Modelling reality

How thinking theoretically revealed the hidden secrets of Messier 87’s black hole

3 May 2019
What you’ll discover in this blog post:
  • How it feels to take an image of a black hole
  • How 60,000 simulated images uncovered the properties of the black hole
  • Why theorists are so important in major scientific discoveries
The recent release of the first “image” of a black hole can’t have escaped your notice. But how can we use this image to find out about the physical characteristics of the black hole? ...to unearth more details about these mysterious objects? The secret is in the simulations that theoretical physicists have spent years working on. Two theoreticians involved in the Event Horizon Telescope (EHT) tell us more.
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Luciano Rezzolla is a theoretician in the Event Horizon Telescope collaboration. He uses sophisticated numerical simulations to deduce the characteristics of the black hole.
Credit: ESO

Name: Luciano Rezzolla
Job: Professor of relativistic astrophysics and numerical relativity at Goethe University, Germany
Roles in the EHT project: Theoretician

What were the biggest challenges you faced whilst getting to this result?

Early on, it was already very clear that our task as theorists in the EHT collaboration was to provide a physical explanation for what would be observed using the telescopes, and to use sophisticated numerical simulations to deduce the properties of the black hole. We were always aware that this would be a huge challenge. So following this strategy, we performed the most extensive numerical exploration of the dynamics of the plasma surrounding the black hole, to figure out how it swirls around and shines when it actually falls onto the black hole.

Once we established what physical properties were needed to create a “best-match simulated image”, we could guess the real physical properties of the black hole.

We considered hundreds of different physical and astrophysical scenarios by studying different properties (masses and spins) of the black hole, but also different thermodynamic properties of the orbiting plasma. For each scenario we constructed thousands of synthetic images that could theoretically be the result of the complex bending and lensing of the light near the black hole. At the end of this effort we had built a library of more than 60,000 synthetic images!

We had thought that based on this huge image library, we would easily be able to decide which scenario was the correct one. But what we instead realised is that many combinations of physical parameters can lead to simulated images that, once blurred with the telescope resolution, would match the actual observed image very well.

This was a relief because it meant that whatever our conclusions on the properties of the observed image, these would be very robust. But although we were very confident that this was a black hole, we weren’t sure what its characteristics would be. This meant that we had to go back to the blackboard and sharpen the techniques we were using to compare the theory with the observations. In practice this has meant a lot of sleepless nights especially for Dr. Fromm, a member of our team in Frankfurt, who eventually managed to single out the simulated images that best matched the observations after exploiting a sophisticated genetic algorithm.

Once we established what physical properties were needed to create a “best-match simulated image”, we could guess the real physical properties of the black hole. Therefore we managed to explain the physical origin of the emission ring surrounding the black hole — a very rewarding and reassuring feeling!

Monika Mościbrodzka is co-coordinator of a research group that looks at the polarisation of the light from the ring surrounding the black hole. She also contributes significantly to theoretical modelling and interpretation of EHT data.
Credit: ESO

Name: Monika Mościbrodzka
Job: Assistant professor, Radboud University, the Netherlands
Roles in the EHT project: Co-coordinator of a research group that looks at polarisation of the light from the ring surrounding the black hole. Leader of theoretical research, major contributor to theoretical modelling and interpretation of EHT data.

What emotions have you been through whilst getting to this result?

My work as a theoretical astrophysicist in the EHT project focuses on modelling the appearance of black holes depending on what is around them. Over many years of my academic research prior to making the EHT observations I sort of got used to seeing simulated pictures of black holes.

After months of secret-keeping, we could finally speak openly about our black hole, this extraordinary wonder of nature, with the rest of the world.

Then when the EHT data was calibrated and ready for imaging, I joined one of the imaging teams out of curiosity. I sensed that this would be a perfect opportunity for a theorist to get in touch with reality. This experience turned out to be very rewarding. It is hard to find words that describe the emotions that I felt when I first saw the initial images of M87 last summer. These images were MIND BLOWING.

But as a theorist I felt an additional thrill when looking at the image. The black hole appeared almost exactly how I imagined it to be, just exactly how I had predicted it would look in my models of its host galaxy, Messier 87. So in some sense this experiment did not reveal something completely unexpected.

During the press conference in Brussels, I was honestly moved. After months of secret-keeping, we could finally speak openly about our black hole, this extraordinary wonder of nature, with the rest of the world. I feel extremely privileged to be a part of the EHT team that made such a big impact on science. It’s truly a life-changing experience.

The next step for EHT scientists is to analyse the polarisation of light from the ring that surrounds the black hole. This will give us critical information about the magnetic fields at the black hole's event horizon, helping us to understand the exact nature of the light that is visible in the image that we see. We could start to discover, for example, how exactly this light is produced.