Planetary Vision
With Robert Gerard Pietrusko,
Stewart Smith,
and Lukas Gianocostas.
to see
a black hole.
The Event Horizon Telescope is a collaboration of telescopes and people spanning the Earth: it took a planet to see a black hole.
Back in 1969, when astronauts first set foot in the Mare Tranquillitatis, the best resolution telescope in the world was the 200”-aperture Hale Telescope on Mt. Palomar in San Diego County. Hale could just about make out a football stadium-sized feature on the Moon. The first black hole imaged, Messier 87*, is larger than our solar system, but so far away (55 million light-years from us, and therefore tiny in the sky) that to resolve it required an aperture the size of the Earth: not 100 inches, but 500 million inches. That is what it would take to resolve an image of M87*—to pick out not a stadium on the Moon as seen from the earth, but the straps on Neil Armstrong’s lunar boots. Building such an instrument was the goal of the Event Horizon Telescope (EHT), set even before its formal establishment in 2015.
Planetary in scale
Sadly, we couldn’t build an instrument with an 8,000-mile aperture. Instead, the EHT linked existing radio telescopes all over the Earth to synthesize a single, virtual telescope the size of our planet. In fact, we not only needed that earth-sized virtual machine; we also required the rotation of the Earth on its axis as part of the instrument. In a fundamental way, the earth had to be not so much the object of inquiry (mapping a world atlas or globe) but itself a part of the instrument (the telescope).
No single radio telescope, not even the very largest in the world, could possibly image a black hole on its own. It requires a network of them integrated, brought together across the whole of the Earth in the western United States, Mexico, Chile, Hawaii, Spain, the South Pole, and more recently Greenland, France, and Korea. Using advanced computational methods, this extended network provides a skein over the Earth itself, a virtual planet-sized telescope.
Planetary in collaboration
A second meaning of “planetary”: there was a human and institutional structure that covered the world. More than 200 scientists and engineers participated in the effort that led to the first image of a black hole, distributed over more than 30 countries and regions. That large-scale collaboration was vital so that the team could attend to the needed range of tasks, from designing the instrument to operating telescopes on 15,000ft mountain peaks, from correlating the astronomical data on supercomputers all the way through the production of images and their theoretical interpretation.
Planetary interdependence
Though the third planetary aspect of the M87* image is subtle, it is extremely important. We were not just sampling a myriad of different pieces, the way you might send out a hundred photographers to take pictures of a park, each photographer snapping images of a distinct square on a grid, each of their images a perfectly valid picture. The park photographers could stitch their images together aggregatively, making a large-scale montage out of constituent images. No, in this sense, the Event Horizon Telescope needed the assembly of data from the telescopes integratively to mean anything at all; data from any single device signified nothing.
Indeed, the basic unit for the EHT is not any particular telescope or even all the telescopes taken one by one. Instead, it is the baseline, the correlation between two different telescopes. It is only these baselines that created a set of frequency capturing devices, the way a violin string pulled taught between the peg-box and the bridge each would vibrate in the presence of tones at G, D, A, and E. (A string fixed only to the bridge would not vibrate; it would only flap in the wind.) Were one to have a sufficient number and variety of such string lengths—if one could measure how much each one was vibrating and when—well, then it would be possible to reconstruct a song. So it is visually: each pair of telescopes added information about the image. Put together a sufficient number of telescope pairs with a variety of distances between each duo, and the image of the black hole could be constructed: integration, not aggregation.
For that first, now famous image of M87* released in April 2019, we had telescopes on five sites. Correlating the signals falling on each pair of telescopes is vital. This is because a single telescope captures vastly more noise than signal; it was essentially a bucket collecting all the light that fell on it from any source in view. The signal—light coming from around the black hole—could only be extracted by correlating the results of two telescopes at a time, so if some unwanted light fell into one and not into another, that junk would be eliminated in the process of comparing the two telescopes.
Only with these pair-wise correlations could we go from the 5 petabytes of data collected in the observing campaign to the tiny fraction of those data relevant for the black hole image.
Three senses of a planetary vision are all at play: the world-spanning array of telescopes, the intercontinental distribution of scientists and institutions, and the fundamentally correlative nature of the instrument. The EHT experiment was planetary in scope and essence. It uses the size of the Earth, the spin of the Earth; it engages institutions and people from dozens of countries and regions around the planet. It processes the data by correlating these various telescopes to produce the fundamental data that undergirds the whole image-making project. Together, the three realms of planetary vision led to the image of the supermassive black hole at the center of the distant galaxy M87, and the smaller, but still supermassive black hole at the core of our galaxy, the Milky Way.
Measuring Earth
Of course, there were earlier, important scientific efforts that covered the Earth long before Albert Einstein’s work pointed toward black holes.
Beyond the mapping of Earth’s geography, Isaac Newton predicted that the spinning of the Earth would cause the planet to be oblate, like a ball squished down very slightly so it became wider at the equator. To check Newton’s prediction, expeditions sought to measure the distance along a longitude line over, say, a one-degree arc of latitude near the equator. If the Earth were flattened, as Newton said it would be, that arc would be shorter than a one-degree arc farther north or south from the equator.
Already in the 18th century, scientists went to the field near Quito, Ecuador, to undertake such measurements to a much greater degree of accuracy with a full armamentarium of new scientific instruments: telegraphs, theodolites, pendulums, and platinum rulers. Mapping the planet took all the technology each generation could muster. By the 21st century, geodesists had determined, using satellites, that the equatorial bulge was some 43 kilometers (27 miles) greater than the polar diameter.
Another example of measurement directed at (not using) the Earth was the distribution of magnetic fields. If you were on a boat somewhere out on the ocean, looking at your compass as it points north, you would want to know how that relates to true north. For mariners as well as land-based explorers, measuring the deviation of the magnetic fields relative to true north, everywhere across the Earth for display in maps, became a vital, even life-or-death project if you wanted to avoid the shoals of Ireland or the sandbars off Cape Cod. Even today, as Inertial Navigation Systems and Global Positioning Satellites become ever more ubiquitous, small crafts and many larger ones have magnetic compasses as primary or backup tools.
So too was it for scientists in the 1930s measuring the intensity of cosmic rays. The discovery that cosmic rays were arriving with greater intensity near the magnetic poles militated for the hypothesis that the earth was a giant magnet. Magnetic field lines shepherd the charged particles from space away from the equator and toward the north and south poles, which, incidentally, is protective for humans who live away from the poles.
In these survey examples, the planetary scale was critical: the flattening of the Earth, the magnetic geography of the Earth, the worldwide cosmic ray distribution. But with the EHT, something else emerges: Earth as an essential part of our instrument looking outward, not Earth as the object of our technical gaze.
Earth as instrument
Why the Earth’s rotation matters: Recall the analogy invoked earlier: each pair of radio telescopes acts as though you are suspending a violin string between two fixed points. That tells you a certain frequency would cause each string to vibrate. If you had lots of different strings, different lengths and tensions, you could, by measuring how much they were vibrating, reconstruct a song that was playing—even if all you had were the oscillations of these violin strings. There’s a visual analog. Each pair of telescopes picked up different visual frequencies, so to speak, and by combining how much there was in response on these different baseline lengths, it is possible to reconstruct an image, in our case, of the black hole.
When the Earth turns, each baseline moves with respect to the black hole. The baseline might be perpendicular to the incoming signal from a black hole, or it might be angled to offer the incoming light a much shorter effective length. (Think of slowly twirling a pencil—when it is perpendicular to one’s eyes, it is seven inches long, but when one gazes down its length, it appears no bigger than its built-in eraser head.) So it is for light arriving from the black hole: the length of the baseline varies as the Earth turns, from a perpendicular maximum to a minimum. The diurnal motion of our planet gives us many more baselines, allowing us to image the black hole.
You are seeing on these three panels are the different baselines as they come into view and rotate relative to the black hole, the filling-in of the virtual image as we fill in more, making a better and better virtual telescope. In the right-hand panel, the image that is produced as you get this additional information. Vividly, the animation shows how necessary the rotation of the Earth is to produce enough information to end up with this image of the black hole. The Earth is part of the instrument, in its size and its rotation. Without both there would have been no image to show the world of M87* in April 2019, nor one of Sagittarius A* May 2022.
Measuring a black hole
One of the hardest things to grasp about imaging a black hole like Messier 87*, this monster of a black hole with a mass six and a half billion times that of the sun, is the difficulty of imagining how far away it is. What does it mean to be 55 million light years away? 55 million years ago, the light recorded in the M87* image had just left the black hole. By geological standards, dinosaurs had only recently laid down their heads for the last time in the aftermath of the Chicxulub impactor’s catastrophic collision with Earth.
Another way of thinking about how distant M87* is from us, is to imagine the distance from the Earth to the Sun, and then zoom out to the outer orbits of planets around the Sun; to zoom out to the various stars that are in our nearby Milky Way neighborhood. To zoom out to the edge of Milky Way itself with its 100 billion stars, to zoom out past the Milky Way to where we're in our local cluster of galaxies, to zoom out to the distance from our galaxy to the giant galaxy M87*. It is worth taking a moment to zoom in and out to absorb the scale of distances as one pulls back, and back, and back.
Philosophy of the black hole: The photon ring
Shortly after the April 2019 release of the M87* image, a group of us were sitting in a seminar room, staring at the image of the black hole wondering: What more could we do? What physics could we extract from this extraordinary technology and the image it made? What astronomy could you pull from the orange donut?
What if we had a telescope not Earth-size, but bigger than the Earth? What if we had an orbiting radio telescope 12,000 miles up joined to the terrestrial array? It would yield a resolution three times that of an earth-bound array, and would be the highest-resolution telescope in the history of astronomy. We are working all-out on this orbiting mission, the Black Hole Explorer (BHEX), reckoning what we would see inside that orange gas donut.
With BHEX, we could see not just the hot, billion-degree gas that is orbiting around the black hole. If we could peer with BHEX + Earth telescopes deep into the hot gas swirl, we would see a thin ring of pure light (the photon ring) orbiting the black hole. Some of the photons arriving in our instrument at any given time would have come from near the black hole, been deflected by it and headed to Earth. Some of the photons would have gone around once before veering our way. Some would have gone around twice before coming to us. Photons could have gone around three or four or any number of times before aiming toward our planet.
Meanwhile, the ever-expanding terrestrial array of telescopes is constantly improving, adding stations around the globe where they will do the most to improve our imaging capabilities. BHEX will deepen its gaze. Together, we will have the eyes of a cosmic lynx, a hybrid Earth-Space telescope capable of seeing the first of these rings of pure light orbiting the black hole, just outside its horizon.
Even BHEX is just the beginning. One day, sometime in the coming decades, we’ll put a radio telescope on the Moon, giving us a chance to see the second photon ring. Down the line, we’ll set one at the second Lagrange point, a million miles from earth, where the James Webb Telescope lives. It will permit a glimpse of the third photon ring. We have in this progression something extraordinary. First, the EHT made the earth into an instrument, offering a planetary vision. With BHEX and its successors, we can envision a beyond-planetary form of sight. A new epoch in our understanding.
Back to the photon ring: Each batch of photons orbiting a given number of times corresponds to a different concentric ring. If we could decrypt the images in these rings (not remotely practical, don't try this at home!), you would see light that had hung around the black hole at given intervals—for the biggest of black holes (there’s one ten times bigger than M87*), each light-orbit takes a week. Looking at the first ring, we would be seeing light that had gone around once and come to us; at the same time, we’d see a second ring from light that had orbited the black hole twice, taking an additional week before coming to us. The third ring would be from photons that had orbited three times (taking three weeks), and therefore we’d be looking at light that had arrived at the black hole three weeks earlier than a photon that had just been deflected by the black hole without orbiting.
ultimate movie.
It is the memory
of everything.
As we glanced at the first, then second, then third, and higher-order rings, we would see photons that were from ever further back in the past. It would be as though they were frames of a movie—all the light that gathered there from all over the universe in successively farther-back times. With a frame rate of one frame per week, we can picture the photon ring series as a film of the history of the visible universe, seen from the black hole, running backward.
Each black hole around the universe is gathering light from all the places that light can shine to it. We can see each black hole as a storehouse of memories. Of memories of the whole universe. I love that idea. That the black hole is not just the place outside the causal history of humankind, disconnected from the universe. It is not just the place that would tear a star to shreds if it wandered too close. It is something else. It is the ultimate movie. It is the memory store-house of all that can be seen, whether light coming from the Earth or light coming from distant galaxies. Perhaps the photon rings gathering light and setting it in orbit, will be the last thing that's recorded of everything we do.
Black Hole Explorer
- 3.4m antenna.
- 100+300GHz simultaneous dual-band observation.
- 100 Gb/s downlink using laser communications.
- 20,000km orbital altitude.
- 6μas angular resolution via space-ground interferometry.
black hole.
watches us.