This month marks the widely celebrated centennial of Albert Einstein’s special theory of relativity, which he began developing in May of 1905. It is also the anniversary of a less famous—yet even more momentous—Einstein event. On May 29, 1919, astronomers observing a solar eclipse provided the initial confirmation of his second and grander relativity theory, called general relativity. This theory holds that gravity is not really a force but more like a warp in the structure of space and time. Eighty-six years later, scientists are still grappling with this wildly counterintuitive idea.
From the start Einstein knew that verifying general relativity would not be easy. Under most conditions, gravitational effects would be subtle at best. In 1911, while in the early stages of developing the theory, he thought of a possible observable consequence: Gravity should bend light. The powerful pull of the sun, for instance, should slightly shift the path of starlight passing close to its surface. Normally, the sun’s glare would obscure the bending. During a solar eclipse, however, astronomers could examine stars next to the sun in the sky and see if they appeared to be displaced from their usual locations.
At first, fate seemed to conspire against Einstein. Rain clouds prevented a Brazilian and British team from watching a 1912 eclipse in Brazil. (That actually proved a lucky break for Einstein, since his early version of general relativity predicted the wrong amount of deflection.) Two years later a German eclipse expedition to southern Russia was thwarted by the outbreak of World War I. Einstein impatiently wrote his friend Arnold Sommerfeld, “Only the intrigues of miserable people prevent the execution of this last, new, important test of the theory.”
Finally in 1919 a solar eclipse occurred while the sun was in Taurus, surrounded by the Hyades star cluster—an ideal setting for the test. Two eclipse expeditions, one in Brazil, the other on an island off West Africa, strained to photograph moderately dim stars under marginal weather, within the twilightlike glow of the sun’s corona. The stars deflected by a tiny angle, about the width of a quarter viewed from two miles away, and the results were none too precise. Nevertheless, Einstein’s giddy supporters deemed them good enough to confirm that the sun’s gravitational field does warp space in its vicinity. Overnight, Einstein became a household name.
General relativity predicts that gravity warps not only space but time as well: The stronger the gravity, the more slowly time progresses. This effect proved even tougher to study than the bending of light, but in 1971 two physicists managed to measure it by lofting atomic clocks in airplanes (and subtracting the time-altering effects of their motion). Because Earth’s gravity is slightly weaker at high altitudes, Einstein’s theory predicted that the clocks would run slightly fast—which is exactly what the experiment showed.
Under extreme conditions, the warping of time can split reality asunder. Consider the monster black hole in the center of our galaxy. It does not merely lurk in a different place than we do; it occupies a wholly different time frame. Most black holes are thought to form when a massive star collapses. As it shrinks, its surface gravity becomes more intense, so a clock on the star would appear to run increasingly slower. Just at the point where the star becomes a black hole, the clock would stop entirely from our perspective. We would have to wait an infinite duration to see that final stage; therefore, we can never see a fully formed black hole.
In 1918 Austrian physicists Joseph Lense and Hans Thirring uncovered yet another strange consequence of warped space and time, one that even Einstein had overlooked. The gravity of a spinning body should pull on space-time, they concluded, twisting it into a spiral. NASA’s Gravity Probe B satellite is now searching for this elusive effect by monitoring how Earth’s spin influences a set of ultrasensitive gyroscopes. “Think of Earth as immersed in honey, dragging it and the spacecraft’s gyroscopes around as the planet turns,” says Francis Everitt of Stanford University.
Everitt expects to present the first results from the probe early next year. Then we will know if Einstein’s ideas are once again yanking us in new directions.
What's up in the May sky
May 8: The thin crescent moon hovers next to Venus, creeping out of the sun’s glare right after sunset.
May 8: Ceres, the largest asteroid, makes its best appearance of the year. You’ll still need binoculars and a star chart.
May 14-17: Medium-bright Mars meets faint Uranus, in Aquarius. The planets make a pretty orange-and-green contrast.
May 19: The moon closely meets Jupiter, bright in the south-east at twilight and easily visible all night long.
May 24: The full moon crosses in front of the prominent red star Antares at 4 a.m. eastern daylight time.
