The gravity near a black hole is so strong that it warps the very fabric of space and time. Black holes sound more like science fiction than fact, but there has been considerable indirect evidence that they exist. They are accepted by the scientific community in spite of an embarrassing admission: nobody has ever directly seen one. Well, until now.
Scientists have announced the first direct observation of a black hole at the center of a galaxy named M87. M87 is a supergiant elliptical galaxy in the constellation Virgo. It is one of the largest galaxies in the nearby universe. (Where "nearby" is the staggering distance of 53 million light years. Astronomers really do think big.)
Now a little bit of care is necessary to understand exactly what was done. Black holes are, well, black. By definition, they do not emit any light. So, the black hole was not observed directly. However, black holes are also surrounded by ordinary matter that is caught in the hole's gravitational grip.
This matter, which is typically just gas of the same type that makes up our sun, orbits the black hole at very high speeds. All of that fast-moving gas gets heated up to the point where it glows and emits all sorts of forms of electromagnetic radiation, from heat to light to radio waves. Intervening gas blocks the visible heat and light, so astronomers look for the radio waves.
You'd think that astronomers would announce that they detected this halo of radio waves surrounding the hole, and that is part of the story. However, it's more complicated than that. Because of the very strong gravity near the black hole, some of the light and radio waves are captured by it and don't escape. The result is that a black hole looks like a ring of light, with a shadow in the middle. Essentially, from a distance, picture astronomers released of the M87 black hole looks like a coffee ring left on a piece of paper, albeit a colored one.
Since the astronomers used radio waves to see the black hole, the colors aren't what you would see with your eye. But they do have meaning. What we are seeing is the gas surrounding the black hole. One side is bright and one is dim because the black hole is spinning. The yellow shows the side of the black hole spinning toward us and the reddish side is spinning away.
Aside from the difficulties associated with seeing something that is perfectly black, another difficulty is their size. Ordinary black holes, which have a mass the few times as big as our Sun, are only about as big as the city of Chicago. Combined with their great distances, they are simply too small to see with modern technology. Seeing the closest known black hole is as difficult as a telescope in New York City seeing a single molecule in Los Angeles. This is well beyond current technical capabilities.
Luckily, the center of nearly all galaxies contain an enormous black hole. For example, the one in the center of our Milky Way galaxy has the mass of about 4 million times that of our sun with a radius about 30 times that of the sun.
However, the black hole at the center of M87 is truly gigantic. Its mass is about 7 billion times the mass of our sun. And its dimensions are huge as black holes go. It is a sphere with a radius about 130 times that of the Earth's orbit or about three times bigger than the average orbit of Pluto.
That sounds large, but the distance to M87 is so huge that the black hole at the center of that galaxy subtends a tiny angle. It is unbelievably small -- it's equivalent to the width of a line drawn by a sharpened pencil seen from the distance separating New York and Los Angeles, a task that is possible if scientists use an incredibly clever technique that uses the entire Earth as a telescope. And, luckily, the shadow cast by the black hole is about 2.5 times wider than the hole itself.
In 2006, an international consortium of astronomers formed a group called the Event Horizon Telescope. The name is misleading, as their equipment isn't a telescope in the way we ordinarily think of it. Instead, the equipment they use is called a radio telescope, which is just an ultra-sensitive radio antenna.
And another level of confusion is that the group didn't employ a single antenna. Instead what they did was to tie together a web of radio telescopes spread across the entire planet. The reason they did that is simple. How small an object a telescope can see depends crucially on the size of the telescope. The bigger the telescope, the smaller objects it can resolve.
A world-class radio telescope is only a few hundred feet across. However, by tying together a worldwide network of radio receivers, astronomers can effectively make a telescope the size of the Earth -- essentially a radio telescope about 8,000 miles wide. And by using ultra-precise atomic clocks to synchronize the observations made from around the world, astronomers were able to resolve the shadow of the black hole at the center of the M87galaxy.
Science is all about pushing the limits -- studying what was once impossible to do. And, being perfectly black, tiny and very distant, black holes certainly qualify. Yet black holes are a key laboratory for testing Einstein's theory of relativity, which is our best theory of gravity. Because of this, scientists have indirectly studied them for decades, from observing their effect on nearby stars, to seeing how they heat up giant clouds of gas, to detecting how their motion sends ripples through space and time.
But seeing one directly is a new thing and a huge advance in our ability to understand the behavior of matter under the strongest gravitational forces imaginable. And it's important to note that this work wouldn't be possible without generous support from taxpayers and science funding agencies across the world, including the National Science Foundation here in the United States. (Disclosure: Fermilab colleagues of mine are collaborators on this project and are funded by the US Department of Energy Office of Science.)
We should all take a bit of pride in our individual role in making possible this breathtaking scientific observation. In the next weeks and months, we're sure to learn even more.
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