Physics Help Needed?

Question

Consider a person standing on ground and a train moving at a high velocity in front of him. Which of these bodies' time is slowed? Please explain me the reason. Thanks.

Or let us consider a person standing on the ground and a person moving in a train in uniform velocity. It moves for a while and comes to stop. At this time both, the person on the ground and the person in the train are in same reference frame. Now who has aged more.
[Consider they both are twins]

Answer 1

Whoever is being observed will look as if his time has slowed. So for the person on the ground, time on the train is slower. For a person on the train, time on the ground is slower. It is the basis for the "twin paradox", since it seems both can't be right (but they are).

EDIT: to answer the new part of the question as to who has aged more after the two are both in the same reference frame: this requires application of the general theory of relativity, since accelerations are involved. As it turns out, the person who undergoes acceleration ages less. This is true even though during the uniform velocity phase each one sees the other as aging slower. If you fail to include the acceleration component, you end up with conflicting results (the twin paradox). But in the real world, the acceleration cannot be ignored.

Answer 2

The one that moves will age slower relative to the one standing still. This would only be noticeable if the train speed is near c though.

Answer 3

Both. That is relativity!
Wherever you are and however you move,
everybody has the same physics. (Einstein)

Th

Answer 4

you need to specify the observation frame!