Avi Loeb, 56, is chair of the astronomy department at Harvard University and has published more than 700 papers on astrophysical phenomena. His areas of interest include black holes and the birth of the first stars in the universe. More recently, Loeb has focused on the possible existence of extraterrestrial intelligent life, a topic on which he is currently writing a textbook.
DER SPIEGEL: Professor Loeb, do you have a favorite alien?
Loeb: To be honest, I don't like science fiction personally. I have a problem when the action in a movie violates the laws of physics. In those cases, I cannot enjoy the experience aesthetically.
DER SPIEGEL: If you don't like aliens, why are you exploring the question of intelligent extraterrestrial life?
Loeb: Because it's one of the really big questions. I have always been interested, even as a young kid, in thinking about the big picture. And the most fundamental of all questions is: Are we alone?
DER SPIEGEL: Why do you consider the question to be so fundamental?
Loeb: If at some point we encounter other intelligent beings, it would radically change our perspective of our importance in the big scheme of things. In addition, extraterrestrial intelligences could enormously increase our knowledge. Learning from them would improve our understanding of the world. It could be a shortcut -- just as if somebody from the Middle Ages were to get a glimpse of our world in the 21st century.
DER SPIEGEL: Are the prospects of encountering aliens better today than they were in earlier times?
Loeb: We live in a special time. For one thing, we've only found out in the past few years how widespread life might be in our galaxy. The Kepler satellite taught us that every fourth star has an Earth-like planet with temperatures favorable for the emergence of life. And secondly, today we have the necessary technology to answer the question of whether we are alone.
DER SPIEGEL: If we assume that some extraterrestrial civilization was really discovered, how significant would this be compared to other important discoveries in human history?
Loeb: I think it would be the biggest of them all. If you think about the history of humans, the perspective has changed as we have evolved -- from a single individual to a family, a tribe, a country, and finally we even found other continents with people living there. If we were now to find other beings beyond planet Earth, this would be the biggest step ever.
DER SPIEGEL: Do you think there will be a moment when we receive some intelligent signal from space, and suddenly everything will be different?
Loeb: I can't tell you what this moment will look like. But it will be shocking. Because we are biased by our own experiences. We imagine other beings to be similar to us. But maybe they are radically different. For example, it is quite possible that we won't encounter the life forms themselves, but rather only their artifacts. In any case, we ourselves are not designed for interstellar journeys. The only reason astronauts survive in space is that they are under the protection of the Earth's magnetic field. Even when traveling to Mars, cosmic rays will become a major problem.
DER SPIEGEL: So E.T. is just a robot sent out by extraterrestrials?
Loeb: Why not? Or the extraterrestrials could, instead of traveling themselves, send 3-D printers which then could print artifacts, or even living organisms, using the material that they find on other celestial bodies.
DER SPIEGEL: Do you think that the big moment of first contact will happen in your lifetime?
Loeb: My guess is that we will first detect some technological debris rather than biological life itself.
DER SPIEGEL: In what form? Stray spaceships? Abandoned junk?
Loeb: Exactly. Most civilizations might be very short-lived. In other words: I'm afraid that they don't take good care of their home planet and that in the end they will destroy themselves -- by nuclear wars, by interventions in the climate, by environmental destruction
DER SPIEGEL: Astronomers would be like inner-galactic historians, reconstructing lost high-tech cultures from the ruins?
Loeb: That's why I call it "space archaeology."
DER SPIEGEL: In the publication Astrophysical Journal Letters, you recently presented the hypothesis that the first extraterrestrials might already be here.
Loeb: Yes. On Oct. 19, 2017, the PanSTARRS telescope in Hawaii detected an unusual object in the sky. It was moving so fast that it must have originated from some place beyond our solar system, making it the first visitor from outer space that we know of. It was christened "'Oumuamua."
DER SPIEGEL: Is it astonishing to find an asteroid wandering across the galaxy and finally finding its way to us?
Loeb: Quite astonishing, in fact. Ten years ago, before PanSTARRS began operating, I calculated together with two colleagues whether this instrument would be able to find any objects from outer space. Assuming that such objects originate from solar systems similar to ours, we came to the conclusion that the probability is somewhere between 1:100 and 1: 100,000,000.
DER SPIEGEL: 'Oumuamua shouldn't exist at all?
Loeb: That's correct. Besides, it is quite a mysterious object in other respects, too. Its brightness changes dramatically, suggesting a very strange shape. A sphere would always reflect the same amount of sunlight. Only a disc or a cigar-shaped body would sometimes turn its edge and sometimes its broad side towards us, which would make it flicker while rotating.
DER SPIEGEL: But just because it has a strange shape doesn't mean that 'Oumuamua is a spaceship.
Loeb: I agree. But the more we found out about this object, the weirder it got. In June, a paper was published in the journal Nature describing the orbit of 'Oumuamua. It differs significantly from an orbit shaped just by the gravitational field of the Sun.
DER SPIEGEL: What do you conclude from this?
Loeb: That there was some additional force acting on 'Oumuamua. For example, if it was a comet, it might have emitted gases while flying past the Sun. This would provide a kickback like in a rocket. However, a considerable part of its mass would have evaporated -- about 10 percent.
DER SPIEGEL: Is that unusual for a comet?
Loeb: Not necessarily. But we should have seen these gases. Despite an intensive investigation, no comet tail was discovered. And there is something else that is really strange: The rotation of 'Oumuamua should have changed during outgassing, but this effect was not observed either.
DER SPIEGEL: What conclusion do you draw from this?
Loeb: I began wondering whether there might be another force acting on 'Oumuamua. And the only one I can think of is the pressure of the sunlight.
DER SPIEGEL: Is that force sufficiently strong to have an observable effect on such a body?
Loeb: This is the critical point. The power of solar radiation is relatively weak. It could only have a visible effect if 'Oumuamua is a very thin object.
DER SPIEGEL: What do you mean by "very thin?"
Loeb: Less than a millimeter thick.
DER SPIEGEL: How could something like that have come about? It's difficult enough to imagine how a giant cigar could have formed, but a giant sheet of paper?
Loeb: I can't imagine such a thing either. I would be grateful if someone came along to make a suggestion. But as long as there is no natural explanation, we are left with the possibility that it is an artificial product. A light sail made by intelligent beings.
DER SPIEGEL: That sounds like quite an outrageous suggestion. If the very first object found from outside our solar system turns out to be an artifact, it would lead to the conclusion that such artifacts might be much more common out there than natural asteroids or comets. Could interstellar space be just a gigantic junkyard littered with debris of alien civilizations?
Loeb: You are addressing an important point here. We've worked it out: If 'Oumuamua is a randomly wandering object, every solar system would have to produce millions of billions of such objects. Unless it's not random. It could be a targeted mission.
DER SPIEGEL: You mean to say that 'Oumuamua is an active space probe and it's spying on us?
Loeb: I'm just saying that it's a possibility. I follow a detective's logic, and Sherlock Holmes said: "If you have ruled out the impossible, then what's left is the truth, however unlikely it may seem."
DER SPIEGEL: It's not exactly common for a respected scientist to make such suggestions. How did other scientists react?
Loeb: Most of them agree that 'Oumuamua is very weird. But still, they hesitate to say that it might be an artificial object.
DER SPIEGEL: Speculating about aliens is a pretty exotic idea. You might as well say that there was magic involved, or some divine creator.
Loeb: Not at all. Thinking about distant civilizations is not speculative. We know that we exist, so at least one life form has developed advanced technology. And we know that there are conditions similar to those on Earth in a quarter of all planetary systems around other stars. Why, then, shouldn't there be intelligent beings elsewhere? I follow what I call the principle of cosmic modesty. We humans tend to think of ourselves as something special, but history has shown time and time again that this is an illusion. We thought that Earth was at the center of the universe, but it revolves around the Sun, which revolves around the center of our galaxy, which is only one of billions of galaxies in the universe. The principle of cosmic modesty requires us to assume that we are nothing special from a biological point of view either. And if there are other intelligent beings out there, chances are that they are technologically much more advanced than we are.
DER SPIEGEL: Some people accused you of just trying to attract attention with your hypothesis of spaceship 'Oumuamua.
Loeb: That's nonsense. I didn't even issue a press release on our paper. If I had been interested in attracting attention, I would have done so. But that's not what it was about. My only motivation is to think about the world. I don't care what people think.
DER SPIEGEL: Could it be that one reason you think 'Oumuamua might be a light sail is because you are working on one yourself as part of the Breakthrough Starshot project?
Loeb: I don't deny that my imagination is limited by what I know. Certainly my ideas are influenced by what I'm working on, but this is true for everybody.
DER SPIEGEL: What is the Breakthrough Starshot project all about?
Loeb: It all started when Yuri Milner, an entrepreneur from Silicon Valley, came to my office in May 2015 and asked if I would be willing to lead a project to send a probe to the nearest star, the condition being that it should arrive within our lifetime.
DER SPIEGEL: That sounds rather absurd. A rocket would take tens of thousands of years to reach our nearest neighbor, Proxima Centauri.
Loeb: I told Yuri I had to think about it. Proxima Centauri is located four light years away from Earth. To get there in 20 years, a probe would have to travel at a fifth of the speed of light. After six months, I had a clear idea of the concept.
DER SPIEGEL: Which is?
Loeb: One thing was clear: I had to rule out any kind of propulsion with the probe's fuel on board. Even with plutonium, the energy density is far too low to reach such speeds. That's why a light sail, accelerated by a powerful laser from Earth, seemed to be the only feasible way. The idea is to accelerate the probe with a 100 gigawatt laser beam for a few minutes. Once it is five times as far away as the moon, it will have reached a fifth of the speed of light.
DER SPIEGEL: Provided the probe is very small and light.
Loeb: Yeah. We plan a payload of about one gram. The probe needs a camera, and also a navigation and a communication device. The miniaturization of electronics allows us to accommodate all this.
DER SPIEGEL: The laser would have to be extremely powerful. Isn't there a danger that your mini-probe would evaporate as soon as it is hit by the beam?
Loeb: To prevent this from happening, we need a material that almost completely reflects the incoming laser light. It must be a perfect mirror that absorbs less than one 100,000th of the light. Such materials do exist.
DER SPIEGEL: Your plan calls for continuing to accelerate the probe even when it is a million kilometers from Earth. Is it possible to focus laser light with such precision?
Loeb: Yes, it is. Our concept is to install many small infrared lasers across an area of one square kilometer. This will allow us to focus the beam up to about five times the distance from the moon. This is an important limitation, by the way. If we could focus the beam to a greater distance, we could use less powerful lasers, because then we could accelerate the probe over a longer period of time.
DER SPIEGEL: Twenty years later, if the probe really does reach its destination, it has to take pictures and send them back to Earth. But the transmitter on board is unlikely to be more powerful than those in a normal mobile phone. Is that enough to transmit a signal across four light years?
Loeb: Traveling across such a distance, the beam of the radio signal will spread out to more than the size of the Earth's orbit around the Sun. So the probe only has to target the Sun and Earth will be within the signaling footprint. However, we will need a big receiver on Earth.
DER SPIEGEL: Let's assume that all this really works and you receive a photo of Proxima Centauri sometime in 2050 or so. Is such a picture worth the $10 billion the project is estimated to cost?
Loeb: For one thing, it will be possible to send a lot of probes into space once the launch system is constructed, because the expensive part is the infrastructure of the laser beam. The probes themselves will be relatively cheap. Moreover, project Starshot is not about the photo itself. It's about demonstrating that we can leave the solar system. This will open the door to a completely new frontier of space exploration.
DER SPIEGEL: Could you also send such a probe after 'Oumuamua to find out whether it really has the shape of a sail?
Loeb: Yes, this is possible. 'Oumuamua is moving too fast for a rocket, but with a laser-driven light sail it would be possible to catch up with it, even at a 10th of a percent of the speed of light.
DER SPIEGEL: Still, it seems unlikely that will ever happen. Are there any other ways to find out if there is any truth to your 'Oumuamua hypothesis?
Loeb: Absolutely. In a few years, the Large Synoptic Survey Telescope will go into operation, the successor to PanSTARRS. If objects like 'Oumuamua are randomly scattered in space, this instrument should see thousands of them, and we would then be able to study them much more thoroughly. Plus, celestial bodies of distant origin are likely to already be orbiting within our solar system. The gravitational fields of the Sun and Jupiter act like a fishing net that can capture bodies arriving from outer space.
DER SPIEGEL: Can they be distinguished from the many homegrown asteroids within the solar system?
Loeb: Yes. Because they come to us from arbitrary directions, their orbits are likely to be inclined relative to the plane of the planetary orbits. We have even identified some candidates.
DER SPIEGEL: And you would now like to investigate those more closely?
Loeb: Yes, I see this as a whole new way of studying distant planetary systems without the hassle of traveling to them. It's like a dinner party where some guest turns out to come from a distant country. Just by interviewing the guest, you can learn a lot about this country without the need to pay airfare and go to there. When it comes to 'Oumuamua, we only became aware of it after the guest had already left and disappeared out the door. As to the next visitor, we will study it earlier and more thoroughly.
DER SPIEGEL: To change perspective for a moment: If there really is a highly advanced civilization somewhere out there, will they already have learned that there are some intelligent beings here on planet Earth?
Loeb: If this civilization is as technically advanced as we are, they should be able to detect our radio signals from a distance of tens of light years.
DER SPIEGEL: Would it be desirable to contact them?
Loeb: This depends on how optimistic you are regarding their intentions. If these beings are peaceful, we could learn a lot from them.
DER SPIEGEL: The historical event here on Earth which probably comes closest to the arrival of aliens was the landing of Christopher Columbus in the Americas. For the peoples living there, that event turned out to be a catastrophe.
Loeb: I agree. Therefore, the best thing to do is to listen first. Once we detect a signal, we can figure out what to do.
DER SPIEGEL: Professor Loeb, thank you for this interview.