Mr. Spiering, what motivated you to become a neutrino astrophysicist? 

That was still in GDR times. I worked on accelerators in Dubna and Protvino and was annoyed that we were always ten years behind the West. So I thought: in neutrino astronomy, at least with the accelerator, everyone has the same starting conditions: the cosmic particle accelerator! And then a supernova exploded in 1987, close enough to detect neutrinos. That was a huge event, a goldmine for astronomy and astrophysics. The field of research suddenly became extremely attractive – for me, too. 

And what is the original idea behind a neutrino telescope like IceCube?

The search for the source of cosmic rays! Cosmic rays have incredibly high energies; an accelerator with LHC technology would have to have the circumference of the Earth's orbit to do that. The question is: how does nature do it, how does it bring particles to such incredible energies? If we could determine the origin of high-energy neutrinos with a neutrino telescope, then we'ld also know the sources of cosmic radiation.

What does that mean for us?

Let me expand a little at this point. Let's take supernovae as an example: stars explode because a neutrino wind pushes outwards from inside. In the process, elements such as carbon and iron are thrown into the universe – like seeds – which ultimately make our own existence possible. We thus learn something about where we ourselves, or our components, come from. With cosmic rays, this isn't quite so obvious. But: without them, our galaxy would probably have a somewhat different structure – and perhaps this little star called the sun would not exist at all. Cosmic rays also play a role in the existence of us humans. They pass through us permanently, cause cell mutations and ultimately contribute to evolution. And finally, they are by far the most energetic thing the universe has produced since the Big Bang. We want to understand how the cosmos manages this feat! 

Let's talk about detectors in Antarctica, about IceCube... 

... and AMANDA. That was the prototype for IceCube. In 1995, we became an official member of this multinational telescope project at the South Pole. At that time we were no more than 30 people – a band of dreamers, including a handful of DESY scientists. One of them was me. The "collaboration dinner" in 1997 took place in the living room of my flat in Prenzlauer Berg. Today, you have to book large halls for IceCube dinners.

That sounds like times long gone.

It was a kind of Wild West atmosphere coupled with a certain naiveté. But if you knew all the problems beforehand, then some things wouldn't even start.

Why was AMANDA so important for IceCube?

The AMANDA story is full of mishaps, today we would call them "challenges". Sometimes this almost brought the project to the brink of failure. But we learned from the mistakes and didn't repeat any of them at IceCube. Thank God, even in the difficult moments we had leaders like in Paul Söding in Zeuthen and DESY Director Björn Wiik who believed in us – and also looked to the future. The AMANDA project, for example, was not yet finished when Björn Wiik took me aside after critical questions about our very first IceCube plans and said: "Are you convinced that you need a cubic kilometre? Then build the thing and don't listen to doubters!" 

To date, more than 100 cosmic neutrinos have been detected; in one case, an active galaxy was identified as a possible source: Can you classify the results of the first ten years of IceCube for us? 

After AMANDA, I, like many of us, would have expected to see something at IceCube right at the beginning. But nothing came. It wasn't until 2013 that we discovered cosmic neutrinos – that was the breakthrough, and we popped bottles of champagne.... But we are only at the beginning of assigning our neutrinos to known astrophysical sources. Of course I would have liked to have four or five crystal-clear sources by now. But sometimes you just have to collect more data and not give up too quickly. Neutrino physicists are a determined lot. 

... and tough. The South Pole is one of the loneliest and coldest places on our planet: what gave you the hardest time?

In the first few days, it was the high altitude air and shortness of breath. After all, you're at 3,000 metres. In addition, the sleeplessness is unpleasant, in which the sun, which is permanently above the horizon, certainly also plays a role. The medical test before departure is extremely thorough. But you don't have to be Reinhold Messner to work there for four weeks.

Speaking of medical tests: we hear you play a decisive role in a thriller involving a wisdom tooth...

A legendary story! I had a fantastic, hard-working Ph.D. student who was supposed to go to the South Pole station in the last AMANDA season (1999/2000). But he had a wisdom tooth problem. You must know that problematic wisdom teeth often become infected at high altitudes. The young man had already had three of them extracted, one of which had massive complications, so he didn't dare go for the fourth. One day before departure, he was told that he was not medically qualified because the fourth wisdom tooth was still there. When he came to me in despair, I said: "Give me the dentist's certificate and some Liquid Paper and we'll turn the three into a four".  That really showed my age because he just said, "We'll do it in Photoshop." No sooner said than done. We then waited together for the answer: medically qualified. The next day he flew off – all went well. I guess he still has the tooth today.

Would you like to go to the South Pole again yourself?

No, I wouldn't pass the medical qualification tests. Besides, I've been there four times, some 13 weeks in total. I'd be much happier for the young people to go. Back then, by the way, the Ph.D. students didn't just come to us because they thought physics was so great, but also because they wanted to go to the South Pole. 

What is the most urgent task for the next generation of researchers – including IceCube Gen II?

We have opened a window into a hitherto completely unknown landscape in the universe. So this landscape does exist! Over the next few decades, we must now map it with individual sources and all the information we can get so that a completely new picture emerges from all the pieces of the mosaic. A landscape of particle accelerators governed by mechanisms that we can only speculate about at the moment.

Beyond all expectations...

We follow the old principle: you build something ten or more of times bigger than what existed before and then you see something completely unexpected. This phenomenon runs like a common thread through the centuries from Galileo to the first radio telescopes. And so we hope that with IceCube-Gen2 we will also see things that we had not yet thought of. Something completely unknown, something crazy. That would be great!