Extraterrestrial Civilizations (35 page)

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 to be located in convenient places, which doesn’t seem at all likely.

Another way of imagining a spaceship’s gaining acceleration without fuel is to picture a laser beam shining upon a large “sail” surrounding the vessel. The laser beam, based on some convenient body in the Solar system, would be trained continually on the sail and would act as one continuous push serving to steadily accelerate the vessel. The laser beam, to remain in being would, of course, consume the vast quantities of energy that the ship was not consuming. (You can’t beat the system when it comes to energy.) In addition, it would be more and more difficult to remain on target as the ship moved farther and farther from home base. Finally, the laser beam could not be used to decelerate unless someone at the destination point up ahead could supply an obliging beam in the opposite direction.

Still, if all nonfuel methods failed and a speed-of-light vessel had to use fuel, it might perhaps not have to carry that fuel. It might be able to pick it up as it went along. After all, interstellar space is not truly empty, not an utter vacuum. There are occasional atoms of matter present, mostly hydrogen.

In 1960, the American physicist Robert W. Bussard suggested that this hydrogen might be picked up as the spaceship plowed through space. The ship would be a kind of “interstellar ramjet” but since space has much less matter in it than Earth’s atmosphere does, the ship would have to sweep up the matter from a far larger volume of space, compress it, and extract energy through hydrogen fusion.

The ship’s scoop, in order to be effective, would have to be at least 125 kilometers (80 miles) in diameter when it is passing through those volumes of space where there are clouds of dust and gas, and matter is strewn most thickly. In clear interstellar space, the scoop would have to be as much as 1,400 kilometers (870 miles) in diameter, and in intergalactic space, 140,000 kilometers (87,000 miles) across.

Such scoops, if we imagine them built of even the flimsiest materials, would be prohibitively massive. How would the materials
in those scoops be carried out into space; or how much time and effort would it take to assemble them out of matter already in space?

Even if the energy problem is somehow beaten by methods we can’t in the least foresee, it remains true that a huge ship traveling very near the speed of light is peculiarly vulnerable. There may be no danger of striking a star, but it may well be that space is fairly full of relatively small bodies from planets down to gravel.

From the viewpoint of the ship, every object in the Universe that happens to be approaching it will be doing so at the speed of light. Such objects will be impossible to avoid, for any conceivable message that heralds their approach (x-rays or anything else) will be traveling only at the speed of light so that the object itself will be hot on the heels of the message. No sooner will a collision warning sound than the collision will take place.

And any massive object colliding with the ship, where the velocity of one relative to the other is that of light, would leave a neat hole in the ship where it entered, where it emerged, and at all intersections in between. The ship might be a Swiss cheese before long.

Even if we discard sizable particles and assume there is nothing but very thin gas in the volume of gas being passed through—that is enough to make trouble.

As the spaceship accelerates and goes faster and faster, the atoms in interstellar space strike harder and harder, and more and more of them do so per second.

From the standpoint of the spaceship, the oncoming particles will be approaching at very near the speed of light and that will make them, to all intents and purposes, cosmic-ray particles.

Under ordinary conditions, cosmic-ray intensity in space is not particularly deadly. Astronauts have remained in space for more than 3 months continuously and have survived handily. Moving through interstellar space at the speed of light, however, with every oncoming particle striking with cosmic-ray speed, the ship will be subjected to an intensity of radiation several hundred times that produced by one of our modern nuclear reactors.

Some scientists suspect that this interference by interstellar matter will itself be sufficient to keep space vessels from ever reaching speeds of over 1/10 that of light—and at that speed the time-
dilatation effect is very minor.

Even if all difficulties are overcome, there remains another problem that lies at the very core of relativity. The slowed time sense affects only the astronauts,
not
the people back on the home planet.

Making use of 1-g acceleration and deceleration, and time dilatation, to the full, a trip to the star Deneb and back will take astronauts 20 years (even allowing one year in the Deneb system for exploratory purposes). When they return, however, they will find that 200 years have passed on Earth. The longer they travel at this acceleration, the more closely they will creep up to the speed-of-light limit and the more slowly time will pass for them. Thus, the discrepancy between ship-time-passage and Earth-time-passage rapidly increases with distance. A round trip to the other end of the Galaxy will seem to take 50 years to the astronauts, but they will find that some 400,000 years will have passed on Earth. (This would be true to an even greater extreme in the case of the photonic drive.)

One has the feeling that this alone would suffice to make it certain that there would be no great popular demand among the people on Earth (or on any home planet) for investing in stellar exploration by time dilatation. It is difficult enough to get people to deprive themselves of anything now for the sake of having something desirable or even essential come about in 30 years. To invest an enormous effort in something that will return centuries later or hundreds of thousands of years later would not seem to be something we would count on people doing.

Considering, then, the difficulties in energy requirements, in radiation danger, and in time differential, our conservative standards would make it seem that time dilatation is not a practical means, either physically or psychologically, for reaching the stars.

Since all methods for traveling near the speed of light or actually beyond it seem to be impractical, we must see what can be done at low speeds.

The advantage there, of course, is that the energy requirements
are not exorbitant, nor is the environment of interstellar space then dangerous. The disadvantage rests in the time such a voyage would take.

Suppose a ship were to be accelerated to a speed of 3,000 kilometers (1,860 miles) per second. This would be very fast by ordinary standards since at that speed a ship could travel from the Earth to the Moon in 2 minutes. Still, it is only 1/100 the speed of light, so that the time-dilatation effect is negligible, and it would take nearly 900 years for the round trip to Alpha Centauri, the nearest star.

Are there any conditions under which a 900-year trip could be endurable?

Suppose the astronauts are immortal. We might decide that in that case, coasting there and back (with comparatively small intervals of acceleration and deceleration) for 900 years would represent a trivial fraction of an endlessly protracted life and would offer no problem.

However, even if the astronauts are immortal, we presume they would have to eat, drink, breathe, and eliminate wastes. That means there would have to be a complex life-support system that would work without fail for nearly 1,000 years. We might imagine it being done, but surely it would be expensive.

Then, too, the astronauts would have to have something to occupy their minds. Comparatively close quarters with no chance for a change in company for nearly 1,000 years could be very difficult to tolerate. It might not be too cynical to suppose that murder and suicide would empty the ship long before the trip is over, for it is much easier to imagine a victory over death than a victory over boredom.

And, of course, we have no real reason to think—at least so far—that we will ever be able to achieve immortality.

But then we can, perhaps, short-circuit some of the difficulties of immortality by changing it to a temporary death followed by a resurrection, In other words, suppose we freeze the astronauts and place them into suspended animation, bringing them back to life only when the destination is in view.

Under such circumstances, the ship can proceed by coasting at low speeds, avoiding the disadvantages of speed-of-light travel, while
the astronauts remain as unaware of the passage of time as they would in the case of time dilatation. To them, a voyage of thousands of years would pass in an eyeblink, and when brought back (it is to be presumed), they would not have aged perceptibly. In that way there would be no need for an inordinately reliable life-support system of the usual form; nor would there be the problem of keeping the astronauts occupied and unbored during the long flight.

There are obvious catches, though. The problem of freezing a human being without killing the person and then bringing about a successful revival is not a problem we seem (so far) to have much hope of solving.

Even if we could solve it, there might well be limits as to how long the frozen body could be kept with its spark of life intact. It might not be possible to maintain it throughout the long voyage between the stars. And if we could do that, then we would have to supply the ship with some foolproof system for maintaining the frozen state (a new form of life-support system) and for acting automatically to revive the astronauts at some appropriate moment. A device that can spring to life after some centuries of remaining dormant is not an easy thing to imagine.

The difficulties are enormous, and while we cannot insist that they will never be overcome given enough time, neither can we be certain that the problem will be solved inevitably.

Then, too, while the frozen astronauts are in suspended animation and, as a result, do not age, and are not aware of the passage of time, this is not true for the people back home who sent them off (unless the entire population of the planet undergoes freezing, which we may dismiss as ridiculous). This means that, exactly as in the case of time dilatation, the astronauts will return generations later and will experience a profound “future shock.”

In fact, even in the case of immortality, there would be difficulties. We might assume that if the astronauts are immortal, then the general population of the planet is immortal as well and that after the long trip the astronauts will return to report to the very people who sent them off long ages ago. But life is sure to have followed very different directions on the ship and on the planet, and the two groups of people are certain to be strangers to one another.

It seems quite likely that, under any circumstances so far mentioned,
there will be no point to the astronauts’ returning home. Exploration of the stars would have to be undertaken on the understanding that the astronauts and the ships will never be seen again. Messages may be sent and received as the centuries and millennia pass, but that would be all.

The question, though, is whether, in that case, human beings would be willing to go into permanent exile. Or whether the home planet would be willing to undergo the expense of sending intelligent beings if all that will come of it are occasional messages received far in the future.

Might it not in that case be more economical, less difficult, and actually more productive, if automatic probes are sent to the stars? The astronomer Ronald N. Bracewell (1921–) suggested as early as 1960 that other civilizations might well have used this strategy.

We ourselves have taken this tack in connection with the planets. While astronauts have only been able to go as far as the Moon, automatic probes have landed on Mars and Venus and gone past Mercury and Jupiter. We have gained considerable knowledge as a result of these probes, and even if we were of the opinion that human exploration would be preferable, we must admit that where human exploration is impossible, the probes are a reasonable substitute. So far they have produced results that are by no means negligible.

We might, therefore, send stellar probes outward. The expense would still be enormous, but it would be far less than that involved in sending human beings. We can indulge in greater acceleration, eliminate life-support systems for either living or frozen astronauts, and feel no concern for the psychological welfare of astronauts. Nor need we fear future shock, since there would be no particular reason for an automatic probe to return—and even if it did, it would not matter to it that generations had passed.

We can imagine advanced civilizations sending out very advanced probes, but surely there must come a point of diminishing returns. The more elaborate the probe, the more difficult and uncertain its maintenance would have to be. Over thousands or even millions of years, it is hard to suppose that anything really elaborate would keep working faultlessly. (Surely even the most advanced civilization could not alter the second law of thermodynamics or the uncertainty principle.)

If we go to an extreme, we might imagine a crew of advanced robots as intelligent as human beings, for instance, exploring the Universe as human beings themselves could not. And yet if robots are
that
intelligent, might they not also find themselves vulnerable to the diseases of intelligence—boredom, depression, rage, murder, and suicide?