Authors: Ph. D. Philip Plait

Death from the Skies! (27 page)

BOOK: Death from the Skies!

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The one drawback with SETI is that the conversations will tend to be a bit boring. If we detect a signal from a star that is really close on the galactic scale, say, a thousand light-years away, the dialogue will really be a monologue. We’d receive the signal, reply, and have to wait two thousand years for them to get back to us (the time it takes for our signal to reach them plus theirs to come to us again). While SETI is a great and worthwhile endeavor—and if they find a signal it will be one of the most important events in the history of science—we’re still used to thinking of aliens actually
coming here.
Face-to-face, as it were, assuming they have faces.

But a thousand light-years is a long way (6,000,000,000,000,000 miles). That’s quite a hike, yet it’s practically in our laps compared to the size of the Milky Way.

Is that why we haven’t been visited? Maybe the distances are simply too great!

Actually, not so much. A trip to the stars wouldn’t take that long at all, if you maintain a sense of scale.

TO BOLDLY GO

Let’s assume that we humans suddenly decide to fund the space program. And fund it
really
well: we want to send probes to other stars. That’s no easy feat! The nearest star system, Alpha Centauri (which has a Sunlike star and is worth a look-see), is 26
trillion
miles away. The fastest space probe ever built would take thousands of years to get there, so we couldn’t really expect a payoff in the form of pretty pictures anytime soon.

However, that’s the fastest probe ever built
so far.
There are ideas out there on the drawing board that would make much faster unmanned probes, even ones that can move at a goodly fraction of the speed of light. Some of these include fusion power, ion drives (which start off slowly but accelerate continuously over years, building up ferocious speeds), and even a ship that explodes nuclear bombs behind it to provide a huge impulse in speed.
⁷¹These methods can drop the trip time from millennia to mere decades.

This might be worth doing. It’s expensive, sure. But there are no
technological
barriers to this idea, just social ones (funding, politics, etc.). Let me be clear: if we had the will,
we could build spaceships like these right now.
In less than a century we could be sending dozens of interstellar emissaries to other stars, investigating our own neighborhood in the galaxy.

Of course, the trip times and the actual construction of the fleet make it difficult to explore much real estate. The galaxy has billions upon billions of stars, and building that many starships is impossible. Sending one probe per star isn’t cost-effective. Even if we let the probe simply sweep through a star system on a fly-by, moving on to the next star, exploring the galaxy would take forever. Space is big.

But there’s a solution: self-replicating probes.

Picture this: an unmanned spaceship from Earth arrives at the star Tau Ceti after a journey of fifty years. It finds a series of small planets and begins its scientific observations. This includes a census of sorts—taking measure of all the bodies in the system, including planets, comets, moons, and asteroids. After some months of surveying, the probe will move on to the next star on its docket, but before it leaves, it sends a package down to a particularly promising nickel-iron asteroid. This package is in fact a self-starting factory. Once it lands, it mines the asteroid, smelts the metal, refines out the necessary substances, and then autonomously
builds more probes.
Let’s say it builds just one probe that, after a few years of construction and testing, blasts off for another star system. Now we have two probes. A few decades later they arrive at their destinations, find appropriate accommodations, and then go forth and multiply again. Now we have four probes, and the process repeats.

The number of robot ambassadors builds very rapidly; it’s exponential. If this takes exactly one hundred years per probe, by the end of a millennium we have 2
¹⁰= 1,024 probes. In two millennia there are a million probes. In three thousand years there will be more than a
billion.
Now, in reality, it’s not that simple, of course, but even a pessimistic approach shows that we can explore every single star in this galaxy in something like 50 million years, maybe a bit less.

Well, that sounds like a long wait! And we’re still a long way off from being able to do it. The technology is formidable.

But hang on—remember that civilization we considered, just 100 million years in advance of us? Given that much time, they could easily have examined every single star in the Milky Way, looking for life. If they saw our warm, blue world, one would think they’d make some note of it. Still, it’s possible they came here 50 million years ago and missed us humans (mining the Moon for a monolith à la
2001: A Space Odyssey
maybe isn’t as silly as it sounds), or maybe they just haven’t gotten here yet.

But given the time scales, that seems unlikely. It just doesn’t take that long to map out a whole galaxy and visit the appropriate planets. That’s why I don’t think the “millions of civilizations” number from the Drake Equation is correct. We’d have seen them by now, or at least heard from them.
⁷²

IN SPACE NO ONE CAN HEAR YOU SCREAM

But sometimes I wonder. Given all this information: the likelihood of life, the relative ease of galactic exploration, the time spans involved . . . and the fact that we have not detected any other life in our galaxy at all, there is another possibility that is worth mulling over.

Consider: what spurs technological advancement more than anything else?

War.

The first Cro-Magnon who beat an opponent over the head with a tree branch was also the one who was most likely to live a bit longer, and be able to reproduce. The army with rifles will (in general) beat the one with spears. The country with missiles will (in general) beat the one with cannons. The ones with electronic remote-controlled drones, spy satellites, and instantaneous communication will outmaneuver the ones without.

Nothing advances technology like good old-fashioned aggression. Even one of the most noble events in human history—man walking on the surface of the Moon—was initiated because of the cold war, the space race with an enemy vast and powerful. Americans imagined Soviet missile bases in orbit and on the Moon, and the motivation to beat them was in place.

When I was a kid, it was fashionable in more intelligent science-fiction stories to assume that any aliens we meet were bound to be friendly—no warlike race would be able to get their act together long enough to reach the stars.

Humans are on the verge of falsifying that statement.

Putting the pieces together, we find that warlike races are perhaps
more
likely to achieve space travel. The ones with a history of victories will have the best technology, and will be most motivated to be at the very least wary of outsiders, if not openly hostile. This case can certainly be made for our own provincial example.

This hypothetical advanced civilization will be xenophobic, fearful of aliens. We’ve already seen that it’s technologically possible to create interstellar starships, and it’s also possible to engineer them to create duplicates of themselves, to speed up the time it takes to comb over the whole galaxy.

What happens when you take a paranoid species and give them the ability to build such spaceships?

Uh-oh.

The scenario plays out in the vignette at the start of this chapter. It makes a creepy kind of sense to me: any aliens that are that aggressive would want to wipe out potential enemies before they got sophisticated enough to pose a threat. The easy way to do it is to create space probes like the ones described, and use them to ruthlessly wipe out all life they find.

Death from the skies, indeed.

I’ve wrestled with this idea, wondering if it’s possible. One potential saving grace is the same as before: exploring the whole galaxy in this way doesn’t take long compared with the age of the galaxy itself. Therefore, according to the same logic above, it’s likely that if such a xenophobic civilization were to evolve,
it would have been here by now.

Yet we’re still here. We know life has been around for billions of years. There have been the odd interruptions, but we’ve never been sterilized back down to the microscopic level. Like so many of the natural disasters we’ve seen, this puts a pretty good damper on the odds of being wiped out by nasty aliens. Simply put, if they were out there, we wouldn’t be here.
⁷³

I honestly don’t know if we’re alone in the Universe; no one does. However, given the immensity of space, and the grand depth of time, it sure seems unlikely. And if we do get out there, it also seems unlikely we’ll meet any nasty races like Klingons, Romulans, Vogons, Reavers, Daleks, or Kzinti. Natural disasters will still probably be our biggest worry.

But the galaxy is big, with room enough for lots of things. I may not know if we’re alone, but I’d love the chance to find out.

CHAPTER 7

The Death of the Sun

THE PLANET IS FAIR-SIZED, CLEARLY BIG ENOUGH TO
sustain a healthy atmosphere, though none is currently present. Given its distance from its parent star, it could easily have held liquid water on its surface too, once, in the far distant past. The outlines of continents are visible on its surface, though difficult to make out because of the lack of contrast. Were those deep, broad basins once ocean floors?

It’s hard to tell now. The planet may have once been green, or even blue, but now it’s all browns and grays and blacks. If any liquid water—or even water vapor—once existed there, it’s long gone, evaporated a billion years before. Without an atmosphere there can be no liquid water.

The planet’s star begins to peek over the planet’s horizon. Swollen, distorted, nebulous, and very, very red, the star rises ponderously. It almost appears flat, it’s so large. But after a few minutes the gently curved nature of the limb becomes more obvious, clarifying just how big the star is. An hour later it still hasn’t fully risen, less than half of it showing above the horizon. It looms menacingly there, glaring like a bloody half-closed eye.

Finally, once the bottom limb clears the horizon, the cause of the planet’s utter stillness and sterility is obvious. The star hangs over the landscape eating up a full 30 degrees of sky, as big as a dinner plate held at
arm’s length. The glowering eye of the star bears down on the planet’s surface, which begins to heat up with the day. By midafternoon, the temperature is above the melting point of rock, and the surface of the dead planet begins to glow a soft red and liquefy once again. Mountains continue their slump, and continental shelves flow slowly, blurring into the dry ocean basins.

Finally, after hours of unleashing its crippling heat, the star sets, though its distended red glow lingers for hours. The rock begins to cool a bit, and by midnight is starting to resolidify. As the sky finally turns black, low wisps of rock vapor are illuminated from below by the still-molten lava shining through cracks in the ragged surface.

In a few hours, the cycle will start again. Every day, the star is marginally bigger, marginally brighter, marginally radiating more heat on the distressed planet. In a few more millennia the rocks will heat so much during the day that there won’t be time for them to become solid again during the ever-briefer respites of night. The entire planet will become molten, erasing any possible hope of discovering its past history.

It’s a shame. The planet’s past is a rich one indeed, in its full and lively role as the third planet from the Sun. But the Sun has since started its long descent into death, and the past of Planet Earth will be lost forever.

SUNRISE, SUNSET

When you look at the Sun, it appears constant, stable, unchanging. But this is an illusion. Deep in its heart, an epic battle has been ongoing for billions of years, and will continue for billions more: the struggle between gravity and pressure. This war is fought with the weapons of contraction and expansion; more than anything else, the life of the Sun is defined by the balance between these two ancient foes.

Right now they are in rough equilibrium. The Sun’s gravity is able to hold it together, counteracting its internal pressure that is trying to blow it up like a bomb. This uneasy balance has existed for eons, and will be maintained for a long time to come.

But not forever.

There is an old phrase, very old:
This, too, shall pass.
We use the Sun to measure the length of our days and years. These time scales are natural to us. But on much, much longer time scales the Sun itself will prove to be a clock that is running down.

The constancy of the Sun is an illusion. Like any other star, the Sun was born, and it will live out its life.

And someday, it will die.

SWIFTLY FLOW THE
EONS