Why does light change its path in different frames of reference?

Question

A similar question has previously appeared on this site:

Light in Different Reference Frames

But my question is slightly different in nature. The difference would be clear after a read of this question.

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While observing an object in different frames of reference it is observed that different observers find different paths of motion for the same object. For example example consider the situation of two observers in a gravity free space. Each has a speed v in other's frame of reference. Now, consider that one of them is enclosed in a rigid transparent compartment of appropriate size so that when he throws a ball vertically upward the ball reflects back from the ceiling to his hands. Now the observation of parth by both are different:

• The one who throws the ball vertically upward observes the ball to follow a path where the path of incidence is parallel to the path of reflection.

• The other sees that the path of incidence is not parallel to the path of reflection, rather they have some angle between them.

The following image taken from this answer from the stated question clearly depicts this situation:

Now, when I first encountered this type of situation in mechanics (where there was train instead of this setup and the path was parabolic under earth's gravity), I thought that the friction between the hand and ball is the cause of this motion. But then while studying special relativity, where a similar case of pulse clock is introduced. In such case one can easily see that no friction is involved and yet the "different path for different observers" case is observed. Clearly this property seems to be related to the space itself.

So, please explain why this occurs? That is in the current understanding of the universe what seems to be the cause of this?

Sorry, if any or all of this seems silly or trivial.

Answer 1

It is nothing to do with friction or with light per se, but simply because motion causes objects to have different relative positions over time.

Consider the simplest case. Imagine you are standing still. As far as you are concerned, you are not moving. However, if I am walking past you, in my frame of reference you are moving relative to me. The underlying principle is as simple as that, although with more complicated arrangements it can be a little harder to picture.

Now imagine you are walking north up your street. If I am crossing your street from east to west, you are moving north west relative to me. If you think about this in the right way, you will realise that it is the same principle as the simplest case I mentioned.