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  1. #1
    Steve Dufour's Avatar
    Steve Dufour Guest

    Default Could Aliens See Us?

    Seeing Extraterrestrials

    By Seth Shostak
    Senior Astronomer, SETI Institute

    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

    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

    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. #2
    Bobsprit's Avatar
    Bobsprit Guest

    Default Could Aliens See Us?

    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.


  3. #3
    Harry Leopold's Avatar
    Harry Leopold Guest

    Default Could Aliens See Us?

    On Tue, 2 Dec 2003 10:24:57 -0600, Steve Dufour wrote
    (in message <>) :


    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. #4
    Peter Gennaro's Avatar
    Peter Gennaro Guest

    Default Could Aliens See Us?

    Yes, In fact they're looking at you right now.

  5. #5
    Stinger's Avatar
    Stinger Guest

    Default Could Aliens See Us?

    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...

  6. #6
    Wally Anglesea™'s Avatar
    Wally Anglesea™ Guest

    Default Could Aliens See Us?

    On 02 Dec 2003 16:48:54 GMT, (Bobsprit) wrote:

    Then the aliens better have a freaking big retina.


    Find out about Australia's most dangerous Doomsday Cult:

    "You can't fool me, it's turtles all the way down."

  7. #7
    Flwrite's Avatar
    Flwrite Guest

    Default Could Aliens See Us?

    Stinger wrote...

    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?


    Checked by AVG anti-virus system (
    Version: 6.0.547 / Virus Database: 340 - Release Date: 12/2/03

  8. #8
    Mr. 4X's Avatar
    Mr. 4X Guest

    Default Could Aliens See Us?

    What about the city lights?

  9. #9
    Stinger's Avatar
    Stinger Guest

    Default Could Aliens See Us?


    "Mr. 4X" <> wrote in message

  10. #10
    Bobsprit's Avatar
    Bobsprit Guest

    Default Could Aliens See Us?


    Bingo game lights?



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