Time travel is one of the unfulfilled dreams of man. Science is breaking the conceptual as well as practical barriers to one dream after another. Manmade machines can now fly better than birds, computers are the intellectual aid to man and genetics has opened up the possibility of tinkering with life itself. And after conquering the moon, man, with the help of rocket, has sent scientific probes beyond the solar system. The time to travel from one place to another is being shortened with the introduction of faster vehicles on the roads, rail tracks, waterways and in the sky. But all these travels take place in space and not in time. That means one has to follow the arrow of time during any travel in space. The object thus moving appears to be just floating on the stream of time. While travelling with time there is no way one can leapfrog along the river of time either into the future or into the past. Such kind of travel that defies the arrow of time is strictly prohibited in the theories of classical science.
In earlier times, time was thought to be something universal as well as mystical. It was the same everywhere. So, all the local clocks of the universe had to be adjusted with the universal clock that ticked away at a constant pace towards eternity. In a similar vein, the length of time between any two moments was inflexible like the distance between any two points of space. These were immutable and inviolable truths. But Einstein corrected this deeply ingrained notion in human psyche. Time does not flow independently of the space and the objects in motion are not delinked from time. Time, space and material objects in motion are intermingled with one another. The clock of time will run slowly for an object at a very high speed. Time will slow down in a similar fashion for an object flying past a very massive object. In the world of Einstein, time lost its earlier independence. But how then is it possible to violate the inexorable dictates of time and thereby bend it to enable us to travel in the past and the future at our sweet will?
Even Einstein could not stand the idea of time travel as it would create a chaotic universe and violate the law of causality. If time travel was possible, then a person could easily travel into the past, kill his younger self and return into his present unscathed. But then how could a person who was already dead in the past would be living in the present time as his older self? If the causal connection between the past, present and future is true, then time travel should be an impossible proposition.
Let us narrate a few other such instances of the violation of the causal structure of the universe to collapse and thereby create a topsy-turvy world by allowing the free movement of an object in the river of time, whether it is independent as in classical science or intermingled with space in the Einsteinian universe.
Consider the following paradoxes involving time travel. Here is the case of a man with no parents: What happens when you go back in time and kill your parents before you are born? If your parents died before you were born, then how could you have been born to kill them in the first place?
There is also the paradox of the man with no past. For example, let's say that a young inventor is trying futilely to build a time machine in his garage. Suddenly, an elderly man appears from nowhere and gives the youth the secret of building a time machine. The young man then becomes enormously rich playing the stock market, race tracks, and sporting events because he knows the future. Then, as an old man, he decides to make his final trip back to the past and give the secret of time travel to his youthful self. Where did the idea of the time machine come from?
Dr. Michio Kaku, Professor of Theoretical Physics at the City University of New York narrates here a few more mind-bending consequences of time travel. He also tries to look into the hard science and find out if time travel is at all possible with the latest discoveries of theoretical physics.
Here is another paradox of the man who is his own mother. "Jane" is left at an orphanage as a foundling. When "Jane" is a teenager, she falls in love with a drifter, who abandons her but leaves her pregnant. Then disaster strikes. She almost dies giving birth to a baby girl, who is then mysteriously kidnapped. The doctors find that Jane is bleeding badly, but, oddly enough, has both sex organs. So, to save her life, the doctors convert "Jane" to "Jim."
"Jim" subsequently becomes a roaring drunk, until he meets a friendly bartender (actually a time traveller in disguise) who whisks "Jim" way back into the past. "Jim" meets a beautiful teenage girl, then accidentally gets her pregnant with a baby girl. Out of guilt, he kidnaps the baby girl and drops her off at the orphanage. Later, "Jim" joins the time travellers' corps, leads a distinguished life, and has one last dream: to disguise himself as a bartender to meet a certain drunk named "Jim" in the past. So, who is "Jane's" mother, father, brother, sister, grandfather, grandmother, and grandchild?
Before Einstein died, he was faced with an embarrassing problem. Einstein's neighbor at Princeton, Kurt Gödel, perhaps the greatest mathematical logician of the past 500 years, found a new solution to Einstein's own equations which allowed for time travel!
The "river of time" had whirlpools in which time could wrap itself into a circle. Gödel's solution was quite ingenious: It postulated a universe filled with time that flowed like a rotating fluid. Anyone walking along the direction of rotation would find oneself back at the starting point, but backwards in time!
In 1963, Roy Kerr, a New Zealand mathematician, found a solution of Einstein's equations for a rotating black hole, which had bizarre properties. The black hole would not collapse to a point (as previously thought) but into a spinning ring (of neutrons). The ring would be circulating so rapidly that centrifugal force would keep the ring from collapsing under gravity.
The ring, in turn, acts like Alice's Looking Glass. Anyone walking through the ring would not die, but could pass through the ring into an alternate universe. Since then, hundreds of other "wormhole" solutions have been found to Einstein's equations. These wormholes connect not only two regions of space (hence the name) but also two regions of time as well. In principle, they can be used as time machines.
Recently, attempts to add the quantum theory to gravity (and hence create a "theory of everything") have given us some insight into the paradox problem.
In the quantum theory, we can have multiple states of any object. For example, an electron can exist simultaneously in different orbits (a fact which is responsible for giving us the laws of chemistry). Similarly, Schrödinger's famous cat can exist simultaneously in two possible states: dead and alive. So by going back in time and altering the past, we merely create a parallel universe. So we are changing someone ELSE's past by saving, for example, Abraham Lincoln from being assassinated at the Ford Theatre, but our Lincoln is still dead. In this way, the river of time forks into two separate rivers.
But does this mean that we will be able to jump into H.G. Wells' machine, spin a dial, and soar several hundred thousand years into a future of some England?
No, or at least, not right now. There are a number of difficult hurdles to overcome. First, the main problem is one of energy. In the same way that a car needs gasoline, a time machine needs to have fabulous amounts of energy. One either has to harness the power of a star, or to find something called "exotic" matter (which falls up, rather than down) or find a source of negative energy. (Physicists once thought that negative energy was impossible. But tiny amounts of negative energy have been experimentally verified for something called the Casimir effect, i.e. the energy created by two parallel plates.) All of these are exceedingly difficult to obtain in large quantities, at least for several more centuries!
Interestingly enough, Stephen Hawking once opposed the idea of time travel. He even claimed he had "empirical" evidence against it. If time travel existed, he said, then we would have been visited by tourists from the future. Yet we see no tourists from the future. Ergo: time travel is not possible.
However, Hawking, too, has changed his mind, and now believes that time travel is possible (although not necessarily practical).
In conclusion, don't turn someone away who knocks at your door one day and claims to be your future great-great-great-granddaughter. She may be right.