In red sweater, tan slacks, and off-white walking shoes, goateed Yakir Aharonov does not immediately stand out as he strides down the colorful streets of Berkeley, California. The town--which proudly proclaims itself THE PEOPLE’S REPUBLIC OF BERKELEY on innumerable tie-dyed T-shirts-- has an almost studied casualness, and Aharonov fits right in. Stopping to relight his momentarily neglected cigar, Aharonov accidentally drops it, then stares for several seconds, apparently considering the propriety of retrieving the smoldering butt from the street.
Ultimately--if reluctantly--Aharonov abandons the stogie, lights a fresh one, and moves on, eager to resume his description of his recent work. It is work of an odd sort, even for a quantum mechanical theorist: Aharonov has designed a time machine. Now I’ve really captured people’s attention, the 59-year-old researcher says, beaming.
Since the era of H. G. Wells, science fiction buffs have gone gaga over the idea of traveling through time. All at once, however, they’ve acquired some serious company. Aharonov--a sober, mainstream physicist who is not only a visiting professor at the University of California at Berkeley but also a faculty member at the University of Tel Aviv and the University of South Carolina--is no less than the third prominent researcher to devote a chunk of his career to studying the realities of time travel.
The process began a few years back when Caltech physicist Kip Thorne, famous for his pioneering theories of black holes, and then graduate student Michael Morris came up with a scheme, based on Einstein’s general theory of relativity, for converting a cosmic wormhole into a time machine (see Discover, June 1989). Cosmic wormholes are theoretical tunnels through space that can directly connect two vastly distant locations--and according to Thorne and Morris’s calculations, two points in time as well. Unfortunately wormholes would also serve as cosmic trash compactors, brutally crushing anything unlucky enough to enter.
The two newer time machines, one proposed by Aharonov and the other by Princeton physicist J. Richard Gott, also rely on Einsteinian relativity to provide the necessary distortion of space and time, but both are more accommodating to the physical comfort of their operators. Plans for the two schemes aren’t exactly at the financing stage--in fact, chances are pretty good neither will ever be realized--but the theoretical workability of the basic ideas has physicists buzzing. This whole thing, says Gott, is telling us a lot about some fundamental areas of physics that have never really been explored. Both of the proposed time travel methods also rely on some pretty extreme science. Aharonov’s, which is probably the more plausible of the two, arose from his studies of one of physics’ most intriguing subspecialties: quantum mechanics.
Quantum mechanics involves the study of subatomic particles and is based on the simple, if slippery, principles of observation and randomness. According to quantum mechanical theory, a moving particle like an electron does not travel from point A to point B to point C and back again, but instead exists at all three points--and all points in between-- at any given moment. It has the literal ability to be in more than one place at a time. Similarly the electron need not exist at just one energy level, but at all levels at once.
The only way to fix a particle in a single location is to observe it. Through some process physicists don’t pretend to understand fully, the act of observation not only reveals a particle’s condition but actually determines it, forcing it to select just one of the possible states. As for what goes on between these observations, physicists only shrug and reply, Don’t ask. By which they mean, quite literally, don’t ask. There is no reality outside observation, quantum mechanics is understood to say.
To the uninitiated, such a theory does violence to simple logic, yet physicists have believed for nearly 70 years that this is exactly how things behave at the subatomic level. The dual nature of subatomic particles--they can behave like both particles and waves--is what first led physicists to believe that they might also have dual locations and states. But since the particles are always observed in only one state or the other, something in the act of observation must be forcing them to choose the state.
When quantum theory was first proposed, physicists realized it could be proved with an experiment in which a single electron would be hurled through a wall with two slits in it. If a particle detector looked for evidence of the electron’s trajectory during its flight, the tiny projectile would pass through just one slit. If it looked only afterward, however, the electron would be seen to have passed through both slits at the same time. The idea sounded preposterous, but over the years numerous experiments--such as a Japanese study conducted with electrons and phosphorescent screens--have yielded just these results.
Aharonov was always captivated by the curious world of subatomic particles between observations. From the time he was a graduate student he has been exploring this world by devising, after the style of Einstein, a variety of lavishly constructed imaginary experiments. These thought experiments generally involved gyroscopically spinning particles and used quantum equations to infer their direction of rotation. The advantage to being a theoretical physicist, Aharonov says, is that you never have to worry about the cost of a thought experiment.
Two years ago Aharonov came up with a particularly intriguing experiment, one that would require only one piece of exotic equipment: a massive balloonlike sphere capable of being instantly expanded or shrunk to any of a wide variety of sizes. The benefits of such capabilities were no less exotic than the device itself: the owner of the balloon could climb inside, let it expand and contract, and be transported forward or backward through time.
Why would the inflation and deflation have such an effect? For that you have to turn to general relativity. According to one of the theory’s basic premises, gravity slows time down; when you’re up in a plane where Earth’s gravitational pull is ever so slightly weaker, your watch actually runs a little faster than it does on the ground (though the effect in this case is immeasurably small). The balloon, like any other object, also exerts gravity. As it gets bigger, someone sitting inside would be exposed to a correspondingly lesser gravitational drag on time because the balloon would be exerting the same gravitational effect over a larger volume, in a sense diluting its strength. When the balloon contracted, its gravity per unit of volume would be greater. As a result, time in the expanded balloon would speed up a bit for the occupant; in the contracted balloon, it would slow down.
Normally that time distortion would be far too tiny to measure. But the picture is different if the balloon is rendered quantum mechanical. What’s a quantum mechanical balloon? Actually, anything can be made quantum mechanical simply by linking its behavior to that of one or more quantum mechanical particles. Erwin Schrödinger, one of the fathers of quantum mechanics, illustrated this point six decades ago when he conjured up his famous, slightly macabre experiment in which a cat would be placed in a box along with a radioactive sample, a Geiger counter, a vial of cyanide, and a spring-driven hammer. (Animal rights activists take note: this was a thought experiment.) The apparatus would be set up so that if the Geiger counter detected radiation--an event whose occurrence is strictly a matter of quantum mechanical chance--it would trigger the hammer to smash the vial, sending the innocent cat to a better world.
When, after a while, someone opened the box and peered in, he would find a perfectly normal cat--albeit a dead one, if the sample had emitted radiation. But while the box was sealed and unobserved, the radioactive sample would exist in all its possible states; that is, emitting and not emitting. As a result, the Geiger counter would both detect and not detect radiation; the hammer would both smash and not smash the vial, and the unsuspecting kitty would be--well, both dead and alive. The cat’s ultimate fate would be determined only by the act of observation.
In the same manner, Aharonov’s balloon can be linked to a quantum mechanical event; a particle’s state (the way it’s spinning, for example) would be associated with a different size for the balloon. The whole setup would behave perfectly respectably every time that state is observed, resulting in a particular degree of inflation for the balloon. But being quantum mechanical, the particles aren’t in any particular state in between observations; they simultaneously exist in all states. And that means that the balloon, like Schrödinger’s cat, would also be left in limbo, existing simultaneously in all its possible sizes--and the occupant of the balloon would simultaneously exist in many, slightly different, rates of time.
But Aharonov’s balloon is more than just an exotic version of Schrödinger’s cat. For Aharonov determined that once in a very great while the simultaneous tiny distortions of time can add up to one enormous distortion in one direction or the other. Any two particular sizes for the balloon, determined by two observations of the particle, are like the two possible paths for the electron in the two-slit experiment. Classically the two possibilities are independent, but quantum mechanically they affect each other. At any moment, says Aharonov, the balloon exists in a sort of superposition of many states. When the observations are made, all those states affect one another. In a sense the overlapping expansions and contractions of the balloon are like the overlapping peaks and troughs of waves, which can sometimes amplify themselves into one superpeak or supertrough. In this case, for anyone inside the balloon, a superpeak would correspond to his being hurtled into the future. A supertrough, on the other hand, would correspond to time running backward.
How exactly does this last part work? It’s just a consequence of quantum mechanics, one which has no classical analogue, says Aharonov patiently. Sometimes you get a particular interference pattern that corresponds to going backward in time.
In other words, don’t ask.
The only hitch in this scenario is that the effects of this tinkering with time would not be like those seen in popular fiction. Rather than sending an occupant into the rest of the world’s past or future, Aharonov’s balloon would instead make him younger or older--in effect sending him into his own personal past or future. At least one of the hazards of piloting such a machine is obvious: you could end up being time translated into a pile of decaying bones. (As a small consolation, you would still feel as if you had experienced your normal life span--although, of course, you would have spent your life inside a balloon.)
As for getting younger, well, things get thornier here-- especially if the balloon’s occupant manages to get himself translated right back through birth, without the benefit of having his mother around to keep up her end of the bargain. In that case, says Aharonov, stroking his goatee thoughtfully, it would appear as if the person had disintegrated into the atoms he originally came from. After several more strokes applied in silence, he adds: Actually, I haven’t worked out all the details of what would happen in this situation. I’ve been busy with a lot of other things.
Aharonov isn’t about to rush out and patent his time-translation machine. As it stands now, this would be a very impractical device, he notes. I don’t see how you could build it, and even if you did, the chances of making an interesting jump in time would be extremely small. The theory behind the device may be verifiable, though: Aharonov says it’s not all that unlikely that someone might think of a way to construct a small, simplified version of the time-translation experiment that would send particles into their own future or past.
Unfortunately, J. Richard Gott is a little less hopeful of having his time travel theory confirmed. That’s because his theory requires even more exotic physics than Aharonov’s. Gott is a highly respected Princeton cosmologist whose appearance, like Aharonov’s, tends to belie his exalted position. With his floppy brown hair, a yellow feltlike blazer, and a loud, slightly nasal voice tinged with a Louisville drawl, Gott could more easily pass for a salesman of heavy farm equipment. But the multicolored pens poking out of his shirt pocket are a tip-off; pulling them out one by one in a Chinese restaurant, he frantically scribbles on the plastic projector sheets littering the table in front of him. temporarily he captures the attention of half the diners in the establishment, as well as that of two large pike swimming in a tank behind him. I love visual aids, he bellows.
Gott came across his time travel hypotheses while studying cosmic strings--theorized freaks of space-time that are essentially long, skinny bundles of energy left over from just after the Big Bang. Stumbling across a cosmic string somewhere in the universe would be a little like stumbling across a patch of volcanic, primordial Earth in the middle of New York’s Central Park. Cosmologists believe that cosmic strings would be packed so tightly that a single inch would weigh nearly 40 million billion tons. Since, as general relativity points out, the degree to which an object warps space and time is related to its density, cosmic strings would fairly pretzelize their environs.
One consequence of such cosmic distortion is that strings could forge shortcuts in space, in much the same way that twisting up a piece of paper can provide faster routes for an ant scurrying from one side of the paper to the other. As a result, a relatively slower object traveling near a string could benefit from this shortcut and outrace a faster object traveling a different path--even, under extreme conditions, if the faster object happened to be a ray of light.
If it took the right path near a string, says Gott, a rocket ship moving slightly less than the speed of light could arrive on the other side of the string before a ray of light that had left at the same time from the same place.
But even with the aid of a string, outracing light is a very special feat. According to Einstein, an object moving close to light speed would experience a slowdown in time; an object moving at the speed of light would experience a freezing of time; and an object moving effectively faster than light would move backward in time. By giving a space traveler a chance to head a light beam off at the pass, therefore, a cosmic string would be opening up a time warp--physics doesn’t differentiate between exceeding the speed of light by simply outracing it and exceeding it by taking a shortcut.
Alas, calculations showed that any warp the string opened up would be too small for any real fun: although a space traveler halfway along a loop around a string would be able to peer back and see an image of himself taking off, by the time he completed the loop he would find that his original self had already left. No matter how hard he tried, he would never be able to get back before he left.
Gott and other cosmologists had long known about this sort of time delay phenomenon but had never found it all that interesting. Why bother observing the past if you never actually get to experience it? What was needed was some way to exaggerate the pretzelizing effect of the cosmic string so that the rocket occupants could actually be thrown into their own past. Theory suggested that even the most twisted string could not pack so great a time-warping wallop, but in 1990 Gott had imagined a solution. Suppose there wasn’t just one string involved in whipping the ship around, but two. And suppose those two strings were sent rushing toward and past each other in opposite directions at just a fraction of a second less than the speed of light, so that the approaching distortions in space-time could combine like two passing breezes building into a twister.
The relative motion of two strings adds an additional twist to space-time, Gott says, enabling the rocket ship to arrive back before it left.
Here’s how it would work: The two strings rush past each other, with the first moving toward the rocket ship as the ship departs its home planet, while the second is rushing away from it. The rocket ship blows past the first string along the shortcut that string opens up; then the ship races after the second string, hooks around it, heads back along the shortcut opened up by that string, and lands back on its home planet. The rocket ship’s occupants emerge to find themselves well within their past.
It’s just remarkable, says Gott, that you can perform this trick twice to get back to where you started.
Though the scenario seemed plausible, physicists don’t consider any arrangements in space-time to be workable until they’ve run it through the equations Einstein cooked up to describe general relativity. The equations allow scientists to plug in the various shapes, masses, speeds, and energies populating an area of space and then compute the various ways in which space and time are distorted. In any but the simplest of scenarios, this is far from a trivial feat; the equations are ferociously difficult to work with.
There really aren’t that many situations where we’ve been able to solve them exactly, says Gott. Usually we just settle for a computer simulation that gives a good approximation.
Gott, however, put the horse before the cart, solving the Einsteinian equations that prove his theory before he even proposed it. Indeed, it was by toying around with Einstein’s equations governing string behavior that Gott stumbled upon his time travel theory in the first place. There is no guesswork or approximation here, Gott says. If there are cosmic strings that move this fast, this situation could exist.
Gott believes that you could probably find some string trajectory that could take you back in time nearly as far as you want. You could also simply repeat the loop, going further back each time. One caveat, though: running a loop makes it possible for you to run into your pre-time-travel self, and running, say, 15 loops can put you in the position of confronting 15 selves. Obviously, he says, you wouldn’t want to repeat that too many times.
Now that Gott has worked out the details of his cosmic time machine, all that remains besides building the rocket ship is to find a pair of strings moving at near light speed in opposite directions. Actually, before trying to locate a pair of cosmic strings, it would probably be helpful to locate a single string--something that no one has yet managed to do. About half the theories of how the universe was formed predict their existence, explains Gott, so I guess you’d say there’s a fifty percent chance they exist.
If they do, we’ll probably find one through the gravitational lens effect, which occurs when light from a distant object is bent by the gravity of an intervening object. If two light rays from an especially bright source like a qua-sar were bent in the right way by a string, both would end up heading to Earth. The result would be that we would see the same quasar in two different positions in the sky. A large galaxy could bend the quasar’s light and produce the same effect, notes Gott, but the calling card of a string will be a series of twin quasar images stacked one on top of the other, since the string would divert the light not only side to side, but also top to bottom.
If we see five pairs of identical quasars, we’ll know it’s a string, Gott says. We’re out there looking for images like that right now.
Of course, a handy pair of strings does not a time machine make, unless they happen to be moving at--according to Gott’s calculations-- approximately 99.999999992 percent the speed of light. That’s actually not any faster than we get electrons to move in the Stanford Linear Accelerator, he notes hopefully--choosing to ignore, for the moment, the somewhat troubling fact that electrons are among the lightest objects in the universe, while strings are easily among the heaviest.
It is also possible that a time window could be opened up by a single string curved into a closed loop. If the loop were to collapse in on itself, the violence of the event could itself distort time and space. The only difference would be that a collapsing loop could potentially form a black hole that would engulf a passing rocket ship. Even if a black hole were created, however, and even if a passing rocket ship were sucked in, that wouldn’t necessarily be the end of the story. Contrary to popular opinion, notes Gott, an explorer could live within the grip of a black hole, so long as he was locked in a lazy orbit just inside the event horizon, or outer edge of the hole’s gravitational influence. Eventually, however, the ship would be pulled into the core of the hole and ripped apart. I don’t think there’s any question that a person could travel back in time while in a black hole, Gott says. The question is whether he could ever emerge to brag about it.
It’s exactly this sort of question that has made the investigation of string-based time travel so tantalizing. Every time we find a new solution to Einstein’s equations, Gott says, we find out something new about physics. That was the case with black holes, and now it’s happening again with strings. This situation is trying to tell us something, and we should keep exploring it until we know what it is.
Aharonov couldn’t agree more with his time-exploring colleague. In fact, he says, it’s only too bad that Einstein himself couldn’t be alive to see some of the startling time travel possibilities that his own theories have made possible. Then again, if we could travel in time, couldn’t we somehow let the great man know?