On Wednesday, April 10th, the world was treated to something unprecedented – the first-ever image of a black hole! Specifically, the image captured the Supermassive Black Hole (SMBH) at the center of M87 (aka. Virgo A), a supergiant elliptical galaxy in the Virgo constellation.
Already, this image is being compared to pictures like the “pale blue dot” taken by the Voyager 1 mission or the “Earthrise” image taken by Apollo 8. Like these images, the picture of the M87 black hole has captured the imagination of people all around the world.
This accomplishment drew on years of hard work involving astronomers, observatories and scientific institutions from all around the world. As with most accomplishments of this caliber, countless people played a role and deserve credit for making it happen.
But as always, there were a handful of people whose contributions really stand out. In addition, capturing the first-ever image of a black hole depended a lot of specialized technology and scientific methods, which also deserve attention. You might say that his historic accomplishment had a historic buildup!
Those Who Made it Happen:
Since the EHT project released the first image of a black hole, Katherine Bouman has become something of a household name. But just who is this black hole hunter whose work has helped us to look into the face of one of the most mysterious phenomena in the Universe?
Bouman received her Ph.D. Electrical Engineering and Computer Science at the Massachusetts Institute of Technology (MIT) in 2017. Since then, Bouman has worked as a postdoctoral researcher with the Event Horizon Telescope project, where she applied emerging computational methods to push the boundaries of imaging technology.
Among her contributions is the development of an algorithm which was instrumental in obtaining the image, known as Continuous High-resolution Image Reconstruction using Patch priors (CHIRP). Though CHIRP itself was not used, it inspired the image validation procedures used, which Bouman also played a significant role in developing.
In addition to verifying and selecting parameters for filtering images taken by the EHT, she also helped the imaging framework that compared the results of different image reconstruction techniques. After the publication of the black hole image, a photo of Bouman smiling in front of a computer screen began to go viral on the internet.
After the announcement was made, Bouman posted a photo of her and her and research team (shown above) with the caption:
“I’m so excited that we finally get to share what we have been working on for the past year! The image shown today is the combination of images produced by multiple methods. No one algorithm or person made this image, it required the amazing talent of a team of scientists from around the globe and years of hard work to develop the instrument, data processing, imaging methods, and analysis techniques that were necessary to pull off this seemingly impossible feat. It has been truly an honor, and I am so lucky to have had the opportunity to work with you all.”
Bouman and her group are currently analyzing the Event Horizon Telescope’s images to learn more about general relativity in a strong gravitational field. For her outstanding work, Bouman was also recently given a position as an assistant professor at Caltech’s Computing and Mathematical Science (CMS) department.
Together with Caltech, Bouman will be working to create a laboratory dedicated to experimenting with computational imaging and machine-learning algorithms. This laboratory will be the first of its kind and it is expected to have a significant impact on the study of gravitational singularities and other extreme phenomena.
Then there’s Sheperd Doeleman, a senior research fellow at the Harvard-Smithsonian Center for Astrophysics (CfA), the assistant director for Observation with Harvard’s Black Hole Initiative, and the director of the EHT. He is also a principal research scientist at MIT and the assistant director of the MIT Haystack Observatory – one of eight that participated in the EHT.
It was during his time at MIT’s Haystack Observatory that Doeleman became one of the first people to see the first hints of the black hole at the center of the Milky Way. And it was because of analyses he conducted to make sense of the data that first revealed it.
“That was a moment where there was one person – me – in the world who knew what had just happened,” he said. “That was pretty amazing. Because as soon as we knew there was something there, then the gloves came off and we were ready to start building an Earth-sized array to image it.”
However, it was in May of 2018 that his team accomplished what many thought was impossible. It began with a conference at the BHI, where students and postdocs shared some of the data they had obtained to Doeleman. As he described the moment of discovery:
“We could see the telltale signatures in these data… and we were all just looking at it, saying, ‘Wow.’ I worked until late that night coming up with a model of how big what we were seeing was, and that’s when I knew we had something very, very interesting.”
In addition to his extensive experience studying astrophysical phenomena, Doeleman also brought his expertise in Very Long Baseline Interferometry (VLBI). This process, where radio dishes that are separated by vast distances are combined to form a virtual telescope array, was essential to the EHT’s efforts.
With his group at MIT, Doeleman helped develop the instrumentation that has allowed astronomers to achieve the greatest possible resolution with VLBI at Earth-based observatories. In the past, he and his team have used this technique to study newly-born stars and the atmospheres of dying stars.
But with the imaging capabilities of EHT, the techniques he helped pioneer can now be used to examine how gravity and general relativity work under the most extreme conditions. This effectively opens a new door to understanding how our Universe works.
“This fulfills our dream to take the first picture of a black hole,” said Doeleman. “We now have access to a cosmic laboratory of extreme gravity where we can test Einstein’s theory of General Relativity and challenge our fundamental assumptions about space and time.”
Thanks to the role he played in coordinating the project, Doeleman now leads the EHT project. Beyond Bouman and Doeleman, countless scientists and engineers played a vital role in making this milestone happen. In addition, several key facilities and processes were involved.
How the Image was Taken:
The Event Horizon Telescope (EHT) is essentially a planet-sized radio telescope made up of observatories from around the world. At present, the EHT consists of eight sites, which includes the:
By combining radio antennas and data from several very-long-baseline interferometry (VLBI) stations, the EHT is able to achieve a level of resolution that allows scientists to view the intermediate environment around black holes (aka. the event horizon).
This was no easy task, given the extreme nature of black holes. Originally predicted by Einstein’s Theory of General Relativity (GR), black holes are essentially what becomes of particularly massive stars once they reach the end of their lifespan.
At this juncture, when a star has exhausted the last of its hydrogen and helium fuel, it undergoes gravitational collapse. This leads to a massive explosion known as a supernova, where the star blows off its outer layers. Depending on the mass of the star, the result will either be a stellar remnant (i.e. a neutron star or “white dwarf”) or a black hole.
In fact, the term “black hole” is a bit of misnomer, since they are actually extremely compressed objects that contain an extraordinary amount of matter within a tiny region. Because of their compact nature, they exert an extremely powerful gravitational force from which nothing – not even light – can escape.
Because of this, scientists were only able to infer the existence of black holes based on the effects they have on their surroundings. These include the way they warp spacetime, causing objects around them to fall into eccentric orbits, and the way they will cause the material to fall into a disk around them which is heated to hundreds of billions of degrees.
As Ramesh Narayan, a Harvard University professor and a leader in EHT theory work, summarized:
“For decades, we have studied how black holes swallow material and power the hearts of galaxies. To finally see a black hole in action, bending its nearby light into a bright ring, is a breathtaking confirmation that supermassive black holes exist and match the appearance expected from our simulations.”
The project’s targets were the two black holes with the largest apparent angular size when viewed from Earth. These are the SMBH located at the center of the Milky Way (Sagittarius A*) and the SMBH at the center of the galaxy known as M87 (Virgo A).
In order to capture an image of these SMBHs, astronomers needed a telescope of unprecedented resolution. This is where the ELT came in. Jonathan Weintroub, who co-coordinates the EHT’s Instrument Development Group, explained:
“The resolution of the EHT depends on the separation between the telescopes, termed the baseline, as well as the short millimeter radio wavelengths observed. The finest resolution in the EHT comes from the longest baseline, which for M87 stretches from Hawai’i to Spain. To optimize the long baseline sensitivity, making detections possible, we developed a specialized system which adds together the signals from all available SMA dishes on Maunakea. In this mode, the SMA acts as a single EHT station.”
Through its eight observatories, the EHT recorded millions of gigabytes of data of these two black holes. In total, each telescope took in about one petabyte (1 million gigabytes) of data and recorded it onto several Mark6 units – data recorders that were originally developed at Haystack Observatory.
Once there, the data was cross-correlated and analyzed by 800 computers that are connected through a 40 Gbit/s network. Converting this data into an image though required developing new methods and procedures.
This involved comparing images among four independent groups of scientists using three different imaging methods – which were designed and led by Katie Bouman. In the coming years, the EHT plans to improve the project’s angular resolution by adding two more arrays and taking shorter-wavelength observations.
In addition to the existence of black holes, Einstein’s Theory of General Relativity predicted that a black hole would cast a circular shadow upon the glowing disk of material that surrounds it. Essentially, the region within the black holes event horizon would appear as total blackness, in stark contrast to the very bright disk beyond it.
The chair of the EHT Science Council, Heino Falcke of Radboud University in the Netherlands, explained all that as follows:
“If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow – something predicted by Einstein’s general relativity that we’ve never seen before, This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these fascinating objects and allowed us to measure the enormous mass of M87’s black hole.”
Interestingly enough, this appearance was also accurately predicted by the special effects team behind the movie Interstellar. To add a sense of realism to the film, theoretical physicist and Nobel laureate Kip Thorne developed a new set of equations to guide the special effects team’s rendering software.
To do this, Thorne relied on known scientific principles. These included the fact that the black hole formed from a massive stellar remnant, which would mean that it would be spinning at near the speed of light. This would also mean that the black hole would have a bright accretion disk, which would appear to curve over the top and under the bottom simultaneously.
To simulate the accretion disk, the special effects team generated a flat, multicolored ring and positioned it around their spinning black hole. The end result showed that the warping effect it had on space-time would also warp the accretion disk – creating the illusion of a halo.
Comparing the image of M87’s SMBH to the rendering from Interstellar (see below), one can see some startling similarities. These include the central, shadowy regions and the bright accretion disks surrounding them, which lend them a sort of “ring of fire” or “eye of Sauron” kind of appearance.
Implications for Astrophysics:
As many astronomers have explained since the image’s release, the ability to photograph a black hole is opening up a new era in astrophysics. Much like the first-ever detections of gravitational waves, this accomplishment effectively allows scientists to detect and visualize phenomena that were either theoretical or could only be studied indirectly.
These include more radical tests of Einstein’s Theory of General Relativity. While many tests have been conducted over the past century to verify the effects gravity has on spacetime, the vast majority of these have involved planetary-sized or stellar-sized objects.
With the ability to visualize SMBHs, the scientists will be able to test the predictions of Einstein’s field equations in the most extreme environment possible. In the past, limited tests have been conducted by observing the behavior of S2, one of the stars that orbit Sagittarius A* at the center of our galaxy.
But with the ability to visualize Sagittarius A*’s accretion disk and shadow, astronomers expect to be able to learn so much more. Scientists also expect to learn more about how matter forms disks around black holes and accretes onto them, which is what allows them to grow.
In short, scientists are still not sure how material makes the transition from the rapidly spinning disk to the event horizon. While it is understood that over time, matter in the disk will lose energy and eventually fall in, scientists are uncertain what causes this loss of energy.
Because the matter in a disk is so dilute, traditional friction should not be possible, which suggests that some unknown force might be at play. With the ability to study two SMBHs and their event horizons, scientists will finally be able to test different theories.
In addition, scientists hope to learn why Sagittarius A* is relatively dim compared to SMBHs in other galaxies. In fact, some SMBHs generate so much energy from their rapidly spinning disks that their central region (their galactic nuclei) outshine the stars in their galactic disks many times over.
In fact, the presence of an Active Galactic Nucleus (AGN) is how astronomers have been able to determine that most galaxies have an SMBH at their center. By improving their understanding of the mechanisms that power debris disks and cause SMBHs to grow, astronomers hope to be able to answer this question at last.
All told, roughly 200 astronomers from around the world played a vital role in capturing the first image of a black hole. Bouman described the EHT Team as a “melting pot of astronomers, physicists, mathematicians, and engineers, and that’s what it took to achieve something once thought impossible.”
With additional facilities being added to the EHT network – not to mention regular improvements in terms of computation, imaging and information sharing – scientists expected to be able to see more black holes soon. The insight this will allow into our Universe is sure to be nothing short of mind-blowing!