Living as we do in technologically triumphant times, we are inclined to view interstellar spaceflight as a technical challenge, like breaking the sound barrier or climbing Mount Everest—something that will no doubt be difficult but feasible, given the right resources and resourcefulness. This view has much to recommend it. Unmanned interstellar travel has, in a sense, already been achieved, by the Pioneer 10 and 11 and Voyager 1 and 2 probes, which were accelerated by their close encounters with Jupiter to speeds in excess of the sun’s escape velocity and are outward-bound forever. By interstellar standards, these spacecraft are slow: Voyager 1, the speediest of the four at 62,000 kilometers per hour (39,000 miles per hour), will wander for several tens of thousands of years before it encounters another star.
But the Wright brothers’ first airplane wasn’t particularly speedy either. A manned interstellar spacecraft that improved on Voyager’s velocity by the same 1,000-fold increment by which Voyager improved on the Kitty Hawk flights could reach nearby stars in a matter of decades, if a way could be found to pay its exorbitant fuel bill. But that’s a big “if,” and there is another way of looking at the question: Rather than scaling a mountain, one can always scout a pass. In other words, the technical problems involved in traveling to the stars need not be regarded solely as obstacles to be overcome but can instead are viewed as clues, or signposts, that point toward other ways to explore the universe. First, interstellar space travel appears to be extremely, if not prohibitively, expensive. All the propulsion systems proposed so far for interstellar voyages—fusion rockets, antimatter engines, laser-light sails and so on—would require huge amounts of energy, either in the manufacturing of fusion or antimatter fuel or in the powering of a laser beam for light sails. Second, there is no compelling evidence that alien spacefarers have ever visited Earth. Third, radio waves offer a fast and inexpensive mode of communication that could compete effectively with interstellar travel. The high cost of interstellar spaceflight suggests that the payloads carried between stars—whether dispatched by humans in the future or by alien spacefarers in the past—are most likely, as a rule, to be small. It is much more affordable to send a grapefruit-size probe than the starship Enterprise. If there were any truth in this fancy, what would our galaxy look like? Well, we would find that interstellar voyages by starships of the Enterprise class would be rare, because most intelligent beings would prefer to explore the galaxy and to plumb its long history through the more efficient method of cruising the Net.
When interstellar travel did occur, it would usually take the form of small, inconspicuous probes, designed to expand the network, quietly conduct research and seed infertile planets. Radio traffic on the Net would be difficult for technologically emerging worlds to intercept, because nearly all of it would be locked into high-bandwidth, pencil-thin beams linking established planets with automated nodes. Our hopes for SETI would rest principally on the extent to which the Net bothers to maintain omni directional broadcast antennae, which are economically draining but could from time to time bring in a fresh, naive species—perhaps even one way out here beyond the Milky Way’s Sagittarius Arm. The galaxy would look quiet and serene, although in fact it would be alive with thought. In short, it would look just as it does.