Suppose the amount of mass in a black hole doubles. Does the event horizon change? If so, how much does it change?


Let's have a look at the Physical_properties subsection of the Wikipedia article on Black holes:

The Schwarzschild radius which is defined only for a non-rotating black hole is given there as

$$r_S=\frac{2GM}{c^2}\approx 2.95 \frac{M}{M_{Sun}} \text{km}.$$

The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.

So the Schwarzschild radius increases linearly with mass, by 2.95 kilometers for every solar mass you add, provided you add it carefully and don't spill any.

Trying to express the "size" of the event horizon for an arbitrary black hole is going to be more complicated, so I'm not going to adventure into that territory. However, @Chappo's comment suggests the dependence on mass will also be approximately linear as well.

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    $\begingroup$ "provided you add it carefully and don't spill any" - lol. Actually, come to think of it, that might be a lot harder than it seems, what with the various mechanisms that tend to oppose reaching 100% efficiency for accretion. Once you have a large, superheated accretion disk, things are going to get pretty difficult. $\endgroup$ – Florin Andrei Jan 31 '19 at 6:34
  • $\begingroup$ @FlorinAndrei exactly, thank you! I didn't have my "science words" with me that day, so it's the only way I could think of to say it. ;-) $\endgroup$ – uhoh Jan 31 '19 at 6:48
  • $\begingroup$ @FlorinAndrei I think the question shouldn't have been so heavily down-voted though. $\endgroup$ – uhoh Jan 31 '19 at 6:50

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