Extraterrestrial Civilizations - Isaac Asimov [114]
What’s more, as we shall see, there are serious difficulties involved in so long an acceleration and deceleration and in so high a speed, so that it is clear that interstellar travel is a mighty project that might well defeat the most advanced technology.
That is why earlier in the book I suggested that the inability of any civilization to carry through successful interstellar flights is the most logical reason why Earth has been left unvisited. The difficulty of interstellar flight may be such that no extraterrestrial civilization has ever made physical contact with any other, but that each one is confined, now and forever, to its own planetary system.
—And that we are confined to ours.
BEYOND THE SPEED OF LIGHT
Let us, however, not give up so quickly. Let us consider that perhaps there is some way of beating the speed-of-light limit. I said earlier that there is “no even remotely reasonable ground for doubting it [the existence of the speed-of-light limit] where the matter and the Universe we know are involved.” Would it be possible, then, to suspect there might be matter we don’t know or aspects to the Universe we don’t know?
To begin with, for instance, the speed-of-light limit applies most clearly to objects that—when they are at rest relative to the Universe as a whole—possess mass. This includes all the components of atoms and, therefore, of ourselves, our ships, and our worlds. All these must always travel at less than the speed of light and only infinite force can bring them to the speed of light itself.
That would seem to include everything, but it doesn’t. There are some objects that do not have mass, or would not have any if they were at rest relative to the Universe generally. Such objects with “zero rest-mass” include the photons that are the units of all electromagnetic radiation. It also includes the gravitons that are, in theory at least, the units of the gravitational force. Finally, it includes several different varieties of particles called neutrinos.
All particles with zero rest-mass must, at all times, move through a vacuum at precisely the speed of light, not a hair less, not a hair more. It is because light is made up of photons that go at that speed, that we speak of the “speed of light.”
If slow-moving particles with mass interact in such a way as to produce a photon, that photon darts off instantly at the speed of light without any perceptible interval during which it accelerates. Again, if a photon is absorbed by some particle with mass, its speed vanishes at once without any perceptible interval of deceleration.
It is sometimes speculated that it might be possible some day to convert all the particles-with-mass in a ship, including those in the crew and passengers, into photons of different types. The photons would then, without the necessity of acceleration and without the expenditure of the energy ordinarily required to bring about that acceleration, move off at the speed of light. Ordinarily, they would move off in all directions, but we might imagine the conversion to take place under conditions that would produce a laser beam of light. Such light would all move off in the same direction, for instance that of Alpha Centauri. Once the photons had arrived at Alpha Centauri, they would be converted back to the original particles—something that would require no deceleration, and none of the energy ordinarily required for such deceleration.
In this way, it would appear that any ship engaged in a round trip to some particular star might save the year ordinarily lost in acceleration and deceleration each way and, what is far more important, would be spared the vast energies required.
There are disadvantages, though. In the first place, it would still mean travel at the speed of light only. Saving 2 years might be significant, but only for the nearest stars. Allowing one year for exploration, the round trip to Alpha Centauri would take 9.4 years rather