Beyond Star Trek

by Lawrence M. Krauss

  This is precisely what experimentalists at CERN are trying to do. Money has just been appropriated to build an antiproton decelerator, which will slow down antiprotons produced in CERN’s synchrotron so that they can be trapped, cooled further, and combined with stored positrons in the lab to make small amounts of stable antihydrogen.

  But how, you may ask, can antihydrogen be stored in order to be experimented on? After all, if the antihydrogen atoms hit the walls of a container made of normal matter, they can annihilate with the protons and electrons in the walls. Charged antimatter is simple to confine in a box without having it come in contact with the walls. Charged particles (antiprotons, say) move in circles when put in a magnetic field, so they can be confined in what is called a magnetic bottle—a torus, or doughnut-shaped ring, with a magnetic field inside that keeps the particles traveling in a circle around the center of the torus, away from the walls. This is how we store antimatter particles in particle accelerators at the present time, and in my last book I criticized the Star Trek writers for stowing the Enterprise’s antimatter aboard ship in the form of heavy-hydrogen antiatoms (antideuterium) instead of as charged antiprotons, which would be a lot easier to store.

  I now think I was a little too hard on the writers. One reason you would want to store antimatter fuel as atoms instead of as independent sets of positively and negatively charged particles is the same reason CERN is trying to cool and store neutral antiatoms. If you eventually want to build up a large amount of material—be it thousands of antiatoms, as at CERN, or billions of billions of billions of antiatoms, as in a starship—you can’t continue to work with charged particles. The electrical repulsion between charged objects is so unimaginably great that it is virtually impossible to store large amounts of them at any reasonable density. In fact, if the Earth contained, on average, one additional electron per 5 billion tons of material, the force of repulsion on an electron at the Earth’s surface would counterbalance the gravitational force holding it down. A greater proportional increase and the Earth would blow itself apart!

  Well… so how can you trap and store neutral antiatoms? You use magnetism again, but this time in a trickier way. The nucleus of an antihydrogen atom consists of a single antiproton. Since one of the fundamental properties of antiprotons is that (like protons) they have a property known as nuclear spin, they act like little magnets. In a strong external magnetic field, they spin in such a way that their own internal magnetic fields tend to line up with the external field—because it would take more energy for them to line up in, say, the opposite direction. Now, if you create antihydrogen atoms and then cool them down to a few thousandths of a degree above absolute zero—which, remarkably, we can do nowadays—then essentially none of the antiatoms will have enough energy to line up their nuclear magnetic fields in the opposite direction to the external magnetic field. Imagine then that we have a bunch of very cold antiatoms with their spins all lined up in some direction, and we place them in an enclosure with a strong magnetic field around the outside aligned in the opposite direction. If the atoms are cold enough, none of them will have sufficient energy to hang out in the region of a strong magnetic field, so they will tend to cluster at the center. You will have a magnetic trap.

  Magnetic traps have already been used successfully to confine normal atoms, and the principle should work exactly as well for antiatoms, once we create them. Such a program is planned to be in operation at CERN by 1999. The antiproton decelerator’s estimated cost is about $5 million, and it should allow storage and detection of about 1,000 antiatoms per hour. That’s about 9 million antiatoms per year; at that rate it would take somewhat more than a million times the present age of the universe to make enough antiatoms to propel a flea to near the speed of light.

  So antimatter propulsion is not practical right now. But one day, if we, or any other creatures, want to travel at near light speed and carry enough fuel along to do it, antimatter is the best and perhaps only way to go. Even here though, there are huge problems: To do a round-trip this way would require 16 times the ship’s mass in antimatter! Carrying and containing antimatter 16 times the mass of a large spacecraft for a period of, say, 20 years—probably the minimum amount of time needed for a round-trip to the nearest star system outside our own—is a logistical nightmare. Even the U.S.S. Enterprise has antimatter containment problems on a more frequent basis than that. There has to be a better way!

  CHAPTER FIVE

  THERE, AND BACK AGAIN?

  Einstein had a little theory.

  It had something to do with relativity.

  Well, Einstein put that theory to the test,

  That’s why he looks confused, and his hair’s a mess.

  —New Rhythm and Blues Quartet (NRBQ)

  Practicality is something we often dispense with when it comes to imagining the future. Part of the fun of physics, and science fiction, is recognizing that to make any progress in the world we can’t limit ourselves to thinking about what we’re capable of today. But at the same time we need to keep in mind that whatever people, or aliens, do build will have to be practical in its own time. If we want to speculate on what form interstellar travel (or any future technology, for that matter) will eventually take, we have to try and imagine what will be easiest—given the laws of physics we already understand, along with the possibilities they don’t yet rule out.

  Necessity is always the mother of invention, both in the real world and the world of science fiction, even when what is contemplated appears implausibly extreme. As Jean-Luc Picard once announced to Data, reprising a remark made generations earlier by Captain Kirk to McCoy, “Things are only impossible until they’re not!” If round-trip rocket propulsion seems the most preposterous way of traveling to the stars, we must be willing to consider alternatives, even those that initially seem absurd.

  And we are doing this already, because the same problems that will confront future interstellar travelers are confronting, on a smaller scale, today’s engineers, as they wrestle with the problems of manned round-trip travel through our own solar system. They may well be thinking about how their predecessors handled the same problem. Columbus didn’t need fuel to set sail across the Atlantic; he used the wind. Lewis and Clark didn’t bring along the fuel they needed to explore the innards of North America; they hunted and fished as they went along. The lesson is clear. If you want to explore strange new worlds where no one has gone before, you probably have to live off the land to do it.

  A local version of this strategy has been proposed by rocket engineer Robert Zubrin as a way of getting to Mars at a cost we may be able to afford. It is known as the Mars Direct approach, and it calls for sending astronauts to Mars in a craft containing only the fuel needed for the outbound voyage. Fuel for the return trip could be manufactured on the Martian surface, using very simple technology—technology so simple, in fact, that Zubrin, who is not a chemical engineer, has built a working prototype on Earth.

  The Martian atmosphere is 95 percent carbon dioxide, and atmospheric CO2 can easily be filtered out, pressurized, and stored as a liquid at Martian surface temperatures. Zubrin’s proposal involves bringing along a small amount of hydrogen and reacting it with the carbon dioxide to produce methane and water. Since this reaction is exothermic—that is, heat-releasing—it requires no input of energy to drive it; rather, it occurs spontaneously in the presence of a catalyst made of nickel or ruthenium. The methane and water are easily separated, and the water is then split by electrolysis into hydrogen and oxygen. The hydrogen is recycled, and the oxygen is refrigerated and stored. When it’s time to go home, just mix the oxygen and the methane, and you will have produced a high-performance fuel in a form that can be stored for a long period of time.

  It might be argued that sending people to Mars without fuel for the return journey would not provide the kind of safety margin that keeps NASA in the business of manned spaceflight. Zubrin’s ingenious answer to this is to send a spacecraft containing the fuel fabrication fa
cility to Mars in advance of the manned spacecraft. Only after this automated craft had safely landed and produced the requisite fuel would the manned mission be launched.

  Finally, the question arises of how to transfer the fuel from the fabrication facility to the return vehicle. The answer is that it is simpler to transfer the astronauts. The original landing vehicle containing the fuel facility will become the return vehicle to Earth. The astronauts will land in their module, and when their time on Mars is through they will transfer on the Martian surface to the fully fueled return vehicle. The combined spacecraft will in the meantime serve as the Mars base, housing the astronauts for up to 2 years, until an accessible return trajectory is available.

  Of course, there are a plethora of other concerns to worry about: radiation exposure during the Mars round-trip and power on the Martian surface, artificial gravity during the months in transit so that the astronauts’ muscles don’t atrophy, and so on, but these are solvable in principle once one knows that one can send a crew to Mars and back with sufficient fuel for a reasonable amount of money. Depending on the size of the crew and the necessary radiation shielding, cost estimates for the round-trip are in the neighborhood of $10 billion to $50 billion. Not exactly cheap but realizable—comparable in 1960 dollars to the money spent to send men to the Moon.

  How might one then adapt this idea for travel to the stars? Can we assume that before we’re visited by aliens, we should expect a large ship to land on Earth—in the Mojave, or somewhere outside Las Vegas, or near Roswell, say—and start producing fuel? I don’t think so.

  We know a tremendous amount about Mars, but if we are traveling to another star’s planetary system, we probably won’t know enough details about where we’re going to send an advance hospitality vehicle there to bring us home. I know of no such precedent in human exploration. However, various optimistic individuals have proposed that instead of powering a spacecraft using fuel obtained either on Earth or at the destination, one should do as the earliest explorers did and harvest fuel along the way.

  Now, the density of matter in our region of the galaxy is very small—about 1 proton (on average) per cubic centimeter. This makes scooping up matter, or antimatter, for use as fuel impractical. However, the universe is also full of radiation. The first person to propose the use of radiation power was also the first person known to have written a science fiction story involving space travel. Johannes Kepler, the discoverer of the laws of planetary motion, was a busy fellow, with a life full of interruptions. In between his contributions to science, he successfully defended his mother against the charge of witchcraft and wrote a story about traveling to the Moon and back. He also observed something just as timely, given the recent return of comet Hale-Bopp: Whether comets are traveling toward the Sun or away from it, their tails point away from it; hence, the Sun must be exerting a kind of pressure. This prompted Kepler, in 1609, to suggest that we would one day design ships that could sail on these “heavenly breezes.”

  There is indeed a solar wind, a stream of charged particles moving out from the Sun into space at high velocity. However, this velocity is still only about 1/10 of 1 percent of the speed of light. While a solar-wind-powered sailing vessel, coasting along on the initial push it got from the solar wind, might be useful for interplanetary travel, it would not be particularly useful for interstellar travel—at least on a human timescale.

  In addition to the solar wind, sunlight itself produces a pressure—any sort of light produces a pressure. But this pressure is very small. After all, if the Sun’s photon pressure packed a wallop, the Earth would be pushed around by it. Nevertheless, certain bold futurists have suggested using solar sails to carry us to the stars. To get enough propulsion to accelerate a 1,000-ton spacecraft even to 10 percent of the speed of light in a year would require a solar sail perhaps 100 miles across, and in order that it not weigh more than the spacecraft it would have to be less than 1/1000 the thickness of a kitchen garbage bag.

  Others have suggested improving upon the Sun. While the Sun is very bright, it shines in all directions. Think of all the sunlight wasted that way! Why not build a powerful space-based laser, perhaps powered by the Sun, which would direct a concentrated beam of light toward a sail big enough to encompass the beam even at vast distances—perhaps 1/4 the width of Texas? Several years before the spacecraft reached its destination, another beam could be turned on which would reach the spacecraft in time to decelerate it, using a series of reflectors.

  All these ideas have their own problems, of course—some involve open questions of fundamental principle and others involve specific engineering issues. They all also require tremendous resources to build the huge sails and the lasers. And they depend for their success on specific interstellar conditions. Just as one must take wind variations into account in a sailboat, navigating the interstellar winds would be a difficult business. Similarly, one cannot travel under external laser power if one is not within the laser’s sights. Finally, none of these methods allow for unscheduled stops; missions would have to be planned completely in advance, and the discovery of something interesting along the way would probably have to be recorded for the next mission to explore.

  Now, everything I have talked about thus far, even fusion and antimatter drives, involves “conventional,” well-understood physics. I think I have adequately demonstrated that any aliens who want to get here probably can’t resort to such conventional physics. But who expected them to? As Mulder has noted, rather wistfully, to Scully, “When conventional science offers no answers, may we finally turn to the fantastic as a possibility?” My answer is, “Yes, as long as the fantastic isn’t impossible!”

  OK, then, what about warp drive, wormholes, antigravity, and all the wonderful exciting unknowns associated with the nature of space-time? One could write a whole book about what may be possible, but that has already been done. I want here to set out what might be possible in practice, as opposed to what might be possible in principle. The wonders of general relativity allow all sorts of incredible things to exist in principle, from warp drive to time travel. That alone warrants thinking about them, and writing about them, and I even spend some of my own research time trying to make some progress in this regard. But we began here by asking how the spaceships that might one day actually be built might behave.

  Here is the place to state unequivocally that I think these things will never be practical for real space travel, although they may well be possible in principle. Even glimmers of hope can become blindingly bright when people are intent on maintaining any hope at all. When I and others began popularizing the idea that it’s still an open question whether or not warp drive and time travel are possible in our universe, I was amazed at the excitement and speed with which this idea propagated—not just among the fans of popular science but throughout the academic community. Even NASA seemed to be listening, and has invited me to speak at a symposium on non-propulsive methods of space travel, including warp drive and wormholes.

  The fundamental and formidable energy problems that have thus far kept human beings away from even the closest planets pale in comparison to those that arise when you turn from conventional Newtonian propulsion to the fantastic possibilities opened up to us by Einstein. Let me remind you of some of these, so that I can then tell you about some recent exciting new discoveries, and also reveal a little secret about warp drive which I don’t think has been discussed in print before.

  By now, it’s clear what a Herculean task it is to imagine traveling at near light speed through space in realizable rocket ships. But why bother traveling through space if you can make space do the traveling for you? Einstein’s general theory of relativity tells us that space itself responds to the presence of matter by expanding, contracting, and bending. If this is possible, then a brave new world of “designer universes” opens before us.

  Within the context of general relativity, you don’t have to move at all to travel throughout the universe. You can move at the speed of
light and yet be sitting still. In fact, you are doing that right now as you read these words. While you and I are more or less at rest with respect to each other and to our nearby surroundings, we are traveling at the speed of light relative to a being in a galaxy at the other end of the visible universe reading the Klingon translation of this book. And that being is also at rest with respect to its local surroundings, yet it is traveling away from us at the speed of light.

  How can we be both traveling and sitting still? Simple: The space between us is expanding.

  This idea is what validates warp travel. One can show explicitly in the context of general relativity that the following is possible in principle: Say you want to travel to the nearest star but don’t relish spending 10,000 years in a rocket ship. Well then, all you have to do is travel a little more than 3/4 of the way to the Moon, to the point where the Moon’s gravitational pull balances that of the Earth, and you can remain at rest there with your engines off. Now arrange for the space between you and the nearest star—all 4 light-years worth of it—to collapse in, say, 1 second, while the space between you and the Earth, formerly only about 180,000 miles, now expands correspondingly in the same short period of time. After space has done its thing, you look around and find that you’re now only 180,000 miles away from Alpha Centauri and some 4 light-years away from Earth—all without moving! Then simply turn on your engines and go the rest of the way.

  As fishy as this may sound, the equations of general relativity have been solved to reveal exactly such a possibility. I don’t want to downplay how truly remarkable this is. In fact, more or less the same physics might make even stranger phenomena—traversable wormholes, say, and time machines—possible in principle. However, consider the following: