Monday, August 2, 2010

Sophistication versus Understanding

I attended the SPIE meeting in San Diego over the weekend, where I gave an invited talk about work done in collaboration with David Watkins of the Math Department. The meeting was in a small room with perhaps a couple dozen attendees, all experts in organic nonlinear optics. We had a good time exchanging ideas and throwing about some new thoughts.

As I have mentioned in previous posts, I find travel physically draining. So, spending eight hours en route to San Diego on Saturday and eight hours on Sunday to return home took its toll. While I usually work on weekends, I find it much more relaxing than sitting on an airplane. So, I started the week in an uncharacteristically bad mood.

To add insult to injury, I learned on Sunday and Monday that two of my papers were outright rejected -- not something that I commonly experience. In addition, a paper submitted by my collaborators, to which my contribution was relatively minor, was also rejected. Thus, in a span of three days I had more rejection than in a typical decade. I think that my darker than usual mood was warranted given the extraordinary circumstances. In fact, I entertained the notion of quitting the professional life of physics altogether.

After a good-night's sleep, my mood dramatically improved so on Tuesday morning, I wrote a levelheaded email to the two editors who had rejected my papers. In the process of composing these letters, I realized that science has turned into a big efficient machine, with creativity a reluctant causality.

For more than three decades, lots of people have been expending a great deal of effort to make better molecules. In parallel, computational methods are getting more sophisticated so that theoretical chemists can calculate the properties of ever-larger molecules, taking into account more subtle effects and getting more accurate results. Similarly, chemists have made a huge number of very complex structures, many of which are pieces of art, such as the ever-branching dendrimers.

My own work, which uses sum rules to understand the nonlinear optical response of quantum systems (which I started ten years ago), illustrates how simple but powerful ideas can arise even in a mature field. The basic question that I asked was if there was a fundamental limit to the nonlinear optical response. The answer was a resounding "yes." This limit is not based on practical considerations, but on very fundamental quantum mechanical principles that span the basis of chemical reactions of life and govern the flow of electrons in electronic circuits. If the fundamental theory of quantum mechanics were to be wrong, then the world would be alien to us. In fact, we probably wouldn't exist. In short, I feel that my fundamental limits calculations stand on solid ground.

I expected great admiration for my theory when I first presented a talk on the topic a decade ago. Instead, I got some very nasty comments to the effect that my work was an insult to all the hard-working chemists who were trying to make better molecules. Who was I to say that it was not possible to do any better? Though that sentiment did not reflect my intentions, mother nature DOES place limits on what is possible.

To put this into perspective, my calculation does not imply a single numerical limit, but rather a limit in the presence of a contraint. For example, to investigate the largest possible area is nonsensical. It makes more sense to determine the largest possible area given a fixed perimeter. Similarly, when studying molecules, it is more appropriate to determine the largest nonlinear response for a given molecular size. Since size is not well defined in a quantum system, we used the more abstract concept of scaling.

The bottom line is that after three decades of research, the best molecules fall short of the fundamental limit by a factor of thirty. While people have been making bigger molecules with a larger nonlinear-optical response, the intrinsic nonlinearity has not changed since the birth of the field. That's why one of the reviewer's comments was particularly annoying. (S)he was critical of our simple fundamental approach as passe in light of all the sophisticated and precise methods available. Ironically, our simple approach has been the only one that has led to an improvement in the intrinsic hyperpolarizability.

Being a scientist, my priority is to understand, not to participate in the frenzy of doing the most sophisticated calculations. I prefer to study broad principles that apply to all systems rather than seeking higher precision in more complex calculations that apply to specific molecules. My long-term goal is to build an understanding of the fundamental issues that identify universal properties of systems that approach the fundamental limit. And in this quest, my small effort continues.

I end this post with good news. In response to my emails, both editors reversed their decisions and are giving us the opportunity to submit a revised manuscript. While I am concerned that the trend of sophistication worship is wasteful, I take comfort in the fact that our group is supported to continue our work. Perhaps a time will come when I will become one of the dinosaurs that was left behind, but for now; I take great satisfaction in my research and the potential it has for making a lasting contribution to the body of science.

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