Tag Archives: dissertation

The PhD Defense

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 NimafrigginArkani-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.

What’s in a Thesis?

As I’ve mentioned before, I’m graduating this spring, which means I need to write that most foreboding of documents, the thesis. As I work on it, I’ve been thinking about how the nature of the thesis varies from field to field.

If you don’t have much experience with academics, you probably think of a thesis as a single, overarching achievement that structures a grad student’s career. A student enters grad school, designs an experiment, performs it, collects data, analyzes the data, draws some conclusion, then writes a thesis about it and graduates.

In some fields, the thesis really does work that way. In biology for example, the process of planning an experiment, setting it up, and analyzing and writing up the data can be just the right size so that, a reasonable percentage of the time, it really can all be done over the course of a PhD.

Other fields tend more towards smaller, faster-paced projects. In theoretical physics, mathematics, and computer science, most projects don’t have the same sort of large experimental overhead that psychologists or biologists have to deal with. The projects I’ve worked on are large-scale for theoretical physics, and I’ll still likely have worked on three distinct things before I graduate. Others, with smaller projects, will often have covered more.

In this situation, a thesis isn’t one overarching idea. Rather, it’s a compilation of work from past projects, sewed together with a pretense of an overall theme. It’s a bit messy, but because it’s the way things are expected to be done in these fields, no-one minds particularly much.

The other end of the spectrum is potentially much harder to deal with. For those who work on especially big experiments, the payoff might take longer to arrive than any reasonable degree. Big machines like colliders and particle detectors can take well over a decade before they start producing data, while longitudinal studies that follow a population as they grow and age take a long time no matter how fast you work.

In cases like this, the challenge is to chop off a small enough part of the project to make it feel like a thesis. A thesis could be written about designing one component for the eventual machine, or analyzing one part of the vast sea of data it produces. Preliminary data from a longitudinal study could be analyzed, even when the final results are many years down the line.

People in these fields have to be flexible and creative when it comes to creating a thesis, but usually the thesis committee is reasonable. In the end, a thesis is what you need to graduate, whatever that actually is for you.