In the sixteenth century, Nicolaus Copernicus dared to go against the establishment by proposing that Earth rotates around the Sun. Having demoted Earth from its unique position in the cosmos to one of mediocrity, Copernicus set in motion a revolution in scientific thought. This perspective has influenced our thinking for centuries.
However, recent evidence challenges the Copernican Principle, hinting that we do in fact live in a special place, at a special time, as the product of a chain of unlikely events. But can we be significant if the Sun is still just one of a billion trillion stars in the observable universe? And what if our universe is just one of a multitude of others—a single slice of an infinity of parallel realities?
In The Copernicus Complex—available now from Scientific American/Farrar, Straus & Giroux—renowned astrophysicist Caleb Scharf takes us on a scientific adventure, from tiny microbes within the Earth to distant exoplanets, probability theory, and beyond, arguing that there is a solution to this contradiction, a third way of viewing our place in the cosmos, if we weigh the evidence properly. Bringing us to the cutting edge of scientific discovery, Scharf shows how the answers to fundamental questions of existence will come from embracing the peculiarity of our circumstance without denying the Copernican vision.
In the late 1700s the brilliant William Herschel, a German-born but Anglicized astronomer who discovered the planet Uranus, became enamored of the argument that there was life on other planets. It seemed more reasonable to him, as it did to many other scientists, that other worlds should be full of people and creatures rather than barren and empty. This logic also allowed for the comforting possibility that the same religious and social orders existed everywhere—a clever way to be both decentralized à la Copernicus and still cosmically important by virtue of our participation in a grander scheme. After all, if we drank afternoon tea and went to church on Sunday in bucolic England, surely the same thing would be happening on Mars.
Some of this thinking took even more inventive turns. Herschel mused that the Moon was inhabited by intelligent beings, and went so far as to declare that in his telescopic observations he felt sure he was seeing something akin to a forest on one of the lunar maria, or plains: “My attention was chiefly directed to Mare humorum, and this I now believe to be a forest, this word being also taken in its proper extended signification as consisting of such large growing substances… And I suppose that the borders of forests, to be visible, would require Trees at least 4, 5 or 6 times the height of ours. But the thought of Forests or Lawns and Pastures still remains exceedingly probable with me…”
He even felt that the Sun must harbor a hot atmosphere shielding a cool surface, glimpsed through sunspots that he thought, incorrectly, were gaps in this gas. Naturally there had to be inhabitants. As Herschel explained in 1794, “The sun… appears to be nothing else than a very eminent, large, and lucid planet… [which] leads us to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe.”
Herschel’s ideas about life on the Moon or the Sun were certainly not mainstream, but they weren’t entirely on the fringe, either. Even the famous and brilliant French mathematical physicist Pierre-Simon Laplace discussed the possibility of life on the other worlds of our solar system. But it was a little later, in the 1830s, that a scientifically minded Scottish minister and would-be astronomer by the name of Thomas Dick made some of the most extraordinary efforts to quantify the number of beings elsewhere in the universe. His first step was to assume that the population density of the United Kingdom at the time was representative of the density of beings on any other planet or asteroid—a startlingly mad thing to do, at least to our modern sensibilities.
On this basis he went on to estimate that the planet Venus held more than 50 billion individuals, Mars had 15 billion, and Jupiter a whopping 7 trillion. In a wild bit of speculation he even suggested that Saturn’s rings held something like 8 trillion inhabitants—just in the rings! Having completed all this enthusiastic extrapolation, he pegged the solar system’s net population of living beings at about 22 trillion—not counting the Sun, which he pointed out could alone harbor thirty-one times more creatures. And he didn’t see fit to stop there. He also came up with an estimate of the total number of planets in the universe being in excess of 2 billion, all of which could be inhabited with the same density of beings as those strolling around the sceptered isle in the 1830s. Ironically, this count of planets we now know to be woefully low, but in fairness, no one at the time knew the true scale and extent of the universe.
The motivations behind Dick’s projections (which were at the absolute extreme end of pluralist arguments) are still important to consider, because lots of serious scientists felt a kinship to them. There was no way to obtain incontrovertible proof that other worlds were or were not inhabited, and for many people it was simply easier to assume that they were. Even with the best telescopes of the time, it was unlikely that anyone would be able to genuinely confirm or disprove signs of life elsewhere. No images had the necessary resolution to help astronomers see the comings and goings of creatures on another planet.
Without evidence one way or the other apart from the Earth, an abundance of life on all celestial bodies could be seen as a natural part of planetary existence, like another layer of material that complemented the rocks and soil. If no other worlds were inhabited, then we would have to find a good reason why. The logic of this reasoning is hard to argue with. Once again, anything that sets the Earth apart from other places is awkward if you fully embrace a Copernican worldview, which was the scientific consensus at that time. It was better to populate the cosmos than make Earth unique.
But time has passed, telescopes have improved enormously, and our appreciation of the actual properties of life has changed irrevocably with the realization that organisms are not static entities. They are products of an ongoing and complex process of evolution and natural selection. Somewhere along this line of scientific thought, planets ceased to automatically equal life. Organisms don’t just plop down en masse. We recognize now that life might—or might not—be able to occur in certain places. The most extreme ideas of the plurality of inhabited worlds have faded from view, and today are firmly consigned to the scrap heap. Our exploration of the solar system has quenched the notion of complex life on the Moon, Venus, and other of our neighbor worlds. And even though we now know there are an overwhelming number of other planets in the universe, we also know that organisms like ourselves cannot occupy them all, because conditions on many of those worlds won’t allow it.
But we are left in a curious intellectual spot, because the universe is obviously a big place. Within our observable cosmic horizon—the distance over which light has managed to travel in the 13.8 billion years since the Big Bang—are several hundred billion galaxies and potentially more than a billion trillion stars. And that’s just the number that appear to us at any one instant, a mix of objects in a snapshot assembled from countless cosmic moments when distant light set out across space to us. Ask how many stars have ever existed in the past 13.8 billion years, and, apart from inducing a headache over the concepts of time and space in a relativistic cosmos, you’ll end up having to wave your arms wildly in the air to justify quoting an even bigger number.
This empirical fact is centrally important to our struggles to understand whether or not anyone else is out there. A huge universe motivates a different kind of answer than a tiny one with few suitable places, and it’s the kind of answer that we’ve all heard before, and probably even thought of ourselves. Since the universe is so big, filled as it is with a billion trillion stars, there surely has to be life somewhere else.
But does the gaping enormity of the visible universe really lead to the inevitable conclusion that there must be someone else out there? The question of “aloneness” contains other hidden layers, too. In particular, much like the pluralists of old, when we ask that question we’re usually wondering whether or not there are any other creatures like us in the universe: thinking, contemplating, technological or philosophical beings, with ideas, beliefs, art and poetry, and, of course, science. And, as with so many phenomena in our world that seem obvious, we would do well to step back for a moment and take a careful look at the details. In this case, a vital issue is whether or not we can tackle the implications of a massive universe with a mathematically rigorous analysis. Can we formulate a properly scientific response, one that moves us beyond the imaginings of pluralists or plain old knee-jerk optimism?
We can. And formulating such a response starts in the unlikely world of probability theory.
Excerpted from THE COPERNICUS COMPLEX: Our Cosmic Significance in a Universe of Planets and Probabilities by Caleb Scharf, published this week by Scientific American/Farrar, Straus and Giroux, LLC. Copyright © 2014 by Caleb Scharf. All rights reserved.