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Watsisname
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### The most massive black holes in the Universe

A-L-E-X wrote:
Source of the post Intuitively, I had always thought that black holes were macro versions of quantum objects, which is why they seem to be singularities.

Eh, black holes don't have much to do with quantum mechanics.  Their macroscopic properties are general relativistic almost all the way through and are also completely independent of a singularity at the center.  (An argument could be made that quantum mechanics is important for understanding Hawking radiation, though it also has a lot to do with thermodynamics.)

If you want to see where quantum mechanics becomes manifest over macroscopic objects, then look at neutron stars.   They are supported by neutron degeneracy pressure, which is a quantum mechanical effect arising from the Pauli exclusion principle (since neutrons are fermions and cannot occupy the same quantum state).  Similarly for white dwarfs, supported by electron degeneracy pressure.

A-L-E-X wrote:
Source of the post About relating the centrifugal force to rotating black holes, would that be a reason why the singularity might be a ring rather than a point?

It's hand-wavy, but you can think of it that way if you like.

To be more precise we need general relativity.  When we derive the Kerr metric we actually start by putting all the mass at a point at the coordinate r=0, just like we do for the Schwarzschild metric, but now we give it some angular momentum.  Then we chug through the field equations and obtain the Kerr solution.  Upon analyzing this solution, we find several singularities!  The outermost one is the event horizon, and its singular nature can be found to simply be a consequence of the coordinates, rather than a place where the space-time is badly behaved.  So it's a "coordinate singularity" that can be removed by switching to a more suitable coordinate system.  Another coordinate singularity is found again at the inner event horizon.  Finally when we look at the central singularity, we discover its location makes no sense.  In polar coordinates it is satisfied at r=0 and a specific angle (θ=π/2), but not at r=0 and any other angle.  Yet those are the same point!

To resolve this we switch again to a new coordinate system, and find that the coordinate r=0 in the Kerr metric is actually describing a disk (x2 + y2 = a2), where a is the spin of the black hole.  The specific angle where the singularity was satisfied (r=0, θ=π/2) describes a ring on the edge of that disk.  This ring turns out to be non-removable by any other coordinate transformation.  It's a true curvature singularity, where the space-time curvature is infinite, rather than a mere coordinate singularity.  This is how the ring singularity of the Kerr metric was discovered -- and it wasn't trivial to find at all.

We must also remember though that this ring singularity is unphysical -- an artifact of the Kerr vacuum solution.  Real rotating black holes do not contain them.  Where the rotation of the Kerr metric does correspond to nature is in the shape of the event horizon, and the ergoregion where spacetime is dragged around so fast that an observer there cannot remain still.

A-L-E-X wrote:
Source of the post Even without the Cauchy horizon, is a new space-time (that is, new universe) still possible once beyond the event horizon?

Probably not, despite how popular and appealing the idea is.  But we don't really know.  What we know with confidence is what happens near, through, and a fair ways below the event horizon.  Near where the inner horizon or Cauchy horizon would be, our understanding breaks down.

I subscribe to the view that physics is always capable of describing "what happens" to something after some initial condition is provided, in principle.  For a black hole interior, this means physics must not break down arbitrarily.  Something happens to a thing falling into a black hole.  Its world line must be extendable to somewhere.  In classical general relativity, that somewhere terminates in a singularity (this is actually how singularities are defined: by the non-extendability of any world lines through it).  A future theory of quantum gravity may replace the singularity with something very dense and exotic but not infinitesimal, where world lines still effectively terminate.

For a rotating black hole, we don't know where that termination point is.  The vacuum solution predicts the world lines can be extended to another universe, but realistically we see many reasons why that should not be so.  More likely it's some chaotic jumble of past and future near-singularities crashing into each other at relativistic speeds like the ultimate cosmic particle collider.  To quote Andrew Hamilton, "what does nature do with such a machine?"

‾\_(ツ)_/‾

A-L-E-X
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Joined: 06 Mar 2017

### The most massive black holes in the Universe

Watsisname wrote:
A-L-E-X wrote:
Source of the post Intuitively, I had always thought that black holes were macro versions of quantum objects, which is why they seem to be singularities.

Eh, black holes don't have much to do with quantum mechanics.  Their macroscopic properties are general relativistic almost all the way through and are also completely independent of a singularity at the center.  (An argument could be made that quantum mechanics is important for understanding Hawking radiation, though it also has a lot to do with thermodynamics.)

If you want to see where quantum mechanics becomes manifest over macroscopic objects, then look at neutron stars.   They are supported by neutron degeneracy pressure, which is a quantum mechanical effect arising from the Pauli exclusion principle (since neutrons are fermions and cannot occupy the same quantum state).  Similarly for white dwarfs, supported by electron degeneracy pressure.

A-L-E-X wrote:
Source of the post About relating the centrifugal force to rotating black holes, would that be a reason why the singularity might be a ring rather than a point?

It's hand-wavy, but you can think of it that way if you like.

To be more precise we need general relativity.  When we derive the Kerr metric we actually start by putting all the mass at a point at the coordinate r=0, just like we do for the Schwarzschild metric, but now we give it some angular momentum.  Then we chug through the field equations and obtain the Kerr solution.  Upon analyzing this solution, we find several singularities!  The outermost one is the event horizon, and its singular nature can be found to simply be a consequence of the coordinates, rather than a place where the space-time is badly behaved.  So it's a "coordinate singularity" that can be removed by switching to a more suitable coordinate system.  Another coordinate singularity is found again at the inner event horizon.  Finally when we look at the central singularity, we discover its location makes no sense.  In polar coordinates it is satisfied at r=0 and a specific angle (θ=π/2), but not at r=0 and any other angle.  Yet those are the same point!

To resolve this we switch again to a new coordinate system, and find that the coordinate r=0 in the Kerr metric is actually describing a disk (x2 + y2 = a2), where a is the spin of the black hole.  The specific angle where the singularity was satisfied (r=0, θ=π/2) describes a ring on the edge of that disk.  This ring turns out to be non-removable by any other coordinate transformation.  It's a true curvature singularity, where the space-time curvature is infinite, rather than a mere coordinate singularity.  This is how the ring singularity of the Kerr metric was discovered -- and it wasn't trivial to find at all.

We must also remember though that this ring singularity is unphysical -- an artifact of the Kerr vacuum solution.  Real rotating black holes do not contain them.  Where the rotation of the Kerr metric does correspond to nature is in the shape of the event horizon, and the ergoregion where spacetime is dragged around so fast that an observer there cannot remain still.

A-L-E-X wrote:
Source of the post Even without the Cauchy horizon, is a new space-time (that is, new universe) still possible once beyond the event horizon?

Probably not, despite how popular and appealing the idea is.  But we don't really know.  What we know with confidence is what happens near, through, and a fair ways below the event horizon.  Near where the inner horizon or Cauchy horizon would be, our understanding breaks down.

I subscribe to the view that physics is always capable of describing "what happens" to something after some initial condition is provided, in principle.  For a black hole interior, this means physics must not break down arbitrarily.  Something happens to a thing falling into a black hole.  Its world line must be extendable to somewhere.  In classical general relativity, that somewhere terminates in a singularity (this is actually how singularities are defined: by the non-extendability of any world lines through it).  A future theory of quantum gravity may replace the singularity with something very dense and exotic but not infinitesimal, where world lines still effectively terminate.

For a rotating black hole, we don't know where that termination point is.  The vacuum solution predicts the world lines can be extended to another universe, but realistically we see many reasons why that should not be so.  More likely it's some chaotic jumble of past and future near-singularities crashing into each other at relativistic speeds like the ultimate cosmic particle collider.  To quote Andrew Hamilton, "what does nature do with such a machine?"

‾\_(ツ)_/‾

The really appealing part of Black Hole cosmology is the duality of the Big Bang singularity vs the singularity described inside Black Holes, but also how the Holographic Principle rather elegantly applies to both.  Being able to describe the universe as existing on the boundary of an event horizon is quite appealing!  Also, if you think about the laws which govern the interior of black holes (for example nothing can escape, not even light) and analogize that to the limitations we have inside our own universe, there are some very interesting parallels. If no Cauchy horizon, then perhaps a different type of horizon?  Question- does our universe have any spin associated with it?  We all know the universe is expanding, but it would be interesting if there was some sort of spin associated with it also.
The frame dragging aspect of the Kerr Metric is fascinating, and I was curious about the spin of a black hole forming a ring singularity, because if there is any way to prove black hole cosmology, one way would be that the spin of the parent black hole should also impart some spin on the baby universe it creates.  Sounds logical, right?  But if past and future collide, the spinning black hole could also be a time machine of sorts, with closed timelike curves all knotted together creating- chaos!  Also perhaps they would be temporally entangled.  Interesting thing is I have always thought of the universe as the ultimate particle collider/quantum computer, perhaps black holes are miniature versions of that.
Interesting fact about neutron stars and white dwarfs!   They often get ignored because black holes are so fascinating- neutron stars and white dwarfs are like their step sisters!   I wonder if we ever discover quark stars, that they too would have quantum mechanical properties.

Stellarator
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### The most massive black holes in the Universe

A-L-E-X wrote:
Source of the post  I wonder if we ever discover quark stars

And if we do, will there be an identical star to it on the other side of the universe ?
Futurum Fusionem

Watsisname
Science Officer
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Joined: 06 Sep 2016
Location: Bellingham, WA

### The most massive black holes in the Universe

A-L-E-X wrote:
Source of the post The really appealing part of Black Hole cosmology is the duality of the Big Bang singularity vs the singularity described inside Black Holes

The problem is that they're not the same kind of singularity.  A black hole singularity is a curvature singularity, where the space-time curvature is infinite at a single point in space.  The Big Bang singularity on the other hand is where the density is infinite at every point in space, and has no curvature.

The space-time metric of a black hole and the universe are also different (Schwarzschild or Kerr metric vs. FLRW metric), due to this difference in how the mass is distributed.

A-L-E-X wrote:
Source of the post Being able to describe the universe as existing on the boundary of an event horizon is quite appealing!

The holographic principle is very appealing, but probably not for that reason.   It isn't what the holographic principle predicts, nor would this idea explain anything useful about how the universe works.

The thrust of the holographic principle is that the entropy of a black hole is proportional to its surface area.  According to observers outside the black hole (emphasis), all the information of everything that fell into the black hole is contained on the event horizon and slowly leaked out as Hawking radiation.  This is pretty interesting, because normally the entropy of a system is proportional to its volume rather than surface area.  This property of black holes is important for understanding the union of deeper principles of physics.  For example, it shows us that the maximum allowed information content of a system depends on the surface area of the region bounding it, because a black hole is the maximum entropy state of any system.  Collapse anything into a black hole, and you maximize its entropy.

But it doesn't mean the universe is literally the horizon of a black hole.  It can't be on the horizon of a black hole, because no allowed observer can remain on the horizon.  When we talk of the information of the material that fell into a black hole lying on its event horizon, that's true only for observers outside the horizon.  If you go to the horizon yourself, then you find there's nothing there at all, and you fall through it and end up at the singularity, or whatever lies at the center.

A-L-E-X wrote:
Source of the post Question- does our universe have any spin associated with it?

Not to any measurable degree, and those measurements are pretty precise (better than 1 degree per billion years).

A rotating universe does lead to measurable effects, though probably not in the way most people would imagine.  This is weird to visualize, but a rotating universe still doesn't have a center.  It is not like a sphere rotating around a point, or a cylinder rotating about an axis.  What it does is make a preferred direction along which we measure an angular velocity.  So we could pick out a point on the sky and say everything is rotating about an axis connecting us to that point.  But observers at a different location in the universe could do the same thing and find an axis of rotation parallel to ours, and there'd be no way to distinguish which of those axes was "the real center of rotation".  Only that those axes must be parallel to each other.

What measures the rotation?  The effect would show up in gyroscopes.  The solar system itself acts as one, and observations rule out any cosmological rotation effect to better than 2 arcseconds per century.  For comparison, the relativistic precession of Mercury's orbit is a whopping 43 arcseconds per century.  Even Earth's relativistic precession is greater: 3.85 arcsec/century.

Cosmological rotation would also show up in the appearance of the Cosmic Microwave Background, as a preferred direction to the anisotropies.  Measurements do not find any such rotation effect to better than 10-8 arcsec/century, or less than 1 arcminute of rotation over the entire history of the universe.

So I think we can rule out a rotation of the universe as being important to any cosmological observations.  Instead we use the FLRW metric which has no rotation in it.

A-L-E-X wrote:
Source of the post Interesting fact about neutron stars and white dwarfs!   They often get ignored because black holes are so fascinating- neutron stars and white dwarfs are like their step sisters!

Yes.   I love black holes, but white dwarfs and neutron stars are fascinating and extreme objects too.  Both relativity and quantum mechanics are crucial to understanding their properties.

People who say "relativity and quantum mechanics clash with each other" often forget that.  They do clash, but only very close to singularities.  Otherwise, we've been able to accurately use relativity and quantum mechanics together for a long time.

A-L-E-X
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Joined: 06 Mar 2017

### The most massive black holes in the Universe

Watsisname wrote:
A-L-E-X wrote:
Source of the post The really appealing part of Black Hole cosmology is the duality of the Big Bang singularity vs the singularity described inside Black Holes

The problem is that they're not the same kind of singularity.  A black hole singularity is a curvature singularity, where the space-time curvature is infinite at a single point in space.  The Big Bang singularity on the other hand is where the density is infinite at every point in space, and has no curvature.

The space-time metric of a black hole and the universe are also different (Schwarzschild or Kerr metric vs. FLRW metric), due to this difference in how the mass is distributed.

A-L-E-X wrote:
Source of the post Being able to describe the universe as existing on the boundary of an event horizon is quite appealing!

The holographic principle is very appealing, but probably not for that reason.   It isn't what the holographic principle predicts, nor would this idea explain anything useful about how the universe works.

The thrust of the holographic principle is that the entropy of a black hole is proportional to its surface area.  According to observers outside the black hole (emphasis), all the information of everything that fell into the black hole is contained on the event horizon and slowly leaked out as Hawking radiation.  This is pretty interesting, because normally the entropy of a system is proportional to its volume rather than surface area.  This property of black holes is important for understanding the union of deeper principles of physics.  For example, it shows us that the maximum allowed information content of a system depends on the surface area of the region bounding it, because a black hole is the maximum entropy state of any system.  Collapse anything into a black hole, and you maximize its entropy.

But it doesn't mean the universe is literally the horizon of a black hole.  It can't be on the horizon of a black hole, because no allowed observer can remain on the horizon.  When we talk of the information of the material that fell into a black hole lying on its event horizon, that's true only for observers outside the horizon.  If you go to the horizon yourself, then you find there's nothing there at all, and you fall through it and end up at the singularity, or whatever lies at the center.

A-L-E-X wrote:
Source of the post Question- does our universe have any spin associated with it?

Not to any measurable degree, and those measurements are pretty precise (better than 1 degree per billion years).

A rotating universe does lead to measurable effects, though probably not in the way most people would imagine.  This is weird to visualize, but a rotating universe still doesn't have a center.  It is not like a sphere rotating around a point, or a cylinder rotating about an axis.  What it does is make a preferred direction along which we measure an angular velocity.  So we could pick out a point on the sky and say everything is rotating about an axis connecting us to that point.  But observers at a different location in the universe could do the same thing and find an axis of rotation parallel to ours, and there'd be no way to distinguish which of those axes was "the real center of rotation".  Only that those axes must be parallel to each other.

What measures the rotation?  The effect would show up in gyroscopes.  The solar system itself acts as one, and observations rule out any cosmological rotation effect to better than 2 arcseconds per century.  For comparison, the relativistic precession of Mercury's orbit is a whopping 43 arcseconds per century.  Even Earth's relativistic precession is greater: 3.85 arcsec/century.

Cosmological rotation would also show up in the appearance of the Cosmic Microwave Background, as a preferred direction to the anisotropies.  Measurements do not find any such rotation effect to better than 10-8 arcsec/century, or less than 1 arcminute of rotation over the entire history of the universe.

So I think we can rule out a rotation of the universe as being important to any cosmological observations.  Instead we use the FLRW metric which has no rotation in it.

A-L-E-X wrote:
Source of the post Interesting fact about neutron stars and white dwarfs!   They often get ignored because black holes are so fascinating- neutron stars and white dwarfs are like their step sisters!

Yes.   I love black holes, but white dwarfs and neutron stars are fascinating and extreme objects too.  Both relativity and quantum mechanics are crucial to understanding their properties.

People who say "relativity and quantum mechanics clash with each other" often forget that.  They do clash, but only very close to singularities.  Otherwise, we've been able to accurately use relativity and quantum mechanics together for a long time.

I think the clashing fault lies with Einstein who literally couldn't stand quantum mechanics lol.  I think he had issues with the idea of a probabilistic universe.  "God does not play dice" and all that!
But there is a way to find patterns even in  randomness and what is random on one level can create fractal beauty on another level in its emergent properties.  Even randomness has certain rules it must abide by!
Wat speaking of the CMBR what do you think of some of the hot and cold spots and assymetries between the north and the south in the CMBR? Is there anything we can glean from that?  The assymetries they found were somewhat more than what they expected to find but not enough to be considered significant? I know statisticians have different standards for what they consider significant vs what a cosmologist might have.
I can visualize what you speak of in terms of rotation, if one considers that every point in the universe can be considered its center since every point was once a single point that was part of the Big Bang singularity.  Therefore the socalled center of the universe has been expanding right along with the rest of the universe!
The thing I like about the holographic principle is that you could literally use 2 dimensions of information to describe the whole universe (thus the term holograph).  Based on black holes maximizing entropy, can they be considered the most stable systems in the universe?  I find it quite eloquent that the principle favors surface area, because a black hole is supposed to have zero volume (as a matter of fact, one of the ways in which to achieve a singularity is a zero volume, because then density becomes infinite [D=M/V] though if Loop Quantum Cosmology is right, space-time has discrete minimum units and V never truly becomes zero- I wonder how this would impact the Holographic Principle?)  Also if the information lies outside the event horizon would an observer outside the event horizon be able to decipher it and tell what fell inside (or what exists inside?) I would think not because from what I have read, the information becomes "scrambled" to the point of not being able to glean any useful information from it?
Have a look at this, I read it awhile back and what you said reminded me of it:
https://www.forbes.com/sites/paulrodger ... 4ae8950c30
http://arxiv.org/pdf/1309.1487v1.pdf
http://www.nature.com/news/did-a-hyper- ... se-1.13743
https://www.express.co.uk/news/science/ ... ingularity
https://www.space.com/32008-five-dimens ... heory.html
One of the compelling properties of a black hole cosmology is that several different physicists have been supporting it from different angles- this group conjectures it from a completely different perspective than Poplawski.

Watsisname
Science Officer
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Location: Bellingham, WA

### The most massive black holes in the Universe

A-L-E-X wrote:
Source of the post Wat speaking of the CMBR what do you think of some of the hot and cold spots and assymetries between the north and the south in the CMBR? Is there anything we can glean from that?

There might be, but the cosmological community has no consensus on the matter.  Figures for how much the anisotropies deviate from what Lambda-CDM predicts are small and also very model dependent, which is to say they are not significant enough for everyone to agree that they even require explanation.  Even if they are important, it isn't clear what they would imply.  So for the time being I do not speculate.

A-L-E-X wrote:
Source of the post Based on black holes maximizing entropy, can they be considered the most stable systems in the universe?

For black holes of stellar mass and larger, they are definitely the longest-lived objects.  I wouldn't say that this is because their entropy is maximized, but rather that their temperatures are so low, so they evaporate extremely slowly.

A-L-E-X wrote:
Source of the post I find it quite eloquent that the principle favors surface area, because a black hole is supposed to have zero volume

When we refer to a black hole and the volume it contains, we are referring to the region within its event horizon, not the singularity (which has neither volume nor surface area.)

Actually the volume enclosed in a black hole is not uniquely defined.  You can compute many different values for the volume, depending on your choice of constant-time slices of the space-time.  The volume can be zero, not zero, or even something that grows with time.  The black hole's surface area, however, is uniquely defined.

The reason the information content of a black hole is contained on the event horizon is that, according to outside observers, the black hole has no interior.  Because of the infinite time dilation at the horizon, all the stuff that formed the black is smeared out in thin shells that (very rapidly) asymptotically approach the horizon and vanish from view.  This argument isn't very rigorous though -- a more correct way to see why it goes by horizon area is to follow the argument given by Bekenstein, which was a brilliant piece of physics.

A-L-E-X wrote:
Source of the post I think the clashing fault lies with Einstein who literally couldn't stand quantum mechanics lol.

Einstein was fine with quantum mechanics (he was even awarded a Nobel Prize for it, with his explanation of the photoelectric effect).  He was also fine with the probabilistic nature of it.  He just didn't like the interpretation that this probability was fundamental to nature.  He preferred that there must be some unknown deeper physics (a hidden variable theory) to explain the outcomes of experiments.

Stellarator
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### The most massive black holes in the Universe

Watsisname wrote:
Source of the post He just didn't like the interpretation

Yeah, a lot of disputes between scientists seem to revolve around this fact. The actual data or theory isn't the problem, just an individuals interpretation of it and reasons for doing so.
Futurum Fusionem

Watsisname
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Location: Bellingham, WA

### The most massive black holes in the Universe

In this case it's not even the interpretation of the theory, but a deeper philosophical question of "what is going on" behind the scenes, which theory and data do not tell us.  It's rather closer to metaphysics, though it does make some practical predictions (for example leading to the Bell inequalities, but even those don't perfectly exclude all hidden variable interpretations -- just the "obvious" ones.)

Most physicists who actually do experimental work in quantum mechanics don't care about these interpretations too much.  They care that the theory allows them to predict the outcomes of experiments.

A-L-E-X
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### The most massive black holes in the Universe

Watsisname wrote:
A-L-E-X wrote:
Source of the post Wat speaking of the CMBR what do you think of some of the hot and cold spots and assymetries between the north and the south in the CMBR? Is there anything we can glean from that?

There might be, but the cosmological community has no consensus on the matter.  Figures for how much the anisotropies deviate from what Lambda-CDM predicts are small and also very model dependent, which is to say they are not significant enough for everyone to agree that they even require explanation.  Even if they are important, it isn't clear what they would imply.  So for the time being I do not speculate.

A-L-E-X wrote:
Source of the post Based on black holes maximizing entropy, can they be considered the most stable systems in the universe?

For black holes of stellar mass and larger, they are definitely the longest-lived objects.  I wouldn't say that this is because their entropy is maximized, but rather that their temperatures are so low, so they evaporate extremely slowly.

A-L-E-X wrote:
Source of the post I find it quite eloquent that the principle favors surface area, because a black hole is supposed to have zero volume

When we refer to a black hole and the volume it contains, we are referring to the region within its event horizon, not the singularity (which has neither volume nor surface area.)

Actually the volume enclosed in a black hole is not uniquely defined.  You can compute many different values for the volume, depending on your choice of constant-time slices of the space-time.  The volume can be zero, not zero, or even something that grows with time.  The black hole's surface area, however, is uniquely defined.

The reason the information content of a black hole is contained on the event horizon is that, according to outside observers, the black hole has no interior.  Because of the infinite time dilation at the horizon, all the stuff that formed the black is smeared out in thin shells that (very rapidly) asymptotically approach the horizon and vanish from view.  This argument isn't very rigorous though -- a more correct way to see why it goes by horizon area is to follow the argument given by Bekenstein, which was a brilliant piece of physics.

A-L-E-X wrote:
Source of the post I think the clashing fault lies with Einstein who literally couldn't stand quantum mechanics lol.

Einstein was fine with quantum mechanics (he was even awarded a Nobel Prize for it, with his explanation of the photoelectric effect).  He was also fine with the probabilistic nature of it.  He just didn't like the interpretation that this probability was fundamental to nature.  He preferred that there must be some unknown deeper physics (a hidden variable theory) to explain the outcomes of experiments.

It's an exciting time to be in the field because so much is in flux there are literally new theories every month- it reminds me of when we had an excessive amount of particles we thought there should be some fundamental reality underlying it all- and then we theorized about quarks in the early 70s and finally discovered them and we formed a periodic table of particles with leptons, neutrinos and quarks and everything else being made of those.
I thought Einstein disliked quantum mechanics because he called it "spooky action at a distance" and things like nonlocality created issues for him.  Do you remember Max Planck's famous quote?  That a new theory doesn't become accepted until the previous generation dies and a new one grows up that is familiar with it? I've always believed that and thought that one of the curses of science is that the old guard is so regressive and resistant to new ideas- but they have to die eventually and replaced by the new, who have better ideas (usually) and are more accepting of change.  And then they need to get replaced one day too......  Although Einstein was working on a multidimensional theory for unification because even back then he must have known that it was necessary.  But I believe the last of the fundamental forces was actually discovered when he was working on that and that made his work even more difficult.
I love a particular area of quantum mechanics called quantum biology that illustrates how quantum mechanics influences DNA and also how it's important in photosynthesis.  Also the various quantum theories of the mind.
I have to read up on Bekenstein now!  I remember something called Bekenstein Bound, I wonder if that was from the same physicist?  I can see why you love black holes so much lol, I've had a love/hate relationship with them since elementary school.  Loved them for the- finality- they represent and hated them because I didn't want them to destroy everything lol.  That's why I've intuitively always looked for another side- where there is destruction there should(must) be creation too - to keep the balance intact?  But like you said it's hard to speculate when we really dont know anything (yet) because the evidence is so sparse.  But it's an interesting possible duality to think about.  Einstein loved to do thought experiments too

As far whats going on behind the scenes, I like Wheeler's quantum foam idea, it explains virtual particles rather well too.  I just hope we can experiment down at those scales one day to see if reality is really like that.  Is surface area uniquely defined because it's on the outside, while volume is the interior?  I remember reading that a supercomputer simulation of colliding supermassive black holes brought forth a structure in the interior of the result that resembled a Hopf Fibration and that shape has been part of some interesting cosmological theories.  What that means- no one knows (though it might be good for proponents of black hole cosmology.)  Similar simulations also predicted a cyclical universe that comes back just before heat death.  I wonder if circular timelike curves are related to that?  When that material and energy vanishes from view asymptotically where does it go?  Being an asymptote implies to me that it "jumps" rather than moves in a smooth direct way.  Like the negative absolute zero temperatures we talked about previously (also, perhaps the speed of light.)

Watsisname
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### The most massive black holes in the Universe

A-L-E-X wrote:
Source of the post  Is surface area uniquely defined because it's on the outside, while volume is the interior?

No, it is much more subtle.

In order to define a volume or an area, we must choose the set of points that form its boundary.  To be logical, those points must also be chosen simultaneously, as if we took a snapshot of the object in an instant with which to measure it.  But in relativity different observers have different notions of what set of points are simultaneous.  That's because simultaneity is relative.  So we have different choices of "constant-time sections" of the space-time.  Different sections may contribute different amounts of area or volume to the total.

For the area of the event horizon this turns out to be irrelevant.  That's because the horizon is a null surface (defined by light rays emitted outward from it being trapped there, and if you recall from earlier the metric for space-time has a "null" distance of zero for the path taken by photons).  All observers agree on the set of events that form the event horizon, and while transformations between different reference frames will change the coordinates of it, it shifts them in a way which does not do anything to the area.  So everyone agrees on the area of the horizon, and we say it is invariant.

The volume enclosed by the horizon turns out not to be invariant.  Different sections do yield different amounts of volume and we can come up with all sorts of different answers.  If you happen to choose the Schwarzschild coordinates, then the t=infinity slice lies on the event horizon, thus none of those sections contribute any volume in the interior, and you get zero.  A different choice might give you something that is not zero.

A-L-E-X wrote:
Source of the post When that material and energy vanishes from view asymptotically where does it go?

Who is asking?

For those watching outside the horizon, they say the material gets closer and closer to the horizon, never quite reaching it, because of the gravitational time dilation becoming infinite.  (No, it never crosses.  By asymptotically approach I am referring to a smooth approach without ever getting there, not a discontinuous jump to the other side which never happens.)  At the same time, it is dimmed and redshifted from view, because of the gravitational redshift effect.  These are actually the same effect.

For those who take the plunge into the black hole themselves, they do not find anything trapped on the horizon.  Nor do they experience being frozen in time at the horizon themselves.  They sail right through, all the way down to the singularity.  Yet anyone remaining behind outside the hole sees them slow down and freeze (and vanish) before the horizon, never crossing it.

Contradictory?  Not really.  Just unintuitive.

Yuno
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### The most massive black holes in the Universe

Maybe im missing it, but i cant find where to create a thread? I just want a mod to contact me in PM, specifically, HarbingerDawn.

SpacyLuke
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### The most massive black holes in the Universe

You cannot create a thread nor edit your comment untill you post 10 comments.

JackDole
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### The most massive black holes in the Universe

Yuno wrote:
Source of the post Maybe im missing it, but i cant find where to create a thread?

And you should not create unnecessary threads. First, see if there maybe a thread that covers your problem. Read the forum rules.
JackDole's Universe 0.990: http://forum.spaceengine.org/viewtopic.php?f=3&t=546
JackDole's Archive: http://forum.spaceengine.org/viewtopic.php?f=3&t=419
JackDole: Mega structures ... http://old.spaceengine.org/forum/17-3252-1 (Old forum)

A-L-E-X
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### The most massive black holes in the Universe

Watsisname wrote:
A-L-E-X wrote:
Source of the post  Is surface area uniquely defined because it's on the outside, while volume is the interior?

No, it is much more subtle.

In order to define a volume or an area, we must choose the set of points that form its boundary.  To be logical, those points must also be chosen simultaneously, as if we took a snapshot of the object in an instant with which to measure it.  But in relativity different observers have different notions of what set of points are simultaneous.  That's because simultaneity is relative.  So we have different choices of "constant-time sections" of the space-time.  Different sections may contribute different amounts of area or volume to the total.

For the area of the event horizon this turns out to be irrelevant.  That's because the horizon is a null surface (defined by light rays emitted outward from it being trapped there, and if you recall from earlier the metric for space-time has a "null" distance of zero for the path taken by photons).  All observers agree on the set of events that form the event horizon, and while transformations between different reference frames will change the coordinates of it, it shifts them in a way which does not do anything to the area.  So everyone agrees on the area of the horizon, and we say it is invariant.

The volume enclosed by the horizon turns out not to be invariant.  Different sections do yield different amounts of volume and we can come up with all sorts of different answers.  If you happen to choose the Schwarzschild coordinates, then the t=infinity slice lies on the event horizon, thus none of those sections contribute any volume in the interior, and you get zero.  A different choice might give you something that is not zero.

A-L-E-X wrote:
Source of the post When that material and energy vanishes from view asymptotically where does it go?

Who is asking?

For those watching outside the horizon, they say the material gets closer and closer to the horizon, never quite reaching it, because of the gravitational time dilation becoming infinite.  (No, it never crosses.  By asymptotically approach I am referring to a smooth approach without ever getting there, not a discontinuous jump to the other side which never happens.)  At the same time, it is dimmed and redshifted from view, because of the gravitational redshift effect.  These are actually the same effect.

For those who take the plunge into the black hole themselves, they do not find anything trapped on the horizon.  Nor do they experience being frozen in time at the horizon themselves.  They sail right through, all the way down to the singularity.  Yet anyone remaining behind outside the hole sees them slow down and freeze (and vanish) before the horizon, never crossing it.

Contradictory?  Not really.  Just unintuitive.

Thats fascinating Wat!  It actually reminds me of how you can asymptotically halve a number and while it will always approach zero it will never quite get there.  That may be a good analogy to what an observer on the outside sees.  As far as what happens for those who take the plunge into a black hole- let's take a supermassive black hole like the one at the center of our galaxy as an example.  Is it true that a person (with an appropriate space suit of course) or a group of people in a space ship could survive for quite a while, because a supermassive black hole's event horizon's surface area is so large and the increase in density so slow that it doesn't even seem that one is inside it?  Until one gets to the singularity of course.  Also, how would the journey through a black hole be different if it was a spinning black hole?  Would the person get crushed as quickly or could that be navigated better (although the crushing would still be inevitable I would think.)
So in relativity-speak, it sounds that while "everything is relative" is a popular cliche, the surface area of an event horizon of a black hole actually isn't relative, while the volume is.  Surface area of an event horizon is invariable just like the speed of light in a vacuum- as a matter of fact, one of the beauties of our universe is that everything else adjusts to make sure it is invariable from the viewpoint of all observers.  It almost seems like our universe was built from the ground up for it to be so, maybe by some sentient advanced alien species  Or maybe the universe had to be that way otherwise it would not be able to exist (most of the theories I've looked at that involve conjecturing about other universes have different values for the constants, but they all agree that the speed of light needs to be a limit for the residents of each universe otherwise the physics of that universe breaks down and it cannot expand beyond the singularity- although, ironically enough- space itself can expand almost instantaneously during inflation and even later during continued expansion.)   It almost sounds like the term "volume" is meaningless when it comes to black holes because no single value applies to it, they ALL do, depending on the coordinate system used.  But does this only apply to black holes or are black holes the extreme case?  Perhaps volume is meaningless but works in Newtonian (classical) physics as a useful approximation.  But when you get to the set of extreme conditions which Newtonian physics cannot explain and only relativity can explain, then you run into issues with volume (similarly you also run into issues with length, as length changes depending upon the velocity of an object, and velocity itself is relative, so length must be also. This also applies to the rate of passage of time. But the discrepancies are insignificant and miniscule in both cases until you reach relativistic velocities.)

Watsisname
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### The most massive black holes in the Universe

A-L-E-X wrote:
Source of the post  Is it true that a person (with an appropriate space suit of course) or a group of people in a space ship could survive for quite a while, because a supermassive black hole's event horizon's surface area is so large and the increase in density so slow that it doesn't even seem that one is inside it?

That's true, but has to do with tidal force, not density.  A black hole is (according to the vacuum solutions) empty except for all the matter at the singularity, and if you fall into one, you never hit anything.  You just feel like you're floating in empty space the whole time, until suddenly the tidal force (the difference in strength of gravity with respect to distance from the center) gets so strong that it tears you apart.  This actually happens very quickly -- so quickly that you wouldn't even have time to notice any discomfort.

For a stellar mass black hole, the lethal range of the tidal force is much larger than the event horizon, but for supermassive black holes, it's well inside the horizon.  If you fell into SgrA*, the time (measured on your clock) between crossing the event horizon and meeting the singularity is about 26 seconds, and tidal force won't become painful or kill you until about 1/10th of a second before the singularity.  For a 10 billion solar mass black hole, you get about 18 hours of time within the horizon, and again death about 1/10th of a second before the singularity.

A-L-E-X wrote:
Source of the post Also, how would the journey through a black hole be different if it was a spinning black hole?

Depends on the spin of the black hole and your trajectory, and how far into the black hole the Kerr vacuum solution is physically valid.  If you want to pretend that the maximally extended vacuum solution for a rotating black hole is realistic, then the journey is very crazy:

For some explanation of what the heck is going on in this video, but using the metric for a charged black hole rather than spinning (still has very similar behavior), I recommend Andrew Hamilton's website.

If you want to know what the real journey would look like, then nobody knows.  It's probably very lethal.

A-L-E-X wrote:
Source of the post Would the person get crushed as quickly or could that be navigated better (although the crushing would still be inevitable I would think.)

Ah, here's a fun fact.  If you want to maximize the amount of time you get to experience inside of a black hole before getting killed by the tidal forces near the singularity, then what should you do?  Do nothing.  Any change in course that you try to make, no matter in what direction, will only make your trip to the center happen faster.  This is because the longest proper time between the horizon and the singularity is a freefall geodesic.

In this way I like to describe black holes as like the proverbial cosmic quicksand: once you're inside it, then the more you struggle, the sooner it kills you.

A-L-E-X wrote:
Source of the post So in relativity-speak, it sounds that while "everything is relative" is a popular cliche, the surface area of an event horizon of a black hole actually isn't relative, while the volume is

Yep.  It's a popular cliche which is also very wrong.  Relativity is about some things like space and time being relative, but it's also about a lot of things not being relative, like the speed of light, or the space-time interval between two events.  Things that are not relative are very important in relativity.

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