GPS & Relativity: Thought Experiment

[August1991]

The first time that I saw a GPS device was on the ferry from Port-aux-Basques to North Sydney. A weird American tourist had this device in his hand and showed me how the captain was steering the boat into port. The screen was monochrome. I was intrigued more than amazed. (It was the mid-1990s.)

The first stand-alone GPS device that I bought (a Mio) was in America (around 2000 or so, at a Best Buy in South Carolina). I was more amazed that this little box knew all the streets of all Europe/America. Since the Mio showed me which satellites were available and signal strength, I understood the process.

A little known fact is that GPS requires both the Specific and General Theory of Relativity.

To know an exact position, a GPS device depends on precise time signals - from several satellites. Since the satellites are travelling at "high speeds" but sending simultaneous signals, the signals arrive at "different times" at a GPS device. Moreover, Earth's gravity alters these signals - depending where a particular satellite happens to be.

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So, here's my thought experiment.

Imagine that Lorentz had never lived, and Einstein had never solved the confusion of the Michelson-Morley experiment. Today, we would just still be confused about the speed of light.

We would not have GPS. But we would have been capable of putting a man on the moon.