Author Archives: 4gravitonsandagradstudent

More Travel

I’m visiting the Niels Bohr Institute this week, on my way back from Amplitudes.

IMG_20170719_152906

You might recognize the place from old conference photos.

Amplitudes itself was nice. There weren’t any surprising new developments, but a lot of little “aha” moments when one of the speakers explained something I’d heard vague rumors about. I figured I’d mention a few of the things that stood out. Be warned, this is going to be long and comparatively jargon-heavy.

The conference organizers were rather daring in scheduling Nima Arkani-Hamed for the first talk, as Nima has a tendency to arrive at the last minute and talk for twice as long as you ask him to. Miraculously, though, things worked out, if only barely: Nima arrived at the wrong campus and ran most of the way back, showing up within five minutes of the start of the conference. He also stuck to his allotted time, possibly out of courtesy to his student, Yuntao Bai, who was speaking next.

Between the two of them, Nima and Yuntao covered an interesting development, tying the Amplituhedron together with the string theory-esque picture of scattering amplitudes pioneered by Freddy Cachazo, Song He, and Ellis Ye Yuan (or CHY). There’s a simpler (and older) Amplituhedron-like object called the associahedron that can be thought of as what the Amplituhedron looks like on the surface of a string, and CHY’s setup can be thought of as a sophisticated map that takes this object and turns it into the Amplituhedron. It was nice to hear from both Nima and his student on this topic, because Nima’s talks are often high on motivation but low on detail, so it was great that Yuntao was up next to fill in the blanks.

Anastasia Volovich talked about Landau singularities, a topic I’ve mentioned before. What I hadn’t appreciated was how much they can do with them at this point. Originally, Juan Maldacena had suggested that these singularities, mathematical points that determine the behavior of amplitudes first investigated by Landau in the 60’s, might explain some of the simplicity we’ve observed in N=4 super Yang-Mills. They ended up not being enough by themselves, but what Volovich and collaborators are discovering is that with a bit of help from the Amplithedron they explain quite a lot. In particular, if they start with the Amplituhedron and do a procedure similar to Landau’s, they can find the simpler set of singularities allowed by N=4 super Yang-Mills, at least for the examples they’ve calculated. It’s still a bit unclear how this links to their previous investigations of these things in terms of cluster algebras, but it sounds like they’re making progress.

Dmitry Chicherin gave me one of those minor “aha” moments. One big useful fact about scattering amplitudes in N=4 super Yang-Mills is that they’re “dual” to different mathematical objects called Wilson loops, a fact which allows us to compare to the “POPE” approach of Basso, Sever, and Vieira. Chicherin asks the question: “What if you’re not calculating a scattering amplitude or a Wilson loop, but something halfway in between?” Interestingly, this has an answer, with the “halfway between” objects having a similar duality among themselves.

Yorgos Papathansiou talked about work I’ve been involved with. I’ll probably cover it in detail in another post, so now I’ll just mention that we’re up to six loops!

Andy Strominger talked about soft theorems. It’s always interesting seeing people who don’t traditionally work on amplitudes giving talks at Amplitudes. There’s a range of responses, from integrability people (who are basically welcomed like family) to work on fairly unrelated areas that have some “amplitudes” connection (met with yawns except from the few people interested in the connection). The response to Strominger was neither welcome nor boredom, but lively debate. He’s clearly doing something interesting, but many specialists worried he was ignorant of important no-go results in the field that could hamstring some of his bolder conjectures.

The second day focused on methods for more practical calculations, and had the overall effect of making me really want to clean up my code. Tiziano Peraro’s finite field methods in particular look like they could be quite useful. There were two competing bases of integrals on display, Von Manteuffel’s finite integrals and Rutger Boels’s uniform transcendental integrals later in the conference. Both seem to have their own virtues, and I ended up asking Rob Schabinger if it was possible to combine the two, with the result that he’s apparently now looking into it.

The more practical talks that day had a clear focus on calculations with two loops, which are becoming increasingly viable for LHC-relevant calculations. From talking to people who work on this, I get the impression that the goal of these calculations isn’t so much to find new physics as to confirm and investigate new physics found via other methods. Things are complicated enough at two loops that for the moment it isn’t feasible to describe what all the possible new particles might do at that order, and instead the goal is to understand the standard model well enough that if new physics is noticed (likely based on one-loop calculations) then the details can be pinned down by two-loop data. But this picture could conceivably change as methods improve.

Wednesday was math-focused. We had a talk by Francis Brown on his conjecture of a cosmic Galois group. This is a topic I knew a bit about already, since it’s involved in something I’ve been working on. Brown’s talk cleared up some things, but also shed light on the vagueness of the proposal. As with Yorgos’s talk, I’ll probably cover more about this in a future post, so I’ll skip the details for now.

There was also a talk by Samuel Abreu on a much more physical picture of the “symbols” we calculate with. This is something I’ve seen presented before by Ruth Britto, and it’s a setup I haven’t looked into as much as I ought to. It does seem at the moment that they’re limited to one loop, which is a definite downside. Other talks discussed elliptic integrals, the bogeyman that we still can’t deal with by our favored means but that people are at least understanding better.

The last talk on Wednesday before the hike was by David Broadhurst, who’s quite a character in his own right. Broadhurst sat in the front row and asked a question after nearly every talk, usually bringing up papers at least fifty years old, if not one hundred and fifty. At the conference dinner he was exactly the right person to read the Address to the Haggis, resurrecting a thick Scottish accent from his youth. Broadhurst’s techniques for handling high-loop elliptic integrals are quite impressively powerful, leaving me wondering if the approach can be generalized.

Thursday focused on gravity. Radu Roiban gave a better idea of where he and his collaborators are on the road to seven-loop supergravity and what the next bottlenecks are along the way. Oliver Schlotterer’s talk was another one of those “aha” moments, helping me understand a key difference between two senses in which gravity is Yang-Mills squared ( the Kawai-Lewellen-Tye relations and BCJ). In particular, the latter is much more dependent on specifics of how you write the scattering amplitude, so to the extent that you can prove something more like the former at higher loops (the original was only for trees, unlike BCJ) it’s quite valuable. Schlotterer has managed to do this at one loop, using the “Q-cut” method I’ve (briefly) mentioned before. The next day’s talk by Emil Bjerrum-Bohr focused more heavily on these Q-cuts, including a more detailed example at two loops than I’d seen that group present before.

There was also a talk by Walter Goldberger about using amplitudes methods for classical gravity, a subject I’ve looked into before. It was nice to see a more thorough presentation of those ideas, including a more honest appraisal of which amplitudes techniques are really helpful there.

There were other interesting topics, but I’m already way over my usual post length, so I’ll sign off for now. Videos from all but a few of the talks are now online, so if you’re interested you should watch them on the conference page.

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.

IMG_20170712_172014

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.

Bootstrapping in the Real World

I’ll be at Amplitudes, my subfield’s big yearly conference, next week, so I don’t have a lot to talk about. That said, I wanted to give a shout-out to my collaborator and future colleague Andrew McLeod, who is a co-author (along with Øyvind Almelid, Claude Duhr, Einan Gardi, and Chris White) on a rather cool paper that went up on arXiv this week.

Andrew and I work on “bootstrapping” calculations in quantum field theory. In particular, we start with a guess for what the result will be based on a specific set of mathematical functions (in my case, “hexagon functions” involving interactions of six particles). We then narrow things down, using other calculations that by themselves only predict part of the result, until we know the right answer. The metaphor here is that we’re “pulling ourselves up by our own bootstraps”, skipping a long calculation by essentially just guessing the answer.

This method has worked pretty well…in a toy model anyway. The calculations I’ve done with it use N=4 super Yang-Mills, a simpler cousin of the theories that describe the real world. There, fewer functions can show up, so our guess is much less unwieldy than it would be otherwise.

What’s impressive about Andrew and co.’s new paper is that they apply this method, not to N=4 super Yang-Mills, but to QCD, the theory that describes quarks and gluons in the real world. This is exactly the sort of thing I’ve been hoping to see more of, these methods built into something that can help with real, useful calculations.

Currently, what they can do is still fairly limited. For the particular problem they’re looking at, the functions required ended up being relatively simple, involving interactions between at most four particles. So far, they’ve just reproduced a calculation done by other means. Going further (more “loops”) would involve interactions between more particles, as well as mixing different types of functions (different “transcendental weight”), either of which make the problem much more complicated.

That said, the simplicity of their current calculation is also a reason to be optimistic.  Their starting “guess” had just thirteen parameters, while the one Andrew and I are working on right now (in N=4 super Yang-Mills) has over a thousand. Even if things get a lot more complicated for them at the next loop, we’ve shown that “a lot more complicated” can still be quite doable.

So overall, I’m excited. It looks like there are contexts in which one really can “bootstrap” up calculations in a realistic theory, and that’s a method that could end up really useful.

The Way You Think Everything Is Connected Isn’t the Way Everything Is Connected

I hear it from older people, mostly.

“Oh, I know about quantum physics, it’s about how everything is connected!”

“String theory: that’s the one that says everything is connected, right?”

“Carl Sagan said we are all stardust. So really, everything is connected.”

connect_four

It makes Connect Four a lot easier anyway

I always cringe a little when I hear this. There’s a misunderstanding here, but it’s not a nice clean one I can clear up in a few sentences. It’s a bunch of interconnected misunderstandings, mixing some real science with a lot of confusion.

To get it out of the way first, no, string theory is not about how “everything is connected”. String theory describes the world in terms of strings, yes, but don’t picture those strings as links connecting distant places: string theory’s proposed strings are very, very short, much smaller than the scales we can investigate with today’s experiments. The reason they’re thought to be strings isn’t because they connect distant things, it’s because it lets them wiggle (counteracting some troublesome wiggles in quantum gravity) and wind (curling up in six extra dimensions in a multitude of ways, giving us what looks like a lot of different particles).

(Also, for technical readers: yes, strings also connect branes, but that’s not the sort of connection these people are talking about.)

What about quantum mechanics?

Here’s where it gets trickier. In quantum mechanics, there’s a phenomenon called entanglement. Entanglement really does connect things in different places…for a very specific definition of “connect”. And there’s a real (but complicated) sense in which these connections end up connecting everything, which you can read about here. There’s even speculation that these sorts of “connections” in some sense give rise to space and time.

You really have to be careful here, though. These are connections of a very specific sort. Specifically, they’re the sort that you can’t do anything through.

Connect two cans with a length of string, and you can send messages between them. Connect two particles with entanglement, though, and you can’t send messages between them…at least not any faster than between two non-entangled particles. Even in a quantum world, physics still respects locality: the principle that you can only affect the world where you are, and that any changes you make can’t travel faster than the speed of light. Ansibles, science-fiction devices that communicate faster than light, can’t actually exist according to our current knowledge.

What kind of connection is entanglement, then? That’s a bit tricky to describe in a short post. One way to think about entanglement is as a connection of logic.

Imagine someone takes a coin and cuts it along the rim into a heads half and a tails half. They put the two halves in two envelopes, and randomly give you one. You don’t know whether you have heads or tails…but you know that if you open your envelope and it shows heads, the other envelope must have tails.

m_nickel

Unless they’re a spy. Then it could contain something else.

Entanglement starts out with connections like that. Instead of a coin, take a particle that isn’t spinning and “split” it into two particles spinning in different directions, “spin up” and “spin down”. Like the coin, the two particles are “logically connected”: you know if one of them is “spin up” the other is “spin down”.

What makes a quantum coin different from a classical coin is that there’s no way to figure out the result in advance. If you watch carefully, you can see which coin gets put in to which envelope, but no matter how carefully you look you can’t predict which particle will be spin up and which will be spin down. There’s no “hidden information” in the quantum case, nowhere nearby you can look to figure it out.

That makes the connection seem a lot weirder than a regular logical connection. It also has slightly different implications, weirdness in how it interacts with the rest of quantum mechanics, things you can exploit in various ways. But none of those ways, none of those connections, allow you to change the world faster than the speed of light. In a way, they’re connecting things in the same sense that “we are all stardust” is connecting things: tied together by logic and cause.

So as long as this is all you mean by “everything is connected” then sure, everything is connected. But often, people seem to mean something else.

Sometimes, they mean something explicitly mystical. They’re people who believe in dowsing rods and astrology, in sympathetic magic, rituals you can do in one place to affect another. There is no support for any of this in physics. Nothing in quantum mechanics, in string theory, or in big bang cosmology has any support for altering the world with the power of your mind alone, or the stars influencing your day to day life. That’s just not the sort of connection we’re talking about.

Sometimes, “everything is connected” means something a bit more loose, the idea that someone’s desires guide their fate, that you could “know” something happened to your kids the instant it happens from miles away. This has the same problem, though, in that it’s imagining connections that let you act faster than light, where people play a special role. And once again, these just aren’t that sort of connection.

Sometimes, finally, it’s entirely poetic. “Everything is connected” might just mean a sense of awe at the deep physics in mundane matter, or a feeling that everyone in the world should get along. That’s fine: if you find inspiration in physics then I’m glad it brings you happiness. But poetry is personal, so don’t expect others to find the same inspiration. Your “everyone is connected” might not be someone else’s.

Where Grants Go on the Ground

I’ve seen several recent debates about grant funding, arguments about whether this or that scientist’s work is “useless” and shouldn’t get funded. Wading into the specifics is a bit more political than I want to get on this blog right now, and if you’re looking for a general defense of basic science there are plenty to choose from. I’d like to focus on a different part, one where I think the sort of people who want to de-fund “useless” research are wildly overoptimistic.

People who call out “useless” research act as if government science funding works in a simple, straightforward way: scientists say what they want to work on, the government chooses which projects it thinks are worth funding, and the scientists the government chooses get paid.

This may be a (rough) picture of how grants are assigned. For big experiments and grants with very specific purposes, it’s reasonably accurate. But for the bulk of grants distributed among individual scientists, it ignores what happens to the money on the ground, after the scientists get it.

The simple fact of the matter is that what a grant is “for” doesn’t have all that much influence on what it gets spent on. In most cases, scientists work on what they want to, and find ways to pay for it.

Sometimes, this means getting grants for applied work, doing some of that, but also fitting in more abstract theoretical projects during downtime. Sometimes this means sharing grant money, if someone has a promising grad student they can’t fund at the moment and needs the extra help. (When I first got research funding as a grad student, I had to talk to the particle physics group’s secretary, and I’m still not 100% sure why.) Sometimes this means being funded to look into something specific and finding a promising spinoff that takes you in an entirely different direction. Sometimes you can get quite far by telling a good story, like a mathematician I know who gets defense funding to study big abstract mathematical systems because some related systems happen to have practical uses.

Is this unethical? Some of it, maybe. But from what I’ve seen of grant applications, it’s understandable.

The problem is that if scientists are too loose with what they spend grant money on, grant agency asks tend to be far too specific. I’ve heard of grants that ask you to give a timeline, over the next five years, of each discovery you’re planning to make. That sort of thing just isn’t possible in science: we can lay out a rough direction to go, but we don’t know what we’ll find.

The end result is a bit like complaints about job interviews, where everyone is expected to say they love the company even though no-one actually does. It creates an environment where everyone has to twist the truth just to keep up with everyone else.

The other thing to keep in mind is that there really isn’t any practical way to enforce any of this. Sure, you can require receipts for equipment and the like, but once you’re paying for scientists’ time you don’t have a good way to monitor how they spend it. The best you can do is have experts around to evaluate the scientists’ output…but if those experts understand enough to do that, they’re going to be part of the scientific community, like grant committees usually already are. They’ll have the same expectations as the scientists, and give similar leeway.

So if you want to kill off some “useless” area of research, you can’t do it by picking and choosing who gets grants for what. There are advocates of more drastic actions of course, trying to kill whole agencies or fields, and that’s beyond the scope of this post. But if you want science funding to keep working the way it does, and just have strong opinions about what scientists should do with it, then calling out “useless” research doesn’t do very much: if the scientists in question think it’s useful, they’ll find a way to keep working on it. You’ve slowed them down, but you’ll still end up paying for research you don’t like.

Final note: The rule against political discussion in the comments is still in effect. For this post, that means no specific accusations of one field or another as being useless, or one politician/political party/ideology or another of being the problem here. Abstract discussions and discussions of how the grant system works should be fine.

Movie Review: The Truth is in the Stars

Recently, Perimeter aired a showing of The Truth is in the Stars, a documentary about the influence of Star Trek on science and culture, with a panel discussion afterwards. The documentary follows William Shatner as he wanders around the world interviewing scientists and film industry people about how Star Trek inspired them. Along the way he learns a bit about physics, and collects questions to ask Steven Hawking at the end.

5834308d05e48__po__ipho__380_568

I’ll start with the good: the piece is cute. They managed to capture some fun interactions with the interviewees, there are good (if occasionally silly) visuals, and the whole thing seems fairly well edited. If you’re looking for an hour of Star Trek nostalgia and platitudes about physics, this is the documentary for you.

That said, it doesn’t go much beyond cute, and it dances between topics in a way that felt unsatisfying.

The piece has a heavy focus on Shatner, especially early on, beginning with a clumsily shoehorned-in visit to his ranch to hear his thoughts on horses. For a while, the interviews are all about him: his jokes, his awkward questions, his worries about getting old. He has a habit of asking the scientists he talks to whether “everything is connected”, which to the scientists’ credit is usually met by a deft change of subject. All of this fades somewhat as the movie progresses, though: whether by a trick of editing, or because after talking to so many scientists he begins to pick up some humility.

(Incidentally, I really ought to have a blog post debunking the whole “everything is connected” thing. The tricky part is that it involves so many different misunderstandings, from confusion around entanglement to the role of strings to “we are all star-stuff” that it’s hard to be comprehensive.)

Most of the scientific discussions are quite superficial, to the point that they’re more likely to confuse inexperienced viewers than to tell them something new (especially the people who hinted at dark energy-based technology…no, just no). While I don’t expect a documentary like this to cover the science in-depth, trying to touch on so many topics in this short a time mostly just fuels the “everything is connected” misunderstanding. One surprising element of the science coverage was the choice to have both Michio Kaku giving a passionate description of string theory and Neil Turok bluntly calling string theory “a mess”. While giving the public “both sides” like that isn’t unusual in other contexts, for some reason most science documentaries I’ve seen take one side or the other.

Of course, the point of the documentary isn’t really to teach science, it’s to show how Star Trek influenced science. Here too, though, the piece was disappointing. Most of the scientists interviewed could tell their usual story about the power of science fiction in their childhood, but didn’t have much to say about Star Trek specifically. It was the actors and producers who had the most to say about Star Trek, from Ben Stiller showing off his Gorn mask to Seth MacFarlane admiring the design of the Enterprise. The best of these was probably Whoopi Goldberg’s story of being inspired by Uhura, which has been covered better elsewhere (and might have been better as Mae Jemison’s similar story, which would at least have involved an astronaut rather than another actor). I did enjoy Neil deGrasse Tyson’s explanation of how as a kid he thought everything on Star Trek was plausible…except for the automatic doors.

Shatner’s meeting with Hawking is the finale, and is the documentary’s strongest section. Shatner is humbled, even devout, in Hawking’s presence, while Hawking seems to show genuine joy swapping jokes with Captain Kirk.

Overall, the piece felt more than a little disjointed. It’s not really about the science, but it didn’t have enough content to be really about Star Trek either. If it was “about” anything, it was Shatner’s journey: an aging actor getting to hang out and chat with interesting people around the world. If that sounds fun, you should watch it: but don’t expect much deeper than that.

You Can’t Smooth the Big Bang

As a kid, I was fascinated by cosmology. I wanted to know how the universe began, possibly disproving gods along the way, and I gobbled up anything that hinted at the answer.

At the time, I had to be content with vague slogans. As I learned more, I could match the slogans to the physics, to see what phrases like “the Big Bang” actually meant. A large part of why I went into string theory was to figure out what all those documentaries are actually about.

In the end, I didn’t end up working on cosmology due my ignorance of a few key facts while in college (mostly, who Vilenkin was). Thus, while I could match some of the old popularization stories to the science, there were a few I never really understood. In particular, there were two claims I never quite saw fleshed out: “The universe emerged from nothing via quantum tunneling” and “According to Hawking, the big bang was not a singularity, but a smooth change with no true beginning.”

As a result, I’m delighted that I’ve recently learned the physics behind these claims, in the context of a spirited take-down of both by Perimeter’s Director Neil Turok.

neil20turok_cropped_photo20credit20jens20langen

My boss

Neil held a surprise string group meeting this week to discuss the paper I linked above, “No smooth beginning for spacetime” with Job Feldbrugge and Jean-Luc Lehners, as well as earlier work with Steffen Gielen. In it, he talked about problems in the two proposals I mentioned: Hawking’s suggestion that the big bang was smooth with no true beginning (really, the Hartle-Hawking no boundary proposal) and the idea that the universe emerged from nothing via quantum tunneling (really, Vilenkin’s tunneling from nothing proposal).

In popularization-speak, these two proposals sound completely different. In reality, though, they’re quite similar (and as Neil argues, they end up amounting to the same thing). I’ll steal a picture from his paper to illustrate:

neilpaperpic

The picture on the left depicts the universe under the Hartle-Hawking proposal, with time increasing upwards on the page. As the universe gets older, it looks like the expanding (de Sitter) universe we live in. At the beginning, though, there’s a cap, one on which time ends up being treated not in the usual way (Lorentzian space) but on the same footing as the other dimensions (Euclidean space). This lets space be smooth, rather than bunching up in a big bang singularity. After treating time in this way the result is reinterpreted (via a quantum field theory trick called Wick rotation) as part of normal space-time.

What’s the connection to Vilenkin’s tunneling picture? Well, when we talk about quantum tunneling, we also end up describing it with Euclidean space. Saying that the universe tunneled from nothing and saying it has a Euclidean “cap” then end up being closely related claims.

Before Neil’s work these two proposals weren’t thought of as the same because they were thought to give different results. What Neil is arguing is that this is due to a fundamental mistake on Hartle and Hawking’s part. Specifically, Neil is arguing that the Wick rotation trick that Hartle and Hawking used doesn’t work in this context, when you’re trying to calculate small quantum corrections for gravity. In normal quantum field theory, it’s often easier to go to Euclidean space and use Wick rotation, but for quantum gravity Neil is arguing that this technique stops being rigorous. Instead, you should stay in Lorentzian space, and use a more powerful mathematical technique called Picard-Lefschetz theory.

Using this technique, Neil found that Hartle and Hawking’s nicely behaved result was mistaken, and the real result of what Hartle and Hawking were proposing looks more like Vilenkin’s tunneling proposal.

Neil then tried to see what happens when there’s some small perturbation from a perfect de Sitter universe. In general in physics if you want to trust a result it ought to be stable: small changes should stay small. Otherwise, you’re not really starting from the right point, and you should instead be looking at wherever the changes end up taking you. What Neil found was that the Hartle-Hawking and Vilenkin proposals weren’t stable. If you start with a small wiggle in your no-boundary universe you get, not the purple middle drawing with small wiggles, but the red one with wiggles that rapidly grow unstable. The implication is that the Hartle-Hawking and Vilenkin proposals aren’t just secretly the same, they also both can’t be the stable state of the universe.

Neil argues that this problem is quite general, and happens under the following conditions:

  1. A universe that begins smoothly and semi-classically (where quantum corrections are small) with no sharp boundary,
  2. with a positive cosmological constant (the de Sitter universe mentioned earlier),
  3. under which the universe expands many times, allowing the small fluctuations to grow large.

If the universe avoids one of those conditions (maybe the cosmological constant changes in the future and the universe stops expanding, for example) then you might be able to avoid Neil’s argument. But if not, you can’t have a smooth semi-classical beginning and still have a stable universe.

Now, no debate in physics ends just like that. Hartle (and collaborators) don’t disagree with Neil’s insistence on Picard-Lefschetz theory, but they argue there’s still a way to make their proposal work. Neil mentioned at the group meeting that he thinks even the new version of Hartle’s proposal doesn’t solve the problem, he’s been working out the calculation with his collaborators to make sure.

Often, one hears about an idea from science popularization and then it never gets mentioned again. The public hears about a zoo of proposals without ever knowing which ones worked out. I think child-me would appreciate hearing what happened to Hawking’s proposal for a universe with no boundary, and to Vilenkin’s proposal for a universe emerging from nothing. Adult-me certainly does. I hope you do too.