What were Stephen Hawking's greatest contributions to science?

Famed physicist Stephen Hawking took gravity to its ultimate limits. In doing so, he made a number of significant advancements in our understanding of black holes, cosmology and quantum gravity. Plus, his contributions to the popularization of science cement his legacy for generations to come.

Hawking began his research career in the 1960s, well before his diagnosis with ALS (amyotrophic lateral sclerosis) led him to use a wheelchair and communicate through technology rather than his voice for most of his adult life. Although it's impossible to neatly summarize Hawking's 200-plus academic papers spanning more than four decades, he was deeply interested in the nature of gravity in extreme environments.

This started with the nature of singularities. Hawking took mathematician and physicist Roger Penrose's proof that singularities do exist in general relativity and extended it to the universe as a whole — proving that, in our models of the evolution of the cosmos, the Big Bang really did begin with a singularity, a point of infinite density.

Hawking then went on to explore black holes in more depth and arrived at his surprising conclusion that black holes aren't entirely black. By artfully combining general relativity with quantum mechanics, Hawking found that black holes emit a tiny amount of radiation, which means they can evaporate and disappear.

Hawking extended this idea to formulate a set of laws of black hole thermodynamics — versions of the familiar laws of thermodynamics but applied to black holes — suggesting a deep link between the nature of heat, energy and entropy with that of gravity.

Back in the cosmological realm, Hawking made major advances in understanding how inflation worked. Although Alan Guth originated the idea that the early universe underwent a period of exceptionally rapid expansion, it was Hawking who fleshed it out and made it a powerful, robust theory of the cosmos.

Besides inflation, Hawking spent a lot of time examining the earliest moments of the Big Bang. He was especially interested in the question of the "beginning” — did the universe have a beginning? Did it even make sense to ask that question? What did quantum mechanics have to say about that?

Gravity is the story of space-time, and Hawking spent many years investigating the deep relationship between space, time and quantum mechanics. For example, he worked a lot on wormholes — shortcuts through space-time — and probed whether they were physically possible. Realizing that wormholes could also be used as time machines, he proposed the chronology protection conjecture, which states that time travel into the past is forbidden because the past has already happened and cannot be changed.

Unlike many of his colleagues, Hawking was not particularly interested in the development of a theory of everything, an all-encompassing set of equations that could explain all of physics. Although he dabbled in aspects of string theory — a promising candidate for a theory of everything — he largely focused on the nature of gravity.

But his work on the quantum aspects of gravity reverberated throughout the entire community. For example, any theory of everything must be able to explain the riddle posed by Hawking radiation, or the slippery nature of cosmic inflation. Hawking's work opened a window into the unification of quantum mechanics and gravity — a goal that researchers are still trying to follow through with today.

To give some perspective on the magnitude of Hawking's work, consider the Nobel prize. The 2019 Nobel Prize in physics was awarded "for contributions to our understanding of the evolution of the universe and Earth's place in the cosmos," with one-half to James Peebles "for theoretical discoveries in physical cosmology." Hawking died in 2018, but if he had lived, his contributions to cosmology would have made him a contender for sharing the prize.

The 2020 Nobel Prize in physics was divided, with one-half awarded to Roger Penrose "for the discovery that black hole formation is a robust prediction of the general theory of relativity.” Hawking could have been a contender here too, for his insight into the fundamental nature of black holes.

Hawking had a decent chance of being in the running for two separate Nobel prizes, which definitely puts him in rare company.

Besides that prodigious scientific output, Hawking was a prolific science communicator. His 1988 book "A Brief History of Time" became an instant hit. It was many people's first introduction to quantum mechanics, gravity and cosmology. He became a cultural icon, with almost everybody able to recognize him and his computerized voice.

All of these contributions, without a doubt, make for a legacy worth remembering.