The Aliens Are Coming!

by Ben Miller

  So a picture is emerging. Since the middle of the last century, the search for aliens has been divided into two camps: the UFO enthusiasts, who definitely aren’t scientists; and SETI, who definitely are. No prizes for guessing whose side I’m on. The irony, for me, is that so far the UFO lot have had the best press. Most people, I would hazard, have heard of Roswell while few have heard of the Wow! Signal. So far, SETI may have drawn a blank, but given the paltry number of stars we’ve searched, that’s exactly what you would expect. And thanks to NASA’s Kepler Space Telescope, in the future we’ll have a much better idea of where to look.

  As the discovery of Kepler 186f makes clear, when it comes to SETI it’s all about timing. Kepler 186f is 500 light-years away, so for a signal to reach us today not only would it have to have been sent by a transmitter ten times more powerful than any we have on Earth, it would have to have been sent 500 years ago. We know that civilizations with the ability to communicate are possible, because we are here. But to detect our neighbors, they need to transmit a signal at just the right moment in the past for us to intercept it in the present. All this is expressed in the Drake Equation, of course, but there’s nothing like a real-world example to bring it into focus.

  It’s stating the obvious, but the reverse is also true. To message our neighbors, we need to allow our signals time to cross the galaxy. And if we want those signals to reach millions of planets, they need to be strong. Back in the day, our TV was broadcast by powerful transmitters. Fawlty Towers, for example, which first aired in 1975, is only just reaching alien worlds thirty-five light-years away. The Office, on the other hand, may never arrive. By that point, we were watching on cable, satellite, or DVD, none of which leak much of a signal into space. If you think about it, as far as radio transmissions go, we were only noisy for a handful of decades.1

  And even if we are lucky enough to catch some neighboring aliens during their equivalent of our twentieth century, there’s no point striking up a conversation if they live a thousand light-years away. No signal—be it radio, laser, or modulated cosmic ray—can ever travel faster than the speed of light, so each time we sent a message, we’d have to wait 2,000 years before we heard anything back. Even if Frank Drake’s guess is correct, and the average civilization lasts 10,000 years, we’d just about have time for “Hello, what’s the secret of life?” . . . “Thank you very much, goodnight.”

  To be truly communicable, our neighbors need to be close by. Luckily, they may well be. As we’ve learned, the Kepler data has shown us that over a fifth of Sun-like stars have Earthlike planets, implying the nearest one might be as little as twelve light-years away. Spookily close, eh? We’ve been scouring the distant galaxy, and they were in our backyard all along. Maybe our nearest M-type star, Proxima Centauri, is home to an Earthlike planet? If that’s the case, then they probably can pick up The Office. In fact, one or two of them may even be in it.2

  Kepler has given us a good feel for the fraction of Sun-like stars that have Earthlike planets, all of which is grist to the mathematical mill that is the Drake Equation. Now we need to know the fraction of Earthlike planets where life blooms. Thankfully, the extraordinary story of how life on this planet got its start provides vital clues as to what we can expect to find out there in the galaxy. Once again, I have to say you are reading this book at just the right time. Our understanding of Earth-based life has been making extraordinary progress over the past few years; in fact, to such an extent that many prominent astrobiologists are beginning to wonder whether it really is Earth-based at all.

  CRAZY LITTLE THING CALLED LIFE

  Like all great endeavors, the story of life on Earth is basically a game of two halves. In the first half, a microscopic universe emerges in a Big Bang of impossibly dense energy, then rapidly inflates, then settles down into a slow expansion, eventually cooling to form stars and galaxies.3 In the second, the Earth forms, develops an atmosphere, and is bombarded by water-laden comets which create the oceans. Life appears, first as single-celled microbes, then becomes multicellular,4 then evolves into complex organisms like you and me. And here’s the rub. In the first half, the game is rigged in life’s favor. In the second, everything is left to chance.

  To see what I mean, let’s look at the first half of that story. It’s often said that the Earth is a “Goldilocks” planet, because, just like Mommy Bear’s porridge, it is neither too hot nor too cold, but just right. “Just right” in the case of the Earth means at the right distance from the Sun to have oceans of liquid water, and in fact we talk about a “habitable zone” as being the band of orbits that a planet can sit in where it is neither too close to its home star so that water vaporizes nor so far away that it freezes.

  But it’s not just the Earth that is incredibly well suited to life. Kick the tires of creation and almost everything you find seems rigged in life’s favor. Consider this: The age of the universe, its mass, the rate at which it’s expanding, its lumpiness (more on that later), and the relative strength of its four fundamental forces are all incredibly finely tuned. Change any one of them slightly and life would cease to exist. Forget about a Goldilocks planet, we live in a Goldilocks universe.

  A LITTLE HISTORY OF THE UNIVERSE

  Let’s just remind ourselves of what an extraordinary place the universe is by brushing up on our basic cosmology. Firstly, no one knows its true size. It may be infinite, it may be one of an infinite number—we don’t know. One thing is certain, however: It has a finite age. The latest data, from Europe’s Planck satellite, clocks the cosmos at 13.8 billion years. For comparison, the age of our home galaxy, the Milky Way, is 13.2 billion years; the Earth is a mere stripling at 4.55 billion years.

  The fact that the universe has a finite age means that there’s a limit to how far out we can see with our telescopes, because this is also the age of the oldest light. At the time of writing, one of the most distant objects known is the galaxy UDFj-39546284, which we are seeing a mere 480 million years after the Big Bang.5 The light from UDFj-39546284 has therefore taken over thirteen billion years to reach us, which is even longer than the average pharmacist takes to locate your medicine after you hand them your prescription.

  A finite age, of course, also implies that the universe had a beginning. We call that beginning the “Big Bang,” a wry phrase coined by the twentieth-century British astronomer Fred Hoyle, who was himself a proponent of the rival “Steady State” theory. Despite his gentle mockery,6 it is the Big Bang which has proved the test of time. The universe is expanding, as we can readily see from the red-shifted light of distant galaxies and supernovae; if we rewind the footage that means it must have originated from a single point. Poetically, cosmologists call such a point a singularity, because it operates outside the known laws of physics.

  Just so you know, the universe has expanded in four distinct phases. Immediately after the Big Bang came an extremely short, slow, steady expansion. The second phase, called inflation, was extremely rapid, lasting only from 10-36 to 10-32 of a second.7 The third, lasting roughly thirteen billion years, was also slow and steady. The fourth, which has lasted roughly a billion years, has seen the universe’s expansion speed up, as if some repulsive force is finally winning out over gravity.8 This is the epoch in which we have the great fortune to find ourselves.

  Some people worry about the Sun running out of fuel, which is predicted to happen in about five billion years’ time. Not me. I worry about the runaway expansion of the universe. The fact that the furthermost galaxies are moving faster and faster away from us means that, in a few billion years or so, they will be moving away from us at the speed of light. At that point, they will disappear from our telescopes, leaving only darkness. Eventually, of course, all but the very closest galaxies will do the same, and one by one the stars will go out, too. Tell me that doesn’t give you the most horrendous claustrophobia. If the Sun dies, we can just move to another star, but jumping ship to another universe is a whole other matter.

  We have a name for
whatever it is that’s causing this runaway expansion of space: dark energy. Many cosmologists believe it’s a property of space itself, called vacuum energy. In essence, the idea is that space is never really empty, but contains a swarm of so-called virtual particles. As the name suggests, they don’t stick around for long. An electron and a positron, for example, will spontaneously appear out of nothingness, have their fun, then annihilate one another within a vanishingly small fraction of a second. Cheeky as this sounds, it’s permitted by quantum theory, where you can borrow any amount of energy provided you borrow it for a short enough time. The snag is that the rate of expansion caused by all the virtual particles in the universe should be much, much greater than that we see. In fact, it should be about 10120 times greater, a discrepancy which is known fondly in the cosmology community as “the worst prediction in physics.”

  Secondly, we aren’t entirely sure what most of the universe is made of. In a nutshell, we can see that nearby galaxies are spinning too fast to be held together just by normal matter in the form of stars, gas, and dust; we call the missing stuff “dark matter.” Dark matter, whatever it may be, exerts a gravitational pull on ordinary matter, but little else. Our best guess is that it is some sort of as-yet-undiscovered particle with no charge and a large mass. The leading candidate is a theoretical particle called a neutralino, though, despite a worldwide effort by thousands of scientists running an impressive array of detectors, no one has yet managed to track one down.9

  Whatever dark matter is, it’s definitely the boss of you. Incredibly, the ordinary matter of the periodic table makes up only 15 percent of the total matter in the universe; the other 85 percent is dark. There are some outlying theorists who suggest that dark matter is made up not of one but a whole zoo of particles, and is therefore also capable of forming dark black holes, dark planets, and possibly even dark life, but given that we have yet to find even one dark matter particle, the jury’s out.

  And thirdly, despite an unpromising start in a chaotic molten fireball, both dark and ordinary matter is full of glorious structure. Like wheels within wheels, planets orbit stars, which are grouped into galaxies, which club together to form galaxy clusters, which in turn federate into galactic filaments. Lumpiness now implies lumpiness earlier, and in fact one of the great triumphs of experimental cosmology has been to trace the structure of stars and galaxies right back to the fireball, only 380,000 years after the Big Bang. That needs a word or two of explanation. You’ll remember that the microwave background radiation is the remnant of the light from the Big Bang, released when neutral atoms formed around 380,000 years after inflation. At the time it was set free it was extremely high-energy—think of x-rays and gamma rays—but as the universe expanded, it cooled, becoming microwaves.

  Since the microwave background radiation was last in thermal equilibrium with the rest of the universe when neutral atoms formed, it should bear the imprint of any lumpiness in the form of temperature variations. Yet back in October 1988, when I started my cosmology course, every effort to find any temperature variation in the microwave background had returned a negative result. If the lumpiness was there, it was too fine-grained for our best telescopes to spot.

  It was NASA’s COBE satellite, launched in November 1989, that finally found what we were looking for. The temperature variations in the cosmic microwave background radiation were tiny, but they were there, winning the COBE team the Nobel Prize in Physics in 2006. At last we knew: The fireball had not been smooth, it had been lumpy. What’s more, the lumpiness had a specific value of 1 part in 100,000, as predicted by our theories of inflation.10

  As always with cosmology, there’s a catch. Inflation theory predicts that on a scale greater than that of filaments there’s no more lumpiness, a phenomenon that cosmologists rather grandly call “The End of Greatness.” Interestingly, no one seems to have told the universe. As we improve our telescopes, bigger and bigger stuff seems to turn up.

  The record used to be held by something called the Great Sloan Wall, a filament of galaxies a billion light-years across. Recently, however, astronomers found the first stirrings of something roughly ten times larger, called the Hercules–Corona Borealis Great Wall, or HCB GW for short. And it’s not an isolated result. The latest data from the Planck satellite seems to show that there is more matter in one half of the universe than the other, which implies there are even bigger objects out there yet to be discovered. It’s beginning to feel like every time we increase the volume of space we survey, we find an even larger structure.

  Yet impressive as such objects undoubtedly are, it’s at the scale of galaxies that the universe holds a truly numinous beauty. Our own discus-shaped Milky Way is a stunning example. A giant swirl of hydrogen gas, dust, and stars circulating an enormous central supermassive black hole, within its spiral arms stars of all sizes are constantly forming as thick clouds of gas and dust collapse under their own gravity.11 And it’s within stars that atoms are made.

  The bigger the star, of course, the hotter and faster it burns, and the bigger the atoms it can make. Smaller stars like our own Sun are capable of making lighter elements, but the biggest stars can make every element in the periodic table, right the way up to uranium. At the end of their lives these giant thermonuclear pressure cookers then explode in what is called a supernova, showering the surrounding gas with freshly minted atoms.

  If this aforementioned gas then collapses under gravity to form a new star, some of these atoms then find themselves clumping together under gravity to form asteroids, moons, and planets. Our own Sun is just such an example, burning hydrogen to make helium, surrounded by a belt of rocky planets, then a rocky asteroid belt, then giant gaseous planets, then giant ice planets. It’s extraordinary to think it, but all stuff that surrounds the Sun is a remnant of ancient supernovae. You, this book, and this planet are literally made from stardust.

  A POCKET UNIVERSE

  All very well, you might think, but what has all this got to do with life? What difference would it make if the universe was an infinite number of years old rather than 13.8 billion? If all we experience from dark matter is the pull of its gravity, couldn’t we do without it? Does it really matter how big galaxies are, or whether stars make atoms?

  Fascinatingly, the answers are: a lot; not in the slightest; yes, enormously; and you’d better believe it. To see what I mean, we are going to create our very own model universe, and then start messing with it. And if you are willing to accept that the very fact that you are reading this book requires stable stars, an abundance of hydrogen, carbon, nitrogen, oxygen, and maybe a little phosphorus, and enough time for evolution to take place . . . well, then, you are going to be in for a bit of a shock.

  GIVE IT A WHIRL

  To get a taste of what I mean, think of that most mysterious of forces, gravity. In daily life it feels like a hindrance, pinning us to our beds in the morning, exhausting us at the gym, and flinging each and every delicious slice of toast on to the kitchen floor butter-side down. One of the great joys of the Moon landings was seeing NASA’s astronauts roam free, leaping across the lunar surface in giant strides despite being mummified in water-cooled suits and carrying backpacks the size of wardrobes.

  Yet compared to the other forces of nature, gravity is extraordinarily weak. A hydrogen atom, for example, is made up of a sole proton orbited by a single electron. There are two forces at work: the electromagnetic force, which attracts the negative charge on the electron to the positive charge on the proton; and the gravitational force, which attracts the heavier proton to the lighter electron. Staggeringly, the gravitational force is roughly 1036 times weaker than the electromagnetic force. If gravity is so feeble, perhaps we could do without it altogether?

  But consider this: Without gravity, there would be no life anywhere. Why? Because there would be no universe. Since the middle of the twentieth century we’ve known that all creation began in a Big Bang, inflating out from a minuscule fireball only a fraction of the size of an atom,
and cooling into the colossal mix of ordinary matter, dark matter, and dark energy that we see today. In other words, though it started with nothing, today the universe contains a great deal of energy, present in several different forms.

  So where did all this energy come from? How do you get something from nothing? Gravity is our best answer. In the broadest of terms, we think that the “positive energy” of all the stuff in the universe is balanced by the “negative energy” of its gravity. The total energy of the universe is therefore zero. For this reason, the originator of inflation theory, the American physicist Alan Guth, has described the universe as the “ultimate free lunch.”

  So without borrowing energy in the form of gravity, the universe would never have got its start. But that, of itself, doesn’t dictate how strong gravity needs to be to produce a universe that is capable of bearing life. And here’s the really interesting thing. If gravity were appreciably stronger, life would simply not exist. Intrigued? Then read on, because, quite frankly, that’s not even the half of it.

  WORKS STRAIGHT OUT OF THE BOX

  OK, let’s play this game. Let’s imagine we are superbeings and we have been given our very own universe for Christmas. It’s basically an empty glass box about half a meter across, sitting on a wooden base. It comes with a row of dials, a green button that resets to empty, and a red button that initiates the Big Bang. No need to look for batteries; it doesn’t need any, because the universe comes for free.

  To begin with, the dials are all pre-set. If we push the red start button, we see the universe emerge in a bright Big Bang, then immediately inflate, then expand and cool to form stars and galaxies. If we are really keen-eyed, we’ll notice that the expansion speeds up over the last second of its 13.8 second lifetime.12 The whole thing then freezes, preserving the universe as it is at the present moment. If we switch the lights out in our superdwelling, we can see the whole observable universe, with the Virgo supercluster at the center. And though it’s much too small to see when viewed on this small scale, at the center of the Virgo supercluster is the Sun, orbited by the Earth.