Hawking’s life — as well as theory — have profound implications


It’s bloody awful.” That’s how a friend of mine recently referred to Stephen Hawking’s best-selling book, “A Brief History of Time.” Having perused it myself, I have to say that in some ways he’s right. Picking up that book with the hopes of actually understanding the physics at play is like being asked to solve a crossword puzzle in a foreign language.

Then why did Hawking, the famed theoretical physicist who passed away two weeks ago at the age of 76, sell 10 million copies? The cynical answer might be coffee table pretense. But I would submit that the public success of the book had more to do with the unique nature of Professor Hawking and his discoveries.

At the age of 21, Hawking was diagnosed with ALS — also known as Lou Gehrig’s disease — a debilitating neuro-degenerative disorder that usually leads to fatality within a few years of its diagnosis. He survived another 56 years, a total mystery to ALS experts, passing away on the same date as Einstein’s birthday. (Hawking was born on the 300th anniversary of Galileo’s death.) Readers familiar with his appearance are aware of how the disease ravaged his body, which for many years served mainly as an incubator for his expansive intellect.

His communication skills were so limited that conversing would have been terribly frustrating, especially for someone with such an active, creative mind. The inability to write would be devastating for most theoretical physicists, as the subject is steeped in mathematics the logical arguments of which are scratched into a cascade of steps coded in mathematical notation. For Hawking it meant adapting his methods to think in new ways, perhaps more geometrically, which allowed him to shortcut mathematical proofs.


Beethoven lost his hearing, Monet his sight, Hawking lost much more, and yet still his spirit was indomitable. He was famously quoted as saying, “My advice to other disabled people would be, concentrate on things your disability doesn’t prevent you doing well, and don’t regret the things it interferes with. Don’t be disabled in spirit as well as physically.”

But it was more than just his singular story that captured the imagination of the public. I think it had just as much to do with the nature of his discoveries as it did with his personal story. While his proofs of singularity theorems with Sir Roger Penrose were of monumental importance, it was his work on black hole evaporation that catapulted him into the pantheon of science.

Shortly after Einstein’s proposal of the theory of general relativity, Karl Schwarchild showed there existed very strange exact solutions to Einstein’s equations that we now call black holes. A few decades later, in 1956, David Finklestein showed that these solutions represented objects from which nothing can escape. There exists a sphere around the black hole that acts as a one way barrier; everything goes in, but nothing, not even light, comes out. The radius of that sphere is called the “Schwarzchild radius” and that invisible sphere is called the “horizon.”

Remarkably, if an astronaut falls thru the horizon, nothing odd happens; they would never know that past that point their fate is sealed. In fact, if they try to forcibly get out, say by firing rockets, it will only serve to hasten their demise, accelerating them in the opposite direction toward the center where the gravitational forces are so strong that atoms get torn apart, as does the fabric of space-time itself.

Enter Hawking, who in 1975 showed that black holes are not truly black. In his landmark paper he showed that black holes actually slowly evaporate. At first glance, this seems more than odd, as we usually attribute evaporation to an object with which we can associate a temperature. But Hawking’s proposal implies that when a black hole forms — the inevitable final state of stars with sufficiently large masses — an observer outside the horizon sees particles bubbling out. We now call this “Hawking radiation.” While this is a remarkable result, it seems like an arcane piece of theoretical physics with little to no implications. It’s not clear that we will ever be able to verify black hole evaporation. (This explains why Hawking has yet to be considered for a Nobel Prize, which requires empirical verification of a proposed theory.)

While the evidence for black holes is now overwhelming, including the measurement of the gravity waves produced in the collision of two black holes in 2016, the Hawking radiation is imperceptible. So why is Hawking’s result so important? The answer lies in what Hawking radiation implies about information.

A fundamental tenet of physics is that information is conserved. If you throw an encyclopedia into the sun, it will burn up, seemingly leading to information loss. But the laws of physics imply the information is indeed there — it’s just encoded in the light emitted by the sun. In principle, with a very careful measurement of the light, we could reconstruct the entire encyclopedia. This conservation of information has profound implications. In particular, information conservation leads to a deterministic universe whereby, given the state of a system at some fixed time, we can predict its state at a later time.

This result is at the heart of the freewill debate. If your future can be determined by your present state, what happens to the notion of choice? Now suppose we throw the encyclopedia into a black hole. If we wait long enough it will completely evaporate. A corollary to Hawking’s result is that the radiation cannot encode the information. In this case, we are forced to conclude that black holes can erase information! The problem is that no one knows how to take the well-known laws of physics, most notably quantum mechanics, and make it consistent with information loss. These issues touch upon the very nature of the human condition and elevate Hawking’s result beyond those of other scientific discoveries.

Thus what makes Hawking such a compelling human is the confluence of a remarkable story of hope and perseverance, combined with a profound discovery that has ramifications regarding the nature of our existence. We may learn as much from the way he lived his life, with appreciation and gratitude despite remarkable disabilities, than we can from the light he shed on the nature of our existence. As such, maybe his book should be, and is, read as much as poetry as science.

Ira Rothstein is a professor of physics at Carnegie Mellon University.

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