Nov. 20, 2003
by Seth Shostak - Senior Astronomer
Chinas Great Wall may, indeed, be a whale of a wall, but you cant see it from space with your naked eye.
I made this point in my column of last month, "Can Aliens Find Us?" where I considered whether sophisticated extraterrestrials could easily discover Homo sapiens. My example was intended to show that even from relatively nearby, the physical artifacts of human society are difficult to detect. Our radio signals are far more conspicuous.
But a reader of that column, Fred Hapgood, wrote to say that, after all, just because the constructions of an advanced culture would be difficult to see directly, this doesnt mean that theyre thoroughly impossible to find, does it? Consider how much better the telescopes of a civilization hundreds of times older than ours would be, Mr. Hapgood suggests.
Hes right, of course. We inevitably tend to envision the capabilities of putative extraterrestrials as being similar to, or slightly more advanced than ours. But what could a society thats many millennia beyond us do? Could they ever map our world and see our ancient walls, our cities, or even us?
A parallel question, albeit in less extravagant form, was posed by former NASA Administrator, Daniel Goldin, shortly after astronomers detected the first extrasolar planets around normal stars. On a 1997 PBS television show, Goldin enthusiastically exclaimed "Could you imagine if in twenty-five, thirty, or forty years, we could take a picture of a planet thats perhaps fifty light years from Earth, and if the resolution was high enough to take a picture of oceans and clouds and continents and mountain ranges breathtaking!"
Indeed it would be. So lets consider this more modest proposal: to map the mountain ranges on a world 50 light-years distant. Whats required?
Roughly speaking, youd need to be able to see detail as small as about 50 miles (this is the necessary pixel size, as digerati would call it). With a bit of high school physics, you can work out that this necessitates a telescope with a mirror two thousand miles across. It wouldnt have to be a one-piece mirror of course: you could borrow a technique in vogue with contemporary astronomers, and construct your instrument out of smaller, widely-separated, individual telescopes. This mammoth spyglass would have to be space-based, to avoid atmospheric blurring; but after all, if you can construct a telescope this size, you undoubtedly have the technology to heft it into space.
In the accompanying figure, Ive plotted the diameters of some existing and proposed mirror and lens telescopes, and not surprisingly, you can see that they have become larger over time. If you make the daring assumption that this growth curve continues into the distant future, then we will be able to build a 2,000-mile telescope in the middle of the next century. In fact, its conceivable that we will do this much sooner, since large telescope arrays will be easier to construct than the filled aperture telescopes of the type shown in the plot. Goldins vision, as it were, is not an impossible one; certainly not for clever extraterrestrials.
But could they up the ante? Could a civilization for whom massive engineering projects are commonplace ever build an instrument that could actually see the life on Earth? Could they have detected the dinosaurs, for instance, simply by imaging them? No, I dont mean the far simpler task of detecting the oxygen or methane in our air that betray biology on this planet. Im asking, could they actually see the animals?
Making even a crude picture of a stegosaurus (or us) would require pixels of about one foot in size. At 50 light-years, that demands a 500 million mile telescope, one that if we built it would just fit between the Sun and Jupiter. Of course thats an instrument of ambitious dimensions. But whats to stop an alien civilization from scattering small telescopes throughout its solar system, thereby achieving this impressive aperture size?
Probably nothing. However, theres another problem. Would enough light from the hide of that stegosaurus actually reach an extraterrestrial telescope? On a clear, sunny day, every square foot of dino skin would reflect about 10 billion billion photons per second back into space. Thats a lot of photons, but they spread out. At 50 light-years, it takes a mirror thats 100 thousand miles in diameter to collect even ONE of those photons each second (and since dinosaurs move, you need short exposure times for the photo).
Bottom line: such a dino-detector would need the equivalent of 100 million billion Keck-size telescopes, spread out over a half-billion miles of space. And we havent even talked about the difficulties alien engineers would face in precisely combining the data from these instruments. Nor have we considered the image-scrambling effects of interstellar gas. This is a project that should boggle the brain of the most ardent futurist.
So what we can say is this: finding mountain ranges isnt terribly hard. But making pictures of extraterrestrial megafauna is.
Of course, theres another approach: send robot probes to worlds with life. Well consider that in a future column.