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Homework Help: Einstein Luminosity and Speed of Light

  1. Feb 6, 2013 #1
    1. The problem statement, all variables and given/known data
    Find the Einstein luminosity (LE) in terms of just c and G (the speed of light
    and the gravitational constant), i.e. determine a power (in watts) from just these two terms
    using dimensional analysis. What is this value? Once determined, you should be able to
    show that an object converting its mass entirely to energy cannot radiate that energy away
    fast enough before becoming a black hole if its luminosity is greater than 0:5LE. Thus LE
    represents an upper limit on how bright anything in our universe can be!

    2. Relevant equations
    There are no particular relevant equations. It helps to know that G is n m^3/(kg s^2) units and that c is m/s. Luminosity also equals Power.

    3. The attempt at a solution
    I solved the beginning. I found that the units of power are J/s which end up being (kg ms^2)/(s^3) and thus solving for a c and G combination yields that L = c^5/G.
    However, I am completely unsure how to procede in the question passed the "Once determined you should be able to show..."
    Please, any help would be great!
  2. jcsd
  3. Feb 6, 2013 #2
    Please do your 250 homework by yourself.
  4. Feb 11, 2013 #3
    Black holes sound ominous...

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  5. Feb 12, 2013 #4
    Hmm, what odd responses you got... So anyway, consider radiation moving away from the source at the speed of light. How much energy is contained in a sphere of radius ct and how much is needed to collapse it into a black hole?
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