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A simulation of what the Event Horizon telescope could find. (Credit: Andrew Chael)
The humanity could soon get its very first image of a black hole. Scientists at the Horizon Event Telescope (EHT) announced this week that they will hold a press conference on Wednesday, April 10th. They should reveal the results of their multi-year quest to capture a black hole in front of the camera.
We still do not know what this image will look like. But scientists think they have a pretty good idea of what is a black hole should look like. For years, astronomers have been doing black hole simulations based on the laws of physics and some basic assumptions about what happens next to the singularity. The resulting images provide a glimpse of what we might expect to see next week when reality is unveiled.
A black hole of interstellar quality, these are not. But they are also part of the most realistic representations of the black holes that we currently have.
Black hole simulations
Two simulations of M87, the image on the right are closer to what we could see. (Credit: Andrew Chael)
This first image was produced by Andrew Chael, an astrophysicist from Harvard University involved in ISE. He used data that simulates what scientists think the telescope will collect to create the above simulation of M87, one of the black holes examined by EHT. The image on the left comes from a simulation of the black hole; The right image is also a simulation, but uses data that more closely matches what the HEH will likely receive.
Simulated images of Sagittarius A *. The leftmost image is a pure simulation, while the following three describe how diffusion can influence the image. (Fish et al. / Astrophysical Journal)
This series of images shows a representation of Sagittarius A * (pronounced A star), the supermassive black hole located at the center of our own galaxy. The first is based on a simulation and the next three incorporate some of the signal scattering that will certainly occur when the light crosses the galaxy to reach us. By using a series of filters, scientists are trying to reduce the spread as much as possible. The two final images include some of these filters.
Anatomy of a singularity
The bright ring visible in all these images is the accretion disk of the black hole, where the gas sucked towards the center is heated until it begins to shine. The disc looks brighter on one side because of the Doppler effect. The light that goes towards us seems to go faster from our point of view, while the light that is moving away from us goes more slowly. So, if we look at an accretion disk that moves from left to right, the light will appear brighter on the left and weaker on the right. The black area in the center of these images is the black hole itself – a region of space where gravity is so intense that light can not escape, which makes it black.
The most realistic of these images are also the least exciting, simply because they take into account that the black holes that the EHT is trying to imagine are so far away. Sagittarius A * is 26,000 light-years away, while the much larger supermassive black hole of M87 is about 55 million light-years away. At these distances, the two objects have an almost minute appearance. Put an apple on the moon, come back to Earth and look at it – that's about all the amount of sky that the two black holes they are trying to imagine take each one. The HEH compensates using a network of observatories from around the world working together, creating a camera the size of our planet.
But, despite this, the EHT is barely big enough to have a picture of these two black holes. Simulations based on realistic data from the EHT produce stained light halos, which is far from being science fiction. This is the reality when you take pictures from so far, of course. For scientists, the true power of the image will be to see its existence. Do not forget that we have never seen a black hole. Until recently, and despite their prevalence in science fiction and science journals, black holes were theoretical. LIGO, the gravitational wave laser interferometer observatory, has already used gravitational waves to detect colliding black holes. But it's not the same as seeing one with our own eyes.
When the image of the EHT is finally available, astronomers will finally be able to compare their simulations to reality. The resulting data will provide invaluable information on how the laws of physics behave in the extreme gravitational environment of a singularity.
The simulations themselves will also play an important role in obtaining this final image. Because scientists are unable to cover the entire planet in observatories, there are gaps in the data collected by the EHT. This means that the results obtained can be interpreted in different ways, depending on what scientists think are in the gaps in their data. Using simulations to guide them, researchers will be able to formulate the most informed hypotheses possible to give us an image of what was previously invisible.
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