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They correctly predicted a Nobel Prize winning discovery. And no one cared.
🕑 Added 2024-01-06 16:37:39 +0000 UTC[This is a transcript with links to references.]
Imagine you make a prediction for a discovery that wins the Nobel Prize. Your prediction turns out to be correct, and no one cares. This is what happened to these two physicists.
I am talking about a prediction from 2009, when Mikhail Shaposhnikov and Christof Wetterich calculated the mass of the Higgs-boson. The Higgs-boson is one of the 25 elementary particles that physicists collect in the standard model of particle physics. It was the last of those particles to be discovered, in 2012 at the Large Hadron Collider. In 2013, a Nobel Prize was awarded for its discovery. While most physicists were quite confident that the Higgs exists, they didn’t know what the mass would be. Or most of them didn’t know anyway.
They did have some clues though. First, they had looked for the Higgs-boson already with the TeVatron collider at Fermilab in the United States. This collider reached a lower energy than the LHC, and didn’t see the Higgs-boson. This allowed physicists to put a lower bound on the mass of the Higgs, which was at around 115 GeV.
GeV stands for Giga electron Volt and is a common unit of energy in particle physics. Yes, it’s actually a unit of energy, not mass, but E equals m c squared, so it’s kinda the same really. To make sense of this unit, it’s helpful to know that the mass of a proton is about 1 GeV and 115 GeV is about the mass of a medium sized atom. In every-day terms, those are really really tiny amounts of masses.
So particle physicists knew that the mass of the Higgs boson needs to be higher than 115 GeV, otherwise Fermilab would have seen it. They also knew that something had to happen about 1000 GeV at the latest, because if one just uses the standard model without the Higgs, that stops working at some point. The maths then spits out probabilities larger than one, which really shouldn’t happen.
Leaving aside typos, that the standard model stops working at around 1000 GeV is why the LHC was such a good investment. Physicists knew that something new had to happen there. If it hadn’t been the higgs, something else would have had to show up. Or otherwise, we might indeed have seen probabilities larger than one, which would also have been interesting I guess.
They had yet another upper bound that was at about 180 GeV. It's called the “triviality bound”. The argument is that if the mass of the Higgs-boson was any larger than this bound, then the theory for the Higgs either breaks down or it can’t create the masses for the other particles.
So we have 115 from below and 180 GeV from above, and a lots of ifs and buts because these bounds depend on the type of theory you assume the Higgs obeys and so on. But other than that, they didn’t know what the mass was.
Now these two physicists came and said, whatever the mass of the Higgs boson is, it must still allow for gravity to become a quantum theory. You see, all the other fundamental forces of nature have quantum properties. But gravity is the odd one out by being entirely non-quantum. If you’ve ever been the only quantum theory at a party, you’ll know how awkward this is.
Most physicists therefore believe that gravity should also have quantum properties, it’s just that we haven’t yet found the right theory. This missing theory in which gravity has quantum properties is called “quantum gravity”. They think it must exist not just because it'd be awkward if gravity was different, but more importantly because without a theory of quantum gravity there are some situations in nature for which we just don’t know what happens, such as at the Big Bang or inside black holes. We need this theory of quantum gravity to figure out what goes on there, and we’d all like to know. Don’t we? Or is it just me who wants to know what’s inside a black hole?
Physicists have tried to turn gravity into a quantum gravity the same way they turned electrodynamics into quantum electrodynamics. This was done by Richard Feynman and Bryce DeWitt already in the 1960s. Unfortunately, it didn’t work. When they used the known techniques for gravity that resulted in a theory that, when extrapolated to high energies, gave them an infinite number of infinities. The so-obtained quantization of gravity was thought to be incurably sick, and was abandoned.
But -- Plot Twist! -- in 1978, Steven Weinberg said that might have been a little too fast. In case the name doesn’t ring a bell, Steven Weinberg won the Nobel Prize just a year later, in 1979. His argument about quantum gravity was quite simple, really. He said that this approximation to higher energies is just wrong. A more sophisticated method has to be used for the extrapolation and then there are no infinites. If one does the calculation correctly, gravity is safely quantum. The idea became to be known as “asymptotically safe gravity”.
The reason you have probably never heard of it is that everyone, including Weinberg himself, thought it was a disappointing solution. Because, you see, physicists hoped that solving this big problem of quantum gravity would require something new. Strings or atoms of space or panpsychism or something, anything exciting. But saying that it’s just a difficult extrapolation, that’s really lame isn’t it.
And so, after Weinberg published his paper in 1978, no one thought much about it, until in the early 1990s, Christof Wetterich and Martin Reuter worked out most of the mathematics.
And this brings me back to the mass of the Higgs boson. Because the mass of the Higgs boson changes how this extrapolation to higher energies in quantum gravity works. And if you use the mathematics that had been developed by Wetterich and Reuter, it turns out that if the mass of the Higgs is either too large or too small, then gravity is no longer safe, and the theory breaks down again.
So these two guys I mentioned in the beginning, Shaposhnikov and Wetterich, they went and calculated this window where quantum gravity works properly, and came up with 126 GeV plus minus 2.2. The measured value of the mass of the Higgs boson is 125 point 3 5 plus minus zero point 15 GeV. Their prediction was spot on.
Now you could say that it wasn’t a terribly precise prediction what with this plus minus 2 point something, but I think one shouldn’t blame them too much. Because this is a difficult calculation which depends on the values of the masses of all the other particles, and each of them has an uncertainty. If one were to redo this calculation today, I suspect the uncertainty would be smaller.
So they correctly predicted the mass of the Higgs boson and no one cared. But what does this all mean? It means that quantum gravity just might not be that big mysterious problem that physicists made it out to be. It really might just be a matter of doing an extrapolation correctly. And that is so boring a solution that most physicists would rather ignore it.
The quiz for this video is here.
Comments
On your first question about the Higgs, the answer is NO. Please read my comment somewhere below yours. On your second: much too soon to tell.
The Higgs' boson was the missing particle in the Standard Model of quantum mechanics. I've never heard that it was anything less or more than that. The corresponding and all-pervading Higgs' field gives some of its mass to some other particles (bosons) immersed in it, so in this way it contributes to gravity, but does not lead directly to quantum gravity, with its elemental particle, the hypothetical graviton, as far as I know: https://en.wikipedia.org/wiki/Higgs_mechanism
Question for Sabine (or other willing to answer) from someone who only understands the principles being discussed (so I just hope I have formulated questions correctly) the constraints that were used to predict Higgs boson mass, as described in paper discussed by Sabine, relate to possible quantum gravity, So is the fact that the prediction of the mass was so accurate, a valid argument for existence of quantum gravity? And another related question: Does the mass of a Higgs boson affects the outcome of unifying quantum forces with stochastic gravity as recent proposed by Oppenheimer (and also discussed by Sabine). Which of these two approaches to unify quantum forces with gravity is most promising?
