These guys occur but why?

Question

We all know that if we consider the assumption of Einstein that is no material object can travel faster than light to be true then two effects that are Length Contraction And Time Dilation seem to be protuding out of this idea. But why this effects were necessary to come into picture? And how this (time dilation and length contraction) even related with the assumption of our impossiblity to travel faster than light?

Answer 1

Length contraction and time dilation are a consequence of the speed of light being the same for everyone.

Imagine standing with a friend who sends out a pulse of light to their left and right. If you decide to walk in the direction of the right hand pulse then at any given instant it will seem to your friend that the pulse has travelled slightly further than it seems to have travelled from your perspective. Likewise, the left hand pulse, which you are walking away from, will seem at any instant to your friend to have travelled slightly less distance than it has travelled from your perspective. Since you both see the speed of light to be the same, the only way you can account for the fact that light has travelled different distances in the two directions is by disagreeing how much time has passed in each direction.

From your perspective, clocks in the frame of your friend are all running out of synch- the further away from your friend in your direction of travel, the more the clocks in your friend's frame are running ahead of time. Conversely, the further away from your friend in the opposite direction, the more the clocks in your friend's frame are running behind time.

The fact that you and your friend disagree on what time it is at any place along your direction of motion is the cause of both time dilation and length contraction. If your friend measures the length of an object in your frame, from your perspective she is measuring the position of the leading edge of the object before she measures the position of the trailing edge, which leads her to underestimate the length of the object. Likewise, if one of your clocks is compared against two clocks in your friends frame, it will seem to be running slow, because the second clock it passes is running ahead of the first.