Two spaceships travel away from the earth.

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The discussion revolves around a problem in special relativity involving two spaceships traveling away from Earth in opposite directions, each at a speed of 0.75c. The original poster seeks to determine the rate at which the distance between the two spaceships changes from the Earth's rest frame.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the appropriate formula for calculating relative speeds in different frames of reference, with one participant suggesting a specific formula for two objects moving in opposite directions. There is also a question about the interpretation of the original problem regarding the rate of distance change.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem and the application of relativistic formulas. Some guidance has been offered regarding the formula to use, but there is no explicit consensus on the correct interpretation of the question.

Contextual Notes

There appears to be some confusion regarding the specific question being asked, as one participant notes a different interpretation of the problem. This may indicate a need for clarification on the original poster's intent.

CarlB
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They go in opposite directions. In the rest frame of the earth, the speed of each spaceship is [tex]0.75 c[/tex].

In the rest frame of the earth, what is the rate at which the distance between the two spaceships changes?

Carl
 
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Surely you should know the formula for this. If, in a given frame, two objects, moving in opposite directions, have speeds, relative to an object motionless in that frame, of v1 and v2 then their speeds, relative to one another in that frame is [tex]\frac{v_1+ v_2}{1+ \frac{v_1v_2}{c^2}}[/tex].
 
CarlB said:
In the rest frame of the earth, what is the rate at which the distance between the two spaceships changes?
[tex]1.5 c[/tex], of course.

(Halls answered a different question: What is the rate at which the distance between the ships changes in the rest frame of either ship.)
 
Oops. Thanks, Doc.
 

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