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?


The truth is told by

both parties.


of the "red shift" phenomenon.

The space police are lurking near the red light, and so they see it as red.

The space car's pilot is travelling very quickly towards the red light, which is shifted towards the blue end of the spectrum. So it doesn't look red.

  • $\begingroup$ Surely, once technology allows for personal vehicles that reach relativistic speeds on interplanetary distances, someone must have managed to integrate a redshift compensator to the windshield, or whatever the corresponding thing is that lets the driver look through to the outside. So the cop should probably fine the driver for driving with a broken compensator :-) $\endgroup$ – Bass Oct 28 at 20:41
  • 1
    $\begingroup$ What driver? Don't they have auto-pilot? $\endgroup$ – Daniel Mathias Oct 28 at 20:51
  • $\begingroup$ @DanielMathias I didn't mention any driver. Anyway, as any fule kno, a driver is a piece of software. $\endgroup$ – Weather Vane Oct 28 at 21:17
  • $\begingroup$ @Bass I hastily assumed this was the first vehicle to use the traffic light, and that the government department entrusted to its development had overlooked the ramifications. They can't even get a pedestrian crossing light on Earth to work properly (the 30-second "suppress" timer is in the wrong part of the sequence: it should be after the light sequence completes, not after the button-push event). $\endgroup$ – Weather Vane Oct 28 at 21:24
  • $\begingroup$ See also what-if.xkcd.com/14. Scroll down to the question by Y. Turniansky. $\endgroup$ – Cloudy7 Oct 30 at 15:58

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