When he unveiled his general theory of relativity, Albert Einstein wasn’t exactly met with applause. Almost no one else could do the math necessary to understand his abstract ideas, and at the time he didn’t have any evidence to back it up. But in the century since it was proposed, Einstein’s theory has continued to pass ever more stringent tests.
It remains our best explanation of the phenomenon of gravity. The theory bears out all sorts of wild predictions, the bulk of which boil down to this: Gravitation behaves the same for all observers, resulting from curving “space-time,” the fabric of the universe.
Einstein’s concepts have been verified — just as he reckoned they would — on scales from a foot-long sub sandwich to galaxy clusters millions of light-years wide. In between, general relativity has made its mark on the Global Positioning System, while explaining anomalous planetary orbits and the whirling death dances of the remnants of giant stars.
“We’re still using the same theory that was invented a hundred years ago, and it still works amazingly well in so many different situations,” says physicist Clifford Will of the University of Florida.
Here are six examples of how Einstein’s landmark theory has stood the test of (space-)time.
Mercury, the Glitch in Newton's Matrix
The Perihelion Precession of Mercury
Isaac Newton’s law of gravity saw perhaps its greatest triumph in the mid-1800s with the discovery of the planet Neptune. In 1846, French mathematician Urbain Le Verrier crunched the numbers on Uranus’ weird orbit, likely caused by another massive body, and just a few months later German astronomers spotted Neptune lurking right where Newton’s laws predicted. Ironically, it was another orbital discrepancy that turned out to be the chink in Newton’s armor, which Einstein’s ideas blew wide open.
In 1859, Le Verrier pointed out that the planet Mercury was arriving at its closest orbital position to the sun, called perihelion, a half-arcsecond behind schedule. “Mercury was not quite behaving the way Newton said it should,” says Daniel Holz, a professor of physics at the University of Chicago.
This so-called precession of Mercury’s perihelion wasn’t much; it worked out to a break per orbit of a mere millionth of a percent from Newtonian predictions. Yet with each go-round (Mercury has an 88-day year), the planet stubbornly appeared out of place during perihelion from where astronomers expected it.
At first they assumed that, as with the Uranus solution, another planet must exist even closer to the sun, affecting Mercury’s orbit. The conjectured world even got a name, Vulcan. Decades of searching failed to reveal the scorched world.
In stepped Einstein. In 1915, his brand-new theory precisely accounted for Mercury’s weirdness, ultimately due to the warping of space-time produced by the substantial mass of the sun.
Similar perihelion precessions, all in perfect agreement with general relativity, have been subsequently documented for other star systems, namely binary pulsars. These pairs of neutron stars — the ultra-dense remains of collapsed, behemoth stars — whip around each other exactly as Einstein said such things should, although no one even conceived of these objects until the 1930s.
Bend It Like Einstein
The Deflection of Light by Cosmic Bodies
Einstein’s initial success with explaining away the Mercury conundrum did not catapult him to superstar status. Those accolades actually came a few years later, with the verification of another of general relativity’s bold prognostications: Massive objects such as the sum should warp space-time enough to throw passing rays of light off course.
Einstein’s work piqued the interest of English astronomer Arthur Eddington, who recognized a great opportunity to test for this light deflection: On May 29, 1919, the sun would conveniently undergo a solar eclipse, which would block out its overwhelming glare, while passing close to a bright group of background stars called the Hyades. If Einstein were right, the sun’s presence would deflect their light, subtly shifting their position in the sky.
Eddington arranged a pair of expeditions (one to Sobral, Brazil, and another to Principe, an island off the west coast of Africa) to look for the bending of the Hyades’ starlight as the eclipse shadow swept through West Africa and Brazil. Sure enough, the tiny predicted displacement of the stars’ light showed up.
The news of this discovery made headlines worldwide, with the Nov. 7 London Times proclaiming: “Revolution in Science/New Theory of the Universe/Newtonian Ideas Overthrown.” Einstein, remarkably for a physicist, became a household name.
The “gravitational lens” created by the bending of light through warped space-time has become a vital tool in probing the cosmos. “I call it Einstein’s gift to astronomy,” says Will. Foreground galaxy clusters can warp and magnify the light of distant, background proto-galaxies, for instance, allowing cosmologists to catch glimpses of early epochs of the universe.
Stretching Light and Time
The Gravitational Redshifting of Light
Along with the two prior predictions, this third example rounds out the three classical tests that Einstein considered critical to prove general relativity, and it’s the only one he didn’t live to see.
Relativity posits that as light moves away from a massive object, gravity’s curving of space-time stretches the light out, increasing its wavelength. With light, wavelength equates to energy and color; less energetic light trends toward the redder part of the spectrum than shorter-wavelength, bluer light. The predicted gravitational “redshifting” effect was too meager for detection for decades, but in 1959, Harvard physicist Robert Pound and his grad student, Glen Rebka Jr., had an idea.
They set up a sample of radioactive iron in an elevator shaft of a Harvard building, letting the radiation travel from the basement to the roof, where they’d set up a detector. Although the span was a measly 74 feet, it was enough for the gamma rays to lose a couple trillionths of a percent of their energy due to our massive planet’s gravitational warping of space-time, in the ballpark of Einstein’s predictions.
To really nail down this relativistic effect, NASA launched its Gravity Probe A rocket in 1976. This time, researchers looked for a change in the frequency of waves — with shorter wavelengths meaning a higher frequency, and vice versa — in a type of laser in atomic clocks. At a peak altitude of 6,200 miles, a clock aboard Gravity Probe A ran ever so slightly faster than a clock on the ground. The difference, a mere 70 parts per million, matched Einstein’s math with unprecedented precision.
In 2010, scientists at the National Institute of Standards and Technology went even further, showing that at just 1 foot higher in elevation, a clock ticks four-hundred-quadrillionths faster per second. The takeaway: Your head ages ever so slightly faster than your feet.
“That was a fantastic experiment, just to be able to measure the difference in the rate of time over that very small amount of distance,” says Will.
On a more practical scale, the same effect impacts the Global Positioning System, whose orbiting satellites have to be adjusted thirty-eight-millionths of a second per day to stay in sync with Earth’s surface. “Without that correction,” says Will, “GPS wouldn’t work.”
The Shapiro Effect: The Relativistic Delay of Light
Often dubbed the fourth classical test of general relativity, and the brainchild of Harvard physicist Irwin Shapiro, this experiment timed how long it took light to travel from A to B and back. If Einstein was on the money, it would take that light longer if there were a massive object near the path.
In the early 1960s, Shapiro proposed testing this by bouncing a radar signal off of Mercury when the planet was situated right next to the sun (from our Earthly perspective). Shapiro calculated that the sun’s gravity well should delay the radar signal by about 200 microseconds, compared with its time back from Mercury without the sun nearby. “That’s not exactly an eternity,” Shapiro says.
Tests began in 1966, using the 120-foot-wide radio antenna at MIT’s Haystack Observatory. The echo from Mercury closely corresponded to Shapiro’s reckonings. Still, close wasn’t good enough; all it took was a teensy anomaly in Mercury’s orbit to overthrow Newton’s laws, after all.
So, to verify the Shapiro effect further, physicists abandoned planets, whose rough surfaces scatter some of the radar signals, for smoother targets: spacecraft. In 1979, the Viking landers on Mars made for a good testing ground for the Shapiro time delay. Then, in 2003, Italian researchers detected a time delay in communication signals to the Cassini spacecraft en route to Saturn. The accuracy achieved was 20 parts per million, 50 times better than even the Viking results, and — wouldn’t you know it — right in line with general relativity.
The Equivalence Principle
At the heart of general relativity lies the equivalence principle. It states that bodies “fall” at the same rate through a gravitational field, regardless of their mass or structure. Building on this idea, the principle also holds that other physical laws within a given reference frame should operate independently of the local strength of gravity; in other words, the coin you flip when cruising on an airplane flips the same as one on the ground. Generally, experiments should reach the same results regardless of where and when in the universe they take place. Therefore, the laws of nature must be the same everywhere and throughout time, stretching all the way back to the Big Bang.
First, the easy part. Evidence supporting the first aspect of the equivalence principle initially came four centuries ago. In 1589, famed Italian astronomer Galileo Galilei, perhaps apocryphally, released balls from atop the Leaning Tower of Pisa. The balls, though made of different materials, met little air resistance and landed at the same time. Presto! Four centuries later, in 1971, a more evocative demonstration took place on — of all places — the moon. During the Apollo 15 mission, astronaut Dave Scott simultaneously let go of a hammer and a feather. In the airless lunar environment, the objects fell together and struck the lunar surface simultaneously, mirroring Galileo’s experiment. The two bodies fell at the same rate, despite their differences.
Apollo astronauts also left behind reflectors on the moon’s surface. These fancy mirrors have enabled scientists to bounce lasers off the moon to precisely measure its position relative to Earth, down to four hundredths of an inch. These readings have offered a rigorous test of the “falling equivalently” concept, as well as its related notion that nature’s laws must apply equally everywhere. To date, decades of data from these lunar laser ranging experiments have agreed with general relativity down to trillionths of a percent.
The setup has also pegged the moon’s acceleration toward the sun as the same as Earth’s, just like Galileo’s and Scott’s dropped objects. After all, according to the equivalence principle, “you are in effect dropping the Earth and the moon around the sun,” says the University of Chicago’s Holz.
Space-Time, Spun and Dragged
The Geodetic and Frame-Dragging Effects
Einstein’s conception of space-time is actually sort of gelatinous. A well-known analogy illustrating this idea is imagining Earth as a bowling ball placed on a trampoline. The massive Earth dents the fabric of the space-time trampoline, such that an object rolling near the planet/ball will have its trajectory altered by Earth’s gravitational warping. But the trampoline analogy is only part of the general relativity picture. If the theory is correct, a spinning massive body pulls space-time along with it, akin to a spoon spun in honey.
Circa 1960, physicists dreamed up a straightforward experiment to examine both of these predictions. Step 1: Place gyroscopes on board a satellite orbiting Earth. Step 2: Align the spacecraft and the gyroscopes with a reference star, serving as a basis for comparison. Step 3: Look for changes in the alignment of the gyroscopes, seeing how far out of alignment they’d been dragged by Earth’s gravitational influence.
Later christened Gravity Probe B (a sequel of sorts to Gravity Probe A), the test only became technologically possible 44 years (and $750 million) later. The results, announced in 2011, were hard won: Despite unprecedented precision and patient waiting, tiny misalignments still made data analysis a challenge. But, in the end, the measurements again buttressed Einstein. Earth’s spin really does drag space-time along with it.
General relativity has held up quite well these last 10 decades. But its trials are far from over. As impressive and rigorous as many of the tests have been, none have taken place in the realm of monstrously strong gravity, in the neighborhood of black holes. In these extreme environments, Einstein’s theories might just come undone, or — given the man’s track record — astound us still more with their predictive power.
“We’re really looking at probing the predictions of general relativity even more deeply,” says Will. “We shouldn’t give up testing it.”