Last year, we finally photographed a black hole. Now what?

Orbiting telescopes can help us imagine black holes like never before.

Shepherd Doleman took almost ten years to accomplish the impossible. As director of the Event Horizon Telescope (EHT), a project involving an international community of hundreds of researchers, he traveled for years on suitcases filled with hard drives around the world to coordinate observations between radio telescopes on four continents, including Antarctica. On April 9, 2019, cooperation finally brought the fruits of their work, and the world looked at the first image of a black hole .

The feat, which James Bardin, a pioneer-theoretician of the black hole, called hopeless in 1973, was an outstanding achievement of astronomical technology. But as soon as the data processing was completed and the champagne was spilled, the EHT collaboration in a sense became like a dog that caught a car. “Everyone was surprised to get such a good shot so fast,” says Andrew Strominger, a theoretical physicist at Harvard University. “Shepard and Michael [Johnson, Harvard-Smithsonian astrophysicist and EHT coordinator], asked me about this. “What shall we do with this? We took a picture, and now what? ”

Now Strominger and an interdisciplinary team of researchers, including theorists, experimenters, and one philosopher, have returned with the wild answer that appeared last week in Science Advances. With access to a fairly remote telescope, the EHT collaboration could distinguish multiple reflections of light flowing around a black hole. By analyzing the exact patterns in these entangled rays, astronomers could directly measure the basic properties of black holes and test Einstein's theory of gravity like never before. In fact, they hope that black holes will become more like stars and planets: not just objects for thought, but for direct observation.

“These are objects that for me were just equations that I tried to mathematically visualize in my mind,” said Alex Lupsaska , a Harvard theoretician who worked on research. “But now we got their real photos.”

The team performed calculations using pencil and paper, based on Einstein's theory of relativity and simulation of unprecedented resolution, to analyze what black holes do with light. Attention spoiler !: something strange happened. “Black holes, they're just the best at everything they do,” says Lupsaska. And that includes bending light rays into loops.

As the densest objects that follow the laws of physics, black holes have tremendous cosmic attraction, and physicists have long known that the abyss is hidden in the envelopes of light. Where the Earth can attract the passing cosmic cobblestone - pulling it into several orbits before it flies back into space - black holes can capture real light particles. Everything that crashes into a black hole gets stuck inside forever, but photons that closely glide along the border can make several revolutions around the black hole. “This is the distorted,“ screaming ”nature of space-time,” says Lupsaska.

Strominger, Lupsaska and their colleagues accurately calculated the specific structure of the light envelope and how it would look when observing it from Earth.

Here's how it works. When the rays of light approach a black hole, its terrifying gravity takes them into orbit. Rays passing at a certain distance make a half turn around a black hole before going into space. Rays going a little closer can make a full circle before returning from where they came from. Rays passing closer can still make one and two turns, the other two turns and so on. Each of these endless groups of light rays can form an image (when they hit the camera or eyeball), so a black hole can create an infinite number of such images. Strominger compares this strange effect to how you would stand between two mirrors in a department store and see how your reflections stretch further there.

“In an ideal world with a perfect telescope, you would look at a black hole and see not only an infinite number of images embedded in other ones, but the whole universe,” he says.

But EHT, like all telescopes, is not perfect. This is not even a telescope, but a technical interferometer. Interferometers work by comparing observations of a distant point from two different places. The farther the places are from each other, the more subtle features of the object they can catch. As successive reflections of black holes (which may appear as rings for the observer) are becoming thinner and thinner, astronomers must use more distant observatories to see them.

To detect reflective rings, EHT will have to go even further. In the end, the authors of the study conclude that collaboration should add a space observatory to their network. Only one has to do it. A satellite orbiting the Earth can clearly identify the first ring, or a device orbiting the moon can see the second. If they could take the spacecraft to the place between the Earth and the Sun, known as the second Lagrange point (the destination of the future James Webb Space Telescope), they could identify the first three rings. Such a mission can cost several hundred million dollars - expensive, but not as expensive as the largest scientific projects. “This is something that someday someone will do,” says Lupsaska. "It is the matter of time."

With this pile of money, astrophysicists will buy a lot of knowledge about the black hole. Observation of the rings would immediately serve as the first test of the general theory of relativity in a medium with strong enough gravity to bend light rays into full loops. The narrowing of the rings is very accurate, so any deviation will signal that something strange is happening. “There is no room for maneuver,” says Lupsaska. “You go there and take a measurement, and it either fits the theory or not.”

A small number of theorists expect the destruction of Einstein's most successful theory. Rather, they are more excited that the rings can prove the existence of two black holes close enough to receive the image in this way. Astronomers have several ways of measuring the basic properties of a black hole, such as its mass and rotation, but they have to make a lot of assumptions to investigate this. The pattern of rings depends only on a black hole - which has nothing to do with luminous plasma and nearby debris - so such observations could provide physicists with a clearer way to answer their most basic questions about these mysterious objects.

And this analysis is only the beginning. After the work was presented last summer (in anticipation of an expert assessment), it caused a wave of subsequent research, as physicists were in a hurry to develop this theory. “It is emphasized that there are many interesting details that we have not yet explored, and they inspired us to possible new studies,” says Elizabeth Himwich , a Harvard graduate student who analyzed how the type of light alternates from one ring to another .

Lupsaska compares the efforts to be made in the initial stages by the example of biology. “Before you want to understand how to arrange DNA and use cluster short palindromic repeats to copy and edit DNA, you first go to the forest and specify:“ This is a tree, this is a flower, ”he says. "It is here that we are in the field of physics in the study of black holes as an experimental science."