Re: Spinning black holes and dragging of space

Anonymous wrote:
I've been reading John Gribbin's "In Search of the Edge of Time", and came upon a section where he says that spinning black holes drag space-time around with it. My question is, if you continuously drag space-time around in a whirlpool, are you ripping apart space, or are you continuously stretching it?

Neither. Space doesn't have the properties you'd normally associate with "stuff", like inertia and tensile strength.

What Gribbin is talking about is called "frame dragging", but that's a somewhat misleading name. Roughly it means that even if a black hole is rotating with respect to the fixed stars, it's not rotating with respect to a local standard of inertial motion. This is interesting because it seems to support Mach's principle, which is the idea that rotation, like velocity, can only be defined with respect to some external reference body. In the vicinity of the event horizon, the nearby black hole seems to dominate the distant fixed stars in setting the standard of inertial motion. Einstein was heavily influenced by Mach's principle, and at least in the early days he thought of general relativity as a physical realization of it. Frame dragging is one case in which general relativity seems to follow Mach's principle, but there are other cases that seem to run counter to it.

-- Ben
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