Robert McNees
Friends, here is a fun little physics story about satellites and relativity. It’s a day late for an #OTD, but please indulge me.
The Navigation Technology Satellite 2 (NTS-2) was launched into orbit on June 23, 1977, an early step in establishing the GPS NAVSTAR network. It was the first satellite to carry a Cesium atomic clock into orbit! 🧵
Image: Institute of Navigation
Robert McNees
By 1977, scientists knew that relativity would cause orbiting and ground-based clocks to beat out of sync.
Two competing effects are at work. The satellite moves rapidly in its orbit, so time dilation makes it tick more slowly compared to a ground-based clock. But clocks higher up in Earth's gravitational field should tick more rapidly than Earth-bound clocks.
Special relativity is trying to slow the satellite clock down relative to the ground, general relativity says it should tick faster.
Robert McNees
Calculating the fractional frequency shift of the satellite clock compared to the ground clock is a homework problem in GR textbooks.
Orbiting at 4.2 Earth radii, SR slows it down by ~ 250 parts in 10¹². The GR effect speeds it up by ~ 697 parts in 10¹².
General relativity produces the larger effect, and the orbiting clocks runs ahead of the earthbound clock by around 446 parts in 10¹². That’s an extra 38,500 nanoseconds per day.
This is a lot for GPS, which requires really precise timing.
Robert McNees
Anyway, folks working on the project weren't entirely sure which effect would dominate.
I'm not sure why there was confusion (engineers).
And apparently there were also people involved with the project who insisted that the effects weren't real, and needn't be accounted for (retired engineers).
Robert McNees
So they decided to just let the clock run in orbit for 20 days and see how it compared to a ground clock.
Once they were sure of the effect, a device installed on the satellite could tune the clock to tick faster or slower, in sync with ground-based clocks.
After 20 days of run time, the satellite clock was ahead of an identical ground-based clock by about 442.5 parts in 10¹². It was a lovely test of special and general relativity: less than 1% off from the predicted value!
Robert McNees
And the people on the project who suggested that relativity wasn't real and could be ignored felt like big dummies.
Greg Whitehead
@mcnees hah, as a retired engineer this made me laugh ;-)
Robert McNees
@grwster 😉
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