The theory you are referring to is Einstein's theory of relativity, specifically the concept of time dilation. The theory of relativity consists of two major parts: the theory of special relativity and the theory of general relativity.
In special relativity, Einstein introduced the idea that time is not absolute but instead is relative to an observer's frame of reference. He proposed that the laws of physics are the same for all observers in uniform motion relative to each other. This theory led to the famous equation E=mc², which describes the equivalence of energy (E) and mass (m) and the role of the speed of light (c) as a fundamental constant.
General relativity, on the other hand, describes gravity as the curvature of spacetime caused by the presence of mass and energy. According to this theory, massive objects, such as planets or black holes, can bend and distort the fabric of spacetime, affecting the motion of other objects in their vicinity.
The aspect of Einstein's theory of relativity that relates to time travel is the phenomenon of time dilation. Time dilation occurs when an object or observer experiences a difference in the passage of time compared to another object or observer due to relative motion or gravitational effects. Essentially, time can pass at different rates for different observers depending on their relative motion or gravitational fields.
This time dilation effect has been experimentally confirmed and is used in various applications, such as in the Global Positioning System (GPS), which relies on precise time measurements. However, the concept of time travel to the past or future as depicted in popular fiction requires significant advancements beyond our current scientific understanding and technological capabilities. While the theory of relativity provides a framework for understanding the possibility of time travel, no scientific evidence or consensus currently supports the existence of practical time travel.