black holes - Picture of Sgr A*

As a lot of you may know, on April 10th we'll get to see the so said "first picture of a black hole" from the EHT. I'm no expert in observational astrophysics, so my questions are: how are they taking this radio image? Is it the same as the "old" radio telescopes with better quality, or is there a new technology behind this picture? Also, during an astrophysics course I was showed a video of some stars orbiting around something that the professor said to be Sgr A*. Isn't that already a photo of a black hole? Do we hope to see anything different? Lastly, will this have consequences on the theoretical study of black holes, or will it just be a better picture of what we already know?

Answer

It is the Event Horizon Telescope (Telescope of the Event Horizon), a joint project of various astronomical observatories around the Earth that have been coordinated to observe the surroundings of a region called Sagittarius A*. These observations would answer some practical questions about the black holes that have long intrigued us, questions like:

• it will be that the black hole has the correct size as predicted by the Theory of General Relativity.


• it will be that the event horizon (the boundary of the black hole) is circular (as predicted) or, in contrast, is oblong (stretched).


• it will be that the radio broadcasts extend more than you think.


• it will be that there is some other deviation from the expected behavior.

The EHT basically aims to convert the entire planet into a large radio telescope antenna. The wavelengths in radio have many advantages as radio waves pass through walls, they also traverse the galactic dust. We could never have seen the center of our galaxy at visible wavelengths, since there is too much material in between. But because of their long wavelengths, radio waves also need large antennas. The largest single-antenna radio telescope in the world has an approximate diameter of about 300 meters, but an image of the Moon produced by it would be more blurred than the image we see through a small optical telescope. A black hole is very far away and very compact so, taking a picture of the black hole in the center of the Milky Way is equivalent to taking a picture of a coin on the Moon, but with a radio telescope.

To take a picture of something so small means that we would need a rather large telescope, about 10,000 kilometers in diameter, which is impractical, because the diameter of the Earth barely exceeds 13,000 kilometers in diameter. The solution adopted by the EHT, is to coordinate the measurements made by radio telescopes located in places very far from each other. But even twice the available telescopes would leave large gaps in the data when they come close to functioning as a 10,000-kilometer antenna.

Normally, an astronomical signal will reach any two telescopes at slightly different times. Taking that difference into account is essential to extract visual information from the signal, but the Earth's atmosphere can also slow down radio waves, exaggerating the differences at the time of arrival, ruining the calculation on which the image depends on interferometry. Scientists then adopted an ingenious algebraic solution to this problem: if you multiply the measurements of three telescopes, the extra delays caused by atmospheric noise compensate each other. This means that each new measurement requires data from three telescopes, not just two, but the increase in accuracy compensates for the loss of information.