TIME TRAVEL

Shaping the Futureby Neil Johnson
The idea of travelling forward into the future or back into the past has always fascinated science fiction writers. The 'grandfather paradox' is the argument many people use to suggest that time travel is impossible. What if you went back in time and prevented your grandfather from meeting your grandmother so that your mother was never born? Then you would never have been born... and so on.
Until very recently such arguments led most scientists to believe that time travel could never exist outside science fiction. But amazingly, some interpretations of the weirdness of the quantum world now suggest that time travel is possible - at least in theory.
Gravity and black holesEinstein's theory of relativity brought space and time together in a single, four-dimensional arrangement that he called spacetime. We know that we can travel forwards, backwards and sideways in space, so why not forwards and backwards in time?
Four dimensions are difficult to imagine, so physicists usually suggest you think of spacetime as a rubber sheet stretched out flat. If there are no large masses around, the sheet stays flat, and so any object placed on it will move around in straight lines. But a large mass, such as the Sun, makes a dip in the sheet because it actually warps spacetime. Now any other object with smaller mass, like our Earth, moving about in spacetime rolls into the dip as it comes past the Sun. It appears 'attracted' to the large mass. This effect of warping spacetime is what gives rise to gravity.
The Universe is full of heavy objects exerting gravitational effects and the net result is that spacetime is not flat at all but curved. Everything, including light, has to follow curved paths in spacetime. We know Einstein was right about this because astronomers can sometimes see distant stars that ought to be masked by nearer objects such as the Sun. Instead of travelling in straight lines and hence being blocked, the light from the stars bends round the obstruction.
When a star reaches the end of its life it may collapse inwards under the influence of its own gravity to such an extent that all its matter becomes concentrated into an extremely dense object a fraction of its original size. This is a black hole. Black holes have such a huge gravitational pull that nothing can escape from them, not even light. We cannot see them but we have good evidence that they exist. We can see stars behaving in ways which suggest that they are being pulled about by a nearby invisible object with enormous mass.
What does a black hole do to space-time? Relativity predicts that at the centre of a black hole is an infinitely dense point, called a singularity, within which all the normal laws of physics no longer apply. Time, space, matter and energy no longer have any well-defined meaning. Einstein's equations show that such a singularity doesn't just make a dip in the imaginary rubber sheet of space-time; it makes a tunnel that goes right through and momentarily opens out on the other side.
Where is 'the other side'? It could be somewhere else in space-time, either in the future or in the past, or it could even be in another Universe! Supposedly it was Dr Who's Time lords who first harnessed the power of a black hole to begin their experiments in time travel. If you could take a spaceship through such a tunnel, or wormhole, you would have discovered the secret of time travel. This is of course impossible with today's technology. But in the future, who knows?
Mini wormholesEinstein's equations describe a space-time that is perfectly smooth, like the rubber sheet. His theory of relativity only deals with the physics of what happens on big scales. It cannot deal with what happens at the centre of a black hole, or what happened during the moment of the Big Bang at the birth of the Universe when spacetime itself was infinitesimally small. That takes us back into the world of quantum physics.
If you could look at spacetime with a magnifying glass so powerful that it reached down to the quantum scale, you would not see the smooth, continuous sheet of Einstein's spacetime. Just as a foam rubber ball looks smooth from a distance but rough and ragged close up. In this picture of spacetime it is quite likely that tiny holes could open up, entrances to little tunnels between now and other times, or between here and other universes. Another option for future time travelers would be somehow to harness these tiny wormholes and expand them.
Many worlds, many futures?To return to the question that has puzzled thinkers since Newton's day, is the future preordained? Or are there an infinite number of futures? One way of looking at the quantum world suggests that not only are there an infinite number of futures, but they are realised in an infinite number of universes.
Photons and electrons sometimes behave as waves and sometimes as particles, but never both at the same time. So far, the argument for interference between one universe and another applies only to events occurring at the quantum level.
But the idea of parallel universes provides a possible resolution to the 'grandfather paradox' that might otherwise cause problems for time travellers. If we travel back in time and change history, we launch ourselves into a new future in a parallel universe - but we have no effect on the present one from which we started out.
Scientists of the future may well pursue a new form of futuristic technology based on quantum effects. Such applications could include quantum teleportation, by which a quantum particle can be teleported from one point in space to another; and quantum computation, where calculations can be carried out which would take many years on a conventional computer. Although we now know how to measure time very accurately, have we come any nearer to answering the basic question 'What is time?'.
Neil Johnson is a Physics lecturer at Oxford University where he heads his own research group.
Source: http://www.bbc.co.uk/science/space/exploration/timetravel/
Brian's Views on Time Travel
and Interdimensional Voyages

Time travel is no longer regarded as strictly science fiction. For years the concept of time travel has been the topic of science fiction novels and movies, and has been pondered by great scientists throughout history. Einstein’s theories of general and special relativity can be used to actually prove that time travel is possible. Government research experiments have yielded experimental data that conclusively illustrate that fast moving aircraft have traveled into the future. This phenomenon is due to the principal of time dilation, which states that bodies moving at high velocities experience a time that ticks slower than the time measured at zero velocity.3 Not as much time elapses for a moving body as does for everything else. Phenomena known as wormholes and closed timelike curves are possible means of time travel into the future and the past.4 Traveling into the past is a task which is much more difficult than traveling into the future. This feat has not yet been accomplished -to our knowledge- and its theory involves complicated scenarios of tears in four dimensional space-time, and traveling near the speed of light. Obstacles which prevent our hubris attempts to cheat time include our inability to move even close to the speed of light, and finding a source of energy as powerful as an exploding star. Simply because the proposal of time travel is backed by scientific theory, is no reason to expect that it is easily achievable. Numerous arguments are proposed that that prevent time travel into the past. Both common sense and scientific fact can be used to paint scenarios that become serious obstacles. Not to fear, we have all the time in the world to overcome these minor limitations.
Imagine if you will, that you are one of the people sill alive today that was born prior to 1903, when the first airplane took flight. When you were young the idea of flying would probably have been quite exciting. Some scientists believe that we may presently be living through an identical scenario. The thing that would be so exciting however, would not be flight, but time travel. Leading scientists believe that our children will live to once again see the impossible become routine. Professor Michio Kaku of the University of New York believes that space flight may one day unlock the secret of time itself. This will require the development of spacecraft that can travel at speeds on the order of two hundred million meters per second, that’s about four hundred and fifty million miles per hour. Craft traveling at this speed will take us near the speed of light, where time actually slows down. This is what’s known as time dilation. Einstein’s theories predict that the faster a spacecraft moves, the slower time ticks inside of it. Imagine that a rocket ship takes off from earth and approaches the speed of light. If we were to watch it from earth with a very powerful telescope as it traveled away from us, we would see everyone inside the ship as being frozen in time. To us their time would slow down, but to them nothing would change! This has been measured in the laboratory and on location using atomic clocks, aircraft, satellites and rockets. It is proven that time slows down the faster you move. In 1975 Professor Carol Allie of the University of Maryland tested Einstein’s theory using two synchronized atomic clocks. One clock was loaded on a plane and flown for several hours, while the other clock remained on the ground at the air base. Upon return, the clock on board the plane was found to be ever so slightly slower that the one on the ground. This was not due to experimental error, and has been repeated numerous times with the same result. This difference in time is even more pronounced in satellites such as the space station. This is because these objects are traveling at speeds much faster and for much longer periods than possible in an airplane. The faster an object moves, the more time is distorted.

Now that we know that it is possible to travel into the future by moving at great speeds, the next problem is how to travel in time a respectable amount without having to sit in a fast moving spaceship for years. This problem is solved by the theoretical existence of what are know as closed timelike curves, and wormholes.

Einstein’s special and general theories of relativity combine three-dimensional space with time to form four dimensional space-time.2 Space-time consists of points or events that represent a particular place at a particular time. Your entire life thus forms a sort of twisting, turning worm in space time! The tip of the worm’s tail would be your birth and its head is the event of your death. The line which this worm creates with its body is called that object’s worldline. Einstein predicts that worldlines can be distorted by massive bodies such as black holes. This is essentially the origin of gravity, remember. Now if an object’s worldline were to be distorted so much as to form a loop that connected with a point on itself that represented an earlier place and time, it would create a corridor to the past! Picture a loop to loop track that smashes into itself as it comes back around. This closed loop is called a closed timelike curve. Timelike means that the body under consideration experiences time that increases in one direction along its worldline.2 Princeton University physicist John A. Wheeler, and Kip S. Thorne of Cal. Tech. have shown that a closed timelike curve is one way to create a kind of shortcut through space-time called a wormhole.2

Wormholes are holes in the fabric of four dimensional space-time, that are connected, but which originate at different points in space and at different times. They provide a quick path between two different locations in space and time. This is the four dimensional equivalent of pinching two pieces of a folded sheet of paper together to make contact across the gap. Distortions in space cause the points separated by the gap to bulge out and connect. This forms a wormhole through which something could instantaneously travel to a far away place and time.4 No more problems of traveling in a rocket ship for years to get into the future! This is essentially what was written about in "Alice in Wonderland’s Through the Looking Glass." Her looking glass was a wormhole that connected her home in Oxford, with wonderland. All she had to do was climb into her looking glass and she would emerge on the other side of forever. In reality however, it would require a much more elaborate scheme to create a wormhole that connects two different points in space-time. First it would require the construction of two identical machines consisting of two huge parallel metal plates that are electrically charged with unbelievable amounts of energy. When the machines are placed in proximity of each other, the enormous amounts of energy -about that of an exploding star- would rip a hole in space-time and connect the two machines via a wormhole. This is possible, and the beginnings of it have been illustrated in the lab by what is known as the Casimir Effect. The next task would be to place one of these machines on a craft that could travel at close to the speed of light. The craft would take one machine on a journey while it was still connected to the one on earth via the wormhole. Now, a simple step into the wormhole would transport you to a different place and a different time.

Wormholes and closed timelike loops appear to be the main ways that time travel into the past would be possible. The limitation on this time travel into the past is that it would be impossible to travel back to a time before the machine was originally created. Although the aforementioned theories of general relativity are consistent for closed timelike curves and wormholes, the theories say nothing about the actual process of traveling through them. Quantum mechanics can be used to model possible scenarios, and yields the probability of each possible output. Quantum mechanics, when used in the context of time travel, has a so-called many-universe interpretation. This was first proposed by Hugh Everett III in 1957.3 It encompasses the idea that if something can physically happen, it does in some universe. Everett says that our reality is only one of many equally valid universes. There is a collection of universes, called a multiverse. Every multiverse has copies of every person, structure, and atom. For every possible event, every possible outcome is said to be played out on a different universe. This interpretation of quantum mechanics is quite controversial however, but does elicit the notion that it may be impossible to travel backward in time to our own universe or dimension. One must consider what past would be the destination of a time traveler. The notion that time travel could link parallel universes, has been anticipated in science fiction novels, and is even depicted in the popular television series "Sliders." In this program, a "sliding machine" creates a wormhole that links two parallel dimensions. Each week the group of "sliders" jump into the wormhole and emerge in the same place and time, but a different dimension. They can run into their other selves and experience a reality that has yielded a vastly different society than their own. The interesting thing is that the stuff of science fiction, can be deduced from existing physical theory. All the claims made about time travel are consequences of basic scientific laws and standard quantum mechanics.

The proposal of time travel is backed by scientific theory, but that is not enough to make it realistically possible. Numerous arguments are proposed that that prevent time travel into the past. Both common sense and scientific fact construct serious obstacles. A major argument against time travel into the past is called the autonomy principle, better know as the grandfather paradox. This paradox is created when a time traveler goes back in time to meet his or her grandfather. Now upon their introduction it would be possible to change the course of events that lead up to your grandfather and grandmother marrying. You could tell him something about a family secret to convince him you are who you say you are, and he may proceed to tell his soon to be wife. She may in turn doubt his sanity and have him committed. Thus your grandparents would never have your mother, and therefore you couldn’t be born! But then how could you have ever existed to travel back in time if you don’t exist? You would have had to have been created via autonomy. The next question would be, if your mother was never born, then when you return to the future would anything you did in your life exist? Or would you, your friends, your home etc. never have existed? This is clearly an inconsistency paradox that would rule out time travel, yet interestingly enough the laws of physics do not forbid such excursions. The multiverse concept eradicates the problem of the autonomy principal, because it allows time travel to the past, but to a different universe. You would meet the person who was your grandfather in your universe, but never married your grandmother in his universe. In the universe that you traveled to, you never existed.

Another argument of impossibility is called the chronology principal. This principal states that time travelers could bring information to the past that could be used to create new ideas and products. This would involve no creative energy on the part of the "inventor." Imagine that Pablo Ruiz y Picasso, the most influential and successful artist of the 20th century, were to travel back in time to meet his younger self. Assuming he stays in his correct universe, he could give his younger self his portfolio containing copies of his paintings, sculptures, graphic art, and ceramics. The young version of Picasso could then meticulously copy the reproductions, profoundly and irrevocably affecting the future of art. Thus, the reproductions exist because they are copied from the originals, and the originals exist because they are copied from the reproductions. No creative energy would have ever been expended to create the masterpieces! 3 This chronology principal rules out travel into the past.

A notion that was once nothing more than science fiction, is now a concept that’s becoming reality. Einstein’s theories of general and special relativity can be used to actually prove that time travel is possible, and research has shown that fast moving craft can travel into the future. Time dilation is the easiest method because it merely requires high velocity motion to experience time travel.3 Phenomena known as wormholes and closed timelike curves are possible means of time travel into the future and the past.4 Traveling into the past is a task which is much more difficult however. Its theory involves complicated scenarios of tears in four dimensional space-time, energy equivalent to that of an exploding star, and traveling near the speed of light. Both common sense and scientific fact can be used to paint scenarios that become serious obstacles. Yet even these hindrances can be explained away! If the multiverse concept is reality, then most present ideas of time travel are based on a false reality. If time travel is completely impossible then the reason has yet to be discovered.

Source: http://www.iit.edu/~bosabri/time.html

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