Renowned physicist Stephen Hawking died peacefully today (March 14) at age 76 at his home in Cambridge, England.
Hawking will be remembered by many as a cultural force — the brilliant thinker who wrote "A Brief History of Time" and played himself on "Star Trek: The Next Generation," "The Simpsons" and "The Big Bang Theory." He published more than 10 popular books (including a series of children's books, co-written with his daughter, Lucy) and was the subject of more than 15 documentary films and biopics. He once threw a party for time travelers (which nobody attended). He did it all while living with the incurable degenerative disease amyotrophic lateral sclerosis (ALS), which put him in a wheelchair for much of his life.
But beyond his inspirational biography and pop-culture bona fides, Hawking will also be remembered as perhaps the most important physicist of the second half of the 20th century. Hawking's work on black holes, quantum mechanics and the origins of the universe advanced the theories of previous thinkers like Albert Einstein and Werner Heisenberg, providing the most comprehensive explanation for the behavior of the cosmos to date.
"My goal is simple," Hawking was quoted as saying in a 1989 biography. "It is a complete understanding of the universe, why it is as it is and why it exists at all." [Stephen Hawking's Most Intriguing Quotes on Humanity, Aliens and Women]
Here, we recall some of Hawking's biggest theories about life, the universe and everything.
Before the Big Bang
Hawking began sharing his vision of the universe in 1966 while studying physics at the University of Cambridge, when he turned in a doctoral thesis on the origin of the universe.
At the time, scientists understood the universe to be expanding, though they did not know why. Hawking proposed the first description of what we now know as the Big Bang: somewhere, at the earliest edges of time, the universe began as a single point of infinitely small, massively dense space-time — a "singularity," as Hawking described it. From this single point, our entire expanding universe burst forth, bringing with it all the laws of time and physics we understand today, Hawking said.
"There is a singularity in our past, which constitutes, in some sense, a beginning to the universe," Hawking eventually wrote in the preface of his first book, "The Large Scale Structure of Space-Time" (1973).
Over the years following his graduation from the University of Cambridge, Hawking attempted to describe that universal starting point mathematically. By applying a complex model inspired by Einstein's theory of general relativity, Hawking and his colleague Roger Penrose created the first of several singularity theorems, which described the cosmological conditions required for a singularity in space-time to exist.
Escaping a black hole
Hawking had a fascination with the places where physics seemingly broke down. This included, famously, his study of black holes — stars that have collapsed into themselves, creating a vortex so gravitationally intense that not even light can escape them.
While working at the Institute of Astronomy at the University of Cambridge, Hawking began studying black holes through the lens of thermodynamics. In 1973, he published (along with several colleagues) a paper detailing the four laws of black hole mechanics, describing the warped physics of the mysterious celestial bodies through a series of complex equations.
Hawking continued studying black holes and, soon after, had the eureka moment for which he is best known.
At the time, it was thought that nothing could escape the strong gravitational pull of black holes. In 1974, Hawking showed that, under certain conditions, black holes actually could create and emit subatomic particles, eventually dissolving and finally exploding in an intense burst of energy. In other words: Black holes were not completely black after all.
This stream of energy radiated by black holes was later named Hawking radiation, after he described it in a 1974 paper titled "Black Hole Explosions?"
A theory of everything
The discovery of Hawking radiation changed the way researchers understand the universe. By attempting to knit together the laws of gravity, thermodynamics, quantum mechanics and relativity, the study of black hole radiation also pointed to the existence of a so-called "theory of everything" — a single, unified theory of physics that described the behavior of the universe.
"Black hole radiation raises serious puzzles we are still working very hard to understand," Sean Carroll, a theoretical physicist at the California Institute of Technology, told New Scientist. "It's fair to say that Hawking radiation is the single biggest clue we have to the ultimate reconciliation of quantum mechanics and gravity, arguably the greatest challenge facing theoretical physics today."
Hawking continued publishing papers for the following four and a half decades, but became most famous in his later career as a science communicator. He has drawn attention lately for his concerns about humanity's future in regards to artificial intelligence, his vocal opposition of wars and his insistence that leaders like U.S. President Donald Trump take the threats of climate change more seriously. A few weeks ago, he told celebrity physicist Neil deGrasse Tyson his theory about what happened before the Big Bang.
Still, Hawking's most lasting quotes may be about the importance of communication itself.
"Mankind's greatest achievements have come about by talking, and its greatest failures by not talking," Hawking said. "It doesn't have to be like this. Our greatest hopes could become reality in the future. With the technology at our disposal, the possibilities are unbounded. All we need to do is make sure we keep talking."
Originally published on Live Science.