duality, exotic physics, physics, Popular science, quantum mechanics, standard model, string theory
With the third and final day of the workshop done, it’s time for me to wrap up this series on Inspiring Science. While this will be my last post about the workshop, I’m sure it won’t be the last time I write about these topics. I learned a great deal in the past few days, enough to increase both my interest in and confidence about covering physics topics. Before getting to the actual science, I want to thank the organisers, sponsors, and participants for an excellent and fruitful workshop. I hope the series continues and I look forward to coming back next year!
Strings and Dualities
The day started with a pair of talks on string theory and how developments in string theory are helping other areas, like the condensed matter physics used to study semiconductors, even though string theory itself still lacks any experimental support. For the moment, let’s leave aside the question of what string theory claims — that may be a topic for a future post — and focus on the benefits it has offered. The main idea goes by several names with a pleasant (or intimidating?) science-fiction ring to them: the holographic duality, the AdS/CFT correspondence (jargon lovers, that’s “anti-de Sitter/conformal field theory”), or gauge/gravity duality. Don’t worry if the names sound a bit scary; the concept itself is actually remarkably straightforward. (I’ve struggled a bit to come up with an effective analogy; please let me know if it’s clear or what parts don’t work so I can improve it.)
Imagine that I took two different Lego sets, let’s say one for building a house and the other for a ship, and thoroughly mixed the pieces together. Your task is to go through the muddled pile of pieces and sort them back into the original sets. It’s not a fun task. You’ll have to get the parts list from each set and go through all the pieces, sorting them into two piles. Of course, some of the pieces will be easy — the sail, for example, clearly belongs to the ship. But some of them might be very similar, so you’ll have to look at those quite carefully before deciding which pile they go in. Others will be identical, so you can put them in either pile but you’ll have to keep track of how many you’ve put in each so both sets end up with the right amount. As you’re looking over the mess and trying to decide how to start, your friend comes up with a clever suggestion. “We don’t actually need to sort this into piles,” she says. “We can just build the house and the ship from this mixed pile. As long as we’re careful and make sure all the pieces fit, we won’t have to worry about losing count or mixing up similar ones. That’ll be easier than going through them all one by one, and besides, we already know how to build Lego stuff, so we won’t have to come up with a whole new way of doing things.” Your friend’s approach isn’t just a different way of getting the task done; it’s a different way of conceiving the task, a different way of thinking about it. She’s changed it from a sorting problem to a building problem, which means you can use different mental tools to solve it.
That’s the heart of the gauge/gravity duality and why it’s called a “duality”. Instead of ways to sort Lego pieces, the gauge/gravity duality is about ways to describe the Universe — mathematically defined theories — and instead of sorting pieces, the goal is to understand something about the Universe. Of course, the duality — the relationship between the two theories — is a lot more strict than the correspondence between the different ways of dealing with the Lego problem. In a 1997 paper, Juan Maldacena showed that two different theories — let’s call them a “gauge theory” and a “gravity theory” — were formally equivalent, which means it’s possible to translate the description of certain physical systems from one theory to the other. If you’re studying such a system using gauge theory, you can use a dictionary to translate everything into the terms of gravity theory and start studying your system in terms of gravity theory instead. To understand why you might want to do that, let’s take another detour through Lego-land.
If the two Lego sets have lots of pieces which are quite similar between them, the building approach might be easier than trying to sort them directly, especially if each set also has many copies of some of the pieces. But what if the house consisted entirely of grey-coloured pieces and the ship of red-coloured ones? In that case, the building approach would certainly be slower than simply sorting the pieces into a red pile and a grey pile. In fact, it might even be harder to build the house and the ship, since you wouldn’t be able to use a piece’s colour as a clue while putting them together. In the same way, a physical system might be easier to study using one theory or another, depending on its properties. Physicists have trouble studying some aspects of semiconductors because they have to deal with strong coupling in quantum field theory. Don’t worry about what that means; the important point is that the strong coupling makes them incredibly challenging to tackle with existing mathematical approaches. The gauge-gravity duality lets physicists translate the strongly coupled quantum field theory into a weakly coupled gravity theory, which is much more tractable. The calculations are still challenging, but at least we know how to do them — we don’t even know how to deal with a strongly coupled quantum field theory problem.
So, what’s the point of all this? Understanding how these semiconductors work could lead to advances in material sciences, but that’s not why I’ve written out this long explanation. I think it’s amazing that we can show that two theoretical frameworks are formally equivalent and then translate between them so we can use either to investigate a problem. Semiconductors belong to the realm of condensed matter; that means you’re working with lots and lots of particles that are close together. That sounds like a problem for quantum mechanics, not gravity. But it’s too difficult for us to solve using the tools of quantum mechanics, so we translate it into a treatment in terms of gravity which we can solve, even though that means thinking about it in very abstract ways. Then we do the calculations and translate back. And the tools to do that weren’t developed to deal with this problem, but as part of string theory — a theory nobody is even able to test. It’s brilliant and incredibly beautiful!
Here Be Dragons!
The Standard Model is the synthesis of our best physical theories, our best description of how the Universe works. Except, as Chad Orzel reminded us, it’s wrong. “I don’t mean wrong in the sense of boringly incorrect,” he said, “but interestingly incomplete.” We know that the Standard Model isn’t a complete theory because, even though it works amazingly well, it doesn’t properly account for the existence of everything we know exists. There are a variety of exotic things that let physicists know that the Standard Model is incomplete, but the most familiar and startling problem with the Standard Model is the fact that we exist. The simplest models predict that the early Universe should have had equal amounts of matter and anti-matter, which would have promptly annihilated each other. The fact that we’re here and that everything we can see is made of matter means there must have been a surplus of matter early on, which is something the Standard Model can’t account for. So physicists know there must be something wrong with — or missing from — our best theories, and they’re hard at work to figure out how to fix them. It’s a tough challenge, though, since the Standard Model is astonishingly good at dealing with all of the things it can account for.
Chad’s talk focused on the quest for new “exotic” physics to push the boundaries of our knowledge and light the path to improving the Standard Model. A lot of the common approaches involve very high energies, like smashing particles together in an accelerator to try to make new kinds particles that would point to new physics. Another approach is to use tools from atomic, molecular and optical physics which can make measurements at incredibly high precisions in the hope of finding a deviation from theoretical predictions. Using these tools, researchers have looked for excitations of cesium which shouldn’t happen according to current theories and even checked for incredibly small changes in a fundamental constant (the fine structure constant) over time. So far, they haven’t found anything, but they keep trying because they know there must be something out there. Like explorers trying to clear dragons off the map, these researchers are labouring to discover a whole new realm for us to map out. “It’s a high risk, high reward approach,” said Orzel. “The most likely result is that you’re going to publish a bunch of papers saying ‘We found nothing better than we’ve ever found nothing before’,” he continued, but whoever does finally find something will have opened the door to new kinds of physics and will likely collect a Nobel one day.
That’s not all I learned at the workshop, but it’s all I’m going to write about for now — this post has already grown much longer than I intended. Of course, I’m not the only one who’s been writing about the workshop after all, it was geared towards science writers!. Some of the other participants have covered things that I didn’t, so be sure to check out Tushna Commissariat’s post about the MAQRO project, a proposal to test whether larger objects can be put into a superposition of states (like Schrödinger’s famous cat), and Chad Orzel’s excellent overviews of the first, second and third days of the conference. Sabine Hossenfelder (who was one of the organizers) reflected back on the conference in posts discussing science communication and the social impact of scientific theories. I’ll update the list to include any more posts about the workshop that I find.
What do I think? As regards the lego bricks I think you’ve had people promoting their useless speculations via a pile of patronising Emperor’s New Clothes twaddle. If you can, email your string guy and ask him this: What’s the string made out of?
As regards Chad and the Standard Model, he’s promoting it via “the mystery of missing antimatter”. But he didn’t tell you that baryon asymmetry is matched by lepton asymmetry. And he didn’t tell you that positronium is like light hydrogen. Or that we label the electron as matter and the proton as matter purely by convention. It’s like there was a game of mixed-doubles tennis, and Chad told you the women won. But fair play to the guy for saying the Standard Model is incomplete and mentioning the word wrong. It’s very incomplete, and it is wrong in some respects. It’s sometimes promoted via “the mystery of mass”, but see A Zeptospace Odyssey by CERN physicist Gian Francesco Giudice:
“The most inappropriate name ever given to the Higgs boson is ‘The God particle’. The name gives the impression that the Higgs boson is the central particle of the Standard Model, governing its structure. But this is very far from the truth…”
“The Higgs sector is that part of the theory that describes the Higgs mechanism and contains the Higgs boson. Unlike the rest of the theory, the Higgs sector is rather arbitrary, and its form is not dictated by any deep fundamental principle. For this reason its structure looks frightfully ad hoc…”
“In summary, the Higgs mechanism accounts for about 1 per cent of the mass of ordinary matter, and for only 0.2 per cent of the mass of the universe. This is not nearly enough to justify the claim of explaining the origin of mass”.
“What’s the string made out of?” I don’t see how that’s any more damning a question than “What’s a quark made out of?”, so I don’t get what the issue is. Current physical theories propose that the fundamental constituents of matter are certain elementary particles. In principle, there’s no problem with considering the idea that matter is fundamentally made of strings, not particles.
In any case, I’m not arguing in support of string theory in this post, and the presentation at the workshop wasn’t an effort to convince us of its validity. In fact, it was very clear that string theory, whatever its merits, is still impossible for us to test.
Having said that, it’s amazing that some of the tools developed in string theory research are finding application in tackling some very, very tough problems in unrelated areas of physics. It’s even more striking since this approach came out of work on certain kinds of string theories that even string theorists think are unlikely to be realistic. Nevertheless, they can provide insights into questions in condensed matter physics and perhaps suggest new experiments for people to try. So regardless of whether string theory is correct or not, it seems to be offering help to condensed matter physicists and giving us an opportunity to discover something new.
Chad is hardly alone in acknowledging that the Standard Model is incomplete and therefore must be wrong in some way. Every physicist with whom I’ve ever discussed the subject has acknowledged that. Yes, there are more issues with the Standard Model than the missing anti-matter, and Chad mentioned some of them — the existence of dark matter and the mass of the neutrino, for example. I stuck to the matter/anti-matter issue in the name of simplicity and brevity.
Having said that, the Standard Model is also amazingly good at predictions when it does work. That’s part of the reason it’s so hard to find new physics beyond the Standard Model. Since there are big questions we can’t handle with the Standard Model, we know something must be missing, so physicists keep trying to find somewhere where predictions and theory don’t match — a new kind of particle, a violation of symmetry, differences in physical constants — but so far they haven’t managed. Everyone knows it’s wrong, but so far nobody can show how it’s wrong, since it keeps passing tests with flying colours.
For quarks, we can point to low-energy proton-antiproton annihilation to gamma photons, or to pions that decay to muons then electrons which we can annihilate with positrons. Then we can say that quarks are made of light/energy/wavefunction. Neutrinos muddy the water, but it’s clear for electrons and positrons, which we can literally make out of light in gamma-gamma pair production. We can diffract electrons, the wave nature of matter is not in doubt. But it’s very different for strings, and it’s only when you ask pointed questions like what are they made of? that you really start to see the issues.
Fair enough about Chad mentioning dark matter and neutrino mass. The Standard Model is certainly incomplete in that it doesn’t recognise that gravitational field energy has a mass equivalence, or that mass is a measure of how much energy and how much less than c. There’s a sliding scale to it. If you slow down a photon to less than c, some of its E=hf energy is exhibited as “effective mass”. If you catch it in a mirror-box, all of its E=hf energy is exhibited as mass, and it is indeed effective. That’s what E=mc² is all about. Note that what’s missing is “within the Standard Model”. And people can show how it’s wrong, you just don’t get to hear about it.
For example, see two-photon physics on Wiki. See how it says two photons cannot couple to one another? And that one photon fluctuates into a charged fermion–antifermion pair, to either of which the other photon can couple? That’s wrong. A 511keV photon does not spontaneously turn into a 511keV electron and a 511keV positron which then magically turn back into a single 511keV photon. Pair production does not occur because pair production occurs. There’s a photon-photon interaction, and it’s missing from the Standard Model.