Artigo de Michio Kaku, professor de física teórica na City University of New York, publicado na Wired:A User's Guide to Time Travel
All it takes is a grasp of theoretical physics, control of the space-time continuum, and maybe a ball of cosmic string.
Did the tech bubble burst in your face? Were you one of those unlucky outsiders who missed the Yahoo! IPO or got stuck with Enron stock long after the execs had dumped theirs? Wouldn't you like to be, just once, in the right place at the right time? Now you can. Follow a few simple instructions to relive the bull market and bail out just in time - then go on to march with Pericles or meet your great-great-great-grandchild.
Once confined to fantasy and science fiction, time travel is now simply an engineering problem. Physicists schooled in Newton's laws believed that time moved along a straight, steady course, like a speeding arrow. Then came Einstein in the early 1900s. His equations showed that time is more like a river. The more mass or energy you possess, the more the current around you varies. By moving at high velocity, for instance, you can make time slow down, and when you come to a stop, you'll be younger than if you'd remained at rest. Thus, a speedy spacecraft makes a fairly basic time machine.
Even after Einstein, most physicists believed the clock ticked in only one direction. While moving faster than the speed of light could, according to Einstein's equations, reverse time's arrow, such motion was impossible, because any object that reached that velocity would become infinite in mass. Trips to the past were preposterous.
Not anymore. Having examined Einstein's equations more closely, physicists now realize that the river of time may be diverted into a whirlpool - called a closed timelike curve - or even a fork leading to a parallel universe. In particular, the more mass you can concentrate at a single point, the more you can bend the flow.
In recent years, new designs for time machines have been flying off drawing boards at the world's top science labs. Exact specifications depend on where in time and space you wish to travel. You'll need a hefty CPU to solve the relevant equations for your machine's precise size, shape, motion, location, surroundings, and so on; the more accurately you can nail down these variables, the closer you'll come to your intended destination.
The designs that follow don't have the panache of Doc Brown's DeLorean in Back to the Future or even H. G. Wells' brass and quartz dream machine, but they do put time travel within reach of anyone with a couple of fast spaceships, a supercomputer, and a solar-system-scale machine shop. Warning: Time-space distortions may not be stable and may collapse as you enter, so approach them at your own risk. Also, when going back in time, do not - repeat - do not kill your parents before you are born. Wired takes no responsibility for parallel universes in which you find yourself trapped for eternity.
When Carl Sagan was writing his 1985 novel Contact, he asked Caltech physicist Kip Thorne how to abbreviate the lengthy flight time required for a trip to a distant star. Thorne suggested a wormhole, a shortcut through space-time that almost certainly exists as a consequence of Einsteinian principles, although one has yet to be detected. A few years later, Thorne suggested that a wormhole's entrances could be positioned in space and time as desired. Unlike some other time machines, this Thorne-inspired design allows round trips. However, it can't take you back to a time before the machine was built. Here's how it works:
•Obtain four large conductive plates at least a few miles in diameter. Arrange them in parallel, very close together. The space between each plate will teem with negative energy - a proven phenomenon known as the Casimir effect - creating slices of identical space-time.
•Separate the plates into two pairs. A wormhole will connect the pairs like an umbilical cord.
•Place one pair in a rocket ship and accelerate to almost the speed of light, preferably in a circular path so the rocket doesn't stray too far. Time will nearly freeze for that set while the other, still on the ground, ages at the usual rate. With each passing moment, the space-borne plates will go farther back in time relative to the others.
•When a sufficient amount of time has passed - preferably decades - step between the earthbound plates. You'll immediately be transported back in time and across space to the other pair.
Fine print: To activate Thorne plates, the distance between each plate must be less than the width of an atom. The resulting wormhole will be equally small, so getting in and out might be difficult. To widen the portal, some scientists suggest using a laser to inject immense amounts of negative energy. In addition, Thorne believes that radiation effects created by gravitons, or particles of gravity, might fry you as you enter the wormhole. According to string theory, however, this probably won't happen, so it's scant reason to cancel your trip.
Many scientists believe the big bang that created the universe left behind cosmic strings - thin, infinitely long filaments of compressed matter. In 1991, Princeton physicist J. Richard Gott discovered that two of these structures, arranged in parallel and moving in opposite directions, would warp space-time to allow travel to the past. He later reworked the idea to involve a single cosmic-string loop. A Gott loop can take you back in time but not forward. The guide to building your own:
•Scan the galaxy for a loop of cosmic string.
•When you find one, fly close to it in a massive spaceship. Use the ship's gravity to shape the string into a rectangle roughly 54,000 light-years long and .01 light-years wide. Gravity exerted by the longer sides of the rectangle will cause it to collapse, bringing the sides closer and closer together at nearly the speed of light.
•As the two sides approach within 10 feet of each other, circle them in a smaller ship. When you return to the start of the circle, you will have traveled back in time.
Fine print: To take you back one year, the string must weigh about half as much as the Milky Way galaxy. You'll need a mighty big spaceship to make that rectangle.
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