Improving the ability to observe the black hole at the center of the galaxy will make it possible to test Einstein's theory of general relativity

Says Dr. Shahar Hadar, a theoretical astrophysicist from the University of Haifa and Oranim College who also works in the development of the next generation of the Event Horizon Telescope. He also explains why it took five years to produce the photo published this week and what is planned to be done to improve the resolution

"Improvements in the capabilities of receiving radio signals and the dedication of dedicated radio telescopes will allow us to obtain a high-resolution image of the black hole at the center of the galaxy and in the future even a video, and to test the theory of relativity in the most extreme extreme conditions in the universe. Says Dr. Shahar Hadar, a theoretical astrophysicist from the University of Haifa at the Oranim campus, explaining why it took five years to produce the photo published this week and what is planned to be done in order to improve the resolution. 

The spectacular image of the black hole at the center of the Milky Way - Sagittarius A* published in a series of articles in the Astrophysical Journal on May 12, 2022, was actually taken on April 6, 2017.

Why did it take so long to decode the data and produce the image? What exactly is the ring of light surrounding the black hole in the area known as the event horizon?

Dr. Shahar Hadar, physics lecturer at the University of Haifa - Oranim Campus, is a theoretical astrophysicist. Although he does not have an official role in the team, but as someone who researches the issue, since his time as a postdoctoral fellow at Harvard University, he specializes in black men, and according to him, is in close contact with the researchers of the "Events Partnership."

In fact, Dr. Hadar is already working on the follow-up project - the next generation of the event horizon telescope which will include a significant expansion. The antennas of the radio telescopes will be designated for the event horizon project. In the current study, the scientists were actually given limited observation time at eight telescopes around the world. They had to make the observations at the same time in all of them, so they had to wait for the time when both the telescopes would be at their disposal and in all the sites where they are placed there would be good weather.

One antenna cannot form an image. The way to create an image like the one we saw is to combine what was measured in several such plates and make a correlation between each pair of plates and thus assemble a reconstruction (reconstruction) for the image. Another problem that delayed the publication of the research is the enormous amount of data that each such radio telescope produces (apparently it was necessary to wait until the big data analysis capabilities reached AB), and the need to transfer it to one place (physically, using hard disks, nevertheless 2017, a .B.)

How to improve? One way is to take one of the plates and put it into orbit around the Earth - one plate will be enough because it will be in contact with the plates on the Earth. As a principle, they did not send a radio telescope into space, there is no need because the radio frequencies are low and the atmosphere does not interfere with them. But this is another distant vision.

The idea behind NGEHT (next-generation Event Horizon Telescope) is to add many telescopes for use by the Event Horizon Partnership, and increase the frequency. This way it will be possible to improve the resolution, maybe not by a factor of a thousand, but respectable enough. The antennas will be aimed at the black hole in the center of the galaxy for a long time (as opposed to the current situation of receiving limited observation times). Observation over time will allow us to produce better statistics. Upgrading the viewing time will make it possible to improve the sensitivity and even "take a video" of the black hole - which is part of my research.

The photon ring

The most prominent factor in the picture is the photon ring. When the photons (particles of light) approach the black hole, its enormous gravitational force causes some of them to circle it a different number of times - depending on the direction and angle from which they come. "Thus, the black hole acts as a lens that concentrates the light passing around it, and the image obtained from the concentration of this light is a bright ring of light. Or, more correctly, the study showed that it is in fact countless rings of different and varying thickness placed on top of each other," explains Dr. Hadar, who adds that measuring the effect of photons will make it possible to better investigate Einstein's theory of general relativity, and for this there is a need, as mentioned, for measurement over time.

Proof of relativity under boundary conditions

"When light rays enter the vicinity of the black hole, they collect more photons. What is beautiful about the photon ring is that it is universal. Unlike the gas in the accretion disk surrounding the black hole, it has a turbulent behavior that is sensitive to many astrophysical parameters, which complicates the pattern of light it creates. On the other hand, the way in which the black hole bends the light rays, or lens (from the language of lens), the image is much simpler and it depends only on the mass and the angular momentum of the black hole, according to the theory of general relativity. This universal priming determines the shape of the photon ring.

In addition, any change (fluctuation) in the material surrounding the black hole causes the image to change at certain points, the places and times are different. Since the photons circle the black hole many times, we see points along the ring of photons "sparkle" in an apparently complicated way. But photography over time will allow us to actually see layers of images and the movement of these points will allow us to derive many insights about the black hole - for example, what is its angular momentum. This information about these fluctuations will make it possible to try to test Einstein's equations even under the most extreme conditions that exist in the universe."

Could the Webb Space Telescope, which can observe the edge of the universe, help photograph the black hole at the center of the Milky Way?

Dr. Hadar: "The Webb space telescope operates in the range of visible light and mainly infrared. Webb can be directed to look towards the center of the galaxy, but it will not be able to 'talk' to the EHT radio antennas in real time."

More of the topic in Hayadan:

• Astronomers reveal for the first time a "radio image" of the black hole at the heart of the Milky Way
• The magnetic field of a black hole was photographed for the first time
• "Without the observations of Ganzel and Gaz (2020 Nobel Prize winners), the whole subject of black holes would have remained theoretical"
• Roger Penrose and the two black hole detectors at the center of the galaxy won the Nobel Prize in Physics For 2020