# When Black Holes "Meet" **URL:** \ **Category:** Factual Questions\ **Created:** [September 5, 2001, 4:50pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884 "2001-09-05T16:50:31Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Spyderman](https://avatars.discourse-cdn.com/v4/letter/s/e79b87/32.png) [@Spyderman](https://boards.straightdope.com/u/Spyderman)\ **Post date:** [September 5, 2001, 4:50pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/1 "2001-09-05T16:50:31Z")
Inspired by [this](http://www.foxnews.com/story/0,2933,33696,00.html) article I was wondering what should happen when two black holes _meet_. Does the bigger black hole swallow the smaller one or do they form a bigger hole? Perhaps all the matter that is stored within these anomolies’ are released and life begins again? Do we have any examples of two (or more) black holes eating each other? I know that black holes do not have an orbit per se but they do rotate and since they _eat_ everything that gets in their way they must be getting bigger and bigger until there is nothing left except other black holes. (pause. deep breath.) So my question then, is, what happens when all that is left are black holes and they start to eat each other? All that matter cannot be stored in one small area, so it must be released somewhere? Is that the end of space/time? There will be no more matter, just one giant gravitational pit that will suck up every random [tachyon](http://www.dictionary.com/cgi-bin/dict.pl?term=tachyon) that burps its way into existance? (Okay, so maybe I have more than one question) Where is the **BadAstronomer** when you need one? > [@](#): > > Warning: Poster is under the influence of cough and cold medicine. Post is not to be read while operating powerful machinery ---
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [September 5, 2001, 4:54pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/2 "2001-09-05T16:54:33Z")
Black holes are physical objects with size and mass. That their local gravitational pull is strong enough to keep light from escaping doesn’t change that. If they ran into each other, they’d form a greater whole, much like slapping together two pieces of playdough. ---
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [September 5, 2001, 5:15pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/3 "2001-09-05T17:15:13Z")
Colliding black holes, IIRC, do lose a bit of their mass from spinning. As they spin around each other they emit gravitational waves, right up until when their horizons meet, after which they form a single black hole. Important to note, it is impossible to tell whether a black hole has formed from a star or from the collision of two black holes. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [September 5, 2001, 5:27pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/4 "2001-09-05T17:27:51Z")
Not everything will be eventually swallowed up by black holes. Things can be orbiting around a black hole without being destined for the ‘event horizon’ - some orbits go slowly toward the object they’re orbiting, and some away. ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [September 5, 2001, 6:35pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/5 "2001-09-05T18:35:44Z")
We went over this a while back, but for some reason, search won’t work with this browser, so I can’t provide the link. A quick answer: when two black holes meet, they form a single black hole of slightly less mass (reason: gravitational radiation, and gravitational binding energy is negative). This does not mean that everything will eventually be swallowed by black holes; only those things that cross the event horizon will be. Further, if Hawking is right, black holes DO radiate, and if you took one and stuck it in the middle of nowhere so that it couldn’t absorb anything, it would shrink while emitting Hawking radiation. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [September 5, 2001, 6:51pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/6 "2001-09-05T18:51:20Z")
> [@](#): > > \*Originally posted by \*\*\*Ethilrist \*\* > Black holes are physical objects with size and mass. That their local gravitational pull is strong enough to keep light from escaping doesn’t change that. If they ran into each other, they’d form a greater whole, much like slapping together two pieces of playdough. The mass of a black hole is concentrated at the point-like singularity. The event horizon is not a physical thing; it is where the escape velocity equals the speed of light. When two black holes combine, the singularities merge and the resulting black hole has an event horizon whose surface area is greater than or equal to the sum of the surface areas of the two event horizons. > [@](#): > > \*Originally posted by \*\*\*g8rguy \*\* > Further, if Hawking is right, black holes DO radiate, and if you took one and stuck it in the middle of nowhere so that it couldn’t absorb anything, it would shrink while emitting Hawking radiation. For a black hole caused by stellar collapse, the temperature of the Hawking radiation is less than the temperature of the universe (approx 3[sup]o[/sup]), so it would still grow. > [@](#): > > \*Originally posted by \*\*\*Chronos \*\*in [another thread](http://boards.straightdope.com/sdmb/showthread.php?threadid=55927) > By the way, about Hawking radiation: To put into perspective just how insignificant it is, a typical stellar-mass black hole radiates as though it has a temperature of about a millionth of a degree above absolute zero. The cosmic microwave background radiation has a temperature of about 2.7 degrees absolute. This means that even if you put a black hole in the emptiest region of one of the intergalactic voids, it’d still be growing far, far faster than it could shrink, just by absorbing the scattered leftover photons of the microwave background (a rather meager diet, I should say). Search works in _my_ browser. 😃 ---
**Author:** ![GulDan](https://avatars.discourse-cdn.com/v4/letter/g/22d042/32.png) [@GulDan](https://boards.straightdope.com/u/GulDan)\ **Post date:** [September 5, 2001, 6:56pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/7 "2001-09-05T18:56:04Z")
One other thing that should be remembered (though mostly just to confuse you) is that black holes do actually occupy volume in the center and are not infintesimally (sp?) small. However, the more massive the blackhole, the smaller the volume as the additional gravity is strong enough to compress it further. ---
**Author:** ![tiny\_ham](https://avatars.discourse-cdn.com/v4/letter/t/a183cd/32.png) [@tiny\_ham](https://boards.straightdope.com/u/tiny_ham)\ **Post date:** [September 5, 2001, 7:03pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/8 "2001-09-05T19:03:02Z")
Oh I’m sorry. I thought this was about the Tom Cruise Penelope Cruz relationship. ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [September 5, 2001, 9:23pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/9 "2001-09-05T21:23:49Z")
here ya go… [http://itss.raytheon.com/cafe/qadir/q859.html](http://itss.raytheon.com/cafe/qadir/q859.html) ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [September 5, 2001, 9:45pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/10 "2001-09-05T21:45:47Z")
Yes, you’re absolutely right, **Dr M.** That’s why I specified in the middle of nowhere so that it can’t absorb ANYTHING (I meant including the CMBR, which of course you can’t really escape). What mass would a black hole have to have before the Hawking radiation is enough to cause the BH to shrink despite the CMBR? I’m assuming that’s something really truly hideously large. Let’s see… about 10[sup]7[/sup] solar masses or so? ---
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [September 5, 2001, 10:38pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/11 "2001-09-05T22:38:13Z")
Though I can’t seem to find my book on black holes, I seem to remember that the hawking radiation’s intensity is inversely proportional to the mass… the smaller the black hole, the faster it disintegrates. The cause for this I really don’t remember, but i think it has something to do with the strength of gravity just outside the horizon… the smaller the black hole, the more intense the gravity at the same distance from the horizon. DAMN where is that book ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [September 5, 2001, 11:17pm UTC](https://boards.straightdope.com/t/when-black-holes-meet/79884/12 "2001-09-05T23:17:56Z")
Uhm, right. I screwed that up. In some appropriate set of units, T = 1/(4_M) (h-bar = c = G = k = 1). So M_T is a constant, and if T is 7 orders of magnitude larger, M is 7 orders of magnitude smaller.