Imaging 55 million light-years away

We are fortunate to be witnessing a “eureka” moment of humanity. It opens new doors to our understanding of the universe and subsequently our own self. 100 years ago Einstein’s theory of general relativity predicted the formation of black holes. However, Einstein himself denied the existence of black holes several times. He believed that mass would not allow itself to be compressed to the point of collapse. Thus over the course of time researchers had been trying to “see the useable”. They did find indirect evidence of the black hole’s presence. For instance, capturing the strong X-ray flares, “the burps of the blackholes”, which is believed to be released when black holes consume large matter (like a star). Or by observing trajectories of stars and gas that seem to circle around very dark spots in the sky. Or by hearing their whispers. Yes, you read it right! Just as sound waves disturb the air to make noise, the collision of black holes unleashes massive gravitational waves which disturb the fabric of spacetime to push and pull matter. Scientists had captured these ripples in a spacetime with LIGO and VIRGO experiments in 2015. However, no direct image of a black hole was ever obtained till date.

The problem in imaging blackholes is that these glories of nature behave in a similar manner as some gorgeous humans do. They consider themselves non-photogenic and therefore try their best to remain away from any flashes by being in dark (not letting any light get reflected by them), hiding behind other bright objects and being extremely silent. Additionally, due to their distance, imaging a black hole is like imaging an orange on the surface of the moon. This made them undetectable to date. But in the words of Mick Jagger, “You can’t always get what you want but if you try sometimes, you just might find what you need”. And on April 10^th, 2019 we found what we needed – the first-ever image of a black hole (Figure-1). 

Figure 1. The black hole at the centre of M87 galaxy. (source: EHT Collaboration)

A black hole basically has a singularity at its heart, the point in space where all laws of physics are invalid. Time and space cease to exist. It is perhaps the most mysterious and interesting place in the universe. Singularity is surrounded by the event horizon. It is the radius around singularity within which the black hole’s escape velocity is greater than the speed of light. In other words, beyond this point, even light cannot escape, which makes them literally “black”. The radius of the event horizon for a non-rotating black hole is called the Schwarzschild radius (for rotating black hole it's slightly different). Theoretically, any amount of matter will become a black hole if compressed into a space that fits within its corresponding Schwarzschild radius. For the mass of the Sun, this radius is approximately 3 kilometres and for the Earth, it is about 9 millimetres. The event horizon is surrounded by a disk of bright light known as the “Accretion disk”. It is created by gas and dust swirling around the speed of light, creating intense radiation and being heated by extreme friction to millions of degrees, thereby getting transformed into plasma. The captured image of the black hole with the Accretion disk, whose one side is brighter than the other, suggests a rotation of light. Due to the Doppler effect of the rotating light, the one that’s coming toward us appears brighter, and the light that’s moving away doesn’t appear as bright. The Accretion disk has a stable inner circular orbit lying around a distance of 3 Schwarzschild radii beyond which everything falls into the hole. However, light photons being massless orbits at 1.5 Schwarzschild radius (forming the photon sphere), but their orbit is unstable. So eventually light either emits out perpendicularly or is pulled into the black hole.

Figure 2. Simulated image of a black hole. (Source: ESO)

The smallest size that can be captured depends proportionally on the wavelength of light being captured and inversely proportional to the telescope size. Thus to capture such an image of a black hole, the telescope of earth’s size would be required. Event Horizon Telescope (EHT) is an international collaboration that creates a virtual telescope of similar scale by networking across 8 observatories around the globe - Mexico, Chile, Hawaii, France, Spain, Arizona and South pole. Each observatory is like a small mirror in an earth size disco ball which collects light to form the image. However, we only have few mirrors (few observatories) available which was not sufficient to get the image. But since the earth rotates, it basically creates mirrors at different locations and thus making it possible to collect measurements. Thereby, by taking repeated readings as the earth spins, researchers captured more angles from these telescopes.

Figure 3. (a) An earth sized a telescope with the surface of mirrors would be required to image a black hole in M87 (b) Each observatory is like a small mirror, however, insufficient to generate an image (c) Rotating earth rotates the observatories (mirrors) alongside and enables more measurements (source: Katie Bouman at TEDxBeaconStreet)

Making heterogeneous arrays of telescopes work together required lots of hardware updates and to link them atomic clocks were installed in each of these telescopes, in order to ensure consistencies and precision in data collection. Researchers worked over a decade to link all the observatories and finally in April 2017, all dishes of EHT stared towards the galaxy 55 million light-years away named M-87 in the Virgo constellation. Within a span of a few days, the researchers collected thousands of terabytes of data, which is more than any other scientific experiment in history. The data was then flown from different parts of the world to a central facility where algorithms were to add the missing pieces of the puzzle together to form the final big picture. This process is called interferometry.

Crunching 5 petabytes of data to generate the shown image of the black hole of few kilobytes took 2 years of arduous work. But it is worth every penny. It is a remarkable achievement. What we see in the image is a photon that fought against the most extreme forces of gravity, the extremely hot matter being ripped apart as it falls into the event horizon, and emanating from the uttermost challenging environment of the universe, propagating 60,000 years in the M-87 galaxy, followed by another 55 million light years in the intergalactic space before winding in our telescopes and now in front of our eyes.

Interestingly enough, the shape of the black hole's shadow is found to be consistent with the estimations of the Einstein theory of relativity. It is for this reason that the observed image looks similar to the simulation generated images which were depicted in movies like “Interstellar”. But this is just a stepping stone to a new era of understanding the most interesting place in the universe, where neither quantum mechanics nor relativity works. With higher resolution images, several breakthroughs await.