Making the ultimate darkness visible

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.”

Link to original article: here.

Prestigious Fellowship for UA’s Dimitrios Psaltis

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)
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.


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, as is a new video of previous fellows discussing the impact of their Radcliffe Institute experience on their lives and work:  

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)
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)
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.”