Stephen Hawking, who died this March at the age of 76, was a physicist from another time. He had more in common with the celebrity scientists of the first half of the 20th century — especially the politically-inclined scientist-intellectuals like Robert Oppenheimer, Richard Feynman, and Enrico Fermi who came out of the Manhattan Project — than he did with any of his contemporaries. This isn’t so much a question of brainpower as it is of public positioning, as getting the public to understand new scientific ideas is a very different job than coming up with them. On the whole, our most prominent science communicators (Neil deGrasse Tyson, Bill Nye, Michio Kaku, etc.) do not actively produce science. Consequently, they are removed from the everyday slog and department politics that pervade the life of a working researcher. Meanwhile, our greatest researchers (a huge list of people I don’t want to enumerate for fear of leaving someone out) don’t prioritize telling the public what their work is and why it matters. Hawking, with his precise insights about physics and beyond who also possessed the power to capture the attention of the public, was a rare link between these two groups. Though some younger physicists like, for instance, Sean Carroll, a cosmologist at Caltech, and Lisa Randall, a particle physicist at Harvard, have similarly rare combinations of theoretical sophistication and an ability to communicate this sophistication clearly, the Hawking’s wide-ranging skills will be hard to replace. After all, I can’t think of anyone else who was able to both shape the modern understanding of black holes and make the only actually funny joke ever aired on The Big Bang Theory.
However unique it was, the power of Hawking’s celebrity — which meant that every pronouncement he made had the potential to shift public conversation in ways that could affect researchers’ lives — incensed some fellow physicists. For instance, he once made a $100 bet that the Higgs boson, the elusive particle that gives mass to matter, could not be found. This was right after the Large Electron-Positron (LEP) Collider came out with a null result in their search, and some feared that Hawking’s statement might limit future research funding. Peter Higgs, a notorious media recluse, came out to chastise Hawking in 2002 for meddling in a subfield that wasn’t his: “His celebrity status gives him instant credibility that others do not have.” Others said that Hawking’s contributions to astrophysics were not commensurate with his public stature. When the Higgs was found ten years later, those physicists didn’t pile on him again. In a talk given shortly after Peter Higgs and his research partner Francois Englert were awarded the Nobel Prize for their work, Hawking was gracious and funny about the affair, saying, “Congratulations to them both. But the discovery of the new particle came at a personal cost… The Nobel prize cost me $100.”
The Higgs boson wasn’t the only bet Hawking didn’t get right. He was also wrong in thinking that the gamma-ray source Cygnus X-1 was not a black hole (it was). And in believing that black holes destroy information (they don’t). His bet against cyclic inflation is still unresolved (currently, it is untestable). And despite his public intellectual suavity, Hawking was wrong on personal things as well, including his belittling views on women; he sometimes tormented his wife and then left her for his nurse. He also wagered a Penthouse subscription against Kip Thorne to annoy his “liberated” wife and spoke of women as the “complete mystery” that he thought about all day. Not as terrifying as Feynman having basically an incel/predator mindset about women owing him sex, but still definitely rough. Then again, a list of toxic men in science would probably be longer than this article.
By the time A Brief History of Time, one of the first books meant for a general audience about cosmology, broke sales records in the late 80s, Hawking had an impressive academic career and was intent on continuing it. For the last ten years of his life, he proceeded to publish a paper or two on cosmology or black holes a year: not a unique pace, but consistent. It isn’t entirely unusual for physicists to continue active research beyond retirement age, but Hawking also remained director of research at the Cambridge department of applied math and theoretical physics. His last grad student graduated in 2010. And, again, he was literally jet-setting to conferences around the world and staying politically active when most people his age were overwhelmingly voting for Brexit. So while it’s true that Hawking was no Paul Dirac — the Nobelist who quantified the quantum mechanics of matter and has so, so many concepts named after him — he did remain deeply invested in making a mark on physics until the end, though many might not have seen it.
Hawking’s final paper, titled “A Smooth Exit from Eternal Inflation?” which was published in the Journal of High Energy Physics this past week, has been called everything from his “Final Theory of the Universe” to “mind-bending” to proof that “the universe is a hologram”, none of which it really is. Media reports about physics are usually overblown because no one wants to read about papers that build incrementally on scientific knowledge between breakthroughs. But this being Hawking’s final bit of science and all, there exists an incentive to read deep meaning into this bit of what the philosopher of science Thomas Kuhn would call “normal science” — that filling-in-the-blanks that fleshes out the consequences of theories, rather than something worthy of causing a paradigm shift or even a fit.
It doesn’t help that the paper itself is too arcane for laypeople to read or even understand its consequences. To undercut mainstream outlets further (and myself because this article is even later to the party), the paper has been available as a pre-print on arXiv for over two months now, and was made publicly available ten days before Hawking’s passing. It now has gone through peer-review without much change, meaning that the science and methods within are sound enough, and was formally published on May 2. If physicists have already looked it over and haven’t lost their minds, it should be a strong clue that this paper does not actually prove that the universe is a hologram.
To situate “A Smooth Exit from Eternal Inflation?” in the scientific world in which it was written, the practice of theoretical physics is often an exercise in fabulism: you propose something new and strange that appears to fit with established theory and observations and then — this is the hard part — follow the ever-branching consequences of your new idea to see if there are any contradictions that might arise. After justifying those contradictions, you make or someone else makes a prediction, however outlandish or futuristic, and you wait for the moment between tomorrow and one hundred years from now that it is proved or disproved. Consequently, though theorists often get Nobel Prizes for being the first to propose an innovative idea, they tend to actually receive the award late in life; the average age of a Nobel recipient in physics for the past twenty years is 67.
In the case of this paper, Hawking links together weighty ideas like general relativity and string theory to make a case for our universe being a possible product — at least in terms of size and physical continuity — of a strange but consequential form of cosmic inflation. In more specific and complicated physics-speak, he starts with his no-boundary proposal, the idea that the earliest moments of the universe’s life must be described in a quantum wave state rather than in any sense of space and time. He then uses AdS/CFT correspondence — an outrageously popular (among high-energy physicists) and mathematically arcane (among everyone else) theory that combines gravitational and quantum perspectives — to illustrate a form of eternal inflation that creates universes like ours rather than empty, weird ones. Eternal inflation is the idea that the sudden, gargantuan expansion at the earliest stage of our universe’s life kept going outside of our visible bubble, creating other bubbles — other universes — in a multiverse. Think of electricity running through a string of Christmas lights, except instead of lighting up the lights it is also creating them, and none of the lights can detect each other. And it’s happening in a bunch of inconceivable dimensions rather than along a cord. So maybe it isn’t much like Christmas lights, but whatever. And though none of the concepts in the paper are new, you can see that Hawking is indeed grappling with Big Questions about space, time, and general existential stuff.
At the same time, the paper itself doesn’t propose any predictions of what tests we could do to prove or disprove Hawking’s theory. The paper’s co-author Thomas Hertog claims that Cosmic Microwave Background — a relic of the early universe when light and matter were in an interacting plasma state, which is the same measurement that would signal inflation — would be an indicator of a multiverse. But, as theoretical physicist Sabine Hossenfelder evocatively points out on her blog, “the claim that the detection of CMB polarization would mean the multiverse exists makes as much sense as claiming that if I find a coin on the street then Bill Gates must have walked by. And a swarm of invisible angels floated around him playing harp and singing ‘Ode To Joy.’” This, I must say, is a totally sick burn, and, without a more precise prediction from Hertog in a new paper, pretty true. (It’s worth noting that other cosmologists have their own pet ideas outside our experimental paradigm; Sean Carroll has produced the idea that, before the Big Bang, there was a universe similar to ours on large scales that happened entirely in reverse. Again, this is untestable, but it at least fits poetically with the various symmetries that get preserved on the quantum mechanical level.)
Hawking’s first blockbuster idea — one of the few connecting general relativity and quantum mechanics that stands to reason — has also proven incredibly difficult to verify. Hawking radiation, or the idea that black holes send out signals, works something like this: The Heisenberg uncertainty principle allows for the spontaneous creation of tiny particle-antiparticle pairs which pop into existence and then annihilate each other. But when this occurs near the event horizon of a black hole, one particle ends up getting sucked in, leaving the other to broadcast the black hole’s existence. Associated with this is the surprisingly simple calculation, derived by Hawking alongside astrophysicist Jacob Bekenstein, that gives you the temperature and radiative rate of a black hole with a given mass, angular momentum, and charge. The problem is that as of now, it’s technologically impossible to detect the difference between the universe’s very low ambient temperature and the even lower temperature of a black hole itself. The distinction between Hawking’s discoveries about the nature of black holes and his last paper is that Hawking radiation is based on general relativity and quantum mechanics — two of the most experimentally verified, robustly detailed theories in modern physics. Meanwhile, concepts underpinning “A Smooth Exit to Eternal Inflation?” sometimes feel like fan-fiction that attempts to explain the nature of the universe, posing questions that, due to the diligent limits of the scientific method, physics cannot ask. But even if it doesn’t result in good science every time, that kind of moonshot thinking Hawking was prone to is necessary to move the discipline forward.
Whether they were groundbreaking mechanisms or just theoretical baubles, Hawking’s astrophysical ideas fleshed out some the central questions that plague cosmologists: “What happens in a black hole?”; “How was the universe born?”; “What is time?”. Sure, many people want to trap the truth of the universe in their mind as some kind of Infinity Gauntlet power trip, but so few people have the skills and humility to face those questions and chip away at them, day after day, without being overcome with hubristic theories that overpromise and underdeliver. Hawking wasn’t afraid to fail under the brightest scientific spotlight, and for that, he should be celebrated. His final paper doesn’t need to be an overarching theory of everything, anyway. After all, the title ends in a question mark.