Time travel has always been a topic of great mystery which provokes many debates and a lot of interest. It inspires scientists, writers and common people and is an inexhaustible source for researching. Many have tried to find the answer to all questions this subject poses and the debate is still on.
Basics:
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to some moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate).
Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible, given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox), as well as gravitational time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel. [1]
Talk about time travel is puzzling even if it isn’t obviously contradictory. Philosophers however are divided about whether time travel involves empirical paradox or some deeper metaphysical incoherence. It is suggested that time travel requires a Parmenidean four-dimensionalist metaphysical conception of the world in time. The possibility of time travel is addressed (mainly) from within a Parmenidean metaphysical framework, which is accepted by David Lewis in his defence of the coherence of time travel. It is argued that time travel raises formidable difficulties which are not satisfactorily resolved by Lewis’s ingenious defence of time travel. Objections to time travel considered include: (1) travel to other times is impossible because there is nowhere (or “nowhen”) to go to; (2) the problem that upon setting out on a journey to the past a time machine will collide with itself; (3) time travel generates a mysterious temporal dualism between experiential time and physical time; (4) travel to the past permits reverse causation, raising the possibility of causal loops and attendant problems arising, for example, from the prospect of empirical contradiction and the possibility of someone being one of their ancestors. [2]
Once Einstein said that the problem of the Now worried him seriously. He explained that the experience of the Now means something special for man, something essentially different from the past and the future, but that this important difference does not and cannot occur within physics. That this experience cannot be grasped by science seemed to him a matter of painful but inevitable resignation. I remarked that all that occurs objectively can be described in science; on the one hand the temporal sequence of events is described in physics; and, on the other hand, the peculiarities of man’s experiences with respect to time, including his different attitude towards past, present, and future, can be described and (in principle) explained in psychology. But Einstein thought that these scientific descriptions cannot possibly satisfy our human needs; that there is something essential about the Now which is just outside the realm of science. We both agreed that this was not a question of a defect for which science could be blamed, as Bergson thought.[3]
The thoughts and revelations of Einstein triggered and encouraged many theories about spacetime and time travel.
Modern Spacetime Theories
Virtually all modern spacetime theories are now built in the same way. The theory posits a manifold of events and then assigns further structures to those events to represent the content of spacetime. A standard example is Einstein’s general theory of relativity. As a host for the hole argument, we will pursue one of its best known applications, the expanding universes of modern relativistic cosmology.
The Freedom of General Covariance
When Einstein first introduced his general theory of relativity in the 1910s, its novel feature was its general covariance. It was the first spacetime theory in which one was free to use arbitrary spacetime coordinate systems. This feature is now shared by virtually all modern formulations of spacetime theories, including modern versions of special relativity and Newtonian spacetime theory. It its original form, general covariance was understood “passively”; that is, as a freedom to describe structures in spacetime by means of arbitrarily chosen coordinate systems. That freedom proves to be equivalent to another freedom, known as “active” general covariance. According to active general covariance, we are licensed to spread geometrical structures like metrical fields over the manifold in as many different ways as there are coordinate transformations. [4]
Recent years have seen a growing consensus in the philosophical community that the grandfather paradox and similar logical puzzles do not preclude the possibility of time travel scenarios that utilize spacetimes containing closed timelike curves. At the same time, physicists, who for half a century acknowledged that the general theory of relativity is compatible with such spacetimes, have intensely studied the question whether the operation of a time machine would be admissible in the context of the same theory and of its quantum cousins. A time machine is a device which brings about closed timelike curves—and thus enables time travel—where none would have existed otherwise. The physics literature contains various no-go theorems for time machines, i.e., theorems which purport to establish that, under physically plausible assumptions, the operation of a time machine is impossible. We conclude that for the time being there exists no conclusive no-go theorem against time machines. [5]
Time Travel in Literature:
Time travel has been a well exploited topic in literature. It ranges from the the Mahabharatha from ancient Hindu mythology to famous Charles Dickens’s novella A Christmas Carol which depicts Ebeneezer Scrooge being transported back and forth in time to points in his own lifetime by a series of ghosts to visit Christmases Past, Present and Future and many others. However, the one literary work which left maybe the biggest mark on talks about time travel is The Time Machine is a science fiction novella by H. G. Wells, published in 1895 for the first time and later adapted into at least two feature films of the same name, as well as two television versions, and a large number of comic book adaptations. It indirectly inspired many more works of fiction in many media. This 32,000 word story is generally credited with the popularisation of the concept of time travel using a vehicle that allows an operator to travel purposefully and selectively. The term “time machine”, coined by Wells, is now universally used to refer to such a vehicle.[6]
Conclusion:
Time machines may never see daylight, and perhaps so for principled reasons that stem from basic physical laws. But even if mathematical theorems in the various theories concerned succeed in establishing the impossibility of time machines, understanding why time machines cannot be constructed will shed light on central problems in the foundations of physics. [7]
Regardless of the obstacles that scientists meet in present time people will most definitely continue to research time travel as human curiosity has always been the strongest initiator.
References:
[1] http://en.wikipedia.org/wiki/Time_travel
[2] William Grey (1999). Troubles with Time Travel. Philosophy, 74, pp 55-70
[3] Carnap 1963, pp. 37-38
[4] http://www.illc.uva.nl/~seop/entries/spacetime-holearg/index.html#ModSpaTheBegGui
[5] Earman, John and Wüthrich, Christian, “Time Machines”, The Stanford Encyclopedia of Philosophy (Winter 2010 Edition), Edward N. Zalta
[6] http://en.wikipedia.org/wiki/Time_travel_in_fiction
[7] Earman, John and Wüthrich, Christian, “Time Machines”, The Stanford Encyclopedia of Philosophy (Winter 2010 Edition), Edward N. Zalta
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