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

Angular momentum of a singularity?

  1. Nov 25, 2003 #1
    We know that an ice skater pulling her arms in and spinning faster is an example of the conservation of angular momentum.

    As stars collapse to become Neutron stars, the rotational period can be as low as 0.001 seconds, again to conserve angular momentum.

    So what happens to this angular momentum when a very large star collapses to a black hole - ie a singularity? How can a dimensionless point have angular momentum?
    Is it conserved?
  2. jcsd
  3. Nov 25, 2003 #2


    User Avatar
    Science Advisor
    Gold Member

    Well you could ask the same thing of electrons. Yes angular momentum is conserved a black hole with angular momentum is called a Kerr black hole and is no longer spherically symetric. In this situation the singuularity becomes a ring:

    http://www.physics.ubc.ca/~psih/kerr-metric/node5.html [Broken]
    Last edited by a moderator: May 1, 2017
  4. Nov 26, 2003 #3


    User Avatar
    Science Advisor

    The definition of angular momentum in curved spacetime is very dicey, and cannot be made in general. With that warning, its probably best to consider the angular momentum "stored" in spacetime itself, not the underlying matter.
  5. Nov 27, 2003 #4
    To all

    Just a question:

    If the ice skater was spinning in outer space
    with arms straight out and the skater started
    to bring in the arms towards himself or herself,
    would angular momentum still apply in this case?
    or are there other factors to be considered?
  6. Nov 27, 2003 #5
    Thanks for the replys jcsd and stingray... I'm not sure I fully understand the answer, but at least I now know that it was a sensible question... :wink:

    Poorichard2 - It would be just the same in space - the skater would speed up as they moved their arms in - angular momentum would still apply in this case. (in fact, without friction it would be a great example of angular momentum being consered)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook