Tag Archives: LIGO

Amplitudes 2018

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.

IMG_20180620_183339441_HDR

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.

Advertisements

At the GGI Lectures on the Theory of Fundamental Interactions

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.

IMG_20180112_082716844

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

I’m at UCLA this week, for the workshop QCD Meets Gravity. I haven’t worked on QCD or gravity yet, so I’m mostly here as an interested observer, and as an excuse to enjoy Los Angeles in December.

IMG_20171213_082840263_HDR

I think there’s a song about this…

QCD Meets Gravity is a conference centered around the various ways that “gravity is Yang-Mills squared”. There are a number of tricks that let you “square” calculations in Yang-Mills theories (a type of theory that includes QCD) to get calculations in gravity, and this conference showcased most of them.

At Amplitudes this summer, I was disappointed there were so few surprises. QCD Meets Gravity was different, with several talks on new or preliminary results, including one by Julio Parra-Martinez where the paper went up in the last few minutes of the talk! Yu-tin Huang talked about his (still-unpublished) work with Nima Arkani-Hamed on “UV/IR Polytopes”. The story there is a bit like the conformal bootstrap, with constraints (in this case based on positivity) marking off a space of “allowed” theories. String theory, interestingly, is quite close to the boundary of what is allowed. Enrico Herrmann is working on a way to figure out which gravity integrands are going to diverge without actually integrating them, while Simon Caron-Huot, in his characteristic out-of-the-box style, is wondering whether supersymmetric black holes precess. We also heard a bit more about a few recent papers. Oliver Schlotterer’s talk cleared up one thing: apparently the GEF functions he defines in his paper on one-loop “Z theory” are pronounced “Jeff”. I kept waiting for him to announce “Jeff theory”, but unfortunately no such luck. Sebastian Mizera’s talk was a very clear explanation of intersection theory, the subject of his recent paper. As it turns out, intersection theory is the study of mathematical objects like the Beta function (which shows up extensively in string theory), taking them apart in a way very reminiscent of the “squaring” story of Yang-Mills and gravity.

The heart of the workshop this year was gravitational waves. Since LIGO started running, amplitudes researchers (including, briefly, me) have been looking for ways to get involved. This conference’s goal was to bring together amplitudes people and the gravitational wave community, to get a clearer idea of what we can contribute. Between talks and discussions, I feel like we all understand the problem better. Some things that the amplitudes community thought were required, like breaking the symmetries of special relativity, turn out to be accidents of how the gravitational wave community calculates things: approximations that made things easier for them, but make things harder for us. There are areas in which we can make progress quite soon, even areas in which amplitudes people have already made progress. The detectors for which the new predictions matter might still be in the future (LIGO can measure two or three “loops”, LISA will see up to four), but they will eventually be measured. Amplitudes and gravitational wave physics could turn out to be a very fruitful partnership.

 

A LIGO in the Darkness

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.

gw170817_factsheet

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!

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.

Nobel2017

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

It’s always fun when nature surprises you.

This week, the Perimeter Colloquium was given by Laura Nuttall, a member of the LIGO collaboration.

In a physics department, the colloquium is a regularly scheduled talk that’s supposed to be of interest to the entire department. Some are better at this than others, but this one was pretty fun. The talk explored the sorts of questions gravitational wave telescopes like LIGO can answer about the world.

At one point during the talk, Nuttall showed a plot of what happens when a star collapses into a supernova. For a range of masses, the supernova leaves behind a neutron star (shown on the plot in purple). For heavier stars, it instead results in a black hole, a big black region of the plot.

What surprised me was that inside the black region, there was an unexpected blob: a band of white in the middle of the black holes. Heavier than that band, the star forms a black hole. Lighter, it also forms a black hole. But inside?

Nothing. The star leaves nothing behind. It just explodes.

The physical laws that govern collapsing stars might not be simple, but at least they sound straightforward. Stars are constantly collapsing under their own weight, held up only by the raging heat of nuclear fire. If that heat isn’t strong enough, the star collapses, and other forces take over, so the star becomes a white dwarf, or a neutron star. And if none of those forces is strong enough, the star collapses completely, forming a black hole.

Too small, neutron star. Big enough, black hole. It seems obvious. But reality makes things more complicated.

It turns out, if a star is both massive and has comparatively little metal in it, the core of the star can get very very hot. That heat powers an explosion more powerful than a typical star, one that tears the star apart completely, leaving nothing behind that could form a black hole. Lighter stars don’t get as hot, so they can still form black holes, and heavier stars are so heavy they form black holes anyway. But for those specific stars, in the middle, nothing gets left behind.

This isn’t due to mysterious unknown physics. It’s actually been understood for quite some time. It’s a consequence of those seemingly straightforward laws, one that isn’t at all obvious until you do the work and run the simulations and observe the universe and figure it out.

Just because our world is governed by simple laws, doesn’t mean the universe itself is simple. Give it a little room (and several stars’ worth of hydrogen) and it can still surprise you.

What Space Can Tell Us about Fundamental Physics

Back when LIGO announced its detection of gravitational waves, there was one question people kept asking me: “what does this say about quantum gravity?”

The answer, each time, was “nothing”. LIGO’s success told us nothing about quantum gravity, and very likely LIGO will never tell us anything about quantum gravity.

The sheer volume of questions made me think, though. Astronomy, astrophysics, and cosmology fascinate people. They capture the public’s imagination in a way that makes them expect breakthroughs about fundamental questions. Especially now, with the LHC so far seeing nothing new since the Higgs, people are turning to space for answers.

Is that a fair expectation? Well, yes and no.

Most astrophysicists aren’t concerned with finding new fundamental laws of nature. They’re interested in big systems like stars and galaxies, where we know most of the basic rules but can’t possibly calculate all their consequences. Like most physicists, they’re doing the vital work of “physics of decimals”.

At the same time, there’s a decent chunk of astrophysics and cosmology that does matter for fundamental physics. Just not all of it. Here are some of the key areas where space has something important to say about the fundamental rules that govern our world:

 

1. Dark Matter:

Galaxies rotate at different speeds than their stars would alone. Clusters of galaxies bend light that passes by, and do so more than their visible mass would suggest. And when scientists try to model the evolution of the universe, from early images to its current form, the models require an additional piece: extra matter that cannot interact with light. All of this suggests that there is some extra “dark” matter in the universe, not described by our standard model of particle physics.

If we want to understand this dark matter, we need to know more about its properties, and much of that can be learned from astronomy. If it turns out dark matter isn’t really matter after all, if it can be explained by a modification of gravity or better calculations of gravity’s effects, then it still will have important implications for fundamental physics, and astronomical evidence will still be key to finding those implications.

2. Dark Energy (/Cosmological Constant/Inflation/…):

The universe is expanding, and its expansion appears to be accelerating. It also seems more smooth and uniform than expected, suggesting that it had a period of much greater acceleration early on. Both of these suggest some extra quantity: a changing acceleration, a “dark energy”, the sort of thing that can often be explained by a new scalar field like the Higgs.

Again, the specifics: how (and perhaps if) the universe is expanding now, what kinds of early expansion (if any) the shape of the universe suggests, these will almost certainly have implications for fundamental physics.

3. Limits on stable stuff:

Let’s say you have a new proposal for particle physics. You’ve predicted a new particle, but it can’t interact with anything else, or interacts so weakly we’d never detect it. If your new particle is stable, then you can still say something about it, because its mass would have an effect on the early universe. Too many such particles and they would throw off cosmologists’ models, ruling them out.

Alternatively, you might predict something that could be detected, but hasn’t, like a magnetic monopole. Then cosmologists can tell you how many such particles would have been produced in the early universe, and thus how likely we would be to detect them today. If you predict too many particles and we don’t see them, then that becomes evidence against your proposal.

4. “Cosmological Collider Physics”:

A few years back, Nima Arkani-Hamed and Juan Maldacena suggested that the early universe could be viewed as an extremely high energy particle collider. While this collider performed only one experiment, the results from that experiment are spread across the sky, and observed patterns in the early universe should tell us something about the particles produced by the cosmic collider.

People are still teasing out the implications of this idea, but it looks promising, and could mean we have a lot more to learn from examining the structure of the universe.

5. Big Weird Space Stuff:

If you suspect we live in a multiverse, you might want to look for signs of other universes brushing up against our own. If your model of the early universe predicts vast cosmic strings, maybe a gravitational wave detector like LIGO will be able to see them.

6. Unexpected weirdness:

In all likelihood, nothing visibly “quantum” happens at the event horizons of astrophysical black holes. If you think there’s something to see though, the Event Horizon Telescope might be able to see it. There’s a grab bag of other predictions like this: situations where we probably won’t see anything, but where at least one person thinks there’s a question worth asking.

 

I’ve probably left something out here, but this should give you a general idea. There is a lot that fundamental physics can learn from astronomy, from the overall structure and origins of the universe to unexplained phenomena like dark matter. But not everything in astronomy has these sorts of implications: for the most part, astronomy is interesting not because it tells us something about the fundamental laws of nature, but because it tells us how the vast space above us actually happens to work.