As non-intuitive as it may seem, length contraction and time dilation always manage to guarantee that the speed of light is the same no matter what frame we are viewing it in. Because relativistic effects are caused by relative motion, as soon as the car slows down, your friend would see its length and time return to normal.Įinstein's theory of Special Relativity, which has been mainstream science for over a century, is essentially a mathematical formulation of length contraction, time dilation, and the speed of light in vacuum being the same in all frames. In fact, to you in the car, your friend on the ground is the one that is moving and therefore you see him as length-contracted and time-dilated. To you in the car, you see your car as at rest and therefore it looks normal, it is not squashed and does not have its clock running slow. Note that these effects are caused by the relative motion. But this contraction does not hurt you or the car because it is space itself that is contracting, so the car experiences nothing different. The moving car actually gets squashed front to back. This relativistic effect is known as length contraction. Additionally, the space of a moving reference frame, and the objects in that space, become contracted in the direction of motion relative to the rest frame. This relativistic effect is known as time dilation. How is that possible? When a frame of reference goes very fast (close to the speed of light) relative to a rest frame, its time slows down as observed by someone in the rest frame. Your friend at rest on the ground would also measure the light traveling at speed c. To you in the speeding car, the light would be traveling away at speed c. If you drove a car close to the speed of light relative to the ground (neglect air effects) and turn on the headlights, light would leave your headlights at speed c the way it always does. Light in vacuum always travels at the same speed c, exactly 299,792,458 meters per second, no matter how it is created or in what frame it is observed. Public Domain Image, source: Christopher S.