Interstellar Travel

Once interstellar travel is mastered, at what rate might you expect the galaxy to be explored thoroughly. How long before they find us? We have to make some assumptions. Let us assume that an average star-hop is 10 light-years distance and, at 1/10th the speed of light thus takes 100 years, ignoring acceleration and deceleration. Let us imagine that once a planet is reached, it takes only 100 years to produce another two exploratory craft, which are then launched outwards into unexplored space. At this rate a 50,000 light-year (ly) diameter galaxy would be crossed in one million years, with significant exploration by the 2⁵⁰⁰⁰ exploratory craft from 5000 colonies!

You can imagine that the trip and next generation craft production might take longer, say 1000 years each. There are still only 5000 hops of 10 ly in a 50,000 ly galaxy, now taking 2000 years instead of 200. The total time only goes up to 10 million years. An optimized program, interleaving long and short hops for maximum exploration coverage would take less time. These exploration programs might be best left to robots in stripped-down craft, but one can also image follow-up ark-like expeditions.

Faster (robotic) exploration certainly seems feasible. Acceleration to relativistic speeds and then no stopping ensures a speedy journey, crossing a 50,000 ly galaxy in a bit over 50,000 years. A first craft might drop off light-weight sensors onto planets; these sensors would radio data to a follow-up craft trailing by two years or so, which would relay the message back home. Yes, slowing down the probes could be a problem, but stellar radiation pressure on an infalling solar sail might do the trick. You can doubtless think of other schemes.

The WOW Signal

On August 15, 1977 the Ohio State University Observatory detected the signal seen above. It was found at the time that the source was not terrestrial and was moving with the background stars. The coordinates did not match any known solar-type star. This would have been called a valid SETI "contact" except for the fact that the signal did not repeat. This has become known as the "WOW" signal since one of the astronomers made that notation in the margin of the computer printout. (Courtesy: SETI)

At some point the SETI programs may be successful - some signal or other evidence of extraterrestrial intelligence may be discovered. What then? One can only imagine, and many science fiction and screen writers have described several alternatives. Scientists working on these projects have their own dreams of a rational process, which you can read at:

SETI Institute

In brief, the discoverers should first make very sure their signal (or whatever) is indeed extraterrestrial, and presumably from outside our solar system. They should then notify other SETI teams for confirmation. If confirmed, the news is to be spread widely to the world. Formalism for a return message is still in development, presumably because there is no universal agreement on whether we should reply.

The "WOW" printout. (Courtesy: SETI)

It may be noted that the first stage of this process were followed back in 1967, when radio astronomers at Cambridge University discovered a radio source emitting pulses at extremely well-spaced intervals, better than most clocks. For a while there was half-serious talk of "little green men", until three more such sources were found and the phenomenon was attributed to spinning neutron stars emitting searchlight-like radio beams. The rest of the scientific community however was not immediately informed of the positions of the additional pulsars. Many publications and a Nobel Prize followed, but no world-wide panic. These relics of supernovae are very interesting, but not nearly as much so as extraterrestrials would have been!