“I don’t care what people say,” asserts Avi Loeb, chairman of Harvard University’s astronomy department and author of one of the most controversial articles in the realm of science last year (and also one of the most popular in the general media). “It doesn’t matter to me,” he continues. “I say what I think, and if the broad public takes an interest in what I say, that’s a welcome result as far as I’m concerned, but an indirect result. Science isn’t like politics: It is not based on popularity polls.”
Prof. Abraham Loeb, 56, was born in Beit Hanan, a moshav in central Israel, and studied physics at the Hebrew University of Jerusalem as part of the Israel Defense Forces’ Talpiot program for recruits who demonstrate outstanding academic ability. Freeman Dyson, the theoretical physicist, and the late astrophysicist John Bahcall admitted Loeb to the Institute for Advanced Study in Princeton, whose past faculty members included Albert Einstein and J. Robert Oppenheimer. In 2012, Time magazine named Loeb one of the 25 most influential people in the field of space. He has won prizes, written books and published 700 articles in the world’s leading scientific journals. Last October, Loeb and his postdoctoral student Shmuel Bialy, also an Israeli, published an article in the scientific outlet “The Astrophysical Journal Letters,” which seriously raised the possibility that an intelligent species of aliens had sent a spaceship to Earth.
The “spaceship” in question is called Oumuamua. For those who don’t keep up with space news, Oumuamua is the first object in history to pass through the solar system and be identified as definitely originating outside of it. The first interstellar guest came to us from the direction of Vega, the brightest star in the Lyra constellation, which is 26 light-years from us. In the 1997 film “Contact,” it’s the star from which the radio signal is sent to Jodie Foster.
Oumuamua was actually discovered by a Canadian astronomer, Robert Weryk, using the Pan-STARRS telescope at the Haleakala Observatory in Hawaii. “Oumuamua” is Hawaiian for “first distant messenger” – in a word, “scout.” It was discovered on October 19, 2017, suspiciously close to Earth (relatively speaking, of course: Oumuamua was 33 million kilometers away from us when it was sighted – 85 times farther than the moon is from Earth).
Whereas all the planets, asteroids and meteors that originate within the solar system more or less circle what is called the Ecliptic plane, that of our sun, since they were formed from the same disc of gas and dust that rotated around itself, Oumuamua entered the solar system north of the plane, in an extreme hyperbolic orbit and at a speed of 26.3 kilometers per second faster relative to the motion of the sun.
A reconstruction of its trajectory shows that Oumuamua traversed the ecliptic plane on September 6, 2017, when the sun’s gravity accelerated the object to a velocity of 87.8 kilometers per second. On September 9, the object passed closer to the sun than the orbit of Mercury. And on October 14, five days before it was discovered in Hawaii, the object passed 24.18 million kilometers away from Earth, or 62 times the distance from here to the moon.
What does it feel like to sit next to colleagues in a university lunchroom a day after publishing an article arguing that Oumuamua may actually be a reconnaissance spaceship?
Loeb: “The article I published was written, in part, on the basis of conversations I had with colleagues whom I respect scientifically. Scientists of senior status said themselves that this object was peculiar but were apprehensive about making their thoughts public. I don’t understand that. After all, academic tenure is intended to give scientists the freedom to take risks without having to worry about their jobs. Unfortunately, most scientists achieve tenure – and go on tending to their image. As children we ask ourselves about the world, we allow ourselves to err. Ego doesn’t play a part. We learn about the world with innocence and honesty. As a scientist, you’re supposed to enjoy the privilege of being able to continue your childhood. Not to worry about the ego, but about uncovering the truth. Especially after you get tenure.”
Without tenure you wouldn’t have published the article?
“I suppose not. It’s not just the tenure. I’m head of the astronomy department, and founding director of the Black Hole Initiative [an interdisciplinary center at Harvard dedicated to the study of black holes]. In addition, I’m director of the Board on Physics and Astronomy of the National Academies. So it could be that I’m committing image suicide, if this turns out to be incorrect. On the other hand, if it turns out to be correct, it’s one of the greatest discoveries in human history. For us to make progress in understanding the universe, we need to be credible, and the only way to be credible is to follow what you see, not yourself. Besides, what’s the worst thing that can happen to me? I’ll be relieved of my administrative duties? This will bring the benefit that I’ll have more time for science.”
The first friend from another solar system stirred great excitement among scientists, but its form and behavior also raised multiple questions.
“It was subjected to observation, but not enough,” Loeb told me with disappointment, when I met with him in Tel Aviv at the end of December. “It was only under consecutive observation for six days, from October 25 to 31 – namely, a week after its discovery. At first they said, Okay, it’s a comet – but no comet tail was visible. Comets are made of ice, which evaporates as the comet approaches the sun. But we didn’t see a trail of gas or dust in Oumuamua. So the thinking was that it must be an asteroid – simply a chunk of stone. But the object rotated on its axis for eight hours, and during that time its brightness changed by a factor of 10, whereas the brightness of all the asteroids that we’re familiar with changes, at most, by a factor of three. If we assume that the light reflection is constant, that means its length is at least 10 times greater than its thickness.
“There are two possibilities in regard to this extreme geometry,” Loeb continues. “One is that it’s in the shape of a cigar, the other than it has the shape of a pancake. The truth is that the same observers who examined Oumuamua’s light variation reached the conclusion that if it receives a lot of gravitational pushes during the voyage – which is reasonable, because it spent a lot of time in interstellar space – its shape is pancake-flat. Subsequently additional qualities were discovered, such as its origin.”
I wrote above that Oumuamua originated at Vega, but that’s not completely accurate: The universe is a vast place, and even at Oumuamua’s velocity – a velocity that no human spaceship has achieved – a voyage from Vega to the solar system would take 600,000 years. But in the meantime, Vega is orbiting the center of the Milky Way, like the sun and all the other stars, and it wasn’t in that region of the heavens 600,000 years ago.
“If you average the velocities of all the stars in the region,” Loeb explains, “you get a system that’s called the ‘local standard of rest.’ Oumuamua was at rest relative to that system. It didn’t come to us. It waited in place, like a buoy on the surface of the ocean, until the ‘ship’ of the solar system ran into it. To make things clear, only one of 500 stars in the system is as much at rest as Oumuamua. The probability of that is very low. After all, if it were a stone that was simply hurled from a different solar system, we would expect it to have the velocity of its star system, not the average velocity of all the thousands of stars in the vicinity.”
However, the biggest surprise came last June, when new data from the Hubble Space Telescope showed that the mysterious object had accelerated during its visit to the inner solar system in 2017 – an acceleration that is not explained by the sun’s force of gravity.
Acceleration of that sort can be explained by the rocket effect of comets: The comet approaches the sun, the sun warms the ice of the comet and the ice escapes into space in the form of gas, an emission that makes the comet accelerate like a rocket. But the observations did not reveal a comet tail behind Oumuamua. Moreover, gas emission would have brought about a rapid change in the rate of the object’s spin, a change which was also not observed in practice, and it also might have torn the object apart.
If it wasn’t comet outgassing, what force caused Oumuamua to accelerate? It is precisely here where Loeb enters the picture. According to his calculations, Oumuamua’s acceleration was caused by a push.
“The only hypothesis I could think of,” he relates, “is a push from solar radiation pressure. For that to work, the object would have to be very thin, less than a millimeter thick, in other words a type of pancake. In addition, the Spitzer Space Telescope found no evidence of heat emission from the object, and that means that it is at least 10 times more reflective than a typical comet or asteroid. What we have, then, is a thin, flat, shiny object. So I arrived at the idea of a solar sail: A solar sail is a spaceship that uses the sun for propulsion. Instead of using fuel, it is propelled ahead by reflecting light. In fact, it’s a technology that our civilization is developing at this very time.”
Bottles in space
Avi Loeb definitely knows a thing or two about solar sails. In 2016, the physicist and venture capitalist Yuri Milner, together with Stephen Hawking, Mark Zuckerberg and others, established Breakthrough Starshot, an initiative to accelerate solar sails to one-fifth the speed of light in order to explore the neighboring solar system, Alpha Centauri, which is four light-years away from us. Loeb was appointed the project’s scientific director.
“The first question we asked is whether a sail like Oumuamua could survive billions of years in the Milky Way – and we discovered that it could. Being hit by interstellar dust or gas won’t wear it down. Afterward, we tried to calculate the acceleration a solar sail would cause in an object [such as a ship or probe], and we found that the acceleration is consistent with that of Oumuamua.
“We have no way of knowing whether it’s active technology, or a spaceship that is no longer operative and is continuing to float in space. But if Oumuamua was created together with a whole population of similar objects that were launched randomly, the fact that we discovered it means that its creators launched a quadrillion probes like it to every star in the Milky Way. Of course, the randomness is significantly reduced if we assume that Oumuamua was a reconnaissance mission that was deliberately sent to the inner solar system – namely, to the habitable region where life would be feasible. But we need to remember that humanity didn’t broadcast anything tens of thousands of years ago, when the object was still in interstellar space. They didn’t know there was intelligent life here. Which is why I think it’s just a fishing expedition.”
Fishing for what?
“I don’t know. I love walking along the seashore when I’m on vacation, like here in Tel Aviv, and looking at the seashells with my daughters. Occasionally we find a glass bottle among the shells. In my opinion, the ‘bottle’ needs to be investigated. Until now we were looking for signatures of alien cultures in radio broadcasts, because we developed that technology in the last century. But another way is to look for a message in a bottle. Humanity launched Voyager 1 and 2, which are already in interstellar space. They’re messages in bottles. And in this century there will be a great many systems to which a great many bottles will be sent, and at far greater velocities.”
Like Breakthrough Starshot?
“Exactly. Our goal is to accelerate solar sails to one-fifth the speed of light, so that they will reach Alpha Centauri within 20 years. And the reason is clear: I am 56 years old, and Yuri Milner is 57. At that speed we will be able to see the pictures in our lifetime. Of course, the sails will continue on their way long after Milner and I are no longer around, maybe after none of us will still be here. It’s possible that space is filled with sails like these and we just don’t see them. We only saw Oumuamua because this is the first time we’ve had technology that’s sensitive enough to identify objects of a few dozen to hundreds of meters in size from the illumination of the sun. In three years, the building of the LSST telescope will be completed. It will be far more sensitive than Pan-STARRS and certainly we will see many more objects that originate outside the solar system. Then we’ll find out whether Oumuamua is an anomaly or not.
“The importance of my article lies in attracting the attention of astronomers so that they will use the best telescopes and look for the next object, and will even plan an encounter with it in space. The current propulsion technology doesn’t offer us the possibility to chase after Oumuamua. The visitor comes for dinner, goes out into the street and disappears in the dark. It’s possible we will never know what it was looking for.”