Poul Anderson - New America

To generate such fields, A.J. Fennelly of Yeshiva University and G.L. Matloff of the Polytechnic Institute of New York propose a copper cylinder coated with a super-conducting layer of niobium-tin alloy. The size is not excessive, 400 meters in length and 200 in diameter. As for braking, they suggest a drogue made of boron, for its high melting point, ten kilometers across. This would necessarily work rather slowly. But then, these authors are cautious in their assumptions; for instance, they derive a peak velocity of just 0.12c. The system could reach Alpha Centauri in about 53 years, Tau Ceti in 115.

By adding wings, however, they approximately halve these travel times. The wings are two great superconducting batteries, each a kilometer square. Cutting the lines of the galactic magnetic field, they generate voltages which can be tapped for exhaust acceleration, for magnetic bottle containers for the power reaction, and for inboard electricity. With thrust shut off, they act as auxiliary brakes, much shortening the deceleration period. When power is drawn at different rates on either side, they provide maneuverability—majestically slow, but sufficient—almost as if they were huge oars.

All in all, it appears that a vessel of this general type can bring explorers to the nearest stars while they are still young enough to carry out the exploration—and the preliminary colonization?—themselves. Civilization at home will start receiving a flood of beamed information, fascinating, no doubt often revolutionary in unforeseeable ways, within a few years of their arrival. Given only a slight lengthening of human life expectancy, they might well spend a generation out yonder and get home alive, still hale. Certainly their children can.

Robert L. Forward, a leading physicist at Hughes Research Laboratories, has also interested himself in the use of the galactic magnetic field. As he points out, the ion density in interstellar space is so low that a probe could easily maintain a substantial voltage across itself. Properly adjusted, the interaction forces produced by this will allow mid-course corrections and terminal maneuvers at small extra energy cost. Thus we could investigate more than one star with a single probe, and eventually bring it home again.

Indeed, the price of research in deep space is rather small. Even the cost of manned vessels is estimated by several careful thinkers as no more than ten billion dollars each—starting with today’s technology. That’s about 50 dollars per American, much less than we spend every year on cigarettes and booze, enormously less than goes for wars, bureaucrats, subsidies to inefficient businesses, or the servicing of the national debt. For mankind as a whole, a starship would run about $2.50 per head. The benefits it would return in the way of knowledge, and thus of improved capability, are immeasurably great.

But to continue with those manned craft. Mention of using interstellar magnetism for maneuvering raises the thought of using it for propulsion. That is, by employing electromagnetic forces which interact with that field, a ship could ideally accelerate itself without having to expel any mass backward. This would represent a huge saving over what the rocket demands.

The trouble is, the galactic field is very weak, and no doubt very variable from region to region. Though it can be valuable in ways that we have seen, there appears to be no hope of using it for a powerful drive.

Might we invent other devices? For instance, if we could somehow establish a negative gravity force, this might let our ship react against the mass of the universe as a whole, and thus need no jets. Unfortunately, nobody today knows how to do any such thing, and most physicists take for granted it’s impossible. Not all agree: because antigravity-type forces do occur in relativity theory, under special conditions.

Physics does offer one way of reaching extremely high speeds free, the Einsteinian catapult. Later I shall have more to say about the weird things that happen when large, ultra-dense masses spin very fast. But among these is their generation of a force different from Newtonian gravity, which has a mighty accelerating effect of its own. Two neutron stars, orbiting nearly in contact, could kick almost to light velocity a ship which approached them on the right orbit.

Alas, no such pair seems to exist anywhere near the Solar System. Besides, we’d presumably want something similar in the neighborhood of our destination, with exactly the characteristics necessary to slow us down. The technique looks rather implausible. What is likely, though, is that closer study of phenomena like these may give us clues to the method of constructing a field drive.

Yet do we really need it? Won’t the Bussard ramjet serve? Since it picks up everything it requires as it goes, why can’t it keep on accelerating indefinitely, until it comes as close to c as the captain desires? The Fennelly-Matloff vehicle is not intended to do this. But why can’t a more advanced model?

Quite possibly it can!

Before taking us off on such a voyage, maybe I’d better answer a question or two. If the ship, accelerating at one gravity, is near c in a year, and if c is the ultimate speed which nature allows, how can the ship keep on accelerating just as hard, for just as long as the flight plan says?

The reason lies in the relativistic contraction of space and time, when these are measured by a fast-moving observer. Suppose we, at rest with respect to the stars, track a vessel for 10 light-years at its steady speed of 0.9c. To us, the passage takes 11 years. To the crew, it takes 4.4 years: because the distance crossed is proportionately less. They never experience faster-than-light travel either. What they do experience, when they turn their instruments outward, is a cosmos strangely flattened in the direction of their motion, where the stars (and their unseen friends at home) age strangely fast.

The nearer they come to c, the more rapidly these effects increase. Thus as they speed up, they perceive themselves as accelerating at a steady rate through a constantly shrinking universe. Observers on a planet would perceive them as accelerating at an ever lower rate through an unchanged universe. At last, perhaps, millions of light-years might be traversed and trillions of years pass by outside while a man inboard draws a breath.

By the way, those authors are wrong who have described the phenomenon in terms of “subjective” versus “objective” time. One set of measurements is as valid an another.

The “twin paradox” does not arise. This old chestnut says, “Look, suppose we’re twins, and you stay home while I go traveling at high speed. Now I could equally well claim I’m stationary and you’re in motion—therefore that you’re the one flattened out and living at a slower rate, not me. So what happens when we get back together again? How can each of us be younger than his twin?”

It overlooks the fact that the traveler does come home. The situation would indeed be symmetrical if the spaceman moved forever at a fixed velocity. But then he and his brother, by definition, never would meet to compare notes. His accelerations (which include slowdowns and changes of course) take the whole problem out of special and into general relativity. Against the background of the stars, the traveler has moved in a variable fashion; forces have acted on him.

Long before time and space measurements aboard ship differ bizarrely much from those on Earth, navigational problems will arise. They are the result of two factors, aberration and Doppler effect.

Aberration is the apparent displacement of an object in the visual field of a moving observer. It results from combining his velocity with the velocity of light. (Analogously, if we are out in the rain and, standing still, feel it falling straight down, we will feel it hitting us at a slant when we start walking. The change in angle will be larger if we run.) At the comparatively small orbital speed of Earth, sensitive instruments can detect the aberration of the stars. At speeds close to c, it will be huge. Stars will seem to crawl across the sky as we accelerate, bunching in its forward half and thinning out aft.