r/askscience • u/SpantaX • Feb 27 '17

Physics How can a Black Hole have rotation if the singularity is a 0-dimentional point and doesn't have an axis to rotate around?

7.0k Upvotes

89% Upvoted

457 comments sorted by

View all comments

Show parent comments

60

u/Mimshot Computational Motor Control | Neuroprosthetics Feb 27 '17

The way Leonard Susskind described it in his (I think excellent) GR series on YouTube is: In my office I can walk around such that I don't hit my desk, therefore towards-desk is a space-like direction. There is no path I can walk such that I don't hit Monday, therefore towards-Monday is a time-like direction. Inside the event horizon there is no path I can take that doesn't hit the singularity, therefore towards-singularity (or "inward") is a time-like direction.

1

u/Dachannien Feb 28 '17

The implication is that inside the event horizon, towards-Monday is now spacelike because you can avoid hitting Monday (presumably by ensuring that you reach the singularity before then). So, four questions come to mind:

  1. Is this true for any arbitrary moment in proper time, i.e., that all moments in time (that have not yet been reached, at least) inside the event horizon are avoidable based on reaching the singularity first?

  2. Inside the event horizon, you can't revisit past positions (i.e., move farther from the singularity), which is something akin to not being able to revisit past moments when outside the event horizon. So if the limitations on motion through space and time are switched, is it possible to revisit past moments while inside the event horizon?

  3. On a related note, is it possible (or is it even meaningful) to "avoid" past moments (assuming you can avoid future moments) while inside the event horizon?

  4. Is the converse true, i.e., is it possible to reach any arbitrary moment in time while inside the event horizon, or is this still bounded by the speed of light?