Tag Archives: science communication

The Quantum Kids

I gave a pair of public talks at the Niels Bohr International Academy this week on “The Quest for Quantum Gravity” as part of their “News from the NBIA” lecture series. The content should be familiar to long-time readers of this blog: I talked about renormalization, and gravitons, and the whole story leading up to them.

(I wanted to title the talk “How I Learned to Stop Worrying and Love Quantum Gravity”, like my blog post, but was told Danes might not get the Doctor Strangelove reference.)

I also managed to work in some history, which made its way into the talk after Poul Damgaard, the director of the NBIA, told me I should ask the Niels Bohr Archive about Gamow’s Thought Experiment Device.

“What’s a Thought Experiment Device?”

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This, apparently

If you’ve heard of George Gamow, you’ve probably heard of the Alpher-Bethe-Gamow paper, his work with grad student Ralph Alpher on the origin of atomic elements in the Big Bang, where he added Hans Bethe to the paper purely for an alpha-beta-gamma pun.

As I would learn, Gamow’s sense of humor was prominent quite early on. As a research fellow at the Niels Bohr Institute (essentially a postdoc) he played with Bohr’s kids, drew physics cartoons…and made Thought Experiment Devices. These devices were essentially toy experiments, apparatuses that couldn’t actually work but that symbolized some physical argument. The one I used in my talk, pictured above, commemorated Bohr’s triumph over one of Einstein’s objections to quantum theory.

Learning more about the history of the institute, I kept noticing the young researchers, the postdocs and grad students.

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Lev Landau, George Gamow, Edward Teller. The kids are Aage and Ernest Bohr. Picture from the Niels Bohr Archive.

We don’t usually think about historical physicists as grad students. The only exception I can think of is Feynman, with his stories about picking locks at the Manhattan project. But in some sense, Feynman was always a grad student.

This was different. This was Lev Landau, patriarch of Russian physics, crowning name in a dozen fields and author of a series of textbooks of legendary rigor…goofing off with Gamow. This was Edward Teller, father of the Hydrogen Bomb, skiing on the institute lawn.

These were the children of the quantum era. They came of age when the laws of physics were being rewritten, when everything was new. Starting there, they could do anything, from Gamow’s cosmology to Landau’s superconductivity, spinning off whole fields in the new reality.

On one level, I envy them. It’s possible they were the last generation to be on the ground floor of a change quite that vast, a shift that touched all of physics, the opportunity to each become gods of their own academic realms.

I’m glad to know about them too, though, to see them as rambunctious grad students. It’s all too easy to feel like there’s an unbridgeable gap between postdocs and professors, to worry that the only people who make it through seem to have always been professors at heart. Seeing Gamow and Landau and Teller as “quantum kids” dispels that: these are all-too-familiar grad students and postdocs, joking around in all-too-familiar ways, who somehow matured into some of the greatest physicists of their era.

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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.

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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.

Textbook Review: Exploring Black Holes

I’m bringing a box of textbooks with me to Denmark. Most of them are for work: a few Quantum Field Theory texts I might use, a Complex Analysis book for when I inevitably forget how to do contour integration.

One of the books, though, is just for fun.

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Exploring Black Holes is an introduction to general relativity for undergraduates. The book came out of a collaboration between Edwin F. Taylor, known for his contributions to physics teaching, and John Archibald Wheeler, who among a long list of achievements was responsible for popularizing the term “black hole”. The result is something quite unique: a general relativity course that requires no math more advanced than calculus, and no physics more advanced than special relativity.

It does this by starting, not with the full tensor-riddled glory of Einstein’s equations, but with specialized solutions to those equations, mostly the Schwarzschild solution that describes space around spherical objects (including planets, stars, and black holes). From there, it manages to introduce curved space in a way that is both intuitive and naturally grows out of what students learn about special relativity. It really is the kind of course a student can take right after their first physics course, and indeed as an undergrad that’s exactly what I did.

With just the Schwarzchild solution and its close relatives, you can already answer most of the questions young students have about general relativity. In a series of “projects”, the book explores the corrections GR demands of GPS satellites, the process of falling into a black hole, the famous measurement of the advance of the perihelion of mercury, the behavior of light in a strong gravitational field, and even a bit of cosmology. In the end the students won’t know the full power of the theory, but they’ll get a taste while building valuable physical intuition.

Still, I wouldn’t bring this book with me if it was just an excellent undergraduate textbook. Exploring Black Holes is a great introduction to general relativity, but it also has a hilarious not-so-hidden agenda: inspiring future astronauts to jump into black holes.

“Nowhere could life be simpler or more relaxed than in a free-float frame, such as an unpowered spaceship falling toward a black hole.” – pg. 2-31

The book is full of quotes like this. One of the book’s “projects” involves computing what happens to an astronaut who falls into a black hole. The book takes special care to have students calculate that “spaghettification”, the process by which the tidal forces of a black hole stretch infalling observers into spaghetti, is surprisingly completely painless: the amount of time you experience it is always less than the amount of time it takes light (and thus also pain) to go from your feet to your head, for any (sufficiently calm) black hole.

Why might Taylor and Wheeler want people of the future to jump into black holes? As the discussion on page B-3 of the book describes, the reason is on one level an epistemic one. As theorists, we’d like to reason about what lies inside the event horizon of black holes, but we face a problem: any direct test would be trapped inside, and we would never know the result, which some would argue makes such speculation unscientific. What Taylor and Wheeler point out is that it’s not quite true that no-one would know the results of such a test: if someone jumped into a black hole, they would be able to test our reasoning. If a whole scientific community jumped in, then the question of what is inside a black hole is from their perspective completely scientific.

Of course, I don’t think Taylor and Wheeler seriously thought their book would convince its readers to jump into black holes. For one, it’s unlikely anyone reading the book will get a chance. Still, I suspect that the idea that future generations might explore black holes gave Taylor and Wheeler some satisfaction, and a nice clean refutation of those who think physics inside the horizon is unscientific. Seeing as the result was an excellent textbook full of hilarious prose, I can’t complain.

Join the Dark Side: Become a Seminar Organizer

Attending talks is the bane of many a physicist’s existence. Taking an hour out of your busy schedule to listen to someone you know you’ll only understand for fifteen minutes, hoping that they’ll at least give you a vague idea of why you should care but expecting that they won’t…who would willingly subject people to that?

Well, I would.

I’ve signed up to be the High Energy Theory Seminar organizer for the Niels Bohr Institute this year. Most physics institutes hold regular seminars, usually once or twice a week, where they invite speakers from the surrounding region and all over the world. Organizing these seminars is a job often handed to one of the local postdocs: in this case, me.

In the past I’ve put some thought into the purpose of seminars, but mostly from the perspective of someone attending and occasionally giving them. Now that I’m involved in organizing them, entirely new questions present themselves.

Are seminars for work, or for fun? On the one hand, seminars can be a way to keep up with your own field and pick up useful techniques from others. Looked at in that way, I should invite speakers whose interests line up with the researchers at NBI. On the other hand, seminars can be a good way to find out what’s going on outside of your own field, to satisfy your curiosity about the “next big thing”. Sometimes you see a paper and wish you could ask the author what they were thinking, seminars let you ask face to face.

Is it better to invite big names, or grad students? The big-name people might give better talks on more interesting topics, and they enhance the prestige of the seminar series. They also tend to be busy, and don’t need the talks as much as the grad students do.

People from nearby, or far away? It’s cheaper to invite people from nearby, but you want at least a few big names from farther away.

For most of these, the right approach is a balanced one. You want to invite people whose interests line up with your colleagues, but also a few more distant people for breadth. You want a mix of established big-name people and younger researchers, nearby people and far away ones.

The Niels Bohr Institute does a lot of seminars, typically two per week. Even with a co-organizer filling half of them, that’s a lot of ground to cover, a lot of room to balance all of these goals.

Seminar organizers get exposed to a wide range of researchers working on a wide range of topics. It’s supposed to be good for the career, the ultimate networking experience. For myself, I’m still quite specialized, so I’m hoping this will be a good opportunity to broaden my interests and learn about what others are doing. Along the way, perhaps I’ll get a better idea of what seminars are really for.

Shades of Translation

I was playing Codenames with some friends, a game about giving one-word clues to multi-word answers. I wanted to hint at “undertaker” and “march”, so I figured I’d do “funeral march”. Since that’s two words, I needed one word that meant something similar. I went with dirge, then immediately regretted it as my teammates spent the better part of two minutes trying to figure out what it meant. In the end they went with “slug”.

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A dirge in its natural habitat.

If I had gone for requiem instead, we would have won. Heck, if I had just used “funeral”, we would have had a fighting chance. I had assumed my team knew the same words I did: they were also native English speakers, also nerds, etc. But the words they knew were still a shade different from the words I knew, and that made the difference.

When communicating science, you have to adapt to your audience. Knowing this, it’s still tempting to go for a shortcut. You list a few possible audiences, like “physicists”, or “children”, and then just make a standard explanation for each. This works pretty well…until it doesn’t, and your audience assumes a “dirge” is a type of slug.

In reality, each audience is different. Rather than just memorizing “translations” for a few specific groups, you need to pay attention to the shades of understanding in between.

On Wednesdays, Perimeter holds an Interdisciplinary Lunch. They cover a table with brown paper (for writing on) and impose one rule: you can’t sit next to someone in the same field.

This week, I sat next to an older fellow I hadn’t met before. He asked me what I did, and I gave my “standard physicist explanation”. This tends to be pretty heavy on jargon: while I don’t go too deep into my sub-field’s lingo, I don’t want to risk “talking down” to a physicist I don’t know. The end result is that I have to notice those “shades” of understanding as I go, hoping to get enough questions to change course if I need to.

Then I asked him what he did, and he patiently walked me through it. His explanation was more gradual: less worried about talking down to me, he was able to build up the background around his work, and the history of who worked on what. It was a bit humbling, to see the sort of honed explanation a person can build after telling variations on the same story for years.

In the end, we both had to adapt to what the other understood, to change course when our story wasn’t getting through. Neither of us could stick with the “standard physicist explanation” all the way to the end. Both of us had to shift from one shade to another, improving our translation.

Popularization as News, Popularization as Signpost

Lubos Motl has responded to my post from last week about the recent Caltech short, Quantum is Calling. His response is pretty much exactly what you’d expect, including the cameos by Salma Hayek and Kaley Cuoco.

The only surprise was his lack of concern for accuracy. Quantum is Calling got the conjecture it was trying to popularize almost precisely backwards. I was expecting that to bother him, at least a little.

Should it bother you?

That depends on what you think Quantum is Calling is trying to do.

Science popularization, even good science popularization, tends to get things wrong. Some of that is inevitable, a result of translating complex concepts to a wider audience.

Sometimes, though, you can’t really chalk it up to translation. Interstellar had some extremely accurate visualizations of black holes, but it also had an extremely silly love-powered tesseract. That wasn’t their attempt to convey some subtle scientific truth, it was just meant to sound cool.

And the thing is, that’s not a bad thing to do. For a certain kind of piece, sounding cool really is the point.

Imagine being an explorer. You travel out into the wilderness and find a beautiful waterfall.

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Example:

How do you tell people about it?

One option is the press. The news can cover your travels, so people can stay up to date with the latest in waterfall discoveries. In general, you’d prefer this sort of thing to be fairly accurate: the goal here is to inform people, to give them a better idea of the world around them.

Alternatively, you can advertise. You put signposts up around town pointing toward the waterfall, complete with vivid pictures. Here, accuracy matters a lot less: you’re trying to get people excited, knowing that as they get closer they can get more detailed information.

In science popularization, the “news” here isn’t just news. It’s also blog posts, press releases, and public lectures. It’s the part of science popularization that’s supposed to keep people informed, and it’s one that we hope is mostly accurate, at least as far as possible.

The “signposts”, meanwhile, are things like Interstellar. Their audience is as wide as it can possibly be, and we don’t expect them to get things right. They’re meant to excite people, to get them interested in science. The expectation is that a few students will find the imagery interesting enough to go further, at which point they can learn the full story and clear up any remaining misconceptions.

Quantum is Calling is pretty clearly meant to be a signpost. The inaccuracy is one way to tell, but it should be clear just from the context. We’re talking about a piece with Hollywood stars here. The relative star-dom of Zoe Saldana and Keanu Reeves doesn’t matter, the presence of any mainstream film stars whatsoever means they’re going for the broadest possible audience.

(Of course, the fact that it’s set up to look like an official tie-in to the Star Trek films doesn’t hurt matters either.)

They’re also quite explicit about their goals. The piece’s predecessor has Keanu Reeves send a message back in time, with the goal of inspiring a generation of young scientists to build a future paradise. They’re not subtle about this.

Ok, so what’s the problem? Signposts are allowed to be inaccurate, so the inaccuracy shouldn’t matter. Eventually people will climb up to the waterfall and see it for themselves, right?

What if the waterfall isn’t there?

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Like so:

The evidence for ER=EPR (the conjecture that Quantum is Calling is popularizing) isn’t like seeing a waterfall. It’s more like finding it via surveying. By looking at the slope of nearby terrain and following the rivers, you can get fairly confident that there should be a waterfall there, even if you can’t yet see it over the next ridge. You can then start sending scouts, laying in supplies, and getting ready for a push to the waterfall. You can alert the news, telling journalists of the magnificent waterfall you expect to find, so the public can appreciate the majesty of your achievement.

What you probably shouldn’t do is put up a sign for tourists.

As I hope I made clear in my last post, ER=EPR has some decent evidence. It hasn’t shown that it can handle “foot traffic”, though. The number of researchers working on it is still small. (For a fun but not especially rigorous exercise, try typing “ER=EPR” and “AdS/CFT” into physics database INSPIRE.) Conjectures at this stage are frequently successful, but they often fail, and ER=EPR still has a decent chance of doing so. Tying your inspiring signpost to something that may well not be there risks sending tourists up to an empty waterfall. They won’t come down happy.

As such, I’m fine with “news-style” popularizations of ER=EPR. And I’m fine with “signposts” for conjectures that have shown they can handle some foot traffic. (A piece that sends Zoe Saldana to the holodeck to learn about holography could be fun, for example.) But making this sort of high-profile signpost for ER=EPR feels irresponsible and premature. There will be plenty of time for a Star Trek tie-in to ER=EPR once it’s clear the idea is here to stay.

What’s in a Conjecture? An ER=EPR Example

A few weeks back, Caltech’s Institute of Quantum Information and Matter released a short film titled Quantum is Calling. It’s the second in what looks like will become a series of pieces featuring Hollywood actors popularizing ideas in physics. The first used the game of Quantum Chess to talk about superposition and entanglement. This one, featuring Zoe Saldana, is about a conjecture by Juan Maldacena and Leonard Susskind called ER=EPR. The conjecture speculates that pairs of entangled particles (as investigated by Einstein, Podolsky, and Rosen) are in some sense secretly connected by wormholes (or Einstein-Rosen bridges).

The film is fun, but I’m not sure ER=EPR is established well enough to deserve this kind of treatment.

At this point, some of you are nodding your heads for the wrong reason. You’re thinking I’m saying this because ER=EPR is a conjecture.

I’m not saying that.

The fact of the matter is, conjectures play a very important role in theoretical physics, and “conjecture” covers a wide range. Some conjectures are supported by incredibly strong evidence, just short of mathematical proof. Others are wild speculations, “wouldn’t it be convenient if…” ER=EPR is, well…somewhere in the middle.

Most popularizers don’t spend much effort distinguishing things in this middle ground. I’d like to talk a bit about the different sorts of evidence conjectures can have, using ER=EPR as an example.

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Our friendly neighborhood space octopus

The first level of evidence is motivation.

At its weakest, motivation is the “wouldn’t it be convenient if…” line of reasoning. Some conjectures never get past this point. Hawking’s chronology protection conjecture, for instance, points out that physics (and to some extent logic) has a hard time dealing with time travel, and wouldn’t it be convenient if time travel was impossible?

For ER=EPR, this kind of motivation comes from the black hole firewall paradox. Without going into it in detail, arguments suggested that the event horizons of older black holes would resemble walls of fire, incinerating anything that fell in, in contrast with Einstein’s picture in which passing the horizon has no obvious effect at the time. ER=EPR provides one way to avoid this argument, making event horizons subtle and smooth once more.

Motivation isn’t just “wouldn’t it be convenient if…” though. It can also include stronger arguments: suggestive comparisons that, while they could be coincidental, when put together draw a stronger picture.

In ER=EPR, this comes from certain similarities between the type of wormhole Maldacena and Susskind were considering, and pairs of entangled particles. Both connect two different places, but both do so in an unusually limited way. The wormholes of ER=EPR are non-traversable: you cannot travel through them. Entangled particles can’t be traveled through (as you would expect), but more generally can’t be communicated through: there are theorems to prove it. This is the kind of suggestive similarity that can begin to motivate a conjecture.

(Amusingly, the plot of the film breaks this in both directions. Keanu Reeves can neither steal your cat through a wormhole, nor send you coded messages with entangled particles.)

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Nor live forever as the portrait in his attic withers away

Motivation is a good reason to investigate something, but a bad reason to believe it. Luckily, conjectures can have stronger forms of evidence. Many of the strongest conjectures are correspondences, supported by a wealth of non-trivial examples.

In science, the gold standard has always been experimental evidence. There’s a reason for that: when you do an experiment, you’re taking a risk. Doing an experiment gives reality a chance to prove you wrong. In a good experiment (a non-trivial one) the result isn’t obvious from the beginning, so that success or failure tells you something new about the universe.

In theoretical physics, there are things we can’t test with experiments, either because they’re far beyond our capabilities or because the claims are mathematical. Despite this, the overall philosophy of experiments is still relevant, especially when we’re studying a correspondence.

“Correspondence” is a word we use to refer to situations where two different theories are unexpectedly computing the same thing. Often, these are very different theories, living in different dimensions with different sorts of particles. With the right “dictionary”, though, you can translate between them, doing a calculation in one theory that matches a calculation in the other one.

Even when we can’t do non-trivial experiments, then, we can still have non-trivial examples. When the result of a calculation isn’t obvious from the beginning, showing that it matches on both sides of a correspondence takes the same sort of risk as doing an experiment, and gives the same sort of evidence.

Some of the best-supported conjectures in theoretical physics have this form. AdS/CFT is technically a conjecture: a correspondence between string theory in a hyperbola-shaped space and my favorite theory, N=4 super Yang-Mills. Despite being a conjecture, the wealth of nontrivial examples is so strong that it would be extremely surprising if it turned out to be false.

ER=EPR is also a correspondence, between entangled particles on the one hand and wormholes on the other. Does it have nontrivial examples?

Some, but not enough. Originally, it was based on one core example, an entangled state that could be cleanly matched to the simplest wormhole. Now, new examples have been added, covering wormholes with electric fields and higher spins. The full “dictionary” is still unclear, with some pairs of entangled particles being harder to describe in terms of wormholes. So while this kind of evidence is being built, it isn’t as solid as our best conjectures yet.

I’m fine with people popularizing this kind of conjecture. It deserves blog posts and press articles, and it’s a fine idea to have fun with. I wouldn’t be uncomfortable with the Bohemian Gravity guy doing a piece on it, for example. But for the second installment of a star-studded series like the one Caltech is doing…it’s not really there yet, and putting it there gives people the wrong idea.

I hope I’ve given you a better idea of the different types of conjectures, from the most fuzzy to those just shy of certain. I’d like to do this kind of piece more often, though in future I’ll probably stick with topics in my sub-field (where I actually know what I’m talking about 😉 ). If there’s a particular conjecture you’re curious about, ask in the comments!