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So a black hole is a something that has enormous gravity, therefore mass accelerates faster towards it. But if the black hole double it's mass, will it's event horizon increase in length , or will it decrease?

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    $\begingroup$ Did you read the Wikipedia article about the radius of black holes and its dependence on the mass? $\endgroup$ – pela Feb 14 '16 at 14:35
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    $\begingroup$ Not even a minimal amount of research. As a guide - if you type your headline question into a search engine and the first link it provides directly gives you the answer, then it is not a very interesting stack exchange question. $\endgroup$ – Rob Jeffries Feb 15 '16 at 13:49
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The radius of the event horizon ($r_\mathrm{s}$) is directly proportional to the mass of the black hole (M). More exactly:

$$r_\mathrm{s} = \frac{2 G M}{c^2}$$

The black holes whose merger was detected by LIGO would each have been about 90 km in radius, and after merger, a little less than 180km.

Read about the Schwarzchild radius

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    $\begingroup$ So apparently, if my waist line is smaller than $6.5×10^{-25}$ meters, I collapse into a black hole. $\endgroup$ – SE - stop firing the good guys Feb 14 '16 at 15:05
  • $\begingroup$ Wow, thanks a lot for that.Really helpful $\endgroup$ – Vase Dodevski Feb 14 '16 at 16:29
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    $\begingroup$ Additional fun fact: If you consider the density of a black hole $\rho$ as its mass over it's volume given as a sphere with a radius equal to the Schwarzschild-radius $\rho = \frac{M}{\frac{4}{3}\pi r_S^3}$, than you′ll find $\rho \sim M^{-2}$. It's getting less dense, the more massive it becomes. $\endgroup$ – rtime Feb 14 '16 at 20:29
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particle and anti-particle pair - basically it has a particle that is attracted to the black hole and one that is repelled by it. Since these particles contain mass, the particle that is repelled is left as radiation, and the one that is attracted is added mass, and by adding this mass it will decrease the total density, since it isn't just an addition problem, there is lost mass increase that makes the density and the mass of the black hole a inverse relationship. This states that the more mass the less dense which means that is will shrink and decrease in length.

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    $\begingroup$ Your answers could be improved by a bit better formatting. Btw, what do you mean by decrease in length? The length of what? $\endgroup$ – zephyr Mar 2 '17 at 13:59

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