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ZL2VAL > SETI 10.09.04 16:00l 141 Lines 7383 Bytes #999 (0) @ WW
BID : 500144ZL2VAL
Read: GUEST
Subj: Would ET have used snail mail?
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<ZL2BAU<ZL2BAU<ZL2WA<ZL1AB<ZL2AB
Sent: 040910/1121Z @:ZL2AB.#46.NZL.OC #:47380 [New Plymouth] FBB7.00g
From: ZL2VAL@ZL2AB.#46.NZL.OC
To : SETI@WW
*Does ET Use Snail Mail?*
*By Seth Shostak*
SETI Institute
posted: 09 September, 2004
7:00 a.m. ET
Virtually all SETI experiments probe the skies looking for broadcasts
from afar: radio or light signals that would tell us that someone as
sharp-witted as ourselves is out there. But could it be that while we
use binoculars to scan the cosmic sea, bottled messages have washed up
unnoticed at our feet?
As Robert Roy Britt reports elsewhere, the Aug. 25 cover story in the
journal `Nature' suggests that the most efficient method of sending
messages between the stars is not to broadcast them, but to use snail
mail. Rutgers University researchers Christopher Rose and Gregory Wright
reckon that clever aliens aren't going to prattle into a microphone.
Instead, they'll inscribe their messages onto some hunk of matter (film,
floppy disks, and flash cards are simple examples of information-bearing
media from our own technology), pack it all into an interstellar rocket,
and launch it towards their extraterrestrial pen-pals.
The two computer scientists claim that, compared to broadcasting radio
to someone else's solar system, going postal could be enormously
cheaper, requiring only a trillionth as much energy (or thereabouts) for
the same message.
Does this mean that SETI experiments are misguided? Should we be using
rakes instead of telescopes to search for messages from other worlds? Is
it possible that an advanced civilization has littered solar systems
like ours with packaged dispatches we have yet to find?
Answering these questions requires considering a few realistic scenarios
for interstellar communication.
First off, there's the indisputable fact that physically transporting
information can be quite efficient. Imagine packing a tanker ship with
DVDs, and sailing it to Australia. You could jam approximately 10
billion disks into the tanker, which might take a week to cross the
Pacific. That's an average `data rate' of 600 trillion bits per second,
and a cost per bit of roughly 0.02 trillionths of a cent! Those are
impressive numbers that not only blow away your internet dial-up, they
clobber broadcasting, too: sending the same amount of information with a
TV transmitter would take two million years.
OK, bussing the bits might beat broadcasts in some circumstances. But
what about interstellar communication? Consider an example derived from
Rose and Wright's article: a message sent by missile mail to a recipient
100 light-years away. Suppose the delivery rocket travels at
one-thousandth the velocity of light, faster than any of our own
spacecraft, but hardly an unthinkable speed. Clearly, it will be 100
thousand years en route. Suppose that during all that time, your
telecommunication colleagues have a powerful transmitter switched on,
using an antenna comparable to the stadium-sized Arecibo dish to beam
radio waves at the same recipient. Both schemes are assumed to send an
equal number of bits, and both take the same amount of time (100
thousand years) to deliver them. But the cost per bit - in terms of
energy - will be 100 billion trillion times less for the rocketed
message, according to Rose and Wright.
This example certainly seems to suggest that snail mail beats hail mail
by a large margin. But it's worth checking out a few important details
of this argument. To begin with, the Rutgers researchers assume that the
encoded message is very efficiently packaged, with a density of 2
million billion billion bits per kilogram. That's the information
density of single-stranded RNA (like a polio virus), in case you
wondered where the number came from. Of course, it might not be entirely
obvious how to encode information at this enormous density (the aliens?
problem) or decode it (our problem), but the point is that the `total'
information on all the hard disks in the world, if packed this tightly,
would weigh less than a single gram! Yes, that's right: you could send
the contents of all the libraries on Earth in one envelope, if you could
package it as efficiently as Rose and Wright have assumed.
Ergo, it should be obvious that the postal `rocket' will be far bulkier
than any reasonably sized `message' it will carry. But this necessary
packaging (the rocket) must be sent, too, and that takes energy. In
addition, there's a real delivery problem. The nearest star systems of
interest to extraterrestrial correspondents (for example, those known to
have planets with biology) would probably be at least 100 light-years
away.
As SETI astronomer Frank Drake points out, it's not easy - indeed, it's
excruciatingly hard - to launch a spacecraft with adequate precision to
make a soft landing, or go into orbit around, a planet that's that far
off. During the 100 thousand years transit time, the planet's motions,
and consequently, its position, will be slightly perturbed in complex
ways by gravitational interactions within its solar system. The only
hope of a precision arrival is to use a `smart' rocket that can maneuver
once it reaches the vicinity of the target. But maneuvering requires
sensors, circuits, and fuel, and that adds to the weight of the rocket,
further decreasing efficiency.
Such practical considerations boost the cost of message delivery. At the
same time, there are plenty of ways to lower the cost of broadcasting. A
larger antenna, for instance (composed of an array of widely separated
dishes) could direct its radio energy at the recipient's inner solar
system. At 100 light-years, that simple stratagem would reduce the
energy cost by about a million at microwave frequencies, compared to the
Arecibo-sized antenna. But the real trick in broadcasting bits into
space, rather than blasting them, is to transmit light, rather than radio.
Radio is a great way to get another world's attention, but if you really
want to send your cosmic correspondents your encyclopedia, you can do it
faster on a light beam. It's technically feasible to semaphore 10
gigabits per second this way, which means that the texts of all the
books in the Library of Congress could be rattled off in less than a day.
Sure, a light beam, even a well-aimed one, would require a good deal of
power if loaded with information. You'd need ten trillion watts, even if
you concentrated it on the recipient's inner solar system. On the other
hand, you might be able to use solar energy as your power source,
limiting the costs of the project to initial construction and maintenance.
But the real point is this: even with messages that are as large as all
Earth's libraries combined, the delivery time from sender to receiver in
our example is scarcely more than a century. Snail mail might be able to
convey more info, yes, but would take a `thousand' centuries to do so.
So while Rose and Wright make an interesting point, it seems only
reasonable to expect that a lot of interstellar messaging is going to be
broadcast rather than delivered. Sometimes it's better to eschew the
pony express, and saunter down to the telegraph office.
73, Alan, ZL2VAL @ ZL2AB.#46.NZL.OC (Sysop)
IP: zl2val@qsl.net
APRS: 3906.34s/17406.45e]
Message timed: 23:15 on 2004-Sep-10
Message sent using WinPack-AGW V6.80, written by Roger Barker G4IDE, SK
9/9/04, RIP.
Old Age
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You know you are getting old when everything either dries up or leaks.
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