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Could Aliens See Us?
Seeing Extraterrestrials
By Seth Shostak Senior Astronomer, SETI Institute Space.com China's Great Wall may, indeed, be a whale of a wall, but you can't 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 doesn't mean that they're 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. He's 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 that's 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 that's 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 let's consider this more modest proposal: to map the mountain ranges on a world 50 light-years distant. What's required? Roughly speaking, you'd 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 wouldn't 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, I've 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, it's 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. Goldin's 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 don't mean the far simpler task of detecting the oxygen or methane in our air that betray biology on this planet. I'm 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 that's an instrument of ambitious dimensions. But what's to stop an alien civilization from scattering small telescopes throughout its solar system, thereby achieving this impressive aperture size? Probably nothing. However, there's 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. That's a lot of photons, but they spread out. At 50 light-years, it takes a mirror that's 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 haven't 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 isn't terribly hard. But making pictures of extraterrestrial megafauna is. Of course, there's another approach: send robot probes to worlds with life. We'll consider that in a future column. |
#2
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China's Great Wall may, indeed, be a whale of a wall, but you can't
see it from space with your naked eye. You're speaking of the human eye, which means little in this context. RB |
#3
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On Tue, 2 Dec 2003 10:24:57 -0600, Steve Dufour wrote
(in message ) : From: (Steve Dufour) Newsgroups: alt.culture.outerspace, alt.sci.seti, alt.alien.research, alt.astronomy Seeing Extraterrestrials By Seth Shostak Senior Astronomer, SETI Institute Space.com China's Great Wall may, indeed, be a whale of a wall, but you can't see it from space with your naked eye. snip Why worry about them seeing us when with a radio telescope no better than the first one would point us out from as far away as 90 or 100 light years, certainly it would find us from 50 light years. -- Harry F. Leopold aa #2076 AA/Vet #4 The Prints of Darkness "Your God wears fuzzy, pink, bunny slippers." |
#4
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Yes, In fact they're looking at you right now.
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#5
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Admittedly, it would only work for "intelligent" life, but seeing
luminescence on the dark side of the planet on a regular basis would definitely float my boat. -- Stinger "Steve Dufour" wrote in message om... Seeing Extraterrestrials By Seth Shostak Senior Astronomer, SETI Institute Space.com China's Great Wall may, indeed, be a whale of a wall, but you can't 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 doesn't mean that they're 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. He's 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 that's 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 that's 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 let's consider this more modest proposal: to map the mountain ranges on a world 50 light-years distant. What's required? Roughly speaking, you'd 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 wouldn't 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, I've 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, it's 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. Goldin's 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 don't mean the far simpler task of detecting the oxygen or methane in our air that betray biology on this planet. I'm 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 that's an instrument of ambitious dimensions. But what's to stop an alien civilization from scattering small telescopes throughout its solar system, thereby achieving this impressive aperture size? Probably nothing. However, there's 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. That's a lot of photons, but they spread out. At 50 light-years, it takes a mirror that's 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 haven't 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 isn't terribly hard. But making pictures of extraterrestrial megafauna is. Of course, there's another approach: send robot probes to worlds with life. We'll consider that in a future column. |
#6
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On 02 Dec 2003 16:48:54 GMT, (Bobsprit) wrote:
China's Great Wall may, indeed, be a whale of a wall, but you can't see it from space with your naked eye. You're speaking of the human eye, which means little in this context. RB Then the aliens better have a freaking big retina. -- Find out about Australia's most dangerous Doomsday Cult: http://users.bigpond.net.au/wanglese/pebble.htm "You can't fool me, it's turtles all the way down." |
#7
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Stinger wrote...
... seeing luminescence on the dark side of the planet on a regular basis would definitely float my boat. Well, if we put one of those hugemongous telescopes on the other side of the universe, it would be looking at the dark side of all of the planets... Would that help? -Neil- --- .. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.547 / Virus Database: 340 - Release Date: 12/2/03 |
#8
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What about the city lights?
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#9
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Bingo.
"Mr. 4X" wrote in message ... What about the city lights? |
#10
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Bingo.
Bingo game lights? RB |
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