Extraterrestrial Civilizations - Isaac Asimov [118]
Suppose, then, we are in a spaceship that is accelerating at 1-g. (That is, at a rate that would make us feel pushed against the rear of the ship with the same force that gravitation now pulls us against Earth’s surface. At this acceleration we would feel perfectly normal. The back of the ship would seem down, the front up.)
After about a year of this, the ship would be moving at nearly the speed of light and by that time, although everything on board would seem normal to us, the outside world would seem very strange. It would become impossible, really, to watch many of the stars, for the light from stars ahead would shift far into the x-ray range and would be invisible. (In fact, the ship would have to be shielded from their radiation.) The light from the stars behind would shift into the radio-wave range and would be invisible, too.
If the people on board ship measured their speed against the distances they were covering, they would seem to be going at many times the speed of light, for it would take them only a week perhaps to cover the distance between two stars known to be ten light-years apart. If we could watch them from Earth, we would see that it actually took a little over ten years for the ship to cover the distance, but to the time-slowed sense of the people on board, those ten years would seem only a week long.
By making use of time dilatation, then, a space vessel would cover enormous distances in times that would seem comparatively short to the people on board. In a length of time that they would experience as 60 years, they would reach the Andromeda Galaxy, which is 2,300,000 light-years away from us.* Does time dilatation solve the problem?
Perhaps not, for there are difficulties. First, to maintain a 1-g acceleration for an extended period of time (or a 1-g deceleration, for that matter) takes enormous quantities of energy, as I indicated earlier.
Suppose we assume the most efficient way of getting energy, interacting equal quantities of matter and antimatter. Such a mixture undergoes mutual annihilation and the total conversion of matter to energy. For a given mass of fuel such a reaction would yield 35 times as much energy as hydrogen fusion, and if there is any way of getting more energy than that out of anything, we have no hint of what it might be at present.
And yet to accelerate a ton of matter to 0.98 times the speed of light would mean the conversion of about 25 tons of mixed matter and antimatter into energy, or the conversion of 100 tons for any round trip, counting two accelerations and two decelerations. If hydrogen fusion were used as the propulsion medium, something like 3,500 tons of hydrogen would have to undergo fusion. In other words, to carry one ton of matter to Alpha Centauri and back—just one ton-would take 10 times as much energy as the people of Earth consume right now in one year.
There is the possibility that one need not use fuel to attain the needed energy. The British-American physicist Freeman John Dyson (1923–) points out that a spaceship whipping around a planet like Jupiter can be enormously accelerated without any ill effects on the astronauts, since every atom of the ship and its contents will be accelerated alike (barring insignificant tidal effect). Indeed, the Jupiter probes, Pioneer 10 and Pioneer 11, were accelerated in this fashion, gaining energy at the expense of the vast pool of gravitational energy of Jupiter and gaining enough speed in this way to be hurled out of the Solar system.
We can imagine spaceships en route to some distant star, slipping past a giant planet now and then to gain huge increments of speed—if such giant planets happened