April 25, 2019
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The black hole photo was no big surprise to scientists. It’s still a big deal.

George Danby | BDN
George Danby | BDN

This was an amazing week to be an astronomer, as we all looked at the first image of a supermassive black hole. Black holes are objects so dense that not even light can escape their gravitational pull. They are a consequence of general relativity, in which massive objects bend space, and thus the paths of light rays. At the center of the black hole is a singularity, which is not described by physical laws as we know them. But the point of no return, the boundary between physics as we know it and the unknown, is a larger radius called the event horizon.

I am a black-hole hunter. But, until now, much of the evidence has been circumstantial. We see mass concentrations in the centers of galaxies, but in most cases we cannot prove that the densities are those of a black hole. Even so, these supermassive black holes capture our imagination. My husband always asks me, what happens inside a black hole. Last week, astronomers came as close to that edge as we can get, in a triumph of creativity, passion and dedication — as well as a confirmation of our faith in the existence of supermassive black holes.

In 2012, early data from the Event Horizon Telescope about a black hole in the galaxy Messier 87, or M87, was released. The Event Horizon, a network of telescopes from the South Pole to North America, had combined radio waves to peer into the very heart of the black hole, with its mass of 6.5 billion suns. It looked like … a blob. So I was not quite prepared for the beautiful ring unveiled last week. I was awed by the tremendous work that went into producing that image. My friend and colleague Dan Marrone spent many months at the South Pole, far from his family, with little time to eat or sleep, to make this happen, and he was only one of 200 scientists working hard over the past decade.

However, what has really stuck with me is a comment from Avery Broderick, a theorist in astrophysics who makes the model predictions of what light rays should do right at the edge of the event horizon. The models were exactly right — a triumph for the theory of general relativity that Albert Einstein formulated more than a century ago.

“I have to admit that I was a little stunned that it matched so closely the predictions that we had made,” Broderick said during Wednesday’s news conference announcing the result. “It’s gratifying, sometimes frustrating.” I get this frustration — did the modeling have to work so well?

After all that work, couldn’t we just get a hint of new physics? A challenge to general relativity that might open fresh theoretical horizons? Indeed, I was asked by a journalist whether there was really anything interesting about the M87 image, when we were just seeing exactly what we expected to see.

So after the decade of investment, the sleepless nights, the petabytes of data, the heroic imaging efforts, was the Event Horizon Telescope worth the investment? Yes, absolutely. We can never know what surprises nature holds until we push right up against the edge of what is possible.

Confirming the predictions of general relativity, and seeing the event horizon, is magical. Yes, finding new physical laws would be splashy, but strong and clear confirmation of the theoretical is both fundamentally important and surprising in its own right. I am confident that as the Event Horizon Telescope incorporates more telescopes and observes at higher resolution, more surprises will emerge. Meanwhile, I will sleep better knowing that there really is a singularity at the center of M87.

Jenny Greene is a professor of astrophysics at Princeton University.


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