|Energy | Pages 132-133 ||
longest wavelengths and are the simplest types of light to combine accurately. Huge radio dishes in arrays that span many miles or even entire continents take exquisitely detailed images of the centers of galaxies and other distant objects. Satellites orbiting Earth use a similar technique with radar waves to gauge subtle shifts in land elevation. The satellites can spot the strain building along earthquake faults, as well as bulges in volcanoes that foretell eruptions.
Improvements in the way we record light have gone hand in hand with advances in telescope design. In the mid-nineteenth century, the American astronomer Henry Draper was the first to mount photographic plates on the back of a telescope. This was a leap forward from drawings and written notes, which depended on the observer's skill and accuracy. To this day, photographs produce stunning views of the heavens. For research, however, most astronomers now use digital detectors called charge-coupled devices, or CCDs. CCDs are silicon chips that convert most of the photons striking them into electronic pictures. That's much more efficient than the measly few percent conversion rate of film. CCDs also erase quickly to record the next image--no reloading necessary--and produce files that store easily. If you own a video camera, an inexpensive version of this device is your camera's electronic retina.
Not all observatories use mirrors or giant dishes to detect energy. Gamma-ray telescopes in space intercept their quarry with clear crystals, which then emit flashes of visible light. Similar flashes occur within the huge tanks of water at neutrino observatories like the ones that detected the neutrinos from Supernova 1987A. Astronomers also have lowered long strings of light sensors beneath the ice of Antarctica to detect neutrinos that stop there. A successful neutrino "telescope" would allow a direct view of the Sun's core, which flings out more than 100 trillion trillion trillion neutrinos every second. But the challenge to see neutrinos efficiently is steep indeed. After all, they can penetrate a light-year of solid lead (nearly 6 trillion miles) without effort.
Yet another nonelectromagnetic window on the universe may open soon: gravitational waves. According to Einstein's general theory of relativity, these elusive ripples in the fabric of space-time spread outward from the motions of massive objects. Two black holes spiraling around each other, or two neutron stars colliding, would stir space-time like an egg beater. As the gravitational waves passed Earth, they would alternately compress and expand the space between two objects. We would hardly notice the change, though. Objects separated by several miles would move relative to each other by less than the diameter of a single proton.
Spurred by the challenge of detecting such tiny motions, physicists are building gravitational wave observatories in the United States and Italy. At each of the two U.S. sites, pairs of mirrors hang 2.5 miles apart along both arms of an L-shaped tunnel. Laser beams course through vacuum tubes between the mirrors to measure their positions. Plans call for a similar observatory in deep space but with arms spanning millions of miles on a side. Ultimately, this approach may let us perceive the subtle ripples in space-time that reverberate through the universe from the Big Bang itself.
Probing Space with SPECTRA
Gravitational waves would provide a clever way to "see" the universe without photographs. Even so, beautiful pictures of heavenly objects will continue to inspire awe and wonder about the cosmos. There's something extraordinary about seeing rocks on the surface of another planet, the nurseries of new stars, or pinwheels of a hundred billion suns like our own. Astronomers enjoy such pictures as well as learn from them. But they have another tool to help unveil the scientific details about distant objects: spectra, which can be worth a thousand pictures.
Spectra are the patterns created by spreading the light from a glowing object into its component colors. Shining sunlight through a prism is an easy way to see this effect. Isaac Newton was the first to use a prism in this way. He also showed that a second prism, inverted with respect to the first one, recombined the colors of the spectrum into the original beam of sunlight. Rainbows are spectra of sunlight painted against the sky. Millions of raindrops act like spherical prisms. They refract and reflect sunlight back to your eyes at a precise but slightly different angle for each color. The geometry of this bending process creates a circle of brilliant colors. However, we usually only see the top halves of the circles because the ground gets in the way.
The difference between these spectra and the ones produced by telescopes is a matter of detail. Instruments called spectrographs divide the light of stars or galaxies into sharply focused spectra that spread up to several feet long. These spectra are not (continued)