If recent rumors are true, scientists have finally detected gravitational waves—shockwaves rippling through space and time itself.
Albert Einstein first proposed the existence of gravitational waves 100 years ago, and directly observing them would provide the final vindication for his masterwork: the theory of general relativity.
On Thursday, we’ll find out if Einstein is right one last time. Researchers from Caltech and MIT will convene for a press conference where they may announce that they’ve picked up the tiny wobble of gravitational waves produced by two colliding black holes.
Einstein wasn’t always seen as the genius he is today. When he first proposed his trippy ideas about relativity, some scholars staged protests. Others badmouthed Einstein in the press, decrying both his dangerous ideas and Jewish identity.
His bombshell studies reworked physics from its foundations. Einstein’s universe plays fast and loose with notions of position and speed—except for light, which always zooms through a vacuum at 300 million meters per second. Space and time are stirred together into a four-dimensional molasses called spacetime that matter can stretch and warp. And moving matter must follow spacetime’s curves—a hidden geometry that we experience as gravity.
It sounds like nonsense.
But for the last 100 years, experiments have shown over and over: Einstein’s right. He’s been vindicated too many times to list here, but even the highlights are impressive.
Light Is a Wave—And a Particle
Einstein is best known for relativity, but his only Nobel Prize comes from his revolutionary work on light. Classical physics held that light was a wave, but that theory couldn’t explain how and why metals emit electrons when illuminated—a phenomenon called the photoelectric effect.
Einstein explained the wacky behavior by proposing that light was actually made of discrete wave packets called photons, each with an energy associated with its frequency. The discovery sparked today’s quantum physics—which also holds that everyday atoms can get weirdly wavy, a discovery Einstein helped make.
Spacetime Can Bend
Einstein’s first big win for general relativity came when he explained a mysterious extra wobble in the planet Mercury’s orbit. In 1859, brilliant French astronomer Urbain Le Verrier had ascribed the effect to a yet-unseen planet, dubbed “Vulcan,” tugging on Mercury. But years of searching failed to turn up any credible evidence for Vulcan’s existence.
To Einstein’s extreme excitement, his new theory of general relativity brought Vulcan to its knees, by showing that the sun’s mass curves nearby spacetime, much like a bowling ball stretching a divot into a taut trampoline. Since Mercury is so close to the sun, its wobbling orbit is the shortest path through the spacetime curved by the sun’s mass. There wasn’t any extra planet: just a geometry to the universe of which Newton hadn’t conceived.
Spacetime Can Act Like a Lens
Einstein was proved right again in May 1919, during a full solar eclipse. According to relativity, the spacetime curved by the sun’s mass would bend incoming starlight like a lens.
British astronomer Arthur Eddington snapped large photographs of the eclipse and found that the sun appeared to stretch out the Hyades star cluster, bending the individual stars’ light by roughly one two-thousandth of a degree—in line with Einstein’s prediction, which called for twice the bending predicted by Newtonian physics.
Even Einstein didn’t anticipate how useful the phenomenon would be to astronomers: By using galaxies themselves as giant lenses, astronomers can peer back in time, to the earliest years of the universe. And when astronomers see lensing caused by apparently invisible mass, the distortions allow them to map vast fields of dark matter.
Rotating Masses Swirl Spacetime Like Syrup
Not only does matter warp spacetime with the bowling-ball effect, but rotating masses like Earth also subtly drag spacetime around them like spoons spinning in molasses. This affects the orbits of nearby satellites—a bizarre effect called frame dragging.
Predicted in 1918 using general relativity, frame dragging eluded confirmation until 2004, when researchers found that Earth’s rotation slightly shifted two satellites’ orbits. In 2011, NASA’s Gravity Probe B confirmed the find, and put better numbers on it.
Gravity Slows Time
Einstein’s equations also endow matter with the ability to speed up or slow down time—and change the color of light.
We can see that his freaky prediction was right because from Earth’s perspective, light from distant stars takes on higher frequencies—or looks bluer —than it would to an observer in deep space. And the farther away you get from Earth’s gravitational well, light beamed from Earth appears to take on lower and lower frequencies, a phenomenon called a gravitational redshift.
It’s subtle, but ignore relativity at your smartphone’s peril: Without relativistic corrections, the clocks on GPS satellites would tick 38 microseconds faster every day than those on Earth’s surface, ruining the system’s accuracy after two minutes, and adding 10 kilometers of error per day thereafter.
Follow Michael Greshko on Twitter.
Correction: An earlier version of this story incorrectly stated that the speed of light in a vacuum is 3 million meters per second. It is 300 million meters per second.