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You must be using the word motion in a very specific sense, because in the more general sense of the word motion is clearly observable. Motion is a change of position over time, and we can clearly ob …

If A changes their velocity then you can say absolutely that A has accelerated, since acceleration is not a relative effect. That said, you still cannot say in any absolute sense who is moving, since …

No, you misunderstand. Either the lightning can strike the two ends of the train simultaneously in the frame of the train- in which case it strikes at two different times in the frame of the platform- …

It is a common misconception that time dilation is some effect that impedes the action of clocks as a result of their motion, which then leads people to ask how come different types of clocks are affe …

The answer is straightforward. The formula shows that there is no dilation when your speed is zero, which is what you would expect, otherwise length dilation would be observable in connection with sta …

There is no paradox. The question you pose is exactly analogous to this: Two presidents must sign a treaty at 6pm or a bomb will go off. They sign it at 6PM London time, so the bomb doesn't go off. B …

You are right to say that the box and its contents will slide to the left when the attractive external force is applied, and will continue to slide at a constant speed when the force is switched off. …

Assuming the blocks are elastic, when the lighter block hits the heavier one, the two momentarily compress then uncompress and come apart. It is that interaction that transfers momentum and energy fro …

If you were at the centre of the balloon, it would appear to be a sphere. The speed at which the balloon was expanding away from you would appear to be reduced owing to the Doppler red-shift effect. I …

At the risk of making the experiment sound more complicated, there are two considerations here. The first is that the local duration of a flash is frame dependent. In a frame in which the flash occurs …

You are overlooking the relativity of simultaneity and misunderstanding the symmetry of time dilation. Suppose the light flashes when the mid-point of the observers on the train passes the mid point o …

The light goes wherever you point it. If you point the light vertically in either frame it follows a diagonal path in the other. The same applies if I climb a ladder vertically in one frame, I move di …

There is no paradox. You are confusing two separate quantities, 1) being the time taken for information to travel to the moon from Angie, and 2) being the elapsed time for parts of the message to arri …

Where you are going wrong is to assume that the time axis of the observer on the train is the same as the time axis of the observer on the platform. If the lightning strikes are simultaneous in the p …

You are mixing up observations from the two frames of reference. Let's say Bolt A hits the rear of the train and leaves Mark A on the adjacent platform, while Bolt B hits the front of the train and le …