Special theory of relativity, theoretical problem

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Homework Statement


Spaceship A, traveling past us at 0.7c, sends a message capsule to spaceship B, which is in front of A and is traveling in the same direction as A at 0.8c relative to us. The capsule travels at 0.9c relative to us. A clock that measures the proper time between the sending and receiving of the capsule travels: a) in the same direction as the spaceships at 0.7c relative to us b) in the opposite direction from the spaceships at 0.7c relative to us c) in the same direction as the spaceships at 0.8c relative to us d) in the same direction as the spaceships at 0.9c relative to us e) in the opposite direction from the spaceships at 0.9c relative to us


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The Attempt at a Solution


There's not much math involved in solving this problem, and it's all theoretical, but I think I'm missing a lot of something here. I don't even know where to begin when thinking about this question. Would the correct time simply be measured by the one that is moving at the exact same speed and direction as the capsule, but why?
 
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PeroK
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The Attempt at a Solution


There's not much math involved in solving this problem, and it's all theoretical, but I think I'm missing a lot of something here. I don't even know where to begin when thinking about this question. Would the correct time simply be measured by the one that is moving at the exact same speed and direction as the capsule, but why?

The key word in the question is "proper" time. What do you know about proper time?
 
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The key word in the question is "proper" time. What do you know about proper time?
Honestly, I don't know anything. I don't understand if it means proper time as in measured by us... I just don't understand. Is proper time measured by a clock that is said to be at rest with respect to the object?
 
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Honestly, I don't know anything. I don't understand if it means proper time as in measured by us... I just don't understand. Is proper time measured by a clock that is said to be at rest with respect to the object?

Yes, that's the proper time of the capsule, which is also the spacetime distance between the events of the capsule being sent and received.
 
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I don't understand if it means proper time as in measured by us...

If an observer is involved their proper time would be called "coordinate" time as far as the experiment is concerned. "Proper" time would be the time of a particle or object involved in the experiment. That's the conventional terminology.
 

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