Tag Archives: DoingScience

The Quantum Kids

I gave a pair of public talks at the Niels Bohr International Academy this week on “The Quest for Quantum Gravity” as part of their “News from the NBIA” lecture series. The content should be familiar to long-time readers of this blog: I talked about renormalization, and gravitons, and the whole story leading up to them.

(I wanted to title the talk “How I Learned to Stop Worrying and Love Quantum Gravity”, like my blog post, but was told Danes might not get the Doctor Strangelove reference.)

I also managed to work in some history, which made its way into the talk after Poul Damgaard, the director of the NBIA, told me I should ask the Niels Bohr Archive about Gamow’s Thought Experiment Device.

“What’s a Thought Experiment Device?”

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This, apparently

If you’ve heard of George Gamow, you’ve probably heard of the Alpher-Bethe-Gamow paper, his work with grad student Ralph Alpher on the origin of atomic elements in the Big Bang, where he added Hans Bethe to the paper purely for an alpha-beta-gamma pun.

As I would learn, Gamow’s sense of humor was prominent quite early on. As a research fellow at the Niels Bohr Institute (essentially a postdoc) he played with Bohr’s kids, drew physics cartoons…and made Thought Experiment Devices. These devices were essentially toy experiments, apparatuses that couldn’t actually work but that symbolized some physical argument. The one I used in my talk, pictured above, commemorated Bohr’s triumph over one of Einstein’s objections to quantum theory.

Learning more about the history of the institute, I kept noticing the young researchers, the postdocs and grad students.

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Lev Landau, George Gamow, Edward Teller. The kids are Aage and Ernest Bohr. Picture from the Niels Bohr Archive.

We don’t usually think about historical physicists as grad students. The only exception I can think of is Feynman, with his stories about picking locks at the Manhattan project. But in some sense, Feynman was always a grad student.

This was different. This was Lev Landau, patriarch of Russian physics, crowning name in a dozen fields and author of a series of textbooks of legendary rigor…goofing off with Gamow. This was Edward Teller, father of the Hydrogen Bomb, skiing on the institute lawn.

These were the children of the quantum era. They came of age when the laws of physics were being rewritten, when everything was new. Starting there, they could do anything, from Gamow’s cosmology to Landau’s superconductivity, spinning off whole fields in the new reality.

On one level, I envy them. It’s possible they were the last generation to be on the ground floor of a change quite that vast, a shift that touched all of physics, the opportunity to each become gods of their own academic realms.

I’m glad to know about them too, though, to see them as rambunctious grad students. It’s all too easy to feel like there’s an unbridgeable gap between postdocs and professors, to worry that the only people who make it through seem to have always been professors at heart. Seeing Gamow and Landau and Teller as “quantum kids” dispels that: these are all-too-familiar grad students and postdocs, joking around in all-too-familiar ways, who somehow matured into some of the greatest physicists of their era.

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One, Two, Infinity

Physicists and mathematicians count one, two, infinity.

We start with the simplest case, as a proof of principle. We take a stripped down toy model or simple calculation and show that our idea works. We count “one”, and we publish.

Next, we let things get a bit more complicated. In the next toy model, or the next calculation, new interactions can arise. We figure out how to deal with those new interactions, our count goes from “one” to “two”, and once again we publish.

By this point, hopefully, we understand the pattern. We know what happens in the simplest case, and we know what happens when the different pieces start to interact. If all goes well, that’s enough: we can extrapolate our knowledge to understand not just case “three”, but any case: any model, any calculation. We publish the general case, the general method. We’ve counted one, two, infinity.

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Once we’ve counted “infinity”, we don’t have to do any more cases. And so “infinity” becomes the new “zero”, and the next type of calculation you don’t know how to do becomes “one”. It’s like going from addition to multiplication, from multiplication to exponentiation, from exponentials up into the wilds of up-arrow notation. Each time, once you understand the general rules you can jump ahead to an entirely new world with new capabilities…and repeat the same process again, on a new scale. You don’t need to count one, two, three, four, on and on and on.

Of course, research doesn’t always work out this way. My last few papers counted three, four, five, with six on the way. (One and two were already known.) Unlike the ideal cases that go one, two, infinity, here “two” doesn’t give all the pieces you need to keep going. You need to go a few numbers more to get novel insights. That said, we are thinking about “infinity” now, so look forward to a future post that says something about that.

A lot of frustration in physics comes from situations when “infinity” remains stubbornly out of reach. When people complain about all the models for supersymmetry, or inflation, in some sense they’re complaining about fields that haven’t taken that “infinity” step. One or two models of inflation are nice, but by the time the count reaches ten you start hoping that someone will describe all possible models of inflation in one paper, and see if they can make any predictions from that.

(In particle physics, there’s an extent to which people can actually do this. There are methods to describe all possible modifications of the Standard Model in terms of what sort of effects they can have on observations of known particles. There’s a group at NBI who work on this sort of thing.)

The gold standard, though, is one, two, infinity. Our ability to step back, stop working case-by-case, and move on to the next level is not just a cute trick: it’s a foundation for exponential progress. If we can count one, two, infinity, then there’s nowhere we can’t reach.

Visiting Uppsala

I’ve been in Uppsala this week, visiting Henrik Johansson‘s group.

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The Ångström Laboratory here is substantially larger than an ångström, a clear example of false advertising.

As such, I haven’t had time to write a long post about the recent announcement by the LIGO and VIRGO collaborations. Luckily, Matt Strassler has written one of his currently all-too-rare posts on the subject, so if you’re curious you should check out what he has to say.

Looking at the map of black hole collisions in that post, I’m struck by how quickly things have improved. The four old detections are broad slashes across the sky, the newest is a small patch. Now that there are enough detectors to triangulate, all detections will be located that precisely, or better. A future map might be dotted with precise locations of black hole collisions, but it would still be marred by those four slashes: relics of the brief time when only two machines in the world could detect gravitational waves.

On the Care and Feeding of Ideas

I read Zen and the Art of Motorcycle Maintenance in high school. It’s got a reputation for being obnoxiously mystical, but one of its points seemed pretty reasonable: the claim that the hard part of science, and the part we understand the least, is coming up with hypotheses.

In some sense, theoretical physics is all about hypotheses. By this I don’t mean that we just say “what if?” all the time. I mean that in theoretical physics most of the work is figuring out the right way to ask a question. Phrase your question in the right way and the answer becomes obvious (or at least, obvious after a straightforward calculation). Because our questions are mathematical, the right question can logically imply its own solution.

From the point of view of “Zen and the Art”, as well as most non-scientists I’ve met, this part is utterly mysterious. The ideas you need here seem like they can’t come from hard work or careful observation. In order to ask the right questions, you just need to be “smart”.

In practice, I’ve noticed there’s more to it than that. We can’t just sit around and wait for an idea to show up. Instead, as physicists we develop a library of tricks, often unstated, that let us work towards the ideas we need.

Sometimes, this involves finding simpler cases, working with them until we understand the right questions to ask. Sometimes it involves doing numerics, or using crude guesses, not because either method will give the final answer but because it will show what the answer should look like. Sometimes we need to rephrase the problem many times, in many different contexts, before we happen on one that works. Most of this doesn’t end up in published papers, so in the end we usually have to pick it up from experience.

Along the way, we often find tricks to help us think better. Mostly this is straightforward stuff: reminders to keep us on-task, keeping our notes organized and our code commented so we have a good idea of what we were doing when we need to go back to it. Everyone has their own personal combination of these things in the background, and they’re rarely discussed.

The upshot is that coming up with ideas is hard work. We need to be smart, sure, but that’s not enough by itself: there are a lot of smart people who aren’t physicists after all.

With all that said, some geniuses really do seem to come up with ideas out of thin air. It’s not the majority of the field: we’re not the idiosyncratic Sheldon Coopers everyone seems to imagine. But for a few people, it really does feel like there’s something magical about where they get their ideas. I’ve had the privilege of working with a couple people like this, and the way they think sometimes seems qualitatively different from our usual way of building ideas. I can’t see any of the standard trappings, the legacy of partial results and tricks of thought, that would lead to where they end up. That doesn’t mean they don’t use tricks just like the rest of us, in the end. But I think genius, if it means anything at all, is thinking in a novel enough way that from the outside it looks like magic.

Most of the time, though, we just need to hone our craft. We build our methods and shape our minds as best we can, and we get better and better at the central mystery of science: asking the right questions.

Topic Conferences, Place Conferences

I spent this week at Current Themes in High Energy Physics and Cosmology, a conference at the Niels Bohr Institute.

Most conferences focus on a particular topic. Usually the broader the topic, the bigger the conference. A workshop on flux tubes is smaller than Amplitudes, which is smaller than Strings, which is smaller than the March Meeting of the American Physical Society.

“Current Themes in High Energy Physics and Cosmology” sounds like a very broad topic, but it was a small conference. The reason why is that it wasn’t a “topic conference”, it was a “place conference”.

Most conferences focus on a topic, but some are built around a place. These conferences are hosted by a particular institute year after year. Sometimes each year has a loose theme (for example, the Simons Summer Workshop this year focused on theories without supersymmetry) but sometimes no attempt is made to tie the talks together (“current themes”).

Instead of a theme, the people who go to these conferences are united by their connections to the institute. Some of them have collaborators there, or worked there in the past. Others have been coming for many years. Some just happened to be in the area.

While they may seem eclectic, “place” conferences have a valuable role: they help to keep our interests broad. In physics, there’s a natural tendency to specialize. Left alone, we end up reading papers and going to talks only when they’re directly relevant for what we’re working on. By doing this we lose track of the wider field, losing access to the insights that come from different perspectives and methods.

“Place” conferences, like seminars, help pull things in the other direction. When you’re hearing talks from “everyone connected to the Simons Center” or “everyone connected to the Niels Bohr Institute”, you’re exposed to a much broader range of topics than a conference for just your sub-field. You get a broad overview of what’s going on in the field, but unlike a big conference like Strings there are few enough people that you can actually talk to everyone.

Physicists’ attachment to places is counter-intuitive. We’re studying mathematical truths and laws of nature, surely it shouldn’t matter where we work. In practice, though, we’re still human. Out of the vast span of physics we still pick our interests based on the people around us. That’s why places, why institutes with a wide range of excellent people, are so important: they put our social instincts to work studying the universe.

Amplitudes 2017

I’ve been at Amplitudes this week, in Edinburgh. There have been a lot of great talks, most of which should already have slides online. (They’ve been surprisingly quick about getting slides up this year, with many uploaded before the corresponding talks!) Recordings of the talks should also be up soon.

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We also hiked up local hill Arthur’s Seat on Wednesday, which was a nice change of pace.

I’ll have more time to write about the talks later, a few of them were quite interesting. For now, take a look at some of the slides if you’re curious.

Visiting LBNL

I’ve been traveling this week, giving a talk at Lawrence Berkeley National Laboratory, so this will be a short post.

In my experience, most non-scientists don’t know about the national labs. In the US, the majority of scientists work for universities, but a substantial number work at one of the seventeen national labs overseen by the Department of Energy. It’s a good gig, if you can get it: no teaching duties, and a fair amount of freedom in what you research.

Each lab has its own focus, and its own culture. In the past I’ve spent a lot of time at SLAC, which runs a particle accelerator near Stanford (among other things). Visiting LBNL, I was amused by some of the differences. At SLAC, the guest rooms have ads for Stanford-branded bed covers. LBNL, meanwhile, brags about its beeswax-based toiletries in recyclable cardboard bottles. SLAC is flat, spread out, and fairly easy to navigate. LBNL is a maze of buildings arranged in tight terraces on a steep hill.

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I forgot to take a picture, but someone appears to have drawn one.

While the differences were amusing, physicists are physicists everywhere. It was nice to share my work with people who mostly hadn’t heard about it before, and to get an impression of what they were working on.