For some of us, sandwiched between the small world of physicists and the larger world of people who frankly didn’t bother, Stephen Hawking was a spectacle. A hologram. We could glimpse the ethereal surface of his research, read paper titles and marvel at other physicists’ befuddlement at whatever he was proposing. By virtue of the reception of his work, Hawking was a legend, and ours was an admiration by proxy. For as long as he glowed with the light of a thousand Suns, he was a star.
The closest many of us have come to really understanding his ideas was through his 1988 book, A Brief History of Time. As an exercise in science communication, it was remarkable – but it was more powerful by far in demonstrating to its readers that the universe possessed an aesthetic grammar of its own. That through their permanence and indefatigability, the laws of nature were constantly yet passively building beauty. A Brief History of Time was Hawking’s elucidation of his mastery of this grammar.
If not for the book itself, his research was proof enough that he was an artist. Foremost, to have been able to confront a blank page, stare down the paradox of choice, and begin. To have been able to bring layer after layer of semes to a picture that stubbornly stays out of focus, while trusting only in the laws of physics to ultimately coalesce into meaning. Finally, to produce a picture astoundingly greater than the sum of its parts, without having let the nervous hands of imagination undermine the assembly of a truly sublime thing, something humans have not fully understood even to this day.
Albert Einstein’s theories of gravity predicted that objects called blackholes inhabited the same universe we did. That they’d form when massive-enough stars ran out of fuel to fuse and imploded, sometimes with such intensity that the spacetime continuum at its heart gets messed up. To an observer sufficiently distant from the star, time inside the sphere appears to flow in a direction perpendicular to its journey on the outside, as if the star’s innards are in forever free-fall. Thus, a blackhole is born.
Leon Lederman and Christopher Hill, both physicists, have likened this moment to the first stanza of W.B. Yeats’s ‘The Second Coming’. I prefer the last:
The darkness drops again but now I know
That twenty centuries of stony sleep
Were vexed to nightmare by a rocking cradle,
And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?
In 1970, Hawking and Roger Penrose showed that our universe could have been born of a singularity, the single point at a blackhole’s centre where spacetime laws have no meaning.
Since the 1910s, the classical view of physics had said that blackholes would have to be absolutely black, emitting no radiation, and absolutely immortal, living forever. In this paradigm, blackholes were marvellous objects – but they could not be touched. Physicists didn’t just lack the tools to study them, they did not know if they should even bother. In 1974, Hawking (together with Jakob Bekenstein) changed this, with enormous consequences for the study of classical and quantum mechanics alike. He added the laws of thermodynamics to the mix and, in the words of Freeman Dyson, “brought blackholes back out of the domain of mathematical abstraction into the domain of things we can see and measure”.
The equations he had uncovered suggested that blackholes were neither black nor immortal. To use the words of physicist Matthew Buckley, “By bending spacetime, the blackhole made the vacuum [of outer space] itself radiate away energy. The source of this energy is the blackhole’s mass, and eventually this process … would cause the blackhole to evaporate.”
Suddenly, blackholes could evaporate.
And just like that, with a more physical understanding of blackholes within reach, physicists also got the tools they had been waiting for.
The mathematics of Hawking’s contribution is neither fully known nor completely understood to this day, nor have his predictions been experimentally verified. But it was clear from the straightforwardness of his study that Hawking had struck a chord loud enough for his peers to be able to wade in with confidence, no longer groping but actively searching in the dark for new insights into the inner lives of these cosmic aberrants.
However, after having deconstructed the legerdemain, is it any longer possible to appreciate a sleight of hand? Can painters marvel at their own paintings? Was Hawking still able to wonder?
It seems he was. Of course, there was also a lot left to wonder about; we as a species have not come that much closer to figuring out nature to its smallest specifics since Hawking’s discovery. But it was still true that Hawking had found something profoundly fundamental. For, embedded within his equations was a direct relationship between a blackhole’s entropy – the amount of heat it could carry at a given temperature – and its surface area.
Entropy and surface area don’t even have the same units: calories per degree versus squared metres. They have no need to be related like this. Imagine having an octopus for a cousin! But such a thing was indeed possible – or at least hoped into possibility – after physicists began to suspect that a deeper, more diffident logic might have shown itself.
To get to his solution, Hawking had solved problems using theories of gravity and quanta both, together with the second law of thermodynamics. There are very few problems in physics that allow physicists to draw from these three domains at once. Hawking wasn’t just able to find one of them; he found it and cracked it.
Meanwhile, the idea that a blackhole could evaporate was setting off alarm bells of its own. Say a blackhole had swallowed a chair, followed by a dozen bananas, and then a few books. In time, the energy it gets from all these objects is radiated out as Hawking radiation. Quantum mechanics is fine with this. What it isn’t fine with is information being treated the same way. Working theories of the quantum realm have all been founded on many assumptions – one of them is that information is always conserved. While a chair, a banana and a book can all be converted into the same kind of energy and spit back out, different pieces of information were required to stay distinct.
So when a blackhole swallows two pieces of information and then evaporates, where do the pieces go? Are they released back into the universe? Do they get trapped in a baby universe inside the blackhole? Or do they traverse an inter-blackhole wormhole and emerge in another part of the universe? We don’t know for sure. Hawking first believed that, yes, the information would be lost forever – an idea he would later call his “biggest blunder”. In 2004, he did a volte face and argued that the information would be encoded on the blackhole’s event horizon, that it wouldn’t ‘fall in’, that its energy would be spent as Hawking radiation, that a foundational assumption of quantum mechanics would be spared.
These are not trivial considerations – or perhaps they would be if a piece of land smaller than Chennai was a trivial consideration if it had the Indian army on side and the Chinese on the other. No; this is a wildly contested space, the no-theory’s-land between the demesnes of gravity and quanta, each ruled by a singular monarch, each culture so unique as to be alien to the other. Physicists have been trying to erase this border for decades in an attempt to unify the kingdoms, and have failed. What Hawking did was identify a small but important patch along the border, rub it out, replace it with a blackhole and offer his equations as clues to deciphering the diplomacy within.
In fact, his work in blackhole thermodynamics and quantum cosmology has allowed him to straddle the line between the kingdoms often enough to have taken him – and for him to have taken us – to many strange places of varying importance in the quest to realise a theory of quantum gravity. Consider: the no-hair conjecture, the blackhole information paradox, N=8 supergravity, the cosmic censorship conjecture and top-down cosmology (the last is a proposition that the universe ‘selects’ its present from a superposition of multiple pasts, sort of like Schrödinger’s cat but with the whole cosmos), among others.
In all this time, Hawking also maintained a high public profile. He made wager after wager with his peers over predictions about the universe’s evolution, and lost most of them. (Shortly before the Higgs boson was discovered at the Large Hadron Collider in late 2011, Hawking had bet that the particle would never be found). He has also been affiliated with large astrophysics projects – such as Breakthrough Listen – as a consultant, often addressed general audiences at awards ceremonies and public events, and appeared in TV shows like The Simpsons and The Big Bang Theory.
In fact, if one were to cast one’s mind back, Hawking seems the cosmic bard, a traveller and poet curious only to know where the universe came from and why it is what it is, while he tom-tommed its feats as well as his own in his journeys through space and time. With his passing, the world grows that much quieter.
Social image credit: Alexandar Vujadinovic/Wikimedia Commons, CC BY-SA 4.0.