Just Six Numbers: The Deep Forces That Shape the Universe

Natural History, Feb, 2000 by Jeremiah P. Ostriker

Just Six Numbers: The Deep Forces That Shape the Universe, by Martin Rees. Basic Books/Perseus; $21; 304 pp.

Mathematical laws drive not only the microworld of atoms--and the forces linking them together--but the whole fabric of the cosmos.

Cosmology is a subject that no culture has done without. For most of Western history, placing the observable world in the context of time and space has been a speculative, philosophical endeavor--not a field of quantitative reasoning.

When I received my Ph.D. in astrophysics in 1964, some aspects of the subject had entered the modern era, but the flavor of speculative ideology was still strong. At that time, galaxies essentially defined the objects to be observed, and the properties to be discussed were their number, their apparent brightness, their distance from us, and the velocity at which they were speeding away. Such information was combined and manipulated to answer two questions raised by Einstein's general theory of relativity: What is the age of the universe (following the big bang)? Will the universe expand forever, or will it stop expanding, reverse course, and recollapse?

Since that time, our knowledge has grown enormously, as has the complexity of the questions. The features of big bang cosmology have been so well confirmed, both by the distribution of the light chemical elements cooked up in that early furnace and by direct measurements of cosmic background radiation, that the majority of cosmologists have no doubt about the basic accuracy of the model.

But now we recognize that the galaxies are not eternal and unchanging and that to use them in empirical studies, we need to understand their formation and evolution. We a]so know that ten times more dark matter of a mysterious nature exists than does normal matter, which is detectable by the radiation it emits or absorbs. What we know as normal matter (on the basis of measured gravitational forces) accounts for perhaps only 10 percent of what's out there.

Martin Rees, England's astronomer royal, addresses these questions at the level of fundamental physics. Starting with Earth, other Solar System planets, and nearby stars, he carries us through our galaxy and out into the cosmos on its grandest scale. Although this is a brief, clear, wise, and witty book, it is challenging to read. Translating the fundamental laws of physics into accessible language, Rees explains why the chemistry necessary for life is possible; why nuclear-powered stars can exist in equilibrium; why structure developed at all (as opposed to the universe's evolving in a featureless void); and why there may exist a repulsive force (unlike gravity, which makes all matter attract all other matter) that causes the universe to expand at an ever accelerating rate. Kees does not directly address the so-called anthropic principle--that the universe had to be just so to allow complex life to form--but leaves it for the reader to discover (or rediscover) unaided.

The story is presented in terms of dimensionless numbers (those whose values are independent of the units used). One of these is pi, familiar to us all as the ratio of the circumference or a circle to its diameter. Another is [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]--the ratio of the electrostatic forces pushing apart two electrons to the gravitational forces pulling them together--one of the famous "big numbers" of physics, roughly ten followed by forty zeroes. (Gravity, although it dominates at cosmic scales, is extraordinarily weak compared with atomic forces.) Rees shows, without using any mathematics, how such numbers determine the structure of our universe.

It is all here, with the tale told--for once--by someone who understands the story.

Jeremiah P. Ostriker, a professor of astrophysics and provost at Princeton University, is a trustee of the American Museum of Natural History.