Quanta Magazine

What was your first encounter with scientific reasoning?

When I was in second grade we lived in Calcutta, India, and would go shopping for vegetables on the weekend. We would pass these booksellers with stalls built into the walls of buildings. I treasured the books I was able to get my hands on. My dad made a little bookshelf out of packing crates from local shops, and I would line up my handful of books and count them. My ambition in life was to someday own 100 books.

One day I picked up a book called The How and Why Wonder Book of Famous Scientists. In particular I remember Antonie van Leeuwenhoek. He didn’t have a degree. He didn’t work in a fancy place. He was a lens grinder. But he was an intelligent and curious guy, and so he used his lenses to assemble the first microscope and discovered the entire world of microscopic life. I remember reading this and realizing that science was a thing you could do. It wasn’t received wisdom from ages past. From then on, I knew what I was going to do in life.

You studied both physics and computer science at the Massachusetts Institute of Technology. Why the latter?

I had begun to feel that the laws of nature as we write them down are fundamentally constrained by the thoughts we can think. A cat can’t understand calculus, and it’s not obvious that humans can understand everything either. I studied computer science because I wanted to understand the limits of the computer in my head.

How did your alternative life as a neuroscientist get started?

As a postdoctoral student at Harvard, I did my main research in string theory, but I also moonlighted in biology labs. I’d spend evenings at my friend’s lab looking at his data and reading papers. I’d wormed my way out of all my high school biology courses, so I had a lot to learn.

Then I came to Penn and met Peter Sterling, a neuroscientist. He would ask me questions like, “Why does this circuit look like this?” or “Why does this neuron look like that?” These are excellent questions because form follows function in biology.

What are some of the forces you’ve identified that have sculpted the form of the brain?

Neural information is very expensive. It costs a lot of power, so if you think about the theory of energy-efficient ways of encoding information, maybe that should tell you about the brain’s structure.

For example, there are different pathways in the brain for processing light patches and dark patches, and the brain devotes more resources to dark spots. Why would it do that? We showed that if you look at the statistical structure of natural images, there are more dark spots than light spots. We developed a quantitative theory predicting how many light detectors and dark detectors you should have to maximize your visual information if you were to set up an artificial system on a fixed budget. And you pretty much end up spot on with what you see in animals.

Have you answered your question about the constraints of the computer in your head?

My neuroscience research has given me insights into why the computer is organized the way it is, but I don’t think I’ve learned much about the limitations of the processes in my mind that let me do physics. I still have hope of getting there. I think those higher-level processes are repurposing more elementary things we need to do to survive. It’s far more important to eat than it is to do physics.

So do you feel like the cat trying to learn calculus? Is the entropy of disjoint universes pushing our cognitive limits?

You would think that, but we keep finding more! There’s a genuine mystery about this. We’re just using simple mathematical theories, but we’re talking about whether space-time is put together in an emergent way by entanglement. It’s so removed from our daily experience that it’s preposterous we’re able to extrapolate so far.

There’s a problem here to be understood. The power of simple theories is a gift we don’t deserve, basically.

How’s your book collection doing? Have you built your Library of Babel?

After my wife and I had our second child, we wanted to move to the Philadelphia suburbs. One afternoon we drove out here and saw four houses, and the moment I walked into this house and saw this room full of shelves, I knew it would be our home.

Now it’s full of hundreds of books. My wife’s a historian here at Penn, so she accumulates books too. And I still have all my storybooks from when I was a kid — dog-eared, with thumbprints and food marks and all.

Editor’s note: Balasubramanian has received funding from the Simons Foundation, which also funds this editorially independent magazine.


https://www.quantamagazine.org/pondering-the-bits-that-build-space-time-and-brains-20220420/