science.ORF.at: Mr. Danzmann, we live in exciting times: celestial observation is now entering the era of “multi-messenger astronomy”. What is happening now?
Karsten Danzmann: For a long time, gravitational waves and Einstein’s theory of relativity had nothing to do with astronomy. The discovery only came in 2015, when the first sign of gravitational waves was discovered – and with it supermassive black holes. At first, people mainly discussed: Is this really true? Are these signs real? Only when we were sure did everything explode. Therefore, gravitational wave astronomy has only been around for a few years. We now also use gravitational waves to learn about the world.
We now have more than 300 published events, including the discovery of merged neutron stars: This was probably the largest astronomical event ever carried out, as not only gravitational wave astronomers installed their measuring instruments there, but also all others. I don’t know how many telescopes around the world were involved, from optical wavelengths to ponds and others. This marked the basis of what we today call multimessenger astronomy.
In addition to gravitational waves, neutrinos are now also being added as an additional sensory organ in astronomy: What does this mean for our picture of the cosmos?
APA/dpa/Julian Stratenschulte
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Karsten Danzmann is director of the Max Planck Institute for Gravitational Physics. The German astrophysicist was instrumental in developing detectors that allowed direct observation of gravitational waves at the LIGO observatory.
Danzmann: Things are a little different with neutrinos. We knew that at some point we would also use them for astronomy. But we also knew: it would be very difficult. Because neutrinos are so light and so difficult to detect, it is necessary to build huge detectors. But people learned to deal with it – coincidentally, at about the same time as the first detection of gravitational waves. For example, today I heard a talk about detecting black holes with a hundred times the mass of our Sun. According to theory, they shouldn’t be there. And I consider this an excellent example of multi-messenger astronomy: without gravitational waves we would never have seen them.
When gravitational waves were first detected, people were happy to have received a signal. The performance of LIGO observatories has improved significantly. How many events do you see per year?
Danzmann: One signal every 2.8 days.
So an incredible increase.
Danzmann: Yes, that’s it. And nothing will change that. It is always necessary to weigh: do we want to turn off the machine to improve it? This usually takes one to two years. Or do we want to take the time and collect data? We are in the process of recording LIGO O4 and it will probably take two years. After the next renovation, everything will become even more intense. Why? Because for every factor of 10 I improve, the event rate increases by a factor of 1,000. This is worth it. And then, at some point, the next generation of detectors will emerge. 1,000 events per day would therefore be entirely conceivable. I hope our neutrino colleagues make similar progress. But I don’t think it will happen so quickly.
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Successor to the LIGO keyword: what will these machines look like?
Danzmann: Ideas are different in Europe and the US and are still subject to change. But it’s relatively clear that things have to be bigger. Size is hard to beat when it comes to detecting gravitational waves. The Americans plan an observatory 40 kilometers long, ten times the size of LIGO. In Europe the philosophy is a little different: the so-called Einstein Telescope, Europe’s next-generation gravitational wave observatory, will be built underground.
Previously, it was said that the Einstein Telescope would be built in the Netherlands or Sardinia. Has the location issue been decided yet?
Danzmann: No one can solve this because at least a dozen nations are involved and no nation can handle the project alone. Americans have it easier there. An institution decides and then says: This is what we are doing now. This will still be a difficult problem in Europe, we have to reach an agreement. The Dutch, the Belgians, the Germans and the Italians all have their favorite places and I don’t have a solution either. In any case, the Dutch made the most progress; they put a billion euros on the table from the beginning. Real money they would give if everyone else participated. I can’t predict that. I have been in the market for a long time and have seen many developments from it.
Returning to astronomy, neutrinos and gravitational waves have one thing in common: they can be used to look all the way into the cosmos, potentially all the way back to the Big Bang. Is this realistic from your point of view?
Danzmann: With electromagnetic radiation it is very difficult to observe the first 400 thousand years of the universe. Simply because the universe was too dense and opaque to begin with. This is different for neutrinos and gravitational waves. I cannot estimate at the moment how far we will get in practice with neutrinos, but when it comes to gravitational waves I am sure: we will do it, we are in the process of developing detectors that can see that far. Nobody knows what we will see.
How long will this last?
Danzmann: decades.
Let’s speculate: what could we see?
Danzmann: This will depend on the signals detected. There are all kinds of theories about the early phase of the universe. One thing is certain: each theory produces a certain type of signal, both in neutrinos and gravitational waves. But they all look different. And then some of these models will emerge.
Ideally, it would be necessary to approach time zero within a fraction of a second.
Danzmann: We will too, I’m sure. I intend to live to be 120 years old. Well, then I probably won’t participate as actively anymore, but I believe: I will see the Big Bang. Or listen. Or somehow understand.