Beyond Centaurus

a deep-space probe into ways of escaping our solar system and the relative effects of intergalactic travel on the human aging process

As we have yet to reach the Moon and planets, it may seem slightly premature to worry about flights to the million-times-more-distant stars. The exploration of the Solar System will keep us busy for centuries; why bother about Alpha Centauri and points beyond?

For the best of reasons. Although the discoveries we shall make on our neighboring worlds will revolutionize our knowledge of the Universe, and probably transform human society, it now seems most unlikely that we shall find intelligent life on the other planets of this Sun. The odds are fantastically against it, when we consider the immense vistas of geological—and astronomical—time. The Solar System was formed at least five billion years ago; modern man is less than a million years old, and his civilization stretches back for little more than five thousand years. We will not be far out if we say that our Earth is a million times older than the culture it now precariously carries. It is, therefore, ridiculous to hope that, only next door, we shall find creatures anywhere near our level of development at this fleeting moment of time. The Martians may have been contemporaries of the great reptiles; the Cythereans may lie even further in the future.

We shall know the truth by the end of this century, but today it appears overwhelmingly likely that we are alone in the Solar System. With the exception of the dolphins, who may have better things to do, there is no one to talk to us within light-years of the Sun. And so, inevitably, farsighted scientists are turning their minds toward the stars.

Several recent developments have almost forced astronomers—some with reluctance, some with enthusiasm—to think seriously about communication with the planets of other suns. Only a generation ago, there was grave doubt that such planets existed; the still popular works of such pre-War writers as Jeans and Eddington asserted that the Solar System was probably unique, and that Earth might be the only abode of life in the Universe.

Today, thanks to improved knowledge of astronomy and biology, the position is completely reverséd. It is now believed that planetary systems are extremely common, and that life will arise inevitably on any world where it is given half a chance. How often life will evolve toward intelligence, and how often intelligence will take the path that leads to technology and science, there is no way of computing. One can only guess; and since our Galaxy—which is only one of countless galaxies—contains a hundred thousand million stars, it may well hold—at this very moment—millions of societies superior to our own.

Until about 1960, such speculations were only of theoretical interest, since there seemed no way in which they could be either proved or disproved. Then came the invention of the maser, which permitted a degree of noiseless radio amplification never before possible. The communications engineers did a few sums, and arrived at an astonishing result. If anyone put up the money—and if there was someone listening at the other end—it would not be difficult to build a microwave system that could send messages to the nearer stars. And if we can do this, less than a century after we have invented the telephone, then interstellar signaling should be a trivial engineering feat for a really advanced culture.

Since it is much easier—and cheaper—to receive radio messages than to transmit them, it will be some time before we attempt to send signals to the stars; but there is no reason why we should not start listening. As is well known, such experiments have already begun; however, even the optimists behind Project Ozma do not expect results for decades, perhaps for centuries. The odds against success are very high, but the prize is so great that the experiment is worth attempting. If we are lucky, we may in our own lifetimes hear the first intelligent signals from outer space. At the same time, we may well be relieved to know that all the radio and TV programs yet launched from Earth sank below the level of cosmic noise before passing beyond the orbit of Pluto ...

All these speculations, occurring simultaneously with the rising tempo of space research and preparations for manned interplanetary flight, have provided a basis for still more ambitious schemes. To talk to the stars—or even to exchange visual images, which is only slightly more difficult—will certainly be exciting, but it will also be very frustrating. Man, the inveterate explorer, will never be content with secondhand information; he will want to see for himself. In a century or so, he will have visited all the planets of this Sun—and, almost certainly, by advanced astronomical techniques, he will have detected planets of other stars. For every world that harbors beings capable of signaling across interstellar space, there must be many that cannot do so, yet would be well worth investigating. The societies that produced Lincoln, Shakespeare and Socrates were scarcely primitive, yet they could not make their presence known even upon the nearby Moon. Space must be full of fascinating cultures and life forms that can be studied only by direct, physical contact.

Today, no competent person doubts that we can reach the planets; but can we reach the stars? There is now a splendid fight shaping up among the space scientists over this very question. Rumors of this battle, which so far has been conducted decorously in the pages of the technical journals, have as yet scarcely reached the public, and whatever its outcome, no one expects that NASA will be providing funds for interstellar flight in the foreseeable future. Yet this is no esoteric controversy among specialists; it affects our entire outlook upon the Universe, upon our place in it—and, conceivably, upon our origin.

For though we may be centuries from achieving interstellar travel, if it is possible, someone must have done it already. And not once, but many, many times in the history of our huge and ancient Galaxy. How often have we had visitors in the past? How often may we expect them in the future? These are not frivolous questions; their answers may shake our civilization to its very roots.

Many scientists are so appalled by the sheer size of the Universe that they flatly deny the possibility of flight to the stars. Their attitude has been breezily defined by the Harvard radio astronomer Edward Purcell as follows: "All this stuff about traveling around the Universe in space suits—except for local exploration —belongs back where it came from, on the cereal box." Similar views have been expressed by other eminent scientists who have looked into the mathematics of interstellar rocketry. To anyone who—like myself—spent most of the Thirties and all of the Forties trying to convince people that we could fly to the Moon, such negative predictions have a depressingly familiar ring. And they are just as ill-founded as the assertions—remember them?—that man would always be confined to the planet of his birth. The remark that "the only thing we learn from history is that we learn nothing from history" is sometimes as true of scientists as of statesmen.

Almost 40 years ago, the British physicist J. D. Bernal pointed out, in a brilliantly imaginative booklet, The World, the Flesh and the Devil, that flight to the stars would be possible by the use of self-contained "Space Arks," virtually miniature worlds that could make voyages lasting thousands of years. Generations would live and the aboard them, knowing no other existence until the voyage drew to its end. The building of such vessels would not be an impossible task for an advanced, stable society, and if there is no other way of exploring the Universe, this is how it will be done. A variation on this theme is the shipload of deep-frozen voyagers, Rip van Winkles awakened by robots when their destination is in sight. All such projects would be expensive and time-consuming and, although they may appear very unattractive to us, one can easily imagine cultures that would undertake them.

Travel to the stars in a reasonable fraction of a human lifetime is a much more difficult proposition, and it is this that arouses the ire of the physicists. For it necessarily involves speeds approaching that of light, and this appears to be beyond the bounds of engineering possibility. To see why, let us look at a few figures.

The nearest star—Alpha Centauri, a triple-star system probably not suited for life—is about 25,000,000,000,000 miles away. Since such a string of zeroes is meaningless, the astronomers have invented the convenient unit of the light-year, or the distance that light travels in one year. (Let me emphasize that it is a unit of distance; many people seem to think it a measure of time.) Because light travels at 186,282 miles a second, simple arithmetic shows that a light-year is 5,880,000,000,000 miles—or six trillion, in round figures. So Alpha Centauri is 4.3 light-years away; there are about a dozen stars within 10 light-years of us.

At the speed of light itself, therefore—assuming no time at all for starting and stopping, still less for sight-seeing at the other end—the round trip to Alpha Centauri would take nearly nine years. This would not be impossible from the human point of view, though it would raise (continued on page 176)beyond centaurus(continued from page 116) some pretty psychological problems. But it is, by many orders of magnitude, out of the question in terms of known engineering and known energy sources.

The best that our present rockets can do, and that with the greatest difficulty, is about nine miles a second. This is only one twenty-thousand of the velocity of light—but there is worse to come, for energy increases with the square of velocity. To move a rocket twenty thousand times faster than the present limit we would need four hundred million times more energy. Even nuclear power comes nowhere near to providing this. After some centuries of technical development, perhaps the most that we can hope for from hydrogen fusion is a tenth of the speed of light (say 60,000,000 miles an hour!). This performance, which most physicists would consider highly optimistic, would just allow us to reach Alpha Centauri in a lifetime, for the one-way voyage would last some 50 years.

Note that these calculations have nothing to do with any speed limit set by the theory of relativity; they are based purely on energy considerations. We simply do not know a source of energy sufficiently concentrated to drive a rocket anywhere near the speed of light.

However, it is always very dangerous to argue, on the basis of existing or even conceivable technology, that something can never be done, as I pointed out in The Hazards of Prophecy in this journal a few years back. In the past, those who have done so have almost invariably been proved wrong. What seemed to be insuperable obstacles have either been overcome, or simply bypassed by the development of new techniques. You cannot bridge the Golden Gate with wood—you have to wait until the steel age arrives; you cannot operate a TV system with ropes and pulleys—you have to wait until electronics comes along. If the rocket is inadequate for flight to the stars, which certainly appears to be the case, then we shall have to think of something better.

That there are several directions in which we may look is encouraging, but perhaps misleading; major breakthroughs are almost always quite unpredictable and occur in areas where no one would dream of finding them. (My favorite example: One of the greatest advances ever made in medicine resulted from a physicist's attempts to pass electricity through a vacuum. What had that to do with medicine? X rays.) In the case of interstellar flight, what we obviously need is a propulsion system that does not have to carry its source of energy with it, but can tap external supplies. The rocket is like a diesel or steam locomotive, limited in performance by the fuel it can carry. We require the equivalent of the electric locomotive—or, perhaps, the fuelless sailing ship.

Although electric fields, and swift but infinitely tenuous "winds," do exist in space, they are too feeble to be of any practical use. However, there are other cosmic forces and properties that we may someday utilize, as long ago we learned to use the moving airs and waters of this world for transportation. One of these forces, as was pointed out recently by Dr. Freeman J. Dyson, a highly imaginative mathematician at the Princeton Institute for Advanced Study, is gravity.

Dr. Dyson's conclusions are stimulating—and tantalizing. He suggests that the gravitational fields of certain double stars might be used, by sufficiently ingenious astronauts, to launch themselves out across interstellar space. Two stars, spinning rapidly round each other, could be used as a kind of cosmic slingshot, and during the period of acceleration the travelers would feel no force whatsoever. For a gravitational field acting upon a freely falling body produces no sense of weight: Even if the astronauts were experiencing 10,000 g, and were thus increasing their speed at the enormous rate of 200,000 miles per hour every second, they would feel nothing at all as the stellar twins shot them off into space.

Unfortunately, we don't happen to have this particular type of double star (a white dwarf binary) in our immediate neighborhood. It is not even certain if such systems exist anywhere, but Dr. Dyson has an answer to this. To quote his words: "There may come a time in the remote future when engineering on an astronomical scale [my italics] will be both feasible and necessary." In other words—if these "gravitational machines" do not exist in nature—they can be made.

Let us pause to give three hearty cheers to Dr. Dyson. His ideas may seem so farfetched that most people will regard them as extravagant fantasies, but when seen against the background of our incredible Universe, they are entirely realistic. If we do not perform such feats in the millions of years that lie ahead, others will.

Another scheme for very-high-speed cosmic flight depends on the fact that space is not entirely empty, but contains about ten atoms of hydrogen per cubic inch. For all ordinary purposes this is a perfect vacuum; however, a spaceship cruising at hundreds or thousands of miles a second would sweep up appreciable quantities of hydrogen. This leads to the daring concept of the "interstellar ram jet"—a device which would scoop up the hydrogen scattered between the stars, feed it into a fusion reactor, and spew out the resulting heated gases in a propulsive jet. It would, therefore, derive both its fuel and its working fluid from the space around it, and would thus have unlimited range.

Though the interstellar ram jet involves such fearsome technical problems that the first scientists to investigate the scheme rejected it out of hand, more recent studies have brought it back into favor. It is certainly centuries in the future, but it violates no fundamental principles. Even if it is never more than a theoretical concept, it is of great interest; for if we can think of slightly plausible ways of tapping the energies of space, we can be sure that our descendants will find much more practical ones.

And someday—perhaps by the use of beamed—energy systems already glimpsed in the blinding light of the laser—we may learn to power our spacecraft from fixed ground stations. The analogy with the electric railroad would then be complete; spaceships need carry no fuel, as all their energy would be provided by planet-based installations which could be of unlimited size. This would again involve technologies far beyond our present horizon, but violating no basic laws. We need something like this to make space flight commercially practical even in the Solar System; and what commerce needs, it eventually gets. If the rocket lasts as long as the steam engine, I shall be most surprised.

To sum up, then: Interstellar flight at speeds approaching that of light is not necessarily impossible, and those who have claimed that it is are being prematurely pessimistic. They may be right, but we shall not know for some centuries. Meanwhile, we will assume that they are wrong—and see just where this conclusion leads us.

In the old-fashioned Newtonian Universe, which all scientists took for granted until the advent of Einstein, the situation was very straightforward. At the speed of light it would take you 10 years to reach a star 10 light-years away, and 10 years to come home again. Total voyage time-20 years. So if you were prepared to spend most of your life spacefaring, you might roam 30 or 40 light-years from Earth, and still return to your birthplace. If you wanted to do better than that, you had to travel faster than light. This would certainly be very difficult; but no one dreamed that it might be impossible for fundamental reasons concerned with the nature of the Universe.

The special theory of relativity, published by Einstein in 1905, established a speed limit in space. There is nothing very mysterious about this, once it is understood that mass and energy are two sides of the same coin. If we accelerate an object, it gains energy by virtue of its speed. Therefore, it also gains mass—and the next time we try to increase its speed, we will find it correspondingly harder to accelerate. The effect is negligible at low velocities—that is, up to a few scores of millions of miles an hour!—which is why it was never detected in the past. For all ordinary purposes, the laws of motion laid down by Galileo and Newton still apply, as they always will.

But near the speed of light, the mass increase rises steeply. A law of diminishing returns sets in; though you may keep on pushing an object, its gain in speed is infinitesimal—all the additional energy goes into increasing its mass. There is nothing theoretical about this; few laws have been more thoroughly tested in practice, for billions of dollars' worth of engineering are now designed around it. The giant atom smashers—the bevatrons, cosmotrons and so forth—are machines for accelerating nuclear particles to almost the speed of light. It requires thousands of tons of magnets and vacuum tubes to push the infinitely tiny electrons and protons up to these speeds, at which they may be hundreds of times heavier than when at rest. For this reason it has been suggested that nuclear accelerators should really be called "ponderators"; the increase in speed that they can produce at the end of their operating range is trivial, but the increase in mass is enormous.

At Berkeley and Brookhaven, and in myriads of high-energy electronic devices (including the picture tube of your TV set), the Einstein equation is obeyed exactly. It predicts that even if we burned up the whole Cosmos to accelerate a single electron, our infinitesimal "pay load" would still fail to reach the speed of light. The solitary electron would have the mass of all the suns and galaxies that had been destroyed to propel it; but its speed would be only 99.99999999999999..., and not 100, percent of that unattainable 186,282 miles per second. And what applies to one electron is true, a fortiori, to large-scale objects such as men and spaceships.

A sufficiently advanced civilization, by a prodigal expenditure of energy, might be able to drive its ships at 99 percent of the speed of light. And since the remaining one-percent increase could never be attained anyway, there would seem to be no point in striving after it, merely to cut three days off every year of travel time.

However, matters are not as simple as this, if you will pardon the expression. The same equations that appear to limit us to journeys of a few dozen light-years in a single human lifetime also provide a loophole. Einstein once condemned a theory of which he did not approve with the words: "The good Lord is subtle, but He is never malicious." Nowhere is that subtlety more evident than in the laws that Einstein himself discovered.

Granted that we can never exceed the speed of light, it follows that a round trip from Earth to a star fifty light-years away can never take less than a hundred years. However—and this is something that no one had suspected before 1905—there is a profound ambiguity in our definition of time. Do we mean one hundred years to the crew of the spaceship, or to their friends waiting back on Earth?

For there is a distinction, and it took Einstein's genius to perceive it. The same equations that predict an increase of mass with velocity, also predict a stretching or dilatation of time, and according to precisely the same mathematical law. The discrepancy is negligible at low speeds, but becomes infinite at the speed of light. To a beam of light, time stands still; it can travel round the Cosmos in one eternal instant.

The consequences of this are now well known; everyone has heard of the astronaut who sets out for the stars at almost the speed of light—and is still a young man when he returns to meet his aged twin brother, 40 or 50 years later. Fantastic though this seems, it would actually occur if we could reach 99 percent of the velocity of light, and within the narrow span of that last one percent even more astonishing paradoxes would arise.

Here are some examples, given by the Harvard biophysicist Carl Sagan in a paper with the splendid title "Direct Contact Among Galactic Civilizations by Relativistic Interstellar Space Flight." If a spaceship took off from Earth at a steady acceleration of one gravity (so that its occupants would feel their normal weight for the duration of the voyage), in five years by ship time it could reach a star ten light-years away. Yet it would not have exceeded the speed of light; to observers back on Earth, it would appear that the voyage had really lasted the full ten years. In effect, the clocks (and the people) in the spaceship would have run at half the speed of their counterparts on Earth—though they themselves would have noticed no change at all.

Ten years of ship time at one g acceleration would take the voyagers more than a hundred light-years, and thereafter the range goes up very steeply with time. Twenty years of cruising would bring them to the star clouds at the center of the Galaxy, some thirty thousand light-years from Earth. And in less than thirty years they would reach the Andromeda Nebula, more than a million light-years away! Of course, when the travelers returned home sixty years older, two million Earth years would have passed...

There is now no serious dispute about these conclusions; like the mass-increase laws, the time-dilatation effect has been demonstrated experimentally. But perhaps I had better take a few minutes (Earth time) to dispose of an objection that is often raised to the so-called "Clock Paradox" by those who have a nodding acquaintance with relativity.

Because, they argue, Einstein stated that "all velocity is relative," it is just as legitimate to say that the spaceship is standing still and the Earth is moving. So the people on Earth should stay young, while the travelers age at the normal rate—which is obviously absurd.

Of course it is; but Einstein never said that all velocity is relative. That statement is true only of uniform velocities, and we are not dealing with these. The Earth is moving at a uniform speed—but the spaceship is steadily accelerating. So the two systems are not equivalent, and the paradox does not arise.

The theory of relativity, therefore, allows us to explore the Universe without limit, by trading energy for time. Once again, it must be emphasized that the amounts of energy needed for such projects are gigantic, even by the standards of thermonuclear explosions. But they are not, in principle, beyond attainment or control; as Dr. Sagan concludes in his stimulating essay, "Allowing for a modicum of scientific and technological progress within the next few centuries, I believe that interstellar space flight at relativistic velocities to the farthest reaches of our Galaxy is a feasible objective for humanity."

There will, of course, be a price to pay, and it is not one that many of us would be prepared to face. Time would flow sluggishly in the speeding spaceship but on Earth its progress would be inexorable. The voyagers would have cut themselves off forever from their friends and families, perhaps even from the culture that had launched them into space, if they returned hundreds or thousands of years in its future. For relativistic space flight is a kind of one-way time travel; though you can vary the rate at which the clock moves forward, you can never turn it back.

If Odysseus had sailed for Deneb, and not for Troy, we might expect him back at any moment, less grizzled than from his wanderings over the wine-dark sea. And how strange to think that, if ships from the galactic center visited our world in the remote past, there may at this very moment be a family of our Cro-Magnon ancestors on display in some celestial zoo ...

Most scientists who have convinced themselves that interstellar flight is possible believe that such visits must have occurred—perhaps many times in the long history of Earth. The astronomer Thomas Gold has even suggested that terrestrial life arose from garbage dumped by one of these early expeditions. I should love to see somebody found a religion on this inspiring belief; but odder faiths have flourished in the past.

• • •

There are many who will be profoundly dissatisfied with these conclusions, and will feel aggrieved because we can never race back and forth across the Universe as we now do over the face of this Earth. They may even doubt the eternal validity of the Einstein equations, though these have stood unchallenged for half a century, and are now backed by the awesome authority of the mushroom cloud.

After all, many other apparent limits have proved to be no more than temporary roadblocks. Less than 20 years ago, we were worrying about the sound barrier; tomorrow, grandmothers will be cruising at Mach 3. Will the "light barrier" go the same way?

I am afraid I cannot offer much hope. If you have followed me so far, you will have realized the utterly fundamental nature of this barrier. And it is no good asking why we cannot travel faster than light, and why time dilatation occurs; our Universe is simply built that way. Anyone who doesn't like it can go somewhere else.

Perhaps that last sentence offers the one faint chance of beating Einstein. If other universes—other space-time continua—do exist, light may propagate in them at higher speeds than our familiar 186,282 miles a second. We may be able to get to the Andromeda Nebula and back again in a few years of Earth time, by taking a spatial detour through another dimension. But this is pure fantasy, with no scientific basis; so is the suggestion that we might be able to tap the so-called psi or paranormal forces which some students claim to have detected. If cosmic teleportation is practical, the current paucity of visitors becomes even more difficult to explain. Unless we are under quarantine (a highly plausible assumption), it really looks as if interstellar travel is expensive, time-consuming and, therefore, infrequent.

We had better cooperate with the inevitable—and, after all, we have no great reason to complain. This planetary system will keep us busy for quite a while, and beyond that, there are some four hundred stars of roughly solar type within a hundred light-years.

So even if we cannot exorcise the ghost of Einstein (and what were those dying words of his, lost forever because his nurse understood no German?), we have a prospect before us that will daunt whole armies of biologists and historians. Columbus is not yet five hundred years in the past; yet before another five centuries have gone, we may have complete records of a hundred civilizations, most of them far older than our own.

We may well be grateful, then, that our sphere of knowledge cannot expand more swiftly than light. That speed limit may be the only thing that can save us, when the real Space Age dawns, from being utterly overwhelmed by the richness and complexity of our many-splendored Universe.