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

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Discussion Overview

The discussion revolves around the nature of black holes, specifically how the mass of a black hole is perceived from outside its event horizon. Participants explore concepts related to gravity, the propagation of gravitational fields, and the interaction between black holes and the external universe.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the gravitational field around a black hole does not need to propagate from inside the event horizon, as it is determined by the mass that originally collapsed to form the black hole.
  • Others argue that gravitational forces can still be felt outside the event horizon, as the field is "frozen" from the time of the black hole's formation.
  • A participant suggests that changes in the gravitational field propagate at the speed of light, but in a static field, nothing is moving, so gravity can be felt without needing to escape the black hole.
  • Some contributions emphasize that gravity is a bending of spacetime rather than a force that can be seen or touched, and that the interaction with the outside universe occurs through gravitational lensing.
  • A later reply questions the assumption that nothing inside the event horizon can interact with the outside, suggesting that gravitational forces may still have an effect.
  • Another participant notes that the information about the mass of a black hole comes from the history of the object that collapsed to form it, rather than from anything inside the event horizon.
  • There is mention of gravitational waves and how they might interact with the event horizon, with some suggesting that they can alter the black hole's event horizon without needing to escape it.

Areas of Agreement / Disagreement

Participants express multiple competing views on how black holes interact with the outside universe and the nature of gravitational fields. The discussion remains unresolved, with no consensus reached on the specifics of these interactions.

Contextual Notes

Some statements rely on assumptions about the nature of gravity and spacetime that may not be universally accepted, and there are unresolved questions regarding the propagation of gravitational fields and the effects of gravitational waves near the event horizon.

Slamfu
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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?
 
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Slamfu said:
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.
 
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.
 
Feodalherren said:
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.

Khashishi said:
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?
 
Feodalherren said:
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...
 
  • #10
Slamfu said:
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.
 
  • #11
Khashishi said:
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".
 
  • #12
phinds said:
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.
 
  • #13
Feodalherren said:
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.
 
  • #14
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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  • #15
Naty1 said:
good job, Peter! they seem consistent!

Thanks! I've been getting plenty of practice... :wink:
 
  • #16
Slamfu said:
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.
 
  • #17
turbo said:
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.

What about General Gaia Theory? :wink:
 
  • #18
PeterDonis said:
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 :).
 
  • #19
Feodalherren said:
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. :rolleyes:

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.
 
  • #20
Ahh... I can't say I fully understand but at least I have a better idea. Thank you yet again good Sir.
 
  • #21
Feodalherren said:
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.
 
  • #22
turbo said:
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?
 
  • #23
Slamfu said:
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.
 
  • #24
Considering quantum entanglement. I'm not sure we can say information cannot exceed the speed of light. In normal circumstances that's certainly true till one considers entanglement that's one form that Einstein aptly named spooky action lol
 
  • #25
Mordred said:
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.
 
  • #26
Yeah I' ve seen that argument before. In the case of using entanglement for communications. I have no wish to misdirect this thread. Howevee to me that's like saying a particle either has all or no information till examined
 
  • #27
Mordred said:
I have no wish to misdirect this thread. Howevee to me that's like saying a particle either has all or no information till examined

This should really be discussed in the quantum physics forum, not here; getting into it further here would indeed misdirect the thread. If you want to post this as a separate thread in the quantum physics forum, feel free to put a link here so whoever is interested can follow up.
 
  • #28
Agreed though there is no need to add another post to something that's already been disussed here countless times lol
 
  • #29
PeterDonis:
...The outside universe doesn't have to "see" any changes in the singularity; it "sees" the mass falling into the hole...,
Yes. For those trying to get a handle on BH descriptions, consider that our math
covers the BH except for the singularity at the center...that is where we have no description, not via relativity nor quantum mechanics...On the other hand, the horizon of a black hole, another type of singularity, a surface where causality between inside the BH to the outside is lost and where we think we understand the physics.

A BH is the ultimate 'roach motel': you can get in, but you can't get out!

So whatever model and space time space-time structure you consider, like Schwarzschild for example, or the more general Kerr [with some rotation] nobody knows just what happens when the radial component of the metric goes to infinity...that's the center singularity.

There is another 'singularity' when the temporal component of the metric goes to infinity...the horizon...time appears to stop at the horizon for a distant outside observer...but this is a coordinate effect, not a physically real local effect... so another coordinate set, like those of a free falling observer continue smoothly thru the 'apparent singularity'...
 
  • #30
Here are excerpts of a description I don't see too often in these forums:
Black Hole Complementarity

Leonard Susskind, THE BLACK HOLE WAR (his arguments with Stephen Hawking)

In this view, all the information ever accumulated by a BH is encoded on a stretched horizon...a Planck length or so outside the event horizon and about a Planck length thick. This is a reflection of the Holographic principle: all the information on the other side of an event horizon is encoded on the surface area of that event horizon...

Of every 10,000,000,000 bits of information in the universe, all but one
are associated with the horizons of black holes. [So if you can lose information via black holes, it a really,really,really big deal.](p238) Today a standard concept in black hole physics is a stretched horizon which is a layer of hot microscopic degrees of freedom about one Planck length thick and a Planck length above the event horizon. Every so often a bit gets carried out in an evaporation process. This is Hawking radiation. A free falling observer sees empty space.

(p258) From an outside observer’s point of view, an in falling particle gets blasted apart….ionized….at the stretched horizon…before the particle crosses the event horizon. At maybe 100,000 degrees it has a short wavelength and any detection attempt will ionize it or not detect it!

(p270)…. eventually the [incoming] particle image is blurred as it is smeared over the stretched horizon and….and the image may (later) be recovered in long wavelength Hawking radiation.

PS; In his argument over the years with Stephen Hawking, Susskind was correct in his interpretation of BH !Here is an online source describing BH recommended to me by others in these forums...some good insights..:

http://www.jimhaldenwang.com/black_hole.htm
Spacetime Geometry Inside [and around] a Black Hole

by Jim Haldenwang
written Nov. 12, 2004
revised July 30, 2012
 
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