To the editors:
For a very long time, a period that can be measured in centuries, the autonomy of chemistry from physics has been accepted as a given. Dirac’s pronouncement notwithstanding, and despite its having spawned a cottage industry of irredeemably dull articles, reducing chemical structure and/or dynamics to quantum physics remains a non-issue.1 It is dead as a doornail.2
All the evidence Marc Henry has gathered in his paper supports the above statement.
But is it the end of the story? Not entirely.
There is one area where physics has not only been beneficial, but, in fact, absolutely crucial to the development of chemistry: the so-called physical methods.3 In the aftermath of World War II, chemical laboratories were revolutionized by instrumentation devised by physicists. In the mid nineteenth century, the polarimeter forever changed chemical practice; similarly, in the mid twentieth century, the laboratory work of chemists was revolutionized by many devices invented by physicists:4 mass spectrometers, nuclear magnetic and electronic spin resonance machines, optical rotatory dispersion and circular dichroism apparatuses, photoelectron spectroscopy and ESCA, X-ray diffractometers, and chromatographs of every ilk.5 Note in passing that this instrument revolution antedated the arrival of computers, by about twenty years.
I shall focus here exclusively on the NMR revolution, in order to distinguish the relative inputs from physics and physicists on the one hand, and from chemistry and chemists on the other.6
When applied to the molecular systems of chemistry, NMR first affected structural elucidation. This was in the early days, the fifties and early sixties, when protons were the only nuclear particles accessible to commercial spectrometers. Chemists quickly evolved standardized procedures to provide the attendant connectivity matrix from the observed NMR spectrum.7 Why was it a revolutionary instrument? Never before had such a tool existed, enabling the mapping of a set of resonance frequencies, with their fine structure, onto a chemical graph. During the ensuing years, studies of carbon-13 provided marvelous additional insights into the structures of, typically, organic molecules. Again, chemists such as Paul Lauterbur, Jack Roberts and Dave Grant were the pioneers.8 Early on, macromolecular proteins beckoned, challenging scientists to unravel their complexities with the tool of NMR. And chemists pushed their instruments to their limits, seeking a foothold for scaling protein structure: Bill Phillips and Aksel Bothner-By were two such pioneers, again coming from the ranks of chemists.9
Relatively early on, the then-poor sensitivity of NMR became an issue. A first solution, borrowing a procedure from electroencephalography, was to record and add successive traces in a multichannel analyser, then termed a CAT (computer of average transients). Where the noise N is random, the signal S is coherent, and so the S/N ratio grows with the square root of the number of runs.10
A far superior solution was Fourier transform (FT) NMR. Here pulse excitation is followed by a FT, which converts it from a time domain to a frequency domain. Both these hikes in sensitivity were devised by physicists, LeRoy Johnson and Richard Ernst.11 Ernst was awarded a Nobel prize for it.
Structural analysis was a major application of NMR. Another was the monitoring and measurement of kinetic processes in molecules and in coordination complexes.12 Pioneers here were Herbert Gutowsky, Sture Forsén and Ragnar Hoffman, who were physical chemists.13
One should not fail to mention also fluxionality, where a fluxional atomic assembly undergoes rearrangements, converting it into itself while exchanging atomic types. The organic molecule bullvalene was the original entity for which the concept and the term were coined. Not long afterwards, pentacoordinate species were discovered to undergo similar fluxional processes. Chemists Martin Saunders, Paul Lauterbur and R. S. Berry were early students of fluxional entities.14
I could not help injecting this brief description of an instance of cross-disciplinary fertilization. It is reminiscent, conceptually, of the mathematical procedure of a convolution product. Would it have been different had it concerned itself with physics in astronomy, chemistry in biology (or vice-versa)? Probably not. Just like irrigation for agriculture in an arid climate, cross-disciplinarity is a must. It demands a keen perception of what constitutes the core of a discipline—as the article by Marc Henry so nicely exemplifies.
Pierre Laszlo
Marc Henry replies:
Pierre Lazlo’s response reminds us that physicists and chemists have been walking hand in hand since World War II to develop very powerful instruments. Among all the proposed structural tools, Nuclear Magnetic Resonance (NMR) plays obviously a prominent role. This was a win-win situation with chemists providing new challenges to physicists and physicists responding to these challenges with still more sophisticated methods that found immediate applications in chemistry and biology.
NMR is, in fact, a perfect illustration that what we call physics in this particular case is not a single discipline, but a subtle interweaving of at least three different autonomous basic sciences: quantum mechanics, electromagnetism, and thermodynamics. These three disciplines should, of course, be mastered by any good NMR spectroscopist. However, as explained in the essay, this does not imply that a NMR spectroscopist should be identified as a physicist. He could also have skills in chemistry, biology, and even in relativity for a correct theoretical handling of heavy elements.
It thus seems important to me to stress that a unification of the six basic sciences should not be sought in a GUT covering all fields, but rather in the men that practice science. My personal opinion is that scientists are made of six basic essences, or perfumes—a little bit like visible light, which may be thought of as a mixture of six basic colors, according to Goethe’s viewpoint. Consequently, if I can easily perceive what the stuff is that makes up a scientist with a wide range of shades, I have considerable difficulty in defining exactly what is a physicist.
The essay was written to pinpoint the fact that the equation Science = Physics is fundamentally wrong, a more useful one being:
Science = quantum mechanics ⊕ relativity ⊕ electromagnetism
⊕ thermodynamics ⊕ chemistry ⊕ biology
With such an equation in mind, the development of NMR techniques should be considered as a milestone reached through a collaboration between scientists and not as the exclusive prerogative of an ambiguous class of people named physicists. In other words, in contrast to Dirac’s famous statement, there should be no master-slave relationship in science.
