Authors: Poul Anderson
If we could go very much faster—
At almost the speed of light, we’d reach Alpha Centauri in about four and a third years. But as most of you know, we who were faring would experience a shorter journey. Both the theory of relativity and experimental physics show that time passes “faster” for a fast-moving object. The closer the speed of light, the greater the difference, until at that velocity itself, a spaceman would make the trip in no time at all. However, the girl he left behind him would measure his transit as taking the same number of years as a light ray does; and he’d take equally long in coming back to her.
In reality, the velocity of light
in vacuo
, usually symbolized by c, cannot be attained by any material body. From a physical viewpoint, the reason lies in Einstein’s famous equation E = mc
2
. Mass and energy are equivalent. The faster a body moves, the more energy it has, and hence the more mass. This rises steeply as velocity gets close to c, and at that speed would become infinite, an obvious impossibility.
Mass increases by the same factor as time (and length) shrink. An appendix to this essay defines the terms more precisely than here. A table there gives some representative values of the factor for different values of velocity, v compared to c. At v = 7c, that is, at a speed of 70% light’s, time aboard ship equals distance covered in light-years. Thus, a journey of 10 light-years at 0.7c would occupy 10 years of the crew’s lives, although to people on Earth or on the target planet, it would take about 14.
There’s a catch here. We have quietly been supposing that the whole voyage is made at exactly this rate. In practice, the ship would have to get up to speed first, and brake as it neared the goal. Both these maneuvers take time; and most of this time is spent at low velocities where the relativistic effects aren’t noticeable.
Let’s imagine that we accelerate at one gravity, increasing our speed by 32 feet per second each second and thus providing ourselves with a comfortable Earth-normal weight inboard. It will take us approximately a year (a shade less) to come near c, during which period we will have covered almost half a light-year, and during most of which period our time rate won’t be significantly different from that of the outside cosmos. In fact, not until the eleventh month would the factor get as low as 0.5, though from then on it would start a really steepening nosedive. Similar considerations apply at journey’s end, while we slow down. Therefore a trip under these conditions would never take less than two years as far as we are concerned; if the distance covered is 10 light-years, the time required is 11 years as far as the girl (or boy) friend left behind is concerned.
At the “equalizing” v of 0.7
c,
these figures become 10.7 years for the crew and 14.4 years for the stay-at-homes. This illustrates the dramatic gains that the former, if not the latter, can make by pushing
c
quite closely. But let’s stay with that value of 0.7
c
for the time being, since it happens to be the one chosen by Bernard Oliver for his argument against the feasibility of star travel.
Now, Dr. Oliver, vice president for research and ‘ development at Hewlett-Packard, is definitely not unimaginative, nor hostile to the idea as such. Rather, he is intensely interested in contacting extraterrestrial intelligence, and was the guiding genius of Project Cyclops, which explored the means of doing so by radio. The design which his group came up with could, if built, detect anybody who’s using radio energy like us today within 100 light-years: Or it could receive beacon signals of reasonable strength within 1000 light-years: a sphere which encloses a million suns akin to Sol and half a billion which are different.
Still, he does not fudge the facts. Making the most favorable assumption, a matter-antimatter annihilation system which expels radiation itself, he has calculated the minimum requirement for a round trip with a stopover at the destination star, at a peak speed of 0.7c. Assuming 1000 tons of ship plus payload, which is certainly modest, he found that it must convert some 33,000 tons of fuel into energy—sufficient to supply the United States, at present levels of use, for half a million years. On first starting off from orbit, the ship would spend 10 times the power that the Sun gives to our entire Earth. Shielding requirements alone, against stray gamma rays, make this an absurdity, not to speak of a thousand square miles of radiating surface to cool the vessel if as little as one one-millionth of the energy reaches it in the form of waste heat.
Though we can reduce these figures a good deal if we assume it can refuel at the other end for its return home, the scheme looks impractical regardless. Moreover, Dr. Oliver, no doubt deliberately, has not mentioned that space is not empty. Between local stars, it contains about one hydro; gen atom per cubic centimeter, plus smalls-amounts of other materials. This is a harder vacuum than any we can achieve artificially. But a vessel ramming through it at 0.7c would release X-radiation at the rate of some 50 million roentgen units per hour. It takes less than 1000 to kill a human being. No material shielding could protect the crew for long, if at all.
Not every scientist is this pessimistic about the rocket to the stars, that is, a craft which carries its own energy source and reaction mass. Some hope for smaller, unmanned probes, perhaps moving at considerably lower speeds. But given the mass required for their life support and equipment, men who went by such a vehicle would have tp reckon on voyages lasting generations or centuries.
This is not impossible, of course. Maybe they could pass the time in suspended animation. Naturally radioactive atoms in the body set an upper limit to that, since they destroy tissue which would then not be replaced. But Carl Sagan, astronomer and exobiologist at Cornell University, estimates that a spore can survive up to a million years. This suggests to me that humans should be good for anyway several thousand.
Or maybe, in a huge ship with a complete ecology, an expedition could beget and raise children to carry their mission on. Calculations by Gerard K. O’Neill, professor of physics at Harvard, strongly indicate that this is quite feasible. His work has actually dealt with the possibility of establishing permanent, self-sustaining colonies in orbit, pleasanter to live in than most of Earth and capable of producing more worldlets like themselves from extraterrestrial resources. He concludes that we can start on it
now
, with existing technology and at startlingly low cost, and have the first operational by the late 1980’s. Not long afterward, somebody could put a motor on one of these.
The hardened science fiction reader may think such ideas are old hat. And so they are, in fiction. But to me the fact is infinitely more exciting than any story—that the accomplishment can actually be made, that sober studies by reputable professionals are confirming the dream.
True, I’d prefer to believe that men and women can get out there faster, more easily, so that the people who sent them off will still be alive when word arrives of what they have discovered. Is this wishful thinking? We’ve written off the rocket as a means of ultra-fast travel, but may there be other ways?
Yes, probably there are. Even within the framework of conventional physics, where you can never surpass c, we already have more than one well-reasoned proposal. If not yet as detailed and mathematical as Oberth’s keystone work on interplanetary travel of 1929, the best of them seem equivalent to Tsiolkovsky’s cornerstone work of 1911. If the time scale is the same for future as for past developments, then the first manned Alpha Centauri expedition should leave about the year 2010____
That’s counting from R. W. Bussard’s original paper on the interstellar ramjet, which appeared in 1960. Chances are that a flat historical parallel is silly. But the engineering ideas positively are not. They make a great deal of sense.
Since the ramjet has been in a fair number of stories already, I’ll describe the principle rather briefly. We’ve seen that at high speeds, a vessel must somehow protect its crew from the atoms and ions in space. Lead or other material shielding is out of the question. Hopelessly too much would be required, it would give off secondary radiation of its own, and ablation would wear it down, incidentally producing a lot of heat, less readily dissipated in space than in an atmosphere. Since the gas must be controlled anyway, why not put it to work?
Once the ship has reached a speed which turns out to be reasonable for a thermonuclear rocket— and we’re on the verge of that technology today—a scoop can collect the interstellar gas and funnel it into a reaction chamber. There, chosen parts can be fusion-burned for energy to throw the rest out backward, thus propelling the vessel forward. Ramjet aircraft use the same principle, except that they must supply fuel to combine with the oxygen they collect. The ramjet starcraft takes everything it needs from its surroundings. Living off the country, it faces none of the mass-ratio problems of a rocket, and might be able to crowd
c
very closely.
Needless to say, even at the present stage of pure theory, things aren’t that simple. For openers, how large an apparatus do we need? For a ship-plus-pay load mass of 1000 tons, accelerating at one gravity and using proton-proton fusion for power, Bussard and Sagan have both calculated a scoop radius of 2000 kilometers. Now we have no idea as yet how to make that particular reaction go. We are near the point of fusing deuterons, or deuterons and tritons (hydrogen nuclei with one and two neutrons respectively), to get a net energy release. But these isotopes are far less common than ordinary hydrgen, and thus would require correspondingly larger intakes. Obviously, we can’t use collectors made of metal.
But then, we need nonmaterial shielding anyway. Electromagnetic fields exert force on charged particles. A steady laser barrage emitted by the ship can ionize all neutral atoms within a safety zone, and so make them controllable, as well as vaporizing rare bits of dust and gravel which would otherwise be a hazard. (I suspect, myself, that this won’t be necessary. Neutral atoms have electrical asymmetries which offer a possible grip to the forcefields of a more advanced technolgoy than ours. I also feel sure we will master the proton-proton reaction, and eventually matter-antimatter annihilation. But for now, let’s play close to our vests). A force-field scoop, which being massless can be of enormous size, will catch these ions, funnel them down paths which are well clear of the crew section and into a fusion chamber, cause the chosen nuclei to burn, and expel everything aft to drive the vessel forward, faster and faster.
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.