The following is a guest post by our AP Physics and Advanced Pre-Calculus class instructor and tutor, Taryn.
“There is no more fulfilling way to spend our time on Earth than exploring the limits of the Universe by expanding the limits of our minds.”
March 14th of this year was a devastating day for modern physics and for the world at large. After an illustrious career, a long life, and discoveries that fundamentally altered our understanding of the universe, the physics world lost one of its greatest heroes: Stephen Hawking.
Of course, there are many, many brilliant physicists around the world today still deeply immersed in the inscrutable mysteries that Hawking devoted his life to untangling. And while to the greater public, Hawking was probably the most famous scientist of our lifetime, as Hawking once put it, “to my colleagues, I’m just another physicist.” Still, it’s not an understatement to say that losing Hawking is an incredible blow to the progress of modern physics.
Why was he great?
So what exactly made this man so great and made his death, at a mature age and after a fulfilling life, so keenly and despondently felt around the world?
Part of what made him such an icon is that he was so distinctive and recognizable. Hawking once explained his fame as “partly because I fit the stereotype of a disabled genius. I can’t disguise myself with a wig and dark glasses–the wheelchair gives me away.”
Since the actual science of Hawking is very complex and esoteric, what obviously hits home to us with the most immediacy is Hawking’s triumph over a debilitating disease which predicted a life expectancy of a couple years when he was first diagnosed. But it would be a grave mistake if this was what Hawking was remembered for. Though it’s hard not to be awed by what Hawking accomplished in the face of what appears to be unimaginable adversity, his greatness is simply what he accomplished, not the fact that he accomplished it “in the face of adversity.”
Hawking himself didn’t view his disease as an impediment to his research–quite the opposite in fact. In his autobiography, he explained, “My disability has not been a serious handicap in my scientific work. In fact, in some ways I guess it has been an asset: I haven’t had to lecture or teach undergraduates, and I haven’t had to sit on tedious and time-consuming committees. So I have been able to devote myself completely to research.”
Einstein is considered the “father of modern physics” because his groundbreaking discoveries opened up two totally new fields of physics: relativity (the physics of the very, very large) and quantum physics (the physics of the very, very small). Since then, almost all of cutting-edge physics research has been devoted to developing our understanding of these two fields. But after over a century of research and an enormous amount of progress in our understanding of these fields down to the tiniest of details, there remains one big unsolved problem: the rules that we apply in the relativity-governed world of the very large don’t work in the quantum-governed world of the very small, and vice versa.
In 1974, Hawking made a discovery that is still one of our best examples of synthesizing the tiny world of quantum physics with physics at the cosmic scale. This discovery continues to inspire physicists in the pursuit of one grand unified theory of physics: one single, elegant theory that could explain our entire universe.
The quote at the top of this article was a comment Hawking made upon his receipt of the 2013 Special Breakthrough Prize in Fundamental Physics (the most lucrative academic prize in the world, with recipients taking home more than twice what a Nobel Prize Laureate is awarded). This was primarily awarded for that groundbreaking 1974 discovery: Hawking radiation.
What is Hawking radiation?
Step 1: What is a black hole?
To understand Hawking radiation, we have to start with probably Hawking’s favorite object in the universe: black holes.
We all have some kind of image in our heads of a black hole. We know that it’s an awe-inspiring thing that’s both mysterious and enormous, and maybe even a little frightening.
In short, a black hole is the final stage in the life of a supermassive star. Our Sun, for example, is never going to become a black hole because it’s an incredibly ordinary star of very average size. Billions of years from now when it reaches the end of its life, it will settle down as a quiet white dwarf, about the size of our Earth.
Stars, just like living things, have a life cycle. Our Sun is about in the middle of its life right now, which means that, deep in the core of our Sun, it’s furiously burning its vast stores of hydrogen into helium. This nuclear process results in the immense radiation that warms our little planet millions of miles away. When it runs out of hydrogen and its core is all helium, it will start burning the helium into iron.
When the helium runs out, though, that’s it. You can’t burn iron because that nuclear process requires an input of energy (endothermic), whereas the hydrogen and helium burning gave off energy (exothermic). So that means its fuel is totally exhausted and there’s nothing to push back against the gravitational attraction of its incredible mass, so it will immediately collapse. Inside the core, all of the atoms will get squished next to each other until it’s just an incoherent mess of electrons, protons, and neutrons. Fortunately, electrons are extremely hard to push together too closely. So if the star is just of average mass, like our Sun, the “repulsion” of the electrons will be strong enough to stop the gravitational collapse at some point, and the star will settle down as a tiny white dwarf.
But for stars that are very, very massive and very dense, the force of the gravitational collapse after the fuel has been tapped is so intense that absolutely nothing can fight against it. The star (in a matter of seconds, actually) just collapses and collapses and collapses, with nothing to stop it, until it becomes the only thing it can become: a singularity–the core of a newly-created black hole.
To be honest, we don’t really understand anything about what happens at a singularity. All we know is that it’s a point in space of infinite density. The rest of the black hole outside of this singularity is just empty space. The “edge” of a black hole is a surface called the “event horizon.” This isn’t a real physical “surface” like the surface of our Earth–it’s just an imaginary line drawn around the singularity at a certain radius away. The event horizon is that magical point in space where, if you’re inside of it, even if you were travelling at the speed of light and pointing your rocket ship directly outwards, there’s no way you could move fast enough to escape the black hole’s gravitational pull. But, if you’re just ever so slightly outside the event horizon, you could get away (as long as you’re travelling at the speed of light). Nothing that crosses the event horizon can ever escape — it will fall immediately into the singularity and be lost forever.
Back to Hawking radiation
So with that more fleshed-out picture of a black hole, we can go on to the totally shocking discovery Stephen Hawking made in 1974: Hawking radiation.
When we think about the great vacuum of space out there in our universe, we naturally imagine that this vacuum (by definition) is total emptiness–no particles, no movement, nothing. But according to quantum physics, this isn’t an accurate picture. Quantum physics theory tells us that there’s actually little fluctuations happening all the time. For example, an electron and a positron are spontaneously created and then immediately annihilate each other, and it’s as though nothing happened. That “it’s as though nothing happened” part is what makes a vacuum a vacuum. The fluctuations are short-lived with no major long-term consequences, so to the untrained eye it still looks like an unchanging mass of nothingness.
But at the edge of a black hole, the vacuum becomes unstable and, rather than the fluctuations just coming and going without any lasting effect, they can actually produce particles. Instead of the electron and positron coming together and re-annihilating before anyone is the wiser, one particle falls across the event horizon and falls into the singularity, and the other manages to escape. This escaped particle manifests as radiation from the black hole.
This radiation is Hawking radiation. It’s completely a consequence of quantum physics and the quantum fluctuations of our vacuum that that theory predicts, and yet it has very real and very devastating effects on a super large-scale object– a black hole. Every time a black hole radiates one of these particles, it’s losing some of its mass. As it loses its mass, it shrinks. And, to make matters worse, the smaller the black hole gets the higher energy particles it emits in Hawking radiation, so the whole process exponentially revs up towards the end. So there you have it: the first meeting point of quantum physics and relativity–the fluctuations produced in the tiny-scale world of quantum physics can ultimately kill a black hole.
This shocking discovery spurned decades of research and continues to inspire physicists all over the world in the pursuit of that elusive grand unified theory of physics.
Beyond Hawking radiation
Though Hawking radiation is one of Hawking’s greatest scientific contributions, it’s not the only reason he was a beloved popular icon and will be missed.
He lived a long and interesting life, visiting every continent in the world, including Antarctica, but excluding Australia. He visited Japan 6 times and the Soviet Union 7 times, including a memorable first trip where he was persuaded to smuggle in Russian-language bibles and was caught by the authorities and detained on his way out. He was the anchor for the Paralympic Games in London in 2012. And of course, he’s also well-known for his bestseller A Brief History of Time, which gave a thorough picture of how far we’ve come in our understanding of the workings of the universe, and, most importantly, did it in a way that was accessible and understandable by the general public. It sold over 10 million copies worldwide, was translated into 40 languages, and placed on The New York Times bestseller list for 147 weeks.
Hawking also had a very memorable and engaging personality. He combined a feverish passion for physics with a very irreverent sense of humor. To that end, he loved making “cosmology bets” with his colleagues. And not just off-the-cuff challenges, but formal wagers with an agreement, including terms and awards, typed up and signed by participants and witnesses. One of the most memorable was a bet with his colleague Kip Thorne when they were at Caltech together. Hawking bet that the binary star system Cyngus X-1 did not have a black hole in it. If he won (though he says he was actually hoping to lose), Thorne would have to buy him a 4-year subscription to Private Eye magazine. If he lost, he’d have to buy Thorne a 1-year subscription to Penthouse. When Hawking did lose, he honored his bet admirably, much to the chagrin of Thorne’s wife.
The passing of a genius and major icon like Hawking is a remorseful occasion, and will be felt as too early no matter when it happens. But Hawking himself was grateful for all that he was able to accomplish in his not-so-brief 76 years.
At the close of his autobiography, he ends thus:
“It has been a glorious time to be alive and doing research in theoretical physics. I’m happy if I have added something to our understanding of the universe.”
If you want to learn more about Hawking and the science he devoted his life to, check out his autobiography My Brief History or his bestseller A Brief History of Time.