Theoretical Physics in the 21st Century

In 2021, March 8–13 will be “Sustainability Week” in Switzerland. During this week, students at all Swiss universities will come together to present their current work, promote a sustainable lifestyle and draw extra attention to changes that must be made at the institutional level. Anna Knörr, a third year Physics Bachelor student at ETH Zürich, is president of the Student Sustainability Commission. She and Professor Niklas Beisert invited me to give the Zurich Theoretical Physics Colloquium on Monday the 8th of March.

She proposed the modest title “Theoretical Physics in the 21st Century”. I like this idea because it would give me a chance to think about the ways in which theoretical physics is stuck, the ways it’s not, and the ways theoretical physics can help us adapt to the Anthropocene. So, I could blend ideas from these two talks:

Unsolved mysteries in fundamental physics, Cambridge University Physics Society, October 3, 2018.

Energy and the environment—what physicists can do, Perimeter Institute, April 17, 2013.

but update and improve the second one. I think it’ll be pretty easy for me to explain that the Anthropocene is about much more than global warming. The hard part is giving suggestions for “what physicists can do”.

Of course we can all resolve to fly less, etc.—but none of those suggestions take advantage of special skills that physicists have. Anna Knörr correctly noted that many theoretical physicists have trouble seeing what they can do to help our civilization adapt to the Anthropocene, since many of them are not good at designing better batteries, solar cells, fission or fusion reactors comes easily. To the extent that I’m a theoretical physicist I fit into this unhappy class. But I think there are more theoretical activities that can still be helpful! And I have more to say about this now than in 2013.

One lesson I may offer is this:

If something is not working, try something different.

This applies to the Anthropocene as a whole, all the social problems that afflict us, and also fundamental physics. I just ran into a talk that the famous particle physicist Sheldon Glashow gave 40 years ago, called “The New Frontier”. He said:

Important discoveries await the next generation of accelerators. QCD and the electroweak theory need further confirmation. We need to know how b quarks decay. The weak interaction intermediaries must be seen to be believed. The top quark (or the perversions needed by topless theories) lurks just out of range. Higgs may wait to be found. There could well be a fourth family of quarks and leptons. There may even be unanticipated surprises. We need the new machines.

That was in 1980. The ‘weak interaction intermediaries’—the W and Z—were found three years later, in 1982. The top quark was found in 1995. The Higgs boson was found in 2012. No fourth generation of quarks and leptons, and we now have good evidence that none exists. To the great sorrow of all physcists, particle accelerators have found no unanticipated surprises!

On the other hand, we have for the first time an apparently correct theory of elementary particle physics. It may be, in a sense, phenomenologically complete. It suggests the possibility that there are no more surprises at higher energies, at least at energies that are remotely accessible.

He’s proved right on this, so far.

Proton decay, if it is found, will reinforce belief in the great desert extending from 100 GeV to the unification mass of 1014 GeV. Perhaps the desert is a blessing in disguise. Ever larger and more costly machines conflict with dwindling finances and energy reserves. All frontiers come to an end.

You may like this scenario or not; it may be true or false. But, it is neither impossible, implausible, nor unlikely. And, do not despair nor prematurely lament the death of particle physics. We have a ways to go to reach the desert, with exotic fauna along the way, and even the desolation of a desert can be interesting.

Proton decay has not been found despite a huge amount of effort. So, that piece of evidence for grand unified theories is missing, and with it a strong piece of evidence that there should be a “desert” of new phenomena between the electroweak unification energy scale and the GUT energy scale.

But, we’re not seeing anything beyond the Standard Model: no “exotic fauna”.

Glashow’s “new frontier” was the “passive frontier”: non-accelerator experiments like neutrino measurements, and this is indeed where the progress came since 1980: we now know neutrinos are massive and oscillate, and there is still some mystery here and room for surprises—though frankly I suspect that neutrino masses will work very much like quark masses, via coupling to the Higgs. (This is in a sense the most conservative, least truly exciting scenario.)

So, very little dramatic progress has happened in particle physics since 1980—except for a profusion of new theories that haven’t made any verified predictions. I’ll argue that physicists should turn elsewhere! There are other things for them to do, that are much more exciting.

6 Responses to Theoretical Physics in the 21st Century

  1. Vlad says:

    I’ll argue that physicists should turn elsewhere!

    Wow, that seems like a pretty radical statement. Is not particle physics doomed to die if there is not continued work in the field?

    • John Baez says:

      If I thought my advice could completely shut down the field, I’d say “almost all physicists should turn elsewhere—but please, a few of you keep working on particle physics.” However, I don’t think I need to say that: no matter what I say, some physicists will keep working on particle physics. If I persuade a few hundred young physicists to turn from particle physics to something more productive and exciting, I’ll count that as a huge success.

      • Vlad says:

        I can understand that, but it does make me a bit sad. It feels like everything else either goes in the direction of engineering or in the direction of pure math, with no need to correspond to physical reality at all.

      • John Baez says:

        I don’t feel that way at all! I work on applied category theory, which is building up a unified approach to networks of many kinds: chemical reaction networks, electrical circuits, and much more. But if someone prefers traditional physics, condensed matter physics is incredibly exciting these days. It’s using almost all the wonderful techniques of quantum field theory that particle physics does, but with the advantage that you can test your theories on a small tabletop! It’s moving much faster than high-energy particle physics, and it’s full of revolutionary new ideas. I feel I have a duty to point young physicists toward condensed matter physics, where they have a much better chance of making important discoveries than if they do high-energy particle physics.

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