The Order of Time, by Carlo Rovelli.

Billy_T wrote:

Oh, and did you guys know “time” is different up in the mountains than it is at sea level? I didn’t before reading Rovelli.

I tend to pay some attention to physics, and am always glad when someone new joins in.

Yes gravity warps time as well as space. So time moves slower the closer it is to a major massive object and its gravity.

In fact–if we did not know that, then, GPS devices would not work. The satellites that relay info to a GPS device has to calculate the difference between time up there where the satellite is, and time down here on the ground.

from http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion.

Further, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth’s mass is less than it is at the Earth’s surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away. As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.

The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day