John Gribbin

genetic engineering possible depends on our present understanding of quantum mechanics, without which we would not be able to interpret X-ray diffraction data, apart from anything else. To understand how to construct, or reconstruct, genes, we have to understand how and why atoms join together only in certain arrangements, certain distances apart and with chemical bonds of a certain strength. That understanding is the gift of quantum physics to chemistry and to molecular biology.

I’ve labored the point a little more than I might have had it not been for a member of the University College of Wales. In March 1983, in a review in New Scientist, I mentioned in passing that “without quantum theory there would be no genetic engineering, no solid-state computers, no nuclear power stations (or bombs).” This drew a complaint from a correspondent in that respected academic institution to the effect that he was fed up with seeing genetic engineering dragged in everywhere as the new scientific buzzword, and that John Gribbin shouldn’t be allowed to get away with such outrageous remarks. What possible connection, however tenuous, could there be between quantum theory and genetics? I hope the connection is clear this time. At one level, it is delightful to be able to point out the fact that Crick’s conversion to biophysics was directly inspired by Schrödinger, and that the work that led to the discovery of the DNA double helix was carried out under the formal, if sometimes unwelcome, direction of Lawrence Bragg; at a deeper level, of course, the reason for the interest of pioneers like Bragg and Schrödinger, and the next generation of physicists such as Kendrew, Perutz, Wilkins, and Franklin in biological problems is that these problems are, as Schrödinger pointed out, simply another kind of physics, one that deals with collections of large numbers of atoms in complex molecules.

This misconception still arises today, partly because of the way the idea of uncertainty is often taught. Heisenberg himself used the idea of observing an electron to make his point. We can only see things by looking at them, which involves bouncing photons of light off them and into our eyes. A photon doesn’t disturb an object like a house very much, so we don’t expect the house to be affected by looking at it. For an electron, though, things are rather different. To start with, because an electron is so small we have to use electromagnetic energy with a short wavelength in order to see it (with the aid of experimental apparatus)at all. Such gamma radiation is very energetic, and any photon of gamma radiation that bounces off an electron and can be detected by our experimental apparatus will drastically change the position and momentum of the electron—if the electron is in an atom, the very act of observing it with a gamma ray microscope may knock it out of the atom altogether.

All this is true enough, and it does give a general idea of the impossibility of measuring precisely both the position and momentum of an electron. But what the uncertainty principle tells us is that, according to the fundamental equation of quantum mechanics, there is no such thing as an electron that possesses both a precise momentum and a precise position.

This has far-reaching implications. As Heisenberg said at the end of his paper in the Zeitschrift, “We cannot know, as a matter of principle, the present in all its details.” This is where quantum theory cuts free from the determinacy of classical ideas. To Newton, it would be possible to predict the entire course of the future if we knew the position and momentum of every particle in the universe; to the modern physicist, the idea of such perfect prediction is meaningless because we cannot know the position and momentum of even one particle precisely. The same conclusion comes out of all the different versions of the equations, the wave mechanics, the Heisenberg-Born-Jordan matrices, and Dirac’s q numbers, although Dirac’s approach, which carefully avoids any physical comparisons with the everyday world, seems the most appropriate. Indeed, Dirac very nearly came to the uncertainty relation before Heisenberg. In a paper for the Proceedings of the Royal Society in December 1926 he pointed out that in quantum theory it is impossible to answer any question that refers to numerical values of both q and p, although “one would expect, however, to be able to answer questions in which only the q or only the p are given numerical values.”

It was only in the 1930s that the philosophers took up the implications of these ideas for the concept of causality—the idea that every event is caused by some other specific event—and the puzzle of predicting the future. Meanwhile, although the uncertainty relations had been derived from the fundamental equations of quantum mechanics, some influential experts began to teach quantum theory by starting out from the uncertainty relations. Wolfgang Pauli was probably the key influence in this trend. He wrote a major encyclopedia article on quantum theory that began with the uncertainty relations, and he encouraged a colleague, Herman Weyl, to begin his textbook Theory of Groups and Quantum Mechanics in much the same way. This book was first published in German in 1928 and in English (by Methuen) in 1931. Together, the book and Pauli’s article set the tone for a generation of standard texts. Students raised on those texts became, in some cases, professors in their turn, and passed on the same style of teaching to subsequent generations. As a result students at university today are still, more often than not, introduced to quantum theory via the uncertainty relations...