Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Timelike geodesic

  1. Mar 30, 2010 #1
    My lecturer has written:

    [itex]\ddot x^{\mu} + \Gamma^{\mu}{}_{\alpha \beta} \dot x^{\alpha} \dot x^{\beta} = 0 [/itex] where differentiation is with respect to some path parameter [itex]\lambda[/itex].

    If we choose [itex]\lambda[/itex] equal to proper time [itex]\tau[/itex] then it can be readily proved that

    [itex]c^2 = g_{\mu \nu}(x) \frac{dx^{\mu}}{d\tau} \frac{dx^{\nu}}{d\tau}[/itex]

    Only problem is I can't quite see how to go from the first to the second, can someone explain for me please?
     
  2. jcsd
  3. Mar 30, 2010 #2
    The second does not follow from the first. The second is just a statement that proper time is normalized in such a way that the magnitude of the 4-velocity [itex]dx^{\mu} / d\tau[/itex] is c.
     
  4. Mar 31, 2010 #3
    Ahar, thank you hamster, makes sense now.

    My lecturer has written something like this:

    [itex]R_{\mu \nu} - \frac{1}{2}R g_{\mu \nu} + \Lambda g_{\mu \nu} = 0[/itex]

    "Now contract indices on both sides:

    [itex]R^{\mu}{}_{\mu} - \frac{1}{2} g^{\mu}{}_{\mu}R + \Lambda g^{\mu}{}_{\mu} = 0[/itex]

    Can someone explain exactly what "contraction" he has done he? I assume he means multiplying by the metric tensor but I'm not sure exactly what metric tensor multiplication has gone on here?
     
  5. Mar 31, 2010 #4

    nicksauce

    User Avatar
    Science Advisor
    Homework Helper

    It is just multiplying both sides by [itex]g_{\mu\nu}[/itex].
     
  6. Mar 31, 2010 #5
    I was just coming online to say don't bother replying I figured it out but you beat me to it haha, thank you.

    I figured out it was just multiplying by [itex]g_{\mu\nu}[/itex] and then the fact you have nu's instead of mu's makes no difference since it's just a dummy index. Thanks anyway :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook