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A picture is worth a petabyte of data

Speed read
  • The image of the black hole is the result of a virtual telescope the size of the Earth
  • Multiple telescopes had to be synchronized to watch the same part of the sky
  • Data analysis and image reconstruction required cloud computing

There are few unifiers quite like the night sky. From ancient Mayan observations of the heavens to Galileo’s defiance of the Church, gazing into the world above our heads has long been a focus of human endeavor. And modern astronomy now requires collaboration from researchers around the world.

Zooming in. This animation of M87 shows its accretion disk, ray-traced images, and blurring due to instrument noise, and demonstrates how the image obtained with the Event Horizon Telescope fits into a bigger picture. Courtesy EH Telescope..

Few projects exemplify this principle better than the Event Horizon Telescope (EHT.) Created to directly observe the immediate environment of a black hole, the EHT aims to gain a new perspective on the study of general relativity. The EHT’s most famous recent accomplishment is the amazing image taken of a black hole in the M87 galaxy that was announced on April 10, 2019.

The reason teamwork was so important has to do with how the image was created. Dan Marrone, an associate professor in the Department of Astronomy and Steward Observatory at the University of Arizona (UA), explains that the collaboration starts with the technology behind the image.

<strong>The first image of a black hole</strong> shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. Courtesy: Event Horizon Telescope Collaboration.“We're using a technique called interferometry, in particular something called very-long baseline interferometry (VLBI) ,” says Marrone. “Interferometers are ways to combine separate telescopes into one big telescope. VLBI can connect telescopes that aren't even at the same location.”

By combining the data received from telescopes all over the world, scientists were able to piece together the EHT, which acts like a telescope the size of the entire Earth. As you might guess, stitching together complex devices like this isn’t easy.

The right tools for the job

One of the requirements for a VLBI device like EHT is for all the linked telescopes to be working in harmony. Every telescope is different in size and varies subtly in how it’s pointed at the sky. Additionally, the scientists have to be very specific in the timing of their observations.

“Everyone has to be looking at the exact same time in order to see the same wave current,” says Marrone. “All of the telescopes all over the world, at the start of our first observation or even slightly before, were synchronized with GPS.”

<strong>The South Pole Telescope</strong> is located at the Amundsen-Scott South Pole Station Antarctica, the most extreme location of the eight telescopes in the Event Horizon Telescope Array. Courtesy Junhan Kim, University of Arizona.Marrone and the team also had to rely on atomic clocks to generate a high-fidelity recording of the light arrival they were trying to capture. But as you might imagine, close observation of a black hole generates a lot of data.

“In the experiment, we collected 5 petabytes,” says Marrone. “But that's from about eight sources. Probably most of a petabyte is just on M87 and then we distill that down. The image is natively tens of kilobytes of true information content. So it's quite a distillation.”

But data collection is just the beginning. Analysis of that data and subsequent image reconstruction requires computation. In this case, that means the cloud.

Although the final infrastructure was ultimately built on Google Cloud, that work wouldn’t have been possible without early prototyping of the data analysis pipeline on Jetstream, a cloud-based tool that gives scientists access to computing and data analysis resources.

“We used Jetstream to prove that cloud computing is efficient in analyzing EHT data,” explains UA astronomer Dr. Chi-Kwan Chan. “For the data analysis and image reconstruction works for the EHT, it's overkill to develop supercomputer codes to perform these works. But laptops and workstations are clearly not enough. This is the place that the cloud such as the Jetstream shines.”

Why this black hole?

With so much work going into the creation and distillation of data, the question remains as to why the black hole in M87 was chosen. As it turns out, scientist didn’t have many options to choose from – and they were lucky to have them.

<strong>Getting their hands cold.</strong> Dan Marrone and Junhan Kim install a mirror on the roof of the South Pole Telescope in Antarctica. Courtesy Junhan Kim, University of Arizona.“It very easily could have turned out that there were zero black holes in the universe that were big enough in the sky for a telescope the size of the Earth to observe,” says Marrone. “But, fortunately, it turns out there are two that are just barely resolvable.”

The other black hole scientists were interested in was Sagittarius A*, which is actually at the center of the Milky Way. The scientists also studied this black hole, and that data is currently being analyzed, but M87 was approximately 1000 times larger in mass and seemed a more likely candidate for imaging.

Despite the good fortune in finding a black hole to image, Marrone knows that luck played only a small part in this astronomical milestone. In reality, it was the hard work of every single person on the project that made it all happen.  

“For many science journalists, the story is always about the one genius, the great hero, and that's not this story,” says Marrone. “It's probably not any story, but it's certainly not this story. This was a big collective effort and a lot of people have worked very hard to take this picture.”

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