Black Hole Question

[xelasnave]

They are out there, apparently up to 20 billion solar masses, maybe larger,
Past the event horizon how much is known.
It seems the consensus suggests a singularity where the matter resides ..infinitely small and infinitely dense.
Is that view a math construct only or is it sciences view of reality.
I find it impossible to imagine 20 billion solar masses compacted to a point smaller than the head of a pin.
I could imagine an object of some reasonable size having the mass observed but to suggest something infinitely small seems unrealistic.
What is at the core of a black hole?
Do scientists believe to find a singularity or something more akin to a pulsar..

[mithrandir]

You should consider a black hole to be the entire space inside the Schwarzschild radius.

The Schwarzschild radius is proportional to the mass with a proportionality constant involving the gravitational constant and the speed of light:

r_s=2*G*m/(c**2)

where:
r_s is the Schwarzschild radius;
G is the gravitational constant (6.67384e-11);
m is the mass of the object;
c is the speed of light in vacuum (299792458).

For 20 billion solar masses that is 5.9e13 metres or 395 Au

[PeterEde]

Didn't a physicist recently say black holes are impossible

[xelasnave]

Thank you Andrew.
I have up until recently considered a black hole as you suggest.
Such that a super massive black hole may engulf our solar system, at least the event horizon would be of such size.
It was pondering the core or the singularity that I started to think about.
It seems to me that irrespective of the size of the black hole when considered at the event horizon the core would be the same size for all irrespective of the S radius..which I think is the event horizon..maybe I am off the mark there I don't know if the two are the same.
My question is I suppose..Does mainstream view the core as a singularity as reality or is it their way of saying they deal with the core as a point rather than possibly a larger compact body.
Say for a super massive black hole is the core seen as a point or say a larger object say as large as Mercury but very dense.

[xelasnave]

There is no way to observe the core so I expect it is determined by what the math suggests.
Is it reasonable to suggest nothing can prevent the collapse such that a singularity is the only conclusion.

[xelasnave]

People say all sorts of things but mainstream has accepted black holes and offer many observations suggesting their existence.
Recently it has been reported black holes may not exist however it appears to me that the physist only suggested some stars may not form a black hole when experiencing collapse.
Her research was sensationalised by a journalist and she has yet to publish in a peer revieved journal...so she has not said they do not exist.
It is clear objects with massive gravitational influence have been observed and mainstream believe they are black holes.
My question is about their make up not if they have found any..because clearly observations show many galaxies appear to have a super massive black hole at the galactic.centre.

[PeterEde]

I'm all for them. Make for great movies :)

[sjastro]

Hello Alex,

As mithrandir pointed out a BH is defined as an object whose physical size is less than S (Schwarzschild) radius.
The physical singularity is an artefact or problem in GR.
GR is a scale dependent theory and breaks down at small scales, the physical singularity is an example.

In Quantum mechanics a physical singularity cannot exist.
A simple analogy using astrophotography can help explain this.
Stars are considered theoretical point sources but when starlight passes through the Earth's atmosphere the point source jumps around and is smeared over a small area. When a star is imaged it is not a point source but has a definable size.
Space time at small scales "behaves" in the same way.

We can think of black holes as being compact bodies but only from our perspective of space time outside the the S radius.
Space time inside the S radius is very different. For example an object travelling away from a BH inside the S radius is moving backwards in time, spatial dimensions have time like characteristics and time has spatial like characteristics.

The fact is we don't know what's going on inside the S radius.

Regards

Steven

[xelasnave]

Thank you Steven that helped immensely.

[Dave2042]

Agree with all this, but a slight expansion on the last point.

It is certainly true that we don't know what is going on inside S, in the sense that it is not something we have observed, and potentially can't observe even in principle, and so it's entirely possible that our current understanding of this is not entirely correct (ie some aspects of GR will require some modification). Of course this is a subset of the broader truth that we know that both GR and QM can't be completely correct and that one or both will need some adjustment.

That said, I don't think it's fair to say that we don't know in the sense of being entirely clueless. We know that GR is a very good description of most things. In a large enough black hole, well away from the singularity, space locally doesn't look different from space outside a black hole. Therefore it is reasonable to assume that it's pretty unlikely that GR isn't a good description of it.

I do get annoyed when people (not directed at you, Steven, or anyone else in particular) suggest that just because we don't perfectly understand some extreme phenomenon, that means all science is up for grabs and scientists can't be trusted on anything.

Harrumph.

[sjastro]

Hello Dave,

About the only the thing we do know as you have stated is that for a sufficiently massive black holes the local space time on either side of the horizon is essentially equivalent.

The problem is how one measures space and time inside the horizon.
For example if you ask astrophysicists how they would measure the distance between the singularity to a spacecraft orbiting outside the event horizon of a BH, they will give a convoluted answer known as the circumferential distance. In other words they can only calculate the distance through the circumference of the orbiting spacecraft.

Suppose the orbiting astronauts attempted to directly measure the distance using a "gigantic" measuring stick. Outside the event horizon each pointer on the measuring stick is separated by a distance interval but not separated in time as a clock at each pointer is measuring the same time.
What happens when the measuring stick is passed over the horizon.
Assuming the stick isn't destroyed by tidal forces, part of the stick outside the horizon is still measuring distance at the same time.
The part of the stick inside the horizon, something very strange is happening, if the mathematics describing what is occurring inside the horizon is correct, then the pointers are no longer separated by distance but are separated by time.
Hence it is impossible to directly measure the distance as time and space inside the horizon is very different from the astronaut's own frame of reference.

The point is we don't understand what's going on inside a BH because we can't even measure what we trying to define.

I think the point Alex was making is "what is it like on the inside" rather than what is it like from the perspective of someone looking in.

I have absolutely no doubt on the existence of BHs given the overwhelming evidence.

Regards

Steven

[Dave2042]

I think we are coming at this from slightly different angles. I was simply thinking about the theory side of things (ie does the maths of GR apply), whereas you seem to be thinking more practically about what one would experience inside S, which I'd agree is a whole lot less certain.

That said, I still feel a bit more 'optimistic' than you. If I recall correctly (GR classes 20 years ago), the 'standard' way of looking at this is that in the frame of reference of the observer inside the black hole, nothing much changes as you cross the event horizon, other than that you become unable to stop moving towards the singularity. That seems comprehensible to me.

Interested to hear if that's not actually the case.

[speach]

No is was a mathematician, and she didn't say that there were impossible but unlikely

[xelasnave]

Steven you are correct...I want to know what the core looks like close up.
How to somehow visualise a singularity and how matter behaves therein.
A godseyeview explained to a five year old child.
A speculation at best based on what we know and delivered via analogy.

And Dave never let folk annoy you there is nothing gained .
I live in an area where folk are anti vaccination, believe in crystal healing, pyramid power, tarot cards etc.

You just have to leave them be and accept they are and will remain like that ..Don't raise your blood pressure to accommodate their beliefs.
Science at least works upon the unknown seeking a testable reality and it matters not if the uninformed bleet one way or another. What matters is maintaining your health and happiness ..annoyances destroys both.

[xelasnave]

I see your point but perhaps Hollywood causes problems similar to the sensational reporting of most science such that the message taken away by casual viewers or readers is not the message science delivered.
I read the science news first then try and read the paper..if you find that there is no paper well remember a journalist may have cherry picked to get only exciting bits so his story becomes well read. Even as a layman I can get a grip on what the researchers have said which often is different from the journalists report.
Next I seek forums where the matter is being discussed by real scientists and get a feel for how they regard the research.
Take the recent news...black holes don't exist..well the research has not been peer reviewed, and the research did not say ...black holes don't exist...it seemed to say no more than..in certain situations Hawking radiation may act to prevent the formation of a black hole...
Then when I read the science forums you find out more...background on the physicist etc...well guess what I am able to conclude the original reporting was very misleading.
Years of research supported by peer reviewed papers is rarely..if ever..overturned by one journalist.

[xelasnave]

Hi Simon I also called her a physist sorry about that.

[sjastro]

It's case of making sense of the mathematics and making predictions that can be supported or refuted through observation or experiment.
BH properties such as mass, charge or spin are characteristics that we can measure as observers outside the horizon.
Knowledge of BHs at a fundamental level is completely lacking.
We don't know what BHs are made out of.
Do BHs violate the Pauli Exclusion Principle?
Then there is space-time inside the S-radius. Since the time like and spatial like terms of the Schwarszchild metric become interchanged the velocity of particles can exceed c with all the associated logical paradoxes that would occur outside the horizon.



That may be comprehensible but let's replace the observer with a photon just inside the horizon that has been scattered away from the singularity. According to the maths, the photon is now travelling back into it's own past before it scattered.... but the scattering process sent it in that direction hence a logical paradox.

Space time inside the horizon is a very weird place.

Regards

Steven

[PeterEde]

Reporters only ever cherry pick. To incite opposing pov and often argument over debate.

I love science especially anything to do with space and getting off this rock and out of this solar system.

[speach]

Now I've started to think of this black hole thing and the question that leap to mind is:-
Now as I understand it Black Holes are the result of super compaction, until even photons cannot escape the from the gravity well imposed by them. Ok so that makes there escape velocity, black Holes, at c or above. Now the photon is a massless particle/wave. Also the graviton is a massless particle/wave that does escape the black hole so it's velocity must be greater that c. "Nothing can exceed c", so the conclusion I come to is that c is not unbreakable or that gravity is not propagated by particles/waves but by ???????????? What?

[PeterEde]

If a photon has no mass why would gravity affect it?

[xelasnave]

General Relativity does not use force to explain gravity ..it is a geometric property of space and applies to both mass and energy. I suspect the photon is affected because it is energy.
I think gravitons are posited via quantum mechanics not General Relativity.
GR is the best model for gravity and QM has yet to offer a
better model

[speach]

It does, look and gravitational lensing

[Solitarian]

... and the best pinball machine :P

[sjastro]

A few issues raised here......

Firstly gravitons have never been observed in nature.
This is compounded by quantum gravity theory that predicts gravitons, but is a mathematical train wreck unable to predict the interactions between gravitational fields without giving absurd results.

If gravitons do exist, they are created when gravitational fields interact with each other. Since a BH's gravitational field extends beyond it's event horizon, gravitons would be created outside the horizon. So there is no issue with gravitons having to escape the horizon.

Secondly as Alex alluded to gravity is not a real force in GR but a fictitious force which is dependant on the observer's frame of reference.
All particles including photons move along geodesic paths in a gravitational field. This explains why the trajectories of photons appears to be bent in a gravitational field.

Furthermore all geodesic paths converge and disappear at the BH singularity.
Space and time appear to end at the singularity.

A photon or any other particle reaching the singularity is literally kaput if our understanding of BHs is correct.

Steven

[xelasnave]

Well put Steven.
What happens when something goes kaput.
Say a hydrogen atom. Can you describe what happens to the structure of the atom and it's components under such energy.

[sjastro]

Alex,

The atom moves on a world line which is the space-time version of the atom moving along a trajectory in space.

From a mathematical perspective the world line ends at the singularity which for all intents and purposes means the atom ceases to exist.

From a physics perspective this cannot be right as the hydrogen atom or its constituent energy becomes part of the BH mass and doesn't "disappear".
It highlights the problem of GR at small scales and the singularity is an artefact of the theory which disappears when QM is applied.

While we can't speculate what happens at the singularity we can make an educated(?) guess what happens as the hydrogen atom approaches the singularity. We know the tidal forces acting on matter increase as one gets closer to the singularity and at progressively smaller scales.

At a certain distance from the singularity the tidal forces are strong enough to separate the electron from the proton in the hydrogen atom.
At smaller distances the tidal forces become so great the quarks themselves that form the proton begin to separate.
Now however we are in the murky world of quantum chromodynamics which states the force between quarks doesn't diminish as their distance increases. So whether free quarks are produced or not is anyone's guess.

There are experts at GR such as Kip Thorne who speculate worm holes, port holes to other Universes etc but these are non mainstream ideas.

Steven

[xelasnave]

That was excellent,
Thanks Steven

[rustigsmed]

ok originally I thought the singularity was a tiny ball of infinitely heavy matter, and now I know that's not the case.

how do black holes grow in size (circumference) and gravitational pull? if the matter that goes 'into' a black hole effectively becomes massless energy (I hope I've got that right), you would think (well some may) that there is therefore no reason for the black hole to change from its original size which was dependent upon its original host star's size?

[sjastro]

It's E=mc^2 at work.
BHs even gain mass when massless photons pass over the even horizon.

The conversion of energy into mass has been demonstrated by the production of positron/electron pairs from high energy photons.

Regards

Steven

[rustigsmed]

thanks steven, i'm still a bit confused i understand that energy can be converted into mass and vice versa. in BH am i right in saying that all mass is converted to energy? so that would mean energy would have to have an affect on gravity in 'energy form' (as opposed from mass)?

cheers

russ

[sjastro]

Russ,

Since our knowledge of what happens inside the event horizon is limited we can only speculate on whether all mass is converted into energy.
We know in the case of x ray binary stars where one of the companions is a BH, a large percentage of in-falling mass is converted into energy before it passes through the horizon.

One of the fundamental differences between Newton's theory of gravity and GR is the role of energy.
Suppose you have a planet under the gravitational influence of a binary star. In Newton's theory the total gravitational force between the planet and binary star is simply the sum of the gravitational forces between each component star and planet.
In GR the total gravitational force is the sum of the gravitational force of each component star and planet plus the gravitational binding energy between the stars.
The gravitational binding energy is the amount of energy required to separate the binary star.

So in GR energy does contribute to gravity.

Regards

Steven

[rustigsmed]

Ok that helps a lot now thanks Steven.