New America

OUR MANY ROADS TO THE STARS

There are countless varieties of science fiction these days, and I would be the last to want any of them restricted in any way. Nevertheless, what first drew me to this literature and, after more years than I like to add up, still holds me, is its dealing with the marvels of the universe. To look aloft at the stars on a clear night and think that someday, somehow we might actually get out among them, rouses the thrill anew, and I become young again. After all, we made it to the Moon didn’t we? Mean wile, only science fiction of the old and truly kind takes the imagination forth on that journey. Therefore I put up with its frequent flaws; and so does many another dreamer.

But are we mere dreamers, telling ourselves stories of voyages yonder as our ancestors told of voyages to Avalon and Cibola? Those never existed, and the stars do; but, realistically, does any possibility of reaching them?

The case against interstellar travel traditionally begins with the sheer distances. While Pioneer 10 and 11, the Jupiter flybys, will leave the Solar System, they won’t get as far as Alpha Centauri, the nearest neighbor sun, for more than 40,000 years. (They aren’t actually bound in that direction.) At five times their speed, or 100 miles per second, which we are nowhere close to reaching today, the trip would take longer than recorded history goes back. And the average separation of stars in this galactic vicinity is twice as great.

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 = mc². 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.

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.

Doppler effect, perhaps more widely familiar, is the shift in observed wavelength from an emitting object, when the observer’s velocity changes. If we move away from a star, we see its light reddened; if we move toward a star, we see its light turned more blue. Again, these changes become extremely marked as we approach c.

Eventually our relativistic astronaut sees most of the stars gathered in a ring ahead of him, though a few sparsely strewn individuals remain visible elsewhere. The ring itself, which Frederik Pohl has dubbed the “starbow,” centers on a circle which is mainly dark, because nearly all light from there has been blue-shifted out of the frequencies we can see. The leading or inner edge of the ring is bluish white, its trailing or outer edge reddish; in between is a gradation of colors, akin to what we normally observe. Fred Hollander, a chemist at Brookhaven National Laboratories, has calculated the starbow’s exact appearance for different v. It gets narrower and moves farther forward, the bull’s eye dead ahead gets smaller and blacker, the faster we go—until, for instance, at 0.9999c we perceive a starbow about ten degrees of arc in width, centered on a totally black circle of about the same diameter, and little or nothing shows anywhere else in the sky.

At that speed, 0.9999c, we’d cross 100 light-years in 20 months of our personal lifetimes. So it’s worth trying for; but we’ll have to figure out some means of knowing where we are! Though difficult, the problem does not look unsolvable in principle.

It may become so beyond a certain velocity. If we travel under acceleration the whole way, speeding up continuously to the half-way point, thereafter braking at the same rate until we reach our goal: then over considerable distances we get truly staggering relativity factors. The longer a voyage, the less difference it makes to us precisely how long it is.

Thus, Dr. Sagan points out that explorers faring in this wise at one gravity will reach the nearer stars within a few years, Earth time, and slightly less, crew time. But they will cross the approximately 650 light-years to Deneb in 12 or 13 years of their own lifespans; the 30,000 light-years to the center of our galaxy in 21 ship years; the two million light-years to the Andromeda galaxy in 29 ship years; or the 10 million light-years to the Virgo cluster of galaxies in 31 ship years. If they can stand higher accelerations, or have some way to counteract the drag on their bodies, they can cross these gulfs in less of their own time; the mathematical formula governing this is in the appendix.

But will the starbow become too thin and dim for navigation? Or will they encounter some other practical limit? For instance, when matter is accelerated, it radiates energy in the form of gravity waves. The larger the mass, the stronger this radiation; and of course the mass of our spaceship will be increasing by leaps and bounds and pole-vaults. Eventually it may reach a condition where it is radiating away as much energy as it can take in, and thus be unable to go any faster.

However, the real practical limit is likelier to arise from the fact that we have enough stars near home to keep us interested for millennia to come. Colonies planted on worlds around some of these can, in due course, serve as nuclei for human expansion ever further into the universe.

Because many atoms swept through its force-fields are bound to give off light, a ramjet under weigh must be an awesome spectacle. At a safe distance, probably the hull where the crew lives is too small for the naked eye. Instead, against the constellations one sees a translucent shell of multicolored glow, broad in front, tapering aft to a fiery point where the nuclear reaction is going on. (Since this must be contained by force-fields anyway, there is no obvious reason for the fusion chamber to be a metal room.) Thence the exhaust streams backward, at first invisible or nearly so, where its particles are closely controlled, but becoming brilliant further off as they begin to collide, until finally a nebula-like chaos fades away into the spatial night.

It’s not only premature, it’s pointless to worry about limitations. Conventional physics appears to tell us that, although nature has placed an eternal bound on the speed of our traveling, the stars can still be ours… if we really want them.

Yet we would like to reach them more swiftly, with less effort. Have we any realistic chance whatsoever of finding a way around the light-velocity barrier?

Until quite recently, every sensible physicist would have replied with a resounding “No.” Most continue to do so. They point to a vast mass of experimental data; for instance, if subatomic particles did not precisely obey Einsteinian laws, our big accelerators wouldn’t work. The conservatives ask where there is the slightest empirical evidence for phenomena which don’t fit into the basic scheme of relativity! And they maintain that, if ever we did send anything faster than light, it would violate causality.

I don’t buy that last argument, myself. It seems to me that, mathematically and logically, it presupposes part of what it sets out to prove. But this gets a bit too technical for the present essay, especially since many highly intelligent persons disagree with me. Those whom I mentioned are not conservatives in the sense of having stick-in-the-mud minds. They are among the very people whose genius and imagination make science the supremely exciting, creative endeavor which it is these days.

Nevertheless we do have a minority of equally qualified pioneers who have lately been advancing new suggestions.

I suppose the best known idea comes from Gerald Feinberg, professor of physics at Columbia University. He has noted that the Einsteinian equations do not actually forbid material particles which move faster than light—if these have a mass that can be described by an imaginary number (that is, an ordinary number multiplied by the square root of minus one. Imaginary quantities are common, e.g., in the theory of electromagnet-ism). Such “tachyons,” as he calls them, would travel faster and faster the less energy they have; it would take infinite energy to slow them down to c, which is thus a barrier for them too.

Will it forever separate us, who are composed of “tardyons,” from the tachyon part of the cosmos? Perhaps—but not totally. It is meaningless to speak of anything which we cannot, in principle, detect if it exists. If tachyons do, there must be some way by which we can find experimental evidence for them, no matter how indirect. This implies some kind of interaction (via photons?) with tardyons. But interaction, in turn, implies a possibility of modulation. That is, if they can affect us, we can affect them.

And… in principle, if you can modulate, you can do anything. Maybe it won’t ever be feasible to use tachyons to beam a man across space; but might we, for instance, use them to communicate faster than light?

Needless to say, first we have to catch them, i.e. show that they exist. This has not yet been done, and maybe it never can be done because in fact there aren’t any. Still, one dares hope. A very few suggestive data are beginning to come out of certain laboratories—

Besides, we have other places to look. Hyper-space turns out to be more than a hoary science fiction catchphrase. Geometrodynamics now allows a transit from point to point, without crossing the space between, via a warp going “outside” that space—often called a wormhole. Most worm-holes are exceedingly small, of subatomic dimensions; and a trip through one is no faster than a trip through normal space. Nevertheless, the idea opens up a whole new field of research, which may yield startling discoveries.

Black holes have been much in the news, and in science fiction, these past several years. They are masses so dense, with gravity fields so strong, that light itself cannot escape. Theory has predicted for more than 40 years that all stars above a particular size must eventually collapse into the black hole state. Today astronomers think they have located some, as in Cygnus X-l. And we see hypotheses about black holes of less than stellar mass, which we might be able to find floating in space and utilize.

For our purposes here, the most interesting trait of a black hole is its apparent violation of a whole series of conservation laws so fundamental to physics that they are well-nigh Holy Writ. Thus many an issue, not long ago considered thoroughly settled, is again up for grabs. The possibility of entering a black hole and coming out “instantly” at the far end of a space warp is being seriously discussed. Granted, astronauts probably couldn’t survive a close approach to such an object. But knowledge of these space warp phenomena and their laws, if they do occur in reality, might well enable us to build machines which—because they don’t employ velocity—can circumvent the c barrier.

Black holes aren’t the sole things which play curious tricks on space and time. An ultra-dense toroid, spinning very rapidly in smoke ring fashion, should theoretically create what is called a Kerr metric space warp, opening a way to hyper-space.

The most breathtaking recent development of relativity that I know of is by F.J. Tipler, a physicist at the University of Maryland. According to his calculations, not just near-instantaneous crossings of space should be possible, but time travel should be! A cylinder of ultra-dense matter, rotating extremely fast (velocity at the circumference greater than 0.5c) produces a region of multiple periodic spacetime. A particle entering this can, depending on its exact track, reach any event in the universe during which the cylinder exists.

The work was accepted for publication in Physical Review, which is about as respectable as science can get. Whether it will survive criticism remains to be seen. But if nothing else, it has probably knocked the foundation out from under the causality argument against faster-than-light travel: by forcing us to rethink our whole concept of causality.

The foregoing ideas lie within the realm of accepted physics, or at least on its debatable borders. Dr. Forward has listed several others which are beyond the frontier… but only barely, and only by date. Closer study could show, in our near future, that one or more of them refer to something real.

For instance, we don’t know what inertia “is.” It seems to be a basic property of matter; but why? Could it be an inductive effect of gravitation, as Mach’s Principle suggests? If so, could we find ways to modify it, and would we then be held back by the increase of mass with velocity?

Could we discover, or produce, negative mass? This would gravitationally repel the usual positive kind. Two equal masses, positive and negative, linked together, would make each other accelerate in a particular direction without any change in momentum or energy. Could they therefore transcend c?

A solution of Einstein’s field equations in five dimensions for charged particles gives an electron velocity of a billion trillion c. What then of a spaceship, if the continuum should turn out to have five rather than four dimensions?

Conventional physics limits the speed of mass-energy. But information is neither; from a physical standpoint, it represents negative entropy. So can information outrun light, perhaps without requiring any medium for its transmission? If you can send information, in principle you can send anything.

Magnificent and invaluable though the structure of relativity is, does it hold the entire truth? There are certain contradictions in its basic assumptions which have never been resolved and perhaps never can be. Or relativity could be just a special case, applying only to local conditions.

Once we are well and truly out into space, we may find the signs of a structure immensely more ample.

These speculations have taken us quite far beyond known science. But they help to show us how little known that science really is, even the parts which have long felt comfortingly, or confiningly, familiar. We can almost certainly reach the stars. Very possibly, we can reach them easily.

If we have the will.