In a distant future, human beings are travelling between planets and even solar systems with ease. Their advanced technology allows them to drive their own space-cars, which have a cruising speed of 20% of the speed of light.
Due to the increasing amount of traffic and the danger of collision, an officer of the space traffic agency (STA) came up with the idea of setting up a traffic light at a highly frequented space road crossing. Whenever the traffic light shows red, space cars have to stop in front of them. When they show a green light, the way is free.
The STA celebrates their first space traffic light with a big opening ceremony. But when the first space car approaches the crossing, it does not stop. It just runs over the red light at full speed. Upset about this shameless act of ignorance, two STA cops rush after the vehicle, catching it at the next planet. They confront the driver:
Cops: You just ignored a red traffic light!
Driver: I would never do so, the traffic light was green!
Who of them tells the truth, and why?