Tag Archives: academia

arXiv, Our Printing Press

Johannes Gutenberg, inventor of the printing press, and possibly the only photogenic thing on the Mainz campus

I’ve had a few occasions to dig into older papers recently, and I’ve noticed a trend: old papers are hard to read!

Ok, that might not be surprising. The older a paper is, the greater the chance it will use obsolete notation, or assume a context that has long passed by. Older papers have different assumptions about what matters, or what rigor requires, and their readers cared about different things. All this is to be expected: a slow, gradual approach to a modern style and understanding.

I’ve been noticing, though, that this slow, gradual approach doesn’t always hold. Specifically, it seems to speed up quite dramatically at one point: the introduction of arXiv, the website where we store all our papers.

Part of this could just be a coincidence. As it happens, the founding papers in my subfield, those that started Amplitudes with a capital “A”, were right around the time that arXiv first got going. It could be that all I’m noticing is the difference between Amplitudes and “pre-Amplitudes”, with the Amplitudes subfield sharing notation more than they did before they had a shared identity.

But I suspect that something else is going on. With arXiv, we don’t just share papers (that was done, piecemeal, before arXiv). We also share LaTeX.

LaTeX is a document formatting language, like a programming language for papers. It’s used pretty much universally in physics and math, and increasingly in other fields. As it turns out, when we post a paper to arXiv, we don’t just send a pdf: we include the raw LaTeX code as well.

Before arXiv, if you wanted to include an equation from another paper, you’d format it yourself. You’d probably do it a little differently from the other paper, in accord with your own conventions, and just to make it easier on yourself. Over time, more and more differences would crop up, making older papers harder and harder to read.

With arXiv, you can still do all that. But you can also just copy.

Since arXiv makes the LaTeX code behind a paper public, it’s easy to lift the occasional equation. Even if you’re not lifting it directly, you can see how they coded it. Even if you don’t plan on copying, the default gets flipped around: instead of having to try to make your equation like the one in the previous paper and accidentally getting it wrong, every difference is intentional.

This reminds me, in a small-scale way, of the effect of the printing press on anatomy books.

Before the printing press, books on anatomy tended to be full of descriptions, but not illustrations. Illustrations weren’t reliable: there was no guarantee the monk who copied them would do so correctly, so nobody bothered. This made it hard to tell when an anatomist (fine it was always Galen) was wrong: he could just be using an odd description. It was only after the printing press that books could actually have illustrations that were reliable across copies of a book. Suddenly, it was possible to point out that a fellow anatomist had left something out: it would be missing from the illustration!

In a similar way, arXiv seems to have led to increasingly standard notation. We still aren’t totally consistent…but we do seem a lot more consistent than older papers, and I think arXiv is the reason why.

Most of String Theory Is Not String Pheno

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Last week, Sabine Hossenfelder wrote a post entitled “Why not string theory?” In it, she argued that string theory has a much more dominant position in physics than it ought to: that it’s crowding out alternative theories like Loop Quantum Gravity and hogging much more funding than it actually merits.

If you follow the string wars at all, you’ve heard these sorts of arguments before. There’s not really anything new here.

That said, there were a few sentences in Hossenfelder’s post that got my attention, and inspired me to write this post.

So far, string theory has scored in two areas. First, it has proved interesting for mathematicians. But I’m not one to easily get floored by pretty theorems – I care about math only to the extent that it’s useful to explain the world. Second, string theory has shown to be useful to push ahead with the lesser understood aspects of quantum field theories. This seems a fruitful avenue and is certainly something to continue. However, this has nothing to do with string theory as a theory of quantum gravity and a unification of the fundamental interactions.

(Bolding mine)

Here, Hossenfelder explicitly leaves out string theorists who work on “lesser understood aspects of quantum field theories” from her critique. They’re not the big, dominant program she’s worried about.

What Hossenfelder doesn’t seem to realize is that right now, it is precisely the “aspects of quantum field theories” crowd that is big and dominant. The communities of string theorists working on something else, and especially those making bold pronouncements about the nature of the real world, are much, much smaller.

Let’s define some terms:

Phenomenology (or pheno for short) is the part of theoretical physics that attempts to make predictions that can be tested in experiments. String pheno, then, covers attempts to use string theory to make predictions. In practice, though, it’s broader than that: while some people do attempt to predict the results of experiments, more work on figuring out how models constructed by other phenomenologists can make sense in string theory. This still attempts to test string theory in some sense: if a phenomenologist’s model turns out to be true but it can’t be replicated in string theory then string theory would be falsified. That said, it’s more indirect. In parallel to string phenomenology, there is also the related field of string cosmology, which has a similar relationship with cosmology.

If other string theorists aren’t trying to make predictions, what exactly are they doing? Well, a large number of them are studying quantum field theories. Quantum field theories are currently our most powerful theories of nature, but there are many aspects of them that we don’t yet understand. For a large proportion of string theorists, string theory is useful because it provides a new way to understand these theories in terms of different configurations of string theory, which often uncovers novel and unexpected properties. This is still physics, not mathematics: the goal, in the end, is to understand theories that govern the real world. But it doesn’t involve the same sort of direct statements about the world as string phenomenology or string cosmology: crucially, it doesn’t depend on whether string theory is true.

Last week, I said that before replying to Hossenfelder’s post I’d have to gather some numbers. I was hoping to find some statistics on how many people work on each of these fields, or on their funding. Unfortunately, nobody seems to collect statistics broken down by sub-field like this.

As a proxy, though, we can look at conferences. Strings is the premier conference in string theory. If something has high status in the string community, it will probably get a talk at Strings. So to investigate, I took a look at the talks given last year, at Strings 2015, and broke them down by sub-field.

strings2015topics

Here I’ve left out the historical overview talks, since they don’t say much about current research.

“QFT” is for talks about lesser understood aspects of quantum field theories. Amplitudes, my own sub-field, should be part of this: I’ve separated it out to show what a typical sub-field of the QFT block might look like.

“Formal Strings” refers to research into the fundamentals of how to do calculations in string theory: in principle, both the QFT folks and the string pheno folks find it useful.

“Holography” is a sub-topic of string theory in which string theory in some space is equivalent to a quantum field theory on the boundary of that space. Some people study this because they want to learn about quantum field theory from string theory, others because they want to learn about quantum gravity from quantum field theory. Since the field can’t be cleanly divided into quantum gravity and quantum field theory research, I’ve given it its own category.

While all string theory research is in principle about quantum gravity, the “Quantum Gravity” section refers to people focused on the sorts of topics that interest non-string quantum gravity theorists, like black hole entropy.

Finally, we have String Cosmology and String Phenomenology, which I’ve already defined.

Don’t take the exact numbers here too seriously: not every talk fit cleanly into a category, so there were some judgement calls on my part. Nonetheless, this should give you a decent idea of the makeup of the string theory community.

The biggest wedge in the diagram by far, taking up a majority of the talks, is QFT. Throwing in Amplitudes (part of QFT) and Formal Strings (useful to both), and you’ve got two thirds of the conference. Even if you believe Hossenfelder’s tale of the failures of string theory, then, that only matters to a third of this diagram. And once you take into account that many of the Holography and Quantum Gravity people are interested in aspects of QFT as well, you’re looking at an even smaller group. Really, Hossenfelder’s criticism is aimed at two small slices on the chart: String Pheno, and String Cosmo.

Of course, string phenomenologists also have their own conference. It’s called String Pheno, and last year it had 130 participants. In contrast, LOOPS’ 2015, the conference for string theory’s most famous “rival”, had…190 participants. The fields are really pretty comparable.

Now, I have a lot more sympathy for the string phenomenologists and string cosmologists than I do for loop quantum gravity. If other string theorists felt the same way, then maybe that would cause the sort of sociological effect that Hossenfelder is worried about.

But in practice, I don’t think this happens. I’ve met string theorists who didn’t even know that people still did string phenomenology. The two communities are almost entirely disjoint: string phenomenologists and string cosmologists interact much more with other phenomenologists and cosmologists than they do with other string theorists.

You want to talk about sociology? Sociologically, people choose careers and fund research because they expect something to happen soon. People don’t want to be left high and dry by a dearth of experiments, don’t feel comfortable working on something that may only be vindicated long after they’re dead. Most people choose the safe option, the one that, even if it’s still aimed at a distant goal, is also producing interesting results now (aspects of quantum field theories, for example).

The people that don’t? Tend to form small, tight-knit, passionate communities. They carve out a few havens of like-minded people, and they think big thoughts while the world around them seems to only care about their careers.

If you’re a loop quantum gravity theorist, or a quantum gravity phenomenologist like Hossenfelder, and you see some of your struggles in that paragraph, please realize that string phenomenology is like that too.

I feel like Hossenfelder imagines a world in which string theory is struck from its high place, and alternative theories of quantum gravity are of comparable size and power. But from where I’m sitting, it doesn’t look like it would work out that way. Instead, you’d have alternatives grow to the same size as similarly risky parts of string theory, like string phenomenology. And surprise, surprise: they’re already that size.

In certain corners of the internet, people like to argue about “punching up” and “punching down”. Hossenfelder seems to think she’s “punching up”, giving the big dominant group a taste of its own medicine. But by leaving out string theorists who study QFTs, she’s really “punching down”, or at least sideways, and calling out a sub-group that doesn’t have much more power than her own.

Amplitudes for the New Year

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Ah, the new year, time of new year’s resolutions. While some people resolve to go to the gym or take up online dating, physicists resolve to finally get that paper out.

At least, that’s the impression I get, given the number of papers posted to arXiv in the last month. Since a lot of them were amplitudes-related, I figured I’d go over some highlights.

Everyone once in a while people ask me for the latest news on the amplituhedron. While I don’t know what Nima is working on right now, I can point to what others have been doing. Zvi Bern, Jaroslav Trnka, and collaborators have continued to make progress towards generalizing the amplituhedron to non-planar amplitudes. Meanwhile, a group in Europe has been working on solving an issue I’ve glossed over to some extent. While the amplituhedron is often described as calculating an amplitude as the volume of a geometrical object, in fact there is a somewhat more indirect procedure involved in going from the geometrical object to the amplitude. It would be much simpler if the amplitude was actually the volume of some (different) geometrical object, and that’s what these folks are working towards. Finally, Daniele Galloni has made progress on solving a technical issue: the amplituhedron gives a mathematical recipe for the amplitude, but it doesn’t tell you how to carry out that recipe, and Galloni provides an algorithm for part of this process.

With this new algorithm, is the amplituhedron finally as efficient as older methods? Typically, the way to show that is to do a calculation with the amplituhedron that wasn’t possible before. It doesn’t look like that’s happening soon though, as Jake Bourjaily and collaborators compute an eight-loop integrand using one of the more successful of the older methods. Their paper provides a good answer to the perennial question, “why more loops?” What they find is that some of the assumptions that people made at lower loops fail to hold at this high loop order, and it becomes increasingly important to keep track of exactly how far your symmetries can take you.

Back when I visited Brown, I talked to folks there about some ongoing work. Now that they’ve published, I can talk about it. A while back, Juan Maldacena resurrected an old technique of Landau’s to solve a problem in AdS/CFT. In that paper, he suggested that Landau’s trick might help prove some of the impressive simplifications in N=4 super Yang-Mills that underlie my work and the work of those at Brown. In their new paper, the Brown group finds that, while useful, Landau’s trick doesn’t seem to fully explain the simplicity they’ve discovered. To get a little partisan, I have to say that this was largely the result I expected, and that it felt a bit condescending for Maldacena to assume that an old trick like that from the Feynman diagram era could really be enough to explain one of the big discoveries of amplitudeology.

There was also a paper by Freddy Cachazo and collaborators on an interesting trick to extend their CHY string to one-loop, and one by Bo Feng and collaborators on an intriguing new method called Q-cuts that I will probably say more about in future, but I’ll sign off for now. I’ve got my own new years’ physics resolutions, and I ought to get back to work!

Knowing Too Little, Knowing Too Much

Scooped Is a Spectrum

The Theorist Exclusion Principle

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There are a lot of people who think theoretical physics has gone off-track, though very few of them agree on exactly how. Some think that string theory as a whole is a waste of time, others that the field just needs to pay more attention to their preferred idea. Some think we aren’t paying enough attention to the big questions, or that we’re too focused on “safe” ideas like supersymmetry, even when they aren’t working out. Some think the field needs less focus on mathematics, while others think it needs even more.

Usually, people act on these opinions by writing strongly worded articles and blog posts. Sometimes, they have more power, and act with money, creating grants and prizes that only go to their preferred areas of research.

Let’s put the question of whether the field actually needs to change aside for the moment. Even if it does, I’m skeptical that this sort of thing will have any real effect. While grants and blogs may be very good at swaying experimentalists, theorists are likely to be harder to shift, due to what I’m going to call the Theorist Exclusion Principle.

The Pauli Exclusion Principle is a rule from quantum mechanics that states that two fermions (particles with half-integer spin) can’t occupy the same state. Fermions include electrons, quarks, protons…essentially, all the particles that make up matter. Many people learn about the Pauli Exclusion Principle first in a chemistry class, where it explains why electrons fall into different energy levels in atoms: once one energy level “fills up”, no more electrons can occupy the same state, and any additional electrons are “excluded” and must occupy a different energy level.

Those 1s electrons are such a clique!

In contrast, bosons (like photons, or the Higgs) can all occupy the same state. It’s what allows for things like lasers, and it’s why all the matter we’re used to is made out of fermions: because fermions can’t occupy the same state as each other, as you add more fermions the structures they form have to become more and more complicated.

Experimentalists are a little like bosons. While you can’t stuff two experimentalists into the same quantum state, you can get them working on very similar projects. They can form large collaborations, with each additional researcher making the experiment that much easier. They can replicate eachother’s work, making sure it was accurate. They can take some physical phenomenon and subject it to a battery of tests, so that someone is bound to learn something.

Theorists, on the other hand, are much more like fermions. In theory, there’s very little reason to work on something that someone else is already doing. Replication doesn’t mean very much: the purest theory involves mathematical proofs, where replication is essentially pointless. Theorists do form collaborations, but they don’t have the same need for armies of technicians and grad students that experimentalists do. With no physical objects to work on, there’s a limit to how much can be done pursuing one particular problem, and if there really are a lot of options they can be pursued by one person with a cluster.

Like fermions, then, theorists expand to fill the projects available. If an idea is viable, someone will probably work on it, and once they do, there isn’t much reason for someone else to do the same thing.

This makes theory a lot harder to influence than experiment. You can write the most beautiful thinkpiece possible to persuade theorists to study the deep questions of the universe, but if there aren’t any real calculations available nothing will change. Contrary to public perception, theoretical physicists aren’t paid to just sit around thinking all day: we calculate, compute, and publish, and if a topic doesn’t lend itself to that then we won’t get much mileage out of it. And no matter what you try to preferentially fund with grants, mostly you’ll just get people re-branding what they’re already doing, shifting a few superficial details to qualify.

Theorists won’t occupy the same states, so if you want to influence theorists you need to make sure there are open states where you’re trying to get them to go. Historically, theorists have shifted when new states have opened up: new data from experiment that needed a novel explanation, new mathematical concepts that opened up new types of calculations. You want there to be fewer string theorists, or more focus on the deep questions? Give us something concrete to do, and I guarantee you’ll get theorists flooding in.

Why I Spent Convergence Working

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Convergence is basically Perimeter Institute Christmas.

This week, the building was dressed up in festive posters and elaborate chalk art, and filled with Perimeter’s many distant relations. Convergence is like a hybrid of an alumni reunion and a conference, where Perimeter’s former students and close collaborators come to hear talks about the glory of Perimeter and the marvels of its research.

Physics Is a Small World

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Earlier this week, Vilhelm Bohr gave a talk at Perimeter about the life of his grandfather, the famous physicist Niels Bohr. The video of the talk doesn’t appear to be up on the Perimeter site yet, but it should be soon.

Until then, here is a picture of some eyebrows.

This was especially special for me, because my family has a longstanding connection to the Bohrs. My great grandfather worked at the Niels Bohr Institute in the mid-1930’s, and his children became good friends with Bohr’s grandchildren, often visiting each other even after my family relocated to the US.

These kinds of connections are more common in physics than you might think. Time and again I’m surprised by how closely linked people are in this field. There’s a guy here at Perimeter who went to school with Jaroslav Trnka, and a bunch of Israelis at nearby institutions all know each other from college. In my case, I went to high school with an unusually large number of mathematicians.

While it’s fun to see familiar faces, there’s a dark side to the connected nature of physics. So much of what it takes to succeed in academia involves knowing unwritten rules, as well as a wealth of other information that just isn’t widely known. Many people don’t even know it’s possible to have a career in physics, and I’ve met many who didn’t know that science grad schools pay your tuition. Academic families, and academic communities, have an enormous leg up on this kind of knowledge, so it’s not surprising that so many physicists come from so few sources.

Artificially limiting the pool of people who become physicists is bound to hurt us in the long run. Great insights often come from outsiders, like Hooke in the 17th century and Noether in the early 20th. If we can expand the reach of physics, make the unwritten rules written and the secret tricks revealed, if we work to make physics available to anyone who might be suited for it, then we can make sure that physics doesn’t end up a hereditary institution, with all the problems that entails.

Who Plagiarizes an Acknowledgements Section?

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I’ve got plagiarists on the brain.

Maybe it was running into this interesting discussion about a plagiarized application for the National Science Foundation’s prestigious Graduate Research Fellowship Program. Maybe it’s due to the talk Paul Ginsparg, founder of arXiv, gave this week about, among other things, detecting plagiarism.

Using arXiv’s repository of every paper someone in physics thought was worth posting, Ginsparg has been using statistical techniques to sift out cases of plagiarism. Probably the funniest cases involved people copying a chunk of their thesis acknowledgements section, as excerpted here. Compare:

“I cannot describe how indebted I am to my wonderful girlfriend, Amanda, whose love and encouragement will always motivate me to achieve all that I can. I could not have written this thesis without her support; in particular, my peculiar working hours and erratic behaviour towards the end could not have been easy to deal with!”

“I cannot describe how indebted I am to my wonderful wife, Renata, whose love and encouragement will always motivate me to achieve all that I can. I could not have written this thesis without her support; in particular, my peculiar working hours and erratic behaviour towards the end could not have been easy to deal with!”

Why would someone do this? Copying the scientific part of a thesis makes sense, in a twisted way: science is hard! But why would someone copy the fluff at the end, the easy part that’s supposed to be a genuine take on your emotions?

The thing is, the acknowledgements section of a thesis isn’t exactly genuine. It’s very formal: a required section of the thesis, with tacit expectations about what’s appropriate to include and what isn’t. It’s also the sort of thing you only write once in your life: while published papers also have acknowledgements sections, they’re typically much shorter, and have different conventions.

If you ever were forced to write thank-you notes as a kid, you know where I’m going with this.

It’s not that you don’t feel grateful, you do! But when you feel grateful, you express it by saying “thank you” and moving on. Writing a note about it isn’t very intuitive, it’s not a way you’re used to expressing gratitude, so the whole experience feels like you’re just following a template.

Literally in some cases.

That sort of situation: where it doesn’t matter how strongly you feel something, only whether you express it in the right way, is a breeding ground for plagiarism. Aunt Mildred isn’t going to care what you write in your thank-you note, and Amanda/Renata isn’t going to be moved by your acknowledgements section. It’s so easy to decide, in that kind of situation, that it’s better to just grab whatever appropriate text you can than to teach yourself a new style of writing.

In general, plagiarism happens because there’s a disconnect between incentives and what they’re meant to be for. In a world where very few beginning graduate students actually have a solid research plan, the NSF’s fellowship application feels like a demand for creative lying, not an honest way to judge scientific potential. In countries eager for highly-cited faculty but low on preexisting experts able to judge scientific merit, tenure becomes easier to get by faking a series of papers than by doing the actual work.

If we want to get rid of plagiarism, we need to make sure our incentives match our intent. We need a system in which people succeed when they do real work, get fellowships when they honestly have talent, and where we care about whether someone was grateful, not how they express it. If we can’t do that, then there will always be people trying to sneak through the cracks.

The Hardest Audience Knows Just Enough to Be Dangerous

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You’d think that it would be hard to explain physics to people who know absolutely nothing about physics.

And you might be right, if there was anyone these days who knew absolutely nothing about physics. If someone didn’t know what atoms were, or didn’t know what a physicist was, then yes it would take quite a while to explain anything more than the basics. But most people know what atoms are, and know what physicists are, and at least have a basic idea that there are things called protons and neutrons and electrons.

And that’s often enough. Starting with a basis like that, I can talk people through the Large Hadron Collider, I can get them to picture Feynman Diagrams, I can explain, roughly, what it is I do.

On the other end, it’s not all that hard to explain what I do to people in my sub-field. Working on the same type of physics is like sharing a language, we have all sorts of terms to make explaining easier. While it’s still possible to trip up and explain too much or too little (a recent talk I gave left out the one part that one member of the audience needed…because everyone else would have gotten nothing out of it), you’re protected by a buffer of mutual understanding.

The hardest talks aren’t for the public, and they aren’t for fellow amplitudes-researchers. They’re for a general physics audience.

If you’re talking to physicists, you can’t start with protons and neutrons. Do that, and your audience is going to get annoyed with you rather quickly. You can’t rely on the common understanding everyone has of physics. In addition to making your audience feel like they’re being talked down to, you won’t manage to say anything substantial. You need to start at a higher level so that when you do describe what you do, it’s in enough detail that your audience feels like they really understand it.

At the same time, you can’t start with the jargon of your sub-field. If you want to really explain something (and not just have fifteen minutes of background before everyone tunes out) you need to build off of a common understanding.

The tricky part is, that “common understanding” is more elusive than you might think. For example, pretty much every physicist has some familiarity with Quantum Field Theory…but that can mean anything from “uses it every day” to “saw it a couple times back in grad school”. Too much background, and half your audience is bored. Too little, and half your audience is lost. You have to strike the proper balance, trying to show everyone enough to feel satisfied.

There are tricks to make this easier. I’ve noticed that some of the best speakers begin with a clever and unique take on something everyone understands. That way, people in very different fields will still have something they recognize, while people in the same field will still be seeing something new. Of course, the tricky part is coming up with a new example in the first place!

In general, I need to get better at estimating where my audience is. Talking to you guys is fun, but I ought to also practice a “physics voice” for discussions with physicists (as well as grants and applications), and an “amplitudes voice” for fellow specialists. The key to communication, as always, is knowing your audience.

4 gravitons

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