Can black holes grow?

[MeMeX]

Hello everybody,

I'am just new at this forum and want to ask (you physics minded) a simple question (certainly the question indicates a lack of understanding ).
How do black holes ever grow?
For is it not at the event horizon where time freezes (according to einsteins theory) for masses speeding up to the velocity of light? Clocktime at the horizon freezes relative to our clocks. Then, how can black holes ever accumulate mass in the timeperiods our clocks register? Growing implies that it occurs in our time.
Certainly there are black holes in the universe which must have accumulated mass in the lifespan of our universe. It has become clear that almost every galaxy contains a supersized black hole at its center.
Is this a real paradox? What is the fault in my reasoning?
The question bothers me already a long time!

MeMeX (the Netherlands)

PS: What a great website, what a great machine!

[Harbles]

I don't have the theoretical chops to compose a meaningful answer so I leave it up to the experts, like this guy. http://motls.blogspot.com/2009/01/lhc-b ... conds.html
Or if you prefer a video ... http://cdsweb.cern.ch/record/1120625

Don't worry, be happy!

[Anitusar]

I am not into theory either, but i reason the following:

Image an asteriod falling into the black hole. Sitting on the asteroid, you will expierence the ride into the black hole with your "normal" time. But when you want to inform your buddy outside about your ride, the information he will receive him later and later, until at somepoint no information will get to him. So for him, you will be frozen in time as he gets no new information. He will only notice, that the black hole will have gained in mass.

So to my understanding, not the process of the black hole getting mass is delayed in time, but the information from this mass getting into the black hole is not reaching you.

But maybe i think to simple.

[Xymox]

But if the mass of the black hole appears to us to be frozen in time.. How can it get bigger.

Interesting

[Stephen]

Interesting question. You also have to account for Hawking radiation - if black holes can grow, but they evaporate too fast for them to absorb matter, do they really have the possibility of ever growing?

[Anitusar]

Hmmm, we have to drag a theorist to this forum.

**Anitusar goes to stealth mode, closing in on the theory department**

[March_Hare]

Huh... where did Anitusar suddenly go to?

[CharmQuark]

Anitusar where are you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Xymox]

He was absorbed by a black hole in the theory Department...

[CharmQuark]

NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

[March_Hare]

Nah, if he'd be off to the experimental department there would be reason to worry about black holes (and who knows what else!).

No chance there'd be a black hole in the theory dept... except of course for theoretical black holes, but those aren't too dangerous as long as you don't approach them theoretically.

[Anitusar]

I am back from my undercover mission. Unfortunaly an old Prof. caught me and discovered, that i am from the experimental department. I got and tried to my way out again. I lost, and they started to and me. When this guy came in i made a .

So no answer but at least i can celebrate . Happy x-mas to all

[CharmQuark]

Anitusar

I am please you had help in escaping now all that need to happen is for you to come here and spend new year with me anyways thats a different story I think i should get on my and see if i can find my Prince Charming

Have a wonderful Christmas and a even better New Year

[c.h.man]

Based on what I heard at a lecture a year or two ago, though the object falling into the black hole experiences less time than someone who's watching from a safe distance does, it has no effect on it's velocity, or on how long it takes to reach the singularity.

It's kinda like what happened in the relativity clock test. For those of you who don't know what I'm talking about, they discovered that if you take two clocks, let one sit still and have the other move really fast, the faster one will fall behind because the faster an object moves, the slower time occurs for that object. If you could get a person moving fast enough, he or she would age only half as fast as a person sitting still. Maybe I should get my insurance company to cover plane tickets, cause I age a little less when flying really fast

[photino]

>How do black holes ever grow?
>For is it not at the event horizon where time freezes (according to einsteins theory) for
>masses speeding up to the velocity of light? Clocktime at the horizon freezes relative to our
>clocks. Then, how can black holes ever accumulate mass in the timeperiods our clocks
>register? Growing implies that it occurs in our time.

I'll take a stab at this... it's a very good question.

You are referring to the usual description of a "probe" mass falling into a black hole. This description is valid, but there is a hidden assumption in the mathematics used to describe it: namely that the mass of the infalling object is negligible compared to the mass of the black hole ("probe mass" is the technical term for this assumption). As a consequence, it is a reasonable approximation that the black hole is not affected by the infalling mass at all. This is fine if one is interested in questions such as what an infalling observer would experience (she crosses the event horizon in a finite time, experiencing nothing special at that point), or what an external observer would see watching the probe mass fall in (you say "time freezes at the horizon" but it would be better to say that the signal reaching an outside observer becomes more and more redshifted (lower in energy) and thus harder and harder to detect. Eventually, for any given measuring device, you will no longer detect anything.)

BUT if you are interested in a description of the growth of black holes due to infalling matter, the probe mass method is not much use -- as by assumption the black hole remains static and unaffected. The mathematics of the full description (without the probe mass approximation, and including the "backreaction" of the black hole to the infalling mass) is MUCH harder. (In fact I'm not sure there's a fully worked out explicit solution? If anyone knows more please reply...) In the end the mass of the black hole (as observed by an outside observer) will indeed grow. You can perhaps sort of see this by noting that the event horizon will expand when mass falls in the black hole, encompassing the region really close to the old event horizon responsible for the "time freezing/extreme redshift" phenomena you refer to.

Hope this helps!