Scientists also suspect life on other planets in the universe. But for physicist and former astronaut Ulrich Walter, the probability that contact will ever occur is extremely low. “Yes. There are some out there, but we will never meet them. We will never communicate with them ourselves. But they must exist,” says Walter, who celebrated his 70th birthday on Friday. “Logic alone demands that, because there are almost an infinite number of other planets.”

However: in other galaxies. Humanity will never be able to travel there. “In fact, you can show that you will never intercept messages from them if they send them to us.”

In our Milky Way – according to Walter, the “part of the universe accessible to us” – the probability of extraterrestrial life or other civilizations is very low. “We tend to be alone there. If so, there are a handful, maybe ten or so, but even then the distances are so great that other people’s signals disappear in the galactic noise.” A trip is certainly not possible. Such flights would take more than 10,000 years – even if they were only a few light years away.

“The basis for biological life is everywhere”

Water, carbon and energy are the prerequisites for life not only on Earth but also in the universe. “The basis for biological life exists everywhere. The improbability lies, firstly, in the fact that such a planet must receive exactly the right amount of energy from another sun-like star.” A second critical point is that a first cell then emerges from inanimate matter.

Other forms of life are hardly conceivable. This is about the chemistry of information carriers such as DNA. “There is only one chemical element, silicon, that can build information strands similar to carbon.” However, complex silicon compounds do not have the necessary stability – and the most important metabolic product is solid, insoluble silicon dioxide compared to the gaseous, highly soluble carbon dioxide. “So silicon can’t store that much information and it doesn’t have the right chemical environment.”

Walter leaves it open what the “green men” might look like. Like us, they must have a food input and output for their necessary metabolism. In biology there are a wide variety of variations based on carbon chemistry. “And they are everywhere in space.”