Einstein based his entire theory of relativity on two simple assumptions, one about predictability of optical experiments irrespective of lab conditions and the other about constancy of velocity of light in fundamental system/empty space.
Thereafter, Einstein fantasized a space ship travelling along a straight line for further construction of his theory. He assumed a scenario in which space ship can be potentially accelerated to the velocity of light.
It is a common experience that when matter is heated by supplying energy to its atoms and molecules, it results in increase of kinetic energy of its atoms and molecules and increasing kinetic energy of atoms and molecules is associated with not only increasing temperature but also increasing velocity of constituent atoms and molecules. This increasing velocity of atoms and molecules is necessarily associated with change of one or the other kind. Ensuing change may be physical, chemical or nuclear reaction and may be essentially nonlinear in character. Physical change often consists of nonlinear change of state from solid to liquid to gas to plasma state and may be even thereafter. Appropriate chemical reactions, again essentially nonlinear in character commonly occur depending on nature of interacting entities and conditions of the experiment. Very high velocities of atoms and molecules are associated with thermonuclear reactions such as nuclear fusion of Hydrogen into Helium at 50,000Â°K.
Exact nature of change can only be empirically known and is rationally unpredictable. In most general terms, nature of change can be attributed to nature of interacting entities.
But Einstein arbitrarily assumed that increasing velocity of the said space ship shall lead to reduction in speed of time measuring devices such as clocks and watches in order to compensate effects of increased velocity. Einstein stated "A moving clock, no matter what its construction, runs slower than an identical clock that is at rest." Einstein never bothered to explain as to why frequency of a pendulum in linear motion changes when in the equation of simple pendulum length of the pendulum and gravity are the only two variables determining its frequency/time period. Moreover why should errors of observation determine behavior of the observed, i.e. why incorrect recording of time by clocks and watches should have a bearing on behavior of time itself and properties of matter?
Einstein's arbitrary conclusions were extended to Biological domain leading to speculation that one day using a device designated as time machine; it shall be possible to travel backward in time.
Time is linearly progressive. This is the inherent nature of time. No experimental conditions or errors of observation can have a bearing upon intrinsic nature of time. Therefore, it shall never be possible to travel backward in time. Hence time machine is nothing more than science inspired fantasy which shall never materialize.
Backward travel of time by any stretch of imagination, if ever becomes feasible than it is bound to lead to many absurd situations and universal chaos. Imagine the following scenarios:
1. A time traveler goes back to times of Jesus and is able to persuade Pontius Pilate not to crucify Jesus. Consider its impact upon subsequent world history and turmoil it will create in the contemporary world.
2. A time traveler in India travels back to Mahabharata period and is able to persuade Pandavas not to indulge in the game of chess with Kauravas or is able to persuade Duryodhana not to insult Draupadi by removing her saree. Alternatively, another time traveler is able to persuade Duryodhana to justly give Pandavas their share in the kingdom. Imagine impact of these backward journeys in time upon subsequent Indian history and changes it will lead to, in contemporary Indian society.
3. A person meets an accident, fractures several bones, is put in time machine, backward travel in time restores his health. A novel technical innovation, yet to be discovered, shall be used to rehabilitate the person in contemporary world. This technology may be employed to treat a large number of ailments.
4. A person has left for his heavenly abode; his dead body is put in time machine, the person comes to life and is than rehabilitated in the contemporary world by some novel technology yet to be discovered.
5. Consider impact on world history of abortion of Hitler's mother while Hitler was still in the womb!
Clearly, there can be countless number of such examples providing ample food for thought to science fiction writers for infinite generations to come. But it is self-evident that the mere feasibility of backward travel in time is bound to lead to countless absurdities that would disrupt natural order and precipitate universal chaos. Such universal chaos is bound to be in conflict with universal instinct to exist. This will jeopardize existence of universe itself of which we all are a part.
THEREFORE NATURE CAN'T AFFORD TO FUNCTION IN A MANNER THAT HAS THE POTENTIAL OF BACKWARD TRAVEL IN TIME.
Hence, backward travel in time is nothing but a pure fantasy of scientific origin devoid of any foundations in fact and logic. However, this is without prejudice to rational predictability of past based on our understanding of natural laws and intrinsic nature of things and events.