A black hole is a region of spacetime which is so dense that not even light can escape its gravitational pull. All the “images” of black holes we have today are just artist illustrations and not real images. So, how are scientists going to take the first ever image of it?
Since no light can escape from a black hole, we can’t see them directly. But because of their huge gravitational influence, its presence is detected. Black holes are often orbited by stars, gas and other material in tight paths that become more crowded and frantic as they’re dragged closer to the event horizon (the boundary of region from which no escape is possible). This creates a superheated accretion disc around the black hole, which emits vast amounts of radiation of different wavelengths. Astronomers are aiming to capture a black hole’s event horizon. This will put test to some basic predictions of Einstein’s General theory of Relativity and open new frontiers in our understanding of gravity.
Two international collaborations of radio telescopes have linked up to create Earth-sized virtual telescopes: the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA), working at different wavelengths with the goal of capturing first ever image of black hole at the centre of our own Milky Way galaxy. The Event Horizon Telescope (EHT) collects light from the black hole using a small number of telescopes distributed around the Earth. Astronomers will use a technique known as Very-long-baseline Interferometry (VLBI), where telescopes thousands of kilometres apart can link together and act as one.
They will target the supermassive black hole at the heart of the Milky Way, Sagittarius A*. It is 26,000 light years away from the earth and 4 million times more massive than our Sun. The Global mm-VLBI Array is currently investigating the process of how gas, dust and other material accrete onto supermassive black holes, as well as the formation of the extremely fast gas jets that flow from them. The Event Horizon Telescope, on the other hand, is working towards a different goal: imaging the shadow that event horizon casts on the surrounding plasma.
To capture something 26,000 light years away is like capturing an orange on the surface of the moon. This requires a telescope comparable to the size of Earth. Therefore, we need telescopes all around the globe working together. VLBI has a long network of telescopes around the world but still they are collecting light at only a few telescope locations. Some information is still missing about the black hole’s image, so they use imaging algorithms to fill in gaps of data which is missing.
After the data is taken, there are still infinite number of possibilities of images which are consistent with the equations and data. But not all images are equal, some are more reasonable. For example,
Using these algorithms they reconstruct the image from the data collected from all the telescopes.
The observations with the EHT and the GMVA were completed in April 2017. The data collected by the antennas around the world has been sent to the US and Germany, where data processing will be conducted with dedicated data-processing computers called correlators. The data from the South Pole Telescope, one of the participating telescopes in the EHT, had arrived at the end of 2017, and then data calibration and data synthesis was begun in order to produce an image. This process would take several months to achieve the goal of obtaining the first image of a black hole, which is eagerly awaited by black hole researchers and the general astronomical community worldwide.
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