[PatrickHenry]

I would expect that two perfect clocks, one at sea level, one at 175,000 ft orbit would read one second different after about 86 years (the clock at sea level lagging the one in space). Has this difference ever been measured?

Clocks do slow down due to gravity, as they also do when accelerated. Yes, this has been measured, and the effect is exactly as predicted by special relativity. I think synchronized clocks (atomic clocks, not your everyday alarm clocks) will diverge enough to be detected when one is at ground level and the other is taken to the top of a tall building.

[PatrickHenry]

This is the classic demonstration: Hafele-Keating experiment.

[Southack]

"Clocks do slow down due to gravity, as they also do when accelerated. Yes, this has been measured, and the effect is exactly as predicted by special relativity."

Special Relativity and Gravity...are you certain?

[Calvin Locke]

PBS ran a series in honor of Einstein's 100th, and the US Navy did indeed take a matched pair of cesium clocks, and fly
one of them around for a while, and yes, the one in the air "slowed".

I didn't think of using the Earth's rotation at different heights to do the experiment. Pretty constant v for each clock...

[UnbelievingScumOnTheOtherSide]

I think synchronized clocks (atomic clocks, not your everyday alarm clocks) will diverge enough to be detected when one is at ground level and the other is taken to the top of a tall building.

Unless the tall building is at the north or south pole, you have to account for the fact that the top of the building is moving faster than the bottom as earth's rotation carries it through a larger arc even though they are not moving with respect to each other or you are counting the same effect twice. Right? (If the building were were at the equator and 2.561 billion miles high, the top would be moving at the speed of light requiring infinite gravity to hold it.)