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.”