Perspectives on theory at the interface of physics and biology


William Bialek has just posted on the arxiv a thoughtful piece exploring the role of theory in biology. He argues that theory has played a more important role in the development of biology than it is given credit for, and also that there is cause for optimism regarding the role that theory will play in the future. For example, he wrote “What is emerging from our community goes beyond the “application” of physics to the problems of biology. We are asking physicists’ questions about the phenomena of life, looking for the kinds of compelling answers that we expect in the traditional core of physics.” I also liked the introduction to Bialek’s Biophysics textbook, which explores some of the same issues.

One of the topics that Bialek addresses in the arxiv article is the explosion of parameters in biological models. I particularly appreciated his historical discussion of the Hodgkin-Huxley model of action potentials in neurons, and how progress can be made by analyzing the class of behaviors that can be achieved and to consider how the cell can adapt to remain in the proper regime. He summarizes his thoughts on the role of parameters with the following:



Students don’t always understand what you think that they understand

Below is a figure that had a tremendous impact on me when I saw it ten years ago in a paper written by Carl Wieman and Kathy Perkins (figure adapted from work done by Eric Mazur at Harvard). The figure shows two  problems designed to test the same basic physics concept, namely how current flows in circuits. Determining the current through the various resistors in part (a) requires solving a pair of equations, and as a teacher a reasonable assumption is that if students can do the question correctly then they understand the concepts. However, what Mazur found is that many students could solve this challenging problem but were unable to solve the (apparently) much simpler conceptual problem in part (b) asking about how the brightness of the light bulbs will change when the switch S is closed.

The students were simply using a “recipe” to solve the problem given in (a), indicating that they would quickly become confused when attempting to apply their knowledge outside of the artificial realm of the problems that they had practiced. This realization played a significant role in my decision to spend three months as a Christine Mirzayan Fellow at the National Academics with the Board on Science Education , where I became convinced that active learning approaches could help students with these conceptually challenging topics.


Nick Bostrom’s “Great Filter” and the Fermi Paradox

This Fall DARPA invited me to speak at their annual gathering “Wait, What! A Future Technology Forum.” I thought that I was invited to speak about my research on tipping points in biological populations, but as it turns out they wanted me to serve on a panel entitled “Are we alone, and have we been?” together with Lucianne Walkowicz, an astrobiologist, and Mark Norell, a paleontologist. Needless to say, this was pretty far from my area of expertise, but it gave me an exciting opportunity to read about issues that all of us are curious about.

One of the most exciting ideas that I have come across over the last ten years originated in a wonderful essay by Nick Bostrom in 2008 entitled “Where are they? Why I hope the search for extraterrestrial life finds nothing” (pdf). I strongly encourage you to read the article if you are not familiar with the argument, as I will only give a brief sketch here. The starting point is the Fermi paradox, in which the famous physicist Enrico Fermi asked his lunch partners “Where is everybody?” Fermi was referring to the apparent contradiction between estimates of the likely number of intelligent lifeforms throughout the galaxy and the absence of any evidence that they exist.

One resolution of this paradox discussed by Bostrom and others is The Great Filter, which is whatever stops a huge collection of lifeless planets leading to life that expands across the galaxy. Bostrom’s argument is that there must be something that is exceedingly unlikely in this process. Perhaps the extremely difficult step is evolving the first self-replicators, or maybe it is evolving cell-based life, or possibly it is evolving multi-cellularity. On the other hand, it is also possible that essentially all technologically advanced life wipes itself out by powerful nuclear or biological weapons (or by advanced AI, a concern recently expressed by my colleagues Max Tegmark and Frank Wilczek).

This gets us to the crux of the argument made by Nick Bostrom in his 2008 article in Technology Review. The Great Filter is something that is either in our past or in our future. If we go to Mars and find life that evolved independently from us, then this makes it more likely that the Great Filter is ahead of us. Finding life on other planets therefore bodes poorly for humanity’s future. Indeed, the more advanced the life is that we find the worse the news is for us.

There is another “solution” to the Fermi paradox that is more hopeful for our future survival but perhaps more depressing regarding the quality of that future. Many people take it for granted that if we wait long enough then we will develop the technology to reach neighboring star. For example, Marc Millis has calculated based on the world energy budget that we will not be able to send an interstellar probe (much less human colony) to another star for 500 years. Even this depressing estimate is perhaps overly optimistic, as it assumes that the world energy allowance increases indefinitely by about 2% per year. However, recent evidence suggests that worldwide human expenditures will actually stop growing in the next fifty years. Indeed, over the last 30 years total energy consumption per person has been constant in both the United States and Europe. If humanity’s energy budget stops growing then perhaps we will never send a colony to another star, and the resolution of Fermi’s paradox may simply be that traveling between the stars is prohibitively costly.

My New Year’s Resolution

For the last few years I have  joked that my New Year’s Resolution was to start using Facebook. I have had an account for many years, but I only log on once a year, and I have to confess that I find the whole thing rather mystifying. As a professor at MIT this is all a bit embarrassing, so this year my resolution was to at least update my personal website, which hasn’t changed for ten years. This blog is the result.

Part of my motivation has also been the excellent blog by my friend Arjun Raj. We were postdocs together in the van Oudenaarden lab, and his blog has been a fun way for me to keep in touch with him (or at least to hear what he has been thinking about, since I typically don’t respond in any way).