Astronomer Reinhard Genzel is one of the first to use stellar motions to prove the existence of a supermassive black hole in the center of our galaxy – he was awarded the Nobel Prize in 2020 for this. With the first image of a black hole by the Event Horizon Telescope, astronomers have now made this image directly visible for the first time. In an interview, Genzel explains what the recording reveals about Sagittarius A* and how well it fits in with expectations.
Using the Event Horizon Telescope (EHT), astronomers have now made the black hole at the center of our Milky Way visible for the first time. Astronomer Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Garching, has been targeting gravitational giant ‘Sagittarius A*’ for more than three decades. Using the European Southern Observatory’s Very Large Telescope in Chile, he and his team observed the motion of stars orbiting the invisible object like moths around light. In 2020 he was awarded the Nobel Prize in Physics for his results.
In the interview, Genzel now ranks the latest result of the EHT collaboration.
How would you rate the image of the galactic center against the background of your measurements?
Reinhard Genzel: The image is a very good result and a great confirmation of our ultra-resolution observations in infrared light. From the orbital motion of stars around the black hole Sagittarius A *, we determined its mass with an accuracy of 0.1% and its distance to Earth with an accuracy of 0.2%. From this we conclude that the radius of the shadow of the black hole is 26 microarcseconds.
The size of the shade measured by the EHT colleagues agrees well with this prediction within measurement errors. The radius of the body’s so-called event horizon is ten microarcseconds – at this angle one euro coin will appear on the moon. This value also fits perfectly with the four-million-sun black hole model defined by our gravitational team.
What can be inferred from this data?
Combined with what we know so far, we can now confidently rule out many speculations that contain alternative physical explanations for the black hole. These include, for example, mass concentrations made up of heavy bosonian or fermion elementary particles of the same mass as the black hole, but with a much larger diameter. Or very dense star clusters that gather in the heart of the Milky Way.
How are your observations different from those using the Event Horizon Telescope?
Although we also use the method of interferometry, in simple terms overlaying light, we are working in the infrared range with an instrument called Gravity, mounted on the European Southern Observatory’s Very Large Telescope interferometer. Its resolution of detail is not naturally as high as that of the Event Horizon Telescope, which is practically the same as a hypothetical Earth-sized telescope.
But this also has its good side: we were able to observe fluctuations in the brightness of the gas orbiting the black hole at a short distance. However, this was a problem for the Event Horizon Telescope thanks to the extremely high detail resolution of 20 microseconds, because the images became blurry due to the constant contrast of the material.
First your results, now the picture – you might think there is nothing else to notice…
not at all! For example, we would like to know how fast a black hole is spinning, i.e. what is its spin. This cannot be deduced from the photo. The slope of the level of rotation also remains uncertain.
Why is knowing these quantities important?
The general theory of relativity says that black holes are characterized only by mass and angular momentum, that is, rotation. In addition, the so-called lack of hair theory applies to these organisms. This means that the black hole at the event horizon, that is, its front, has no local structure at all. In short: if you know the two mentioned quantities, mass and spin, you are done describing a black hole perfectly.
Will you solve all the puzzles then?
Not quite, because, according to the theory, there is a so-called singularity at the core of a black hole. According to the theory of relativity, this is a point with an infinitely high mass and an infinitely strong gravitational field, where spacetime is no longer defined. These singularities are not accessible and I don’t see how anyone can study them now or in the future. With this problem I have to pass.
Source: Max Planck Society