On April 10, 2019, six papers slid into The Astrophysical Journal, one of the premier publications for astronomy research. Their shared title does not exactly signal a scientific breakthrough to the lay reader: “First M87 Event Horizon Telescope Results.” Dig deeper, though, and you’ll unearth a figure that would have humbled Einstein, Hawking, and all the other great physicists of the past century. Perhaps 22nd-century historians will hold this achievement in the same esteem as the discovery of nuclear fission or the Moon landing. The global populace of 2019 reacted accordingly. Newspapers, TV programs, amateur astronomers, and non-scientists around the world all froze to admire this strange image — a fuzzy orange fireball perforated by an ominous shadow, a cosmic Cyclops as it were. After years of peering into an open sky, scientists caught the extragalactic monster staring right back at us. Humanity had finally come face-to-face with the stuff of wild thought experiments and blockbuster movies — a black hole, previously thought unseeable, now exposed to the whole world.
Even six years later, M87* — sometimes called Powehi, in honor of the Hawaiian land that made this observation possible — is no less captivating, and it might very well headline Google searches and popular science articles for decades to come. Yet the black hole’s existence was old news for the astronomical community.
Without a doubt, the 2019 image carried enormous scientific implications — Powehi’s appearance agreed perfectly with theory, laying Einstein’s century-old predictions about black holes to rest. But 25 years earlier, researchers had detected rapidly swirling gas at the center of the M87 galaxy, implying the presence of a compact object billions of times more massive than the Sun. Astronomers knew exactly where to look for the black hole — they just needed to tease it out of hiding. Why didn’t this discovery happen the other way around? What took astronomers so long to capture this picture?
Black holes ain’t so black?
On their own, black holes are notoriously invisible, devouring all light that crosses a point of no return called the event horizon. As one might imagine, this makes black hole imaging a tricky business.
Nevertheless, the largest black holes, called supermassive black holes (SMBHs), leave signs of their presence. Powehi, for instance, is cloaked in hot plasma that glows in the radio portion of the electromagnetic spectrum. Although human eyes can’t see radio waves, we’ve built radio telescopes over the past few decades that can.
Some of the light particles, or photons, emitted from this plasma have the misfortune of trailing too close to Powehi’s event horizon, and they get swallowed, never to reach our telescopes. Beyond that, radio emission is safe from Powehi’s gravitational grip, free to travel toward Earth.
Therefore, an unambiguous image of a SMBH should feature a shadow — demarcating the point of no return, where photons tumble to their doom — enveloped in a bright ring of light. So when we “see” a black hole, we are really talking about its silhouette. All it takes to image Powehi, then, is to make out a shadow against a blob of emission. Sounds easy, right?
A visit to the optometrist
Have you ever had your eyesight checked? Even if you have perfect vision, you are likely familiar with the Snellen chart, which is often used to assess visual acuity.
If you are reading this article on a large computer screen, you should be able to read off even the smallest letters toward the bottom. Easy enough. Now try reading those same letters 20 feet away.
Clearly, it’s not the physical size of the letters on a Snellen chart that matters, but their angular size, or how much space they take up in your field of view. Past a certain distance, even those with 20/20 vision (or corrective lenses) will struggle to discern the smallest letters. So “perfect vision” is somewhat misleading of a term: no matter how good your vision is, there is a limit to the scale at which your eyes can perceive details. This limit is called resolution.
The resolution limit is unavoidable—it’s the frustrating result of how light behaves. When light passes through any aperture, like your pupil, it will bend around the aperture’s edges. As a result, the light smears as it approaches a light-collecting surface like your retina. This blurring effect is insignificant except on the smallest scales, where fine details (such as the strokes of a letter) hopelessly bleed into each other.
Still, the resolution of the human eye is quite impressive in everyday situations: we can observe details as small as a 1/60th of a degree wide, or an “arcminute” in astronomy lingo. That is the same resolution as a 576 megapixel screen! But our eyes are (unsurprisingly) shabby astronomical instruments, since the galaxies, stars, and planets dotted across the night sky are often far smaller than an arcminute.
That’s exactly why humans invented telescopes. Telescopes are basically just really large eyes. The upshot to increased size is that the larger the diameter of an aperture, the better its resolution. That’s why the latest telescopes keep on getting bigger and bigger — as we learn more about the Universe, we need to observe even finer details.
But are modern-day telescopes up to the task of imaging Powehi’s shadow? Powehi is one of the largest astronomical objects we know, but it’s also 55 million light years away, in a galaxy far from the Milky Way. That gives it an angular size of… 40 microarcseconds. That’s around a hundredth of a millionth of a degree. For some perspective, seeing Powehi is like reading off the Snellen chart on the moon. That’s one hell of an eye exam — you would need eyes the size of the Earth to do that!
Wait… unless astronomers are hiding something from us and Earth really is some Deep Thought-esque supercomputer, I don’t know any telescope out there that’s as big as our planet. Yet that mesmerizing image of Powehi, flaunting its 40-microarcsecond resolution, negates this seeming impossibility, its gaping void clawing away at all semblance of reason… surely, some wizardry is at work.
If you’re feeling suspicious, you’re not entirely misguided. This wizardry has a special name: radio interferometry.
Interferometry, explained with cats
In the far reaches of our solar system lurks a furry monster. Cuddles the Cat — once a happy household pet, now an extraterrestrial wanderer — was the victim of a tragic spacecraft failure in the late 1940s. Well before the Soviet Union launched the first two dogs into space, top American intelligence officers intercepted communications that detailed Soviet ambitions for aerospace innovation. Panic swept through the U.S. government, and the President ordered a top-secret mission to put the Soviets in their place. In a clandestine operation, Cuddles was seized from his home in Columbus, Ohio, and shoved onto the payload of Felinus I. Although the spacecraft successfully reached low Earth orbit, several screws on the ejection latch came loose. As a result, Cuddles was launched into the harsh vacuum of space, where his body ballooned to behemoth proportions.
The accident was covered up for decades, and Cuddles was never to be recovered. But recently, a group of amateur astronomers discovered unusual orbital perturbations in the Asteroid Belt. America’s best astronomers were filled in on the Felinus disaster and took notice. Could Cuddles be responsible? The Atacama Large Millimeter Array (ALMA) — the world’s largest radio interferometer, located on a plateau in Chile’s Atacama Desert — was tasked with hunting down Cuddles and capturing a resolved image of the fearsome feline.
Unlike most telescopes you might be familiar with, ALMA is not a single dish or lens. It is a network of antennae, working together to function as a telescope that’s greater than the sum of its parts. How does that work?
First, consider two of ALMA’s 66 antennae. Cuddles’ radio waves travel through Earth’s atmosphere, reaching the dishes of our two antennae. If Cuddles is not directly overhead, though, as shown in the diagram above, then the radio waves will reach one of the antennae first. Since light travels at a finite speed (the speed of light), there will be a time delay before the second antenna picks up on the signal. So, the second antenna records a light wave with some horizontal shift.
Once recorded by both antennae, these two signals — like ripples in a pond — will combine, or interfere, to produce a new wave. (See where the word interferometry comes from?) A device called a correlator calculates this combined wave.
In this sense, the antennae act like detectives, taking advantage of their spacing to extract information about the image. As the Earth rotates, Cuddles will move across the sky, and the time offsets between the receivers will change. Each new correlated signal is like a clue that the interferometer can use to place limits on how the source’s brightness is distributed. Over time, these waves can be stitched together in a case file of sorts, called an interference pattern. Eventually, using a fancy operation called a Fourier transform, an interferometer can attempt to reconstruct the image based on the “evidence” it has collected.
So, how well do these two lonely antennae fare? Even if they observe Cuddles for an entire day, this resulting image… isn’t great. Does this look like a cat to you?
It seems like we need more evidence. The easiest way forward? Hire more detectives! Expanding our team of antennae to 12, we will now have 66 distinct pairs of antennae, with each pair oriented strategically over an area. At a given time, each pair will experience different time delays and therefore produce a different correlated signal from the others simply by virtue of positioning. This fills in even more blanks in the interference pattern. What image can this larger interferometer produce?
It’s not perfect, but it’s Cuddles!
In an ideal world, an interferometer would sample an entire interference pattern, allowing it to completely restore the image of Cuddles. But there are only so many antenna-detectives we can deploy, and only so many clues they can find. Information is always lost when making an interferometric observation, creating unwanted image artifacts. To iron out these blemishes, astronomers go through a complicated process called deconvolution before arriving at a final image.
The payoff of all this hassle, of course, is improved resolution. If an interferometer is treated like a single telescope, its effective diameter is the maximum separation between two antennae in the array. This unlocks a level of resolution that individual telescopes, which are limited in size, simply can’t live up to.
For some perspective, ALMA boasts a resolution between 20 and 100 milliarcseconds — depending on the frequency of light being observed — in its most extended configuration. Compare that to the Green Bank Telescope (GBT) in West Virginia, the world’s largest steerable radio dish. GBT is still used in cutting-edge research today, but its resolution is over ten times worse than ALMA’s. Moreover, GBT’s dish is over 300 feet in diameter, so it is more difficult (and costly!) to operate and maintain than ALMA’s compact, 40-foot-wide antennae. While single-dish techniques have their merits, interferometry clearly paves the way in modern radio astronomy.
[If you’d like a more rigorous treatment of this high-level introduction, check out these excellent slides from the National Radio Astronomy Observatory! And if you want to play around with different interferometer configurations, check out this web tool, also from NRAO.]
Teamwork makes the dream work!
Let us now turn from Cuddles to a slightly less cuddly creature. Sure, Powehi would dwarf the Solar System if placed side by side, but remember, it’s millions of light years away, so it’s just a speck in the vast night sky. You could pack a million Powehis in a square on the sky, and all ALMA would see (if anything) is a blob. Even ALMA isn’t good enough.
Luckily, ALMA has friends in radio telescopes stationed around the world. If ALMA’s antennae can work together, why can’t ALMA join forces with other observatories to form one mega-interferometer? This is the shockingly simple premise of very-long-baseline interferometry (VLBI), a technique that exploits the strategy behind interferometry over extreme distances to push toward even sharper resolution.
VLBI has been around for decades. But in 2009, an international collaboration of researchers launched a telescope array of record-breaking scale — one that would sweep the entire globe, with an effective diameter matching the Earth’s. For the first time in history, such an observatory could resolve the event horizon — the shadow — of a black hole like Powehi. In pursuit of this goal, the project was aptly named the Event Horizon Telescope, or EHT.
Today, the EHT comprises ALMA and one other famous interferometer — the Submillimeter Array atop Maunakea in Hawaii. The rest of its constituents include state-of-the-art single-dish telescopes, such as the James Clerk Maxwell Telescope (also on Maunakea) and the Large Millimeter Telescope (LMT) in Mexico. (Fun fact: UMass has a 30% stake in the LMT! You can thank our friends next door for helping bring us the Powehi image.)
In principle, the EHT operates like an interferometer — conjoining pairs of signals, it pieces together an interference pattern, which it then converts into an image. Yet the sheer physical extent of this telescope network poses several logistical challenges.
For one, antennae in a single interferometric array are linked electronically, so they are all timed with each other. In the EHT, however, the telescopes make observations independently, and the signals they receive need to be synchronized. To accomplish this, EHT researchers used some of the most precise atomic clocks ever manufactured. Each of these clocks loses one second every 10 million years!
In addition, with such large separations between telescopes, the Earth’s curvature factors significantly into the time delay between receivers. Astronomers had to devise models of the Earth’s curvature in order to correct for these additional offsets.
With that business settled, the EHT telescopes were raring to go in April 2017. After EHT scientists crossed their fingers for good weather, the telescopes began tracking Powehi in synchrony, collecting five petabytes’ worth of radio emission over the course of four nights.
The next step was to process the data. For a single interferometer like ALMA, the data reduction process is fairly straightforward: using a computer backend, the signals from each antenna can be mashed together on-site to produce an interference pattern. But with disconnected telescopes scattered across the world, EHT does not enjoy this convenience. Before even digging into the raw data, EHT scientists had to lug thousands of hard drives to two data processing centers — the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy — to gather all the data from the global telescope network in one place.
The researchers ran into more obstacles while trying to produce the perfect image. In VLBI configuration, the antenna-detectives, spaced thousands of kilometers apart, make a crucial trade-off — they are positioned to feel the fine edges of the black hole and maximize resolution, but in exchange, they miss out on larger brightness variations. Therefore, in addition to a well-known deconvolution algorithm known as CLEAN, the EHT team crafted three new pipelines to ensure that their final images reliably compensated for the limited coverage in their interference pattern.
Improved VLBI imaging methods are still in active development. In 2023, a group of scientists followed up on EHT’s famous 2019 results in an attempt to reconstruct a better image. Using a new algorithm called PRIMO, the researchers returned to EHT’s old 2017 dataset and recovered an even sharper ring feature. Astoundingly, the resolution of the PRIMO image is still the same as it was in 2019 — the algorithm simply plucked out more of the fluff in the ring emission. The updated results speak to the value of these computational techniques, which are arguably as essential to black hole image synthesis as the data collection itself.
EHT’s far-flung future
The capabilities of the EHT will soon be out of this world. Literally!
The EHT recently announced the Black Hole Explorer (BHEX) project, an ambitious endeavor starting this year to push VLBI beyond Earth’s surface. In the coming years, the BHEX mission will expand the EHT with a dedicated space telescope to accompany the ground-based facilities that currently make up the planet-sized interferometer. This new telescope will be deployed in an orbit thousands of kilometers above the Earth, which will nearly triple the maximum extent of the EHT and boost its resolution to a staggering 6 microarcseconds.
With this unprecedented detail, astronomers expect to observe crisper outlines around the shadows of SMBHs like Powehi. These so-called photon rings arise from light that grazes the event horizon of a black hole, whizzing around several times before escaping toward our telescopes. According to general relativity, a photon ring is like the fingerprint of a black hole, inscribing fundamental properties such as its angular momentum. Once BHEX launches in 2031, black hole astrophysicists will be able to dig into high-resolution observations of these photon rings for the first time, which could revolutionize our understanding of black holes and the mysterious processes that form them.
Although BHEX feels a far way off, astronomers won’t have to twiddle their thumbs for the next few years, as new ground-based results from the EHT are already pouring in.
Alongside the famous examples of Powehi and Sagittarius A* (our Milky Way’s SMBH), the EHT has imaged SMBHs in the centers of other galaxies. These cosmic beasts, located in active galactic nuclei (AGN), eject ferocious jet streams of plasma. These phenomena might point to the complex physics that takes place around the event horizon. A paper published this year took a detailed look at these AGN jets, comparing the latest high-resolution EHT images with older data. EHT stumbled upon a perplexing insight: magnetic fields revealed by the new observations seem to contradict a well-known model for black hole jets. If future VLBI measurements confirm these anomalies, astrophysicists might need to reconsider exactly how matter behaves around black holes.
With advancements in VLBI rolling out at lightspeed, we now stand at the dawn of an exciting era for fundamental physics. Just a few decades ago, black holes were mere mathematical entities burrowed in the solutions of Einstein’s field equations, only trickling into the public consciousness as fanciful objects of science fiction. Now, the hard work of a massive international effort has elevated these invisible creatures to the pedestal of reality, bringing our theories of black hole physics into focus. Soon, as observations inch ever closer to the event horizon, we’ll sharpen the horizon of our own knowledge — or perhaps discover that it was all a mirage.
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