Opening the Door on Time Machines

Carl Sagan had a problem. In 1983, the author and astronomer was searching for a rapid interstellar transport system that could whiz the heroine of his science fiction saga “Contact” billions of miles to the star Vega to meet the newly discovered alien and then return her safely home the next day. He toyed with the extraordinary idea of sending Ellie Arroway down a black hole. But he worried: Would the physics be right?

Sagan, who died two years ago, had a friend in Kip Thorne, the Caltech physicist who specializes in space and time warps.

Not only is a black hole a one-way street to oblivion, Thorne told Sagan, it would crush Arroway with a force of billions of tons per cubic inch.

So Thorne began to think about possible alternatives--in particular, “wormholes,” or tunnels through space and time, which few scientists had thought about very seriously. Thorne’s work on wormholes not only gave Sagan a scientifically respectable way to get Ellie to Vega, it also opened up a new area of scientific research: the idea that the laws of physics might allow what Thorne calls “closed time-like curves”--in other words, “time machines.”

What’s a respectable physicist doing studying time travel? Thorne works on the boundary between science and speculation, on the cusp of what is and what might be. Specifically, he’s made a specialty of exploring the often outrageous consequences of Albert Einstein’s theory of gravity: black holes, wormholes and various other bizarre objects that bend the mind almost as much as they distort space and time.

He and his colleagues even talked the National Science Foundation into springing for $365 million to build a machine to study something never seen--gravity waves. If it works, the gravity wave detector might well uncover a universe as astonishing as the mountains on the moon revealed in Galileo’s first primitive telescope.

As early as 2002, the Laser Interferometer Gravity-Wave Observatory, or LIGO, may be gearing up to hear rumbles from exploding stars, colliding black holes and eventually even echoes of creation--ripples in space-time from the Big Bang itself.

Still, Thorne doesn’t see himself as “that far out on the edge.” After all, time machines (possibly) and gravity waves (certainly) are consequences of Einstein’s well-tested theory of gravity. Thorne has made a specialty of stretching Einstein’s laws to their limits to see where they break. In essence, he is crash testing Einstein’s laws of physics.

In this context, even time machines aren’t necessarily crazy ideas. “Ideas are crazy if they don’t have any chance of being right,” he says.

The soft-spoken Thorne’s method walks the tightrope between wild imaginings and established law. His daydreams of whirling tornadoes of space-time are tempered by the hardier truths contained in well-worn equations.

“Only [through] mathematics can you test whether your insight is right,” he says. “But if you had to work strictly on the basis of mathematics, you’d move at a snail’s pace.” The images in his head, he says, help him “see some great distance downstream, to figure out what direction you should explore mathematically.”

Another way he focuses his mental energies on cosmic problems is making bets with his peers. Most notoriously, he bet renowned British physicist Stephen Hawking a four-year subscription to the magazine “Private Eye” (against a one-year subscription to Penthouse for Thorne), that Cygnus X-1--an X-ray emitting object in space--was a black hole. Although the stakes “mortified” his feminist wife and sisters, Thorne said, Hawking later conceded the bet.

The bets, he says, are “partly for fun,” but they also have an important aim in clarifying critical problems.

The Fabric of Space and Time

The universe has not looked the same since Einstein proposed in 1915 that the force of gravity is actually the warpage of a fabric woven of space and time. According to Einstein, gravity is not a long-distance glue that sticks objects to Earth. Instead, it works more like a pothole in the fabric of space, posing a fatal attraction for nearby objects. Earth--and other massive objects--create these indentations in space-time like so many bowling balls sitting on water beds.

As bizarre as it sounds, Einstein’s theory has withstood every test put to it. To name a few: Light does bend as it passes near a star; time does slow down near massive objects; massive rotating objects do (apparently) drag space-time around them as they spin.

Finding out where Einstein’s equations lead is Thorne’s life’s work. The fundamental question that drives him is: What has Einstein’s legacy left us? What kinds of objects should be produced by extremely warped space-time, and how should they behave?

Space-time theorists like Thorne usually rely on an interplay of equations and imagination, but Thorne also pursues experiments that put his ideas to the test. Like Christopher Columbus, this requires having not only the vision, but also equipment and crew. “Then if you need a substantial amount of money, you have to have a certain ability to deal with the politicians, like Queen Isabella, or the National Science Foundation,” Thorne says.

A case in point is LIGO--the gravity wave detector now under construction by Thorne’s colleagues in two separate sites in Louisiana and Washington state. Like Columbus, Thorne says he too was “asking for a very large amount of money for something that has never been seen.”

A joint Caltech-MIT project, the twin LIGO detectors aren’t so much telescopes as nets for snaring gravity waves--undulations in space-time created, like waves in a pond, by colossal cosmic events.

The threads of the nets are pairs of 2 1/2-mile long beams, placed at right angles. The light bounces repeatedly between delicately poised mirrors. If the set-up works, it should be sensitive to ripples in space-time that jiggle the sensors with motions smaller than the diameter of an atomic nucleus.

Of course, so sensitive a sensor will also pick up random noise that has nothing to do with gravity waves. So only two identical signals in two detectors at widely separated locations will ensure that a twang in the beams is the signature of the real thing.

If LIGO succeeds, the payoff could be huge. “I’d lay moderately heavy odds,” Thorne says in typical betting fashion, that the first thing scientists see is crashing and merging of two spinning black holes about a billion years away in space and time. These cosmic smashups warp space-time so violently that the resulting wake of gravity waves should have no problem rattling the detector.

“The spinning holes are like two tornadoes, not made of whirling air but whirling space and warped time,” he says, “orbiting around each other inside a third tornado made of whirling space, and we’re asking what happens when they come together. We don’t know.”

Understanding how colliding black holes behave is important because it will offer insights into the nature of space and time under extreme conditions--like those that existed at the origins of the universe. LIGO and its already-planned successors will also be on the lookout for such strong gravity exotica as “spacequakes” produced by exploding stars or colliding burnt-out stars.

With LIGO under construction, Thorne is thinking about leaving the field. The effort doesn’t really need him anymore, he says, and he likes to work where he can “make a difference.”

More important, he does not like crowds. And the study of gravity waves has attracted scores of young physicists--most of them trained by Thorne or his former students. “It’s not as much fun any more,” he says. So he’s casting about for new directions.

One very appealing new direction is back to the question Sagan first set him to pondering more than 10 years ago: Could extremely warped space-time create a wormhole that people could travel through, and if it could, would time travel be possible? The answer is an issue of intense debate, at least for the few intrepid physicists willing to even consider the issue.

These researchers are not interested in creating time machines so much as exploring the nature of space and time under extreme conditions.

Wormholes are theoretically plausible objects first discovered lurking in Einstein’s equations in 1916. Unlike a black hole, a wormhole has two “mouths,” an entrance and a exit--essential requirements for any viable transport system. A wormhole mouth is a tear in the fabric of space-time; it connects to the other tear, or mouth, which might be many light-years distant. Since the wormhole tunnels through four-dimensional hyperspace, it creates a shortcut through space and time.

As an analogy, imagine an ant walking from the left edge of this page to the right edge. The ant’s flat, two-dimensional “space” (the page) is analogous to our everyday three-dimensional space, and the ant’s route from the left to the right side of the page is analogous to a person’s 26 light-year trip to the star Vega.

If you curl up the newspaper page so that the right edge touches the left edge, the ant could take a shortcut from one edge to the other. But the paper has to curve through three-dimensional space.

In the same way, a wormhole bores through four-dimensional space to make a shortcut from Earth to Vega.

The problem--amid the intense curvature of space-time--is keeping the wormhole open long enough to pass through. But after Sagan posed his question, Thorne put in some time “playing around with Einstein’s equations” and discovered that a wormhole could be kept open. This would require filling it with exotic “negative” energy, which might not even exist.

Starting With Guesses

The methods he used to arrive at this conclusion offer insights into Thorne’s way of approaching problems far beyond the known.

First, he says, he makes some “guesses, based on knowledge and past experience,” about what might happen to time in such a wormhole. He thinks about the problem in pictorial terms, “mostly shapes and curves,” he says.

His thought experiments led him to conclude that time flows differently inside a wormhole than outside in the external universe. Thorne imagined that he and his wife held hands inside such a wormhole. One “mouth” remained with Thorne in his living room while the other “mouth” got packed into the family spaceship, where his wife took it for a spin around the galaxy, while still holding hands through the wormhole.

According to Einstein, time flows differently for people who are moving relative to each other. So for Thorne’s wife, traveling at near light speed, only an hour passed, while for Thorne, sitting in his living room, an hour plus a day passed.

Meanwhile, their hands continued to experience the same time inside the wormhole. When Thorne’s wife returned from her journey, he greeted her and noticed that she was holding hands with someone through the wormhole; that someone was him, the day before. Theoretically, he imagined, she could climb through the wormhole, traveling backward one day in time.

Thorne’s thought experiment suggested that a wormhole could become a time machine.

The next step is to build a simple mathematical model--or description--and then to solve the equations. The math led him to the same conclusion: “The equations say unequivocally that [in the simplest case] if you had such a wormhole, it would convert itself into a time machine.”

In fact, he said, it’s “embarrassingly simple” to make a time machine with worm holes based on Einstein’s laws of gravity.

Alas, in the real world, gravity alone doesn’t rule the universe. There are other forces at work--specifically, the quantum mechanics that govern the inner world of the atom. Exploring the quantum world of the wormhole, Thorne and his colleagues discovered an unexpected disaster. Calculations showed that if a wormhole was used as time machine, subatomic fluctuations would pile up on one another inside the wormhole and explode. The time machine would self-destruct.

No one knows what would actually happen inside the wormhole, because physicists don’t yet understand how gravity and quantum mechanics marry under extreme conditions. The study of so-called quantum gravity is still frontier territory.

However, even if the time machine could survive the explosion, it would pose conundrums for cause and effect. If would seem that if you can travel back in time, you can murder your grandfather, and then you don’t exist. Like Hollywood scriptwriters, Thorne and other physicists have explored various scenarios for getting around this problem. “It is not hopeless,” says Thorne, “but I’d give heavy odds that explosions destroy all time machines, so we needn’t face the conundrums.”

Hawking Shares Pessimism

Thorne’s colleague Hawking shares his pessimism. He believes that the universe protects itself against time travel with a “chronology protection” mechanism that “keeps the world safe for historians,” as he likes to put it.

Hawking has also said the best experimental proof that travel back in time isn’t possible is the noticeable absence of hordes of tourists from the future.

In the end, physicists don’t understand the concept of time very well. They don’t know how it arose in the first place.

The types of questions Thorne explores may well help physicists arrive at an answer, but it won’t come any time soon. “This is tough stuff that I don’t understand very well at all,” he says.

Indeed, the very complexity of the subject matter makes knowing where to go next a difficult issue. “I get enjoyment out of probing [ideas like time machines],” he says, “but it’s not clear whether that’s ripe for real success.”

Not that warped space-time or any of its progeny seem unreal to Thorne. On the contrary, he doesn’t think gravity waves or wormholes are any harder to imagine than the stars or particles that occupy other physicists. “You can’t see them with your eyes, but you can’t see atoms with your eyes,” he says. “You can’t see air with your eyes.”

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Shortcut Through Space

In his book “Black Holes and Time Warps, Einstein’s Outrageous Legacy,” Caltech physicist Kip Thorne describes how a wormhole through hyperspace might be used to take a shortcut through space and time. The journey from Earth to the star Vega would take 26 light -years through everyday space and time-- represented in this graphic by a curved sheet. But through a wormhole, the trip could be vastly shorter. Thorne was inspired to think about wormholes by Carl Sagan, who needed a way to get the heroine of “Contact” from Earth to Vega quickly.