Extraterrestrial Civilizations (33 page)

It might all have been less likely without the Moon beckoning us in our fictional dreams, but given the lapse of additional time it might have taken place. Indeed, without the Moon, we could imagine everything that has taken place so far to have taken place anyway, except for the manned and unmanned flights to and past the Moon. Even the probes to the far-distant planets would have taken place.

But would we then have progressed onward to space settlements? If such things seem impractical to many “hardheaded” human beings now, how much more impractical would it seem if all the material for the construction of settlements had to come from Earth itself; if there were no way of using the Moon as a source of raw materials?

And without the space settlements, the true exploration of the Solar system would, in my opinion, be most unlikely.

If, then, it is true that a large Moonlike satellite is a very unlikely possession for an Earthlike habitable planet, and that Earth is the beneficiary of a very rare astronomical accident in this respect, then we must wonder if other civilizations have ever developed space-flight capacity greater than that which we possess right now.

Are other civilizations, one and all, confined to their planet and its immediate environs, and are they capable, at the most, of sending probes to other planets? And is this true, no matter how advanced their technology? It is a tempting thought. It would so neatly explain why the Universe seems so empty, even though half a million and more civilizations may exist in our own Galaxy alone.

It would also offer a sop to our pride. Thanks to our lucky possession of the Moon, it might be that within the next couple of centuries we will develop space-flight capacities far beyond other
civilizations that may be far older, and in other respects far more advanced than we. Will we, and no other civilization, eventually fall heir to the Universe, thanks to the Moon?

It is hard to believe that, perhaps. Surely, given a little more technological development than we have, and given the driving force of the need for energy, a civilization would somehow launch itself into space even without the presence of a Moon. The planet’s own resources would be used, at whatever reasonable cost, and the direct flight to the nearest planets made no matter how tedious and difficult. Once that was done the resources of the nearby planet could be used to continue matters.

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.

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 in 1905, it is impossible for any object
with mass to exceed the speed of light. Einstein set this limit from purely theoretical considerations and it seemed, when it was first suggested, to go against the dictates of “common sense” (and it seems so to many people even today)—but it is true just the same. The speed-of-light limit has been verified in innumerable experiments and observations, and there is no even remotely reasonable ground for doubting it where the matter and the Universe we know are involved.

The “common sense” that makes it so difficult to accept the limitation is based on our experience with everyday phenomena. We notice that if we keep pushing an object, it goes faster and faster and faster. In fact, Newton’s second law of motion specifically states that this is so and that an equal push will always result in an equal speedup regardless of how fast the object is already moving. It would therefore seem that no matter how fast we make an object move, we can always make it go still faster by giving it an additional push. Indeed, careful observation and measurement bear this out under ordinary circumstances.

But that is because we deal with objects that go only a tiny fraction of the speed of light, and under such circumstances Newton’s second law does indeed hold as far as we are able to measure and “common sense” reigns supreme.

The truth is, though, that if we give an object a push and make it speed up, and then give it a second push of just the same size, the amount by which the object speeds up the second time is
not quite
as high as the first time. Some of the force of the push goes into increasing the speed, yes; but some goes into increasing the mass as well.

At ordinary speeds, so little of the force goes into increasing the mass that that portion is undetectable. As the speed goes higher and higher, however, a larger and larger fraction of the force goes into increasing the mass and a smaller and smaller fraction into increasing the speed, according to a formula worked out by Einstein. When the speeds are high enough, so much of the force goes into mass and so little into additional speed that we begin to notice that Newton’s second law and “common sense” aren’t working anymore.

It wasn’t until the opening of the twentieth century that scientists knew of any objects that moved fast enough to begin to show the imperfection of the second law. The fast-moving objects then discovered were subatomic particles, and careful studies of these tiny
objects showed that Einstein’s equation relating force and speed was exactly right.

By the time the speed of any object gets close to the speed of light, hardly any of the force applied to it goes into additional speed. Almost all of it goes into additional mass. The speeding object becomes much more massive, but hardly any more speedy. In the end, even if you put an infinite amount of force into the speeding object, you can only serve to give it an infinite mass and raise the speed only to the speed of light.

That means that even if you accelerate to maximum speed in an instant by some magic device, it would still take you 4.40 years to reach Alpha Centauri. If you could then decelerate to zero in an instant, turn around, accelerate to maximum speed in an instant, it would still take you 8.80 years to make a round trip.

In actual fact, you would have to accelerate to a very high speed, and that would take a long time if you confine yourself to an acceleration low enough for the human body to endure. It would then take an equally long time to decelerate so that it would be possible to land on a planet in the Alpha Centauri system.

The need to accelerate and decelerate would add about a year to the time it would take to reach a star if we were to travel at the speed of light all the way. Another year would have to be added on the return, and a third year, perhaps, for the time taken in exploration.

Thus, if we count in acceleration, deceleration, and exploration, the time taken to go to any star and return is the speed-of-light round trip plus three years. To travel to Alpha Centauri, explore the system, and return would take 11.80 years—and Alpha Centauri is, I repeat, the
nearest
star.

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.

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 than 11.4 years, which is a significant saving; but the round trip to Rigel would take 1,081 years instead of 1,083, which is not.
^*
Secondly, I am not at all sure that it is possible to divorce speed and energy expenditure as I have so glibly stated. I have a strong suspicion that if we arranged to convert a quantity of matter into photons, we would find that the amount of energy we had to expend to do so would be equal to the amount we would have had to expend to accelerate the matter to near the speed of light in the first place. The same would be true of the conversion back into matter, where we would have to expend as much energy as we would have had to in decelerating the matter from the speed of light. Therefore, it could be that the “photonic drive” would save us no time to speak of, and no energy either.