Question

can someone explain why traveling forward in time is possible but traveling back in time (reversing time) is not possible? (using Einstein’s special theory of relativity)

Answer 1

Time travel is a concept of travelling between two points in time, analogous to that of travel in distance between two points in space. It is possible with the help of a time machine. It is not certain if time travel to the past is physically possible. Forward time travel, is an extensively observed phenomenon and well-understood within the framework of special relativity and general relativity. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current methods.

There is a great deal of observable evidence for time dilation in Einstein's special theory of relativity.The theory of relativity states that the speed of light is invariant for all observers in any frame of reference; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light which states that it may be regarded in a limited sense as time travel into the future, that is a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere.For two identical clocks moving relative to each other without acceleration each clock measures the other to be ticking slower. This is possible due to the principle of simultaneity.

Also, if the motion is time-like, we know that for 4D cone, the space-time equation can be written as:-

r2-c2t2<0

where r= position between two points in (x,y,z) coordinates.

c= velocity of light

t= time interval between two events

Thus from this t should be positive, indicating that we can travel forward in time, but not backward. Thus, backward time traversal is impossible in Special theory of relativity.