For the third time in four years, the 2020 Nobel Prize went to gravitational physics. Oldenburg expert Jutta Kunz explains why she finds nothing more exciting than black holes, neutron stars and gravitational waves.
Prof Kunz, what fascinates you so much about astrophysics?
Even as a child, I was fascinated by questions about the universe. Where the origin of everything lies, what the universe is made of, how it continues, I found that very exciting, and it fascinates many people. There is no field of work that I would ever want to swap for it. With three Nobel Prizes in such quick succession, you can see that a lot is happening here at the moment.
This year, the Nobel Prize in Physics was awarded for research into black holes. What kind of celestial bodies are these?
Black holes are very astonishing objects. Their gravity is so strong that they swallow up any matter that comes too close to them. Beyond a certain limit, the so-called event horizon, not even light can escape from a black hole. This is why they appear completely black in the sky. Light can only come from excited matter in their neighbourhood.
You have been studying black holes for a long time and started studying physics in the 1970s. What image did people have of these objects back then?
A lot was already known about black holes. The general theory of relativity, formulated by Albert Einstein in 1915, had already identified black holes as solutions to the field equations. However, they were initially regarded as curious mathematical objects. They were not yet associated with the real universe.
When did that change?
At the end of the 1930s, the work of nuclear physicist Robert Oppenheimer and his colleagues made it clear what happens when very massive stars have used up their nuclear fuel: They collapse due to their own gravity. These considerations showed that these heavy stars should then theoretically become black holes. However, it was widely believed at the time that this did not really happen in nature. However, Roger Penrose, one of this year's Nobel Prize winners, showed in 1965 that no special conditions are necessary for such a collapse, meaning that it should actually happen.
When were the first black holes observed?
As early as the 1950s, very strange radio sources were discovered, i.e. celestial bodies that emit radio waves. They were christened quasars, quasi-stellar objects. In 1963 it was discovered that these were very distant objects. These quasars are relatively small but outshine their entire galaxy. Rotating black holes were then identified as the source of the huge amount of energy they emit. The radiation is released by the so-called accretion disc. This is matter that rotates around the black hole, heats up strongly in the process and eventually falls into it. We still assume today that quasars are powered by supermassive black holes at the centre of early galaxies.
One half of this year's Nobel Prize goes to the American astronomer Andrea Ghez and the German astrophysicist Reinhard Genzel, who have proven that there is also a supermassive black hole at the centre of the Milky Way.
The two have observed very bright stars there that move around a dark centre in a similar way to the planets around the sun. The orbits therefore look almost like ellipses, but the stars reach incredibly high speeds on their orbits. This all points to a supermassive black hole of around four million solar masses in the centre of the Milky Way, around which these stars move.
In 2019, astrophysicists published an image of a black hole for the first time. What do you see there?
Strictly speaking, the team from the Event Horizon Telescope imaged the so-called shadow of the black hole, an area in which light itself is captured by the black hole. The image shows that the dark shadow is surrounded by a luminous accretion disc. The Event Horizon Telescope consists of several networked radio telescopes, which together form a virtual telescope the size of the Earth. This was the only way to image the supermassive black hole at the centre of the galaxy M87, the most massive galaxy in our galaxy cluster. This black hole is over a thousand times heavier than the one in our Milky Way; it has more than six billion solar masses.
What conditions prevail inside a black hole?
That is a great mystery. If you solve Einstein's equations, a so-called singularity arises in the centre - physical quantities are no longer defined there, the curvature of space-time becomes infinite due to the strong gravity, and the equations collapse. However, we do not expect such infinities in nature.
How could we find out what is really going on there?
We need a new theory that is compatible with the general theory of relativity on an astronomical scale and with quantum theory on a microscopic scale. That would be a theory of quantum gravity. The hope is that the interior of black holes can be described in a physically meaningful way with such a theory, i.e. that singularities no longer occur there.
Why does quantum theory play a role for black holes?
When we look at processes in microscopic areas, we have to take quantum mechanics into account. This also applies to the centre of black holes or to the entire universe, such as the Big Bang. However, we don't yet know how to bring quantum theory and general relativity together correctly. In the case of electromagnetism, it worked to generalise a classical theory using quantum mechanics, and it was possible to do the same with the theories of particle physics. Gravitation, however, is more complicated.
What possibilities are there?
In order to arrive at a theory of quantum gravity, many researchers have long focussed on string theory. We have also been working on this in our "Models of Gravity" research training group, in which four other universities are involved in addition to the University of Oldenburg. String theory introduces additional spatial dimensions. This also makes other, completely new types of black holes possible. For example, we can find black Saturns in which a black hole with a round event horizon is surrounded by a black ring with a ring-shaped event horizon, and even more complicated black objects.
Can such predictions be tested by observations?
This is precisely the central topic of our Research Training Group. We are investigating whether the predictions of certain gravitational theories are observable. For example, we calculate how large the shadow of a black hole would have to be if a certain theory is correct - and check whether this agrees with the image of M87, the only measuring point that exists for this so far. Such theories also make predictions about the movements of stars in the centre of the Milky Way, or about gravitational waves that are released when neutron stars or black holes merge.
And the result?
So far, everything we have measured agrees with Einstein's theory. Within the margin of error, the general theory of relativity has always been confirmed. We therefore concentrate on theories that predict only very small deviations from general relativity in relation to all known measurement data. Otherwise, we already know from the existing observations that we can forget about the theory.
When do you expect a breakthrough?
We will only make real progress with the next generation of instruments. The first gravitational wave detector in space, LISA, is one example. This instrument from the European Space Agency ESA is due to be launched in 2034. It will then be able to detect gravitational waves from the merging of supermassive black holes, which is not possible with detectors on Earth. In future, it will be possible to measure many things even more precisely. I therefore firmly believe that gravitational physics has a bright future ahead of it.
Interview: Ute Kehse
