# do Black Holes grow?

• November 11th, 2009, 05:08 AM
Booms
do Black Holes grow?
I'm contemplating again 8)

I was wondering if Black Holes can increase in size.

also if they do is it proportional to how much they comsume?
• November 11th, 2009, 05:31 AM
John Galt
Enthusiastic duplicate post.
• November 11th, 2009, 05:32 AM
John Galt
• November 11th, 2009, 12:27 PM
DrRocket
Re: do Black Holes grow?
Quote:

Originally Posted by Booms
I'm contemplating again 8)

I was wondering if Black Holes can increase in size.

also if they do is it proportional to how much they comsume?

Yes

Yes

Black holes are characterized by mass, charge and angular momentum.
• November 12th, 2009, 10:53 AM
Heinsbergrelatz
i have read that black holes can merge to form a bigger black hole, but thats a different story
well,a black hole can increase its gravitational force by pulling more matter into the singularity at its center.
• November 12th, 2009, 12:29 PM
MagiMaster
Whether it reaches the singularity (or whether a singularity actually exists) is a moot point. For all practical purposes, the black hole increases in mass as soon as anything enters its event horizon.
• November 12th, 2009, 01:40 PM
DrRocket
Quote:

Originally Posted by MagiMaster
Whether it reaches the singularity (or whether a singularity actually exists) is a moot point. For all practical purposes, the black hole increases in mass as soon as anything enters its event horizon.

Absolutely.

Black holes pretty clearly exist.

It is not at all clear that singularities exist. I would personally bet against it, since a singularity is not reall a "thing" but only a hypersurface on which the mathematical prediction is that curvature ceases to be defined.
• January 3rd, 2010, 12:28 AM
jasonlang
One interesting point : as matter approaches the event horizon of a black hole it undergoes time dilation, effectively freezing it before it hits!

So, one wonders how the black hole is able to grow at all in finite time?

The answer seems to be that as sufficient mass approaches the event horizon, the effective event horizon is relocated outwards! So the black hole grows not by matter hitting the event horizon so much as the event horizon growing outwards to encompass more space.
• January 3rd, 2010, 12:36 AM
inow
Jason - You're forgetting that the "freezing" is frame dependent. The object only appears to freeze to outside observers. From the reference frame of the object or the black hole, there is no freezing, time dilation is essentially rendered irrelevant, and everything progresses as per usual.

Also, the mass of the BH very much increases as matter enters it, so your last few comments are really somewhat misguided.

Anyway... Welcome to the site. Thank you for taking part in the discussions. :)
• January 3rd, 2010, 12:45 AM
jasonlang
If the infalling matter is "frozen" for a non-finite time according to the external observer, but the external observer sees the black hole growing in finite time, then it must grow before the infalling matter has a chance to fall past it, in anyones frame of reference.

My point is that enough matter coming close to the black hole will actually be sufficient to allow a larger event horizion. So a black hole could grow without any matter actually "hitting" the event horizion (i.e more mass came within a new, larger, schwarzchild radius).

PS i never ever said the mass of a black hole didn't increase, i just said an the event horizon moves as the physical arrangement of mass allows.

---------------------

Edit : Using arbitrary units, but relations from the schwarzchild equation :

consider a black hole with mass 1 and diameter 1 ( a sphere of diameter 1)
if I had 4 of these masses they could form a black hole of diameter 4 ( a sphere of diameter 4)

To form the new larger black hole it would be sufficient to place the original 4 black holes within the larger spherical volume, and none of the original black holes would actually have to touch, so showing that black holes can grow without any matter actually having to physically hit the event horizion.
• January 8th, 2010, 01:25 AM
kakarot
Of course.
You better read Mr.Stephan Hawkings books.
you know they help a lot.
• January 8th, 2010, 04:10 PM
Arcane_Mathematician
it would actually be a diameter of \sqrt[3]{4}