Tag Archives: physics

Physical Truths, Lost to the Ages

The Hardest Audience Knows Just Enough to Be Dangerous

You’d think that it would be hard to explain physics to people who know absolutely nothing about physics.

And you might be right, if there was anyone these days who knew absolutely nothing about physics. If someone didn’t know what atoms were, or didn’t know what a physicist was, then yes it would take quite a while to explain anything more than the basics. But most people know what atoms are, and know what physicists are, and at least have a basic idea that there are things called protons and neutrons and electrons.

And that’s often enough. Starting with a basis like that, I can talk people through the Large Hadron Collider, I can get them to picture Feynman Diagrams, I can explain, roughly, what it is I do.

On the other end, it’s not all that hard to explain what I do to people in my sub-field. Working on the same type of physics is like sharing a language, we have all sorts of terms to make explaining easier. While it’s still possible to trip up and explain too much or too little (a recent talk I gave left out the one part that one member of the audience needed…because everyone else would have gotten nothing out of it), you’re protected by a buffer of mutual understanding.

The hardest talks aren’t for the public, and they aren’t for fellow amplitudes-researchers. They’re for a general physics audience.

If you’re talking to physicists, you can’t start with protons and neutrons. Do that, and your audience is going to get annoyed with you rather quickly. You can’t rely on the common understanding everyone has of physics. In addition to making your audience feel like they’re being talked down to, you won’t manage to say anything substantial. You need to start at a higher level so that when you do describe what you do, it’s in enough detail that your audience feels like they really understand it.

At the same time, you can’t start with the jargon of your sub-field. If you want to really explain something (and not just have fifteen minutes of background before everyone tunes out) you need to build off of a common understanding.

The tricky part is, that “common understanding” is more elusive than you might think. For example, pretty much every physicist has some familiarity with Quantum Field Theory…but that can mean anything from “uses it every day” to “saw it a couple times back in grad school”. Too much background, and half your audience is bored. Too little, and half your audience is lost. You have to strike the proper balance, trying to show everyone enough to feel satisfied.

There are tricks to make this easier. I’ve noticed that some of the best speakers begin with a clever and unique take on something everyone understands. That way, people in very different fields will still have something they recognize, while people in the same field will still be seeing something new. Of course, the tricky part is coming up with a new example in the first place!

In general, I need to get better at estimating where my audience is. Talking to you guys is fun, but I ought to also practice a “physics voice” for discussions with physicists (as well as grants and applications), and an “amplitudes voice” for fellow specialists. The key to communication, as always, is knowing your audience.

A Nobel for Blue LEDs, or, How Does That Count as Physics?

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When I first heard about this year’s Nobel Prize in Physics, I didn’t feel the need to post on it. The prize went to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura, whose discoveries enabled blue LEDs. It’s a more impressive accomplishment than it might seem: while red LEDs have been around since the 60’s and 70’s, blue LEDs were only developed in the 90’s, and only with both can highly efficient, LED-based white light sources be made. Still, I didn’t consider posting on it because it’s pretty much entirely outside my field.

Shiny, though

It took a conversation with another PI postdoc to point out one way I can comment on the Nobel, and it started when we tried to figure out what type of physicists Akasaki, Amano, and Nakamura are. After tossing around terms like “device physicist” and “condensed matter”, someone wondered whether the development of blue LEDs wasn’t really a matter of engineering.

At that point I realized, I’ve talked about something like this before.

Physicists work on lots of different things, and many of them don’t seem to have much to do with physics. They study geometry and topology, biological molecules and the nature of evolution, income inequality and, yes, engineering.

On the surface, these don’t have much to do with physics. A friend of mine used to quip that condensed matter physicists seem to just “pick whatever they want to research”.

There is something that ties all of these topics together, though. They’re all things that physicists are good at.

Physics grad school gives you a wide variety of tools with which to understand the world. Thermodynamics gives you a way to understand large, complicated systems with statistics, while quantum field theory lets you understand everything with quantum properties, not just fundamental particles but materials as well. This batch of tools can be applied to “traditional” topics, but they’re equally applicable if you’re researching something else entirely, as long as it obeys the right kinds of rules.

In the end, the best definition of physics is the most useful one. Physicists should be people who can benefit from being part of physics organizations, from reading physics journals, and especially from training (and having been) physics grad students. The whole reason we have scientific disciplines in the first place is to make it easier for people with common interests to work together. That’s why Akasaki, Amano, and Nakamura aren’t “just” engineers, and why I and my fellow string theorists aren’t “just” mathematicians. We use our knowledge of physics to do our jobs, and that, more than anything else, makes us physicists.

Edit: It has been pointed out to me that there’s a bit more to this story than the main accounts have let on. Apparently another researcher named Herbert Paul Maruska was quite close to getting a blue LED up and running back in the early 1970’s, getting far enough to have a working prototype. There’s a whole fascinating story about the quest for a blue LED, related here. Maruska seems to be on friendly terms with Akasaki, Amano, and Nakamura, and doesn’t begrudge them their recognition.

Feeling Perturbed?

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You might think of physics as the science of certainties and exact statements: action and reaction, F=ma, and all that. However, most calculations in physics aren’t exact, they’re approximations. This is especially true today, but it’s been true almost since the dawn of physics. In particular, approximations are performed via a method known as perturbation theory.

Perturbation theory is a trick used to solve problems that, for one reason or another, are too difficult to solve all in one go. It works by solving a simpler problem, then perturbing that solution, adjusting it closer to the target.

To give an analogy: let’s say you want to find the area of a circle, but you only know how to draw straight lines. You could start by drawing a square: it’s easy to find the area, and you get close to the area of the circle. But you’re still a long ways away from the total you’re aiming for. So you add more straight lines, getting an octagon. Now it’s harder to find the area, but you’re closer to the full circle. You can keep adding lines, each step getting closer and closer.

And so on.

This, broadly speaking, is what’s going on when particle physicists talk about loops. The calculation with no loops (or “tree-level” result) is the easier problem to solve, omitting quantum effects. Each loop then is the next stage, more complicated but closer to the real total.

There are, as usual, holes in this analogy. One is that it leaves out an important aspect of perturbation theory, namely that it involves perturbing with a parameter. When that parameter is small, perturbation theory works, but as it gets larger the approximation gets worse and worse. In the case of particle physics, the parameter is the strength of the forces involves, with weaker forces (like the weak nuclear force, or electromagnetism) having better approximations than stronger forces (like the strong nuclear force). If you squint, this can still fit the analogy: different shapes might be harder to approximate than the circle, taking more sets of lines to get acceptably close.

Where the analogy fails completely, though, is when you start approaching infinity. Keep adding more lines, and you should be getting closer and closer to the circle each time. In quantum field theory, though, this frequently is not the case. As I’ve mentioned before, while lower loops keep getting closer to the true (and experimentally verified) results, going all the way out to infinite loops results not in the full circle, but in an infinite result instead. There’s an understanding of why this happens, but it does mean that perturbation theory can’t be thought of in the most intuitive way.

Almost every calculation in particle physics uses perturbation theory, which means almost always we are just approximating the real result, trying to draw a circle using straight lines. There are only a few theories where we can bypass this process and look at the full circle. These are known as integrable theories. N=4 super Yang-Mills may be among them, one of many reasons why studying it offers hope for a deeper understanding of particle physics.

The Many (Body) Problems of the Academic Lifestyle

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I’m visiting Perimeter this week, searching for apartments in the area. This got me thinking about how often one has to move in academia. You move for college, you move for grad school, you move for each postdoc job, and again when you start as a professor. Even then, you may not get to stay where you are if you don’t manage to get tenure, and it may be healthier to resign yourself to moving every seven years rather than assuming you’re going to settle down.

Most of life isn’t built around the idea that people move across the country (or the world!) every 2-7 years, so naturally this causes a few problems for those on the academic path. Below are some well-known, and not-so-well-known, problems facing academics due to their frequent relocations:

The two-body problem:

Suppose you’re married, or in a committed relationship. Better hope your spouse has a flexible job, because in a few years you’re going to be moving to another city. This is even harder if your spouse is also an academic, as that requires two rare academic jobs to pop up in the same place. And woe betide you if you’re out of synch, and have to move at different times. Many couples end up having to resort to some sort of long-distance arrangement, which further complicates matters.

The N-body problem:

Like the two-body problem, but for polyamorous academics. Leads to poly-chains up and down the East Coast.

The 2+N-body problem:

Alternatively, add a time dimension to your two-body problem via the addition of children. Now your kids are busily being shuffled between incommensurate school systems. But you’re an academic, you can teach them anything they’re missing, right?

The warm body problem:

Of course, all this assumes you’re in a relationship. If you’re single, you instead have the problem of never really having a social circle beyond your department, having to tenuously rebuild your social life every few years. What sorts of clubs will the more socially awkward of you enter, just to have some form of human companionship?

The large body of water problem:

We live in an age where everything is connected, but that doesn’t make distance cheap. An ocean between you and your collaborators means you’ll rarely be awake at the same time. And good luck crossing that ocean again, not every job will be eager to pay relocation expenses.

The obnoxious governing body problem:

Of course, the various nations involved won’t make all this travel easy. Many countries have prestigious fellowships only granted on the condition that the winner returns to their home country for a set length of time. Since there’s no guarantee that anyone in your home country does anything similar to what you do, this sort of requirement can have people doing whatever research they can find, however tangentially related, or trying to avoid the incipient bureaucratic nightmare any way they can.

Amplitudes on Paperscape

What’s up with arXiv?

New Guide, Taking Suggestions

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Hello readers!

Some of you have probably read the guide to N=4 super Yang-Mills theory linked at the top of my blog’s home page. A few of you even discovered this blog via the guide.

I’m thinking about doing another series of posts, like those, explaining a different theory. I’d like to get an idea of which theory you guys are most interested in seeing described. Whichever I choose, it will be largely along the same lines as the N=4 posts, so focused less on technical details and more on presenting something that a layman can understand.

Here are some of the options I’m considering:

N=8 Supergravity: Very broadly speaking, this is gravity’s equivalent of N=4 super Yang-Mills. It’s connected to N=4 in a variety of interesting ways, and it’s something I’d like to work more with at some point.

The (2,0) Theory: This was the motivation behind my first paper. It’s harder to explain than some of the other theories because it doesn’t have an easy analogy with the particles of the real world. It’s also even harder to work with, to the point that saying something rigorous about it is often worthy of a paper on its own.

String Theory/M Theory: This is a big topic, and there are many sites out there already that cover aspects of it. What I might try to do is look for an angle of approach that others haven’t covered, and try to explain some slightly more technical aspects of the situation in a popularly approachable way.

I could also give a more detailed description of some method from amplitudeology, like generalized unitarity or symbology.

Finally, I could always just keep posting like I have been doing. But this seems like a good time to add to my site’s utility. So what do you think? What should I talk about?

Dammit Jim, I’m a Physicist not a Graphic Designer!

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Over the last week I’ve been working with a few co-authors to get a paper ready for publication. For my part, this has mostly meant making plots of our data. (Yes, theorists have data! It’s the result of calculations, not observations, but it’s still data!)

As it turns out, making the actual plots is only the first and easiest step. We have a huge number of data points, which means the plots ended up being very large files. To fix this I had to smooth out the files so they don’t include every point, a process called rasterizing the images. I also needed to make sure that the labels of the plots matched the fonts in the paper, and that the images in the paper were of the right file type to be included, which in turn meant understanding the sort of information retained by each type of image file. I had to learn which image files include transparency and which don’t, which include fonts as text and which use images, and which fonts were included in each program I used. By the end, I learned more about graphic design than I ever intended to.

In a company, this sort of job would be given to a graphic designer on-staff, or a hired expert. In academia, however, we don’t have the resources for that sort of thing, so we have to become experts in the nitty-gritty details of how to get our work in publishable form.

As it turns out, this is part of a wider pattern in academia. Any given project doesn’t have a large staff of specialists or a budget for outside firms, so everyone involved has to become competent at tasks that a business would parcel out to experts. This is why a large part of work in physics isn’t really physics per se; rather, we theorists often spend much of our time programming, while experimentalists often have to build and repair their experimental apparatus. The end result is that much of what we do is jury-rigged together, with an amateur understanding of most of the side disciplines involved. Things work, but they aren’t as efficient or as slick as they could be if assembled by a real expert. On the other hand, it makes things much cheaper, and it’s a big contributor to the uncanny ability of physicists to know about other peoples’ disciplines.

Hawking vs. Witten: A Primer

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Have you seen the episode of Star Trek where Data plays poker with Stephen Hawking? How about the times he appeared on Futurama or the Simpsons? Or the absurd number of times he has come up in one way or another on The Big Bang Theory?

Stephen Hawking is probably the most recognizable theoretical physicist to laymen. Wheelchair-bound and speaking through a voice synthesizer, Hawking presents a very distinct image, while his work on black holes and the big bang, along with his popular treatments of science in books like A Brief History of Time, has made him synonymous in the public’s mind with genius.

He is not, however, the most recognizable theoretical physicist when talking to physicists. If Sheldon from The Big Bang Theory were a real string theorist he wouldn’t be obsessed with Hawking. He might, however, be obsessed with Edward Witten.

Edward Witten is tall and has an awkwardly high voice (for a sample, listen to the clip here). He’s also smart, smart enough to dabble in basically every subfield of theoretical physics and manage to make important contributions while doing so. He has a knack for digging up ideas from old papers and dredging out the solution to current questions of interest.

And far more than Hawking, he represents a clear target for parody, at least when that parody is crafted by physicists and mathematicians. Abstruse Goose has a nice take on his role in theoretical physics, while his collaboration with another physicist named Seiberg on what came to be known as Seiberg-Witten theory gave rise to the cyber-Witten pun.

If you would look into the mouth of physics-parody madness, let this link be your guide…

So why hasn’t this guy appeared on Futurama? (After all, his dog does!)

Witten is famous among theorists, but he hasn’t done as much as Hawking to endear himself to the general public. He hasn’t written popular science books, and he almost never gives public talks. So when a well-researched show like The Big Bang Theory wants to mention a famous physicist, they go to Hawking, not to Witten, because people know about him. And unless Witten starts interfacing more with the public (or blog posts like this become more common), that’s not about to change.

4 gravitons

Stories about physics from someone who's been there