Project Cyclops, A Design... - Department of Earth and Planetary ...

THEPROBABILITY
COMMUNICATION
OF INTERSTELLAR
To have a 63 percent chance of success we must
search lip target stars. If the search takes r years per
star, the mean search time will be
r rT
Ls - - (17)
P FL r
Pr
t
f(t)
_s
6
1----_
where we have written L r for L to indicate the length of
the radiative phase.
If interstellar communication is not an already
established reality in the galaxy and various races, like
ourselves, make sporadic attempts at both searching and
radiating, we might assume that Lr=L s in (17) and
obtain
Ls_>_
Taking r = 1000 sec ---3X 10-s years, we find
L s >
rT
560 years
This represents a truly formidable effort, but one we
might be willing to make if we were sufficiently
convinced of the existence of extraterrestrial intelligent
life, of the potential value of contact, and of the validity
of our search technique.
On the other hand, if interstellar communication
already exists, the situation is likely to be very different
as a result of what Von Hoerner (ref. 26) has called the
"feedback effect." The radiative history of a typical
civilization might then be as depicted in Figure 2-11.
After an initial search phase of duration S, during which
the civilization radiates a beacon signal, with probability
Pr, contact is established for reasons that will soon
become clear. The civilization then finds itself part of a
galactic community that shares the obligation to facilitate
acquisition by other civilizations. This might mean
radiating a beacon for a fraction o of the time for a very
long period L. (Or such beacons might be established for
reasons we are completely unable to foresee or understand.)
The radiative phase would then have an effective
duration:
L r = prS + eL
Figure 2-11. Radiative history of a civilization assuming
interstellar communication exists.
We might, not unreasonably, expect eL to be as great as
108 years. Taking r = 3× l 0--s years we then find from
(17)
3X 10-a
L s = _ years (20)
F
(18) which means we might have to search 100 to 10,000
stars, requiring only from 1 day to 4 months, depending
on F. If interstellar communication already exists and its
participants have good reason to continue it, the
acquisition of our first contact may be far easier than we
would otherwise expect.
We thus come to the viewpoint that interstellar
communication is a phenomenon with some of the
attributes of life itself. It is difficult to explain how it
got started, but once started it tends to perpetuate itself.
Just as for life itself, interstellar communication is
unlikely to have originated in any single trial (by some
earlier race), but there have probably been millions of
attempts over billions of years only one of which needs
to have been successful to start the whole process. We
can think of many situations that may have triggered
interstellar communication initially:
1. A race realizes its primary star is nearing the end
of its main sequence lifetime and simply transmits
its history and knowledge with no hope of reply.
Detection of these messages would provide other
races with the existence-proof of other life needed
to justify a prolonged search.
2. Planetary systems exist for which ne > 1, and
advanced life on one planet discovers reasonably
evolved life on another planet, thus encouraging
the effort at contact with other planetary systems.
3. The random distribution of stars places two or
more advanced cultures fortuitously close to each
other so that first efforts quickly bear fruit.
Perhaps leakage signals are detected first. This
situation is more likely in dense regions such as
(19) star clusters or the galactic nucleus.
27