Time Travel Research Center © 2005 Cetin BAL - GSM:+90  05366063183 - Turkey / Denizli 

Issue 6.07 - Jul 1998

To Infinity ... and Beyond!

By Jeff Greenwald

Warp drive, wormholes, and the power of nothing.

The NASA Lewis Research Center - known by the nerdy acronym of LeRC - is a spread of beige brick buildings and hulking aluminum ducts located beside the roar of the Cleveland Hopkins International Airport. In front of the visitors center, with its Apollo 8 spacesuit and gift shop selling freeze-dried pizza, a broad sign proclaims the Center's mandate: Powering Our Nation's Future - For the Benefit of All. In Building 60 on the LeRC lot, in the bright, cramped office he shares with two other NASA engineers, Marc Millis displays the fleet of spaceships poised atop his filing cabinet.

"This is a DCA 271 Infector Probe." He lifts an efficient-looking warbird. "It's an alien vessel, 31st century, that I assembled from spoon trays, Christmas ornaments, and other household scraps. As for that Space Bus -" he points to an exquisitely detailed model of an Earth-Mars commuter ship, dated 2060 - "believe it or not, virtually the entire fuselage is made from Yoplait yogurt cups."

With his round, unlined face and neat triangular beard, Millis could audition for Hamlet. An expert model builder, his plans appear often in FineScale Modeler and the International Plastic Modelers' Society Journal. His command of rocket science is equally impressive. Until recently his day job was in aerospace engineering, with a focus on cryogenic propellants like liquid hydrogen, a volatile fluid you wouldn't want to spill on the rug.

Two years ago, Millis's assignment at LeRC became the ideal complement to his hobby. In the summer of 1996, he was asked to create NASA's Breakthrough Propulsion Physics (BPP) program - an attempt to make progress, no matter how slight, toward the epoch of interstellar travel. There are more details about the program, he informs me, on his LeRC Web site. I've seen the header: "Warp Drive, When?"

The site's name is more than a mere conceit. Amazing as it may seem, there are already physicists working on such ideas. A physicist named Miguel Alcubierre recently published a well-received paper showing that warp drive - a way to manipulate the fabric of space-time and move faster than the speed of light - may actually be possible within the limits of general relativity. Meanwhile, Kip Thorne, a scientist at Caltech, has investigated the theoretical behavior of wormholes, examining their potential as shortcuts through interstellar space. And in labs and universities around the planet, quantum theorists - physicists who navigate a world many orders of magnitude smaller than the atom - study a bizarre form of energy that, limitless and free, may drive future starships.

Last year, NASA assembled a cross-section of such scientists and brought them to Cleveland for a Breakthrough Propulsion Physics workshop. The event, organized by Millis, was an attempt to review and debate revolutionary new ideas in space transportation. Fourteen respected American physicists and engineers presented papers. Some, like Arkady Kheyfets and Raymond Chiao, were well-known theorists. Others, including Robert Forward and Frank Tipler, were popular science and science fiction writers. Not all the speakers were sympathetic to the cause. Both Lawrence Krauss (The Physics of Star Trek) and Peter Milonni (of the Los Alamos National Laboratory) oppose the program, contending that its objectives are too far-fetched to merit serious attention. In February, another such conference was held in Huntsville, Alabama. Nearly 100 engineers and scientists from NASA's Marshall Space Flight Center, other aerospace facilities, and national labs congregated for a lecture series called "Physics for the Third Millennium."

Despite their detractors, the fact that such conferences are taking place at all is exhilarating. Our culture is at the point, it seems, where the commitment to space travel is an imperative. A collective destiny to roam the galaxy has been etched so deeply into our souls that reality leaves a sour taste in our mouths. But while our vision of the cosmos has continued to expand - thanks to the Hubble Space Telescope and interplanetary probes - our physical reach is no greater than it was when Nixon took office.

The fact is, we're stuck. And the more we learn about the neighborhood, the more frustrating it gets. Take Barnard's star, the second nearest to the Sun. It appears a fascinating place. A wiggle in the star's motion hints that it may have its own solar system - with at least two planets, both about two-thirds the size of Jupiter. What's more, the star is tantalizingly close. On a cosmic scale, it's no farther than the corner mailbox: not even six light-years away. One really would think we could get there in a sensible amount of time - couldn't we?

Do the math. Even if we had a spaceship that could cruise 50 times faster than the fastest speed ever attained by a human-made object (37,000 miles per hour, the speed of the interplanetary probe Voyager as it left our solar system), a trip to Barnard's star would take more than 2,000 years - one way.

"Sagan-type questions"

In his book Black Holes & Time Warps, physics luminary Kip Thorne recalls the time when astronomer Carl Sagan asked him for advice. Sagan was working on a novel - Contact - and wanted a character to be able to travel from Earth to the Vega system in no time flat.

Sagan's call motivated Thorne to think about wormholes (which we'll get to later). It also inspired Thorne to broaden his style of thinking in general, and consider what he now calls a "Sagan-type question": What things do the laws of physics permit an infinitely advanced civilization to do, and what things do those laws forbid?

By "infinitely advanced," Thorne imagined a civilization for which no amount of energy or know-how is out of reach. The only obstacles are those dictated, irrevocably, by the laws of physics themselves.

"We physicists," Thorne writes, "have tended to avoid such questions because they are so close to science fiction. While many of us may enjoy reading or even write some, we fear ridicule from our colleagues for working on research close to the science fiction fringe."

A number of the papers presented at Millis's BPP workshop certainly walked that line. Research into time travel and infinite-energy sources would be a deadly career move for most scientists. But if there was one thing the presenters at the workshop had in common, it was a willingness to take our most basic preconceptions about space travel - and physics itself - by the horns.

Such out-of-the-box thinking can pay dividends. Fifty years ago, the transistor - a stunning breakthrough in electronics - was invented at Bell Labs. Today, we sneer at the memory of house-sized mainframes and bark into digital cell phones without a second thought. What Millis and his loosely knit think tank hope to do is launch a similar revolution in the field of propulsion. The three goals of the BPP program, he declares, point to the major breakthroughs needed if we hope to reach the stars.

"Goal one," Millis states, "is finding a way to fly a spacecraft without using vast amounts of propellant. This means discovering fundamentally new ways to create motion, possibly by manipulating space-time itself." Goal two is achieving the ultimate transit speed. This means travel up to or beyond the speed of light, if possible - and Millis would like to believe that it is. "Goal three," he concludes, "is finding new methods of generating energy, so that we can meet the power requirements of the first two goals."

All three goals are daunting, but goal two may prove the most formidable of all.

Up against the law

For decades, the speed of light itself - 186,000 miles per second - has been the Holy Grail of sci-fi spaceships, and like the Grail, it is unattainable. No matter what miracles we humans may achieve - peace on Earth, artificial intelligence, or a Cubs World Series victory - we will never, ever accelerate a vehicle to light speed.

Why not? Because special relativity, formulated by Albert Einstein in 1905, forbids it.

Einstein discovered that the speed of light, like a fine scotch, never varies. It is absolute for an observer, whether that observer is moving or still. As we approach that velocity, we enter a very different world from the one we're used to. Driving through Baja California, for instance, we constantly modify our speed. Time flows at a constant pace, and the distance between Tijuana and Cabo San Lucas holds steady. As we approach the velocity of light, however, all that changes. With speed locked down, space and time become malleable, bending and stretching like Silly Putty.

No one denies how wacky this seems, or how unfair this speed limit is to Han Solo wannabes. If special relativity were a religion, we could argue with it, we could contest the arrogant notion that Einstein's opinion is a universal truth.

But special relativity is not a religion, or a matter of faith at all. It's the law - for humble earthlings and infinitely advanced civilizations alike. Every one of Einstein's predictions about the seemingly absurd relationship between velocity, time, and mass have proven true - not just on Earth, but as deep into space as we can see.

One example has to do with mass. Unlike weight - which depends on gravity - mass is a measure of an object's resistance to motion. Even a weightless satellite has enough mass to clobber you (remember what happened to astronaut Frank Poole when the space pod rammed him in 2001: A Space Odyssey). Einstein predicted that an object's mass must approach infinity as it nears the speed of light. Experiments have shown that this is so. Tiny electrons, flung through particle accelerators at close to light speed, whack into their targets with the momentum of elephants. And as an object grows more massive, it takes more and more energy (i.e., fuel) to keep it accelerating. The graph rises off the scale; there is not enough energy in the entire universe to push even a lentil to the light speed limit.

Einstein also predicted that time itself must slow down for objects in motion. The faster you move, the slower your clocks would appear to tick - relative to someone watching from a remote location. If you could actually reach light speed, time would crawl to a stop. It's wildly counterintuitive, but experiments have proved it true. Radioactive particles decay much more slowly as they are accelerated toward light speed; atomic clocks carried aboard an aircraft return home lagging slightly behind their earthbound twins. So even if a crew of explorers could zip to Deep Space Nine at the speed of light, human civilization would have aged a millennium by the time they returned, still youthful, to Earth.

The long and short of it is that nothing - nothing made of stuff, at least - can reach light speed.

So if there were nothing more to the universe than special relativity, Starfleet would be in trouble. Humanity would be doomed to spend its existence marooned in this remote arm of the Milky Way, gazing mournfully at the bright lights of the galactic center. But general relativity - which describes the geometry of space-time itself - provides us with some loopholes. And some of them, as Lawrence Krauss points out in The Physics of Star Trek, might be "large enough to drive a Federation starship through."

To appreciate what he means, let's revisit Einstein. A decade after coming up with special relativity, the dean of theoretical physics published his theory of general relativity.

The premise of general relativity is that mass and energy create gravity, and thus curve the fabric of space-time. Einstein's equations make it possible to look at the mass and energy at loose in the cosmos and see how they ought to bend space. We've actually measured this during solar eclipses; starlight bends slightly as it passes the sun. But the most radical example of this effect is a black hole: a region around an imploded star where gravity is so intense that not even light can escape.

What all this means (or at least implies) is that travel through space-time need not depend on the line-of-sight distance between two points. If space-time bends, it may be possible to find shortcuts through it.

Many cosmologists, Millis observes, like to look at mass and energy and study the quirky ways that space-time bends in their vicinity. It's a passive role. Millis's interest is strictly proactive: Can one demand that space-time behave a certain way, and then figure out the recipe, the exact mass and energy requirements, to make it so?

Such questions, he assures me, are creeping up even in the top physics journals. The best example - and a source of inspiration to at least half the presenters at the BPP workshop - is a paper called "The Warp Drive: Hyper-Fast Travel Within General Relativity." Written by Miguel Alcubierre, a theoretical physicist at the University of Wales, it was published in Classical and Quantum Gravity in 1994.

Warp drive, of course, is the favored means of propulsion on Star Trek. Field-tested by space pioneer Zefram Cochrane in 2063, the technique involves distorting the space-time continuum enough to drive a starship past the speed of light. How it works isn't clear, but it's pretty damned fast; a velocity of warp eight translates to 1,000 times light speed.

Alcubierre had set out to determine whether the concept of faster-than-light travel was feasible within the constraints of general relativity. Plus, he wanted to sidestep the annoying time and mass distortions of special relativity altogether. His starship had to be able to cross the galaxy at nearly infinite speed, seek out new life and new civilizations, and return home in time for The X-Files. Finally, the issue of inertia - the force that pushes you back in your seat during acceleration - had to be resolved. Jump-starting from zero to light speed, a human crew would be flattened against the bulkhead like so many communion wafers.

His solution was so crisp and elegant that it delighted physicists around the world. It is theoretically possible, Alcubierre calculated, to distort space, specifically to allow warp speed travel: to literally expand the volume of space-time behind a starship, while compressing it up ahead. The best analogy I can think of is when you feed a tent pole through its sleeve by bunching up the fabric ahead, and pulling it along behind.

Alcubierre showed that space-time can be similarly manipulated. The position of a starship within such a distortion would change, relative to its destination - yet the ship itself need not actually "move" at all.

Wormhole world

Equally inspiring to the BPP lobby is the concept of "wormholes." These are tunnels - formed, according to one theory, when two singularities meet - that an infinitely advanced civilization might use to link far-flung regions of the cosmos.

And what, you might ask, is a singularity? It's a place, strictly theoretical, where gravity is so intense - i.e., in the pit of a black hole - that space-time is funneled into an infinitely small point. Since the universe is curved, there might be a spot where two such points chance to touch.

To get a visual handle on this idea, think again of a dome tent. Someone pushes a pencil point into the fabric on one side; someone else pushes one through on the other side. In hyperspace, the two points, or singularities, would annihilate each other on contact and create a possible passageway. Like Alcubierre's warp drive, such wormholes - unlikely as they might be - are permitted by general relativity. Their potential usefulness, however, has been greatly exaggerated. Current research suggests that these wormholes can exist only at the quantum level, living immeasurably short lives before they are destroyed by gravitational forces.

But what if a wormhole of a different kind could exist over a period of time? Kip Thorne and fellow physicist Michael Morris - while studying these theoretical phenomena with an eye to Sagan's book - discovered that a wormhole could theoretically be held open. This might be done by coating its throat with some exotic material that possessed a "negative" energy density. Unlike black holes - out of which nothing, not even light, can emerge - these wormhole mouths allow two-way traffic: of Federation starships, Jem'Hadar warships, or the dreaded Dominion fleet.

It looks great on paper. Since "normal," or positive, energy produces wormhole-clenching gravity, "negative" energy could counteract this force. The only problem is that nothing on Earth can produce more than a hint of this negative energy.

All of this begs a Sagan-like question: Is there any hope at all that such theories - of warping space and propping open wormholes - may someday become practice, allowing us to navigate the cosmos?

Matt Visser, a Saint Louis astrophysicist who reviewed current wormhole research in Lorentzian Wormholes: From Einstein to Hawking, provides an answer: "If you emphasize the 'someday' and 'maybe,' what we've got, mainly, are nice toy models that teach us the limits of general relativity and quantum theory as we know it. The researchers in the field don't expect to see an interstellar drive coming out of the stuff anytime soon."

"Have you seen any evidence at all," I ask, "that wormholes can be artificially created, or even used to send a signal to a distant part of space?"

"The possibility is there," he sighs. "But there's a big, big difference between possibility and probability.

"Naturally occurring wormholes," explains Visser, "if they even exist, are probably so tiny - 25 orders of magnitude smaller than an atom - that we'd never be able to squeeze a message through them." As for the larger variety, "we know that the minimum requirement for keeping a wormhole open is to have large amounts of negative energy floating near the throat. Suppose you want a wormhole just 1 meter across - big enough to shove a human through. The amount of negative mass you would need at the throat of that wormhole is about minus one Jupiter, OK?"

"One Jupiter?" I'm stumped. "I've never heard of that unit."

"You basically need to take the mass of the planet Jupiter, turn that into negative mass, and shove it down the throat of the wormhole. That gives you some feel," Visser concludes dryly, "of how technologically challenging this is."

So: Is traveling via warp drive or wormholes possible? Maybe somehow, for some "infinitely advanced civilization." Is it likely to happen in our little corner of the galaxy any time soon? In the immortal words of Donnie Brasco: "Fuggedaboudit."

The wall that these two ideas smack into is energy. Warp drive and wormholes require so much of it that when the droll and erudite Lawrence Krauss presented his paper at the BPP conference, he ridiculed such schemes as "the most inefficient way to fly." Such energies may be theoretically possible, Krauss claims, but they will be forever beyond human reach.

Not everyone agrees. Theoretical physicist Hal Puthoff, for one, claims to be on to something. Nothing major - just a limitless source of free energy.

The subject of Puthoff's investigations is space itself. According to some renowned theorists - including the late, great Richard Feynman - the cosmic vacuum is a boiling sea of quantum fluctuations, rife with untapped energy from "virtual" particles popping in and out of existence. This "zero-point energy" (ZPE) pervades the universe. A single cup of the vacuum, Feynman once remarked, contains enough energy to boil all the world's oceans.

Puthoff, founder and director of the Institute for Advanced Studies at Austin in Texas, has no doubt that this is so. "For the chauvinists in the field like ourselves," he declares, "the 21st century could be the Zero-Point Energy Age."

Puthoff's claims for ZPE go far beyond its utility as a possible energy source. Vacuum fluctuations, he believes, may also be responsible for that mysterious drag force called inertia. If this is so, we can potentially find a way to manipulate it - or cancel it entirely. A 1994 paper, published in Physical Review by Puthoff and his colleagues on this very subject, impressed at least one fellow visionary. In 3001: The Final Odyssey, Arthur C. Clarke employs an inertia-canceling SHARP Drive: named after Sakharov, Haisch, Alfonso Rueda, and Puthoff. It made the physicist's day.

The hard proof for zero-point energy comes from a phenomenon called the Casimir effect. If you bring two metal plates close enough together, Dutch physicist H. B. G. Casimir calculated in 1948, the ZPE "pressure" outside the plates should be greater than the force inside, and the plates should come together. Indeed they do. This effect was measured very precisely by physicist Steve Lamoreaux in August 1996. The New York Times reported the news in an article with a headline that must have delighted Seinfeld fans: "Physicists Confirm Power of Nothing."

So the big debate about ZPE isn't whether it exists; experiments have shown that it does. The question is, how much is there? Could a cup of ZPE really boil the oceans - or would it take an ocean of the stuff just to boil a cup of water? On this point, quantum theorists like Puthoff and cosmologists like Lawrence Krauss are bitterly divided. Their arguments are totally opposed, and you don't need a PhD to understand them.

Remember, once again, that mass - which is just a fancy form of energy (i.e., E=mc2) - curves space. If there's so much energy in the vacuum, cosmologists insist, space would be so tightly curled up into a ball that you wouldn't be able to see your nose in front of your face. From the quantum theorist's standpoint, however, the incontrovertible presence of so much ZPE - which they calculate, in part, by studying the behavior of excited atoms - shows that cosmologists are clinging to wrong assumptions. Who says that all energy, no matter what form, has to curve space?

While that chicken-or-egg argument rages, there's at least one thing that everyone agrees on. So far, there is no practical way to extract energy from this phenomenon. When a pair of Casimir plates have bonked together, for example, it takes just as much energy to pry them apart.

But hope springs eternal, and the possibility of mining the vacuum is irresistible. Zero-point energy is out there - and whoever figures out how to tap it will become very, very rich. Inventions that claim to exploit this force - ostensibly producing more energy than they use up - arrive at Puthoff's door on a regular basis. The strongest argument for his own objectivity, Puthoff remarks with irony, is that he has personally tested - and debunked - all but two of them so far (and tests on those two are still in progress).

Nonetheless, some well-known physicists are pursuing or have already obtained patents on ZPE devices. Players include plasma researcher Ken Shoulders, science fiction author and physicist Robert Forward, and the venerable hot-gas dynamist Frank Mead. Even Arthur C. Clarke, who didn't patent his own 1945 invention (the Clarke Orbit), has jumped on the bandwagon. He claims to have invested more than $50,000 in various zero-point energy devices. "I'm hoping for big developments next month," Clarke told me in a December 1997 email, "but then I've been doing that for years!"

Warped thinking

Marc Millis's new job description with the BPP program includes a lot of outreach. Part of this involves drawing scientists in, part entails figuring out whether the public thinks NASA should be working on breakthrough propulsion at all. "When we asked that question of the aerospace community," Millis says, "I was expecting either 'You guys are crazy ... this is way premature' or 'It's about time!' And the response has been mainly, 'It's about time. Let's take a look at these things.'" He pauses. "The only people with real reluctance toward this program are within the physics community."

Millis pins this on the money issue. "They suspect we're putting a lot of bucks into this," he says, "which we're not." The 1997 workshop, he explains, was cheap by government standards: the cost of a one-year college grant. February's lecture series was underwritten with NASA training funds.

Another reason for the reluctance - and the main one, I imagine - is that established physicists are loath to associate themselves with research that could attract a fringe (or even a popular) following. You can bet your last slab of gold-pressed latinum that anything connected with warp drive, wormholes, or free energy is going to draw that element. (Millis knows this all too well; a small but persistent cadre of crackpots assails him weekly.)

And there is a vocal minority in the aerospace community, Millis concedes, who do feel that NASA's investment in breakthrough propulsion is premature. The space agency, after all, is mainly an engineering shop. Why should it put a dime into speculation? Why not wait until the physics is hammered down, and take it from there?

The most vocal of these skeptics is Lawrence Krauss. Aside from writing four successful books on popular physics, Krauss holds an eminent position in the sciences: chair of the Department of Physics at Case Western Reserve University.

He's an affable guy, with a wide mouth and an ironic smile. "When I wrote about warp drive in The Physics of Star Trek, it got a lot of people excited," Krauss says. "And the fascinating thing is that it might be possible, in principle. That is amazing, if true. But it's not possible in practice. And in the near-term - in the lifetime of anyone working on this project - it's impossible."

"Has there been any progress at all in the last few years," I ask, "as far as faster-than-light travel goes?"

"Yeah, there's been progress," he nods. "And it's tended to be negative. The recent work that's been done with wormholes and warp drive is all pessimistic. A calculation by Larry Ford and his colleagues at Tufts University says that even if it were possible to create a sort of 'warp bubble,' it would require something like 10 billion times the mass of the visible universe in energy."

I ask Krauss how he felt about the Breakthrough Propulsion Physics workshop. He was, after all, the first to present a paper at the event - even though his comments were plainly adversarial.

"NASA spent 50 grand on the BPP workshop," Krauss reflects. "That's a reasonable thing to do, because, what the heck, something interesting might come out of it. In my opinion, it was a very good idea. But then - having done the workshop - it seems to me that it's not really worth following up on any of the ideas presented there. Warp drive is not the kind of thing for this century, or the next century, or the century afterward. It's in the realm of vague ideas."

"So you think Millis is off track?"

"Millis is an honest engineer whose attitude is, 'We've got to think outside the box.' He has the right idea: We should try to be open-minded and investigate these questions. In the end, personally, I think the right answer is that they're not worth working on. The kind of energies required for interstellar travel are so far beyond anything we can achieve at the present time that it's just not worth worrying about.

"Look," he says. "The main reason people talk about breakthrough, nonpropulsion-type methods like warp drive and wormholes is because propulsion requires so much energy. Just to accelerate to half the speed of light, then stop again, you'd need nearly 7,000 times the mass of your spacecraft in fuel - and that's using fusion power. But what they don't seem to realize is that these other techniques require even more energy."

"Do any of these breakthrough ideas," I ask, "seem even remotely feasible to you?"

"No." He shakes his head. "The only mechanisms that are going to get us anywhere roundtrip in a reasonable amount of time are those that are going to get us around the solar system: nuclear electric propulsion, or, eventually, fusion. Laser-powered sails is another idea that's semirealistic."

"Laser-powered sails?" The concept seems languid, absurd.

"Obviously, what you want is propellants that go faster and faster - and you can't do better than light. So you can either shoot a laser beam out the back of a spacecraft or bounce one off a big solar sail to drive something. The size of the sails you'd need is immense; about a quarter the size of Texas. Some people might find that impractical," Krauss allows, "but it's far more practical than what I've heard in terms of warp drive."

"So what should we be looking at," I demand, "that could potentially get us across the galaxy within a human lifetime?"

"I think there isn't anything that's practical for round-trip interstellar travel." Krauss lays his cards on the table. "The only way we're going to go to the stars is one-way - when we're willing to go slow, and take a long time to get there."

He observes my baleful expression. "But I don't think that's a big problem," he adds. "Throughout history, people have always been willing to take one-way trips into the unknown - because they have no choice."

As in baseball, so in physics

A few years ago, the New York Yankees had a terrific slogan: "At any moment, a great moment." The same is true in physics. No one knows what the immediate future holds, or what grand-slam inventions, like the transistor, wait around the corner. While asking Sagan-type questions doesn't guarantee dramatic results, it does serve as a conceptual springboard. In physics, that's a big part of the ball game.

At the same time, survival in the sciences depends on consensus. Peer review is everything. Some researchers exploring the more remote reaches of physics will indeed lose their footing and plunge into obscurity. But there will always be a few who make it to the next ledge and pull the rest up with them.

One of these is Raymond Chiao, a quantum physicist at the University of California at Berkeley. Chiao recently performed an experiment in which a photon - a single, massless unit of light energy - was clocked at 1.7 times light speed. The phenomenon, called "quantum tunneling," had long been predicted; Chiao was among the first to demonstrate it. A bemused-looking professor with salt-and-pepper hair, he explains his experiment (which took two years to set up) in detail, but then adds, "You cannot use this method to send a signal faster than the speed of light. The reason is, you cannot control whether the photon tunnels or not. At the quantum level, you cannot manipulate. It's random - and a signal is something that you have to be able to control. So it's not faster-than-light communication we're talking about here.

"In Star Trek," Chiao laughs, "everything is still classical thinking. In quantum mechanics, determinism is out the window."

Chiao shares Krauss's belief that NASA's BPP program is of no practical use at this point. Unlike Krauss, though, he won't take a stand against the possibility of superluminal travel. "I'm loath to say that something is absolutely impossible," Chiao says. "There are many examples in history where very good physicists said that something possible was impossible. Ernest Rutherford, who won the Nobel Prize for chemistry in 1908, said that the idea of extracting energy from the nucleus of the atom was pure 'moonshine!'"

Krauss would roll his eyes at such a statement. "Sometimes," he remarked during our meeting, "when you say 'this will never happen,' it actually never happens."

Still, Marc Millis will continue to push for breakthrough propulsion solutions - as long as the money holds out, or until he's convinced that no civilization, no matter how advanced, can move between the stars.

"There might be experiments, relatively affordable things we can do today, to show us if the three goals of the BPP program are conquerable," he states. His wish list includes three critical questions: Is there some way of repeating Chiao's experiment using an electron, to learn if matter itself - i.e., "stuff" - can tunnel at superluminal speeds? Is it possible to create even a microscopic wormhole in the laboratory? Can zero-point energy be exploited to create power in a way that's cost-effective?

Research into these questions will take time, and the results are bound to be controversial. Theoreticians and engineers on both sides have strong convictions. A few careers - not to mention humanity's future in space - may hang in the balance.

"That's why we're trying to do things step by step, and be open-minded," says Millis. "But open-minded goes two ways: to where things like warp drive might work - and to where they might not. You'd be surprised," he offers a wry grin, "how many open-minded people are open only one way."

Jeff Greenwald (jeffji@compuserve.com) is a contributing editor at Wired. His new book, Future Perfect: How Star Trek Conquered Planet Earth, has just been released by Viking.

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