# A few relativity questions

#### TheScienceOrca

After learning more about relativity, and the vast amount of knowledge that is on this forum, I would like to clear up a few questions I had about relativity.

1) If Alice is on a planet and sees Bob fly away from her through the universe at .9c relative to her and bob then throws a baseball at .9c in the same direction he is travelling in I have a few questions;

If you let each object move for 1 second relative to the object itself would you see from a fourth frame of reference; Alice, Bob, and Bob's baseball should be moving for different amounts of time relative to you as they are travelling at different speeds.

If you measured the distance from Alice and Bob would it be .9ls?

If you measured the distance from Bob and Bobs Baseball Would it be .9ls?

If you measured the distance between Alice and Bobs Baseball would it be 1.8ls?

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#### PeterDonis

Mentor
1) If Alice is on a planet and sees Bob fly away from her through the universe at .9c relative to her and bob then throws a baseball at .9c in the same direction he is travelling in
Is the baseball moving at 0.9c relative to Bob? I assume so, but you should make these things explicit. General rule: any time you specify a distance, time, or speed, you need to specify what it's relative to. For distances, you also need to specify at what time the distance is measured; and for times, you need to specify between which two events the time is measured.

If you let each object move for 1 second relative to the object itself would you see from a fourth frame of reference; Alice, Bob, and Bob's baseball should be moving for different amounts of time relative to you as they are travelling at different speeds.
Yes, 1 second for each of these objects, according to the object's own clock, will correspond to a different amount of time according to you since all three of them are moving relative to you, and at different speeds.

If you measured the distance from Alice and Bob would it be .9ls?

If you measured the distance from Bob and Bobs Baseball Would it be .9ls?

If you measured the distance between Alice and Bobs Baseball would it be 1.8ls?
All of these questions are not well-defined, because you haven't said what you're measuring the distance relative to, nor have you specified at what time the distances are being measured.

#### jbriggs444

Science Advisor
Homework Helper
If you let each object move for 1 second relative to the object itself would you see from a fourth frame of reference; Alice, Bob, and Bob's baseball should be moving for different amounts of time relative to you as they are travelling at different speeds.
What is the fourth frame of reference? How fast are Alice, Bob and Mr. Baseball travelling with respect to it?

If you measured the distance from Alice and Bob would it be .9ls?
It could be, if the fourth frame happens to be the one in which Alice is at rest or the one in which Bob is at rest. Try calculating it from some other frame.

If you measured the distance from Bob and Bobs Baseball Would it be .9ls?
It could be, if the fourth frame happens to be the one in which Bob is at rest or the one in which Bob's baseball is at rest. Try calculating it from some other frame.

If you measured the distance between Alice and Bobs Baseball would it be 1.8ls?
It could be, if the fourth frame happens to be the one in which Bob is at rest. Try calculating it from some other frame.

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