preskill and others argued that information must be preserved if quantum mechanics is correct. In 2004, Hawking conceded — citing the AdS/CFT correspondence — that information is preserved, and paid Preskill a bet. The mechanism by which information escapes a black hole remains an active area of research; recent work on quantum extremal surfaces and “islands” has made progress, but a complete resolution requires a quantum theory of gravity that does not yet exist.

The no-boundary proposal and the three arrows

The no-boundary proposal (1983, with James Hartle). In quantum cosmology, the wave function of the universe is computed by summing over all possible histories. The Hartle-Hawking proposal specifies which histories to include: compact Euclidean geometries — spacetimes where the time dimension is replaced by an imaginary-time coordinate, and the geometry smoothly rounds off rather than terminating in a singularity. In ordinary spacetime, this translates to a universe that begins without an initial boundary or edge. “The boundary condition of the universe is that it has no boundary.” The proposal attempts to explain the initial state of the universe — the low-entropy beginning that the arrow of time depends on — as a consequence of the wave function rather than a contingent fact. Whether it succeeds is debated; Penrose has argued that the no-boundary condition does not select for the low gravitational entropy that the early universe actually had.

The three arrows. In A Brief History of Time (1988), Hawking organised the arrow-of-time question around three distinct arrows: the thermodynamic arrow (entropy increases), the cosmological arrow (the universe expands), and the psychological arrow (we remember the past, not the future). His central argument: the psychological arrow is determined by the thermodynamic arrow — storing a memory is a thermodynamic process that increases entropy, so memories point toward lower-entropy states, which is the past. The three arrows align in our universe. Hawking argued, on anthropic grounds, that they must align in any universe containing observers: “The thermodynamic and cosmological arrows of time must point in the same direction. The conditions in the contracting phase would not be suitable for the existence of intelligent beings who could ask the question: why does disorder increase in the same direction of time as that in which the universe expands?”

Where Hawking stops

The singularity theorems prove that general relativity, under generic conditions, produces singularities — points where the theory breaks down. They are results about where general relativity fails, not about what replaces it. The theorems point toward the need for a quantum theory of gravity; Hawking spent much of his career pursuing one, but no satisfactory theory exists. The singularities remain as markers of incompleteness.

The information paradox is Hawking’s most consequential open question. His 2004 concession — that information is preserved — resolved the question of principle but not of mechanism. How information that fell into a black hole is encoded in the outgoing radiation is not understood. The recent “island” calculations reproduce the expected behaviour of information (the Page curve) within certain models, but they rely on specific assumptions about quantum gravity and do not constitute a general resolution. The paradox remains the sharpest point of contact between quantum mechanics and gravity, and it was Hawking who sharpened it.

The no-boundary proposal offers a specific answer to the initial-condition problem, but whether it selects the right initial condition is contested. Penrose argues that the no-boundary state does not have the low gravitational entropy the early universe actually had — that the proposal smooths away the singularity without explaining why gravity started in so special a configuration. The anthropic argument for the alignment of the three arrows — that misaligned arrows are incompatible with observers — explains why we observe alignment but not why the universe is arranged so as to produce it. It constrains what we can see; it does not explain what is there.

Key works

See also: Penrose · Boltzmann · Rovelli