Right, it's been a long time since i've done anything physics related, and i've never been an expert to begin with, just a level standard, but was having a bit of a chat with a friend earlier and wanted to see if what we were saying makes any sense atall? We were having a bit of an argument over altitude and boiling eggs, and weather it's the cooking time, or the boiling of the water that takes longer. i got this cleared up and was saying that (not taking temperature into account) water should boil faster, and in a jokey way that even time would travel slightly faster therfor speeding up the boiling process by an incredibly small amount. My friend then mentioned that he though it was velocity difference that can cause difference in the perception of time for an observer and we started talking about the famous concord experiment. so, what i think happend was, 1 atomic clock on the ground, stationary, and another in a concord flying round the world at a higher altitude than the other clock. what exactly was causing the difference in time here, was it gravititional time dilation due to the concord being further away from the earth, or was it velocity based time dialation, since the concord was traveling at a faster velocity compare to the other clock. or, as we througt, is it a combination of both? Then we were thinking about GPS satillites, and we know that they would have to be calibrated for small time differences, which form of time dialation would have the most effect on the satillite? Anyways, then we started thinking about what would happen the closer you got to the center of the earth, would time get slower and slower, and then eventually reach a point where it couldn't slow down anymore? i'd imagine this speed would be related to the gravitational force of the earth/mass, and may even possibly be concidered the "correct" time for earth? if we then though about the time at the center of jupiter, i'd imagine it to be slower again than our "earth time" and so on as mass and gravitiotinal forces incresed, this must carry on until time slows down sooo much, that it would almost have stopped to the observer. would at this point we be talking about the occurance of a black hole? when the object's gravity becomes too much that it collapses? or is this gravitational based time dialation a lot more subtle than i am imagining, and even comparing very small and incredible large gravitational forces, time doesn't actually chance much?