Imagine turning the whole Earth into a giant telescope, capable of visualizing an orange on the surface of the Moon. This is what astronomers from around the world have done with the Event Horizon Telescope (EHT), a string of huge radio telescopes that covers the surface of the globe. Only through the combined power of many such radio antennas can scientists reach the resolution needed to break new ground in astronomy: to visualize the neighborhood of a black hole.
And they have done just that, as revealed today in a worldwide press conference. The results are released in six papers in the Astrophysical Journal Letters. “We report the first image of a black hole,” says the opening sentence in the introductory text for the focus issue. (Watch the explainer videos at the bottom of this article to better understand the magnitude of this news—and to learn more about how it all came together.)
The results are stunning: the contours of the giant black hole at the center of the galaxy M87, located in the nearby Virgo galaxy cluster at 55 million light-years from Earth. With a mass of 6.5 billion times larger than our Sun, the intense gravity around the object bends light to create a bright ring surrounding a dark object, the black hole horizon.
“This is a remarkable achievement,” said Shep Doeleman, an astronomer at Harvard University.
Black holes deserve their reputation of being the weirdest objects in the universe. After all, at their core lies the “singularity,” a point in space where the laws of physic as we know them presumably break down. What goes on there remains a mystery: we can’t really travel into the singularity and come back to tell the tale. Mechanical probes wouldn’t fare any better. However, we can study black holes in a different way, by looking at what goes on sufficiently close to them, near their “event horizon.”
The event horizon is, essentially, the ultimate point of no return: if you pass it, you can’t get back out. Not even light. This is why black holes have their names. You can roughly picture the event horizon as being like the surface of a balloon surrounding the singularity at the very center. (Strictly speaking, this is only true for a non-rotating black holes. Black holes that spin around, known as Kerr black holes, have more complicated singularity structures. But let’s leave these details aside for now.)
The team at the EHT trained their antennas at the huge M87, which houses the giant black hole captured in the image. Just for comparison, our own Milky Way galaxy also houses another candidate black hole for the EHT group, but with “only” four million solar masses. The behemoth at the heart of M87 is so far away that to visualize it would be equivalent to standing in New York and counting the dimples of a golf ball in Los Angeles. The nearby gas and stars spiral into the black hole, forming a plasma heated to billions of degrees, resulting in a glowing disk (we call it an “accretion disk”) around the event horizon. This is what the theory predicted and, now, what observations confirmed in spectacular fashion.
To “see” this heated gas and, hence, the event horizon, it is best to capture it in the millimeter wavelength—that is, in the radio waves range. The larger the antenna to capture the radiation, the better the resolution of the image. Hence the Earth-covering radio telescopes that make up the EHT network, from the South Pole to Hawaii to Chile. Other wavelengths, including the much shorter visible light, get blocked on their way to Earth.
For the composite image to be as sharp as possible, the radio telescopes need to be almost perfectly synchronized. To achieve such a feat, the telescopes are equipped with atomic clocks that lose only about one second every hundred million years. The data is collected into a supercomputer that is programmed to reconstruct the composite image.
Visualizing the event horizon offers a direct test of Einstein’s general theory of relativity, the one that describes gravity as due to the curvature of space about a mass. The shape and properties of the event horizon depend on the details of the theory. The more massive the object, the more radical the distortion in the geometry of space and the flow of time. So far, Einstein’s theory has passed all its tests with flying colors. But we never know what a new window to the Universe will disclose to us.
Confirming the theory would be very exciting, of course, but even more exciting would be to find some departure from the usual prediction, pointing toward a new way of thinking about gravity. After all, progress in science comes from the breakdown of old theories, from the need for new ideas. From what we now know, at least preliminarily, Einstein’s theory remains successful, as stated in the sixth paper: “[the measurement] is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.”
If there are hidden secrets, we need to probe even deeper. Which is exactly what will be done in the coming years.
If you could fly next to the supermassive black hole M87*, this is what you would see:
Eight telescopes around the world are synchronized with atomic clocks, creating a virtual telescope dish as large as the Earth itself:
Here’s an overview of the Event Horizon Telescope project and the first black hole image:
[Images: Event Horizon Telescope Collaboration; videos: NSF]