Exploring Black Holes: Part 3

Seeing the Invisible

Black holes are invisible, but astronomers have developed a lot of ways to see them through the matter that surrounds them

Part 3 of a four-part series on black holes. Part 1 can be found here, and Part 2 is here.

Pioneering radio astronomer and professional weirdo Grote Reber. While his discoveries shaped our understanding of astronomy, he rejected a lot of modern theories about the universe in favor of his own peculiar ideas. (His vocabulary extended further than Groot’s, though.) Photo: National Radio Astronomy Observatory

In 1937, a deeply weird engineer named Grote Reber built a telescope in the lot next to his mother’s house in Wheaton, Illinois. Home observatories aren’t unusual, but Reber’s project was the first telescope designed to look for radio waves from space, and he was only the second person in history to find them. Karl Jansky, the first radio astronomer, had accidentally discovered astronomical radio waves while working on shortwave radio communications.

But Reber set out deliberately to study the cosmos in radio light. He found that the center of the Milky Way emitted a lot of radio waves and discovered an intense radio source in the constellation Cygnus. By the 1950s, astronomers found many other radio galaxies (as they were creatively named) that emitted very powerful radio waves from small regions at the centers of those galaxies.

Cygnus A, the earliest known radio galaxy. The red lobes are radio emissions from gas jetted out by the galaxy’s central supermassive black hole (at the center of the image). X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Radio: NSF/NRAO/AUI/VLA

As we learned in Part 2 of this series, the sources of the radio waves in the Milky Way and beyond turned out to be supermassive black holes: powerful gravitational dynamos millions or billions of times the mass of our sun. As with Reber’s discoveries, the study of black holes has been driven by invention and creativity. In fact, every new advance in astronomy has led to new discoveries about black holes, and new technologies are being invented for the purpose of studying these weird objects.

The centers of galaxies are chaotic jam sessions of stars and other objects, which means that in visible light—the stuff our human eyes can see—everything is opaque and confusing. By contrast, stars don’t emit much radio light, so other sources of radio waves stand out — including black holes.

But seeing the center of a galaxy isn’t just a matter of building more radio telescopes. The resolution of a telescope — how well it can see the edges and details of whatever it’s pointing at — depends on the wavelength of the light and the size of the telescope. The longer the wavelength of light, the larger the telescope must be to achieve the same resolution, and even supermassive black holes are physically smaller than many stars.

To overcome that obstacle, astronomers started combining signals from two or more telescopes to create a virtual telescope called an interferometer. By using these signals to boost each other (via constructive interference), interferometers achieve much higher resolution than any individual instrument ever could have. In the early 1970s, Bruce Balick and Robert Brown used an interferometer made from a pair of radio telescopes at the Green Bank observatory in West Virginia to make the first clear observation of the black hole at the center of our galaxy.

The Jansky Very Large Array (VLA), in New Mexico, is the best-known radio interferometer. It consists of 27 radio telescopes, seven of which are visible here. Together, the telescopes act like one huge telescope, providing high-resolution radio images of various objects in the universe. Photo: Matthew R. Francis

But this type of observation couldn’t tell us exactly how massive the black hole is, only that it exists. Starting in 1995, two groups of astronomers, led by Reinhard Genzel in Germany and Andrea Ghez in the United States, began a series of observations using infrared interferometers. Though the black hole itself is dim in infrared light, the astronomers identified several stars orbiting very close to it. The stars are moving quickly enough to allow Ghez and her colleagues to track how far they move from year to year.

As a result, they can recreate entire orbits for multiple stars. Using this data and a slightly modified version of an equation first discovered by Johannes Kepler in the 17th century, they estimated that the Milky Way’s black hole is about 4 million times the mass of the sun. (Though it is mind-blowingly massive, that’s only a middleweight by supermassive black hole standards.)

Additionally, Ghez’s team found one faint star that approaches the black hole at a distance about 260 times the size of Earth’s orbit, which means the black hole must be physically much smaller than that. According to general relativity, the Milky Way’s black hole would be at most 24 million kilometers in diameter. For comparison, the sun is 1.4 million kilometers in diameter, and Earth’s orbit is 30 billion kilometers across, so this huge black hole would easily fit inside our solar system — and we’re trying to see it from halfway across the galaxy.

The orbits of several stars near the Milky Way’s supermassive black hole, plotted from 1995 through 2012. Image: Keck/UCLA Galactic Center Group

With more than two decades of data and many more years to come, astronomers will have enough information to see the effects of the black hole’s strong gravity on the orbits of these stars. That’s a new way to test general relativity—the theory of gravity that describes black holes. While few doubt we’ll see any big deviations from predictions, there’s still a small chance of finding something new and unexpected.

The truest test of theory, though, would be to see the black hole itself. Black holes themselves are invisible, of course, but as we learned in Part 2 of this series, the matter they pull into orbit glows very brightly. The closer that matter is to the event horizon — the boundary separating the interior of the black hole, where nothing can escape, from the rest of the universe — the brighter the glow. Additionally, the black hole’s strong gravity twists the paths of light, making the emissions even more intense.

As a result, a black hole should appear as a black circle or oval (depending on how fast it’s spinning) surrounded by a ring of light. But to see this, we need extremely high resolution. That’s the goal of the Event Horizon Telescope (EHT), a huge interferometer consisting of dozens of telescopes, including the South Pole Telescope in Antarctica, the Atacama Large Millimeter Array (ALMA) in Chile, the Submillimeter Telescope in Arizona, and many more. The EHT is designed to span as much of the planet as possible to effectively create an Earth-sized observatory.

In the spring of 2017, the EHT telescopes pointed at the Milky Way’s center for about two weeks. As I write this, astronomers are processing the huge amount of data they collected, and hopefully they will present the world with the first image of a black hole. With that data, we’ll know exactly how big the black hole is, and we may even be able to extract information about how fast it’s spinning and in what direction. That’s as exciting a prospect as any since we learned black holes exist.

Powerful as it is, the EHT is only good enough to resolve the supermassive black hole in the Milky Way and maybe one or two other galaxies. Other black holes are too small, too far away, or both. But there are other ways to learn about what happens at the event horizon.

Gravitational waves are made when two objects orbit each other. Most of the time, those waves aren’t powerful enough to be detected, but when one or both objects are black holes, a lot of energy can be emitted as gravitational radiation. The first three detections by the Laser Interferometer Gravitational-Wave Observatory (LIGO) all involved black holes orbiting each other at ever-faster rates until they collided and merged into a single new black hole.

LIGO’s black holes were at most a few dozen times the mass of our sun. But astronomers think supermassive black holes must collide as well, and that’s how the universe may have formed the most massive black holes. Gravitational waves produced by supermassive black hole pairs, however, have wavelengths too large to be visible to LIGO.

The 100-meter-diameter Green Bank Telescope is the largest radio telescope that can be pointed in any direction. It’s part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which uses flashes of light from pulsars to detect pairs of supermassive black holes. Photo: NRAO/AUI/NSF

That brings us back to radio telescopes. Astronomers realized gravitational waves will affect pulsars — the extremely fast-rotating cores of dead stars. Because they emit pulses of radio light that arrive so predictably, pulsars are some of the most precise clocks in the cosmos. Using pairs of pulsars separated by many light-years in the galaxy, radio astronomers can look for signs of gravitational waves affecting the pulses.

These observations, known as pulsar timing arrays, let us determine how many supermassive black holes are orbiting each other. It’s a slower and less flashy process than LIGO, but it’s likely that a pulsar timing array — such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) will see its first gravitational wave signal from orbiting supermassive black holes within a few years.

As they improve their observations, LIGO and other gravitational wave observatories will be able to measure the effects of gravity at the scale of event horizons. In the final installment of this series, I’ll look at why scientists are interested in the physics of event horizons and what it might tell us about the nature of physics itself.