Nothing. Not a trace, not a hint, not a glimmer. We may not be alone, but we might as well be.
That could change, any day. One day, tomorrow or the next century, we'll catch that whisper, we'll detect that first hint of life, we'll discover a primitive microbe buried under a kilometer of ice. That will truly be a wondrous day, to be remembered throughout the future annals of history: the day we finally proved that there are others. That will surely be the first day of a new era for humankind. Or the last. You know, it's a toss-up.
Again, what are the chances for life appearing on another world? I boldly stated that it was slim but not zero, without any, you know, proof to back that up.
Well then, let's rewind.
What does it take for life to appear? What's the right cocktail mix, the right balance of sweet and sour, to get life going? The answer, of course, is “It depends.” So far we have access to only one kind of life to study: the life on Earth. Energy from sunlight or deep-sea vents. Carbon for structures. Water for a solution for chemical reactions. Limited to a narrow range of temperatures and pressures. We may find microbes on the bottom of the ocean and drifting through the upper reaches of the atmosphere, but for all that range, life thrives in just a thin delicate shell. Let's just say that all of the life on Earth could be obliterated and the rest of the universe wouldn't even notice. Do you mourn for that fleck of skin when you scratch your armpit? If you do, you're strange, and the universe is not strange. It wouldn't weep for us.
What did life on Earth need to get the ball rolling? First, it needed liquid oceans. All life on the planet requires water in some way. Life started in the oceans, and the graduation to land happened when organisms could carry their own little bits of ocean with them—the invention of skin.
To get a liquid ocean, a planet has to have a heat source. But not too much of a heat source. The sun is a nice, handy heat source. Too far from the sun and all your water's locked up in ice, but too close and the water molecules are too agitated to stick around. Astronomers, always the eager label makers, identify the region around each star where water has the best chances of being, well, water as the habitable zone. Not to be confused with the Twilight Zone or the end zone, it's the ring in each system where Goldilocks finds her favorite soup: not too cold, not too hot.
The habitable zone isn't the only zone, though. Tides can heat up a place too: the constant flexing, stretching, and bending from gravity can warm a world nice and toasty, even if it's far from a sun. To make that work, your candidate planet actually has to be a moon—preferably of a fat gas giant. That's the only way to get enough gravitational tug to turn your core into Play-Doh. Hence all the interest in the (subsurface) liquid water oceans of Europa, Enceladus, and more. But until we know for sure that life has found a holdfast there, we'll leave it to the side here.
Sunlight itself seems kind of useful. After all, it is a giant, constant source of energy available for free to anybody. Some kinds of life on Earth get their energy from other places, like deep-sea hotspots, and while it's currently up for debate whether life got started in those extreme conditions or not,3 the light-eating kind certainly proved more popular on this planet.
Next, life needs an atmosphere to keep out the constant storm of cosmic rays. I haven't talked about cosmic rays, but for the purposes of this discussion, just understand that the universe is swimming in a constant bath of death-dealing high-energy particles and radiation,4 and atmospheres make for a great security blanket.
The pressure from the atmosphere helps to keep the water on the Earth…on the Earth. Without that pressure, oceans would simply boil away into the vacuum.
Life needs planets with a thick atmosphere, yes, but not too thick! Then the pressures would be too great for delicate life to form extended structures, and the energy of the sun would go into powering storms and winds instead of photosynthesis. Temperatures in those atmospheric pressure cookers can be so high that they can vaporize any water that might have been brave enough to reach the surface. Just look at poor, poor Venus, forever choking in its own haze, too hot and too intense for life to ever get a running start.
Don't forget the magnetic fields. Those cosmic rays are made of charged particles, and magnetic fields can steer them from hitting the precious surface of a planet, either bouncing them away like bullets off Superman's chest or funneling them (relatively) harmlessly into the polar regions. Most planets, especially inner rocky ones, don't get a strong magnetic field. Earth did.
A nice large moon helps, too. See all those craters on Earth's moon? Those are all comets and asteroids that hit the moon instead of hitting the Earth. A few impacts here and there can be a good thing, spurring evolution or delivering some useful compounds. But too much of anything can be a bad thing. One scar on your face can give you character. A hundred? That's a lot of character.
The moon isn't the only thing in the system playing solar goalie for cosmic deathballs headed for the Earth. Jupiter, with its massive gravitational pull, is the sheriff of the outer system, pulling rogue comets into its orbit or kicking them entirely out of town. With a massive planet like Jupiter in the outskirts, the life-bearing inner worlds are that much less vulnerable.
I should note that occasionally Jupiter pitches a rock from the asteroid belt into the inner solar system, so maybe it's a wash there.
Your host star needs to be stable over millions, even billions of years. It can't be young and volatile, throwing energetic tantrums that are no good. It can't be old and senile, filling the inner system with blasts of radiation. Life takes a long time to get its foothold and start running. If the race is over too soon, there's nowhere to go.
And of course, to get water on rocky planets you need (a) some water and (b) a lot of rocks. Not just any old rocks will do: you need good amounts of carbon, oxygen, nitrogen, sulfur, and hydrogen. Life is complex, depending on all sorts of reactions and processes—and those elements are at the heart of them.
You have to pick the right spot in your galaxy too. Too far away from the center, and there aren't enough of those precious elements to make a wet rock. Too close, and you risk being blasted by the intense radiation of the dense stellar neighborhoods.
Taken altogether, the chances of life appearing on any one planet do appear slim.
Really weird orbit? Too bad, no life.
Highly variable star? Too bad, no life.
Too massive giving you a thick atmosphere? Too bad, no life.
Too small for an atmosphere? Too bad, no life.
Not a lot of carbon? Too bad, no life.
No large outer planets? Too bad, no life.
Too much axial tilt and no stable weather patterns? Too bad, no life.
No magnetic field? Too bad, no life.
No plate tectonics? Too bad, no life.
Unlucky massive comet strike? Too bad, no life.
No rotation? Too bad, no life.
Nearby supernova? Too bad, no life.
That's just to get life started. The barest, simplest set of criteria needed to add a bio in front of chemistry. Single-celled organisms ruled Earth for something like a billion years. A billion years! And putting your DNA in a nucleus was once considered a hot new fashion trend. Complex, multicellular life? Land-dwelling life? Life that can bang rocks together? Life that likes to think it can think? That takes time. Deep time. Millions upon billions of years of stability. Look how many times life on Earth narrowly escaped complete extinction. How many times the total gene pool was more like a shallow pond.
Given all the opportunities life needs to get started and evolve, it's surprising there's life anywhere at all, let alone intelligent life, let alone life that can write a chapter in a cosmology book about the possibility of life.
And yet, like I said, here we are.
We beat the odds, so far at least. And if we can beat the odds, so can someone—or something—else.
You most likely did not get in a car accident today. If you did, at least you can read this while you w
ait to get your arm in a cast. Car accidents are rare, on a personal, individual level. Think of all the tiny little coincidences that have to line up to make you get into an accident on this drive. Leaving the house fifteen seconds later than normal. Being distracted by that repetitive song on the radio. Looking left at the intersection first instead of right. The sweat on the palm of the other driver's hand, reducing his ability to turn by a few microseconds. The brake pads worn down by 50 percent instead of 49 percent.
Take any one of those elements away, and boom. Well, the opposite of boom. No accident.
The chances of getting in an accident are so low that you don't even think about it. Run out of milk, pop over to the nearest store. Time for practice? Pile up in the back. Date night? Make sure you cleaned out the cheeseburger wrappers.
Despite the chances being so incredibly low, accidents happen every single day. Not to you, but to somebody, somewhere. The odds are low but not zero. And that tiny number gets multiplied by the incredible number of cars on the road at any time.
Accidents find a way, and life is an accident waiting to happen.
And so we have a bit of a paradox, named the Fermi paradox in honor of Enrico Fermi, who (naturally) first articulated it.5 The odds of any planet hosting life are incredibly tiny, almost but not quite zero. So we do occupy a privileged position in the universe. Aha, the revenge of Ptolemy! We may not be at the center—that ship sailed a long time ago—but there is something unique, something special about us, about Earth. A little unlike the other planets in the solar system, perhaps the galaxy, and dare I say…the universe?
But! Time and time again we've found that we live in a Copernican universe, one where we are not at the center and we are emphatically not special. The physics surrounding you right now—the pull of gravity, the photons entering your eyes, the chemistry and thermodynamics—it's all exactly the same across the universe. So life, while rare, can't be too rare. If a process or interaction is forbidden in our cosmos, it simply doesn't happen, full stop. If it's allowed, it must be commonplace, because the universe is so freakishly gigantic.
But it looks like life is right on the razor's edge between allowed and not-allowed. Not strictly forbidden in the universe but definitely frowned upon.
So if we're not special after all, and life has a halfway decent shot, where is everybody? Hence, the paradox.
Before resolving it, let's first visit some ways we might be able to spot anybody else.
Freeman Dyson suggested that as we grow up as a species, we would have to go to extraordinary lengths to satisfy our unquenchable thirst for energy. How else are we going to play mind-controlled online poker in the far future? We would find wind power and nuclear power far too wimpy and be inspired to try something truly fantastic: encasing the sun in a giant sphere of rock, collecting 100 percent of that juicy solar output for our nefarious purposes. Of course, such an engine wouldn't be absolutely perfect (even a superadvanced civilization still has to obey thermodynamics), so it would leak a little heat. Actually, a lot of heat. From a great distance, you wouldn't see the star itself (encased in rock, etc.), but you would see something like a blurred-out, surprisingly red, probably infrared, starlike object. If we got overly ambitious, we could do the same to every star we came across, which would be so dramatic it would shift the characteristic hues of the galaxy.
We have found no signs of such constructions, either in the stars we can individually observe or in our deep galaxy surveys. Perhaps that's no surprise when you dig into the details of a so-called Dyson sphere. They require a lot of material to build, and you need to spend a lot of energy assembling it—those rocks ain't gonna collect themselves. And as cosmic energy sources go, stars are nice but not that nice. They only last a few billion years (the small red dwarfs are too puny to be worth the effort). Nah, if you were an interstellar civilization on the go, you'd head over to the nearest white dwarf, neutron star, or black hole. Now those babies can harness some serious gravitational energy punch.
Or not. We're kind of just making stuff up at this point.
Maybe the aliens, whatever they might be, are already here! Look, I have to mention this possibility just so you can't accuse me of not being 100 percent comprehensive, but the chances of alien life actually taking a trip to Earth are so incredibly small under any reasonable understanding of physics that it's so easy to dismiss, I almost forgot to do it. You remember how big-with-a-capital-B the universe is? The incredible distances to even the nearest star? The travel time measured in—at minimum—tens of thousands of years? Distance = time = energy. Colonizing another star, especially with a clunky spaceship big enough to hold some meatbags and their required nutrients, is just about number one on the list titled “Technically Possible but So Infeasible It Might As Well Be Impossible.” I'm not one to dismiss romantic thinking, but I'm holding back a serious scoff—and possibly a pshaw—at the thought of interstellar travel. It's just not a thing, folks.6
OK, so visiting isn't an option. What about just blabbing on the radio? A good old-fashioned chit-chat. We send out a big blast of “Howdy, universe!” on all the frequencies, wait a few dozen (hundred?) years, and get a response back of “How's it going, Earth?” Seems reasonable. Any radio transmitter worth its salt should cut through thousands of light-years of interstellar junk like butter. Our “radio bubble,” the ever-expanding sphere of transmissions we've been blasting out ever since we've been able to blast out, is rather small, barely a hundred light-years across. Should someone Out There happen to tune in to the right frequencies, they'll immediately know that something funky is going on, Earthwise.
But any older, or just simply previous, civilizations in the galaxy would have been jammin’ for far longer than us, so we should be awash in obviously artificial and obviously foreign radio waves. While we occasionally hear a random bleep or bloop on our radio antennas, they always end up having a rather boring explanation. Reflections from a comet, a new class of unknown star, or even the microwave in the visitor's center (I'm not joking about this one!7). Even if we couldn't recognize the source of an odd radio signal, aliens are never the answer; a natural explanation, even if it's not completely satisfactory, is always logically preferred over “Aliens did it.” Extraordinary claims and so on. You know the deal.
So we've been on both ends of it (the radio blasting and the radio listening) for naught. There are no signs of any superadvanced civilizations reimagining the galaxy with their technological marvels. Nobody's stopped by for a visit. As far as we can tell, and I hope I've made my point clear enough by now, we're alone. What's going on?
Perhaps we're the first sentient species to arrive on the galactic scene. Perhaps there's some sort of filtering action that snuffs out sentient life (whether by self-harm or other, more vague and nefarious, causes).
More likely, we're not comfortable with two things: statistics and large distances.
When it comes to statistics, I suppose I should mention the Drake equation. Drake what? If you're not familiar with it, don't fret. Originated by Frank Drake a few decades ago, probably first on the back of a bar napkin, it purports to quantify the chance of us discovering life, based on variables like the number of stars hosting planets, those planets being in the habitable zone, life surviving long enough to build a radio dish, and so on.8 The game plan is to make measurements on understanding all the little numbers, and out pops a final probability of getting to make an interstellar handshake. While the Drake equation sees lots of replay action in the discussion on life in the universe, I'm going to be a little blunt here and say that it's absolutely useless.
That's right, I'm going bold: useless.
The Drake equation gives the illusion of knowledge and understanding. You make some assumptions about the requirements for life (like the discussion above, in case you skipped it) and go out making measurements to pin down all those numbers. The problem is that it doesn't really lead to a confident prediction. For example, if you have all the numbers measured to
incredible precision and accuracy except for one, your final result is still unclear; you have to make precise measures on all numbers, or you might as well not have even started. And we have absolutely no way of confidently estimating most of the numbers in the Drake equation.
What's more, the very act of trying to parameterize ignorance commits you more than you might desire to a particular line of thinking. What if you missed some crucial but nonobvious requirement for life and didn't put it into the Drake stew? You may think you have an answer at the end of the day, but really you're way off the mark. And that doesn't even begin to address the issues of finding life—even life that we might readily recognize—in an unfamiliar and surprising environment. Like, say, the liquid water oceans of the icy moons in our solar system.
In the end, you pour a lot of work into fretting over the Drake equation parameters, only to end up with…a guess. You could have just started with the guess and moved on with your life. Or not even bothered playing the game.
We honestly have no clue how rare/unrare life is in the galaxy, let alone the universe. The chances of life appearing on any planet are obviously not zero and also obviously not extremely large. The ultimate answer to why nobody else appears to be home is probably very mundane: life is somewhat common, but intelligent life is rarer, and space is big.
There very well could be at least one other intelligent species hanging out on some rock or two within the Milky Way. But we probably haven't heard from them, or any other past civilizations, because sending radio signals is simply hard. Our own radio bubble, hundreds of light-years across, isn't even distinguishable from the background hum and hiss of the galaxy at the distance of our nearest neighbor. In the interstellar regime, a loud and clear shout very quickly just ends up looking like another bit of noise.
Also, the galaxy is huge. Gigantic. Supremely large. Other synonyms would be appropriate, but I think you get the idea. And it's constantly evolving, with new stars appearing on the scene and others dying. Perhaps it's just the case that the galaxy is far too large and far too complex for any species to “colonize,” even if they really wanted to. Civilizations will appear, grow, decay, and die, making their mark in their little neighborhood and accomplish nothing more.
Your Place in the Universe Page 27