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Please explain this light speed question.

  1. Jan 18, 2013 #1
    If you are traveling .6C away from your point of origin and your friend is traveling .6C away from the same point of origin but in the opposite direction, why are you not traveling 1.2C away from your friend?
  2. jcsd
  3. Jan 18, 2013 #2


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    Staff: Mentor

    Same question was asked and answered in another thread just a few hours ago... Look here
  4. Jan 20, 2013 #3
    I want to ask a couple more here that may have been covered in that other thread, but In an effort to keep things clear and not to hijack that other thread I am going to post them here.

    in my original question that was ansered in the other thread it shows that .6c+.6c is not 1.2c.

    However, lets go a step further. Lets say the two objects start coasting once they hit .6c. So I'm in my space ship and I no longer feel any acceleration. I'm floating around in the cabin. My frame of refference is that I am not moving at all. I'm just sitting there floating.

    Now I hit my thruster again and I accelerate to .6c again from my original frame of refference.

    Why can't I do this over and over until I am going faster than light as measured from my original starting point? Is it that every time I make a speed jump time dialates in response?
  5. Jan 20, 2013 #4


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    In your original frame, the energy you impart would mostly go into apparent mass. This is how the LHC and other devices work. Energy is imparted to the protons, but as they get close to the speed of light, it shows up as mass rather than speed increase.
  6. Jan 20, 2013 #5


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    Just try it and see what the Velocity Addition formula gives you for each additional burst of acceleration.
  7. Jan 20, 2013 #6
    So instead of going faster, my mass increases. As my mass increases, I exert more gravity on the space around me. And as my gravity grows time dialates. So in essence I would turn myself into a black hole about the same time I reached the speed of light, if I could get that fast.

    Any good free on line physics courses out there? Ones that expain how a particular mathamatician came up with their equasions and the proofs they used?

    I have a lot of probably stupid questions that I'm sure would be answered by taking some classes.
  8. Jan 20, 2013 #7


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    No, that's not what happens. You will go faster. But no observer watching you will see you reach c. You definately won't become a BH.

    I recommend classes, but I can't name any specifics wothout knowing your level of mathematics.
  9. Jan 20, 2013 #8


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    Staff: Mentor

  10. Jan 21, 2013 #9
    That would conflict with the basics of mathematics: .6c+.6c=1.2c by mathematical definition of addition. What was perhaps not made clear is that you asked about a transformation between coordinate systems.

    See for an elaboration some earlier threads, for example:
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