Talk:Ergosphere

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Ergosphere smergosphere[edit]

The article reads: "The outer surface of the ergosphere is called the static surface or static limit. At the static surface, a particle, moving against the flow of space at the speed of light, is static relative to a distant observer. This is because world lines change from being time-like outside the static limit to being space-like inside it. Outside this surface, space is still dragged but at a lesser rate."

From this I assume that the outer limits of the ergosphere are defined by the speed of light. Is that correct? In a sense that is arbitrary as the effects of space-time dragging extend forever. The Sun-Earth system is radiating gravitational waves at 200 watts. As the Earth is well outside of the Sun's ergosphere, I assume that the Penrose effect applies out to infinity with the cavet that it diminishes to a trivial amount quite rapidly. Please correct me if I am wrong. Zedshort (talk) 18:28, 22 February 2016 (UTC)

If a test particle at the edge of the ergosphere is to be stationary relative to the coordinate bookkepper (for example an observer at infinity) it has to move with local c in the retrograde direction of the black hole's spin. If the test particle was to be locally at rest, in the reference frame of a distant observer it would corotate in the prograde direction of the black hole's spin. Since the edge of the ergosphere rotates with local c, in the system of the bookeeper it rotates slower than c because of the time dilation in the vicinity of the black hole's gravity. The sun doesn't have an ergosphere at all because it's radius is not inside the critical radius to form one (there is no distance from the sun in which you are forced to have local c in order to stay at rest relative to the bookkeeper, which is the definition of the ergosphere). --Yukterez (talk) 12:03, 27 November 2016 (UTC)

Shape[edit]

The description "The ergosphere has an oblate spheroidal shape that touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator" is only a valid approximation for low spin parameters. With higher spin parameters the form is not an ellipsoid or oblated spheroid at all, see comparison and animation. Only the event horizon is an ellipsoid in cartesian Kerr-Schild coordinates, but not the ergosphere (very easy to calculate with equation 2 from here). The confusion between both is a common misconception. --Yukterez (talk) 08:20, 26 November 2016 (UTC)