Extraterrestrial Civilizations - Isaac Asimov [112]
Perhaps every one of the civilizations would do this—not as easily as we would do it, but perhaps all the better, thanks to the greater intensity of the challenge. Perhaps every civilization develops space flight and explores and settles its planetary system.
In that case, why haven’t we heard from them? Why hasn’t any civilization come calling?
What would be needed for a visit is not merely the capacity to flit from one planet to another, but from one planetary system to another, and this might represent a completely different order of difficulty.
CHAPTER 12
Interstellar Flight
THE SPEED OF LIGHT
The farthest objects we can see in our Solar system are the planet Pluto and its moon, Charon. There are comets that recede to distances far greater than that of Pluto. Perhaps many billions circle the Sun at distances far greater than Pluto at every point. No comet, however, has ever been seen past Pluto’s orbit—or past Saturn’s, for that matter. The width of Pluto’s orbit can therefore be taken as the diameter of the visible Solar system and that comes to 11,800,000,000 kilometers (7,330,000,000 miles).
This is an enormous distance, for the diameter of the visible Solar system is nearly 80 times the distance from the Earth to the Sun. Nevertheless, the distance to even the nearest star, Alpha Centauri, is about 3,500 times that diameter.
If the Solar system were so shrunk that the orbit of Pluto would just fit around the Earth’s equator (on which scale the Earth would be 160 kilometers, or 100 miles, from the Sun), Alpha Centauri would be at just the distance of Venus at its closest—and Alpha Centauri is the nearest star. Sirius is twice as far away as Alpha Centauri; Procyon 2.5 times as far away; Vega 6 times as far away; Arcturus 9 times as far away; Rigel well over 100 times as far away.
We can look upon these distances in another fashion. Consider the speed of light and of electromagnetic radiation (x-rays, radio waves, and so on). That speed is 299,792.5 kilometers (186,282.4 miles) per second. This is important, since our fastest means of communication is through use of electromagnetic radiation. We know no signals that travel faster.
It takes 1.25 seconds for light (or any similar radiation) to travel from Earth to the Moon. That means that when someone on Earth speaks to an astronaut on the Moon, he cannot possibly get an answer in less than 2.5 seconds, even if the astronaut answers instantly in a mere eyewink on hearing what is said to him.
If we define a “light-second” as the distance light can travel in one second, then the Moon is 1.25 light-seconds from the Earth.
It takes light 10.93 hours to travel the full width of Pluto’s orbit. If we imagined a space settlement at each side of the orbit, with one attempting to establish communication with the other, then the one who spoke first could not expect to get an answer, under any circumstances we now know of, in less than 21.86 hours.
Therefore, the diameter of the visible Solar system is equal to 10.93 light-hours; that is, 10.93 times the distance light can travel in one hour.
Using that system, Alpha Centauri, the nearest star, is 4.40 light-years away, or 4.40 times the distance light can travel in one year. If someone on Earth sent a message to a planet circling Alpha Centauri and an answer was sent back the very instant the message was received, the person on Earth would have to wait 8.8 years after sending his message to get an answer.
As for other stars, Sirius is 8.63 light-years away; Procyon, 11.43 light-years; Rigel (still a comparatively close star), 540 light-years away. It would take over 1,000 years to get an answer from a planet circling Rigel.
This might seem irrelevant to the problem of getting to the stars. If light takes 4.40 years to reach Alpha Centauri, need we not merely build up our speed to where it is faster than light and thus outrace the signal and get there in less time than light does?
However, as Albert Einstein (1879–1955) first pointed out in his Special Theory of Relativity