The Dark Matter: Contemporary Science's Quest for the Mass Hidden in Our Universe by Wallace and Karen Tucker (William Morrow: $16.95; 254 pages)
The history of science has been marked by periods when observations did not square with theories. When this happens, it is a sign that the underlying theory is wrong. Putting things right has often required a revolution in thought.
In the 16th Century, observations of the motions of the planets in the sky did not coincide with the predictions of the Ptolemaic universe. This was the sign that something was wrong with humanity's picture of the cosmos. It took Copernicus to put the sun in the center of the solar system and straighten things out.
Similarly, by the early 20th Century, observations of electromagnetic phenomena were at odds with the predictions of Newtonian physics. Einstein's theories of relativity explained the discrepancies and in the process changed the way we think about reality.
Too Much Gravity
In the last decade or so, astrophysics has come up against a similar and deeply troubling problem: There is more gravitational force at work in the universe than can be accounted for by the amount of matter we observe. Much more gravitational force. Something is drastically wrong either with the way we observe mass or with our theories of gravitation or both. The missing mass has been dubbed dark matter, and it poses one of the most bedeviling problems that scientists face today.
"Astronomers now suggest that as much as 90% of the matter in the universe must be in some dark form that has so far escaped detection," Wallace and Karen Tucker write in "The Dark Matter," their intelligent survey of this problem and its possible solutions.
"The mystery of the dark matter has become one of the most important issues of the day for anyone who wants to understand what the universe is and how it works," they write. "The dark-matter problem is so pervasive that the solution promises to revolutionize astronomy and cosmology."
Simply put, the shape and rotational speed of spiral galaxies, including our own Milky Way, are substantially out of whack with predictions based on the amount of matter in the visible stars. This is not a minor discrepancy. Estimates vary, but it seems that somewhere between 50% and 90% of the mass of the universe has somehow escaped detection up till now. That is, what we see may be only one-tenth of what there is!
Where's the Mass?
This state of affairs raises numerous questions, not least of which are: What form is the rest of the mass in, and where is it? The Tuckers (he's an astrophysicist and she's a science writer) address these questions and several others with verve, style and illuminating analogies.
First they show that the dark matter must be everywhere--in the disk of our galaxy, in the envelope or cloud surrounding our galaxy, in similar envelopes around galaxies of all sizes and shapes and in groups or clusters of galaxies.
Then, one by one, they run down the various theories that have been offered to explain the missing mass, and one by one they show how each of these theories fails to solve the problem. There have even been suggestions that there is something wrong with the theory of gravitation itself, an idea that has met with much disfavor among scientists reluctant to give up a theory that has so much evidentiary support.
What makes the book particularly compelling is that it operates on two levels. On the one hand, it is a detailed but comprehensible account of the problem of dark matter, a problem that grows in significance with every new discovery.
At the same time, it is the story of how science works. Observations that clash with theory are at first dismissed as experimental errors. Then, as they begin piling up and can no longer be ignored, simple explanations are offered. When they don't work, the attempted explanations become more complicated. Scientists are very, very reluctant to abandon a theory that has worked in the past until they have no choice and until they have something better.
'We Can't Understand'
At the moment, astrophysicists have nothing better, no way to explain the dark-matter problem. "New ideas proliferate like weeds," the Tuckers write. "Which ideas should be kept and which ones discarded? . . . It is as if nature is trying to tell us something very obvious and we can't understand."
The Tuckers are convinced that the solution to the dark-matter problem will change our view of reality as much as Copernicus or Einstein did. "Either the basic laws of physics may have to be revised, or we may have to accommodate ourselves to a universe in which most of the matter is concealed from us in some dark form," they write.
Their book is a compelling account of a problem on the frontiers of thought.