Behind the black hole

Name: Sera Markoff

Job: Professor of Theoretical High Energy Astrophysics, University of Amsterdam, the Netherlands

Roles in the EHT project: Member of the Science Council, co-coordinator of the Multiwavelength Working Group, co-coordinator of Proposals Working Group and leads a research group that contributed to theoretical modelling and interpretation.

What has been the most exciting part of this project so far?

Without a doubt, the most exciting part of the project so far was to make the big discovery — to show the world that black holes really exist, and to quite literally be able to gaze down into that sinkhole. I have been working on modeling black holes in one way or another for most of my career, and I think that one gets a bit blasé at some point, since we use the concept of black holes all the time without having ever actually seen one directly. To really “look it in the eye” is fascinating but also a bit maddening! And now I dream of seeing what it looks like close up, without the distortions of a telescope in between! I want to understand how such a thing can be possible, when our understanding of physics at the moment is not complete and cannot yet explain gravity or black holes at a quantum level.

I also found working with a big team focused on a single, major goal very exciting. There were so many researchers, particularly PhD and postdoctoral students who dedicated a huge amount of time to making this project a success, and I am very happy to see it pay off for them, since it will boost their careers massively.

Name: Heino Falcke

Job: Professor of Astroparticle Physics and Radio Astronomy, Radboud University, the Netherlands

Roles in the EHT project: Coiner of the term “black hole shadow” and proposer to try to image a black hole using millimetre-wavelength Very Large Baseline Interferometry (VLBI). Chair of the EHT science council and co-Principal Investigator (together with Luciano Rezzolla and Michael Kramer) of the European Research Council Synergy Grant BlackHoleCam that co-funded the EHT.

How did it feel when you saw the first image of the black hole?

Twenty-five years ago, back in the pioneering days of millimetre-wavelength VLBI, I was doing my PhD at the Max-Planck Institute in Bonn. Modeling the black hole at the centre of the Milky Way, I realised that light of millimetre-wavelength or below would be emitted from close to the black hole’s event horizon. Alas, black holes are surprisingly tiny, so the event horizon seemed too small to see, even with an Earth-sized telescope.

But then, one lonely afternoon in the library, I stumbled across an article that described how a black hole would look much bigger when illuminated from behind. I was electrified. I hadn’t considered gravitational lensing — that a black hole could actually magnify itself due to the bending of light by its own mass. This would make it look much bigger!

I worked with two other scientists, Eric Agol and Fulvio Melia, to calculate what a black hole would look like if it was engulfed by a glowing transparent region and, lo and behold, we found that a dark area would appear, surrounded by a bright ring that would be just large enough to be detected. We called the dark area the “shadow of the black hole” and claimed it could be detected within the following ten years!

Well, not quite. But 19 years later my own PhD student, Sara Issaoun, showed me the first raw data from the EHT project. The plot was a complicated and incomplete one-dimensional mathematical transformation of an image. But doing the mathematical inversion in my head, as we have all learned to do during this project, my heart started beating faster: this could be a ring!

Weeks later, we could finally make the actual image and there it was — the shadow inside a ring. All these years after predicting that it would be possible to image a black hole in this way, this huge collaboration of scientists had finally done so! For an hour I felt like I was hovering above the ground, but then it hit me that we still had many rough months to go before we could be certain. I sent up a brief “thank you” prayer to heaven and continued the day with a smile on my face.

Name: Sara Issaoun

Job: Graduate student at Radboud University, the Netherlands

Roles in EHT project: EHT observing staff at the Submillimeter Telescope (SMT), core contributor in EHT data processing and calibration, active contributor in imaging efforts

Describe some of the emotions you went through whilst getting to this result.

Although I’ve gone through many emotions during this project, the most common is probably exhaustion! During our 2017 observing campaign at the SMT in Arizona, I was excited to be carrying out observations, hearing the equipment roar as blinking green lights indicated the successful collection of data. And the weather was excellent, meaning that we could observe on multiple days in a row. The downside to this? Back-to-back 16 hour observing shifts, with preparation time in between, and very, very little sleep. Combined with the high altitude, this made it an exhausting expedition. But then I saw the messages rolling in from Chile, the South Pole, Spain, Mexico and Hawaii, which made me feel part of a truly historic moment; all these telescopes and people, all staring towards the centre of Messier 87 — just one galaxy in amongst several trillion that exist in the Universe.

After we packed up our recordings and drove down the mountain, it took a few months before we got the results of our observations. But when we heard that the telescope had worked well, and that we had worked well, I felt extreme relief.

But it also meant that our data was ready for calibration, which involved a lot more hard work, exhaustion and stress. I will never forget the day when I first saw the fully calibrated data; the quality was so high that it took only seconds for me to understand that this could lead to a groundbreaking image. Four imaging teams worked separately to create the final image, and I was part of one of these teams.

A mere few minutes after I started processing the data, I saw the ring structure appear. It was jaw-dropping, even thrilling. Six weeks of hard work passed, during which we perfected our image and improved our understanding of the data, before all the imaging teams met at a workshop in July 2018. We were all extremely anxious to see if everyone had seen the same structure. Once again, it turned out that everyone saw the same thing, even though we had all been using different software. This was real. This was it. The room filled with applause and laughter and general awe at being part of this incredible project. We were fully aware of the huge amount of work ahead of us, to understand what we were seeing and convince the rest of the community, but that moment was really special.