In addition to being a world-renowned cosmologist, Hawking has become something of a pop culture icon. He's a striking figure: a genius curled up in a body largely immobilized by amyotrophic lateral sclerosis, or ALS. He's supported by an advanced wheelchair, and communicates to the world through a rare and specialized system that converts the movements of a single muscle in his cheek into speech. In that mode, he appeared on "Star Trek: The Next Generation," "The Simpsons" and "The Big Bang Theory."
But Hawking's most lasting legacy will be as the most important physicist of the second half of the 20th century — a researcher who took the earlier works of figures like Albert Einstein and Werner Heisenberg and knitted them together into something approaching a coherent explanation for the behavior of the cosmos. [8 Shocking Things We Learned from Stephen Hawking's Book]
No good genius story starts with anything less than a bang, so it's appropriate that Hawking's first great achievement was also his doctoral thesis at the University of Cambridge.
Hawking's thesis, approved in 1966, made a dramatic argument: that the entire universe began as a single point, infinitely small and dense and curled up upon itself — a point at the beginning of everything. Or, as he would later write, succinctly: "There is a singularity in our past."
It was the first description of the Big Bang as it's commonly understood today: an infinitely small point at the far reaches of time that burst into our modern, ever-expanding cosmos.
As Hawking described in his 2005 lecture "The Origin of the Universe," his thesis arrived at a moment when scientists had seen that the vast empty reaches of space, the chasms between galaxies, were expanding. But they weren't sure why. Some physicists proposed weaker versions of the Big Bang concept, minus the singularity. But another theory, called the Steady State Universe, was dominant.
"As galaxies moved apart," Hawking said of the Steady State Universe, "the idea was that new galaxies would form from matter that was supposed to be continually being created throughout space. The universe would have existed forever and would have looked the same at all times."
In other words, many scientists thought that the universe was expanding, but in a way that gave it no beginning and no end.
As biographer Kitty Ferguson wrote in her book "Stephen Hawking: An Unfettered Mind" (St. Martin's Griffin, 2012), Hawking struggled with depression in the months after his 1963 ALS diagnosis at age 21, and had that mental illness persisted, he might never have arrived at his thesis. But his depression subsided as it became clear that he was outliving expectations and when he was granted an exception from Cambridge rules governing graduate students, allowing him to marry his first wife, Jane Wilde, according to Ferguson.
During that period before he arrived at the subject of his doctoral thesis, Hawking reported feeling frustrated with the way researchers busied themselves with work he considered ultimately trivial.
"People were so pleased to find any solution to [Einstein's] field equations; they didn't ask what physical significance, if any, it had," he later said in his 2002 birthday lecture.
That frustration led him to his first brush with notoriety. As Ferguson recounted, Hawking traveled in June 1964 to hear a lecture from Fred Hoyle, a famous astronomer and advocate of the Steady State Universe theory. During the lecture, Hawking became so frustrated that he hauled himself to his feet, leaning on his cane, to challenge one of Hoyle's results. [Portrait of Genius: Stephen Hawking Exhibit Photos]
"An astonished Hoyle asked Hawking how he could possibly judge whether the result was right or wrong," Ferguson wrote. "Hawking replied that he had 'worked it out.'"
The audience was impressed, and Hoyle was "infuriated," by this unknown graduate student who had appeared to tear apart the professor's research in his head at the lecture, Ferguson wrote. (In fact, Hawking had befriended one of Hoyle's students and begun to attack the idea long before the lecture.)
Soon afterward, Ferguson wrote, Hawking learned about a cosmological theory developed by mathematician Roger Penrose: that singularities, the points of infinite density and space-time curvature theorized in general relativity, might actually appear when sufficiently large stars collapse on themselves. [8 Ways You Can See Einstein's Theory of Relativity in Real Life]
"Hawking took off from there," Ferguson wrote, "reversed the direction of time, and considered what would happen if a point of inifinite density and infinite curvature of space-time — a singularity — exploded outwards and expanded. Suppose the universe began like that. Suppose space-time, curled up tight in a tiny, dimensionless point, exploded in what we call the Big Bang and expanded until it looks the way it does today. Might it have happened like that? Must it have happened like that?"
Hawking got to work, backing up his train of speculation with robust supporting calculations. His doctorate thesis, based on those calculations, was approved in 1966. Those calculations, along with follow-up research conducted in partnership with Penrose over the decade that followed, formed the foundation for scientists' modern understanding of the Big Bang.
Around the same time, certain key predictions of the Steady State Universe theory began to fail experimental tests, cementing Hawking's status as discoverer of the true history of the early universe.
If Hawking's only achievement in his career were discovering the historical shape of the universe, he would still be a giant — the kind of person mentioned alongside Rosalind Franklin, who discovered the double-helix shape of DNA, or Nicolaus Copernicus, who first proposed the heliocentric model of the solar system. But that was just the first of Hawking's two defining achievements.
The second, Hawking radiation, requires a bit of understanding of two things: black holes and the quantum mechanics of empty space.
First, about black holes: A black hole is a star that has collapsed on itself and become so gravitationally intense that not even light can escape a region around its center. Beyond that point, called the event horizon, space-time is so curved that everything that falls behind the shroud is lost forever. A black hole, according to this understanding in the early 1970s, never emits light, never shrinks, never loses mass; it only gains mass and draws more of space into its shrouded territory.
Second, on quantum mechanics: By the time of Hawking's career, scientists had long known that Heisenberg's uncertainty pricinple implied that empty space isn't really empty. Instead, it roils with "virtual" particles — matter-antimatter pairs that appear together, separate and then crash into each other and annihilate in a span of time too short to measure. (Scientists argue to this day whether those virtual particles really exist or turn up only in quantum equations due to their weird, probabilistic nature.)
In the late summer of 1973, Stephen and Jane Hawking attended a lecture series in Warsaw, Poland, celebrating Copernicus' 500th birthday, Ferguson wrote. There, Hawking encountered two Soviet physicists, Yakov Borisovich Zel'dovich and his student Alexei Alexandrovich Starobinsky, who had shown that the energy of spinning black holes would create particles just outside their event horizons. Those particles would careen away into space, Zel'dovich and Starobinsky said in their lecture, sapping some of the spin of the black hole as they went. Eventually, Zel'dovich and Starobinsky said, the black holes would stop spinning.
The idea stuck in Hawking's head, Ferguson wrote, and he returned to Cambridge to repeat and refine Zel'dovich and Starobinksy's calculations. But when he took his first stabs at their results, something new unfolded.
"I found, to my surprise and annoyance, that even nonrotating black holes should apparently create and emit particles at a steady rate," he later wrote in his 1988 book "A Brief History of Time." [The Best Science Books]
Here's why, as he explained in that book:
If black holes exist in space and have defined event horizons, and if space roils constantly with virtual" pairs of self-annihilating particles, then sometimes those particles must pop into existence right at the edges of black holes' event horizons. In fact, some of those particle pairs must appear perfectly positioned with one negative-mass antimatter particle separated onto one side of the event horizon and the other positive-mass matter particle separated onto the other side.
That strange circumstance would effectively "boost" the particles from their virtual semiexistence into full reality, Hawking realized, as they would have separated enough not to annihilate. That meant that particles of energy and mass would appear to stream from the surface of black holes' event horizons. And that stream of energy, radiating outward from what physicists had previously believed were eternally dark bodies, took the name Hawking radiation, after he described it in a 1974 paper in Nature titled "Black Hole Explosions?"
Hawking radiation profoundly changed the way physicists understood the universe. Before Hawking's realization, scientists believed that any matter or energy lost to a black hole was gone from the wider universe forever, such that black holes' event horizons would act as walls from beyond which some of the universe's stuff would never return.
But Hawking's discovery showed that black holes would decay faster and faster over time. For each positive particle that streamed from the surface of an event horizon out into the wider universe, a negative particle with negative energy and mass would fall back into the space beyond the event horizon, reducing the total mass and energy locked away there. Over time, that process would cause black holes to shrink. And as they shrank, they would become more active with Hawking radiation and shrink faster.
Hawking predicted that the universe must contain "primordial black holes" that emerged not from collapsing stars but from the extreme pressures of the early universe. These black holes, he reasoned, would have shrunk considerably over the intervening billions of years and their small event horizons would churn out powerful rays of Hawking radiation.
"Such holes hardly deserved [to be called] black: they are really white hot," he wrote in "A Brief History of Time."
Eventually, Hawking decided, they would explode.
As Hawking began to share this idea, Ferguson wrote in "An Unfettered Mind," his peers received it as either brilliant or heretical. When Penrose heard whispers of it, he called Hawking just as the physicist was sitting down to his 1974 birthday dinner and congratulated him for so long that his dinner got cold. But months later, the moderator at the symposium where Hawking presented his proposal rose to declare it "utter rubbish."
Today, it's considered a basic scientific fact.
In the four and a half decades since "Black Hole Explosions?" Hawking has continued to publish research that picks away at the underpinnings of the universe — including ideas that attack his own earlier contributions. (See, for example, the startling 2014 headline in Nature, "Stephen Hawking: There Are No Black Holes'".)
Hawking has become most famous in his later career as a science communicator. He has followed up his 1988 classic "A Brief History of Time" with 10 more works of popular science and a memoir, titled "My Brief History" (Random House, 2013).
It's impossible to talk about Hawking's enormous contributions to the human understanding of the universe without acknowledging the context of his long-declining health. Hawking's two seminal contributions to physics came during the same period in which he transformed from a young person who was able to walk on his own to a man who was confined to a wheelchair, slurred his speech and was reliant on his wife to transcribe his thoughts.
ALS paralyzes the body, but — at least in Hawking's case — it doesn't damage the mind. And for that, Ferguson wrote, Hawking has long counted himself "supremely lucky."
"It was true in 1964, and it is today," Ferguson wrote, "that as far as Hawking is concerned, the less made of his physical problems, the better. I recognized in 1989, during interviews for my first book about him, that if I were to write about his scientific work and fail entirely to mention that doing such work possibly represented more of an achievement for him than it would for most people, that would have suited him fine."
Hawking has appeared most comfortable discussing disability in the context of his activism, which has been significant. In 1999, he joined a group of 12 prominent figures, including South African activist Desmond Tutu, in signing a charter calling on the world's governments to transform their relationships with their disabled populations and expand services that improve the lives of people with disabilities.
Hawking has also been a prominent defender of universal health care and the United Kingdom's National Health Service (NHS), going so far as to attack Conservative Party Health Secretary Jeremy Hunt in an August 2017 speech for insufficiently funding and supporting the program.
"I wouldn't be here without the NHS," Hawking said.
Hawking tends to get the most attention for his ideas on humanity's future when he comments on artificial intelligence or aliens. But the bulk of his pronouncements on the subject have been more down-to-earth: opposing wars, worrying that U.S. President Donald Trump's dismissal of climate change could damage the planet, and joining the global academic boycott of Israel.
Live Science wishes Hawking a very happy birthday and many more.
Originally published on Live Science.