# How does a black hole know how big it should be?

I was reading an article the other day about the second largest black hole found, something around 17 billion solar masses big, with an estimated event horizon 11 times the orbit of Neptune and it got me thinking. If all the matter generating the gravity lies in the singularity, how does that fact communicate itself thru the event horizon, where no communication can occur, to the area outside of the event horizon? Essentially, how does the outside universe know how much mass is contained in the singularity if there is no way for anything inside an event horizon to interact with anything outside of it?

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phinds
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I was reading an article the other day about the second largest black hole found, something around 17 billion solar masses big, with an estimated event horizon 11 times the orbit of Neptune and it got me thinking. If all the matter generating the gravity lies in the singularity, how does that fact communicate itself thru the event horizon, where no communication can occur, to the area outside of the event horizon? Essentially, how does the outside universe know how much mass is contained in the singularity if there is no way for anything inside an event horizon to interact with anything outside of it?
Black holes interact with the outside world via gravity and the resulting gravitational lensing.

Khashishi
As long as the object stays still, the gravitational field around the object doesn't have to propagate, and thus is not affected by the speed of light. It is only changes in the gravitational field that have to propagate at the speed of light, and therefore cannot escape the black hole. So, I suppose the gravitational field is somehow frozen in from when the black hole formed. I'm just guessing.

Gravity is not "something" such as a photon or a piece o matter. Gravity is bending of space-time. Light cannot escape because light has a maximum speed. The fabric of space has no limiting speed as far as I know. Although that's irrelevant. Suffice it to say that gravity is nothing you could see or touch, it's an indirect result of weight. When you sit down on your bed the mattress bends beneath you, gravity does the same thing in a 3D perspective.
The black hole essentially punches a hole through the space. A hole to where? Nobody knows.

Drakkith
Staff Emeritus
Gravity is not "something" such as a photon or a piece o matter. Gravity is bending of space-time. Light cannot escape because light has a maximum speed. The fabric of space has no limiting speed as far as I know.
Changes in gravity do propagate at c. Note that in a static field nothing is moving. Nothing even needs to come out of the black hole at all for its gravity to be felt.

Gravity waves themselves don't even need to "exit" the black hole itself either depending on how you look at it. A gravity wave that meets the event horizon of a black hole will alter the black hole itself in such a way that the event horizon stretches and bends just slightly, which will alter the metric and the resulting gravitational wave from the event horizon bending back and forth will be indistinguishable from a gravitational wave passing right on through the black hole.

So, I suppose the gravitational field is somehow frozen in from when the black hole formed. I'm just guessing.
One way I've heard it described is that infalling material never reaches the event horizon from out point of view, so you don't even need to consider what happens from "beyond" the event horizon because nothing ever passes beyond it anyways. It will be redshifted beyond detection however, so you would still be unable to actually see the material that is very very close to the event horizon.

Wow you lost me :-). The way people have explained it to me is that space has be altered at any speed.
Isn't that the idea behind NASA's faster-than-the speed of Light project?

Drakkith
Staff Emeritus
Wow you lost me :-). The way people have explained it to me is that space has be altered at any speed.
Isn't that the idea behind NASA's faster-than-the speed of Light project?
Hmmm. I admit I don't know enough to comment on it. I know GRAVITY propagates at c. If there are other types of metric changes that don't obey that, I am unaware of them.

Your mistake lies in the assumption that "there is no way for anything inside an event horizon to interact with anything outside of it". Are you saying gravitational forces can't escape the event horizon because.... gravitational forces pulls them back in? Hmm...

PeterDonis
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Essentially, how does the outside universe know how much mass is contained in the singularity if there is no way for anything inside an event horizon to interact with anything outside of it?
Because the outside universe knows how much mass originally collapsed to form the black hole. That is where the information about the mass comes from: from the past history of the object that collapsed. It doesn't come from inside the hole.

PeterDonis
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So, I suppose the gravitational field is somehow frozen in from when the black hole formed.
This is more or less correct. A better way to say it is that, as the object that originally formed the hole collapses, the field in the vacuum region outside the object becomes more and more "frozen"; and once the collapsing object has formed an event horizon around itself, the field outside the horizon is entirely "frozen".

PeterDonis
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Black holes interact with the outside world via gravity and the resulting gravitational lensing.
But this is all due to the field outside the horizon, which is formed by the original collapsing object. Nothing has to "propagate" from inside the horizon.

PeterDonis
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The fabric of space has no limiting speed as far as I know.
Yes, it does; it's the speed of light. "The fabric of space" is just another word for spacetime (or at least that's the only meaning I can usefully assign to that phrase), and the limiting speed in spacetime is the speed of light. The reason that doesn't stop the black hole's gravity from affecting objects outside it is that the gravity doesn't come from inside the hole; it comes from the object that originally collapsed to form the hole. Information about that collapse, before the horizon forms, can travel to the rest of spacetime outside the hole at the speed of light, although that's not really the best way to phrase it since the information doesn't "travel"; a better way to state it is that the field at a given point outside the hole is entirely determined by what's in the past light cone of that point, which includes the history of the collapsing object outside the horizon.

From an earlier discussion posted by PeterDonis:

The field doesn't have to propagate from inside the EH to outside. The EM+ field outside the hole that makes charged particles move differently from uncharged ones is not coming from inside the hole; it's coming from the charge-current density of the object that originally collapsed to form the hole. Similarly, the gravity felt outside the hole isn't coming from inside the hole; it's coming from the stress-energy of the object that originally collapsed to form the hole. The field was present prior to the BH formation.
+ my note: or gravitational curvature

edit: oops..I see Peter posted while I was finding his prior explanation.....good job, Peter! they seem consistent!!

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PeterDonis
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good job, Peter! they seem consistent!!
Thanks! I've been getting plenty of practice...

turbo
Gold Member
I was reading an article the other day about the second largest black hole found, something around 17 billion solar masses big, with an estimated event horizon 11 times the orbit of Neptune and it got me thinking. If all the matter generating the gravity lies in the singularity, how does that fact communicate itself thru the event horizon, where no communication can occur, to the area outside of the event horizon? Essentially, how does the outside universe know how much mass is contained in the singularity if there is no way for anything inside an event horizon to interact with anything outside of it?
Nature does not "know" anything. Rules are rules, and the rules do not allow for anthropomorphism WRT to black holes, planets, etc. These bodies do not "know" anything regarding their formation.

PAllen
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Nature does not "know" anything. Rules are rules, and the rules do not allow for anthropomorphism WRT to black holes, planets, etc. These bodies do not "know" anything regarding their formation.

Yes, it does; it's the speed of light. "The fabric of space" is just another word for spacetime (or at least that's the only meaning I can usefully assign to that phrase), and the limiting speed in spacetime is the speed of light. The reason that doesn't stop the black hole's gravity from affecting objects outside it is that the gravity doesn't come from inside the hole; it comes from the object that originally collapsed to form the hole. Information about that collapse, before the horizon forms, can travel to the rest of spacetime outside the hole at the speed of light, although that's not really the best way to phrase it since the information doesn't "travel"; a better way to state it is that the field at a given point outside the hole is entirely determined by what's in the past light cone of that point, which includes the history of the collapsing object outside the horizon.
Interesting. But now I don't understand how NASA could build what they were talking about in the article that I posted. Could you explain that? Thanks :).

PeterDonis
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But now I don't understand how NASA could build what they were talking about in the article that I posted. Could you explain that? Thanks :).
The Alcubierre "warp drive" is highly speculative; I would not take the fact that NASA has a project to try to build one as good evidence that one can actually be built.

That said, the "warp drive" spacetime does not violate the law I stated; in other words, it does not involve anything moving outside the local light cones. What it claims to be able to do is to warp the light cones themselves. This in itself is not speculative; ordinary gravity does the same thing. But ordinary gravity, meaning gravity caused by ordinary matter or radiation, can't warp the light cones enough to allow anything that looks, globally, like "faster than light" travel; that requires a kind of substance called "exotic matter", which most physicists do not believe is physically possible--at least not in the quantities that would be needed for an FTL ship. When the article talks about a "negative vacuum energy ring", it's talking about trying to generate this kind of substance, which, if it were to exist, could warp spacetime in a way that ordinary gravity can't. Personally, I'll believe it when I see it.

Ahh... I can't say I fully understand but at least I have a better idea. Thank you yet again good Sir.

Drakkith
Staff Emeritus
Ahh... I can't say I fully understand but at least I have a better idea. Thank you yet again good Sir.
It's pretty much saying: "If we can find some kind of material to do something that we've never ever seen done, ever, then according to the math it will make spacetime our plaything".
Except it uses much more sciency sounding words.

Nature does not "know" anything. Rules are rules, and the rules do not allow for anthropomorphism WRT to black holes, planets, etc. These bodies do not "know" anything regarding their formation.
I maybe was unclear, I didn't mean that nature was self aware about it, but the fact is everything is about information transmission. Position, mass, etc.... and information only travels at most at the speed of light. A black hole of a given mass creates an event horizon of a certain size, add another solar mass and it gets a bit bigger. The characteristics of the singularity have changed. However, since space-time is bent completely back on itself, there is no way forward in time that doesn't go back to the singularity, how can any changes in it be communicated to the outside universe?

PeterDonis
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A black hole of a given mass creates an event horizon of a certain size, add another solar mass and it gets a bit bigger. The characteristics of the singularity have changed. However, since space-time is bent completely back on itself, there is no way forward in time that doesn't go back to the singularity, how can any changes in it be communicated to the outside universe?
See my posts #10 - #13. The outside universe doesn't have to "see" any changes in the singularity; it "sees" the mass falling into the hole, and that is sufficient for the outside universe to now "see" an increased mass where the hole is.

Considering quantum entanglement. I'm not sure we can say information cannot exceed the speed of light. In normal circumstances thats certainly true till one considers entanglement thats one form that Einstein aptly named spooky action lol

PeterDonis
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Considering quantum entanglement. I'm not sure we can say information cannot exceed the speed of light.
Yes, we can. You can't use quantum entanglement to transmit information faster than light; you can run spacelike separated experiments that are statistically correlated more than seems classically possible, but you only know about the correlations after you've compared the experimental results, and the comparison can only be done at or slower than the speed of light.