Monthly Archives: May 2014

Experimentalist Says What?

I’m a theoretical physicist. That means I work with pencil and paper, or with my laptop, or at most with a computer cluster. I don’t have a lab, and even if I did I wouldn’t have any equipment to store there.

By contrast, most physicists (and most scientists in general) are experimentalists, the people who actually do experiments, actually work in labs, and actually use piles and piles of expensive equipment. Naturally, these two groups have very different ways of doing things, spawned by different requirements for their jobs. This leads to very different ways of talking. We theorists sometimes get confused by the quaint turns of phrase used by experimentalists, so I’ve put together this handy translation guide:

Lab: Kind of like an office, but has a bunch of big machines in it for some reason. Also, in some of them they don’t even drink coffee, some nonsense about toxic contaminants. I don’t know how they get any work done with all those test tubes all over the place.

PI: Not Private Investigator, but close! The Primary Investigator is the big cheese among the experimentalists, the one who owns all the big machines. All of the others must bow before him or her, even fellow professors must grovel if they want to use the PI’s expensive equipment. Naturally, this makes experimentalists very hierarchical, a sharp contrast to theorists who are obviously totally egalitarian.

Poster: Let me tell you a secret about experimentalists: there are a lot of them. Way more than there are theorists. So many, that if they all go to a conference it’s impossible for them all to give talks! That’s where posters come in: some of the experimentalists all stand in a room in front of rectangles of cardboard covered in charts, while the others walk around and ask questions. Traditionally, these posters are printed an hour before the conference, obviously for maximum freshness and not at all because of procrastination.

Group: Like our Institutes, but (because there are a lot of experimentalists) there isn’t just one per university and (because of the shared lab) they actually have something to talk about. This leads to regular group meetings, because when you’re using expensive equipment you actually need to show you’re doing something worthwhile with it.

IRB: For the medical and psychological folks, the Internal Review Board is there to tell you that, no, you can’t infect monkeys with flesh-eating bacteria just to see what happens. They’re also the people who ask you whether a grammatical change in your online survey will pose risks to pregnant women, which is clearly exactly as important. Theorists don’t have these, because numbers are an oppressed underclass with no rights to speak of. EHS (Environmental Health and Safety) fills a similar role for those who only oppress yeast and their own grad students.

Annual Meeting: Experimentalists tend to be part of big organizations like the American Physical Society. And that’s all well and good, occupies a space on the CV and so forth. What’s somewhat more baffling is their tendency to trust those organizations to run conferences. Generally these are massive affairs, with people from all sorts of sub-fields participating. This only works because experimentalists have the mysterious ability to walk into each other’s talks and actually understand what’s going on, even if the subject matter is very different from what they’re used to. Experts suggest this has something to do with actually studying real things in the real world, but this is a hypothesis at best.

Insert Muscle Joke Here

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I’m graduating this week, so I probably shouldn’t spend too much time writing this post. I ought to mention, though, that there has been some doubt about the recent discovery by the BICEP2 telescope of evidence for gravitational waves in the cosmic microwave background caused by the early inflation of the universe. Résonaances got to the story first and Of Particular Significance has some good coverage that should be understandable to a wide audience.

In brief, the worry is that the signal detected by BICEP2 might not be caused by inflation, but instead by interstellar dust. While the BICEP2 team used several models of dust to show that it should be negligible, the controversy centers around one of these models in particular, one taken from another, similar experiment called PLANCK.

The problem is, BICEP2 didn’t get PLANCK’s information on dust directly. Instead, it appears they took the data from a slide in a talk by the PLANCK team. This process, known as “data scraping”, involves taking published copies of the slides and reading information off of the charts presented. If BICEP2 misinterpreted the slide, they might have miscalculated the contribution by interstellar dust.

If you’re like me, the whole idea of data scraping seems completely ludicrous. The idea of professional scientists sneaking information off of a presentation, rather than simply asking the other team for data like reasonable human beings, feels almost cartoonishly wrong-headed.

It’s a bit more understandable, though, when you think about the culture behind these big experiments. The PLANCK and BICEP2 teams are colleagues, but they are also competitors. There is an enormous amount of glory in finding evidence for something like cosmic inflation first, and an equally enormous amount of shame in screwing up and announcing something that turns out to be wrong. As such, these experiments are quite protective of their data. Not only might someone with early access to the data preempt them on an important discovery, they might rush to publish a conclusion that is wrong. That’s why most of these big experiments spend a large amount of time checking and re-checking the data, communicating amongst themselves and settling on an interpretation before they feel comfortable releasing it to the wider community. It’s why BICEP2 couldn’t just ask PLANCK for their data.

From BICEP2’s perspective, they can expect that plots presented at a talk by PLANCK should be accurate, digital plots. Unlike Fox News, scientists have an obligation to present their data in a way that isn’t misleading. And while relying on such a dubious source seems like a bad idea, by all accounts that’s not what the BICEP2 team did. PLANCK’s data was just one dust model used by the team, kept in part because it agreed well with other, non-“data-scraped” models.

It’s a shame that these experiments are so large and prestigious that they need to guard their data in such a potentially destructive way. My sub-field is generally much nicer about this sort of thing: the stakes are lower, and the groups are smaller and have less media attention, so we’re able to share data when we need to. In fact, my most recent paper got a significant boost from some data shared by folks at the Perimeter Institute.

Only time will tell whether the BICEP2 result wins out, or whether it was a fluke caused by caustic data-sharing practices. A number of other experiments are coming online within the next year, and one of them may confirm or deny what BICEP2 has showed.

How do I get where you are?

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I’ve mentioned before that this blog will be undergoing a redesign this summer, transitioning from 4gravitons.wordpress.com to just 4gravitons.wordpress.com. One part of that redesign will be the introduction of new categories to help people search for content, as well as new guides like the ones for N=4 super Yang-Mills and the (2,0) theory for some of those categories. Of those, one planned category/guide will discuss careers in physics, with an eye towards explaining some of the often-unstated assumptions behind the process.

I’ve already posted on being a graduate research assistant and on what a postdoc is. I haven’t said much yet about the process leading up to becoming a graduate student. In this post, I’m going to give an overview of a career in theoretical physics, with a focus on what happens before you find an advisor. This is going to be inherently biased, based as it will be on my experiences. In particular, each country’s education system is different, so much of this will only be relevant for students in the US.

Let’s start at the beginning.

A very good place to start.

If you want to become a theoretical physicist, you’d better start by taking physics and math courses in high school. Unfortunately, this is where socioeconomic status has a big effect. Some schools have Advanced Placement or International Baccalaureate courses that let you get a head-start on college, many don’t. Some schools don’t even have physics courses at all anymore. My only advice here is to get what you can, when you can. If you can take a physics course, do it. If you can take calculus, do it. If you can take classes that will give you university credit, take them.

After high school, you go to college for a Bachelor’s degree in physics. Getting into college these days is some sort of ridiculous popularity contest, and I don’t pretend to be able to give advice on that. What I can say is that once you’re in college, coursework is important, but research is more important. Graduate schools will look at how well you did in your courses and how advanced those courses were, but they will pay special attention to who you get recommendations from, and whether you did research with them. Whether or not your college has anyone who you can research with, you should consider doing summer research somewhere interesting. With programs like the NSF’s Research Experience for Undergraduates (or REU) you can apply to get hooked up with interesting projects and mentors. In addition to looking good on an application to grad school, doing research helps boost your self-confidence: knowing that you can do something real really helps you start feeling like a scientist. Research also teaches you specialized skills much faster than coursework can: I’ve learned much more about programming from having to use it on projects than from any actual programming course.

That said, coursework is also useful. You want courses that will familiarize you with basic tools of your field, physics courses on classical mechanics and quantum mechanics and electromagnetism and math courses on linear algebra and differential equations. You want to take a math course on group theory, but only if it’s taught by a physicist, as mathematicians focus on different aspects. More than any of that, though, you want to try to take at least a few graduate-level courses in while you’re still in college.

That’s important, because grad school in theoretical physics is kind of a mess. You’ll be there for around five years in total (I was in at the low end with four, some people take six or seven). However, you take most if not all of your courses in the first two years. In general, during that time you are paid as a Teaching Assistant. The school pays your tuition and a livable (if barely) wage, and in return you lead lab sections or grade papers. Teaching experience can be a positive thing, but you don’t want to keep doing it for too long, because the point of grad school isn’t teaching or courses, it’s research. Your goal is to find an advisor who is willing to pay you out of one of their (usually government) grants, so that you can transition from Teaching Assistant to Research Assistant. This is hard to do while you’re still taking courses: you won’t have time, and worse, you won’t know everything you need. Theoretical physics requires a lot of background, and much of it gets taught in grad school. Here at Stony Brook, you’d be taking graduate-level quantum mechanics, quantum field theory, and string theory. Until recently, each one of those was a one-year course, and the most logical way to take them was one after the other. Add that up, and that’s three years…kind of a problem when you want to start research after two. That’s why getting ahead in courses, however and whenever you can, is so important: not so much for the courses themselves, but so you can get past them and do research.

Research is what you do for the rest of your time in grad school. It’s what you do after you graduate, when you become a postdoc. It (and teaching) are what you do as a professor, what you are judged on when they decide whether or not you get tenure. Working through research is going to teach you more than any other experience you will have, so get as much of it as you can. And good luck!

Look what I made!

The PhD Defense

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Last Wednesday I completed the final stage of my PhD, the Defense. I booted up a projector and, in a room filled with esteemed physicists, eager grad students, and a three foot sub, I summarized the last two years of my work. A few questions later, people were shaking my hand and calling me “Doctor von Hippel”.

Now that I’m transitioning out of the grad student world, my blog will be transitioning too. I’ll be starting work as a Postdoctoral Fellow in the Fall at the Perimeter Institute for Theoretical Physics. Some time in between, probably in July, this blog will undergo a redesign, hopefully becoming easier to navigate. I’ll also be dropping the “and a grad student” from the title, switching to a new URL, 4gravitons.wordpress.com. Don’t worry, traffic from the old address will be forwarded, so infrequent readers won’t lose track. That said, if anyone with more experience has some advice about making the transition more seamless I’d love to hear it.

There are a lot of stereotypes about the PhD Defense, and mine broke almost all of them. My advisor hadn’t been directly involved in my work, my committee chair was one of the nicest, mellowest professors I’ve ever known, my experimentalist asked me a theoretical physics question, and my external member was Nima–friggin–Arkani-Hamed.

That said, I’ve also seen several other PhD Defenses, and I have to say that the stereotypes are usually right on the money. And since I’m on a bit of a list-based comedy kick recently, let me introduce you to the four members of your PhD committee:

First, of course, is your advisor. If you two collaborate closely, you may find yourself presenting material that your advisor had a hand in. Naturally, the other committee members will ask questions about this material, and naturally you will answer them. Naturally, those answers will not be how your advisor would have explained it, so naturally your advisor will start explaining it themselves. (After all, it’s their work that’s being questioned!) Manage things well and the whole defense will be an argument between your advisor and the other committee members, and you won’t have to say anything at all!

Second is your committee chair. This is someone from your field, chosen for their general eminence and chair-ish-ness. They’ve done a lot of these before, and in their mind they’ve developed a special bond with the students, a bond forged by questions. See, if you have a typical committee chair, they will ask you the toughest, most nitpicky, most downright irrelevant lines of questions possible. The chair’s goal isn’t to keep things moving, it’s to make sure that you took their class and remember everything from it, no matter how much time that takes away from discussing your actual dissertation.

Third you must face your experimentalist. According to the ancient ideals of academia (ideals somehow unbreakably important for grad students and largely irrelevant for top-level university administrators), a dissertation must be judged not only by the yes-men of your own sub-field, but also by someone from the rest of your department. For a theoretical physicist, that means bringing in an experimental physicist. You may try to make things accessible to this person, but eventually you have to actually start talking about your work. This is healthy, as it will allow them much-needed sleep. Once they awake, they will bless you with a question that represents the most tenuous link they can draw between their own work and yours, generally asking after the mass of some subatomic particle. Once you have demonstrated your ignorance in some embarrassing fashion the experimentalist may return to sleep.

Finally, the defense brings in a special individual, the external member. Not only must you prove your worth to an experimentalist, but to someone from outside of your department altogether! For the lucky, this means someone who does similar work at a nearby university. For the terminally rural, this instead means finding the closest department and bringing in someone who will at least recognize some of the words in your talk. For us, this generally means a mathematician. Like the experimentalist, they will favor you with bewildered looks or snores, depending on temperament. Unlike the experimentalist, they are under no illusion that anything they do is relevant to anything you do, so their questions will be mercifully brief.

Grilled by these four, you then leave the room, allowing them to talk about the weather or their kids or something before they ask you back in to tell you that, of course, you’ve got your PhD. Because after all that, anything else would just be rude.

4 gravitons

Stories about physics from someone who's been there