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Questions around Black Holes


geordief
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Did Newtonian physics predict Black Holes?

 

As I am belatedly understanding it was GR that predicted them but what would Newtonian physics have made of the situation ?

 

Would it have envisaged explosions happening eventually as the mass increased ? How would it have viewed the scenario differently?

 

Were Black Holes something that were theorized and discovered only after Newtonian Physics was "out dated" ?

 

 

Coming back to GR ,might it be ,as I think I may have heard that we are not talking about a build up of matter but a change in spacetime curvature?

 

Is there any theoretical way such a curvature can be reversed? Is that Hawking radiation? The BH will slowly "empty out" ?

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Did Newtonian physics predict Black Holes?

 

Kind of:

 

The idea of a body so massive that even light could not escape was first put forward by John Michell in a letter written in 1783 to Henry Cavendish of the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
— John Michell

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[10][11] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[12]

https://en.wikipedia.org/wiki/Black_hole#History

 

I think the calculated radius would be different among other things.

 

Edit: Apparently not!

 

 

Coming back to GR ,might it be ,as I think I may have heard that we are not talking about a build up of matter but a change in spacetime curvature?

 

Those are really just different descriptions of the same thing. Mass is the curvature of spacetime.

https://en.wikipedia.org/wiki/Dark_star_(Newtonian_mechanics)

 

Yes, 'Dark Stars' were predicted before general relativity by John Michell in 1783. His idea was simple, if the escape velocity for some body is greater than the speed of light, then that body cannot be seen.

 

That is a much better link. And it also highlights the problem with the "escape velocity" description that is often still used wrt black holes.

Edited by Strange
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Those are really just different descriptions of the same thing. Mass is the curvature of spacetime.

 

 

I haven't had time to go into the replies yet but do you stand by that description?

 

I am familiar with the "mass curves spacetime and spacetime tells mass how to move" description but this seems different .

 

Is your word "is" to be taken loosely? " Mass and the curvature of spacetime are intimately related" is what you are saying ? Or "mass" and "curvature of spacetime " describe the same thing"?

Edited by geordief
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Or "mass" and "curvature of spacetime " describe the same thing"?

 

I think it is a bit more than mass being some distinct property that somehow causes space-time to curve. It is more that the curvature of spacetime defines what mass is.

 

I may be wrong in thinking of it in that way - it will be interesting to see what ajb has to say ...

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One says things like 'mass tells space-time how to curve and space-time tells mass how to move'. They are not the same thing, but via the equations of general relativity they are tied together.

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Subsequent to the detection of gravitational waves which seemed (to me) extraordinary even if ,apparently it was to have been expected would it be correct to say that the detection of Hawking radiation would be something so difficult that we might even suppose with a huge degree of certainty that it may be that scientists will never,ever actually detect them?

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Subsequent to the detection of gravitational waves which seemed (to me) extraordinary even if ,apparently it was to have been expected would it be correct to say that the detection of Hawking radiation would be something so difficult that we might even suppose with a huge degree of certainty that it may be that scientists will never,ever actually detect them?

I seriously doubt we will find a black hole small enough to radiate Hawking radiation. The blackbody temperature of the BH must be higher than the blackbody temperature of its surroundings.

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I seriously doubt we will find a black hole small enough to radiate Hawking radiation.

The only sort of thing that could be possible to to detect them in particle physics colliders or higher energy cosmic rays. I don't expect any results soon either way.

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  • 2 weeks later...

I think I may have another couple of different questions/observations about black holes that maybe I can ask here.

 

1: Is it possible to say that from the point of view of an outside observer the black hole never actually reaches any supposed "singularity ?

 

Does the process slow down at an exponentially increasing rate and does the BH simply keep approaching the singularity without ever reaching it (from the observer's pov) ?

 

2:There seems to be a major distinction between a small BH and a large BH..Is it possible to imagine that ,as a BH grows we can say it approaches an effective diappearance?

 

BH> infinity = no BH?

Edited by geordief
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I think the calculated radius would be different among other things.

 

Edit: Apparently not!

No, you were right the first time :P. In Newtonian gravity "r" is a distance, whereas when describing a Schwarzschild black hole "r" is merely a coordinate. Radial distances need to be found via an integral of ds. The r-coordinate only asymptotically approaches a measure of distance as it increases. So, even though the two equations look identical, they mean different things.

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1: Is it possible to say that from the point of view of an outside observer the black hole never actually reaches any supposed "singularity ?

An observer at infinity - so very far from the black hole - will not see anything reach the horizon let alone the singularity. Time dilation effects loosley mean that it will take an infinite amount of time for the object falling into the black hole to reach the event horizon - as measured by the far away observer. As far as the object falling in, it takes finite time to pass the horizon and reach the singularity (mod what is actually at this classical singularity)

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. As far as the object falling in, it takes finite time to pass the horizon and reach the singularity

And my question is... let say that this object that is falling into singularity observers the rest of the universe. What does it see? Does it see the whole universe speeds up? I find it puzzling.

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And my question is... let say that this object that is falling into singularity observers the rest of the universe. What does it see? Does it see the whole universe speeds up? I find it puzzling.

Pretty sure I have heard that.

 

In Special Relativity observers in mutually moving (at relativistic speeds) frames of reference both see each other slowed down but with time dilation due to gravity wells it is as you described and one will see the other speeded up. (must update my diary :) )

An observer at infinity - so very far from the black hole - will not see anything reach the horizon let alone the singularity. Time dilation effects loosley mean that it will take an infinite amount of time for the object falling into the black hole to reach the event horizon - as measured by the far away observer. As far as the object falling in, it takes finite time to pass the horizon and reach the singularity (mod what is actually at this classical singularity)

 

Do you think we can extrapolate from that that a singularity never happens and so cannot represent a problem in GR?

 

I mean we cannot change our FOR qualitatively in this regard and do all our theories have to take account of that fact?

 

Is that just playing with words?

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And my question is... let say that this object that is falling into singularity observers the rest of the universe. What does it see? Does it see the whole universe speeds up? I find it puzzling.

 

 

By chance, there is a very good answer to this just posted on another forum: http://cosmoquest.org/forum/showthread.php?155307-Black-holes-Can-anything-ever-fall-into-it&p=2362833#post2362833

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Do you think we can extrapolate from that that a singularity never happens and so cannot represent a problem in GR?

It is a problem in GR - but we expect quantum gravity will remove this singularity. But still, at distances not too close to 'r=0' the physics will not change much from the classical situation.

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It is a problem in GR - but we expect quantum gravity will remove this singularity. But still, at distances not too close to 'r=0' the physics will not change much from the classical situation.

Strange's link in the above post#16 seems to deal with this issue(not that I could follow the discussion).

 

I think my idea was ,kind of "out of sight out of existence" (to paraphrase) but it seemed to have been taken on board and dismissed there.

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If time dilation is effectively infinite past the event horizon, would that imply that once a body has collapsed past its Schwarzschild radius it could be seen as effectively frozen in time - and therefore once past that threshold it will never have time to collapse further and become a singularity because of the infinite time dilation?

 

Ah, that sounded a lot less garbled in my head.

Edited by Daecon
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If time dilation is effectively infinite past the event horizon, would that imply that once a body has collapsed past its Schwarzschild radius it could be seen as effectively frozen in time - and therefore once past that threshold it will never have time to collapse further and become a singularity because of the infinite time dilation?

If you mean 'does this mean black holes don't form?' then the answer no. This does not imply that black holes take an infinite amount of time to form as see by near by observers.

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  • 2 weeks later...

Is the BH filled with anything? I have heard that it is better described as a deformation in Space Time .

 

Is that true ? Are there no objects " floating about " or gravitating towards the "centre".

 

I think I have also heard that ,for a large BH an object would not notice anything as it passed through the event horizon and would continue all the way to the "singularity" (I personally understand the "singularity" to be the physical equivalent or counterpart to a mathematically undefined term)

 

That might imply that there are objects inside a BH

 

Does the fact that gravitational waves were so successfully and accurately predicted from the recent Binary BH merger say anything about the understanding of the internal working of Black Holes? Was anything confirmed in that regard. as a result of that successful observation ?

 

Just as an observation in the dark (an uninformed guess),is it possible that in a BH ,as in the universe per se the "centre is everywhere" ? Would that ,if it was true have any relevance to the problem with the singularity?

Edited by geordief
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