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<blockquote data-quote="freyar" data-source="post: 6688589" data-attributes="member: 40227">Well, that's quite a few duplicate posts! No saving time or space there... <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=";)" title="Wink&nbsp; &nbsp; ;)"&nbsp; data-smilie="2"data-shortname=";)" />There are two different effects at work.&nbsp; The first, which is what I mentioned before, is about gravitational time dilation.&nbsp; It's really pretty simple though counter-intuitive: suppose you take two precise atomic clocks (or whatever kind of perfect clock you want to imagine).&nbsp; You make sure in a lab that they are running at the same rate when they are sitting side by side.&nbsp; Then you put one on the surface of the earth and one on a rocket that holds position above the earth (ie, not moving compared to the earth).&nbsp; While the one on the ground ticks off one second, the one in the rocket will tick off more time according to general relativity.&nbsp; If you put the clock in an orbiting satellite, you also have to worry about special relativity since the earth and satellite are moving relative to each other, so that would make the clock in the satellite tick off less time. The special relativity and general relativity effects don't quite cancel.&nbsp; The way it effects GPS is that the GPS satellite signal is basically broadcasting the satellite's time, so your phone can compare to its own time and figure out how far the signal travelled.&nbsp; If GPS did not account for how relativity affects the running of time, it just wouldn't work.&nbsp; So this is not only tested but used all over the world every day.&nbsp; But just to be clear, this isn't altering the time for the signal to travel a certain distance, it's really altering how time flows in different places.The other thing you stumbled on, the change of frequency, as AbduAlhazred says, is the gravitational Doppler shift, which is also due to this time dilation effect but can be thought of as the photon gaining kinetic energy as it falls.&nbsp; This has been measured, first by Robert Pound and Glen Rebka, who &quot;dropped&quot; a photon off the Harvard University physics building.&nbsp; It's actually quite a clever experiment, but I won't get into the details or else this post will go on for too long.&nbsp; But note that the normal Doppler shift is due to the relative speed of emitter and receiver while this is again due to location in a gravitational field.</blockquote>

[QUOTE="freyar, post: 6688589, member: 40227"] Well, that's quite a few duplicate posts! No saving time or space there... ;) There are two different effects at work. The first, which is what I mentioned before, is about gravitational time dilation. It's really pretty simple though counter-intuitive: suppose you take two precise atomic clocks (or whatever kind of perfect clock you want to imagine). You make sure in a lab that they are running at the same rate when they are sitting side by side. Then you put one on the surface of the earth and one on a rocket that holds position above the earth (ie, not moving compared to the earth). While the one on the ground ticks off one second, the one in the rocket will tick off more time according to general relativity. If you put the clock in an orbiting satellite, you also have to worry about special relativity since the earth and satellite are moving relative to each other, so that would make the clock in the satellite tick off less time. The special relativity and general relativity effects don't quite cancel. The way it effects GPS is that the GPS satellite signal is basically broadcasting the satellite's time, so your phone can compare to its own time and figure out how far the signal travelled. If GPS did not account for how relativity affects the running of time, it just wouldn't work. So this is not only tested but used all over the world every day. But just to be clear, this isn't altering the time for the signal to travel a certain distance, it's really altering how time flows in different places. The other thing you stumbled on, the change of frequency, as AbduAlhazred says, is the gravitational Doppler shift, which is also due to this time dilation effect but can be thought of as the photon gaining kinetic energy as it falls. This has been measured, first by Robert Pound and Glen Rebka, who "dropped" a photon off the Harvard University physics building. It's actually quite a clever experiment, but I won't get into the details or else this post will go on for too long. But note that the normal Doppler shift is due to the relative speed of emitter and receiver while this is again due to location in a gravitational field. [/QUOTE]

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