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<blockquote data-quote="freyar" data-source="post: 6688387" data-attributes="member: 40227">Well, getting to work on time and the time you'd use in high school physics are pretty well the same.&nbsp; The way we think of time in our daily lives and the way physicists did until Einstein is as an absolute quantity that passes at a fixed rate.&nbsp; But Einstein changed that.&nbsp; First, the theory of special relativity showed that time passes at different rates for objects moving with respect to each other.&nbsp; Generally, we don't move very fast, so this doesn't impact us much.&nbsp; But if, say in the future, future you wanted to do your daily commute from earth to mars in half an hour, you'd need to move at a significant fraction of the speed of light, and the amount of time that passes in the clock on your space ship would be noticeably different than the time that passes on a clock that stays on earth while you make your round trip.&nbsp; Then, the theory of general relativity says time is affected also by gravity.&nbsp; So, for example, for every one second measured by a clock on earth, a clock in space ticks a bit more than one second.&nbsp; The extreme example is that, while a clock on the event horizon of a black hole ticks one second, an infinite amount of time passes on a clock far away from the black hole.This does have some practical impact.&nbsp; If you have a smart phone, it most likely has a GPS unit, which you might use to track your location.&nbsp; GPS works by figuring distances from a set of satellites orbiting earth, and it figures distances by finding the length of time a radio signal takes to travel from the satellites to your phone.&nbsp; That takes very accurate time-keeping, and it actually has to be accurate enough to notice both of those &quot;time dilation&quot; effects I just mentioned.This &quot;flexibility&quot; in the behavior of time is a very important and fundamental part of gravitational physics.&nbsp; It also causes some complications in our calculations, though the details are a bit complex.Of course, there are lots of other speculative theoretical ideas about the role of time in a quantum theory of gravity.&nbsp; A popular thought is that time (and space) is not really a fundamental concept but emerges from something else.&nbsp; So maybe time is just an approximate notion altogether.</blockquote>

[QUOTE="freyar, post: 6688387, member: 40227"] Well, getting to work on time and the time you'd use in high school physics are pretty well the same. The way we think of time in our daily lives and the way physicists did until Einstein is as an absolute quantity that passes at a fixed rate. But Einstein changed that. First, the theory of special relativity showed that time passes at different rates for objects moving with respect to each other. Generally, we don't move very fast, so this doesn't impact us much. But if, say in the future, future you wanted to do your daily commute from earth to mars in half an hour, you'd need to move at a significant fraction of the speed of light, and the amount of time that passes in the clock on your space ship would be noticeably different than the time that passes on a clock that stays on earth while you make your round trip. Then, the theory of general relativity says time is affected also by gravity. So, for example, for every one second measured by a clock on earth, a clock in space ticks a bit more than one second. The extreme example is that, while a clock on the event horizon of a black hole ticks one second, an infinite amount of time passes on a clock far away from the black hole. This does have some practical impact. If you have a smart phone, it most likely has a GPS unit, which you might use to track your location. GPS works by figuring distances from a set of satellites orbiting earth, and it figures distances by finding the length of time a radio signal takes to travel from the satellites to your phone. That takes very accurate time-keeping, and it actually has to be accurate enough to notice both of those "time dilation" effects I just mentioned. This "flexibility" in the behavior of time is a very important and fundamental part of gravitational physics. It also causes some complications in our calculations, though the details are a bit complex. Of course, there are lots of other speculative theoretical ideas about the role of time in a quantum theory of gravity. A popular thought is that time (and space) is not really a fundamental concept but emerges from something else. So maybe time is just an approximate notion altogether. [/QUOTE]

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