Seeing into a black heart

Black holes have fascinated me since I was a teenager: enormous, powerful entities that we couldn’t see, sucking everything in, playing by different physical rules to anything known before.  

Some are so massive that they govern the dynamics of galaxies, yet we cannot directly observe them, only ‘seeing’ infalling matter that is superheated, producing electromagnetic radiation across all wavelengths.  They are like giant cosmic plugholes, swirling space and time around their event horizons, trapping all matter and light that strays too close.  

Black holes came to life during my PhD in Astrophysics, when I took pen and paper equations and solved them on supercomputers, creating images and theoretical predictions of what the light around a black hole would look like, if we had telescopes large enough to resolve it. To produce an image of a black hole, one must solve the equations describing the trajectories of the light (i.e. geodesics), the evolution of the matter which produces this light, and ones that describe the attenuation of this light as it travels though the matter itself.  

There are proposals to place radio telescopes in space, creating a telescope even larger than the Earth to achieve much greater resolutions 

Unlike conventional ray-tracing and radiative transfer, the light around a black hole no longer travels in straight lines and is bent, in addition to being absorbed, re-emitted and scattered. I calculated how light moved using radiative transfer of light in strong gravity, using Einstein’s theory of general relativity. 

Earth-sized telescope 

I joined the Event Horizon Telescope (EHT) in 2014 a few months into my first postdoctoral position, working within the theory and imaging teams. The EHT is a global collaborative project presenting an innovative solution to the longstanding problem of not having a telescope large enough to ‘see’ a black hole. 

The problem is solved by creating a global network of radio telescopes. Earth’s atmosphere is optically thin at the radio frequencies at which the EHT telescopes operate, allowing the radio signals from near the event horizons of supermassive black holes to be detected. While no one single telescope has the resolution to image a black hole, by combining data from several different radio telescopes around the world, this data can be combined to create an image as if obtained from a single radio telescope the size of the Earth.  

This is a technique called Very Long Baseline Interferometry (VLBI). The signal from the black hole is recorded at these telescopes at slightly different times due to the telescopes’ different geographical locations. Atomic clocks record the precise arrival times at each telescope: these times are used to sync the data once they arrive at central data processing facilities. 

To gather, process and interpret this data, the EHT combines efforts from teams from over sixty research institutes worldwide. There are several working groups, each working on different aspects of the science behind the project: there are engineers, astronomers, physicists, mathematicians, computer scientists, and even philosophers. There are people who are responsible for retrofitting old telescope hardware with new recorders, people who work at the telescope sites and perform the observations, those who create theoretical predictions, and those who analyse and correlate the petabytes of data recorded at each telescope and combine it. This data is put together to create an image of a black hole that is then compared with theoretical predictions to aid in its interpretation.

Huge, yet invisible  

The final image providing the first direct evidence of black holes’ existence was released in April of this year. It was of M87, a supermassive black hole 55 million light years away, weighing 6.5 billion times the mass of the Sun. This is one of the largest known black holes in the Universe. It combined data collected from eight telescopes globally. The first images were created in July 2018, but it took many months for us to test and cross-validate the different image reconstruction algorithms used to produce the image to ensure the image features were robust and not an artefact of any particular choice of algorithm. 

The 2020 Breakthrough Prize in Fundamental Physics, was awarded to all EHT researchers involved in the publications surrounding the first black hole image of M87. The three million dollar prize from the Breakthrough Prize Foundation and its founding sponsors – Sergey Brin, Priscilla Chan and Mark Zuckerberg, Ma Huateng, Yuri and Julia Milner, and Anne Wojcicki – will be shared equally between all of the three hundred and forty-seven EHT researchers. 

What about future steps? Our theoretical predictions using Einstein’s general relativity are, so far, in keeping with what has been seen, but we would like to test this further to understand the physical properties of black holes. There is a finite resolution to the image produced by the EHT: to further test our predictions we will need to have more telescopes on Earth in order to provide better coverage of a black hole as the Earth rotates. There are also proposals to place radio telescopes in space, extending the EHT to even longer baselines, creating a telescope even larger than the Earth to achieve much greater resolutions than possible on Earth alone.

We have also imaged the black hole in our Galactic Centre (Sagittarius A*), over two thousand times closer but nearly fifteen hundred times smaller than M87. Its place in the centre of our galaxy presents its own unique set of challenges. We must not only look through all the interstellar dust between ourselves and the black hole, which scatters light produced near the black hole, but also account for the rapid variation of the black hole’s immediate environment, as this is moving over a thousand times faster than that of M87. Stay tuned! 

Author: Dr Ziri Younsi, Astrophysicist, UCL-MSSL. He was part of the Event Horizon Telescope collaboration behind the image of the supermassive blackhole M87.