Tag Archives: science communication

The (but I’m Not a) Crackpot Style Guide

Ok, ok, I believe you. You’re not a crackpot. You’re just an outsider, one with a brilliant new idea that would overturn the accepted paradigms of physics, if only someone would just listen.

Here’s the problem: you’re not alone. There are plenty of actual crackpots. We get contacted by them fairly regularly. And most of the time, they’re frustrating and unpleasant to deal with.

If you want physicists to listen to you, you need to show us you’re not one of those people. Otherwise, most of us won’t bother.

I can’t give you a foolproof way to do that. But I can give some suggestions that will hopefully make the process a little less frustrating for everyone involved.

Don’t spam:

Nobody likes spam. Nobody reads spam. If you send a mass email to every physicist whose email address you can find, none of them will read it. If you repeatedly post the same thing in a comment thread, nobody will read it. If you want people to listen to you, you have to show that you care about what they have to say, and in order to do that you have to tailor your message. This leads in to the next point,

Ask the right people:

Before you start reaching out, you should try to get an idea of who to talk to. Physics is quite specialized, so if you’re taking your ideas seriously you should try to contact people with a relevant specialization.

Now, I know what you’re thinking: your ideas are unique, no-one in physics is working on anything similar.

Here, it’s important to distinguish the problem you’re trying to solve with how you’re trying to solve it. Chances are, no-one else is working on your specific idea…but plenty of people are interested in the same problems.

Think quantum mechanics is built on shoddy assumptions? There are people who spend their lives trying to modify quantum mechanics. Have a beef against general relativity? There’s a whole sub-field of people who modify gravity.

These people are a valuable resource for you, because they know what doesn’t work. They’ve been trying to change the system, and they know just how hard it is to change, and just what evidence you need to be consistent with.

Contacting someone whose work just uses quantum mechanics or relativity won’t work. If you’re making elementary mistakes, we can put you on the right track…but if you think you’re making elementary mistakes, you should start out by asking help from a forum or the like, not contacting a professional. If you think you’ve really got a viable replacement to an established idea, you need to contact people who work on overturning established ideas, since they’re most aware of the complicated webs of implications involved. Relatedly,

Take ownership of your work:

I don’t know how many times someone has “corrected” something in the comments, and several posts later admitted that the “correction” comes from their own theory. If you’re arguing from your own work, own it! If you don’t, people will assume you’re trying to argue from an established theory, and are just confused about how that theory works. This is a special case of a broader principle,

Epistemic humility:

I’m not saying you need to be humble in general, but if you want to talk productively you need to be epistemically humble. That means being clear about why you know what you know. Did you get it from a mathematical proof? A philosophical argument? Reading pop science pieces? Something you remember from high school? Being clear about your sources makes it easier for people to figure out where you’re coming from, and avoids putting your foot in your mouth if it turns out your source is incomplete.

Context is crucial:

If you’re commenting on a blog like this one, pay attention to context. Your comment needs to be relevant enough that people won’t parse it as spam.

If all a post does is mention something like string theory, crowing about how your theory is a better explanation for quantum gravity isn’t relevant. Ditto for if all it does is mention a scientific concept that you think is mistaken.

What if the post is promoting something that you’ve found to be incorrect, though? What if someone is wrong on the internet?

In that case, it’s important to keep in mind the above principles. A popularization piece will usually try to present the establishment view, and merits a different response than a scientific piece arguing something new. In both cases, own your own ideas and be specific about how you know what you know. Be clear on whether you’re talking about something that’s controversial, or something that’s broadly agreed on.

You can get an idea of what works and what doesn’t by looking at comments on this blog. When I post about dark matter, or cosmic inflation, there are people who object, and the best ones are straightforward about why. Rather than opening with “you’re wrong”, they point out which ideas are controversial. They’re specific about whose ideas they’re referencing, and are clear about what is pedagogy and what is science.

Those comments tend to get much better responses than the ones that begin with cryptic condemnations, follow with links, and make absolute statements without backing them up.

On the internet, it’s easy for misunderstandings to devolve into arguments. Want to avoid that? Be direct, be clear, be relevant.

Fun with Misunderstandings

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Perimeter had its last Public Lecture of the season this week, with Mario Livio giving some highlights from his book Brilliant Blunders. The lecture should be accessible online, either here or on Perimeter’s YouTube page.

These lectures tend to attract a crowd of curious science-fans. To give them something to do while they’re waiting, a few local researchers walk around with T-shirts that say “Ask me, I’m a scientist!” Sometimes we get questions about the upcoming lecture, but more often people just ask us what they’re curious about.

Long-time readers will know that I find this one of the most fun parts of the job. In particular, there’s a unique challenge in figuring out just why someone asked a question. Often, there’s a hidden misunderstanding they haven’t recognized.

The fun thing about these misunderstandings is that they usually make sense, provided you’re working from the person in question’s sources. They heard a bit of this and a bit of that, and they come to the most reasonable conclusion they can given what’s available. For those of us who have heard a more complete story, this often leads to misunderstandings we would never have thought of, but that in retrospect are completely understandable.

One of the simpler ones I ran into was someone who was confused by people claiming that we were running out of water. How could there be a water shortage, he asked, if the Earth is basically a closed system? Where could the water go?

The answer is that when people are talking about a water shortage, they’re not talking about water itself running out. Rather, they’re talking about a lack of safe drinking water. Maybe the water is polluted, or stuck in the ocean without expensive desalinization. This seems like the sort of thing that would be extremely obvious, but if you just hear people complaining that water is running out without the right context then you might just not end up hearing that part of the story.

A more involved question had to do with time dilation in general relativity. The guy had heard that atomic clocks run faster if you’re higher up, and that this was because time itself runs faster in lower gravity.

Given that, he asked, what happens if someone travels to an area of low gravity and then comes back? If more time has passed for them, then they’d be in the future, so wouldn’t they be at the “wrong time” compared to other people? Would they even be able to interact with them?

This guy’s misunderstanding came from hearing what happens, but not why. While he got that time passes faster in lower gravity, he was still thinking of time as universal: there is some past, and some future, and if time passes faster for one person and slower for another that just means that one person is “skipping ahead” into the other person’s future.

What he was missing was the explanation that time dilation comes from space and time bending. Rather than “skipping ahead”, a person for whom time passes faster just experiences more time getting to the same place, because they’re traveling on a curved path through space-time.

As usual, this is easier to visualize in space than in time. I ended up drawing a picture like this:

IMG_20160602_101423

Imagine person A and person B live on a circle. If person B stays the same distance from the center while person A goes out further, they can both travel the same angle around the circle and end up in the same place, but A will have traveled further, even ignoring the trips up and down.

What’s completely intuitive in space ends up quite a bit harder to visualize in time. But if you at least know what you’re trying to think about, that there’s bending involved, then it’s easier to avoid this guy’s kind of misunderstanding. Run into the wrong account, though, and even if it’s perfectly correct (this guy had heard some of Hawking’s popularization work on the subject), if it’s not emphasizing the right aspects you can come away with the wrong impression.

Misunderstandings are interesting because they reveal how people learn. They’re windows into different thought processes, into what happens when you only have partial evidence. And because of that, they’re one of the most fascinating parts of science popularization.

Particles Aren’t Vibrations (at Least, Not the Ones You Think)

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You’ve probably heard this story before, likely from Brian Greene.

In string theory, the fundamental particles of nature are actually short lengths of string. These strings can vibrate, and like a string on a violin, that vibration is arranged into harmonics. The more energy in the string, the more complex the vibration. In string theory, each of these vibrations corresponds to a different particle, explaining how the zoo of particles we observe can come out of a single type of fundamental string.

Particles. Probably.

It’s a nice story. It’s even partly true. But it gives a completely wrong idea of where the particles we’re used to come from.

Making a string vibrate takes energy, and that energy is determined by the tension of the string. It’s a lot harder to wiggle a thick rubber band than a thin one, if you’re holding both tightly.

String theory’s strings are under a lot of tension, so it takes a lot of energy to make them vibrate. From our perspective, that energy looks like mass, so the more complicated harmonics on a string correspond to extremely massive particles, close to the Planck mass!

Those aren’t the particles you’re used to. They’re not electrons, they’re not dark matter. They’re particles we haven’t observed, and may never observe. They’re not how string theory explains the fundamental particles of nature.

So how does string theory go from one fundamental type of string to all of the particles in the universe, if not through these vibrations? As it turns out, there are several different ways it can happen, tricks that allow the lightest and simplest vibrations to give us all the particles we’ve observed.* I’ll describe a few.

The first and most important trick here is supersymmetry. Supersymmetry relates different types of particles to each other. In string theory, it means that along with vibrations that go higher and higher, there are also low-energy vibrations that behave like different sorts of particles. In a sense, string theory sticks a quantum field theory inside another quantum field theory, in a way that would make Xzibit proud.

Even with supersymmetry, string theory doesn’t give rise to all of the right sorts of particles. You need something else, like compactifications or branes.

The strings of string theory live in ten dimensions, it’s the only place they’re mathematically consistent. Since our world looks four-dimensional, something has to happen to the other six dimensions. They have to be curled up, in a process called compactification. There are lots and lots (and lots) of ways to do this compactification, and different ways of curling up the extra dimensions give different places for strings to move. These new options make the strings look different in our four-dimensional world: a string curled around a donut hole looks very different from one that moves freely. Each new way the string can move or vibrate can give rise to a new particle.

Another option to introduce diversity in particles is to use branes. Branes (short for membranes) are surfaces that strings can end on. If two strings end on the same brane, those ends can meet up and interact. If they end on different branes though, then they can’t. By cleverly arranging branes, then, you can have different sets of strings that interact with each other in different ways, reproducing the different interactions of the particles we’re familiar with.

In string theory, the particles we’re used to aren’t just higher harmonics, or vibrations with more and more energy. They come from supersymmetry, from compactifications and from branes. The higher harmonics are still important: there are theorems that you can’t fix quantum gravity with a finite number of extra particles, so the infinite tower of vibrations allows string theory to exploit a key loophole. They just don’t happen to be how string theory gets the particles of the Standard Model. The idea that every particle is just a higher vibration is a common misconception, and I hope I’ve given you a better idea of how string theory actually works.

*But aren’t these lightest vibrations still close to the Planck mass? Nope! See the discussion with TE in the comments for details.

Starshot: The Right Kind of Longshot

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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.

I Don’t Get Crackpots

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[Note: not an April fool’s post. Now I’m wishing I wrote one though.]

After the MHV@30 conference, I spent a few days visiting my sister. I hadn’t seen her in a while, and she noticed something new about me.

“You’re not sure about anything. It’s always ‘I get the impression’ or ‘I believe so’ or ‘that seems good’.”

On reflection, she’s right.

It’s a habit I’ve picked up from spending time around scientists. When you’re surrounded by people who are likely to know more than you do about something, it’s usually good to qualify your statements. A little intellectual humility keeps simple corrections from growing into pointless arguments, and makes it easier to learn from your mistakes.

With that kind of mindset, though, I really really don’t get crackpots.

For example, why do they always wear funnels on their heads?

The thing about genuine crackpots (as opposed to just scientists with weird ideas) is that they tend to have almost none of the relevant background for a given field, but nevertheless have extremely strong opinions about it. That basic first step, of assuming that there are people who probably know a lot more about whatever you’re talking about? Typically, they don’t bother with that. The qualifiers, the “typically” and “as far as I know” just don’t show up. And I have a lot of trouble understanding how a person can work that way.

Is some of it the Dunning-Kruger effect? Sure. If you don’t know much about something, you don’t know the limits of your own knowledge, so you think you know more than you really do. But I don’t think it’s just that…there’s a baseline level of doubt, of humility in general, that just isn’t there for most crackpots.

I wonder if some fraction of crackpots are genuinely mentally ill, but if so I’m not sure what the illness would be. Mania is an ok fit some of the time, and the word salad and “everyone but me is crazy” attitude almost seem schizophrenic, but I doubt either is really what’s going on in most cases.

All of this adds up to me just being completely unable to relate to people who display a sufficient level of crackpottery.

The thing is, there are crackpots out there who I kind of wish I could talk to, because if I could maybe I could help them. There are crackpots who seem genuinely willing to be corrected, to be told what they’re doing wrong. But that core of implicit arrogance, the central assumption that it’s possible to make breakthroughs in a field while knowing almost nothing about it, that’s still there, and it makes it impossible for me to deal with them.

I kind of wish there was a website I could link, dedicated to walking crackpots through their mistakes. There used to be something like that for supernatural crackpots, in the form of the James Randi Educational Foundation‘s Million Dollar Prize, complete with forums where (basically) helpful people would patiently walk applicants through how to set up a test of their claims. There’s never been anything like that for science, as far as I’m aware, and it seems like it would take a lot more work. Still, it would be nice if there were people out there patient enough to do it.

You Go, LIGO!

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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.

The Higgs Solution

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My grandfather is a molecular biologist. Over the holidays I had many opportunities to chat with him, and our conversations often revolved around explaining some aspect of our respective fields. While talking to him, I came up with a chemistry-themed description of the Higgs field, and how it leads to electro-weak symmetry breaking. Very few of you are likely to be chemists, but I think you still might find the metaphor worthwhile.

Picture the Higgs as a mixture of ions, dissolved in water.

In this metaphor, the Higgs field is a sort of “Higgs solution”. Overall, this solution should be uniform: if you have more ions of a certain type in one place than another, over time they will dissolve until they reach a uniform mixture again. In this metaphor, the Higgs particle detected by the LHC is like a brief disturbance in the fluid: by stirring the solution at high energy, we’ve managed to briefly get more of one type of ion in one place than the average concentration.

What determines the average concentration, though?

Essentially, it’s arbitrary. If this were really a chemistry experiment, it would depend on the initial conditions: which ions we put in to the mixture in the first place. In physics, quantum mechanics plays a role, randomly selecting one option out of the many possibilities.

Choose wisely

(Note that this metaphor doesn’t explain why there has to be a solution, why the water can’t just be “pure”. A setup that required this would probably be chemically complicated enough to confuse nearly everybody, so I’m leaving that feature out. Just trust that “no ions” isn’t one of our options.)

Up till now, the choice of mixture didn’t matter very much. But different ions interact with other chemicals in different ways, and this has some interesting implications.

Suppose we have a tube filled with our Higgs solution. We want to shoot some substance through the tube, and collect it on the other side. This other substance is going to represent a force.

If our force substance doesn’t react with the ions in our Higgs solution, it will just go through to the other side. If it does react, though, then it will be slowed down, and only some of it will get to the other side, possibly none at all.

You can think of the electro-weak force as a mixture of these sorts of substances. Normally, there is no way to tell the different substances apart. Just like the different Higgs solutions, different parts of the electro-weak force are arbitrary.

However, once we’ve chosen a Higgs solution, things change. Now, different parts of our electro-weak substance will behave differently. The parts that react with the ions in our Higgs solution will slow down, and won’t make it through the tube, while the parts that don’t interact will just flow on through.

We call the part that gets through the tube electromagnetism, and the part that doesn’t the weak nuclear force. Electromagnetism is long-range, its waves (light) can travel great distances. The weak nuclear force is short-range, and doesn’t have an effect outside of the scale of atoms.

The important thing to take away from this is that the division between electromagnetism and the weak nuclear force is totally arbitrary. Taken by themselves, they’re equivalent parts of the same, electro-weak force. It’s only because some of them interact with the Higgs, while others don’t, that we distinguish those parts from each other. If the Higgs solution were a different mixture (if the Higgs field had different charges) then a different part of the electroweak force would be long-range, and a different part would be short-range.

We wouldn’t be able to tell the difference, though. We’d see a long-range force, and a short-range force, and a Higgs field. In the end, our world would be completely the same, just based on a different, arbitrary choice.

Newtonmas 2015

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Merry Newtonmas!

I’ll leave up my poll a bit longer, but the results are already looking pretty consistent.

A strong plurality of my readers have PhDs in high energy or theoretical physics, a little more than a quarter. Another big chunk (a bit over a fifth) are physics grad students. All together, that means almost half of my readers have some technical background in what I do.

In the comments, Cliff suggests this is a good reason to start writing more technical posts. Looking at the results, I agree, it looks like there would definitely be an audience for that sort of thing. Technical posts take a lot more effort than general audience posts, so don’t expect a lot of them…but you can definitely look forward to a few technical posts next year.

On the other hand, between people with some college physics and people who only saw physics in high school, about a third of my audience wouldn’t get much out of technical posts. Most of my posts will still be geared to this audience, since it’s kind of my brand at this point, but I do want to start experimenting with aiming a few posts to more specific segments.

Beyond that, I’ve got a smattering of readers in other parts of physics, and a few mathematicians. Aside from the occasional post defending physics notation, there probably won’t be much aimed at either group, but do let me know what I can do to make things more accessible!

Visiting the Blog? Here’s a Poll!

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A few of my recent posts talked about how important it is to know your audience when communicating science. As it turns out, I don’t actually know much about who reads this blog. WordPress tells me which countries you come from (mostly from the US, but large contingents from several other countries, with views from 122 countries last year), and in some cases what links you clicked on to get here (lots of search engines, facebook, reddit, twitter, various other peoples’ blogs). What it doesn’t tell me, though, is what your background is.

That’s what this poll is for. Readers, I’d like you to tell me how much physics background you have. Did you only run into it in high school (if at all), or did you see some college physics too? How many of you are actually physicists? How many of you are mathematicians? (From my observations, even mathematicians with no physics experience favor very different explanations from other people with no physics experience.)

I try to make this blog accessible to as many people as I can, but I do wonder how much of my audience needs that accessibility. So whether you’re just stopping by to read a post linked on reddit, or you’re a long-time reader, vote in the poll and let me know where you stand. And if you’ve got more to say or the poll doesn’t capture some subtlety, feel free to respond in the comments!

The “Lies to Children” Model of Science Communication, and The “Amplitudes Are Weird” Model of Amplitudes

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Let me tell you a secret.

Scattering amplitudes in N=4 super Yang-Mills don’t actually make sense.

Scattering amplitudes calculate the probability that particles “scatter”: coming in from far away, interacting in some fashion, and producing new particles that travel far away in turn. N=4 super Yang-Mills is my favorite theory to work with: a highly symmetric version of the theory that describes the strong nuclear force. In particular, N=4 super Yang-Mills has conformal symmetry: if you re-scale everything larger or smaller, you should end up with the same predictions.

You might already see the contradiction here: scattering amplitudes talk about particles coming in from very far away…but due to conformal symmetry, “far away” doesn’t mean anything, since we can always re-scale it until it’s not far away anymore!

So when I say that I study scattering amplitudes in N=4 super Yang-Mills, am I lying?

Well…yes. But it’s a useful type of lie.

There’s a concept in science writing called “lies to children”, first popularized in a fantasy novel.

This one.

When you explain science to the public, it’s almost always impossible to explain everything accurately. So much background is needed to really understand most of modern science that conveying even a fraction of it would bore the average audience to tears. Instead, you need to simplify, to skip steps, and even (to be honest) to lie.

The important thing to realize here is that “lies to children” aren’t meant to mislead. Rather, they’re chosen in such a way that they give roughly the right impression, even as they leave important details out. When they told you in school that energy is always conserved, that was a lie: energy is a consequence of symmetry in time, and when that symmetry is broken energy doesn’t have to be conserved. But “energy is conserved” is a useful enough rule that lets you understand most of everyday life.

In this case, the “lie” that we’re calculating scattering amplitudes is fairly close to the truth. We’re using the same methods that people use to calculate scattering amplitudes in theories where they do make sense, like QCD. For a while, people thought these scattering amplitudes would have to be zero, since anything else “wouldn’t make sense”…but in practice, we found they were remarkably similar to scattering amplitudes in other theories. Now, we have more rigorous definitions for what we’re calculating that avoid this problem, involving objects called polygonal Wilson loops.

This illustrates another principle, one that hasn’t (yet) been popularized by a fantasy novel. I’d like to call it the “amplitudes are weird” principle. Time and again we amplitudes-folks will do a calculation that doesn’t really make sense, find unexpected structure, and go back to figure out what that structure actually means. It’s been one of the defining traits of the field, and we’ve got a pretty good track record with it.

A couple of weeks back, Lance Dixon gave an interview for the SLAC website, talking about his work on quantum gravity. This was immediately jumped on by Peter Woit and Lubos Motl as ammo for the long-simmering string wars. To one extent or another, both tried to read scientific arguments into the piece. This is in general a mistake: it is in the nature of a popularization piece to contain some volume of lies-to-children, and reading a piece aimed at a lower audience can be just as confusing as reading one aimed at a higher audience.

In the remainder of this post, I’ll try to explain what Lance was talking about in a slightly higher-level way. There will still be lies-to-children involved, this is a popularization blog after all. But I should be able to clear up a few misunderstandings. Lubos probably still won’t agree with the resulting argument, but it isn’t the self-evidently wrong one he seems to think it is.

Lance Dixon has done a lot of work on quantum gravity. Those of you who’ve read my old posts might remember that quantum gravity is not so difficult in principle: general relativity naturally leads you to particles called gravitons, which can be treated just like other particles. The catch is that the theory that you get by doing this fails to be predictive: one reason why is that you get an infinite number of erroneous infinite results, which have to be papered over with an infinite number of arbitrary constants.

Working with these non-predictive theories, however, can still yield interesting results. In the article, Lance mentions the work of Bern, Carrasco, and Johansson. BCJ (as they are abbreviated) have found that calculating a gravity amplitude often just amounts to calculating a (much easier to find) Yang-Mills amplitude, and then squaring the right parts. This was originally found in the context of string theory by another three-letter group, Kawai, Lewellen, and Tye (or KLT). In string theory, it’s particularly easy to see how this works, as it’s a basic feature of how string theory represents gravity. However, the string theory relations don’t tell the whole story: in particular, they only show that this squaring procedure makes sense on a classical level. Once quantum corrections come in, there’s no known reason why this squaring trick should continue to work in non-string theories, and yet so far it has. It would be great if we had a good argument why this trick should continue to work, a proof based on string theory or otherwise: for one, it would allow us to be much more confident that our hard work trying to apply this trick will pay off! But at the moment, this falls solidly under the “amplitudes are weird” principle.

Using this trick, BCJ and collaborators (frequently including Lance Dixon) have been calculating amplitudes in N=8 supergravity, a highly symmetric version of those naive, non-predictive gravity theories. For this particular, theory, the theory you “square” for the above trick is N=4 super Yang-Mills. N=4 super Yang-Mills is special for a number of reasons, but one is that the sorts of infinite results that lose you predictive power in most other quantum field theories never come up. Remarkably, the same appears to be true of N=8 supergravity. We’re still not sure, the relevant calculation is still a bit beyond what we’re capable of. But in example after example, N=8 supergravity seems to be behaving similarly to N=4 super Yang-Mills, and not like people would have predicted from its gravitational nature. Once again, amplitudes are weird, in a way that string theory helped us discover but by no means conclusively predicted.

If N=8 supergravity doesn’t lose predictive power in this way, does that mean it could describe our world?

In a word, no. I’m not claiming that, and Lance isn’t claiming that. N=8 supergravity simply doesn’t have the right sorts of freedom to give you something like the real world, no matter how you twist it. You need a broader toolset (string theory generally) to get something realistic. The reason why we’re interested in N=8 supergravity is not because it’s a candidate for a real-world theory of quantum gravity. Rather, it’s because it tells us something about where the sorts of dangerous infinities that appear in quantum gravity theories really come from.

That’s what’s going on in the more recent paper that Lance mentioned. There, they’re not working with a supersymmetric theory, but with the naive theory you’d get from just trying to do quantum gravity based on Einstein’s equations. What they found was that the infinity you get is in a certain sense arbitrary. You can’t get rid of it, but you can shift it around (infinity times some adjustable constant 😉 ) by changing the theory in ways that aren’t physically meaningful. What this suggests is that, in a sense that hadn’t been previously appreciated, the infinite results naive gravity theories give you are arbitrary.

The inevitable question, though, is why would anyone muck around with this sort of thing when they could just use string theory? String theory never has any of these extra infinities, that’s one of its most important selling points. If we already have a perfectly good theory of quantum gravity, why mess with wrong ones?

Here, Lance’s answer dips into lies-to-children territory. In particular, Lance brings up the landscape problem: the fact that there are 10^500 configurations of string theory that might loosely resemble our world, and no clear way to sift through them to make predictions about the one we actually live in.

This is a real problem, but I wouldn’t think of it as the primary motivation here. Rather, it gets at a story people have heard before while giving the feeling of a broader issue: that string theory feels excessive.

Why does this have a Wikipedia article?

Think of string theory like an enormous piece of fabric, and quantum gravity like a dress. You can definitely wrap that fabric around, pin it in the right places, and get a dress. You can in fact get any number of dresses, elaborate trains and frilly togas and all sorts of things. You have to do something with the extra material, though, find some tricky but not impossible stitching that keeps it out of the way, and you have a fair number of choices of how to do this.

From this perspective, naive quantum gravity theories are things that don’t qualify as dresses at all, scarves and socks and so forth. You can try stretching them, but it’s going to be pretty obvious you’re not really wearing a dress.

What we amplitudes-folks are looking for is more like a pencil skirt. We’re trying to figure out the minimal theory that covers the divergences, the minimal dress that preserves modesty. It would be a dress that fits the form underneath it, so we need to understand that form: the infinities that quantum gravity “wants” to give rise to, and what it takes to cancel them out. A pencil skirt is still inconvenient, it’s hard to sit down for example, something that can be solved by adding extra material that allows it to bend more. Similarly, fixing these infinities is unlikely to be the full story, there are things called non-perturbative effects that probably won’t be cured. But finding the minimal pencil skirt is still going to tell us something that just pinning a vast stretch of fabric wouldn’t.

This is where “amplitudes are weird” comes in in full force. We’ve observed, repeatedly, that amplitudes in gravity theories have unexpected properties, traits that still aren’t straightforwardly explicable from the perspective of string theory. In our line of work, that’s usually a sign that we’re on the right track. If you’re a fan of the amplituhedron, the project here is along very similar lines: both are taking the results of plodding, not especially deep loop-by-loop calculations, observing novel simplifications, and asking the inevitable question: what does this mean?

That far-term perspective, looking off into the distance at possible insights about space and time, isn’t my style. (It isn’t usually Lance’s either.) But for the times that you want to tell that kind of story…well, this isn’t that outlandish of a story to tell. And unless your primary concern is whether a piece gives succor to the Woits of the world, it shouldn’t be an objectionable one.

4 gravitons

Stories about physics from someone who's been there