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This question already has an answer here:

Here is what I understand by coordinate time.

It is the time difference measured between two events, using two synchronized clocks, one present at each event, and the difference is measured in an inertial reference frame.

My problem is that I am not sure exactly what causes the difference in time between two different inertial reference frames.

Is it correct to say that this is due to the information propagation that needs to happen for the observer in the inertial reference frame to be able to read the time difference?

If not, what is the actual reason for the different coordinate times in different reference frames?

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marked as duplicate by John Rennie, Kyle Kanos, Danu, Brandon Enright, Prahar Dec 3 '14 at 1:34

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Tilting an object in space changes its apparent dimension (think of trying to get furniture through a door: the width of an object depends on its orientation).

Objects in relative motion are tilted in space and time (or rather, spacetime), and different observers will see things unfold under different perspectives. Personally, I find relativity of simultaneity less mysterious when considering it a form of parallax.

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One reason for the difference is time dilation--a given coordinate clock at rest in one frame will be running slow as measured by coordinate clocks at rest in another frame. It can be demonstrated that this follows logically from the two postulates of special relativity, using the "light-clock" thought-experiment detailed here. The second postulate says that both frames must measure light to move at the same same speed of c, so the thought-experiment uses a hypothetical type of "clock" which measures time by light bouncing back and forth between two mirrors, and if both frames measure the light to move at c this implies that the frame in which the clock is moving will measure it to tick slower than the frame where it's at rest. And the first postulate says all laws of physics work the same way in all frames, so if some other type of clock would keep the same time as a light clock if both were at rest in some frame (say, the frame of a lab on Earth), this must be true for a light-clock/other-clock pair in any frame, thus all types of clocks must display the same time slowdown as a light clock when they are in motion relative to the observer.

The other reason for the difference in coordinate time between frames is the relativity of simultaneity, which has to do with the rule that SR uses for how different coordinate clocks in the same frame should be "synchronized", namely the Einstein synchronization convention. This convention says that if I have two coordinate clocks at rest in my frame at different locations, I should synchronize them using the assumption that light signals travel at the same speed in all directions in my frame (the second postulate again). So if a light signal is sent from clock A when it reads time T1, and it's reflected by a mirror next to clock B when that clock reads T, and it returns to clock A when it reads T2, then A and B are defined as "synchronized" if T is exactly halfway between T1 and T2. Another equivalent method would be to set off a flash of light at a point midway between A and B, and then make sure that they both read the same time when the light from the flash reaches them.

But using this synchronization method, it's not hard to deduce that a pair of clocks which are synchronized in their own rest frame can be out-of-sync in the coordinates of other frames, given that an observer in another frame assumes that light travels at the same speed in all directions relative to herself. For example, suppose Bob is aboard a train which is moving forward relative to Alice, and he decides to synchronize clocks at the front and back of his ship by setting off a flash midway between them and setting them to the same time when the light reaches them. From Alice's perspective, the clock at the back of the train is moving towards the point on the rails that was next to the flash when it was set off, and the clock at the front is moving away from that point on the rails, so if she assumes both light rays move at the same speed away from that point, light from the flash must reach the back clock before it reaches the front clock. Thus if Bob sets both clocks to read the same time when the light hits them, in Alice's frame the back clock will end up having a time that's ahead of the front clock's time.

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  • $\begingroup$ So the cause is that clock synchronization itself is frame dependent in part because c is the same in all reference frames. Makes sense. Thanks. $\endgroup$ – picaposo Dec 2 '14 at 17:24
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At the research of the answer, I would recommend to consider the example of an astronaut going very far in a lifetime (example: moving near speed of light, he reaches a star 5000 light years away in only 50 years).

I would ask the reversal of "Why is coordinate time frame dependent?": why proper time is passing slower than the observer's dilated time?

It seems that time is linked with slow movement/standstill. The slower we move, the more time is passing on our clock.

There is a mathematical model expressing this mechanism: you may consider that an object at rest (in space directions) is traveling at speed of light in time direction. If it is moving near light speed through space, it is not traveling in time direction, meaning that no time is passing on its clock.

This is only a simplifying arithmetical model which does not hold with regard to physical laws (in particular, "at rest" is relative, if one is at rest for one observer, he can be moving for another). But it helps understanding the space-time mechanism and time dilation which is inherent to Minkowski spacetime.

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I don't see the difference between your "coordinate time" and just time.

Yes, time is relative.

Since time is relative, it is incorrect to find a reason (or cause).

A cause is, by definition, some local event in the past, which affects some other event (consequence) in the future.

Time relativity is not an event. It is not local, and is not in the past.

So, time relation between events is a global constraint of space time. It has no reason itself, it is eternal property of nature (as we know it).

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