# Can black holes ever be stopped from growing in mass? **URL:** \ **Category:** Factual Questions\ **Created:** [January 17, 2001, 8:06am UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488 "2001-01-17T08:06:10Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![jb\_farley](https://avatars.discourse-cdn.com/v4/letter/j/9d8465/32.png) [@jb\_farley](https://boards.straightdope.com/u/jb_farley)\ **Post date:** [January 19, 2001, 5:46am UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/21 "2001-01-19T05:46:09Z")
um, I am going to take the above post with a grain of salt, especially when it has stuff like: > [@](#): > > The speed of light is not constant in general relativity I will gladly eat crow if I am wrong, but I’m pretty sure i’m right. jb ---
**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:** [January 19, 2001, 1:58pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/22 "2001-01-19T13:58:04Z")
The speed of light is not constant in GR. Consider a photon at the event horizon trying to escape the black hole. It is stationary. Therefore, the speed of light at the event horizon is zero. IIRC if you consider the earth as your reference frame, pluto is travelling faster than 186,000 m/s. What you have in GR is that nothing travells faster than light, but SOL is not constant. Crow doesn’t taste that bad really. ---
**Author:** ![jb\_farley](https://avatars.discourse-cdn.com/v4/letter/j/9d8465/32.png) [@jb\_farley](https://boards.straightdope.com/u/jb_farley)\ **Post date:** [January 19, 2001, 2:02pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/23 "2001-01-19T14:02:20Z")
this will go into my “Something New You learnevery day” list. Thanks, Dr. Matrix. Sorry, Ring. gobble gobble. swallow. euch- can’t I just drink a bottle of Old Crow and call it even? jb p.s.- tastes like bowling pin and spray paint- yuck… ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [January 19, 2001, 2:19pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/24 "2001-01-19T14:19:03Z")
> [@](#): > > \*Originally posted by manhattan \* > Glenoled \*\* I think any resident of Boston will confirm that there is a giant black hole in Massachusetts. They call it “the Big Dig.” \*\* [/QUOTE] Yeah, the black hole singularity is the budget. The messed up roadways are the event horizon that you can’t escape from. ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [January 19, 2001, 2:20pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/25 "2001-01-19T14:20:35Z")
_oops, sorry I messed up the quote_ ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 19, 2001, 4:01pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/26 "2001-01-19T16:01:43Z")
> [@](#): > > The speed of light is not constant in general relativity Maybe I can explain it better. The speed of light in flat spacetime is always constant, which means that, locally, as in special relativity, an observer would always measure it to be c. However, in general relativity not only is spacetime, itself, expanding, but spacetime is also curved (the intrinsic geometry is curved). This makes it almost meaningless to ask how fast anything, including light, is moving. > [@](#): > > The speed of light is not constant in GR. Consider a photon at the event horizon trying to escape the black hole. It is stationary. Therefore, the speed of light at the event horizon is zero. IIRC if you consider the earth as your reference frame, pluto is travelling faster than 186,000 m/s. What you have in GR is that nothing travells faster than light, but SOL is not constant. Yes, except locally light is still traveling at c. Which means that to a faraway observer time must come to a stop at the event horizon in order to escape the paradox. The other way to look at is that, at the horizon, spacetime is infalling at c and light is moving out at c which means that to a faraway observer nothing, including light, can escape the black hole. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [January 19, 2001, 4:33pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/27 "2001-01-19T16:33:40Z")
> [@](#): > > \*Originally posted by DrMatrix \* > **IIRC if you consider the earth as your reference frame, pluto is travelling faster than 186,000 m/s.** Okay, bearing in mind that in this context (we are talking about the speed of light) m/s denotes “miles per second”, are you sure? ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [January 19, 2001, 4:39pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/28 "2001-01-19T16:39:12Z")
The obvious way to stop a black hole increasing in size is to build a Dyson sphere around it. Just set up your construction equipment at various points in a stable orbit and start building a ring, then complete the sphere. Now you have a tame black hole in a cage, which you can tow around at will. You could make the hole smaller by pumping out any stray radiation that is emitted from the event horizon, and you could make it bigger by feeding it. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 19, 2001, 5:44pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/29 "2001-01-19T17:44:36Z")
> [@](#): > > You could make the hole smaller by pumping out any stray radiation that is emitted from the event horizon, and you could make it bigger by feeding it. Except for very tiny holes Hawking radiation is so weak as to be totally neglible. For instance for a 2 solar mass black hole it would take 1x10[sup]66[/sup] years for it to evaporate. That’s 10000000000000000000000000000000000000000000000000000000000000000000000000000 years. ---
**Author:** ![Cerowyn](https://avatars.discourse-cdn.com/v4/letter/c/82dd89/32.png) [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)\ **Post date:** [January 19, 2001, 9:09pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/30 "2001-01-19T21:09:05Z")
There is an interesting discussion of Hawking radiation here: [http://math.ucr.edu/home/baez/physics/hawking.html](http://math.ucr.edu/home/baez/physics/hawking.html). With regard to building a Dysan sphere around a black hole: how would you “tow around at will” that kind of mass??? Even a one solar mass BH (assuming you could put a naturally occuring BH on a diet by pumping energy out of the sphere as described) is about 1.989e30 kg ([http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html](http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html), one of my most favourite sites on the web); that is one **hell** of a mass to start towing around. And, once you start growing or shrinking the BH, it’s gonna play havoc with the Dysan sphere. If the BH gets a little off-centre, or if the sphere is too thick, tides are going to rip it to shreds. Cool idea, though. 🙂 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 20, 2001, 1:28am UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/31 "2001-01-20T01:28:10Z")
Back on to the subject of the OP, a large problem is that you’re trying to consider what’s going on inside the event horizon. From the point of view of someone outside the event horizon, all we can say about the inside is its mass, its angular momentum, its electric charge, and (if there is such a thing) its magnetic charge. It makes no observational difference whether the contents of the hole continue their collapse after passing the event horizon. Even without the absolute barrier of the event horizon, collapsing matter still doesn’t present a problem with conservation of energy. Consider that figure skater again. When she pulls her arms in, she does have to exert a force and expend energy (which turns into rotational kinetic energy), but that energy has to come from somewhere. In the skater’s case, that’s chemical energy in her body, and in the case of an object collapsing due to gravity, it’s gravitational potential energy. If you just take the equations to a (theoretically somewhat suspiscious) limit and assume that something gravitationally collapses to a point, then the rotational kinetic energy would go to infinity, but the gravitational potential energy would go to negative infinity at the same rate, exactly balancing it out. 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). ---
**Author:** ![jb\_farley](https://avatars.discourse-cdn.com/v4/letter/j/9d8465/32.png) [@jb\_farley](https://boards.straightdope.com/u/jb_farley)\ **Post date:** [January 20, 2001, 7:14am UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/32 "2001-01-20T07:14:09Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*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). > \*\* **Chronos** , that is very sad. jb ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [January 20, 2001, 1:12pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/33 "2001-01-20T13:12:34Z")
> [@](#): > > ## quote: > > ## while ago, on this website, folks who know more than me convinced me that all the mass in a black hole is contained in a single, dimensionless point. Spinning? What is the significance of a black hole spinning if all the mass is in a single point? How would you measure it? > > Well the developers of string theory would argue with that point but if the singularity is spinning then so to would be the horizon. But the event horizon is just the area where the escape velocity = c or where gravity is a certain number, there is no substance to it how can it rotate(unless you can come up with a graviton? Anyway does anyone know what the accrleration would be at the event horizon in g’s, or m/s^2 or more likely km/s^2 or whatever units you can find? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [January 20, 2001, 5:16pm UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/34 "2001-01-20T17:16:58Z")
Well, actually what happens is that a spinning black hole causes space to swirl around with it. In other words if you were close to hole’s horizon you would spin with the hole but you would not be spinning with respect to space. This is called frame dragging and it’s the result of a solution to Einstein’s field equations called the Kerr metric ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 21, 2001, 6:06am UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/35 "2001-01-21T06:06:36Z")
The acceleration at the event horizon depends on the size of the hole. Specifically, it’s inversely proportional to the mass: a = c[sup]4[/sup]/(4_G_m), so for a black hole three times the mass of the Sun (total of 1.8_10[sup]31[/sup] kg), the acceleration would be 1.7_10[sup]12[/sup] m/s[sup]2[/sup], but for a million-solar mass hole such as might be found in the core of a galaxy, it’s only 5\*10[sup]6[/sup] m/s[sup]2[/sup]. The tidal force at the event horizon goes as the inverse square of the mass, so for a nice, massive hole, it’d actually be possible to pass the horizon without being torn asunder or pulled into spaghetti. ---
**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:** [January 21, 2001, 8:44am UTC](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488/36 "2001-01-21T08:44:45Z")
> [@](#): > > \*Originally posted by \*\*\*hibernicus \*\* > > > [@](#): > > > > \*Originally posted by \*\*\*DrMatrix \*\* > > IIRC if you consider the earth as your reference frame, pluto is travelling faster than 186,000 m/s. > > Okay, bearing in mind that in this context (we are talking about the speed of light) m/s denotes “miles per second”, are you sure? Lets do some very rough approximations. We will consider a system attached to the (rotating) earth. There are 24 hours per day. (Yeah, I know solar day vs. sidereal. I said rough.) Relative to a non-rotating coordinate system, at a distance of d objects are traveling at c where: d \* 2 pi/24hr = c d \* 2 pi/24 hr \* 1hr/60min = c = 1AU/8min (It takes light 8 mins to reach earth.) Solving for d: d = 90AU/pi (If it’s not too biblical, we can set pi = 3) d = 30AU So, anything further than 30 AU from earth is moving faster than 186,000miles/sec. Pluto is 39.5 AU from the sun. [Previous page](https://boards.straightdope.com/t/can-black-holes-ever-be-stopped-from-growing-in-mass/50488.md?page=1)