The Big Question with Yuri Kovalev, astrophysicist

Yuri Kovalev is an astrophysicist with the Lebedev Physical Institute of the Russian Academy of Sciences in Moscow. He also heads the Laboratory of Fundamental and Applied Research of Relativistic Objects of the Universe at Moscow Institute of Physics and Technology (MIPT). Much of his research has focused on the study of active galactic nuclei. In the past, he has been involved in the launching of a satellite, RadioAstron, designed to observe objects in space, in particular — you guessed it — active galactic nuclei and quasars. Of course, RadioAstron studies much more than that and operates according to an open sky policy, meaning anyone can apply to take advantage of its impressive capabilities.

What is the biggest question facing your field today?

The biggest challenge faced by myself and my colleagues is discovering black holes. There are, of course, many theoretical predictions of their behavior and properties. And the presumed existence of black holes underlies many models. However, there is a reason these elusive objects are called “black”: It is impossible to see a black hole, as it absorbs light. Our best bet in terms of indirect observation is to look for its shadow. Any object that absorbs light casts a shadow, because it does not let light through and does not itself emit light. This shadow should be possible to detect. Besides, the so-called event horizon must result in a sharp edge of the black hole.

Allow me to explain: School physics tells us that for an object in an Earth orbit to fly off into space, it has to achieve what’s known as the escape velocity. The same goes for objects orbiting a black hole, but there is one crucial difference: Within a certain distance of its center, the escape velocity is greater than the speed of light. Now, relativity says that nothing can go faster than the speed of light. As a result, not even light can escape from within the event horizon, so we expect to see a sharp edge.

The mass of the central object in our galaxy, the so-called Sagittarius A*, is relatively easy to estimate by observing the stars that orbit it. All that remains is to prove that what lies there at the center is in fact a black hole. To do this, we need to observe its shadow.

Why is it significant?

This year’s Nobel Prize in physics was awarded for the discovery of gravitational waves, which confirmed a century-old prediction of the general theory of relativity. From an astrophysical perspective, the originally detected gravitational waves are a product of a black hole merger. And this is surely a strong argument in favor of pursuing this line of research.

Most scientists agree that supermassive black holes lie at the centers of galaxies. A black hole discovery would thus confirm a basic assumption made by numerous models describing the evolution of the universe as well as many other observed astrophysical phenomena. Indeed, it would furnish the latest Nobel Prize in physics with further interpretations.

Where will the answer likely come from?

As I have said, black holes are not to be seen, because they absorb all light without emitting any in return. But we can observe them indirectly with a huge radio interferometer. It is a special instrument that allows us to obtain images with a high angular resolution — that is, of the best quality. To make such images, it is necessary to account for the scattering and absorption of light. After all, the galactic center is littered with matter gravitating toward the central object. Light emitted at the center has to come through the surrounding plasma without being absorbed. But even then, it will still be scattered by the electrons in its way.

There are two ways to address this: First, short waves can be used, because shorter wavelengths mean less scattering. This is something being done by the ground-based Event Horizon Telescope. The other way to deal with the issue is by understanding the subtle effects involved in wave propagation and taking them into account. We are working toward this goal within the scope of the Russian space mission RadioAstron. Right now, radio interferometry is seen as the prime candidate for enabling the observation of black hole shadows and thus proving their existence.

This fundamental objective has a practical dimension, too: Every day, we use a network of radio telescopes spread across the globe to measure the rotation parameters of the Earth, which are essential for the operation of any navigation system, such as GLONASS or GPS.

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