Hi

You know how Atomic clocks in orbit have to be reset because bending spacetime or something. What is actually happening? Do the clocks go slower (or faster) in a weaker gravitational field? Is the difference the result of atomic decay being variable in differing gravitational fields?
Also if there is a time difference( say between a clock on Earth and one in orbit) if we were to physically move the clocks so they met in the same gravitational field i.e. the surface of the Earth would the clocks right themselves and become synchronous?

A clock in weak gravitational field will run more quickly than one in a strong gravitational field, all other things being equal. Atoms will decay more quickly in a weak gravitational field. People will age more quickly in a weak gravitational field. Time just runs more quickly in a weak gravitational field.

If you put the clocks side-by-side in the same place, they will run at the same rate. Any differences in their settings will remain, however.

Are you thinking about the GPS Navigation System?
http://en.wikipedia.org/wiki/Global_Positioning_System

in the case of the GPS system, there are at least 4 satellites are always visible from any position on the Earth at any given time (with up to 12 visible).
The satellites each carries an atomic clock that has an accuracy of roughly 1 nanosecond in a given amount of time.

GPS receiver determines its current heading and position by comparing the time signals it picks up from the GPS satellites in orbit. By triangulating on the known positions of the GPS satellites, the receiver can know its positional information with ultra high precision.

in order for this to work, the clock in the GPS satellites must be known to an accuracy within 20 to 30 nanoseconds. Now, because the satellites are constantly moving in relation to people on the ground and each other, there is a discrepancy between the clocks. Special and General theories of Relativity must be taken into account to make sure the clocks stay within acceptable synchronization tolerances.

when looking at a satellite in orbit from the ground, the satellites are in motion relative to the observer (or vice-versa) Special Relativity predicts that we should observe the satellites clocks ticking slower than ours on the ground.
http://www.astronomy.ohio-state.edu/.../Unit5/sr.html
Special Relativity predicts that the clocks on the satellites should fall behind clocks on the ground by roughly 7 microseconds per day. This is because they are ticking slower due to the effects of time dilation due to their relitive motion.


also
satellites in high orbit, like the GPS satellites where the Earth's gravitational field is less than it is at the Earth's surface, General Relativity is asserted and shows that clocks that are on the earths surface will tick away more slowly than the satellites the higher orbits further away from earths gravitational field. and the clocks in orbit will tick faster than the clocks on the ground.
http://www.astronomy.ohio-state.edu/...lackholes.html

when using General Relativity to calculate the clocks in GPS satellites, it is found that the clocks in the high orbit satellite should move ahead of ground-based clocks by ~45 microseconds per day.

when one combines the two relativistic effects where special relativity says that the clocks will tick slower and general relativity says that the clocks will tick faster than the ground based clocks, it is found that the clocks on-board the satellites should tick faster than identical clocks on the ground by about ~38 microseconds per day (45-7=38).

sounds small, but when a system that needs nanosecond information where 38 microseconds is 38,000 nanoseconds, If these effects are not taken into account properly, a navigational position based on the GPS satellite constellation would be in error after 2 minutes. The errors would accumulate to about 10 kilometers per day.

http://www.losangeles.af.mil/library...et.asp?id=5311

Link removed

http://en.wikipedia.org/wiki/Effects...ativity_on_GPS

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-work in progress--

assume this, you and your friend have identical synchronized atomic clocks. you are on board the satellite with your atomic clock. and you friend is on the ground with his atomic clock. both you and him are staring at each others atomic clock with a telescope.

General Relativity shows that when you look at your friends clock back on earth through your telescope from orbit, his clock is moving slower than yours. He is seeing that your clock is moving faster.
the gravitational field of earth is stronger at your friends location which actually makes the frequency of time move slower. so his clock is ticking away slower than yours.

when you meet up with each other and compare the clocks, you will see that his clock setting is indeed different than your clock. while together at the same location again the clocks are ticking away at the same speed, your clock shows that more time has passed.

in Special Relativity, the difference is due to relative motion. taking the gravitational aspect away, if you are just looking at each others clocks and are not moving relative to each other, the clocks will stay synchronized.
but as the two of you move in relation to each other, you will notice each others clocks slow down as compared to each others.

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-work in progress--

Good detail, Sabianq.

A weaker gravitational field causes them to go faster, but their orbital velocity causes them to go slower.

No. For the GPS system the various computers keep track of the ongoing cumulative error, and that error is taken into account during all calculations. Last time I checked, it was about 29 seconds, total, since they launched the fleet nearly two decades ago.

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If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

The clocks are continually re-synched. What is the 29 seconds to which you are referring?

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Long time no see, swansont. Good to see you back.

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Thanks. Work's been keeping me busy, but I'm now on vacation, so I decided to drop in.

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