Stephen Hawking, who sadly passed away in 2018, has repeatedly joked that he could win a Nobel Prize if the Large Hadron Collider produced tiny black holes. For example, here is a recording of a lecture he gave in 2016:
“Some of the collisions can lead to black micro-holes. These would emit particles in a pattern that would be easy to see. Maybe I’ll get a Nobel Prize after all. “
British physicist and science journalist Phillip Ball, who attended the 2016 talk, commented:
“I was impressed by how unusual it was for a scientist to state publicly that his work justified a Nobel Prize … [It] gives an indication of the elusive character of the physicist: shamelessly self-promoting to the point of arrogance and regardless of what others might think. “
I heard Hawking say pretty much the same thing in a public lecture in Stockholm a year earlier. But I had a very different reaction. I did not consider his comment arrogant. I thought he was explaining something few people knew about. And I thought he was right that if the Large Hadron Collider had seen those tiny black holes crumble, it would almost certainly have won a Nobel Prize. But I also thought this wasn’t going to happen. It was much more likely that he won a Nobel Prize for something else. And he almost did it.
This is exactly why Hawking could have won the Nobel Prize, and should he have won it? That’s what we’ll talk about today.
In 1974, Stephen Hawking published a calculation that showed that black holes are not perfectly black, but that they emit thermal radiation. This radiation is now referred to as “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 bigger the black hole, the lower its temperature and the more difficult it is to measure the radiation. For the astrophysical black holes known to us, the temperature is far too small to be measurable. The chances that he would ever win a Nobel Prize for black hole evaporation seemed very slim.
In the late 1990s, however, the idea arose that the Large Hadron Collider could create tiny black holes from particle collisions. This is only possible if the universe has additional spatial dimensions, not just the three that we know, but at least five. These additional spatial dimensions would have to be rolled up to small radii, otherwise we would have already seen them.
Additional dimensions rolled up. Have we never heard that before? Yes, because string theorists are always talking about coiled dimensions. Indeed, string theory was the main motivation to consider this hypothesis of additional dimensions of space. I have to warn you, however, that string theory is NOT telling you that these additional dimensions should be of a size that the Large Hadron Collider could test. Even if they exist, they might be way too small for that.
However, just assuming the extra dimensions are the right size, the Large Hadron Collider could have created tiny black holes. And since they would have been so small, they would have been really, really hot. So hot, in fact, that they would expire pretty much instantly. To be precise, they would decay in about ten to twenty-three seconds long before they could reach a detector.
However, according to Hawking’s calculation, these tiny black holes should decay according to a very specific pattern. Most of all, according to Hawking, black holes can disintegrate into just about any other particle. And there is no other particle decay that looks like this. So it would have been easy to see black hole decays in the data. If they had happened. They have not. But if they did, Hawking would almost certainly have won a Nobel Prize.
However, the idea that the Large Hadron Collider would create tiny black holes was never very plausible. That’s because there was no reason why the additional dimensions, if they existed to begin with, should be exactly the right size to make this production possible. The only reason physicists thought this would be the case was because of an argument of mathematical beauty called “naturalness.” I explained the problems with this argument in a previous video. More information can be found here.
So, yeah, I don’t think tiny black holes on the Large Hadron Collider were Hawking’s best shot in a Nobel Prize.
Are there other ways you can see black holes evaporate? Not really. Without these coiled up extra dimensions that don’t seem to exist, we can’t make black holes ourselves. Without additional dimensions, the energy density that we would have to achieve to create black holes is far beyond our technological limits. And the black holes that are created in natural processes are too large and then too cold to observe Hawking radiation.
One thing you can * do is simulate black holes with superfluids. This was done by Jeff Steinhauer’s group 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 which no light can escape. In a superfluid, you don’t capture light, but sound waves. This is possible because the speed of sound in the superfluid depends on the density of the fluid. And since you can control this density experimentally, you can control the speed of sound.
Then when you let the liquid flow, there are regions from which the sound waves cannot escape because they are simply too slow. It’s like trying to swim away from a waterfall. There is a limit that you just can’t swim over fast enough to get away. This border resembles a black hole horizon. And the superfluid has such a limit, not for swimmers, but for sound waves.
You can do this with a regular liquid as well, but you need the super-liquid in order for the sound to have the correct quantum properties, as is the case with Hawking’s calculation. And in a series of really neat experiments, Steinhauer’s group showed that those sound waves in the superfluid actually have the properties that Hawking predicted. This is because Hawking’s calculation applies to the superfluid as well as to real black holes.
Could Hawking have won a Nobel Prize for it? I do not believe that. That’s because it’s cool to mimic a black hole with a superfluid, but of course it’s not the reality. These experiments are a kind of quantum simulation, which means that they show that Hawking’s calculation is correct. However, the measurements on superfluids cannot show that Hawking’s prediction for real black holes is correct.
To be honest, it never seemed likely that Hawking would win a Nobel Prize in Hawking Radiation. It’s just too difficult to measure. But that wasn’t the only thing Hawking did in his career.
Before working on black hole evaporation, Hawking worked with Penrose on the singularity theorems. Penrose’s theorem showed that contrary to what most physicists at the time believed, black holes are a fairly inevitable consequence of star collapse. Physicists previously thought that black holes were mathematical curiosities that would not arise in reality. Black holes were taken seriously only because of the singularity theorems. Eventually, astronomers went looking 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 also inevitable, unless general relativity somehow breaks down. And that is an absolutely amazing insight into the origin of our universe.
I made a video about the history of the black holes two years ago where I said the singularity theorems are worth a Nobel Prize. In fact, Penrose was one of the recipients of the 2020 Nobel Prize in Physics. If Hawking had not died two years earlier, he and Penrose would have won the Nobel Prize. Or maybe the Nobel Prize Committee was just waiting for him to die so they wouldn’t have to think about how to separate Hawking’s work from Penrose’s? We will never know.
Does it matter that Hawking didn’t win a Nobel Prize? Personally, I see the Nobel Prize primarily as an opportunity to celebrate scientific discoveries. The people we think could win this award deserve it with or without an extra medal. And Hawking didn’t need a Nobel Prize, he’ll be remembered without him.