It was in 1905 that Einstein attempted to break the classical notion of time. Before Einstein, time was considered to be absolute. This meant that there is ONE master clock for the universe and everything in it. Two events observed by different observers at the same time must be simultaneous for both observers irrespective of frame of reference.
In his Special Relativity, Einstein introduced relative time. Hee, every observer has their own interdependent clock. the notion of simultaneous observations was broken. But how did Einstein come to think about relative time? He did a though experiment.
The Thought Experiment
Imagine a clock. A clock with two mirrors parallel to each other, separated by distance d. Now put a photon of light in between the mirrors (see figure below). the photon can bounce from one mirror to the other. one oscillation for this clock is when the photon travels from the first mirror to the second and back to the first. this means that the photon travels distance 2d for one oscillation.
Imagine this clock to be at rest. The Photon’s path right now will be a vertical line as first figure of the image above. now imagine putting this clock in a rocket travelling close to the speed of light horizontally. To an observer inside the rocket, the photon’s path is again a straight line. But, to an observer outside, the photons path is zig-zag. This zig-zag line is a much greater distance than the distance of the two mirrors. therefore, the time taken to cover the zig-zag line is much greater than time t (1 oscillation at rest). Basically, the clock has slowed down.
Using basic algebra and Pythagoras, the following eqaution can be derived for time dilation (change in rate of time):
here, T’ is time dilation, t is stationary time, v is velocity of object and c is speed of light squared. as it it can be concluded from the v squared/ c squared part of the equation, time dilation is significant only when v is close to the speed of light c.