by Avi Loeb
What explains an object being at the LSR? What would need to have happened to place an object at such a distinctive velocity in our vicinity? As with all of ‘Oumuamua’s peculiarities, our answers depend on the assumptions we make concerning its origins.
Let us start with a hypothesis that will be more to the liking of most scientists than my lightsail one: If we suppose that ‘Oumuamua was a dry rock, then perhaps the parent star from which it was ejected was one of those one-in-every-five-hundred stars in the LSR frame.
Could this explain the fact that ‘Oumuamua, too, is at LSR? Yes, perhaps—if its departure from its home system was exceedingly gentle. Understanding why requires only common sense: An object violently ejected from a star system at LSR would be kicked into a different frame of reference. Only an object gently ejected from such a home system would retain the frame of its parent.
At the grave risk of straining the analogy, let us return to the multilane highway. Envision a motorcycle that is among the few vehicles that are “at rest” in comparison to the cars and trucks traveling around them. Now envision that motorcycle with a sidecar attached only by a single, well-greased pin. Immediately after that pin is gently removed, the sidecar, like the motorcycle, would remain at comparative rest. And if—and here is where we really strain the analogy—the highway was frictionless, motorcycle and sidecar would retain their position and velocity in comparison to all the traffic around them. Similarly, if a planet at LSR detaches a piece of itself gently, the detached piece would retain the planet’s position at LSR.
A gentle departure from a parent star is possible but statistically unlikely. Pieces of planets do not detach easily, and the sort of event that would detach a piece of a planet is rarely anything you could describe as gentle. A strike to a planet at LSR that resulted in that planet ejecting an object that remained at LSR would be akin to an extremely careful strike undertaken with the planetary precision of a feather. The probability of it happening is estimated at 0.2 percent.
It’s also possible that ‘Oumuamua came from one of the 99.8 percent of stars that are in significant motion relative to the LSR. But for that to happen, the ejection maneuver would have had to be a haymaker rather than a nudge, and a precise one at that. The kick that would eject an object from a star system not at LSR in a way that resulted in the ejected object being at LSR means a blow exactly equal and opposite to the velocity of the parent star. The blow would need to perfectly cancel out the home system’s movement to produce an object at the LSR. Imagine the challenge of a surgeon attempting a delicate operation with a crude instrument like a hammer.
Either possibility, feather or hammer, renders exceedingly unlikely this hypothesis about a naturally occurring ‘Oumuamua being ejected from a home system at LSR.
That leaves us with a third, only slightly more plausible hypothesis.
An object expelled from a home system at LSR could itself remain at LSR if its expulsion occurred on the extreme outskirts of the home system. There, of course, the gravitational hold of the parent star is far weaker. Indeed, Oort cloud–like outer shells are where most interstellar asteroids and comets that manage to break free of their birth systems probably come from. The more tenuous the gravitational hold of the birth star, regardless of whether it is at LSR or not, the easier it is for some of the debris in its outermost shell to be pulled away by another source of gravity.
Our own solar system’s trillion-comet Oort cloud is a case in point. Its icy shell is 100,000 astronomical units (AUs) distant from the Sun (each AU is the distance from the Earth to the Sun, or roughly 93 million miles). The Sun’s gravitational hold on the material making up the Oort cloud is far weaker than its hold on, say, the Earth. Way out there, a mild nudge of less than 2,200 miles per hour—what an encounter with a passing star might provide, for example—could be enough to send an object on its interstellar way.
So, if ‘Oumuamua originated from an icy Oort cloud–like shell around a host system in the LSR, that could explain the object’s velocity. But this doesn’t explain ‘Oumuamua being a dry rock.
No matter how you look at it, the dynamical origin of ‘Oumuamua—that it was at LSR before encountering our solar system—is extremely rare, and it’s all the rarer given our need for a naturally occurring object dry enough to produce no visible outgassing when it deviates from a trajectory explicable by the gravitational force of the Sun alone.
Which brings us to our second hypothesis, that ‘Oumuamua was a manufactured object specifically designed to be at the local standard of rest. Perhaps long, long ago, ‘Oumuamua was not junk but extraterrestrial technological equipment built for a distinct purpose.
Perhaps it was something closer in intent to a buoy.
…
We think of ‘Oumuamua as hurtling toward us, but it can be more instructive to view things from ‘Oumuamua’s vantage point. From that object’s perspective, it was at rest and our solar system slammed into it. Or, in a way that works both metaphorically and, maybe, literally, perhaps ‘Oumuamua was like a buoy resting in the expanse of the universe, and our solar system was like a ship that ran into it at high speed.
The hypothesis that intelligent extraterrestrials designed ‘Oumuamua to be at LSR raises the obvious question: Why would they bother? I can imagine any number of reasons. Perhaps they wanted to set up the interstellar equivalent of a stop sign. Or maybe it was more like a lighthouse—or, more simply, a signpost or navigation marker. A vast network of such buoys could act as a communication grid. Or it could be used as a trip wire, an alert system triggered when one of them was knocked out of LSR. In that spirit, perhaps its creators wanted to disguise its—and their—spatial origins. Putting an object at LSR effectively camouflages who put it there. Why? Because math and a little knowledge of an object’s trajectory is sufficient to trace that object’s origins back to a launchpad; doing that is one of the primary purposes of the North American Aerospace Defense Command (NORAD). Consider as well that any intelligence with a grasp of math and a good map of the universe could trace back to Earth any of the interstellar ships we’ve launched from our planet’s surface.
That all of these analogies are terrestrial isn’t just a reflection of the fact that so is this book’s author. That human civilization has built buoys, grids of communication satellites, and early-warning detection systems tells us that it is probable other civilizations might do the same thing. What is more, these conjectures are plausible for the simple reason that any one of them is an outcome that humanity could engineer, build, and launch if we wished to. Our reasons needn’t even be interstellar. Just for example, if India had put such an object into space, the scientists at NASA might wonder why, but they wouldn’t wonder how there came to be a small, flat, luminous object marked with the Indian Space Research Organisation’s distinctive logo at LSR.
The hurdle to accepting that this answer applies to ‘Oumuamua is, of course, that it requires us to accept that ‘Oumuamua is of extraterrestrial origin. And the hurdle to that is that we must take seriously the possibility that we’re not the only intelligence in the universe.
…
A buoy. A grid of pods for communication. Signposts that an extraterrestrial civilization could navigate by. Launch bases for probes. Other intelligent living organisms’ defunct technology or discarded technological trash. These all are plausible explanations for the ‘Oumuamua mystery—plausible because here on Earth, humanity is already doing these things, albeit on a far more limited scale, and we would certainly consider replicating them if and when we explore out into interstellar space.
What renders these hypotheses implausible is the inability to conjecture an extraterrestrial intelligence. Foreclose that possibility, and you moot all such explanations. Refuse to look through the telescope and it matters little if it does or doesn’t show any compelling evidence. Perhaps it is the shadow of science fiction stories or just a flaw in some people’s ability to broaden the scope of hypotheses they will entertain, but ent
ertaining an explanation that posits an extraterrestrial civilization is much like presenting to the skeptical a telescope they outright refuse to look through.
The best antidote to such recalcitrance, I have found, is to think for yourself. If any of these ideas seem feverish or over the top or detached from reality, just remind yourself of the evidence before you.
The data we confront tells us that ‘Oumuamua was a luminous, thin disk at the LSR, and when it encountered the gravitational pull of the Sun, it deviated from a trajectory explicable by gravity alone, and it did so without visible outgassing or disintegration.
These data points can be summed up as follows: ‘Oumuamua was statistically a wild outlier.
Using very conservative probabilities, based on its shape, rotation, and luminosity alone, a cometary ‘Oumuamua would be a one-in-a-million naturally occurring object. Attempt to explain its composition so that we can explain its deviation beyond solar gravity by outgassing that was invisible to our instruments and you still have an object that is as rare as one in thousands.
But that’s not all. It’s also very odd, remember, that ‘Oumuamua’s spin rate didn’t change. Maybe just one in every thousand comets keeps a steady spin despite the significant mass loss implied by ‘Oumuamua’s nongravitational acceleration. If ‘Oumuamua was one of those rare comets, we’re now talking about a one-in-a-billion object.
Then there is its lack of jerks. If there was naturally occurring outgassing and disintegration that was somehow invisible to our instruments, those putative jets propelling ‘Oumuamua would have had to perfectly cancel each other out. If that’s another one-in-a-thousand coincidence, ‘Oumuamua is now one in a trillion.
And we still have to take into account ‘Oumuamua’s origin in velocity-position space, the fact that it was at LSR. Recall that an LSR birth star is a 0.2 percent probability, so now the odds that ‘Oumuamua is just a random comet are approaching one in a quadrillion.
These numbers strain credulity and beg for an alternative explanation. They were what led me to suggest to Shmuel Bialy that we search for another, more plausible hypothesis. And we could come up with just one that made sense for the nongravitational acceleration: ‘Oumuamua’s weirdly steady push was provided by sunlight.
This was fully consistent with an important clue. The excess force acting on ‘Oumuamua that caused it to deviate, observers noted, seemingly declined inversely with the square of its distance from the Sun, as one would expect if that force was provided by reflected sunlight.
But solar radiation pressure isn’t very powerful. If it were indeed responsible, we calculated, then ‘Oumuamua had to be less than a millimeter thick and at least twenty meters wide. (The diameter depends on the object’s reflectivity, which is unknown. If ‘Oumuamua were a perfect reflector, bouncing back 100 percent of the sunlight that hit it, in this super-thin scenario it would be twenty meters across.)
There is nothing in nature with those dimensions, as far as we can tell, and no known natural process that can produce them. But of course, humanity has built something that fits the bill, and we’ve even launched it into space: a lightsail.
We arrived at this hypothesis through logic and evidence—in short, by sticking to the facts. But if we take the hypothesis seriously, it allows us to ponder new, incredible questions about how ‘Oumuamua appeared in our universe and where it came from. It even, as I will explain, gives us the opportunity to ask whether we might someday meet the creators of this mysterious visitor.
The lightsail hypothesis opens up a world of possibilities—unlike the comet hypothesis, which closes them off. The fact that scientific consensus strongly favors the more conservative and restrictive of these two possibilities says less about the evidence than it does about the practitioners and culture of science itself.
7
Learning from Children
Are we alone? This question is among the most fundamental humanity confronts. The moment we have a conclusive answer, negative or positive, is the moment we face profound realizations. Indeed, there are few cosmological questions of equal importance.
To be sure, it would be transformative to learn what preceded the Big Bang, where the matter sucked into a black hole goes, or what theoretical insights finally square relativity with quantum physics. Indeed, I have devoted a significant portion of my life and career to answering the first two of these questions. But would the answers to these questions change our sense of ourselves as significantly as learning that we are just one intelligent species among many—or, conversely, that we are the only conscious intelligence to arise in the universe? I doubt it.
Because I believe this question is so consequential, I find it remarkable how rarely, and how cavalierly, scientists have gone about seeking an answer. That didn’t start with the resistance to my lightsail theory; far from it. Scientists’ reluctance to read the messages of ‘Oumuamua long predates its passing through our solar system.
…
The search for extraterrestrial life has never been more than an oddity to the vast majority of scientists; to them, it is a subject worthy of, at best, glancing interest and at worst, outright derision. Few of repute have dedicated their careers to advancing the field, and even at its zenith of academic respectability, in the 1970s, only about a hundred scholars were publicly associated with the SETI Institute. Far more speculative fields of mathematical gymnastics are known to attract bigger communities of physicists.
The more rigorous approach to SETI began in 1959 when two physicists based at Cornell University, Giuseppe Cocconi and Philip Morrison, coauthored a seminal paper titled “Searching for Interstellar Communications.” Their article, which was published in the prestigious scientific journal Nature, made two simple conjectures. First, that extraterrestrial civilizations as advanced as ours if not more so existed. Second, that such civilizations would most likely broadcast their interstellar communication We exist at the radio frequency 1.42 GHz, that “unique, objective standard of frequency, which must be known to every observer in the universe.” Cocconi and Morrison were referring to the twenty-one-centimeter wavelength of neutral hydrogen—the very same radio emission that would preoccupy me and other astrophysicists nearly half a century later as we attempted to peer back in time to the cosmic dawn.
The paper was an immediate sensation, heralding the birth of SETI and establishing the rationale for all subsequent searches for extraterrestrial intelligence with its concluding sentence: “The probability of success is difficult to estimate; but if we never search, the chance of success is zero.” For me, it echoes a much older thought, one attributed to Heraclitus of Ephesus: “If you do not expect the unexpected, you will not find it.”
Cocconi and Morrison’s paper also brings to mind the old saw about people with nothing but hammers seeing only nails everywhere they look. The two men were writing a quarter of a century after the birth of radio astronomy, a fact that surely helped them in their effort to “expect the unexpected.” As with my and Bialy’s lightsail hypothesis, humans seem to be better at seeing the technological signature of alien civilizations after we have developed the technology ourselves.
Cocconi and Morrison’s paper immediately inspired the astrophysicist Frank Drake, also at Cornell. In 1960 he decided to conduct just the kind of search that Cocconi and Morrison encouraged. Using the National Radio Astronomy Observatory of Green Bank, West Virginia, Drake conducted a search of two nearby sun-like stars, Tau Ceti and Epsilon Eridani. For one hundred and fifty hours, spread out over four months, Drake used the radio telescope to seek a discernible signal indicating intelligence, with no success. The sense of whimsy with which Drake undertook his search for extraterrestrial life is captured in the name he gave the project: Ozma. It derives from a character invented by the novelist L. Frank Baum in his Land of Oz books.
Drake’s project became the stuff of broad interest and popular media coverage. That about two hundred hours of observation discovered nothing hardly dimmed public enthusi
asm. And on the updraft of that interest, in early November 1961, Drake participated in an informal conference sponsored by the National Academy of Sciences at the National Radio Astronomy Observatory. It was there that he first articulated the Drake equation, which he used to estimate the number of actively communicating extraterrestrial civilizations.
The equation now graces T-shirts, informs the plots of young adult novels, was misused by Gene Roddenberry to bestow a patina of plausibility to the television series Star Trek, and has been roundly critiqued and tweaked by scientists ever since. Lost in the dust and noise is the simple appreciation that the equation is nothing more than a heuristic, a shorthand tool to factor out the different terms that affect the success of SETI. Its standard expression is:
N = R* × fp × ne × fl × fi × fc × L,
where the terms are defined as:
N: the number of species in our galaxy that possess the technology necessary for interstellar communication;
R*: the rate of star formation in our galaxy;
fp: the fraction of stars with planetary systems;
ne: the number of planets in each system with environmental conditions amenable to life;
fl: the fraction of planets on which life arises;
fi: the fraction of planets on which intelligent life arises;
fc: the fraction of intelligent life that develops sufficiently sophisticated technology to take part in interstellar communication;
L: the duration of time such intelligent life is able to produce detectable signals.
