# Thoughts on Polchinski’s Memoir

I didn’t get a chance to meet Joseph Polchinski when I was visiting Santa Barbara last spring. At the time, I heard his health was a bit better, but he still wasn’t feeling well enough to come in to campus. Now that I’ve read his memoir, I almost feel like I have met him. There’s a sense of humor, a diffidence, and a passion for physics that shines through the pages.

The following are some scattered thoughts inspired by the memoir:

A friend of mine once complained to me that in her field grad students all brag about the colleges they went to. I mentioned that in my field your undergrad never comes up…unless it was Caltech. For some reason, everyone I’ve met who went to Caltech is full of stories about the place, and Polchinski is no exception. Speaking as someone who didn’t go there, it seems like Caltech has a profound effect on its students that other places don’t.

Polchinski mentions hearing stories about geniuses of the past, and how those stories helped temper some of his youthful arrogance. There’s an opposite effect that’s also valuable: hearing stories like Polchinski’s, his descriptions of struggling with anxiety and barely publishing and “not really accomplishing anything” till age 40, can be a major comfort to those of us who worry we’ve fallen behind in the academic race. That said, it’s important not to take these things too far: times have changed, you’re not Polchinski, and much like his door-stealing trick at Caltech getting a postdoc without any publications is something you shouldn’t try at home. Even Witten’s students need at least one.

Last week I was a bit puzzled by nueww’s comment, a quote from Polchinski’s memoir which distinguishes “math of the equations” from “math of the solutions”, attributing the former to physicists and the latter to mathematicians. Reading the context in the memoir and the phrase’s origin in a remark by Susskind cleared up a bit, but still left me uneasy. I only figured out why after Lubos Motl posted about it: it doesn’t match my experience of mathematicians at all!

If anything, I think physicists usually care more about the “solutions” than mathematicians do. In my field, often a mathematician will construct some handy basis of functions and then frustrate everyone by providing no examples of how to use them. In the wider math community I’ve met graph theorists who are happy to prove something is true for all graphs of size $10^{10^10}$ and larger, not worrying about the vast number of graphs where it fails because it’s just a finite number of special cases. And I don’t think this is just my experience: a common genre of jokes revolve around mathematicians proving a solution exists and then not bothering to do anything with it (for example, see the joke with the hotel fire here).

I do think there’s a meaningful sense in which mathematicians care about details that we’re happy to ignore, but “solutions” versus “equations” isn’t really the right axis. It’s something more like “rigor” versus “principles”. Mathematicians will often begin a talk by defining a series of maps between different spaces, carefully describing where they are and aren’t valid. A physicist might just write down a function. That sort of thing is dangerous in mathematics: there are always special, pathological cases that make careful definitions necessary. In physics, those cases rarely come up, and when they do there’s often a clear physical problem that brings them to the forefront. We have a pretty good sense of when we need rigor, and when we don’t we’re happy to lay things out without filling in the details, putting a higher priority on moving forward and figuring out the basic principles underlying reality.

Polchinski talks a fair bit about his role in the idea of the multiverse, from hearing about Weinberg’s anthropic argument to coming to terms with the string landscape. One thing his account makes clear is how horrifying the concept seemed at first: how the idea that the parameters of our universe might just be random could kill science and discourage experimentalists. This touches on something that I think gets lost in arguments about the multiverse: even the people most involved in promoting the multiverse in public aren’t happy about it.

It also sharpened my thinking about the multiverse a bit. I’ve talked before about how I don’t think the popularity of the multiverse is actually going to hurt theoretical physics as a field. Polchinski’s worries made me think about the experimental side of the equation: why do experiments if the world might just be random? I think I have a clearer answer to this now, but it’s a bit long, so I’ll save it for a future post.

One nice thing about these long-term accounts is you get to see how much people shift between fields over time. Polchinski didn’t start out working in string theory, and most of the big names in my field, like Lance Dixon and David Kosower, didn’t start out in scattering amplitudes. Academic careers are long, and however specialized we feel at any one time we can still get swept off in a new direction.

I’m grateful for this opportunity to “meet” Polchinski, if only through his writing. His is a window on the world of theoretical physics that is all too rare, and valuable as a result.