Tag Archives: LIGO

Amplitudes in String and Field Theory at NBI

There’s a conference at the Niels Bohr Institute this week, on Amplitudes in String and Field Theory. Like the conference a few weeks back, this one was funded by the Simons Foundation, as part of Michael Green’s visit here.

The first day featured a two-part talk by Michael Green and Congkao Wen. They are looking at the corrections that string theory adds on top of theories of supergravity. These corrections are difficult to calculate directly from string theory, but one can figure out a lot about them from the kinds of symmetry and duality properties they need to have, using the mathematics of modular forms. While Michael’s talk introduced the topic with a discussion of older work, Congkao talked about their recent progress looking at this from an amplitudes perspective.

Francesca Ferrari’s talk on Tuesday also related to modular forms, while Oliver Schlotterer and Pierre Vanhove talked about a different corner of mathematics, single-valued polylogarithms. These single-valued polylogarithms are of interest to string theorists because they seem to connect two parts of string theory: the open strings that describe Yang-Mills forces and the closed strings that describe gravity. In particular, it looks like you can take a calculation in open string theory and just replace numbers and polylogarithms with their “single-valued counterparts” to get the same calculation in closed string theory. Interestingly, there is more than one way that mathematicians can define “single-valued counterparts”, but only one such definition, the one due to Francis Brown, seems to make this trick work. When I asked Pierre about this he quipped it was because “Francis Brown has good taste…either that, or String Theory has good taste.”

Wednesday saw several talks exploring interesting features of string theory. Nathan Berkovitz discussed his new paper, which makes a certain context of AdS/CFT (a duality between string theory in certain curved spaces and field theory on the boundary of those spaces) manifest particularly nicely. By writing string theory in five-dimensional AdS space in the right way, he can show that if the AdS space is small it will generate the same Feynman diagrams that one would use to do calculations in N=4 super Yang-Mills. In the afternoon, Sameer Murthy showed how localization techniques can be used in gravity theories, including to calculate the entropy of black holes in string theory, while Yvonne Geyer talked about how to combine the string theory-like CHY method for calculating amplitudes with supersymmetry, especially in higher dimensions where the relevant mathematics gets tricky.

Thursday ended up focused on field theory. Carlos Mafra was originally going to speak but he wasn’t feeling well, so instead I gave a talk about the “tardigrade” integrals I’ve been looking at. Zvi Bern talked about his work applying amplitudes techniques to make predictions for LIGO. This subject has advanced a lot in the last few years, and now Zvi and collaborators have finally done a calculation beyond what others had been able to do with older methods. They still have a way to go before they beat the traditional methods overall, but they’re off to a great start. Lance Dixon talked about two-loop five-particle non-planar amplitudes in N=4 super Yang-Mills and N=8 supergravity. These are quite a bit trickier than the planar amplitudes I’ve worked on with him in the past, in particular it’s not yet possible to do this just by guessing the answer without considering Feynman diagrams.

Today was the last day of the conference, and the emphasis was on number theory. David Broadhurst described some interesting contributions from physics to mathematics, in particular emphasizing information that the Weierstrass formulation of elliptic curves omits. Eric D’Hoker discussed how the concept of transcendentality, previously used in field theory, could be applied to string theory. A few of his speculations seemed a bit farfetched (in particular, his setup needs to treat certain rational numbers as if they were transcendental), but after his talk I’m a bit more optimistic that there could be something useful there.

Amplitudes 2018

4 Replies

This week, I’m at Amplitudes, my field’s big yearly conference. The conference is at SLAC National Accelerator Laboratory this year, a familiar and lovely place.

Welcome to the Guest House California

It’s been a packed conference, with a lot of interesting talks. Recording and slides of most of them should be up at this point, for those following at home. I’ll comment on a few that caught my attention, I might do a more in-depth post later.

The first morning was dedicated to gravitational waves. At the QCD Meets Gravity conference last December I noted that amplitudes folks were very eager to do something relevant to LIGO, but that it was still a bit unclear how we could contribute (aside from Pierpaolo Mastrolia, who had already figured it out). The following six months appear to have cleared things up considerably, and Clifford Cheung and Donal O’Connel’s talks laid out quite concrete directions for this kind of research.

I’d seen Erik Panzer talk about the Hepp bound two weeks ago at Les Houches, but that was for a much more mathematically-inclined audience. It’s been interesting seeing people here start to see the implications: a simple method to classify and estimate (within 1%!) Feynman integrals could be a real game-changer.

Brenda Penante’s talk made me rethink a slogan I like to quote, that N=4 super Yang-Mills is the “most transcendental” part of QCD. While this is true in some cases, in many ways it’s actually least true for amplitudes, with quite a few counterexamples. For other quantities (like the form factors that were the subject of her talk) it’s true more often, and it’s still unclear when we should expect it to hold, or why.

Nima Arkani-Hamed has a reputation for talks that end up much longer than scheduled. Lately, it seems to be due to the sheer number of projects he’s working on. He had to rush at the end of his talk, which would have been about cosmological polytopes. I’ll have to ask his collaborator Paolo Benincasa for an update when I get back to Copenhagen.

Tuesday afternoon was a series of talks on the “NNLO frontier”, two-loop calculations that form the state of the art for realistic collider physics predictions. These talks brought home to me that the LHC really does need two-loop precision, and that the methods to get it are still pretty cumbersome. For those of us off in the airy land of six-loop N=4 super Yang-Mills, this is the challenge: can we make what these people do simpler?

Wednesday cleared up a few things for me, from what kinds of things you can write down in “fishnet theory” to how broad Ashoke Sen’s soft theorem is, to how fast John Joseph Carrasco could show his villanelle slide. It also gave me a clearer idea of just what simplifications are available for pushing to higher loops in supergravity.

Wednesday was also the poster session. It keeps being amazing how fast the field is growing, the sheer number of new faces was quite inspiring. One of those new faces pointed me to a paper I had missed, suggesting that elliptic integrals could end up trickier than most of us had thought.

Thursday featured two talks by people who work on the Conformal Bootstrap, one of our subfield’s closest relatives. (We’re both “bootstrappers” in some sense.) The talks were interesting, but there wasn’t a lot of engagement from the audience, so if the intent was to make a bridge between the subfields I’m not sure it panned out. Overall, I think we’re mostly just united by how we feel about Simon Caron-Huot, who David Simmons-Duffin described as “awesome and mysterious”. We also had an update on attempts to extend the Pentagon OPE to ABJM, a three-dimensional analogue of N=4 super Yang-Mills.

I’m looking forward to Friday’s talks, promising elliptic functions among other interesting problems.

At the GGI Lectures on the Theory of Fundamental Interactions

2 Replies

I’m at the Galileo Galilei Institute for Theoretical Physics in Florence at their winter school, the GGI Lectures on the Theory of Fundamental Interactions. Next week I’ll be helping Lance Dixon teach Amplitudeology, this week, I’m catching the tail end of Ira Rothstein’s lectures.

The Galileo Galilei Institute, at the end of a long, winding road filled with small, speedy cars and motorcycles, in classic Italian fashion

Rothstein has been heavily involved in doing gravitational wave calculations using tools from quantum field theory, something that has recently captured a lot of interest from amplitudes people. Specifically, he uses Effective Field Theory, theories that are “effectively” true at some scale but hide away higher-energy physics. In the case of gravitational waves, these theories are a powerful way to calculate the waves that LIGO and VIRGO can observe without using the full machinery of general relativity.

After seeing Rothstein’s lectures, I’m reminded of something he pointed out at the QCD Meets Gravity conference in December. He emphasized then that even if amplitudes people get very good at drawing diagrams for classical general relativity, that won’t be the whole story: there’s a series of corrections needed to “match” between the quantities LIGO is able to see and the ones we’re able to calculate. Different methods incorporate these corrections in different ways, and the most intuitive approach for us amplitudes folks may still end up cumbersome once all the corrections are included. In typical amplitudes fashion, this just makes me wonder if there’s a shortcut: some way to compute, not just a piece that gets plugged in to an Effective Field Theory story, but the waves LIGO sees in one fell swoop (or at least, the part where gravity is weak enough that our methods are still useful). That’s probably a bit naive of me, though.

4gravitons Meets QCD Meets Gravity

A LIGO in the Darkness

5 Replies

For the few of you who haven’t yet heard: LIGO has detected gravitational waves from a pair of colliding neutron stars, and that detection has been confirmed by observations of the light from those stars.

They also provide a handy fact sheet.

This is a big deal! On a basic level, it means that we now have confirmation from other instruments and sources that LIGO is really detecting gravitational waves.

The implications go quite a bit further than that, though. You wouldn’t think that just one observation could tell you very much, but this is an observation of an entirely new type, the first time an event has been seen in both gravitational waves and light.

That, it turns out, means that this one observation clears up a whole pile of mysteries in one blow. It shows that at least some gamma ray bursts are caused by colliding neutron stars, that neutron star collisions can give rise to the high-power “kilonovas” capable of forming heavy elements like gold…well, I’m not going to be able to give justice to the full implications in this post. Matt Strassler has a pair of quite detailed posts on the subject, and Quanta magazine’s article has a really great account of the effort that went into the detection, including coordinating the network of telescopes that made it possible.

I’ll focus here on a few aspects that stood out to me.

One fun part of the story behind this detection was how helpful “failed” observations were. VIRGO (the European gravitational wave experiment) was running alongside LIGO at the time, but VIRGO didn’t see the event (or saw it so faintly it couldn’t be sure it saw it). This was actually useful, because VIRGO has a blind spot, and VIRGO’s non-observation told them the event had to have happened in that blind spot. That narrowed things down considerably, and allowed telescopes to close in on the actual merger. IceCube, the neutrino observatory that is literally a cubic kilometer chunk of Antarctica filled with sensors, also failed to detect the event, and this was also useful: along with evidence from other telescopes, it suggests that the “jet” of particles emitted by the merged neutron stars is tilted away from us.

One thing brought up at LIGO’s announcement was that seeing gravitational waves and electromagnetic light at roughly the same time puts limits on any difference between the speed of light and the speed of gravity. At the time I wondered if this was just a throwaway line, but it turns out a variety of proposed modifications of gravity predict that gravitational waves will travel slower than light. This event rules out many of those models, and tightly constrains others.

The announcement from LIGO was screened at NBI, but they didn’t show the full press release. Instead, they cut to a discussion for local news featuring NBI researchers from the various telescope collaborations that observed the event. Some of this discussion was in Danish, so it was only later that I heard about the possibility of using the simultaneous measurement of gravitational waves and light to measure the expansion of the universe. While this event by itself didn’t result in a very precise measurement, as more collisions are observed the statistics will get better, which will hopefully clear up a discrepancy between two previous measures of the expansion rate.

A few news sources made it sound like observing the light from the kilonova has let scientists see directly which heavy elements were produced by the event. That isn’t quite true, as stressed by some of the folks I talked to at NBI. What is true is that the light was consistent with patterns observed in past kilonovas, which are estimated to be powerful enough to produce these heavy elements. However, actually pointing out the lines corresponding to these elements in the spectrum of the event hasn’t been done yet, though it may be possible with further analysis.

A few posts back, I mentioned a group at NBI who had been critical of LIGO’s data analysis and raised doubts of whether they detected gravitational waves at all. There’s not much I can say about this until they’ve commented publicly, but do keep an eye on the arXiv in the next week or two. Despite the optimistic stance I take in the rest of this post, the impression I get from folks here is that things are far from fully resolved.

Congratulations to Rainer Weiss, Barry Barish, and Kip Thorne!

4 Replies

The Nobel Prize in Physics was announced this week, awarded to Rainer Weiss, Kip Thorne, and Barry Barish for their work on LIGO, the gravitational wave detector.

Many expected the Nobel to go to LIGO last year, but the Nobel committee waited. At the time, it was expected the prize would be awarded to Rainer Weiss, Kip Thorne, and Ronald Drever, the three founders of the LIGO project, but there were advocates for Barry Barish was well. Traditionally, the Nobel is awarded to at most three people, so the argument got fairly heated, with opponents arguing Barish was “just an administrator” and advocates pointing out that he was “just the administrator without whom the project would have been cancelled in the 90’s”.

All of this ended up being irrelevant when Drever died last March. The Nobel isn’t awarded posthumously, so the list of obvious candidates (or at least obvious candidates who worked on LIGO) was down to three, which simplified thing considerably for the committee.

LIGO’s work is impressive and clearly Nobel-worthy, but I would be remiss if I didn’t mention that there is some controversy around it. In June, several of my current colleagues at the Niels Bohr Institute uploaded a paper arguing that if you subtract the gravitational wave signal that LIGO claims to have found then the remaining data, the “noise”, is still correlated between LIGO’s two detectors, which it shouldn’t be if it were actually just noise. LIGO hasn’t released an official response yet, but a LIGO postdoc responded with a guest post on Sean Carroll’s blog, and the team at NBI had responses of their own.

I’d usually be fairly skeptical of this kind of argument: it’s easy for an outsider looking at the data from a big experiment like this to miss important technical details that make the collaboration’s analysis work. That said, having seen some conversations between these folks, I’m a bit more sympathetic. LIGO hadn’t been communicating very clearly initially, and it led to a lot of unnecessary confusion on both sides.

One thing that I don’t think has been emphasized enough is that there are two claims LIGO is making: that they detected gravitational waves, and that they detected gravitational waves from black holes of specific masses at a specific distance. The former claim could be supported by the existence of correlated events between the detectors, without many assumptions as to what the signals should look like. The team at NBI seem to have found a correlation of that sort, but I don’t know if they still think the argument in that paper holds given what they’ve said elsewhere.

The second claim, that the waves were from a collision of black holes with specific masses, requires more work. LIGO compares the signal to various models, or “templates”, of black hole events, trying to find one that matches well. This is what the group at NBI subtracts to get the noise contribution. There’s a lot of potential for error in this sort of template-matching. If two templates are quite similar, it may be that the experiment can’t tell the difference between them. At the same time, the individual template predictions have their own sources of uncertainty, coming from numerical simulations and “loops” in particle physics-style calculations. I haven’t yet found a clear explanation from LIGO of how they take these various sources of error into account. It could well be that even if they definitely saw gravitational waves, they don’t actually have clear evidence for the specific black hole masses they claim to have seen.

I’m sure we’ll hear more about this in the coming months, as both groups continue to talk through their disagreement. Hopefully we’ll get a clearer picture of what’s going on. In the meantime, though, Weiss, Barish, and Thorne have accomplished something impressive regardless, and should enjoy their Nobel.

Simple Rules Don’t Mean a Simple Universe

4 gravitons

The trials and tribulations of four gravitons and a postdoc