How do time dilation and length contraction follow from the constant speed of light?
In the philosophy of physics, time dilation and length contraction are logical consequences of the postulate that the speed of light is a universal constant, independent of the motion of the observer or the source.12
In classical physics, it was assumed that space and time were absolute. However, because the speed of light must remain constant for all observers, Albert Einstein demonstrated that space and time cannot be absolute. He concluded that simultaneity is relative; because observers in different states of motion disagree on which events occur at the same time, they must also disagree on measurements of distance and time intervals.134
To maintain the constancy of the speed of light, the Lorentz transformations dictate that for a moving body:
* Time dilation: Time intervals appear longer to a stationary observer, meaning clocks on the moving body run more slowly.3
* Length contraction: The body appears shorter along its direction of motion.32
These effects ensure that the speed of light remains for all inertial observers, regardless of their relative velocity.3