100 years ago, an eclipse proved Einstein right. Today, black holes do too — for now

A century ago, British astronomer Arthur Stanley Eddington and his colleagues photographed a solar eclipse, and changed the way humankind thought about the heavens.

Those photographs, taken on May 29, 1919, from Sobral, Brazil and Príncipe Island off Africa’s west coast, affirmed for the first time a key prediction of Albert Einstein’s general theory of relativity: Mass bends spacetime. The expeditions marked a revolution in physics and made Einstein a celebrity.

Today, physicists are at it again — on a much larger scale. In April, the Event Horizon Telescope (EHT) collaboration released the first picture of the edge of a black hole (SN: 4/27/19, p. 6). That image again showed that massive objects, such as black holes or the sun, can change how light travels, just as Einstein predicted.

“The EHT has done the exact same thing, but in the most extreme example imaginable,” says physicist and EHT team member Lia Medeiros of the University of Arizona in Tucson. “It’s almost poetic that these two experiments occurred almost exactly 100 years apart.”

So far, the new black hole data have confirmed general relativity. But future EHT images of the gravitational beasts — especially the one at the center of our own galaxy — could potentially poke holes in Einstein’s famous theory.

“Any time we have a theory that works so spectacularly, you just want to push it to its extremes,” says astrophysicist and EHT team member Michael Johnson of the Harvard-Smithsonian Center for Astrophysics. And black holes are “a laboratory of extremes — this is where we can point to new physics and point to cracks in our existing theories,” he says.

A hundred years ago, scientists didn’t have a black hole to test for cracks in general relativity —black holes were just the stuff of imagination back then — but they did have the 1919 total solar eclipse (SN Online: 4/12/19). At the time, the predominant theory of gravity was Newtonian, which says that gravity is a force. Forces can accelerate objects that have mass, but since light has no mass, gravity shouldn’t affect it, the thinking went. But a few years earlier, in 1915, Einstein had proposed his general theory of relativity, which says that gravity comes from matter and energy warping spacetime, generating curves that change objects’ motion or even the path of light itself.

In Eddington’s and his colleagues’ photographs of the eclipse, stars appeared in different positions in the sky during the eclipse, when their light had to pass the sun to reach earthly observers, than on an ordinary night (SN Online: 8/15/17). The sun’s gravity had changed the path that the starlight took. Einstein was right.

These days, the idea that gravity can curve light is so well understood that physicists use it to probe the properties of spacetime itself. Before the EHT started taking data in 2017, for example, scientists had used Einstein’s equations to get a precise idea of what a black hole should look like, if the theory didn’t break down in the extreme environment.

Black holes curve spacetime so extremely that light gets trapped inside them. So physicists can’t see light emitted by the black hole directly. But they can see the black hole’s shadow on bright material around it. Under general relativity, that shadow should have a specific size and shape: a circle whose width is directly related to its mass. “This all falls out of Einstein’s equations,” Johnson says. “If you have a different theory of gravity, you can predict a different ring on the sky.”

The EHT’s first picture captured the black hole in galaxy M87, about 55 million light-years from Earth, and looked like researchers thought it would.  “Again, GR passes with flying colors, as far as we can tell currently,” Johnson says.  

The theory’s next real test will come when the EHT team photographs the black hole in the center of the Milky Way, called Sagittarius A*. “The reason Sgr A* is in many ways a stronger test for relativity is we know very precisely exactly what that ring should look like, if [general relativity] really holds up,” Johnson says.

Sgr A* is close enough, about 26,000 light-years from Earth, that astronomers can see individual stars whipping around the black hole. That gives researchers an extremely accurate estimate of its mass, and thus the size of its shadow inside a glowing ring.

M87 is too far away for physicists to have measured its black hole’s mass precisely in advance of taking the picture. Previous mass estimates differed by a factor of two, and only the EHT measurement told scientists which mass was right (SN Online: 4/22/19). But that mass uncertainty meant that the prediction for the size of the ring was much weaker.

“There was a lot of wiggle room there” for M87, Johnson says. “For Sgr A*, there’s almost no wiggle room.” Either Sgr A*’s shadow is a certain width, or general relativity is broken.

Unfortunately, Sgr A* is a much more difficult black hole to photograph than M87. It’s about one one-thousandth the mass of M87. For perspective, that’s about 4 million times the mass of the sun compared to M87’s 6.5 billion times. That means that material swirls around Sgr A* much more quickly, making the black hole appear to flicker and vary over the course of a single night of observing.

But Medeiros and others on the EHT team are working on computer algorithms to work around that variability. It should take much less than another century to find out what Sgr A* has to say about general relativity.