Cosmic Communications
Cover story blurb:
It's almost a 'given' that remote civilizations will try to contact us
using radio waves. The search for extraterrestrial intelligence was
first mooted by Giuseppe Cocconi and Philip Morrison 45 years ago
('Searching for interstellar communications' Nature 184,
844--846; 1959). Since then, 2 million years of CPU
time has been given to SETI@home in the spirit of that paper: 'The
probability of success is difficult to estimate: but if we never
search, the chance of success is zero.' Christopher Rose and Gregory
Wright propose a new take on extraterrestrial contact. It seems the
'Sounds of Earth' gold disks aboard Voyagers 1 and 2 (see cover, left) were
on the right track. The message-in-a-bottle idea of sending physical
objects across space is highly energy efficient, and we should search
for artefacts in the Solar System now.
Nature
Letter 431, pp.47--49, September 2, 2004 and
Supplementary Information. Full size 9/2/2004 Nature cover
picture (PDF).
NSF
(National Science Foundation)
3/10/2006 Discoveries feature
story (local copy).
If the above blurb made little sense to you, some really nice descriptions of the work by really good science writers can be found here. Some even managed to drag the right words out of me during radio interviews (at least for the most part when they could contain my digressions and technospeak). Here's the Rutgers University press release.
My home page and Gregory Wright's (co-author) home page.
Basic Idea |
Publications |
Press Coverage |
Acknowledgments |
Related |
Quantitative Analogies |
Mass Information Density |
Voicemail For ET? |
Cosmically Selfish Genes? |
Cartoons |
It turns out that it's often MUCH ( many many orders of magnitude ) better from an energy use perspective (and perhaps from others like message persistence at the destination) to write a message down in some medium and LITERALLY toss it to the recipient than it is to radiate the message electromagnetically -- assuming delay beyond light transit time can be tolerated. This result has some implications for terrestrial communications, but the most impressive gains are for interstellar and intergalactic distances. So perhaps the SETI program should be sifting through dust/pebbles/rocks in addition to searching the electromagnetic spectrum for messages from "out there."
This work, an accidental outgrowth of work on terrestrial wireless, is pretty careful in assessing the energetics and provides outer bounds for the ratio of the energy necessary to deliver a message using "inscribed matter" to that for electromagnetic radiation -- and usually favors radiation by assuming such things as infinite bandwidth.
We also worry about directed radiation, aperture sizes, and the
fact that there are many targets --
matter transmission STILL wins.
We've recently included the delivery problem (how do you
slow the particle down at the destination), the construction problem (how
much energy to construct the particle) and a version of the civilization
communication problem (civilizations are almost certainly asynchronous owing
to calamity -- celestial, like meteor strikes, or competition/stupidity-induced).
We'd wanted to wait to add these, but various referees felt these aspects
were necessary to convince readers that evidence of
ET might appear in grains of sand or on comets, meteors, etc..
We also recently considered the ballistic aiming problem since gravitational perturbations might serve to knock a package off course (the effect seems small).
Of course, if energy is no object (consider Freeman Dyson's "sun herding" or building what amounts to a giant blinker around a sun) our energy comparisons are irrelevant. But since such feats are not presently on our technical horizon, we have a bias in favor of worrying about energy.
Hopefully you'll find the work thought provoking, or at least amusing! Talks, publications and earlier "kitchen sink" versions of the work can be found below. Of particular note, some folks have found the following captioned pictures especially amusing.
Communication Theory Workshop 2013, Phuket Thailand
Slides (pdf)
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Northern NJ Junior Science and Humanities Symposium
Slides (pdf)
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CSEE Seminar, Universtiy of Maryland Baltimore County
Slides (pdf)
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Lunch Bunch Seminar, California Institute of Technology
Slides (pdf)
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iCORE Summit'06 Keynote Address
Slides (pdf)
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British InterPlanetary Society
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Princeton University
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Lawrence Berkeley National Laboratory
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University of Texas at Austin
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University of Texas at San Antonio EE seminar talk slides (pdf) |
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Telcordia Midnight Seminar Series talk slides (pdf) ( postscript) |
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Asilomar'04 Conference on Signals, Systems, and Computers talk and paper |
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Nature Letter 431, pp.47--49 and Supplementary Information |
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DIMACS Workshop on Storage talk |
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Crawford Hill Coffee Hour talk |
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VTC Fall 2003 talk and paper |
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Space Archaeology
interview
on
Are We Alone
with Seth Shostak (6/6/2011)
(
local mp3 (at 23:18)
)
National Public Radio
interview
on
All Things Considered
with David Kestenbaum (9/1/2004)
(
local mp3
)
BBC Radio 4
interview on
Leading Edge with Geoff Watts (9/2/2004)
(
local copy of website)
(
local mp3
)
BBC World Service Radio
interview (local mp3)
on
Science in Action
with Dee Palmer (9/3/2004)
National Public Radio LIVE
interview on
Science Friday
with Ira Flatow (9/3/2004)
(
local mp3
)
National Public Radio (WBUR)
interview on
Here and Now
with Gail Harris (9/7/2004)
(
local mp3
)
CBC Radio
interview mp3
(
ogg )
on
Quirks & Quarks
with Bob McDonald (9/11/04)
SETI Institute Radio
LIVE interview on
Are We Alone?
with Seth Shostak (9/26/2004)
(
local copy of website
)
(
local mp3
)
Geoff and his team recently (Aug/Sept 2004) discovered
SMALLish exoplanets -- a big deal if you're interested
in extrasolar life!! They then more recently discovered a
ROCKY extrasolar planet. Of course, it's whizzing around its sun with a period
of about a day and a half so anything on it is likely
to be pretty hot (and dizzy
).
)
Jerry Foschini's paper on the canonical artifact -- how hard is it to create an artifact that cannot be mistaken for something of "natural" origin, and is the universe indifferent to the emergence of life?
HILARIOUS application
of the basic idea of matter-based communication from the
Annals of Improbable Research.
The opening illustration is priceless --
inscribed-matter communication methods scale well
.
Saul Perlmutter at Lawrence Berkeley National Labs brought this article
to my attention. He obviously has a cosmic sense of humor and fun
which probably had some hand in his discovery
that not only is the universe expanding, it's expanding
faster and faster.
Crick's panspermia is an interesting idea. Here's
a
Google search
on the topic. You'll need to pick and choose with your critical
faculties on full alert.
Paul Davies' essay
(
New Scientist, August 7, 2004) on searching our own
genetic backyard for evidence of
ET.
Paul is a rather
famous person.
). In truth, without actually quantifying the reception problem
and figuring out where to look (without spending billions of dollars),
launching a search for inscribed matter will be a tough sell both
within and outside the SETI community. That is, even if we never hear
distant "intelligent" radio signals, at least the SETI telescopes can
do useful astronomy as a sideline. My guess is that inscribed matter,
if any is found, will be an en passant discovery associated
with solar system (including earth) exploration as opposed to the
result of a dedicated search.
) 1997
unpublished paper
For an Arecibo-Arecibo radio communications example (without shielding or other costs for matter), the efficiency gain of inscribed matter over radiation is about 5 X 1015. So, if raising a 2kg mass (roughly a 5lb sack of sugar) one meter on earth were equated with the energy needed to deliver the matter, the relative amount of energy necessary to radiate the same message would be a 24 megaton nuclear blast.
If we include the extra costs for sending matter as in the paper, the relative efficiency drops to 108. Using the sugar sack raised one meter analogy again, the relative effort to send a radiated message would be like raising a 200,000 ton cruise liner about 1 meter on earth.
Radiation would seem to have an advantage since it covers more targets
than a matter transmission. However, for radiation to be even
remotely competitive with inscribed matter, the radiation has to be
collimated (like a laser beam). The tighter the collimation, the
fewer star systems (we're being "solar-centric" )
are illuminated -- and this requirement vitiates any "broadcast
advantage" one might get with radiation. Said another way, if signals
were sent isotropically (everywhere) in a spherical galaxy with uniform
stellar density, even accounting for the fact that shielded messages
would have to be sent to each and every star in a given volume, the
relative efficiency of matter is so enormous that the breakeven
galaxy size (where there are so many stars to reach that radiation again becomes more efficient) is about ten times larger than
the visible universe!!!
In the paper we say that owing to shielding, longer messages are favored over short "we exist" messages. One sharp journalist (Stefan Lovgren of National Geographic) asked the question of exactly how long is long in this context. For the Arecibo example at 10,000 lightyears, inscribed matter beats radiation for messages larger than 1014 bits -- about the size of the written information in the library of congress (see Michael Lesk's treatment).
One question which arose is how well one could aim a particle using mostly ballistic transport. For grossly macroscopic particles, Heisenberg (uncertainty principle) is pretty much irrelevant, but more important, there will be random gravitational perturbations en route owing to the gravitational pull and somewhat random (unknown) placement of stars in our Galaxy. The effect of how well we can aim, however, seems relatively unimportant.
If civilizations at any given stellar location are short-lived owing to various sorts of celestial, geological or self-inflicted calamity, there could be a survival benefit to seeding the cosmos with compelling records of a civilization in the hopes that it would be eventually adopted by other fledgeling civilizations (or subsumed by older ones). However, under the assumption of impermanence, literal biological seeding of likely habitats (similar to Crick's "panspermia") seems much more likely in that it confers a selective advantage -- of sorts. That is, those civilizations that seed may survive in some form, and those that don't may likely perish completely.
Here's a recent (6/30/2005) news article ( local copy ) on the idea that life is everywhere and might even turn up in the comets that NASA is currently stalking ( Wild-2, Tempel-1 ). I have a soft sport for comets as potential message delivery vehicles. They can release material during transit through the inner solar system and they are highly visible. However, for archival data messages, the annoying fact is that their orbits are only stable for a relatively short time -- tens of thousands of years as opposed to hundreds of millions. That doesn't leave the local denizens much time to attain the necessary technological sophistication to reach out and grab one. But if biological seeding is the aim, comets seem (at first blush) like a decent delivery method.
Of course, the context of the selfish gene is evolution,
and since
evolution implies mixing and competition,
(more)
Michael Crichton the novelist made exactly the point that life might be the message (and explicitly that it seems more efficient to send a package than to use electromagnetic radiation) in his 1969 novel The Andromeda Strain (pp.222-225 courtesy of Rich Howard).
One admittedly crazy image I cannot seem to shake is of arrays of computer chips using MEMS catapults to deliver info in lieu of optical/electromagnetic bus lines! Well, I nagged myself into doing the calculations. Specific scenario: wafer-to-wafer communication (10cm) at a speed of about 0.3m/s compared to IR laser communication assuming 5 micron radius apertures.
You work out the numbers and find that the matter efficiency ( &Omega ) figure is about &Omega = 5.2 X 10-14 &rho where &rho is the information density of the storage/transmission medium. So tossing little magnetic chits back and forth (see density figures above) would be about 5000 times more energy-efficient than laser communication. And if an STM tip were employed to inscribe the surface of an information carrier (like an SiO2 chit), the energy efficiency could be more than 1000 times larger still.
So, smart dust tossing inscribed dust may not be crazy after all!
A recent article (local copy) in New Scientist profiles a now bybone service (racket!) whereby you call a "900" number to leave voice messages for ET to be beamed out into space using a satellite dish. New Scientist contacted me and since silence can imply complicity, I did the calculation. I suspect that customers of this service have little hope of getting their messages across. But people bought "pet rocks" in the '70s so perhaps there'll be a lot of takers.
A special note of thanks to Leslie Sage and the editorial staff of Nature who saw something of interest in an overlong and overmathy draft, and shepherded a newbie (me) through the Nature publications process from "presubmission enquiry" to the final paper. I would have formally thanked Leslie in the paper acknowledgements, especially since he made substantive contributions to the paper (for instance, the Voyager comparison was his idea), but it seemed to go against Nature's policy. Overall, this was the most enjoyable editorial experience I've ever had even if the paper had not "made the cut." Bravo Nature! But a special Bravo Leslie!!!"
In addition, without countless invaluable conversations with
G.J. Foschini or
N. Beaulieu's
eye for potential and strong encouragement, I'd never have
approached Nature with this work. Thank you both!
I'd also like to correct two misprints which appeared in the paper
acknowledgments. Special thanks are due to
PAUL HENRY
for being an invaluable sounding board early on, and to
STAVROS TOUMPIS
for careful reading of an early manuscript.
Christopher Rose and Gregory Wright.
Common Question:
How did an independent
consultant/astronomer and a college professor of wireless communications theory
get together?
Answer: We're both part of the post-divestiture diaspora from the old Crawford Hill lab (Bell Labs Research) -- of Telstar and big bang background radiation fame.
Tripped over a non-functioning link? Please email Chris. No guarantees, but I'll try to get to it ASAP .
Last updated June 28th, 2013
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