The Best of Creative Computing Volume 2 (published 1977)

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Communication Across The Universe (transmitting messages into space, SETI, messages on Pioneer 10 and 11 spacecraft)
by Martin Harwit

graphic of page

COMMUNICATION ACROSS THE UNIVERSE
by Martin Harwit
Space Sciences Department
Cornell University

In the past few years a number of messages have gone out from Earth in the hope
that an advanced civilization might find them. We also have searched cosmic
sources of radiation for signs of extraterrestrial intelligence but have not yet
found any.

We are faced with two questions:

What are the best ways for us to transmit messages? What are the best strategies
to employ in searching for messages?

The answers are not simple.

In writing an ordinary postcard, we have to consider three different factors. We
have to decide to who to write, what to write and how to convey the card. These
same decisions -not necessarily in the same order - are also involved in cosmic
communication.

First, we must decide on the best channel of communication. If we plan to
transmit, should we deliver our message by interstellar rocket, should we employ
a modulated laser beam flashing coded signals, or is there some method far
superior to either of these?

Once we decide how the message is to be conveyed, we must decide on the language
to be used. Surely it won't be English, or Chinese, or even Fortran. The
intended recipients won't know English or any other terrestrial tongues. How can
we establish a common language understandable to all?

Lastly we have to decide on an address: If we knew how to choose a civilization
that is alive and active today, would it still be around tomorrow? In fact, what
is the life expectancy of a technically advanced civilization? Judging by recent
Earth history, the step from the development of elementary radio transmission
techniques to an ultimate nuclear explosive extinction of Man may last just one
short century. If that is common, we had best make our message quite short and
to the point. And we should forego asking for a recognition sign; we may not be
here to receive it.

[Image]

Let us first look at the optimum channel for communication. Basically this is
dictated by the structure and contents of the universe we inhabit. If the
universe were different, our options also would differ. Consider, for example,
the messages carried by the spacecraft Pioneer 10 and 11. Each of these vehicles
carries a plaque that describes the location of our planet in the Milky Way
galaxy, and describes the inhabitants of Earth.

What are the chances that these two plaques will ever be discovered? I think the
odds are very low. There is not so much doubt that a sophisticated civilization
would have the ability to detect the spacecraft. Rather there is uncertainty
about the ability to recognize them:

We know that roughly once a year the solar system ejects a comet nucleus about a
mile in diameter. For each of these large chunks of matter there are probably
thousands, and perhaps millions, of smaller fragments that also find their way
out of the solar system. In the past billion years alone, a billion mile-sized
comet nuclei and countless smaller chunks have left our solar system. In this
confusion of debris, how is any civilization going to pick out two tiny Pioneer
spacecraft as having especial significance? Certainly, success will require a
formidable effort. The Pioneer 10 and 11 probes are therefore basically
addressed only to the most sophisticated and ambitious civilizations. That
happens to be the best we can hope to do right now, but as our technology
develops we should be able to do far better.


In the past billion years alone, a billion mile-sized comet nuclei and countless
smaller chunks have left our solar system. In this confusion of debris, how is
any civilization going to pick out two tiny Pioneer spacecraft as having
especial significance?

Similar questions of confusion arise when we think of communicating by radio or
by means of visible lasers. The prime constraint is seldom the sensitivity of
the detector. Instead, we are mainly limited by the profusion of signals emitted
from natural astronomical sources. The detection of a message from another
civilization requires means of discriminating against all this noise.

If we inhabited a quieter universe, in which there were far fewer bright
astronomical sources, this problem would be less severe. On the other hand,
perhaps there would also be correspondingly fewer intelligent civilizations in
the universe.

The optimum means for transmitting signals also depends on the urgency of a
message. Spacecraft are rather slow. At best they can approach the speed of
light if we are willing to budget for the high expense of accelerating these
craft. Even if we do, the destruction rate through collisions

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