Suppose you want to take a trip across the country from Portland, Maine to Portland, Oregon.
That's roughly 3,000 miles. A trip around the world along the equator is only a little over eight times that, 25,000 miles.
To go from the Earth to the moon is only about nine times the equatorial jaunt, about 240,000 miles. Beyond that? Well, Venus at its closest is just over a hundred times the distance to the moon; it is about 25,000,000 miles away. And right now, Pluto is just about as near to Earth as it ever gets, but it is over a hundred times the distance to Venus. It is about 2,800,000,000 miles away.
So far we've stayed in our solar system, but beyond that are the stars. Even the nearest star is nearly 9,000 times as far away as Pluto is right now. The nearest star is Alpha Centauri and it is 25,000,000,000,000 miles away. And that's the nearest star.
The distance across the Milky Way galaxy is 23,000 times the distance from Earth to Alpha Centauri. The distance from here to the Andromeda galaxy, the nearest large galaxy to our own, is about twenty-three times the diameter of the Milky Way galaxy. And the distance from here to the farthest quasar is about 4,000 times that from here to the Andromeda.
What about time? It takes a few days to get to the moon; a few months to get to Venus or Mars; a few years to get to the giant planets of the solar system. But that's about as far as we can go and have it make reasonable sense.
To get to even the nearest star, at the present state of the art, would take hundreds of thousands of years. All that NASA has so far done in sending probes as far as Saturn has been to play games in our backyard. It is interstellar travel, trips to the stars, that represent the longest voyage.
And it is in trips to the stars that science fiction writers and readers are most interested. Our solar system is too well known and too limited. The solar system (outside Earth) is not at all likely to bear life of any kind-certainly not intelligent life. So we've got to take the longest voyage and get to the stars, if we're to find extraterrestrial friends, competitors, and enemies. As long ago as 1928, in The Skylark of Space, E. E. (Doc) Smith took the first science-fictional trip to the stars, and how the readers loved it.
Good old Doc was a little vague on just how his interstellar ships managed to cross those huge spaces, however, and, to tell you the truth, we're not much better off now. Let's list the possibilities:
1. We can keep accelerating; going faster and faster and faster until we're going fast enough to cover vast interstellar and intergalactic distances in a matter of months, or even days. objection: Physicists are strongly of the opinion that the speed of light in a vacuum, 186,000 miles per second, is as fast as anyone can go. At that speed, it will still take years to reach the nearest star, millions of years to reach the nearest large galaxy.
2. Even if we're limited to the speed of light, that could be good enough. As one approaches the speed of light, the rate of time passage on the speeding object slows steadily, and at the speed of light itself, the rate of time passage is zero. At light speed, then, the crew of a starship would cover enormous distance practically instantaneously. objection: Interstellar and intergalactic space is littered with occasional hydrogen atoms. At light speed, these atoms would strike the ship with the energy and force of cosmic ray particles and would quickly kill the starship's crew and passengers. Probably, the ship would have to go no faster than one-tenth light speed, and at that speed the time effects are not great enough to help us much.
3. Suppose we attach a kind of "atom-plow" arrangement in front of the starship. It would scoop up all the atoms in front of it, thus preventing cosmic ray problems and, in addition, gathering material to serve as fuel for its nuclear fusion engines. objection: Such atom-plows would have to be many thousands of miles across to be effective. Building such things would represent enormous and perhaps insuperable problems.
4. We can evade the speed-of-light limit altogether by making use of tachyons, subatomic particles that move much faster than the speed of light and that, as a matter of fact, cannot move slower than the speed of light. objection: Tachyons exist only in theory, and have not actually been detected. Most physicists think they will never be detected. Even if they were detected, no one has even come close to figuring out a way of putting them to use.
5. Perhaps we can evade the speed-of-light limit by going through black holes. They at least are known to exist. objection: Even if black holes exist (and astronomers are not yet unanimous on this), no one is even close to suggesting how any stars hip might approach one without, being destroyed by tidal forces. In addition, there is by no means general agreement that one can negotiate long distances quickly by going through black holes.
6. In that case, we might find some other way of leaving this universe. We could then travel through hyperspace in "jumps" that will carry us enormous distances in zero time. objection: So far hyperspace exists only within the imagination of science fiction writers.
7. Well, then, we can submit to the speed-of-light limit, but freeze the crew and passengers, and arrange to have them restored to conscious life after thousands of years have passed and the destination has been reached. objection: No one really knows how human bodies can be frozen without being killed; or whether such frozen bodies, even if retaining a spark of life, can retain it over a period of thousands of years.
8. In that case, there seems nothing left to do but to coast-to travel at ordinary speeds, considerably less than that of light, with all people aboard thoroughly conscious. This means it will take many thousands of years to reach even the nearer stars, so that many generations will have to spend their lifetimes aboard the starship. That may be bearable if the starship is large enough. objection: None, really, if people want to do it.
So much for hardheaded realism. In science fiction, we tend to have faith that problems that seem insuperable now will be solved-perhaps in ways that are utterly unexpected.
Therefore we are offering you a baker's dozen of stories, all involving starships. In these are explored the various strategies I have described above for covering long distances, and perhaps one or two that are too far-out for me to have even mentioned.
What's more, the stories explore the effect of the long voyages on the people on board the starship, and the kind of events that might take place on them.
Since it is not likely that such voyages will be undertaken in our lifetime (and certainly not completed, if the generations-long coasting starship should indeed prove to be the only practical alternative), these exciting science-fictional speculations are the only way we can experience, if only vicariously, the long voyages that are the quintessential dreams of the far-flung imagination.