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The wikipedia article on Time Dilation (http://en.wikipedia.org/wiki/Time_dilation) has an explanation involving the following two diagrams:

At rest Moving

I have some problems with these diagrams. In the second one the bottom mirror is not only moving, but it's also changed the direction in which it's emitting light so obviously the path is going to be longer. If the bottom mirror in the second diagram wasn't moving but went all the way from point A to C and emitted the light at that same angle it would result in the same 'time' measurement even though it's not actually moving.

Is the diagram implying that light will be emitted at an angle if the light source starts to travel sideways?

Qmechanic
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Kai G
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2 Answers2

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Perhaps the following animation will help:

enter image description here

Image credit

Note that, from both the rest frame of the light clock and from the relatively moving frame, the mirrors are always vertically aligned and the 'photon' is always located along the line through both mirrors.

Alfred Centauri
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Per general intuition, (I do not have a reference).

For the moving frame of reference, space is curved. Therefore, what seems to be vertically up, actually, is at an angle, as observed by a stationary observer, but it appears to be going straight up and straight down for the moving observer.

When we say vertically up, that is with respect to the curved space. So, "Straight up" itself is different for the moving frame Vs for the stationary frame. Therefore, it is the stationary observer that sees the time being slow, not the moving frame.

kpv
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