What’s the hurry?
- Dreams of a Final Theory by Steven Weinberg
Radius, 260 pp, £16.99, January 1993, ISBN 0 09 177395 4
Until roughly the 20th century, physics was concerned with the realities of ordinary experience: light, heat and sound; motion, acceleration, falling bodies; gases, fluids, solids; electricity, magnetism and so on and so forth through the world of phenomena. Then in 1895, Wilhelm Roentgen discovered X-rays; in 1897, J.J. Thompson discovered the electron; in 1914, Rutherford discovered the proton – and all at once a new branch of physics had come into existence: elementary particle theory, dealing with the hidden realities, the fundamental entities that underlie the observed phenomena of everyday life.
During the 20th century, particle theory developed and grew. It was a continual race between theory and experiment, as new particles showed up in accelerators and new theories arose to accommodate and explain them. Now, near the end of the century, and after almost a hundred years of particle theory, physicists are seriously contemplating the end of their subject. Soon, they think, a Final Theory will be at hand, one that will provide an explanation for absolutely everything that matters in physics. Steven Weinberg’s Dreams of a Final Theory tells us how we got to this point, and urges us to take the next logical step, which might also be our last, since if successful it will give us a complete and lasting theory of nature, ‘one that would be of unlimited validity and entirely satisfying in its completeness and consistency’.
This last step, unfortunately, will be rather expensive, involving the building of the Superconducting Super Collider (or SSC), a 53-mile-long $8 billion particle accelerator underneath the wheat fields of Ellis County, Texas. These last few elementary particles, apparently, are extremely bashful, and do not come cheap. Weinberg’s book is gracefully, even elegantly written. As a history of physics, mainly particle physics, it’s clear and authoritative. It describes the so-called standard model: the currently accepted picture of the elementary particles and the various forces – strong, weak and electromagnetic – that govern their interaction. And it explains one of the major outstanding problems with the standard model, the origin of ‘spontaneous symmetry-breaking’.
Nature, mostly, is symmetrical. On one level this means that objects look the same from different points of view. ‘A sphere looks the same from any direction,’ Weinberg says. ‘Empty space looks the same from all directions and all positions.’ On a deeper level, nature is symmetrical in the sense that its laws and regularities are immune to changes in frames of reference. ‘It makes no difference to our results whether we do our experiments in Texas or Switzerland or on some planet on the other side of the galaxy.’ Nevertheless, nature’s symmetries are often spontaneously broken; an ordinary magnet is an example. ‘The equations that govern the iron atoms and magnetic field in a magnet are perfectly symmetrical with regard to direction in space,’ Weinberg says. ‘Nothing in these equations distinguishes north from south or east or up. Yet, when a piece of iron is cooled below 770°C it spontaneously develops a magnetic field pointing in some specific direction, breaking the symmetry among different directions.’ When such asymmetries appear out of nowhere, an explanation is required. The explanation in the case of the magnet is that during the process of cooling, its atoms spontaneously line up in the same direction, producing a magnetic field. Asymmetries have cropped up in particle theory, too: specifically, there is a breakdown of the symmetry that relates weak and electromagnetic forces. The problem is that the explanation of this asymmetry is currently unknown, and constitutes one of the main theoretical gaps (the other being how to incorporate gravity into the overall picture) which a Final Theory of Nature is designed to bridge. Weinberg suggests what such a theory might be like, and ventures to say what it will mean to physics, and to the world at large, if and when it arrives.