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Old 01-09-2020, 02:48 PM
Jackknifed Juggernaut is offline
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Quick Black Hole Mass Question


If I understand correctly, observers outside of the event horizon of black holes never see any objects actually falling in. Due to the immense gravity near it, and the associated time dilation, we can only see the object that is falling in gradually slow down and eventually freeze (from our perspective). Of course, to an observer who is in or on the object, time flows normally and she witnesses herself fall in.

In theory, there could be some observer somewhere in our galaxy who has monitored Sag A* since its formation. And like us, that observer has never actually seen anything fall in. However, that observer would presumably be measuring its growing mass from close-to-zero, at the beginning, to 4 million solar masses now.

Is this mismatch reconcilable? How can something grow to such a large mass if, from the outside, nothing actually falls in? Apologies if this is a dumb question.
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Old 01-09-2020, 04:18 PM
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The result of nothing ever actually crossing the horizon is based on the assumption that the mass of the black hole isn't changing. When you're looking at one infalling object that's very small compared to the existing hole, this is a good approximation. When you're looking at the hole growing from near-zero to supermassive, it's not.

And as an aside, you don't see things frozen on the hole's surface, either. There's still a last photon emitted by the infalling object, and it's not very long after a naive observer would expect the object to reach the horizon. Shining more light on it to illuminate it won't help, because that light will just get swallowed, too.
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Old 01-10-2020, 04:26 PM
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Sorry, perhaps I didnít ask the question the right way.

I thought that as any object approaches the black hole, its clock keeps getting slower and slower, until it basically freezes (from the perspective of the outside observer). So from the outside, the object never falls in.

We only know the mass of the black hole due to its gravitational effect on objects near it (but not by measuring whatís fallen in it). So if the outside observer was measuring its mass growing by just the matter falling in, rather than by its gravitational effect on objects outside of it, she would never measure it as growing (since nothing ever falls in).
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Old 01-10-2020, 05:14 PM
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Even if the black hole were not rapidly growing, you would see the object get redder and dimmer and finally vanish. To me that sounds like you see it fall into a big black hole in space. In this day and age, there should be nice videos simulating exactly what you'd see; I guess that is what this is supposed to be, but I don't see any details given.
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Old 01-10-2020, 05:23 PM
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This paper, for example, reports actual observations of stuff being sucked into a black hole.
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Old 01-10-2020, 05:42 PM
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Objects falling toward a black hole behave exactly like other falling objects. light can reflect off them, and they can be seen. (with considerable difficulty, by the way, even from up close, like one or two light years away. Accretion disks give off a lot of EM radiation as they approach.) At the radius where space is warped enough to make the escape velocity equal c, there is is no up. All the matter falling in is stretched out in proportion to the difference between the gravity at its "top" end and its 'bottom" end. The effect far exceeds the tensile strength of any atomic material. Perhaps even more than the structural integrity of other particles. The mass is the only thing left of the matter. (Or perhaps charge, and angular momentum, but I am not clear about that.)
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Old 01-10-2020, 07:00 PM
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You can't observe anything corssing the event horizon from the external region as, by definition, the event horizon is the horizon seperating events that can be observed from the external region and those that cannot.

However just becuase we cannot observe things crossing the event horizon it does not mean they aren't. The problem is that because the events bounded by the event horizon are not in the past of the infinite future of the external region, it's difficult or meaningless to try say"when" something crosses the event horizon from the point of view of an external observer.
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Old 01-11-2020, 03:42 PM
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Black holes are said to have charge. How is this measured? But I thought that any measure of charge requires emission of a photon. But no photon can escape a black hole, so how does that work?
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Old 01-11-2020, 04:18 PM
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Quote:
Originally Posted by DPRK View Post
This paper, for example, reports actual observations of stuff being sucked into a black hole.
Technically, I believe objects "fall" into black holes. They aren't "sucked" due to a pressure differential like a tornado or giant vacuum cleaner.


Quote:
Originally Posted by Hari Seldon
Black holes are said to have charge. How is this measured? But I thought that any measure of charge requires emission of a photon. But no photon can escape a black hole, so how does that work?
Photons do not have a charge.

It is true that nothing escapes a black hole once it passes the event horizon. However, black holes can throw off photons and other particles as they ingest matter. Quite spectacularly, actually.


I did some quick Googling and, while possible, black holes generally have a negligible charge. The "how" of their charge made my brain hurt so I stopped reading.




Quote:
Originally Posted by Triskadecamus
Objects falling toward a black hole behave exactly like other falling objects. light can reflect off them, and they can be seen. (with considerable difficulty, by the way, even from up close, like one or two light years away. Accretion disks give off a lot of EM radiation as they approach.) At the radius where space is warped enough to make the escape velocity equal c, there is is no up. All the matter falling in is stretched out in proportion to the difference between the gravity at its "top" end and its 'bottom" end. The effect far exceeds the tensile strength of any atomic material. Perhaps even more than the structural integrity of other particles. The mass is the only thing left of the matter. (Or perhaps charge, and angular momentum, but I am not clear about that.)
You are talking about "tidal forces" and the resulting "spaghettification"? That depends on the size of the black hole and your distance from the center. For larger and supermassive black holes you could drift well inside the event horizon before tidal forces have an effect on you.
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Old 01-11-2020, 05:32 PM
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In practice, it's expected that most black holes in the Universe would have a charge of only a few times the charge of an electron.

But in principle, a black hole can have an extremely large charge, large enough that the electromagnetic force between two black holes could be as great as the gravitational force (though no greater).

And the way that a black hole can have an electric field is the same as the way that it can have a gravitational field. You can, if you like, think of it in terms of virtual photons, or virtual gravitons, leaving the hole (though there are other ways of thinking of it that do not involve such particles). As long as no real particles escape, and no information beyond a few very basic points make it out, there's no problem.
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Old 01-13-2020, 02:27 PM
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The OP's question has been asked before, though under a different title. I am far from an expert on the subject but I engaged in some reading on the subject and (perhaps unscientific) speculation that may be interesting to some on that exact paradox, in posts #18, #20, and #22. As well as excellent contributions from others in that same thread.
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