A short history of nearly everything

clockwise and counterclockwise, which are treated differently. The clockwise vibrations live in a ten-dimensional space. The counterclockwise live in a twenty-six-dimensional space, of which sixteen dimensions have been compactified. (We recall that in Kaluza’s original five-dimensional, the fifth dimension was compactified by being wrapped up into a circle.)” And so it goes, for some 350 pages.

String theory has further spawned something called “M theory,” which incorporates surfaces known as membranes-or simply “branes” to the hipper souls of the world of physics. I’m afraid this is the stop on the knowledge highway where most of us must get off. Here is a sentence from the New York Times, explaining this as simply as possible to a general audience: “The ekpyrotic process begins far in the indefinite past with a pair of flat empty branes sitting parallel to each other in a warped five-dimensional space. . . . The two branes, which form the walls of the fifth dimension, could have popped out of nothingness as a quantum fluctuation in the even more distant past and then drifted apart.” No arguing with that. No understanding it either. Ekpyrotic, incidentally, comes from the Greek word for “conflagration.”

Matters in physics have now reached such a pitch that, as Paul Davies noted in Nature, it is “almost impossible for the non-scientist to discriminate between the legitimately weird and the outright crackpot.” The question came interestingly to a head in the fall of 2002 when two French physicists, twin brothers Igor and Grickha Bogdanov, produced a theory of ambitious density involving such concepts as “imaginary time” and the “Kubo-Schwinger-Martin condition,” and purporting to describe the nothingness that was the universe before the Big Bang-a period that was always assumed to be unknowable (since it predated the birth of physics and its properties).

Almost at once the Bogdanov paper excited debate among physicists as to whether it was twaddle, a work of genius, or a hoax. “Scientifically, it’s clearly more or less complete nonsense,” Columbia University physicist Peter Woit told the New York Times, “but these days that doesn’t much distinguish it from a lot of the rest of the literature.”

Karl Popper, whom Steven Weinberg has called “the dean of modern philosophers of science,” once suggested that there may not be an ultimate theory for physics-that, rather, every explanation may require a further explanation, producing “an infinite chain of more and more fundamental principles.” A rival possibility is that such knowledge may simply be beyond us. “So far, fortunately,” writes Weinberg in Dreams of a Final Theory, “we do not seem to be coming to the end of our intellectual resources.”

Almost certainly this is an area that will see further developments of thought, and almost certainly these thoughts will again be beyond most of us.

While physicists in the middle decades of the twentieth-century were looking perplexedly into the world of the very small, astronomers were finding no less arresting an incompleteness of understanding in the universe at large.

When we last met Edwin Hubble, he had determined that nearly all the galaxies in our field of view are flying away from us, and that the speed and distance of this retreat are neatly proportional: the farther away the galaxy, the faster it is moving. Hubble realized that this could be expressed with a simple equation, Ho = v/d (where Ho is the constant, v is the recessional velocity of a flying galaxy, and d its distance away from us). Ho has been known ever since as the Hubble constant and the whole as Hubble’s Law. Using his formula, Hubble calculated that the universe was about two billion years old, which was a little awkward because even by the late 1920s it was fairly obvious that many things within the universe-not least Earth itself-were probably older than that. Refining this figure has been an ongoing preoccupation of cosmology.

Almost the only thing constant about the Hubble constant has been the amount of disagreement over what value to give it. In 1956, astronomers discovered that Cepheid variables were more variable than they had thought; they came in two varieties, not one. This allowed them to rework their calculations and come up with a new age for the universe of from 7 to 20 billion years-not terribly precise, but at least old enough, at last, to embrace the formation of the Earth.

In the years that followed there erupted a long-running dispute between Allan Sandage, heir to Hubble at Mount Wilson, and Gerard de Vaucouleurs, a French-born astronomer based at the University of Texas. Sandage, after years of careful calculations, arrived at a value for the Hubble constant of 50, giving the universe an age of 20 billion years. De Vaucouleurs was equally certain that the Hubble constant was 100.[26] This would mean that the universe was only half the size and age that Sandage believed-ten billion years. Matters took a further lurch into uncertainty when in 1994 a team from the Carnegie Observatories in California, using measures from the Hubble space telescope, suggested that the universe could be as little as eight billion years old-an age even they conceded was younger than some of the stars within the universe. In February 2003, a team from NASA and the Goddard Space Flight Center in Maryland, using a new, far-reaching type of satellite called the Wilkinson Microwave Anistropy Probe, announced with some confidence that the age of the universe is 13.7 billion years, give or take a hundred million years or so. There matters rest, at least for the moment.

The difficulty in making final determinations is that there are often acres of room for interpretation. Imagine standing in a field at night and trying to decide how far away two distant electric lights are. Using fairly straightforward tools of astronomy you can easily enough determine that the bulbs are of equal brightness and that one is, say, 50 percent more distant than the other. But what you can’t be certain of is whether the nearer light is, let us say, a 58-watt bulb that is 122 feet away or a 61-watt light that is 119 feet, 8 inches away. On top of that you must make allowances for distortions caused by variations in the Earth’s atmosphere, by intergalactic dust, contaminating light from foreground stars, and many other factors. The upshot is that your computations are necessarily based on a series of nested assumptions, any of which could be a source of contention. There is also the problem that access to telescopes is always at a premium and historically measuring red shifts has been notably costly in telescope time. It could take all night to get a single exposure. In consequence, astronomers have sometimes been compelled (or willing) to base conclusions on notably scanty evidence. In cosmology, as the journalist Geoffrey Carr has suggested, we have “a mountain of theory built on a molehill of evidence.” Or as Martin Rees has put it: “Our present satisfaction [with our state of understanding] may reflect the paucity of the data rather than the excellence of the theory.”

This uncertainty applies, incidentally, to relatively nearby things as much as to the distant edges of the universe. As Donald Goldsmith notes, when astronomers say that the galaxy M87 is 60 million light-years away, what they really mean (“but do not often stress to the general public”) is that it is somewhere between 40 million and 90 million light-years away-not quite the same thing. For the universe at large, matters are naturally magnified. Bearing all that in mind, the best bets these days for the age of the universe seem to be fixed on a range of about 12 billion to 13.5 billion years, but we remain a long way from unanimity.

One interesting recently suggested theory is that the universe is not nearly as big as we thought, that when we peer into the distance some of the galaxies we see may simply be reflections, ghost images created by rebounded light.

The fact is, there is a great deal, even at quite a fundamental level, that we don’t know-not least what the universe is made of. When scientists calculate the amount of matter needed to hold things together, they always come up desperately short. It appears that at least 90 percent of the universe, and perhaps as much as 99 percent, is composed of Fritz Zwicky’s “dark matter”-stuff that is by its nature invisible to us. It is slightly galling to think that we live in a universe that, for the most part, we can’t even see, but there you are. At least the names for the two main possible culprits are entertaining: they are said to be either WIMPs (for Weakly Interacting Massive Particles, which is to say specks of invisible matter left over from the Big Bang) or MACHOs (for MAssive Compact Halo Objects-really just another name for black holes, brown dwarfs, and other very dim stars).

Particle physicists have tended to favor the particle explanation of WIMPs, astrophysicists the stellar explanation of MACHOs. For a time MACHOs had the upper hand, but not nearly enough of them were found, so sentiment swung back toward WIMPs but with the problem that no WIMP has ever been found. Because they are weakly interacting, they are (assuming they even exist) very hard to detect. Cosmic rays would cause too much interference. So scientists must go deep underground. One kilometer underground cosmic bombardments would be one millionth what they would be on the surface. But even when all these are added in, “two-thirds of the universe is still missing from the balance sheet,” as one commentator has put it. For the moment we might very well call