The Event Horizon Telescope will combine data from a worldwide network of radio telescopes to image the shadow that a black hole casts on the surrounding plasma.
Within months of the publication of Albert Einstein’s general relativistic field equations in 1915, Karl Schwarzschild had derived the equations’ first nontrivial solution—the black hole spacetime. Ever since then, the physics and astronomy communities have had a love–hate relationship with black holes. It took almost half a century before they were considered anything more than a mathematical curiosity. Today the existence of black holes is widely accepted, but they remain perplexing nevertheless. In most attempts to unify quantum field theory and general relativity, black holes present paradoxes that are hard to resolve.Formally speaking, a black hole is a vacuum spacetime with all the mass concentrated in an infinitesimally small region at the center. At large distances from the concentration, the gravitational field behaves like that of any other object. However, a black hole is surrounded by a virtual surface, called the event horizon, from which nothing can escape, not even light. For a nonspinning black hole, the radius of the event horizon, called the Schwarzschild radius RSRS, is equal to 2GM/c22GM/c2, where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light. An ongoing project called the Event Horizon Telescope (EHT) is now attempting to image black holes with horizon-scale resolution.
SEEING IN THE DARK
In the observation of gravitational waves recognized by the 2017 Nobel Prize in Physics, detectors at the Laser Interferometer Gravitational-Wave Observatory listened to spacetime ringing as two black holes coalesced. Imaging black holes will give EHT scientists a different way to investigate physics just outside the horizons of these enigmatic objects. Specifically, an image can provide spatially resolved information about strong-field gravitational effects in stationary spacetimes and about the interaction of the horizon with the surrounding matter. However, by their very definition, horizons do not emit light. It is therefore difficult to see how they lend themselves to imaging.To see black holes, the EHT looks for the silhouettes they cast on background emission. Photons that are directed radially outward from a black hole can escape its gravitational field only if they are outside the event horizon. Photons that are not radially directed can be trapped at even greater distances. In fact, any photon with an inward radial momentum component is destined to cross the horizon once it passes the so-called photon orbit radius. As long as there is a source of photons outside the black hole, such as hot material falling into the black hole, there will be radiation on which the black hole will cast a shadow, a silhouette that can be imaged. Figure 1 shows a simulation of what the EHT might see.
Figure 1.The black hole at the center of the Milky Way radiates as it accretes hot plasma. This three-dimensional simulation of 1.3 mm radiation shows the circular shadow cast by the black hole. The shadow is not fully dark because some radiation is emitted between the black hole and the viewer. (Courtesy of Chi-kwan Chan/University of Arizona.)
A worldwide collaboration has won a highly competitive award from the National Science Foundation to develop new ways of processing unprecedented amounts of data in real time.Daniel Stolte, University Communications
A computer simulation of superheated plasma swirling around the black hole at the center of our galaxy. (Image: Chi-Kwan Chan, Dimitrios Psaltis and Feryal Ozel)
A worldwide collaboration has won a highly competitive award from the National Science Foundation to develop new ways of processing unprecedented amounts of data in real time.Daniel Stolte, University Communications
Feb. 21, 2018
Chi-Kwan Chan waves his hand a few inches above a matchbox-size device. On a dark computer monitor, a million light dots appear as a solid sheet, each dot representing a light particle.
The Event Horizon Telescope is a virtual Earth-size telescope, achieving its globe-spanning baseline by combining precisely synchronized observations made at various sites around the world. (Image: Dan Marrone)
The photon sheet hovers above a black disc simulating a black hole. With a slow turn of the hand, the sheet approaches the black hole. As it passes, the gravitational monster swallows any light particles in its direct path, creating a circular cutout in the sheet of particles. The rest of the particles are on track to move past the black hole, or so it seems. But they don’t get very far: Instead of continuing along their straight lines of travel, their paths bend inward and they loop around the black hole and converge in one point, forming a sphere of photons around it.
“What you see here is light trapped in the fabric of space and time, curving around the black hole by its massive gravity,” explains Chan, an assistant astronomer at the University of Arizona’s Steward Observatory, who developed the computer simulation as part of his research into how black holes interact with things that happen to be nearby.
The demonstration was part of an event at UA’s Flandrau Science Center & Planetarium on Feb. 16 to kick off a UA-led, international project to develop new technologies that enable scientists to transfer, use and interpret massive datasets.
Known as Partnerships for International Research and Education program, or PIRE, the effort is funded with $6 million over five years by the National Science Foundation, with an additional $3 million provided by partnering institutions around the world. While the award’s primary goal is to spawn technology that will help scientists take the first-ever picture of the supermassive black hole at the center of our Milky Way, the project’s scope is much bigger.
What looks like a fun little animation on Chan’s computer screen is in fact a remarkable feat of computing and programming: As the computational astrophysicist drags virtual photons around a virtual black hole, a powerful graphics processor solves complex equations that dictate how each individual light particle would behave under the influence of the nearby black hole — simultaneously and in real time.
Study Relies on Simulations
Unlike the crew in the movie “Interstellar,” astrophysicists can’t travel to a black hole and study it from close range. Instead, they have to rely on simulations that mimic black holes based on their physical properties that are known to — or thought to — govern these most extreme objects in the universe.
Chan belongs to a group of researchers in an international collaboration called the Event Horizon Telescope, or EHT, that is gearing up to capture the first picture of a black hole — not just any black hole, but the supermassive black hole in the center of our galaxy. Called Sagittarius A* (referred to as “Sgr A Star,” pronounced Sag A Star), this object has the mass of more than 4 million suns.
Since nothing, not even light, can escape a black hole, it casts a silhouette in the background of in-falling plasma that is too small to be resolved by any single telescope. So far, the existence of Sgr A* has been inferred from indirect observations only, such as the intriguing choreography of stars in its vicinity, whose orbits clearly outline an unseen, incomprehensibly large mass.
“Imaging the black hole at the center of our galaxy from Earth is like trying to read the date on a dime on the East Coast from the UA campus,” says Feryal Özel, a professor of astronomy and physics at Steward and a co-investigator on the project. “There is not one telescope in existence that could do that.”
The EHT is an array of radio telescopes on five continents that together act as a virtual telescope the size of the Earth — the aperture needed to image “the date on the dime,” or in this case the supermassive black hole Sag A*. To accomplish this, the individual telescopes must be precisely synced in time. Because existing internet cables and even satellite communication are too coarse to ensure this, the researchers rely on atomic clocks and … FedEx (more on that later).
“Our PIRE project is a prime example of the kind of innovation you can only get by leveraging the innovative, intellectual capital in academia,” says Dimitrios Psaltis, the principal investigator on the project. “By its very nature, this project is multidisciplinary and requires expertise in areas ranging from detector development to high-performance computing and theoretical physics.”
At peak activity, the EHT will collect more data than any project before, according to Psaltis, a professor of astronomy and physics at the UA.
“We’re talking petabytes every single night,” he says, and this is comparable to the three petabytes of video uploaded each day on YouTube. “Post-processing is a huge effort, and we will need additional data to improve the science that we hope will come from these observations.”
The team uses graphic processing units, or GPUs — processors developed for gaming that are capable of performing many calculations in parallel. This makes them more efficient and energy-saving than “regular” computer processing units, or CPUs.
“We hope that this technology will transfer to other areas of science and life,” said Joaquin Ruiz, dean of the UA College of Science, at the launch event.
Applications Could Be Extensive
The PIRE project is expected to spin off technologies that go beyond the project’s primary goal. The fast processing of large data in real time and the efficient use of resources distributed across the globe will have applications ranging from self-driving cars to renewable energy production and national defense. Examples also include augmented reality applications that are good at fast computing with real-time input and minimum computing resources, Özel explains.
“This could be used, for example, in visual aids for security efforts around the globe where data connection bandwidth and energy supplies are limited,” she says, “so you want devices that make maximum use of precious resources available in those scenarios.”
The PIRE project team integrates researchers in the U.S., Germany, Mexico and Taiwan. Education of students and early career scientists is a key component, providing internally collaborative, hands-on experience in instrument technology, high-performance computing, and big and distributed data science. There also are monthly webinars and hackathons, as well as summer schools, that will be sponsored every year.
Fast and reliable real-time communication channels are crucial in syncing up telescopes scattered around the globe for observations, and improving such technology is one of PIRE’s goals. For now, EHT scientists rely on video chat, phones and whiteboards to keep track of each telescope location’s status. During a rare stretch of a few days in April 2017, skies were mostly clear in all nine observing sites that are part of the EHT array — including Arizona, Hawaii, Chile, Mexico and Antarctica.
The South Pole Telescope is the latest to join the globe-spanning array of radio telescopes working together to take a direct image of the supermassive black hole at the center of our Milky Way. (Image: Junhan Kim)
The South Pole Telescope, or SPT, site was incorporated under another NSF grant to the UA, with Dan Marrone as principal investigator. Last year was the first year that the full EHT observed as an array, and the first year in which the SPT participated.
During that first observation run, the observing stations that together make up the EHT pointed at the Milky Way’s center and collected radio waves originating from the supermassive black hole over the course of several nights. By obtaining the first-ever images of black holes, researchers will be able to directly test Einstein’s theory of general relativity in extreme conditions.
“Each telescope records its observation data onto a bunch of physical hard drives,” explains Marrone, an associate professor at Steward and a co-investigator on the PIRE award. “Precisely time-stamped, the drives are loaded into crates and delivered to processing centers in Cambridge, Massachusetts, and Bonn, Germany, via FedEx.”
The EHT data are shipped on physical carriers because current internet data pipelines aren’t up to the scope this endeavor requires. Then data experts combine the literal truckloads of data, synchronize it according to their time stamps and process it to extract the signal from the black hole, which in the raw data is buried under a blanket of noise and error — the inevitable side effects of turning the Earth into one giant telescope.
“PIRE is an international project that not only will revolutionize worldwide efforts to study black holes, but usher astronomical projects into the era of big and distributed data science,” Psaltis says. “By awarding the PIRE project, the NSF has tasked the UA and its collaborators to contribute solutions that may inform many areas of technology, including the internet of tomorrow.”
Astronomer Dimitrios Psaltis is woking on black holes as part of the massive Event Horizon effort that will point a number of earth’s telescopes at the Milky Way’s black hole this spring.
Jon Chase/Harvard Staff Photographer
In effort
to image black hole, a chance to rule on Einstein
BY Colleen Walsh; Harvard Staff Writer
DATE: November 3, 2016
Being an
astrophysicist and father of two is no easy task. Just ask Dimitrios
Psaltis.
On a
recent morning, the University of Arizona professor of
astronomy and physics toggled between a recipe for French pancakes and a series
of complex computer simulations tracing the outline of a black hole.
“Life
goes on,” said Psaltis, the 2016–2017 Shutzer Fellow at Harvard’s Radcliffe
Institute for Advanced Study, who is working on capturing the
first-ever image of the massive dark void at the center of the Milky Way, the
one scientists think is sucking up any matter or radiation that wanders too close
to its event horizon, or point of no return.
“In the
morning, you do black holes,” said Psaltis, “in the evening, you make Nutella
crepes for your kids.”
Prioritizing
his time is second nature for Psaltis, a lead scientist on the Event
Horizon Telescope (EHT) project, a multinational effort
involving more than 100 researchers, including his wife, former Radcliffe
fellow Feryal Özel, and a series of super-powered radio
telescopes scattered around the globe. Next spring those telescopes will turn
the Earth into one giant eye when they all point to Sagittarius A* — the black
hole at the center of the galaxy first forecast by Albert Einstein and his
theory of general relativity, and since then the subject of study by countless
theoretical physicists, among them the famous cosmic detective Stephen Hawking.
During
his fellowship Psaltis will refine the computer simulations he and his team
will use when analyzing EHT data to determine the black hole’s size and shape.
Their results could prove that Einstein’s theory — the notion that gravity is
due to the curvature of the continuum known as space-time — is exact. Or,
perhaps, just a little bit off.
“What we
are looking for is not a description of gravity,” he said, “but the description
that happens to be the one that describes our universe.”
To make
those calculations, researchers will need to see what has thus far been
invisible. But how exactly do you capture the image of a spinning, giant black
abyss? You don’t, said Psaltis. You take a picture of its shadow.
Swirling
around Sagittarius A* are charged particles that have been ejected from the
surface of nearby stars. Moving at supersonic speeds, those particles heat up
millions of degrees to form a shining mass of plasma, or “accretion disk,”
around the edge of the black hole before they are engulfed.
“The
plasma is so hot that it is actually glowing in the radio waves detected by the
telescopes,” said Psaltis. “You put a black hole in front of that glowing plasma
and you get a shadow, you get a silhouette.”
But, as
the special effects team for the movie “Interstellar” discovered, producing a
realistic image of a black hole is hugely time-consuming. (Some individual
frames of the film reportedly took 100 hours to render.) Eager to accelerate
the process, Psaltis and his team hacked into their computer’s graphics card,
the circuit board that controls how images appear on the screen, and gave it a
little something extra.
“We made
it program those chips to do the rendering in the presence of a black hole. …
Our codes are so fast that now we use a type of Xbox to control the process
with our hands because there’s no way to type fast enough to do it.”
If the
image Psaltis and his colleagues produce is perfectly round, it will indicate
Einstein was entirely correct. But if the image starts to warp and bend, it
means his theory might need some tweaking.
“That
nice circle that you see here has a particular size, has a particular shape
only because Einstein’s theory told us so,” said Psaltis, pointing to a
simulation on his screen. “If the theory is different, both the size and the
shape will be different.
“The
shape of the shadow can be used to tell us exactly what that gravitational
field looks like outside that black hole,” he added. “And by measuring that,
either we will be able to say if Einstein’s theory predicts it 100 percent, or
if there are small tweaks that we need to add in order to get it right … this
is the smoking gun as far as Einstein’s gravity is concerned.”
Psaltis’
current project has deep Harvard roots. In the 1990s, he and Özel were both on
campus, Psaltis doing postdoctoral research, his future wife pursuing her Ph.D.
Together they collaborated with Ramesh Narayan, the Thomas Dudley Cabot
Professor of the Natural Sciences, on early simulations that explored what
happens to the plasma around a black hole. That research helped determine that
the radio wavelength that would give them the best chance at seeing the black
hole’s event horizon was roughly one millimeter long.
“We found
that the plasma becomes more and more transparent as you go to a higher and
higher frequency and that’s what we calculated, where you need to make that
observation in order to be able to peer through the plasma,” said Psaltis. At
one millimeter you “see the black hole’s shadow,” he said.
The work
builds on research by Sheperd Doeleman, an astrophysicist at the Harvard-Smithsonian
Center for Astrophysics and principal investigator for the
Event Horizon project. It was Doeleman who first measured the size of the
emitting region of the accretion disk, in 2008.
Skeptics
persist. Despite its potential to advance understanding of black holes and
render a key scientific judgment on Einstein’s work, research like Psaltis’
leaves some doubting an effect for life on Earth when Sagittarius A* is 26,000
light-years away. The native of Greece, who said he gets that question “all the
time,” dons his philosopher’s hat to answer it. Such endeavors have a foot in
both the past and future, he noted, and can also illuminate specific events and
ideas, from the Big Bang to investigations into parallel universes.
Equally
important is the notion that today’s research might have its greatest impact
tomorrow, Psaltis said. To make his case, he cited the work of the German
mathematician Bernhard Riemann, who challenged the accepted model of Euclidian
geometry in the 1800s by imagining a world in which two parallel lines
ultimately crossed. Einstein would go on to base general relativity on
Riemann’s mathematical framework.
“Not even
in his wildest dreams could Riemann have predicted that,” said Psaltis. “But if
he had not asked in the 1800s, ‘Is there any way to make two parallel lines
cross?’ we would not have Einstein’s theories, or GPS, since your phone makes
calculations based on Einstein’s theories to determine where you are.
“Abstract thought is good for intellectual curiosity,” he added. “You never can tell where that can take you.”
A black hole 4 million times more massive than our sun, called Sagittarius A* and pictured here in an artist’s impression, is at the center of our Milky Way. (Image: NASA)
The astronomy professor has been awarded a Radcliffe Institute fellowship to explore the edges of known physics by looking at black holes and exchanging ideas with scientists, artists and filmmakers at Harvard University.
Daniel Stolte, University Relations – Communications
June 1, 2016
The Radcliffe Institute for Advanced Study at Harvard University has selected Dimitrios Psaltis, a professor in the University of Arizona’s Steward Observatory, as a Radcliffe Institute fellow. Together with 54 other women and men, Psaltis is in the 2016–2017 fellowship class at the institute, where the acceptance rate to the fellowship program this year was just under 4 percent.
Dimitrios Psaltis is interested in testing Einstein’s Theory of General Relativity outside of the comparably tame conditions found in our solar system, so he he has turned to the universe’s ultimate proving grounds: neutron stars and black holes, the astrophysical systems with the strongest gravitational fields we know of.
EXTRA INFO
Dimitrios Psaltis is among fewer than 4 percent of applicants who were accepted to Harvard’s Institute for Advanced Study. The Radcliffe Institute has awarded more than 800 fellowships since its founding in 1999. The full list of fellows is online at www.radcliffe.harvard.edu/fellows2016, as is a new video of previous fellows discussing the impact of their Radcliffe Institute experience on their lives and work: www.radcliffe.harvard.edu/video/fellowship-experience-radcliffe-institute.
As the 2016–2017 Shutzer Fellow, Psaltis will pursue an individual project at Radcliffe, in a community dedicated to exploration and inquiry across disciplinary boundaries. In addition to receiving the funding, time and space for up to a year of focused work, the fellows — scholars, scientists and artists — benefit from access to Harvard’s libraries and from engaging Harvard undergraduates as research partners.
Some black holes, like this one in a far-away galaxy called M87, shoot out jets of matter at nearly the speed of light. (Image: NASA/STScI/AURA)
Psaltis joins an international group of fellows coming to the institute from Africa, Asia, Australia, Europe and South America, as well as from across North America. During the year, fellows will present their work in lectures and in gallery exhibitions, many of which are open to the public and shared online.
“What is incredible about the fellowship is that it is not specific to science,” Psaltis said. “The majority of the fellows aren’t scientists but artists like writers and filmmakers. They put us together in the same building with access to all sorts of resources. We will interact and try to explain to each other what we do to people who are not involved in science or the technical aspects of it.”
Psaltis is excited about the prospect of such “intellectual collisions” and the new ideas they might bring not only to his research, but also to the ways he and his colleagues talk to the public. A large part of his work involves creating scientifically accurate, visual simulations of black holes and their surroundings, such as accretion disks, accumulations of matter swirling around the black hole before getting sucked into it.
“Being around filmmakers, for instance, who do this for a living, and trying to explain to them what we do to get input from them about how we visualize those things and make them accessible to the generable public is not something you can easily achieve sitting in an astronomy department somewhere,” Psaltis said.
The UA’s Submillimeter Telescope on Mount Graham is one of many linked together to form the Event Horizon Telescope, a virtual telescope as big as Earth. Psaltis and his colleagues are getting ready to use the EHT to take an image of the black hole at the center of the Milky Way and compare it to others, such as the one in M87. (Photo courtesy of Dave Harvey)
Psaltis will focus on his work on the Event Horizon Telescope, or EHT, a global network of telescopes linked to function as if it were one Earth-size observatory. Expected to be fully operational for the first time in April 2017, the EHT is poised to peer through the gas and dust of our Milky Way to observe the supermassive black hole suspected to be at the galaxy’s center.
Together with his colleague and wife, Feryal Ozel, who was a 2012-2013 Radcliffe Fellow, Psaltis is heavily involved in developing the simulations and tools for interpreting the data the telescope will collect, and piecing together the image of the black hole.
The timing is perfect, Psaltis said, as he will be at the center of the action of Harvard’s brand-new Black Hole Initiative, an interdisciplinary program designed specifically to research black holes. The program is housed in vicinity of the Radcliffe Institute on Harvard Square.
“The Black Hole Initiative follows a similar concept to the Radcliffe Institute in that it gathers people with a wide range of expertise related to the astrophysics of black holes, the EHT, string theory and even philosophy of science,” he said. “The idea is to put all the people in the same place with the resources they need and students and postdocs around them, to see what can be achieved out of this confluence of expertise. I hope this will result in ways of seeing things in ways that we don’t normally look at.”
The EHT teams both at the UA and the Harvard-Smithsonian Center for Astrophysics will be working together continuously, trying to merge the activities that are going on in both places in order to maximize the output of the science experiment.
All EHT components are in place and ready, Psaltis said.
“Every single station in the array has been outfitted with the equipment that is required — for example, detectors, atomic clocks, et cetera,” he said. “We successfully completed an engineering dry run last month to make sure everything is working.”
If everything goes well, next April will be the first time that every single telescope in the array will turn to the black hole and we will begin to observe and collect data.
The EHT’s main targets are two very different black holes. One is the black hole in the center of our own galaxy, the Milky Way, and the other is the central black hole estimated to be a thousand times more massive than ours, in a different galaxy called M87.
“The black hole in M87 has one of the most brilliant, long jets that you can see,” Psaltis said, referring to an outflow of matter and energy that spouts from the galaxy’s central black hole into space over a distance of 4,900 light-years.
“The jet goes way, way out of the host galaxy,” he added, “and it’s clearly launched by the black hole itself. Our black hole, on the other hand, has no evidence for any jet or any big outflow like that.”
The EHT scientists are gearing up to actually see the shadow of the black hole that is cast on the emission around it, and measure its properties, which will allow them to learn whether Einstein’s predictions are valid near a black hole.
“The other important thing we want to do is compare a wimpy black hole like ours to a powerful one like the one in M87,” Psaltis said. “There are so many unanswered questions in accretion physics, which is the physics of how black holes gain their mass and grow in size. For example, what happens to the magnetic field near the black hole? Are magnetic fields responsible for launching the jets? What produces the jets? Does most of the matter fall into the black hole or does it get ejected?
“This will be the very first time that we will be able to see those processes happening very close to the black hole and take pictures in real time of how those processes work.”
I had the privilege to coauthor with Shep Doeleman an article on testing General Relativity with the Event Horizon Telescope, that was published in the special September 2015 Scientific American issue on the 100 years of Einstein’s general theory of relativity.
