# Relativity of simultaneity

Suppose two spaceships A (at rest) and B(moving right with velocity v relative to A) one above the other with observer X in A and Y in B,both at the middle of the their spaceship.Let two events event RED and event BLUE strike at the edges(RED at left edge) of the both spaceship when Y is just above X.Then with respect to X the two events will be simultaneous whereas for Y event Blue will occur first than event red.My question if i was Y then what would i predict for X whether X will see two events simultaneously or not??(Does my question make sense?)
p.s. my english so poor sorry for ur inconvenince.Thanks in advance.

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Yes, Y would agree that X does see the events as happening simultaneously, because he also see the light signals from the red and blue events arrive at X's central position at the same time.

Both Y and X agree that the events were not simultaneous in the Y frame but were simultaneous in the X frame.

<Edited yet again. Hope I have X and Y the right way round now>

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ghwellsjr
Gold Member
Suppose two spaceships A (at rest) and B(moving right with velocity v relative to A) one above the other with observer X in A and Y in B,both at the middle of the their spaceship.Let two events event RED and event BLUE strike at the two edges(RED at left edge) of the spaceship when Y is just above X.Then with respect to X the two events will be simultaneous whereas for Y event Blue will occur first than event red.My question if i was Y then what would i predict for X whether X will see two events simultaneously or not??(Does my question make sense?)
p.s. my english so poor sorry for ur inconvenince.Thanks in advance.
If you mean the two events occurred simultaneously in X's rest frame at the two ends of his spaceship A, then, yes, Y will see that the light from those two events will propagate toward X and meet him simultaneously, even though, to Y, those two events are not simultaneous in his rest frame. Whether or not you could predict this, is another question as you would have to have prior knowledge of everything that was going to happen in order to make such a prediction.

If you mean the two events occurred simultaneously in X's rest frame at the two ends of his spaceship A, then, yes, Y will see that the light from those two events will propagate toward X and meet him simultaneously, even though, to Y, those two events are not simultaneous in his rest frame. Whether or not you could predict this, is another question as you would have to have prior knowledge of everything that was going to happen in order to make such a prediction.

But relative to Y ,X is moving towards event red with same velocity 'v'(comparable to c),then won't Y see that Event red reached X before blue??( as before X can perdict that Y will see event blue before event red as Y is moving towards event blue with velocity v relative to X,cannot Y say that X will meet event red at first??)

Yes, Y would agree that X sees the the events as happening simultaneously because he also see the light signals from the red and blue events arrive simultaneously at X's central position.

Both Y and X agree that the events were not simultaneous in the Y frame but were simultaneous in the X frame.

Sorry.but i am not getting this.how could Y agree that X would see both events simultaneously as Y will see that X is moving towards event red with same velocity v??

Sorry.but i am not getting this.how could Y agree that X would see both events simultaneously as Y will see that X is moving towards event red with same velocity v??
Sorry, I accidentally switched X and Y around. I will edit my post to correct that.

Yes, Y would agree that X does not see the events as happening simultaneously, because he also see the light signals from the red and blue events arrive at X's central position at different times.

Both Y and X agree that the events were not simultaneous in the X frame but were simultaneous in the Y frame.

<Edited. Hope I have X and Y the right way round now>

Sorry,but you again messed up with X and Y.The two events are never simultaneous in Y frame!They are simultaneous in X frame.My question is just, will Y predicts that events to be simultaneous in X frame or not?

ghwellsjr
Gold Member
If you mean the two events occurred simultaneously in X's rest frame at the two ends of his spaceship A, then, yes, Y will see that the light from those two events will propagate toward X and meet him simultaneously, even though, to Y, those two events are not simultaneous in his rest frame. Whether or not you could predict this, is another question as you would have to have prior knowledge of everything that was going to happen in order to make such a prediction.
But relative to Y ,X is moving towards event red with same velocity 'v'(comparable to c),then won't Y see that Event red reached X before blue??( as before X can perdict that Y will see event blue before event red as Y is moving towards event blue with velocity v relative to X,cannot Y say that X will meet event red at first??)
That's why I started my post with "If". You have to define which frame the events are simultaneous in which you did not do. If they are simultaneous in frame A (which is what I assumed you meant), then X sees the light from them arrive at his location simultaneously and so does Y. That's the definition of simultaneity in Special Relativity. If you instead meant that they were simultaneous in frame B, then Y will see the light from the events arrive at his location simultaneously. What happens depends on what you say happens but you can't have it be both ways or be ambiguous.

That's why I started my post with "If". You have to define which frame the events are simultaneous in which you did not do. If they are simultaneous in frame A (which is what I assumed you meant), then X sees the light from them arrive at his location simultaneously and so does Y. That's the definition of simultaneity in Special Relativity. If you instead meant that they were simultaneous in frame B, then Y will see the light from the events arrive at his location simultaneously. What happens depends on what you say happens but you can't have it be both ways or be ambiguous.

I have already mentioned that the events are simultaneous in X frame(sorry if it wasn't clear).
But how will Y predicts that both events are simultaneous in X frame as he will see that X is moving towards event red with same velocity v relative to him??

ghwellsjr
Gold Member
I have already mentioned that the events are simultaneous in X frame(sorry if it wasn't clear).
But how will Y predicts that both events are simultaneous in X frame as he will see that X is moving towards event red with same velocity v relative to him??
Y can use the A frame to analyze the situation just like we can. But as far as what Y sees from his rest frame B, he will determine that the events occurred at different times and that's why the light from them reach X simultaneously.

Y can use the A frame to analyze the situation just like we can. But as far as what Y sees from his rest frame B, he will determine that the events occurred at different times and that's why the light from them reach X simultaneously.

yoU mean Y viewing from his rest frame B to X (throug a telescope or by any other) will see events occuring simultaneously or at different times in X??

Sorry,but you again messed up with X and Y.The two events are never simultaneous in Y frame!They are simultaneous in X frame.My question is just, will Y predicts that events to be simultaneous in X frame or not?
It appears I had it right the first time before you threw me off course. I have edited my original post back to how it was.

Further to your questions here. Y does not see the red and blue events as simultaneous in his own reference frame, but he does agree that from X's point of view the events would appear to be simultaneous to X in X's reference frame. Both X and Y agree that the light from red and blue events arrive at X's location simultaneously.

ghwellsjr