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.

A Response from Nielsen, Guffanti and Sarkar

I have been corresponding with Subir Sarkar, one of the authors of the paper I mentioned a few weeks ago arguing that the evidence for cosmic acceleration was much weaker than previously thought. He believes that the criticisms of Rubin and Hayden (linked to in my post) are deeply flawed. Since he and his coauthors haven’t responded publicly to Rubin and Hayden yet, they graciously let me post a summary of their objections.

Dear Matt,

This concerns the discussion on your blog of our recent paper showing that the evidence for cosmic acceleration from supernovae is only 3 sigma. Your obviously annoyed response is in fact to inflated headlines in the media about our work – our paper does just what it does on the can: “Marginal evidence for cosmic acceleration from Type Ia supernovae“. Nevertheless you make a fair assessment of the actual result in our paper and we are grateful for that.

However we feel you are not justified in going on further to state: “In the twenty years since it was discovered that the universe was accelerating, people have built that discovery into the standard model of cosmology. They’ve used that model to make other predictions, explaining a wide range of other observations. People have built on the discovery, and their success in doing so is its own kind of evidence”. If you were as expert in cosmology as you evidently are concerning amplitudes you would know that much of the reasoning you allude to is circular. There are also other instances (which we are looking into) of using statistical methods that assume the answer to shore up the ‘standard model’ of cosmology. Does it not worry you that the evidence from supernovae – which is widely believed to be compelling – turns out to be less so when examined closely? There is a danger of confirmation bias in that cosmologists making poor measurements with large systematic uncertainties nevertheless keep finding the ‘right answer’. See e.g. Croft & Dailey (http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1112.3108) who noted “… of the 28 measurements of Omega_Lambda in our sample published since 2003, only 2 are more than 1 sigma from the WMAP results. Wider use of blind analyses in cosmology could help to avoid this”. Unfortunately the situation has not improved in subsequent years.

You are of course entitled to air your personal views on your blog. But please allow us to point out that you are being unfair to us by uncritically stating in the second part of  your sentence: “EDIT: More arguments against the paper in question, pointing out that they made some fairly dodgy assumptions” in which you link to the arXiv eprint by Rubin & Hayden.

These authors make a claim similar to Riess & Scolnic (https://blogs.scientificamerican.com/guest-blog/no-astronomers-haven-t-decided-dark-energy-is-nonexistent/) that we “assume that the mean properties of supernovae from each of the samples used to measure the expansion history are the same, even though they have been shown to be different and past analyses have accounted for these differences”. In fact we are using exactly the same dataset (called JLA) as Adam Riess and co. have done in their own analysis (Betoule et alhttp://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1401.4064). They found  stronger evidence for acceleration because of using a flawed statistical method (“constrained \chi^2”). The reason why we find weaker evidence is that we use the Maximum Likelihood Estimator – it is not because of making “dodgy assumptions”. We show our results in the same \Omega_\Lambda – \Omega_m plane simply for ease of comparison with the previous result – as seen in the attached plot, the contours move to the right … and now enclose the “no acceleration” line within 3 \sigma. Our analysis is not – as Brian Schmidt tweeted – “at best unorthodox” … even if this too has been uncritically propagated on social media.

In fact the result from our (frequentist) statistical procedure has been confirmed by an independent analysis using a ‘Bayesian Hierarchical Model’ (Shariff et alhttp://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1510.05954). This is a more sophisticated approach because it does not adopt a Gaussian approximation as we did for the distribution of the light curve parameters (x_1 and c), however their contours are more ragged because of numerical computation limitations.

Rubin & Hayden do not mention this paper (although bizarrely they ascribe to us the ‘Bayesian Hierarchical Model’). Nevertheless they find more-or-less the same result as us, namely 3.1 sigma evidence for acceleration, using the same dataset as we did (left panel of their Fig.2). They argue however that there are selection effects in this dataset – which have not already been corrected for by the JLA collaboration (which incidentally included Adam Riess, Saul Perlmutter and most other supernova experts in the world). To address this Rubin & Hayden  introduce a redshift-dependent prior on the x_1 and c distributions. This increases the significance to 4.2 sigma (right panel of their Fig.2). If such a procedure is indeed valid then it does mark progress in the field, but that does not mean that these authors have “demonstrated errors in (our) analysis” as they state in their Abstract. Their result also begs the question why has the significance increased so little in going from the initial 50 supernovae which yielded 3.9 sigma evidence for acceleration (Riess et alhttp://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:astro-ph/9805201) to 740 supernovae in JLA? Maybe this is news … at least to anyone interested in cosmology and fundamental physics!

Rubin & Hayden also make the usual criticism that we have ignored evidence from other observations e.g. of baryon acoustic oscillations and the cosmic microwave background. We are of course very aware of these observations but as we say in the paper the interpretation of such data is very model-dependent. For example dark energy has no direct influence on the cosmic microwave background. What is deduced from the data is the spatial curvature (adopting the value of the locally measured Hubble expansion rate H_0) and the fractional matter content of the universe (assuming the primordial fluctuation spectrum to be a close-to-scale-invariant power law). Dark energy is then *assumed* to make up the rest (using the sum rule: 1 = \Omega_m + \Omega_\Lambda for a spatially flat universe as suggested by the data). This need not be correct however if there are in fact other terms that should be added to this sum rule (corresponding to corrections to the Friedman equation to account e.g. for averaging over inhomogeneities or for non-ideal gas behaviour of the matter content). It is important to emphasise that there is no convincing (i.e. >5 sigma) dynamical evidence for dark energy, e.g. the late integrated Sachs-Wolfe effect which induces subtle correlations between the CMB and large-scale structure. Rubin & Hayden even claim in their Abstract (v1) that “The combined analysis of modern cosmological experiments … indicate 75 sigma evidence for positive Omega_\Lambda” – which is surely a joke! Nevertheless this is being faithfully repeated on newsgroups, presumably by those somewhat challenged in their grasp of basic statistics.

Apologies for the long post but we would like to explain that the technical criticism of our work by Rubin & Hayden and by Riess & Scolnic is rather disingenuous and it is easy to be misled if you are not an expert. You are entitled to rail against the standards of science journalism but please do not taint us by association.

As a last comment, surely we all want to make progress in cosmology but this will be hard if cosmologists are so keen to cling on to their ‘standard model’ instead of subjecting it to critical tests (as particle physicists continually do to their Standard Model). Moreover the fundamental assumptions of the cosmological model (homogeneity, ideal fluids) have not been tested rigorously (unlike the Standard Model which has been tested at the level of quantum corrections). This is all the more important in cosmology because there is simply no physical explanation for why \Lambda should be of order H_0^2.

Best regards,

Jeppe Trøst Nielsen, Alberto Guffanti and Subir Sarkar

On an unrelated note, Perimeter’s PSI program is now accepting applications for 2017. It’s something I wish I knew about when I was an undergrad, for those interested in theoretical physics it can be an enormous jump-start to your career. Here’s their blurb:

Perimeter Scholars International (PSI) is now accepting applications for Perimeter Institute for Theoretical Physics’ unique 10-month Master’s program.

Features of the program include:

• All student costs (tuition and living) are covered, removing financial and/or geographical barriers to entry
• Students learn from world-leading theoretical physicists – resident Perimeter researchers and visiting scientists – within the inspiring environment of Perimeter Institute
• Collaboration is valued over competition; deep understanding and creativity are valued over rote learning and examination
• PSI recruits worldwide: 85 percent of students come from outside of Canada
• PSI takes calculated risks, seeking extraordinary talent who may have non-traditional academic backgrounds but have demonstrated exceptional scientific aptitude

Apply online at http://perimeterinstitute.ca/apply.

Applications are due by February 1, 2017.

“Maybe” Isn’t News

It’s been published several places, but you’ve probably seen this headline:

If you’ve been following me for a while, you know where this is going:

No, these physicists haven’t actually shown that the Universe isn’t expanding at an accelerated rate.

What they did show is that the original type of data used to discover that the universe was accelerating back in the 90’s, measurements of supernovae, doesn’t live up to the rigorous standards that we physicists use to evaluate discoveries. We typically only call something a discovery if the evidence is good enough that, in a world where the discovery wasn’t actually true, we’d only have a one in 3.5 million chance of getting the same evidence (“five sigma” evidence). In their paper, Nielsen, Guffanti, and Sarkar argue that looking at a bigger collection of supernovae leads to a hazier picture: the chance that we could get the same evidence in a universe that isn’t accelerating is closer to one in a thousand, giving “three sigma” evidence.

This might sound like statistical quibbling: one in a thousand is still pretty unlikely, after all. But a one in a thousand chance still happens once in a thousand times, and there’s a long history of three sigma evidence turning out to just be random noise. If the discovery of the accelerating universe was new, this would be an important objection, a reason to hold back and wait for more data before announcing a discovery.

The trouble is, the discovery isn’t new. In the twenty years since it was discovered that the universe was accelerating, people have built that discovery into the standard model of cosmology. They’ve used that model to make other predictions, explaining a wide range of other observations. People have built on the discovery, and their success in doing so is its own kind of evidence.

So the objection, that one source of evidence isn’t as strong as people thought, doesn’t kill cosmic acceleration. What it is is a “maybe”, showing that there is at least room in some of the data for a non-accelerating universe.

People publish “maybes” all the time, nothing bad about that. There’s a real debate to be had about how strong the evidence is, and how much it really establishes. (And there are already voices on the other side of that debate.)

But a “maybe” isn’t news. It just isn’t.

Science journalists (and university press offices) have a habit of trying to turn “maybes” into stories. I’ve lost track of the times I’ve seen ideas that were proposed a long time ago (technicolor, MOND, SUSY) get new headlines not for new evidence or new ideas, but just because they haven’t been ruled out yet. “SUSY hasn’t been ruled out yet” is an opinion piece, perhaps a worthwhile one, but it’s no news article.

The thing is, I can understand why journalists do this. So much of science is building on these kinds of “maybes”, working towards the tipping point where a “maybe” becomes a “yes” (or a “no”). And journalists (and university press offices, and to some extent the scientists themselves) can’t just take time off and wait for something legitimately newsworthy. They’ve got pages to fill and careers to advance, they need to say something.

I post once a week. As a consequence, a meaningful fraction of my posts are garbage. I’m sure that if I posted every day, most of my posts would be garbage.

Many science news sites post multiple times a day. They’ve got multiple writers, sure, and wider coverage…but they still don’t have the luxury of skipping a “maybe” when someone hands it to them.

I don’t know if there’s a way out of this. Maybe we need a new model for science journalism, something that doesn’t try to ape the pace of the rest of the news cycle. For the moment, though, it’s publish or perish, and that means lots and lots of “maybes”.

EDIT: More arguments against the paper in question, pointing out that they made some fairly dodgy assumptions.

EDIT: The paper’s authors respond here.

Starshot: The Right Kind of Longshot

On Tuesday, Yuri Milner and Stephen Hawking announced Starshot, a $100 million dollar research initiative. The goal is to lay the groundwork for a very ambitious, but surprisingly plausible project: sending probes to the nearest star, Alpha Centauri. Their idea is to have hundreds of ultra-light probes, each with a reflective sail a few meters in diameter. By aiming an extremely powerful laser at these sails, it should be possible to accelerate the probes up to around a fifth of the speed of light, enough to make the trip in twenty years. Here’s the most complete article I’ve found on the topic. I can’t comment on the engineering side of the project. The impression I get is that nothing they’re proposing is known to be impossible, but there are a lot of “ifs” along the way that might scupper things. What I can comment on is the story. Milner and Hawking have both put quite a bit of effort recently into what essentially amounts to telling stories. Milner’s Breakthrough Prizes involve giving awards of$3 million to prominent theoretical physicists (and, more recently, mathematicians). Quite a few of my fellow theorists have criticized these prizes, arguing that the money would be better spent in a grant program like that of the Simons Foundation. While that would likely be better for science, the Breakthrough Prize isn’t really about that. Instead, it’s about telling a story: a story in which progress in theoretical physics is exalted in a public, Nobel-sized way.

Similarly, Hawking’s occasional pronouncements about aliens or AI aren’t science per se, and the media has a tendency to talk about his contributions to ongoing scientific debates out of proportion to their importance. Both of these things, though, contribute to the story of Hawking: a mascot for physics, someone to carry Einstein’s role of the most recognizable genius in the world. Hawking Inc. is about a role as much as it is about a man.

In calling Hawking and Milner’s activity “stories”, I’m not dismissing them. Stories can be important. And the story told by Starshot is a particularly important one.

Cosmology isn’t just a scientific subject, it contributes to how people see themselves. Here I don’t just mean cosmology the field, but cosmology in the broader sense of our understanding of the universe and our place in it.

A while back, I read a book called The View from the Center of the Universe. The book starts by describing the worldviews of the ancients, cosmologies in which they really did think of themselves as the center of the universe. It then suggests that this played an important role: that this kind of view of the world, in which humans have a place in the cosmos, is important to how we view ourselves. The rest of the book then attempts to construct this sort of mythological understanding out of the modern cosmological picture, with some success.

One thing the book doesn’t discuss very much, though, is the future. We care about our place in the universe not just because we want to know where we came from, but because we want to have some idea of where we’re going. We want to contribute to a greater goal, to see ourselves making progress towards something important and vast and different. That’s why so many religions have not just cosmologies, but eschatologies, why people envision armageddons and raptures.

Starshot places the future in our sight in a way that few other things do. Humanity’s spread among the stars seems like something so far distant that nothing we do now could matter to it. What Starshot does is give us something concrete, a conceptual stepping-stone that can link people in to the broader narrative. Right now, people can work on advanced laser technology and optics, work on making smaller chips and lighter materials, work that would be useful and worth funding regardless of whether it was going to lead to Alpha Centauri. But because of Starshot, we can view that work as the near-term embodiment of humanity’s interstellar destiny.

That combination, bridging the gap between the distant future and our concrete present, is the kind of story people need right now. And so for once, I think Milner’s storytelling is doing exactly what it should.

You Go, LIGO!

Well folks, they did it. LIGO has detected gravitational waves!

FAQ:

What’s a gravitational wave?

Gravitational waves are ripples in space and time. As Einstein figured out a century ago, masses bend space and time, which causes gravity. Wiggle masses in the right way and you get a gravity wave, like a ripple on a pond.

Ok, but what is actually rippling? It’s some stuff, right? Dust or something?

In a word, no. Not everything has to be “stuff”. Energy isn’t “stuff”, and space-time isn’t either, but space-time is really what vibrates when a gravitational wave passes by. Distances themselves are changing, in a way that is described by the same math and physics as a ripple in a pond.

What’s LIGO?

LIGO is the Laser Interferometer Gravitational-Wave Observatory. In simple terms, it’s an observatory (or rather, a pair of observatories in Washington and Louisiana) that can detect gravitational waves. It does this using beams of laser light four kilometers long. Gravitational waves change the length of these beams when they pass through, causing small but measurable changes in the laser light observed.

Are there other gravitational wave observatories?

Not currently in operation. LIGO originally ran from 2002 to 2010, and during that time there were other gravitational wave observatories also in operation (VIRGO in Italy and GEO600 in Germany). All of them (including LIGO) failed to detect anything, and so LIGO and VIRGO were shut down in order for them to be upgraded to more sensitive, advanced versions. Advanced LIGO went into operation first, and made the detection. VIRGO is still under construction, as is KAGRA, a detector in Japan. There are also plans for a detector in India.

Other sorts of experiments can detect gravitational waves on different scales. eLISA is a planned space-based gravitational wave observatory, while Pulsar Timing Arrays could use distant neutron stars as an impromptu detector.

What did they detect? What could they detect?

The gravitational waves that LIGO detected came from a pair of black holes merging. In general, gravitational waves come from a pair of masses, or one mass with an uneven and rapidly changing shape. As such, LIGO and future detectors might be able to observe binary stars, supernovas, weird-shaped neutron stars, colliding galaxies…pretty much any astrophysical event involving large things moving comparatively fast.

What does this say about string theory?

Basically nothing. There are gravity waves in string theory, sure (and they play a fairly important role), but there were gravity waves in Einstein’s general relativity. As far as I’m aware, no-one at this point seriously thought that gravitational waves didn’t exist. Nothing that LIGO observed has any bearing on the quantum properties of gravity.

But what about cosmic strings? They mentioned those in the announcement!

Cosmic strings, despite the name, aren’t a unique prediction of string theory. They’re big, string-shaped wrinkles in space and time, possible results of the rapid expansion of space during cosmic inflation. You can think of them a bit like the cracks that form in an over-inflated balloon right before it bursts.

Cosmic strings, if they exist, should produce gravitational waves. This means that in the future we may have concrete evidence of whether or not they exist. This wouldn’t say all that much about string theory: while string theory does have its own explanations for cosmic strings, it’s unclear whether it actually has unique predictions about them. It would say a lot about cosmic inflation, though, and would presumably help distinguish it from proposed alternatives. So keep your eyes open: in the next few years, gravitational wave observatories may well have something important to say about the overall history of the universe.

Why is this discovery important, though? If we already knew that gravitational waves existed, why does discovering them matter?

LIGO didn’t discover that gravitational waves exist. LIGO discovered that we can detect them.

The existence of gravitational waves is no discovery. But the fact that we now have observatories sensitive enough to detect them is huge. It opens up a whole new type of astronomy: we can now observe the universe not just by the light it sheds (and neutrinos), but through a whole new lens. And every time we get another observational tool like this, we notice new things, things we couldn’t have seen without it. It’s the dawn of a new era in astronomy, and LIGO was right to announce it with all the pomp and circumstance they could muster.

My impressions from the announcement:

Speaking of pomp and circumstance, I was impressed by just how well put-together LIGO’s announcement was.

As the US presidential election heats up, I’ve seen a few articles about the various candidates’ (well, usually Trump’s) use of the language of political propaganda. The idea is that there are certain visual symbols at political events for which people have strong associations, whether with historical events or specific ideas or the like, and that using these symbols makes propaganda more powerful.

What I haven’t seen is much discussion of a language of scientific propaganda. Still, the overwhelming impression I got from LIGO’s announcement is that it was shaped by a master in the use of such a language. They tapped in to a wide variety of powerful images: from the documentary-style interviews at the beginning, to Weiss’s tweed jacket and handmade demos, to the American flag in the background, that tied LIGO’s result to the history of scientific accomplishment.

Perimeter’s presentations tend to have a slicker look, my friends at Stony Brook are probably better at avoiding jargon. But neither is quite as good at propaganda, at saying “we are part of history” and doing so without a hitch, as the folks at LIGO have shown themselves to be with this announcement.

I was also fairly impressed that they kept this under wraps for so long. While there were leaks, I don’t think many people had a complete grasp of what was going to be announced until the week before. Somehow, LIGO made sure a collaboration of thousands was able to (mostly) keep their mouths shut!

Beyond the organizational and stylistic notes, my main thought was “What’s next?” They’ve announced the detection of one event. I’ve heard others rattle off estimates, that they should be detecting anywhere from one black hole merger per year to a few hundred. Are we going to see more events soon, or should we settle into a long wait? Could they already have detected more, with the evidence buried in their data, to be revealed by careful analysis? (The waves from this black hole merger were clear enough for them to detect them in real-time, but more subtle events might not make things so easy!) Should we be seeing more events already, and does not seeing them tell us something important about the universe?

Most of the reason I delayed my post till this week was to see if anyone had an answer to these questions. So far, I haven’t seen one, besides the “one to a few hundred” estimate mentioned. As more people weigh in and more of LIGO’s run is analyzed, it will be interesting to see where that side of the story goes.

Gravitational Waves, and Valentine’s Day Physics Poem 2016

By the time this post goes up, you’ll probably have seen Advanced LIGO’s announcement of the first direct detection of a gravitational wave. We got the news a bit early here at Perimeter, which is why we were able to host a panel discussion right after the announcement.

From what I’ve heard, this is the real deal. They’ve got a beautifully clear signal, and unlike BICEP, they kept this under wraps until they could get it looked at by non-LIGO physicists. While I think peer review gets harped on a little too much in these sorts of contexts, in this case their paper getting through peer review is a good sign that they’re really seeing something.

Pictured: a very clear, very specific something

I’ll have more to say next week: explanations of gravitational waves and LIGO for my non-expert audience, and impressions from the press release and PI’s panel discussion for those who are interested. For now, though, I’ll wait until the dust (metaphorical this time) settles. If you’re hungry for immediate coverage, I’m sure that half the blogs on my blogroll have posts up, or will in the next few days.

In the meantime, since Valentine’s Day is in two days, I’ll continue this blog’s tradition and post one of my old physics poems.

When a sophisticated string theorist seeks an interaction

He does not go round and round in loops

As a young man would.

Mature, the string theorist knows

That what happens on

(And between)

The (world) sheets,

Is universal.

That the process is the same

No matter which points

Which interactions

One chooses.

Only the shapes of things matter.

Only the topology.

For such a man there is no need.

To obsess

To devote

To choose

One point or another.

The interaction is the same.

The world, though

Is not an exercise in theory.

Is not a mere possibility.

And if a theorist would compute

An experiment

A probability

He must pick and choose

Obsess and devote

Label his interactions with zeroes and infinities

Because there is more to life

Than just the shapes of things

Than just topology.