Interstellar Communication

Poul Anderson, The Boat Of A Million Years (London, 1991).

The ramscoop field blanks out electromagnetic communication but not modulated neutrinos. However:

any neutrino beams cast by Pytheas would disperse too rapidly to reach Earth;

the huge transmitter in the Solar System which would be able to reach Pytheas is tied up beamcasting to remote targets that might respond much later.

The targets are sources of indecipherable messages but are also located near probably uninhabitable sites like black holes or condensing nebulae. Starfaring civilizations would have dispatched robot probes to such sites and the probes in turn would send messages to attract or communicate with others. In fact, Earth has sent some such probes and Pytheas is moving in the direction of the nearest source.

Pytheas receives, from a short distance ahead of it, a broadcast of neutrinos with modulated pulse, amplitude and spin. The Pytheas AI, which reports verbally to and converses with Hanno, must "'...compensate for Doppler shift and time dilation.'" (p. 517) How is it possible to make sense of anything in such conditions? The ship detects a second, weaker but strengthening, source on the opposite side of its own projected course. Hanno infers that the aliens, knowing how Pytheas is headed but not knowing either the precise destination or the boost being used, have distributed several messengers through the zone that Pytheas will probably traverse.

Since Pytheas is moving at near light speed, its radiation is not far enough ahead of it to have given the aliens enough notice to dispatch anything but "'...low-mass, high-thrust robot craft, maybe made for this one purpose.'" (p. 518) Unless these aliens have instantaneous communication, FTL etc? However, Terrestrial scientists claim to know the limits of technological possibility.

The aliens cannot be near Sol or they would already have been detected. Since Pytheas, although decelerating, is still at near light speed, it will pass the messengers as it responds to them and their answers will take years to catch it. While Hanno and Svoboda discuss these problems, the alien transmission, its nearest source a light year distant, changes from "'...mathematically simple...'" (p. 517) to "'...considerably more complex...'" (p. 519). This could mean that the aliens can detect Pytheas instantaneously or just that they alternate between an alert signal and a substantial message.

The screened message is colored lights forming three-dimensional shapes. The ship explains that:

prime numbers define a coordinate space;
digital impulses identify points that are members of fractal sets;
an empirical seach is necessary for combinations providing images;
emerging intelligibility should give clues to refinements that will extract the full content.

Hanno and Svoboda watch dancing curves and surfaces become a picture of stars. They have not yet seen an alien.