Approach for How to Take a Picture of a Black Hole (proven right)

In a fascinating TED talk from 28.April 2017, Katie Bouman (researcher at MIT) explains her approach for how to take a picture of a black hole using the Event Horizon Telescope (EHT). This talk was given nearly two years before the unveiling first-ever photograph of a black hole on the 10th of April 2019 (ref. the photo above). The photo taken shows the supermassive black hole at the center of a neighboring galaxy known as Messier 87. The black hole lies about 55 million light-years from Earth and is billions of times more massive than the sun.

At the heart of the Milky Way, there is a supermassive black hole that feeds off a spinning disk of hot gas, sucking up anything that ventures too close, even light. We can not see the black hole, but its event horizon casts a shadow, and an image of that shadow could help answer some important questions about the universe. Scientists used to think that making such an image would require a telescope the size of Earth. This was until Katie Bouman and a team of astronomers came up with a clever alternative which entailed using an international consortium that linked eight radio observatories around the world called Event Horizon Telescope (EHT), which creates a computational telescope the size of the Earth.

• Each of the telescopes takes pictures from their location.
  
• Since there are very few telescopes as compared to the surface of the Earth, we get few measurements. However, as the earth rotates we get new angles and thereby new measurements.
  
• The pictures from the various telescopes are then used to “triangulate the most valid picture”, or rather reconstruct the full image using a special imaging algorithm.
  
• The algorithm looks for the most likely/reasonable possibilities of a valid image, that also fits the telescope measurements.
  
• As the algorithm is fed with data of what we are expecting to see (based on simulations), this could bias the result. To avoid this, the algorithm is also trained on datasets with e.g. astronomical images and everyday images as they have different features. This way different features from various types of images are imposed on the algorithm to see how this affects its reconstruction ability. If similar results is received regardless of the types of images provided as input, this builds the confidence that the algorithm is less biased and can be trusted to greater degree.
  
• This training can then be done by breaking familiar photos into small pieces and then have the algorithm try to reconstruct them, to check its accuracy.

Black Hole Basics

Stars shine because of the nuclear fusion reactions taking place at their cores. The reactions create an outward pressure that counters the inward pull of gravity. As a result, the star neither expands nor contracts. However, when a star’s fuel supply is exhausted and the outward pressure stops, gravity causes the star to shrink:

• If it is about the mass of our sun or a bit bigger, it will collapse until it is a roughly Earth-size entity, a white dwarf.
  
• Stars that are significantly larger will collapse into an ultra-dense entity, a neutron star.
  
• If it is really big, the collapse cannot be stopped, and a black hole is formed.

The speed needed to overcome the gravitational tug of a particular star or planet and move out into space is called escape velocity. In the case of a black hole, the escape velocity is greater than the speed of light. Since nothing can travel faster than light (ref. Einstein’s theory of relativity), the star disappears. A black hole is bounded by its event horizon, the imaginary sphere that represents the region where the escape velocity is exactly equal to the speed of light.

With light unable to escape, the remnants of the star appears black, which means black holes are invisible even with powerful telescopes. Yet physicists know black holes exist because they’re consistent with time-tested theories, and because astronomers have observed how matter behaves just outside a black hole.

Countless black holes are now thought to exist in the universe, including an estimated 100 million in our galaxy alone.