Time Travel - How Time's Arrow Can Be Reversed

Time travel is easy: All of us do it every day, but only in one direction. For thousands of years scientists and philosophers have talked of time as a river that flows steadily onward year after year. But what if there were a way to swim against the flow, or to run down the bank ahead of the river? Might we be able to journey back and forth in time just as we travel through space? the idea is not as far-setched as it sounds, and the implications for the future are intriguing.

science Fact:
Scientists say building a time machine may be impossibly difficult. But time travel is not against the laws of physics.

Gas Guzzler
One problem for any time-traveler will be the energy bill. Even a small step through time is likely to use up the energy output of a fair-sized star.

But Seriously...
Scientists often talk about time travel in the context of its implications for theoretical physics. But they try not to mention "time machines" in case it gets picked up by the supermarket tabloids.

Super-science, not fantasy
Three Ways To Travel In Time Without Breaking The Rules

Ever since Einstein, scientists have considered 3-dimensional space and time nat as 2 different things, but as different aspects of 4-dimensional "space-time." Quantum physicists, who study the world of subatomic particles, often find it easier to explain events by assuming time runs backward as well as forward, however much if defies common sense.

At the other extreme, cosmologists looking at the universe on a grand scale have found that space and time can be warped by gravity and speed. back in the 1940s, German mathematician Kurt Goedel proved that what he called "closed, timelike curves" - then we could bore tunnels through time itself. But no one know how to do the twisting - until black holes.

The gravitational pull of a black hole is so enormous that it distorts the very fabric of space-time into what is called a singularity. When singularities were found to spin, it was proved that closed, timelike curves not only can occur - they must occur. The singularity forms a doughnut shape in space-time, while the hole in the middle is a perilous gateway to somewhere - or when.

Theory one: Wormholes

Since the 1930s, physicists have speculated about the existence of "wormholes" in the fabric of space. Wormholes are essentially gateways between different parts of the universe and are made by linking a pair of black holes. This effectively creates a tunnel through time and space: a traveler entering at one end would exit the other at a different time as well as a different place.

The difficutly lies in keeping the wormhole open while the travel makes his journey: if the opening snaps shut, he will never survive the emerge at the other end. For years, scientists believed that the transit was physically impossible. But recent research, especially by the U.S. physicist Kip Thorne, suggests that it could be done using exotic materials capable of withstanding the immense forces involved. Even then, the time machine would be limited use - for example, you could not return to a time befroe the wormhole was created. Using wormhole technology woudl also require a society so thechnologically advanced that it could master and exploit the energy within black holes. But the trio would not be impossible - just very, very difficult!

Theory Two: Rotating cylinder

Civilizations withe the technology to harness black holes might be better advised to leave wormholes alone and try the time-warp method suggested by U.S. astronomer frank Tipler. he has a simple recipe for a time machine: First take a piece of material 10 times the mass of the Sun, squeeze it together and roll it into a long, thin, super-dense cylinder - a bit like a black hole that passed through a spaghetti factory. Then spin the cylinder up to a few billion revolutions per minute and see what happens.

Tipler predicts that a ship following a carefully plotted spiral course around the cylinder would immediatly find itself on a "closed, timelike curve." It would emerge thousands, even billions, of years from its starting points and possibly several galaixes away.

There are problems, though. For the mathematics to work properly, Tipler's cylinder has to be infinetely long. Also, odd things happen near the ends and you need to steer well clear of them in your timeship. However, if you make a device as long as you can, and stick close to the middle of the cylinder, you should survive the trip.

Theory Three: Cosmic Strings

As a variation on the rotating cylinder, some scientists have suggested using "cosmic strings" to construct a time machine. At the moment, these are purely theoretical objects that might possibly be left over from the creation of the universe in the Big Bang.

A black hole contains a 1-dimensional singularity - an infinetly small point in the space-time continuum. A cosmic string, if such a thing existed, would be a 2-dimensional singularity - an infinetly thin line that has even stranger effects on the fabric of space and time.

Although no one has actually found a cosmic string astronomers have suggested that they may explain strange effects seen in distant galaxies. By maneuvering 2 cosmic strings close together - or possibly one cosmic string and a black hole - it i s theoretically possible to create a whole array of "closed, timelike curves." Youru best bet is to fire two infinetly long cosmic strings past each other at a high rate of speed, then fly your ship around them in a carefully calculated figure eight. In theory, you would be able to emerge anywhere, anytime!

True Story
There are no such things as free lunch

Many scientists are uncomfortable with time travel, not because it is impossible, but because of the Paradoxes it creates. Imagine that you traveled back in time to visit the compser Ludwig van Beethoven and took with you a cd of his Fifth Symphony. beethoven listens to it (which would be tough considering his deafness) and writes the music down, then later his score is used to record your cd. Where does the music come from? In science, as in life there is no such thing as a free lunch!

Futures What if...
tourists from the future could visit us?

If time machines are possible, it is likely that someone in the future will already have constructed one. After all, in the future there is time to complete even the largest engineering project! even if humans are not up to the task, creatures from other planets may try. So why are we not overrun by visitors from the future?

So far we have managed to avoid being overwhelmed by tourists for the future for good scientific reasons. In fact, time tourisn is unlikely to be a problem: theoretically, time machines have a very limited capacity.

this is the argument used by the famous English pshysicist Stephen Hawking in what he called his "chronology protection conjecture." Like many other scientists, Hawking is troubled by the weird paradoxes of time travel. He argues that the universe simply couldn't allow time travel to happen, because its evolution since the Big Bang cannot be reversed. If the universe were to contract instead of expanding, asks Hawking, would human beings "unevolve" in the same way as they have evolved over millions of years?

A second explanaition for the absence of visitors from the future is that nonr of the time machines envisaged so far lets the voyager go back before the moment the machine was first constructed. So relax. Since no one has built a time machine yet, out-of-time tourists are not a problem!

Technical limitations aside, the "many worlds" theory also solves most ot the paradoxes of time travel. According to this theory, and infinity of universes is constantly being created. In quantum physics, when subatomic particles hace a "choice" of options (such as going through one hole or another in a screen), they select one at random. The "many worlds" theory says that there is a universe for each possible choice made by the particle.

"Many worlds" solves another of the famous time travel paradoxes. Say you went back in time and shot your grandfather before he met your grandmother. Wold you never have been born? If not; you could never have traveled back in time and shot your grandfather? Which means that you were born, so you could have gone back... According to "many worlds," when you go back in time you actually emerge in another universe that develops in parallel to our own. But with an infinity of universes to choose from, how can time travelers ever hope to find their way back to the one they started out from?

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