When will the person at point B see train2 and why?

[dwspacetime]

the original purpose of reference from is due to emptyness of the space itself i think. correct me if i am wrong. in the nothingness space without any matter to reference to you can not tell something is moving or not which brought the necessity of reference frame. by its very nature i think the movement should be always reference to a matter.

but a ball's accelerating is relative to the nothingness space which is really confuses me. maybe i think too much.

 

[atyy]

i do see both sides using legalese. :)

i did some search online. there is a university website about how to define the inertial frame which i don't remember what it is. there are some nonsense in it. but regardless, an inertial frame or a non-inertial frame or any kind of reference frame is an arbitraty coordinate system for us to measure the movement in spacetime. the movement happens in spacetime but not a reference frame. i use a ruler to measure my table to be 3.281ft long means my table takes 3.281ft long spacetime in the english (reference) frame and 1 meter long in the metrics reference system. my table takes space but not ruler. without necessity of measurement there is not need for reference frame. without reference frame things don't move? they move with or without referece frame in spacetime!

when a ball moves from pointA to point B in spacetime under any reference frame. there must be another reference frame in which we measure the spacetime moves from point B to point A relative the ball.

maybe there is not such a thing as a ball gets a force from nowhere and start to accelerate 'cause action and reaction is always a pair. how does a rocket get reaction to propell?

anyway i think i personaly want to go ahead to finish the book. it is really interesting to me.

Well, what do you mean by "move"? In physics, words are defined by saying what experiments we do and what results we get. The moment we do experiments to measure "movement", there is no more empty space. Suppose there is experimenter A, he does an experiment, and sets up a reference frame A in which the ball "moves". And suppose there is an experimenter B, she does an experiment using reference frame B, and the ball does not "move". Who is right? Well, they can both be right, because by using different reference frames, they used different definitions of "move". We can show that they are both right if we have an equation that consistently relates definitions in different reference frames.