Details

The solution to the first traffic paradox lies in the principle of length contraction. Consider what it means for a car to park: the vehicle must come to a halt in the reference frame of the road. This means that every observer, in every frame of reference, will see the relative velocity of the car (and therefore the magnitude of its length contraction) change until it matches that of the parking spaces, revealing whether or not the car can actually park. As it turns out, the cars in this Demonstration are too large to park in the available spaces.

What about the different car-to-parking-space densities observed from a moving reference frame? Their explanation lies in the relativity of simultaneity. In order to determine how many cars are passing each parking space "at the same time", an observer must simultaneously measure the positions of the front and back of each car and each parking space. However, the constancy of the speed of light causes observers in relative motion to disagree about whether or not events separated in space are also separated in time. Therefore, position measurements that a moving car driver makes "at the same time" would appear to happen at different times from the viewpoint of a stationary observer (and vice versa), distorting length measurements and the apparent ratio of cars to parking spaces. In the words of Harvard professor Howard Georgi, "these additional parking spaces are coming in from the past and future".

Looking at moving charges instead of moving cars illuminates the connection between electric and magnetic forces. In a moving frame, observers see an electric force repelling the electrons in each wire; in the rest frame, observers see a magnetic force with the same effect, apparently created by the electric current. Electric and magnetic forces are simply the same force observed in different frames of reference.