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SR - car with headlights - new twist

  1. Jan 20, 2012 #1
    What happens if two objects travel at speed 1/2c in opposite directions?

    Like suppose two ships left the ISS in opposite directions, each going speed 1/2c.

    Wouldn't either of the ships determine that the other is moving away at the speed of light?
    Could they see each other?

    What if they both went 3/4c in opposite directions? would they conclude the speed between them was greater than c?
     
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  3. Jan 20, 2012 #2

    D H

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  4. Jan 20, 2012 #3
    I understand that for two frames, but there's three frames here - one for each ship and one 'stationary' that they move away from.

    We can determine that an object is moving at 1/2c or 3/4c, relative to us, right?

    which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
     
  5. Jan 20, 2012 #4

    Doc Al

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    Yes, the separation rate as measured by that third frame can be greater than c. But that's not the speed of any object.
     
  6. Jan 20, 2012 #5

    D H

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    Just because we would conclude that does not mean the people on the spaceships would. They would see the other spaceship as moving at 4/5 the speed of light in your first example (we see two spaceships each moving at 1/2 c in the opposite direction), and at 24/25 c in your second example (we see two spaceships each moving at 3/4 c in the opposite direction).

    Read the link I provided.
     
  7. Jan 20, 2012 #6

    Vanadium 50

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    By the way, this is not a "new twist". It's over a century old.

    It would be good to read the suggested links.
     
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