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Undisciplined Science

Brian Hayes

"All science is either physics or stamp collecting" said Lord Rutherford, who was not a stamp collector. The remark did nothing to win friends for physics among practitioners of other sciences. But Rutherford got his come-uppance: When he was summoned to Stockholm in 1908, the prize awaiting him there was not in physics but in chemistry.
A century later, surveying the state of physics and its relations with other fields, I am tempted to give Rutherford's quip an even more inflammatory reading, though he never intended it. "All science is physics" might be taken as a territorial claim, annexing other disciplines as provinces to be ruled by the laws of physics and administered by physicists. This imperial vision of the destiny of physics is not entirely without a basis in history, or at least etymology. At one time, the term physics had a very broad meaning, roughly synonymous with natural science. The 18th-century Encyclopédia of Diderot and d'Alembert listed under the rubric physique particuliere everything from astronomy and cosmology to meteorology, mineralogy, chemistry, zoology and botany (but not stamp collecting).

Figure 1. Tree of knowledge . . .Click to Enlarge Image

Browsing through recent issues of Physical Review E (a section of the main journal published by the American Physical Society), one could form an equally expansive view of the scope of 21st-century physics. Within the past year, the Phys Rev E table of contents has included titles such as "Outbreaks of Hantavirus induced by seasonality," "Large-scale structural organization of social networks," "Topology of the world trade web," "Generating neural circuits that implement probabilistic reasoning" and "Number fluctuation and the fundamental theorem of arithmetic." Evidently, the boundaries of physics are elastic enough to take in aspects of viral epidemiology, sociology, market economics, cognitive neuroscience and number theory. Are all of those fields now absorbed into the empire of physics?

The story I want to tell here is not about sleeper cells of militant physicists plotting a coup in the biology department. As a matter of fact, although physics provides the most dramatic examples, several other disciplines also have boundaries that seem to be shifting or growing porous. Intellectual migrants are wandering back and forth across many academic frontiers, generally without stopping for any formalities at the customs house. In some cases, the same paper might be classified as physics, biology, mathematics or computer science, depending more on the author's affiliation and where it was published than on the subject matter.

Departmental reshuffling and realignment goes on all the time, but the present moment seems to be one of particular ferment. Among many possible causes, I would point to the changing role of computation in the various sciences. A number of earlier upheavals in the structure of scientific disciplines have been triggered by new techniques and instruments, sometimes imported from other fields. Today, computation is the common thread in many of the areas that are having a disciplinary identity crisis. Some of these areas rely heavily on computer simulations or experiments, and others analyze large data sets accessible only with computer technology. Computer science also exerts a subtler but deeper influence when laws of nature are expressed in algorithmic form.

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