Black holes are camera shy. Their extreme gravity prevents light from escaping. That means that the dark hearts of these cosmic heavy hitters remain entirely invisible. However, supermassive black holes in the centers of galaxies may give themselves away by spewing bright jets of charged particles. Others may be “seen” by the light of the nearby stars that they fling away or rip apart. Up close, these behemoths are surrounded by accretion disks — glowing disks made from the material being sucked into them.
Scientists have now cleverly created a network of eight radio telescopes. Working as one, they effectively make an Earth-sized eye on the sky. And they have just imaged the silhouette of a black hole’s event horizon — the edge inside which nothing can be seen or escape. It can be seen against the black hole’s accretion disk.
In April 2017, this so-called Event Horizon Telescope, or EHT, collected data that have now yielded the first image of a supermassive black hole. It sits inside the galaxy M87.
“There is nothing better than having an image,” says Avi Loeb. He’s an astrophysicist at Harvard University in Cambridge, Mass. Though scientists have collected plenty of indirect evidence for black holes over the last half century, he notes that “seeing is believing.”
Creating that first-ever portrait of a black hole was tricky, though. Black holes take up a minuscule sliver of sky. And they’re so far away that the halo of light surrounding some of them appears very faint. The project of imaging M87’s black hole required eight observatories across the globe. By working as one virtual radio dish, their vision would be sharper than that of any single observatory working on its own.
Putting the ‘solution’ in resolution
The supermassive black hole inside M87 is no small fry. It weighs in at around 6.5 billion times the mass of our sun. But viewed from 55 million light-years away, it appears as the smallest of blips in the sky. It’s smaller than an orange on the moon as viewed by someone on Earth. Still, besides Sagittarius A* — the black hole at the center of our own galaxy — M87’s black hole is the largest black hole silhouetted on the sky.
Only a telescope with EHT’s resolution could pick out something so tiny. A telescope’s resolution depends on its diameter: The bigger the dish, the clearer the view. So getting a crisp image of even a supermassive black hole needed a planet-sized radio dish.
“The trick is that you don’t cover the entire Earth with an observatory,” explains Loeb, who wasn’t involved in EHT. Instead, astronomers combine radio waves seen by many telescopes at once. This makes the telescopes effectively work as one giant dish. The diameter of that virtual dish is equal to the length of the longest distance, or baseline, between two telescopes in the network. For the EHT in 2017, that was the distance from the South Pole to Spain.
The EHT was not always the hotshot array that it is today. In 2009, a smaller network of just four observatories — in Arizona, California and Hawaii — imaged the base of one plasma jet spewing from the center of M87’s black hole. But this small telescope network didn’t yet have the magnifying power to reveal the black hole itself.
Over time, the EHT program recruited new radio observatories. By 2017, there were eight observing stations in North America, Hawaii, Europe, South America and at the South Pole. Among the newcomers was the Atacama Large Millimeter/submillimeter Array, or ALMA. It is located on a high plateau in northern Chile. With a combined dish area larger than an American football field, ALMA collects far more radio waves than other observatories.
“ALMA changed everything,” says Vincent Fish. He’s an astronomer at MIT’s Haystack Observatory in Westford, Mass. It can offer “really solid detections now,” he says, of “anything that you were just barely struggling to detect before.”
More than the sum of their parts
EHT observing campaigns are best run within about 10 days in late March or early April. That’s when the weather at every observatory promises to offer the clearest images of the sky. Researchers’ biggest enemy is water in the atmosphere, such as rain or snow. It can muddle the millimeter-wavelength radio waves to which the EHT’s telescopes are tuned.
Planning for weather on several continents can be a headache.
Geoffrey Bower works at the Academia Sinica Institute of Astronomy and Astrophysics in Hilo, Hawaii. “Every morning” during an EHT campaign, this astronomer notes, “there’s a frenetic set of phone calls and analyses of weather data and telescope readiness.” Afterward, “we make a go/no-go decision for the night’s observing.” Researchers are picky about conditions. But toward the tail end of the run, they’ll take what they can get.
When the skies are clear enough to observe, researchers steer the network’s telescopes at each EHT observatory toward the vicinity of a supermassive black hole. Then they begin collecting radio waves. M87’s black hole and Sgr A* appear on the sky one at a time. Each one is just about to rise as the other sets. This allows the EHT to switch back and forth between observing its two targets over the course of a single multi-day campaign. All eight observatories can track Sgr A*. Because M87 is in the northern sky, it can’t be seen by the South Pole station’s sight.
On their own, the data from each observing station look like nonsense. But taken together, these data can reveal a black hole’s appearance.
Here’s how it works. Picture a pair of radio dishes aimed at a single target. In this case, it’s the ring-shaped silhouette of a black hole.
Radio waves emanating from each bit of that ring must travel slightly different paths to reach each telescope. These radio waves can interfere with each other. Some will reinforce one another. Others will sometimes cancel each other out. The pattern of waves seen by each telescope depends on how the radio waves from different parts of the ring are interacting when they reach that telescope.
For simple targets, such as individual stars, the radio waves picked up by just two telescopes can provide enough data for researchers to figure out how the light is distributed across the sky. But a black hole is a complex light source. There are too many possible solutions for what the image could be. So researchers need more data to work out how a black hole’s radio waves are interacting.
The ideal array has as many baselines of different lengths and orientations as possible. Telescope pairs that are farther apart can turn up finer details. That’s because there’s a bigger difference between the pathways that radio waves take from the black hole to each telescope. EHT includes telescope pairs with both north-south and east-west orientations. And as Earth rotates, these change, relative to the black hole.
Pulling it all together
In order to braid together the observations from each observatory, researchers need to very precisely record the time they collect their data. So the astronomers use atomic clocks (ones that lose about one second every 100 million years).
There are a lot of data to time stamp. “In our last experiment, we recorded data at a rate of 64 gigabits per second.” Explains Bower, that’s “about 1,000 times [faster than] your home internet connection.”
These data are then transferred to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy in Bonn, Germany. There, the data are processed in a special kind of supercomputer. It’s called a correlator.
Each telescope station amasses hundreds of terabytes of information during a single observing campaign. That’s far too much to send over the internet. So the researchers use the next best option: snail mail. So far, there have been no major shipping mishaps. Still, Bower admits that mailing the disks is always a little nerve-wracking.
Although most of the EHT data reached Haystack and Max Planck within weeks of the 2017 observing campaign, there were no flights from South Pole until November.
Filling in the blanks
Combining the EHT data still isn’t enough to render a crisp image of a supermassive black hole. If M87’s black hole were a song, then imaging it the EHT data would be like listening to the piece played on a piano with a bunch of broken keys. The more working keys — or telescope baseline pairs — the easier it is to get the gist of the melody.
“Even if you have some broken keys, if you’re playing all the rest of them correctly, you can figure out the tune,” says Fish. “That’s partly because we know what music sounds like,” he notes. “The reason we can reconstruct images, even though we don’t have 100 percent of the information, is because we know what images look like” in general.
There are math rules about how much randomness any given picture can contain — such as how bright it should be and how likely it is that neighboring pixels will look similar. Those basic guidelines can inform how computer software decides which data interpretations make the most sense.
Black holes and beyond
EHT’s black hole observations are expected to help answer questions like how some supermassive black holes, including M87’s, launch such bright plasma jets. Understanding how gas falls into and feeds black holes could also help solve the mystery of how some black holes grew so quickly in the early universe, Loeb says.
Loeb says the EHT also could be used to find pairs of supermassive black holes orbiting one another. Two such relatively small black holes collided, creating the gravitational waves detected in 2015. Getting a census of such black-hole pairs may help researchers identify targets for the Laser Interferometer Space Antenna, or LISA. It’s goal will be to search from space for gravitational waves kicked up by the movement of objects like black holes.
EHT has few viable targets other than supermassive black holes, says Daniel Marrone. He’s an astrophysicist at the University of Arizona in Tucson. There are few other things in the universe that appear as tiny but bright as the space surrounding a supermassive black hole. “You have to be able to get enough light out of the really tiny patches of sky,” explains Marrone. “In principle, we could be reading alien license plates or something,” but they’d need to be super bright.
Too bad for alien seekers. Still, spying supermassive black holes is a pretty neat trick.
alien A non-native organism. (in astronomy) Life on or from a distant world.
antenna (plural: antennae) In physics: Devices for picking up (receiving) electromagnetic energy.
array A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
atmosphere The envelope of gases surrounding Earth or another planet.
atomic clock A timekeeping device that relies on the frequency of microwave emissions from excited atoms. For example, for the cesium atom that frequency is 9,192,631,770 hertz (or cycles/oscillations per second). Many common devices including cell phones, computers and GPS-satellite receivers rely on the high accuracy of atomic clocks to regularly reset their time (known as synchronization).
behemoth A term for anything that is amazingly big. The term comes from a monstrous animal described in the Bible’s book of Job.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
census An official count or survey of a population.
continent (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.
cosmic An adjective that refers to the cosmos — the universe and everything within it.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
diameter The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.
event horizon An imaginary sphere that surrounds a black hole. The more massive the black hole, the bigger the sphere. Anything that happens inside the event horizon is invisible, because gravity is so strong that under normal circumstances even light can’t escape. But according to some theories of physics, in certain situations small amounts of radiation can escape.
football field The field on which athletes play American football. Owing to its size and familiarity, many people use this field as a measure of how big something is. A regulation field (including its end zones) runs 360 feet (almost 110 meters) long and 160 feet (almost 49 meters) wide.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
moon The natural satellite of any planet.
nerve A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.
network A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)
observatory (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.
pixel Short for picture element. A tiny area of illumination on a computer screen, or a dot on a printed page, usually placed in an array to form a digital image. Photographs are made of thousands of pixels, each of different brightness and color, and each too small to be seen unless the image is magnified.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles. (in medicine) The colorless fluid part of blood.
plateau A flat area of land, high above sea level. It’s sometimes referred to as “tableland.” Several of its edges tend to be steeply sloped (cliffs).
radio waves Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.
resolution (in optics) A term having to do with the degree of clarity or detail with which some object can be seen. (v. resolve)
software The mathematical instructions that direct a computer’s hardware, including its processor, to perform certain operations.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
tune (in engineering) Adjust to the right level.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
viable Able to live and survive. (in biology) Able to survive and/or live a normal lifespan. (in engineering) Something that should work or operate according to plan, as in a “viable concept.”
virtual Being almost like something. An object or concept that is virtually real would be almost true or real — but not quite. The term often is used to refer to something that has been modeled — by or accomplished by — a computer using numbers, not by using real-world parts. So a virtual motor would be one that could be seen on a computer screen and tested by computer programming (but it wouldn’t be a three-dimensional device made from metal).
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.