You can see it sometimes, if you're out in temperate latitudes on a clear night at dusk or before dawn, when slanting sunlight glints off satellites 300 miles (483 kilometers) high—a dot of light, no brighter than an average star, trudging across the sky in a state of seeming preoccupation like that of the rabbit in Alice's Wonderland, the rippling of Earth's atmosphere (the very distortions that it was designed to rise above) making its smooth, ceaseless fall look halting and perturbed. Which pretty much describes its early career: Repeatedly delayed, then lofted into orbit only to prove myopic, repaired by one space shuttle crew, then improved by others, the Hubble Space Telescope has become the world's most popular scientific instrument, one that has been seen, and seen through, by more people than any before. Scientists feast on its data, while its beautiful images of star clusters, nebulae, and galaxies have made its name—after Edwin Hubble, discoverer of the expansion of the universe—almost as well-known as Google.
It's curiously appropriate that an unmanned telescope should emerge as a symbol of science, since it was instruments generally—and telescopes in particular—that jump-started the scientific revolution. We tend to think of science in terms of great minds conjuring big ideas (an image that Edwin Hubble himself encouraged, at least when it came to his own research), but that paradigm is largely a holdover from prescientific days, when knowledge was sought principally in philosophers' books. In science, instruments can trump arguments. The disinterested verdict of Galileo's telescope did more than Galileo's arguments to lay bare the shortcomings of the regnant Earth-centered model of the cosmos, and Newton's mechanics endured less for their indubitable elegance than for their being able to predict what astronomers would see through their telescopes. Galileo's contemporary Johannes Kepler, whom Immanuel Kant called "the most acute thinker ever born," was quick to grasp that straightforward observations using scientific instruments could sweep away centuries of intelligent but ignorant discourse. Although he was a mathematical theorist who never owned a telescope, Kepler celebrated Galileo's innovation in an ode, addressing the telescope as, "You much knowing tube, more precious than any scepter."
Hubble is Galileo's telescope flung into a Keplerian orbit, and if these two early scientists came back to life today, I expect they would be impressed less by its technological sophistication than by its potential to bring things to light that challenge old ideas—and to publish them on the Internet, science having always been about making knowledge available. That was certainly the attitude of Lyman Spitzer, Jr., the astrophysicist and alpinist who proposed putting a large astronomical telescope in orbit in 1946, nearly a half century before Hubble was launched and long before many of the innovations it relies upon—microprocessors, digital imaging and communications systems, the space shuttle—yet existed. Spitzer stressed that it would serve not just to test and refine existing ideas, but also to spark entirely new ones. "The chief contribution of such a radically new and more powerful instrument," he predicted, "would be, not to supplement our present ideas of the universe we live in, but rather to uncover new phenomena not yet imagined, and perhaps to modify profoundly our basic concepts of space and time."
Selling a billion-dollar project with hand-waving promises that it would alter our basic conceptions of the universe could not have been easy. But Spitzer persisted, lobbying Congress for years while reassuring his fellow scientists that the job could be done without underfunding traditional ground-based astronomy. Ultimately he prevailed, lived to see Hubble fly, and was working in his Princeton office with Hubble data on March 31, 1997, hours before he died suddenly at home that night, at the age of 82. His dream, "that a large space telescope would revolutionize astronomy and might well be launched in my lifetime," had come true. His prophecy that it might alter our conceptions of space and time was fulfilled as well—in ways far more remarkable than anyone could have anticipated.
Before that happened, however, the space telescope did help scientists test and verify many existing astronomical theories. They used Hubble to follow the string of impacts—each more powerful than all this world's combined nuclear warheads—of the disintegrating comet Shoemaker-Levy 9 into the upper atmosphere of the giant planet Jupiter in 1994, a sobering spectacle that helped build a political consensus that NASA ought to inventory asteroids that might one day strike Earth (an effort that is itself threatened by budget constraints, even though most of the potentially dangerous asteroids remain uncharted). Astronomers produced striking images showing the astonishing and unique beauty of planetary nebulae—the shells of gas ejected by unstable, dying stars—which continue to refine astrophysical accounts of how stars evolve in the late stages of their colorful careers. They captured protoplanetary disks in the Orion Nebula and other star-forming regions, confirming that planets begin as disks of dust and gas, as had been theorized. They discovered several of the now more than 200 known planets orbiting other stars and obtained a spectrum for one of them, the first to show the atmospheric composition of an extrasolar planet. They verified the existence of black holes squatting at the centers of galaxies and nailed down a theoretical link between such black holes and the brilliant beacons called quasars. They confirmed that the mysterious high-energy flashes of light called gamma-ray bursts arrive from all over the universe, and that one class of bursts results from the implosion of massive stars.
But the strangest and least expected discovery, the one that really would "modify profoundly our basic concepts of space and time" as Spitzer had predicted, came the year after he died.
Two teams of astronomers were using Hubble to investigate supernovae—exploding stars—in galaxies long ago and far away. Their prey was a particular class of supernovae whose intrinsic brightness makes them suitable "standard candles" to help determine the change in the rate of the universe's expansion since light left the distant explosions. They were expecting to find that the rate of expansion has slowed over the eons. The idea was that cosmic expansion ought to be braked by the combined gravitational attraction exerted by all the galaxies on one another—in much the same way that a ball, thrown into the air, is slowed down by Earth's gravity. If the cosmic deceleration rate was greater than a certain quantity, the universe would eventually stop expanding and collapse, like a ball falling back to Earth; if lower, the universe was destined to expand forever.
Instead, the astronomers were astonished to find that cosmic expansion is not slowing down at all: It is speeding up. What's more, this unheralded acceleration has been going on for the past five billion years. It is as if a ball, thrown into the air, at first slowed but then sped up and simply flew away. No natural force on Earth can do such a thing—and none in the known universe could be accelerating the cosmic expansion rate. Nor is the newly discovered force particularly subtle: Taking to heart Einstein's E=mc²—that energy and matter are two sides of the same coin—scientists calculate that the new force comprises 70 percent of all the matter and energy in the universe.
Physicists have taken to calling this unknown force dark energy. But as University of Chicago cosmologist Rocky Kolb says, "Naming is not explaining," and nobody yet knows what dark energy actually is.
It may well be that dark energy is inherent to space itself. Physicists had long suspected that such a "vacuum energy" must exist, since quantum fields, which contain energy, permeate even the emptiest voids out between the galaxies. Yet when physicists calculate the amount of energy in the vacuum, they get absurdly large results, ranging from infinite ("That can't be right," ex Nobel laureate physicist Steven Weinberg mused) to zillions of times more than is required even to account for the mighty force of dark energy. The disparity troubles them; Weinberg calls it "the worst failure of an order-of-magnitude estimate in the history of science." Clearly, something is wrong with either the observations (but no error has yet been found, in many ongoing studies with Hubble and other telescopes) or with the consensus models of physics and cosmology, which for all their flaws stand as one of the grandest attainments of modern science.
Fortunately, knotty problems often prove to be gateways to scientific breakthroughs. Knowing this, the cleverest scientists are more attracted to vexing questions than to comforting answers. (Physicist Niels Bohr, confronted with one such puzzler, reportedly exclaimed, "How wonderful that we have met with a paradox. Now we have some hope of making progress.") But where might the dark-energy riddle lead?
Surveying the whole panoply of physics, from quasars imaged by Hubble near the edge of the observable universe to the subatomic realms probed by particle accelerators, one increasingly gets the sense that science has as yet detected only the tip of an iceberg. Consider the question of dimensionality: On how many dimensions is the universe built? Newton got by with just the three dimensions of familiar, everyday space. Einstein improved on Newton's accuracy by adding time as a fourth dimension: His gravitational fields bend space within four-dimensional space-time. But gravitation is only one of the many fields that, as Weinberg notes, pervade the vacuum of space. If you're trying to write a unified theory of all the known particles and fields, you may find yourself working in a dozen or more dimensions. And for all we know there are multitudes of as yet undetected particles, each with its own field, implying still more dimensions.
Are these dimensions real, or—as philosophers came to consider Ptolemy's Earth-centered model of the universe—just a handy way of calculating? Increasingly, physicists suspect that they are real. If so, the perceived universe is but a glimmer on the surface of something much larger and more complex, and the known laws of nature are not bedrock but a kind of weather, like the clouds that form over mountain peaks. Dark energy may offer a glimpse of the mountain—or iceberg—beneath. So the next time someone wonders aloud what use it is to spend billions on Hubble and the other space telescopes when we have "problems here at home," the answer may be that their use is to help us understand just what home is, and where it abides in the wider and wilder landscape.
Hubble is getting old. The next shuttle service mission to upgrade and repair it—scheduled for late next year—may be the last. Fortunately it's not alone up there. Its peers include the Spitzer Space Telescope, which detects long-wavelength infrared light invisible from the surface of Earth; the Chandra X-ray Observatory, which probes the short-wavelength part of the spectrum; and little Swift, a satellite that pinpoints short-duration, high-energy gamma-ray bursts and instantly emails word of them to professional and amateur astronomers around the world. None can do all that Hubble does, but coming up is the genuinely gigantic James Webb Space Telescope: Scheduled for launch in 2013 into an orbit a million miles (1.6 million kilometers) high, Webb will gather infrared light with a mirror over 21 feet (6.4 meters) in diameter, stylishly screened from sunlight by an umbrella the size of a tennis court. Together with a growing network of ground-based telescopes and detectors, the space observatories are producing floods of astronomical data, at a constantly increasing rate. They promise, as Lyman Spitzer noted back in 1946, to alter not only what we know, but how we learn.