[This is a transcript of the video embedded below.]
[embedded content]Stephen Hawking, who sadly passed away in 2018, has repeatedly joked that he might get a Nobel Prize if the Large Hadron Collider produces tiny black holes. For example, here is a recording of a lecture he gave in 2016:
“Some of the collisions might create micro black holes. These would radiate particles in a pattern that would be easy to recognize. So I might get a Nobel Prize after all.”
The British physicist and science writer Phillip Ball, who attended this 2016 lecture, commented:
“I was struck by how unusual it was for a scientist to state publicly that their work warranted a Nobel… [It] gives a clue to the physicist’s elusive character: shamelessly self-promoting to the point of arrogance, and heedless of what others might think.”
I heard Hawking say pretty much exactly the same thing in a public lecture a year earlier in Stockholm. But I had an entirely different reaction. I didn’t think of his comment as arrogant. I thought he was explaining something which few people knew about. And I thought he was right in that, if the Large Hadron Collider would have seen these tiny black holes decay, he almost certainly would have gotten a Nobel Prize. But I also thought that this was not going to happen. He was much more likely to win a Nobel Prize for something else. And he almost did.
Just exactly what might Hawking have won the Nobel Prize for, and should he have won it? That’s what we will talk about today.
In nineteen-seventy-four, Stephen Hawking published a calculation that showed black holes are not perfectly black, but they emit thermal radiation. This radiation is now called “Hawking radiation”. Hawking’s calculation shows that the temperature of a black hole is inversely proportional to the mass of the black hole. This means, the larger the black hole, the smaller its temperature, and the harder it is to measure the radiation. For the astrophysical black holes that we know of, the temperature is way, way too small to be measurable. So, the chances of him ever winning a Nobel Prize for black hole evaporation seemed very small.
But, in the late nineteen-nineties, the idea came up that tiny black holes might be produced in particle collisions at the Large Hadron Collider. This is only possible if the universe has additional dimensions of space, so not just the three that we know of, but at least five. These additional dimensions of space would have to be curled up to small radii, because otherwise we would already have seen them.
Curled up extra dimensions. Haven’t we heard that before? Yes, because string theorists talk about curled up dimensions all the time. And indeed, string theory was the major motivation to consider this hypothesis of extra dimensions of space. However, I have to warn you that string theory does NOT tell you these extra dimensions should have a size that the Large Hadron Collider could probe. Even if they exist, they might be much too small for that.
Nevertheless, if you just assume that the extra dimensions have the right size, then the Large Hadron Collider could have produced tiny black holes. And since they would have been so small, they would have been really, really hot. So hot, indeed, they’d decay pretty much immediately. To be precise, they’d decay in a time of about ten to the minus twenty-three seconds, long before they’d reach a detector.
But according to Hawking’s calculation, the decay of these tiny black holes should proceed by a very specific pattern. Most importantly, according to Hawking, black holes can decay into pretty much any other particle. And there is no other particle decay which looks like this. So, it would have been easy to see black hole decays in the data. If they had happened. They did not. But if they had, it would almost certainly have gotten Hawking a Nobel Prize.
However, the idea that the Large Hadron Collider would produce tiny black holes was never very plausible. That’s because there was no reason the extra dimensions, in case they exist to begin with, should have just the right size for this production to be possible. The only reason physicists thought this would be the case was an argument from mathematical beauty called “naturalness”. I have explained the problems with this argument in an earlier video, so check this out for more.
So, yeah, I don’t think tiny black holes at the Large Hadron Collider was Hawking’s best shot at a Nobel Prize.
Are there other ways you could see black holes evaporate? Not really. Without these curled up extra dimensions, which do not seem to exist, we can’t make black holes ourselves. Without extra dimensions, the energy density that we’d have to reach to make black holes is way beyond our technological limitations. And the black holes that are produced in natural processes are too large, and then too cold to observe Hawking radiation.
One thing you *can do, though, is simulating black holes with superfluids. This has been done by the group of Jeff Steinhauer in Israel. The idea is that you can use a superfluid to mimic the horizon of a black hole. If you remember, the horizon of a black hole is a boundary in space, from inside of which light cannot escape. In a superfluid, one does not trap light, but one traps sound waves instead. One can do this because the speed of sound in the superfluid depends on the density of the fluid. And since one can experimentally control this density, one can control the speed of sound.
If one then makes the fluid flow, there’ll be regions from within which the sound waves cannot escape because they’re just too slow. It’s like you’re trying to swim away from a waterfall. There’s a boundary beyond which you just can’t swim fast enough to get away. That boundary is much like a black hole horizon. And the superfluid has such a boundary, not for swimmers, but for sound waves.
You can also do this with a normal fluid, but you need the superfluid so that the sound has the right quantum properties, as it does in Hawking’s calculation. And in a series of really neat experiments, Steinhauer’s group has shown that these sound waves in the superfluid indeed have the properties that Hawking predicted. That’s because Hawking’s calculation applies to the superfluid in just exactly the same way it applies to real black holes.
Could Hawking have won a Nobel Prize for this? I don’t think so. That’s because mimicking a black hole with a superfluid is cool, but of course it’s not the real thing. These experiments are a type of quantum simulation, which means they demonstrate that Hawking’s calculation is correct. But the measurements on superfluids cannot demonstrate that Hawking’s prediction is correct for real black holes.
So, in all fairness, it never seemed likely Hawking would win a Nobel Prize for Hawking radiation. It’s just too hard to measure. But that wasn’t the only thing Hawking did in his career.
Before he worked on black hole evaporation, Hawking worked with Penrose on the singularity theorems. Penrose’s theorem showed that, in contrast to what most physicists believed at the time, black holes are a pretty much unavoidable consequence of stellar collapse. Before that, physicists thought black holes are mathematical curiosities that would not be produced in reality. It was only because of the singularity theorems that black holes began to be taken seriously. Eventually astronomers looked for them, and now we have solid experimental evidence that black holes exist. Hawking applied the same method to the early universe to show that the Big Bang singularity is likewise unavoidable, unless General Relativity somehow breaks down. And that is an absolutely amazing insight about the origin of our universe.
I made a video about the history of black holes two years ago in which I said that the singularity theorems are worth a Nobel Prize. And indeed, Penrose was one of the recipients of the 2020 Nobel Prize in physics. If Hawking had not died two years earlier, I believe he would have won the Nobel Prize together with Penrose. Or maybe the Nobel Prize committee just waited for him to die, so they wouldn’t have to think about just how to disentangle Hawking’s work from Penrose’s? We’ll never know.
Does it matter that Hawking did not win a Nobel Prize? Personally, I think of the Nobel Prize in the first line as an opportunity to celebrate scientific discoveries. The people who we think might win this prize are highly deserving with or without an additional medal. And Hawking didn’t need a Nobel Prize, he’ll be remembered without it.