Every one of us wants to do time travel and also does it but only in dreams or through our heart/brain but not physically. Because our brain/heart (called MANN) acting as a time machine moves with an unlimited speed and can take us in our past as well as in future at any moment for a short time period. There are many stories available in Indian mythology about the people who had their control over time. They made that means time dilation possible not mentally but physically also. We know about great character "Bhishma Pitamaha" of Mahabharata time who was gifted with a time control to end his life. Thus, he served his dynasty for many of his following generations but ultimately he felt sorry for the gift due to unwanted sociological and philosophical changes he witnessed during the period.
Einstein through his special theory of relativity gave the concept of time dilation for a body moving comparable with the velocity of light (C=3x1010cm/second), which said to be constant and same throughout the universe and during relative motion also. Time dilation is given as (t= ( (to , where ( = 1/(1-V2/C2) 1/2 and V is the velocity of the moving body and C is the velocity of light with (t and (to as the time intervals w.r.t. the observer and the moving body itself. Under these conditions, if out of two twins of say 30 years of age each, one decides to travel in a spaceship, which is able to move with a velocity such that ( = 30 (i.e., V=1.5x1010cm/sec, about 50% of C) and going to a distant place where the spaceship takes total two years (one for going and one for coming back) by the watch of the traveling twin. The time dilation for him w.r.t. his twin on earth using equation (t= ( (to will be 2x30 = 60 years. When he will come back to earth his twin brother of age 30 years at the time of departure will be 30+60= 90 years where as the traveler twin himself will be 30+2= 32 years only. Earth and Earthlings would have a time traveling of 60 years by that time and many things might have changed for the traveler which he have left behind at the time of his departure.
The time dilation is proved practically not only for elementary particles (moving with a velocity comparable with velocity of light) in the laboratories but also with atomic clocks (capable of measuring very-2 small time difference) by flying them in air jets around the earth. However, twin paradox (as both of them will be moving relative to each other and time dilation should be same for each of them) is explained on the basis that the twin on earth is moving in an inertial frame (constant velocity, ie, condition for special theory of relativity) where as traveler twin in space ship will be in non-inertial frame (ie, accelerated frame) situations at four times (escaping from earth, entering into new place, escaping from new place and re-entering earth). Under accelerated frame situation, general theory of relativity will be applicable which takes into account the effect of gravity also. Hence, the time period of two years for the traveler twin will not be two years for the twin on the earth but it will be 60 years only. Thus, the twin on earth would have lived a life of total 90 years by the time when his twin come from journey of two years and he will be 32 years of age only.
Though, at present, there are practical difficulties in making such time machines (ie, spaceships) moving with a velocity near to velocity of light. But scientists and engineers have not lost the hope and it may be possible in future that people may be enjoying back to the future with such time machines. Of course, any one will like it (time travel) or not due to its sociological and philosophical effects is a matter of time to come but even then people of this purely materialistic world may be interested to experiment such endeavours.
Dr SS Verma, Writer is with the Dept of Physics, SLIET, Longowal