I heard on a podcast recently that the supermassive black holes at the centre of some galaxies could have densities less than water, so in theory, they could float on the substance they were gobbling up... can someone explain how something with such mass could float?
Please see the below link for the podcast in question:
http://www.universetoday.com/83204/podcast-supermassive-black-holes/
Answer
Well, it can't (float), since a Black Hole is not a solid object that has any kind of surface.
When someone says that a super massive black hole has less density than water, one probably means that since the density goes like $\frac{M}{R^3}$ where M is the mass and R is the typical size of the object, then for a black hole the typical size is the Schwarzschild radius which is $2M$, which gives for the density the result
$$\rho\propto M^{-2}$$
You can see from that, that for very massive black holes you can get very small densities (all these are in units where the mass is also expressed in meters). But that doesn’t mean anything, since the Black Hole doesn’t have a surface at the Schwarzschild radius. It is just curved empty space.
No comments:
Post a Comment