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There is a possibility that the foreign object from interstellar space that visited our Solar System last year may have been an alien probe. That, at least, is the thesis that Abraham Loeb, chair of the astronomy department at Harvard, recently presented together with a postdoc. ZEIT ONLINE wanted to know if it was some kind of a stunt or if it was science in its most transparent form. We met with Loeb in Berlin, where he was taking part in the Falling Walls Conference, and spoke with him about the mysterious object Oumuamua, his assumptions and his motivations.

ZEIT ONLINE: Do you believe that the existence of aliens is likely, especially an intelligent, highly developed extraterrestrial species?

Abraham Loeb: We’re not special. Anyone pretending we’re the only intelligent civilization in the universe is arrogant. I try to be modest – cosmic modesty, as I call it. There are a lot of stars with Earth-like planets that have an atmosphere, liquid water on the surface and the chemistry of life as we know it. There are more Earth-like planets than grains of sand on all the beaches on Earth. What remains is the question: How do we find life, either in the sense of microbes or more highly-developed life, with technologies? That’s the focus of some of my research these days. 

ZEIT ONLINE: You’ve caused quite a stir with your latest paper, publishing a rather far-fetched thesis regarding the mysterious interstellar object Oumuamua. It reads: "We discuss the possible origins of such an object including the possibility that it might be a lightsail of artificial origin." Do you really believe that Oumuamua is not an exocomet of some sort but is in fact an alien probe?

Loeb: Oumuamua is really weird. Some people would say it’s an outlier. But it certainly doesn’t look like asteroids or comets we find in our Solar System. It has an extreme shape. Based on the reflected sunlight, it appears to be much longer than it is wide by a factor of five to 10. And it’s being pushed by a force additional to the gravity of the sun, so something is affecting it, as another recent paper by another research team states. A comet could be affected by the vaporization of ice, but there is no cometary tail or change of spin. So we ask: What could be pushing it? The only idea that came to mind was the sunlight. And in order for the sunlight to affect it, it has to be really thin, just like a sail. A lightsail. It's a technology we’re currently working on, so it’s possible that another civilization has already accomplished it and – taking it one step further – might have sent a probe to the earth on purpose.

Abraham Loeb is chair of the Department of Astronomy at Harvard University and chair of the Advisory Committee of the Breakthrough Starshot Initiative. The theoretical physicist is convinced that aliens exist. One focus of his research: searching for signs of extra-terrestrial life and developing technologies for deep space exploration such as lightsails. © Andreas Prost für ZEIT ONLINE

ZEIT ONLINE: So you are saying it’s a possibility. One of many.

Loeb: Not of many. If the paper that was published about the additional push is correct, and they claim that they detected it beyond a reasonable doubt, then I wonder: What could be the reason? The only other possibility besides a lightsail I could think of was that the object broke apart and one part gave a kick to the other. But that would be an impulsive kick and the team observed a continuous push.

ZEIT ONLINE: Some scientists I spoke with made it clear that a single idea about what this object could be doesn’t make it the only explanation.

Loeb: So they should write a paper suggesting something else and then let’s see if that’s more reasonable. That’s the whole idea of science. I adopt the approach of Sherlock Holmes: When you rule out the impossible, whatever is left, however improbable it is, must be the truth.

ZEIT ONLINE: Other scientists who search for extraterrestrial life, in this case from Seti (Search for Extraterrestrial Intelligence) and Breakthrough Listen, checked out Oumuamua with their radio telescopes to listen for a signal. But they didn’t hear a thing. How do you explain that?

Loeb: It’s a completely different signal. It’s as if you say nobody is there because you tried to hear voices, but didn't hear anything. But what if they just aren't saying anything? A lightsail would be a message in a bottle.

ZEIT ONLINE: Couldn’t you flip this argument around? You said they looked for vaporization, but didn’t find it, so therefore it’s not a comet. But it would’ve been easy to miss vaporization.

Loeb: It’s not easy to miss. We’re talking about an object that passed close by.

ZEIT ONLINE: Other astronomers say it is possible to have missed it, because they didn’t observe the object in the right kind of light, for example. Others say important telescopes weren’t available. Why fill the knowledge gaps with an unlikely thesis instead of using the data that was collected to arrive at the normal, most probable solution? 

Loeb: The importance of the paper is not to claim that this must be the case. The importance is to motivate people next time we have an object of this class from interstellar space to look at it with the best telescopes we have. Last year, no one cared. With the paper, they do. My objective is to find the truth, not to create interest. The nature of the object will not be dictated by the opinion poll you take on Twitter, it will be dictated by what the evidence tells us.