Simple questions about the Universe/cosmology

[zidane1strife]

No center of the Universe:

I get the idea that there isn't a center in the sense of an origin point from where it all expanded(If I'm not mistaken, it all expands in all directions, almost as if you were zooming in more and more on a point with everything within).

But if the elements are finite and if they're arranged in a 3d configuration, there has to be a center position. Only way you're getting out of that is with some weird thing like torus/videogame cube scenario(go up come through the bottom, same with left and right, etc.). If not there has to be a center position, even if we don't know it, and even if it's simply another average chunk of space.

I also have some issues with the idea that there is nothing outside the universe, as in no border/limit at the edge of the finite number of composing elements. how is that possible? There either has to be a void beyond the finite elemets, or at least a torus/videogame esque thing must occur.

Dark Matter:

Ok, so stars can't be at the edge of the galaxy cause there ain't enough mass to keep them from flying apart. So we just throw some massive invisible thingies at the borders and presto. What causes something that's supposed to interact only through gravity to cluster in a halo around the galaxy(aka, why is it concentrated in one region at all, why not dispersed all over?)?

Black holes and information:

Recently Stephen gave some ideas about how information is preserved despite h.radiation, I missed them, how exactly is information preserved?

Black Hole question #2:

Could one engineer a matter collapse around a ship(aka, hollow spherical mass surrounding the ship), such that the ship wasn't affected by the collapse and was right in the singularity? (iirc, objects within a hollow sphere can have the external gravitational force on them cancelling out. )

Big Bang:

From, what I've heard Mr. Tipler(from physics of immortality), in order to re-do the bang during a crunch, one 'd have to collapse everything in an intelligent and precise fashion. Intelligence surviving as things compressed infinitely.

What are the reasons for this need to precisely coordinate the collapse, what's the math involved? What kind of compression is this, as far as we know, molecular machinery hits the limits of miniaturization, will the compression be relative to something(that is not felt by the compressed) else leaving matter unscathed, if not are there really any known means of surviving compression that'd destroy molecular machinery?

 

[Bigus Dickus]

Where is the center of the surface of the Earth? All points are equivalent so far as defining the center of Earth's surface is concerned. The Earth's surface is a two dimensional surface curved in a three dimensional volume. Remember the whole flatlander thing... imagine that people on Earth's surface are oblivious to the concept of up and down, they only know forward/backwards, left/right. Ask them where the center of their world is, and they will probably insist that there must be one even if they don't know where it is.

Now think of a balloon instead of the Earth, but consider the surface just the same. Still no "center" of the balloon's surface. Start blowing the balloon up... the surface expands. Draw dots all over the balloon and keep inflating it; the dots all expand relative to each other, none can be considered the surface.

Now ask yourself (or the flat people on Earth) where the "edge" of the surface of the balloon or Earth lies? Surely something must be beyond that "edge," but in reality there is only more balloon/Earth surface. Keep going, and you do indeed wind up where you started.

These easy to visualize analogies are two dimensional surfaces occupying a three dimensional volume. In the "classical" Einsteinian universe, "space" is a three dimensional "surface" occupying a four dimensional "volume" (our concepts of surface and volume are a bit limited, but close enough for the simplistic explanation I'm giving). Just as the Earth's surface has no center, space (the three dimensions we are familiar with) has no center. And just with the balloon, if you go far enough in one direction in space you may well end up where you started (there are other possibilities, but this is the simplest to visualize and relate to something familiar). And just as all points on the balloon's surface expand when you inflate it, all points in our space expand as the universe's size grows. No point is preferred as a center.

Ahh, but you might point out that the Earth and the balloon do have a center to their volume, so the universe should as well. Quite so. The center does not lie on the surface, however, but instead is found by moving in the "third" dimension. The balloon's center is, for our purposes, where the balloon was before you started inflating it. Similarly, to find the "center" of our four dimensional universe, you must move through the "fourth" dimension, which is conveniently represented as time. This provides the intuitive concept that the only "place" you can find the center of the universe is actually not a place at all, but a "time" - the beginning of the universe.

This answers most of the questions you asked in a fairly simplistic (and probably not completely consistent or rigorously correct, but nevertheless understandable) way.

 

[_xxx_]

But if the elements are finite and if they're arranged in a 3d configuration, there has to be a center position. Only way you're getting out of that is with some weird thing like

You assume that time and space are linear, which they're not.

 

[KimB]

In the "classical" Einsteinian universe, "space" is a three dimensional "surface" occupying a four dimensional "volume" (our concepts of surface and volume are a bit limited, but close enough for the simplistic explanation I'm giving).

Well, not really. You actually need many more extra dimensions if you want to attempt to describe the curvature of spacetime as some surface embedded in a higher-dimensional volume.

So it's not done. Instead we just develop a mathematical formalism for describing curvature without worrying about some sort of surface embedded in higher dimensions. It may be harder to visualize this way, but it's far easier mathematically.

 

[KimB]

And now, on to the original poster's questions, but first I just want to mention that ten years ago I couldn't have given you nearly as complete answers to these questions. Cosmology really is an exploding science.

No center of the Universe:

I get the idea that there isn't a center in the sense of an origin point from where it all expanded(If I'm not mistaken, it all expands in all directions, almost as if you were zooming in more and more on a point with everything within).

But if the elements are finite and if they're arranged in a 3d configuration, there has to be a center position. Only way you're getting out of that is with some weird thing like torus/videogame cube scenario(go up come through the bottom, same with left and right, etc.). If not there has to be a center position, even if we don't know it, and even if it's simply another average chunk of space.

Ah, but is space finite? We really don't know. Basically, we can only see so far, and we are certain that the totality of existence extends far beyond our own limited (15 billion light-year) horizon. So even if there might be some sort of center, there's no sense in talking about it, because it has no meaning.

I also have some issues with the idea that there is nothing outside the universe, as in no border/limit at the edge of the finite number of composing elements. how is that possible? There either has to be a void beyond the finite elemets, or at least a torus/videogame esque thing must occur.

Right, so there has to be a whole lot outside of our universe to explain the homogeneity and isotropy that we notice experimentally. Basically, the universe looks the same everywhere, and looks the same in every direction everywhere. If there was some sort of change beyond the limits of what we can see, we would expect to be able to detect such a change experimentally, which we haven't. But the main point is that there isn't any reason to bother discussing what happens outside of our horizon, because we can't interact with anything outside our horizon, and it can't interact with us.

Ok, so stars can't be at the edge of the galaxy cause there ain't enough mass to keep them from flying apart. So we just throw some massive invisible thingies at the borders and presto. What causes something that's supposed to interact only through gravity to cluster in a halo around the galaxy(aka, why is it concentrated in one region at all, why not dispersed all over?)?

Well, flying away from the galaxy, not apart. Anyway, it's a little bit more subtle than that. First, we take measurements of the velocities of stars. The velocity of the stars, if we assume a circular orbit, gives us a measure how how much mass is inside the orbit of the star. Now, we add up the mass from all of the stars we see and compare. We measure much more mass inside the orbits of these stars than can be explained by other stars, but it's actually worse than that: the mass density as a function of distance from the center doesn't even follow the right profile.

So, enter dark matter. Dark because we can't see it. And we also know that it extends far beyond the galaxies themselves. So, how do we measure it? Well, there are two big experiments that can help. One is gravitational lensing. Gravitational lensing can give strong contraints on what the masses of objects are by looking at how the gravity of the objects bends light from galaxies behind them.

A second measure of dark matter comes from observing the cosmic microwave background. To understand how dark matter can have an effect, I'll have to describe a little bit about what the CMB is. Right now, the universe is cold (~2.7K) and diffuse. Once it was hot and dense, so hot that it was basically a plasma (like the surface of the sun). Now, in a plasma, you have electrons decoupled from the protons, which in turn means that photons cannot travel very far before hitting something. Now, once the universe cools enough, those electrons and protons combine into atoms, and the universe becomes both neutral and transparent: the photons just stream freely (this is why we see the CMB).

But before that, the universe was made up of this fluid composed of photons, electrons, and protons. If there was an area that became slightly overdense, then the pressure from the photons would push it back. So we had this oscillatory motion that gave rise to characteristic sizes of densities.

Now, enter dark matter. Dark matter doesn't see the photons (otherwise we could see the dark matter), so there is nothing preventing it from just collapsing into bound systems. Thus, in the early universe, you have this interplay between matter that sees photons and matter that does not, which gives a very definite statistical distribution of hot and cold spots on the CMB. We measure this statistical distribution, and what we get is pretty striking:
There's about 10 times as much dark matter as ordinary matter. And not only that, but combine that information with galaxy surveys and distance measurements, and you find that the total dark matter and ordinary matter only make up about 30% of the total energy density of the universe. What's the rest? We don't know, so we call it dark energy.

Recently Stephen gave some ideas about how information is preserved despite h.radiation, I missed them, how exactly is information preserved?

Sorry, can't help ya there

Could one engineer a matter collapse around a ship(aka, hollow spherical mass surrounding the ship), such that the ship wasn't affected by the collapse and was right in the singularity? (iirc, objects within a hollow sphere can have the external gravitational force on them cancelling out. )

Ah, but matter is always drawn into a black hole. You can see this by simply looking at the behavior of light: if you pass just within the event horizon and shine a light beam out of the black hole, that light beam will actually keep moving towards the center of the black hole (space is being drawn in at faster than the speed of light!). Since no matter can travel faster than the speed of light, all matter must be drawn inward towards the center of the black hole.

So imagine you are in this ship "protected" by a huge shell of matter that is massive enough to make it into a black hole. Well, that shell of matter cannot help but collapse in on itself, physically crushing your ship.

From, what I've heard Mr. Tipler(from physics of immortality), in order to re-do the bang during a crunch, one 'd have to collapse everything in an intelligent and precise fashion. Intelligence surviving as things compressed infinitely.

What are the reasons for this need to precisely coordinate the collapse, what's the math involved? What kind of compression is this, as far as we know, molecular machinery hits the limits of miniaturization, will the compression be relative to something(that is not felt by the compressed) else leaving matter unscathed, if not are there really any known means of surviving compression that'd destroy molecular machinery?

Well, first of all, it seems extremely unlikely at this point in time that there will be any sort of "big crunch." At the current time, the expansion of the universe is actually getting faster, and there's no reason to believe this will change in the future (according to the simplest models, the universe is going to accelerate faster and faster until the expansion becomes exponential).

As for the Universe needing a very precise collapse to "re-explode," I don't buy it. If the Universe did turn around, then its collapse would be an exact mirror of the expansion. My naiive interpretation of what this person was thinking was simply that if you take a simple gas collapse model, you don't get just a ball of matter at the center: the probability for that low of angular momentum is almost nil, so what you get is a ball at the center, surrounded by a big accretion disk. This accretion disk may later coalesce into planets, asteroids, whatever.

But the collapse of the Universe as a whole is different. One very, very basic argument is simply the conservation of angular momentum: if it wasn't spinning when it started, it won't be spinning when it recollapses. So there's absolutely no problem with a completely symmetrical recollapse.

The problem comes in a repetition of the Big Bang. We just don't know enough physics to say anything about whether the Big Bang would start again.

 

[NANOTEC]

 

[zidane1strife]

Where is the center of the surface of the Earth? All points are equivalent so far as defining the center of Earth's surface is concerned. The Earth's surface is a two dimensional surface curved in a three dimensional volume. Remember the whole flatlander thing... imagine that people on Earth's surface are oblivious to the concept of up and down, they only know forward/backwards, left/right. Ask them where the center of their world is, and they will probably insist that there must be one even if they don't know where it is.

Now think of a balloon instead of the Earth, but consider the surface just the same. Still no "center" of the balloon's surface. Start blowing the balloon up... the surface expands. Draw dots all over the balloon and keep inflating it; the dots all expand relative to each other, none can be considered the surface.

Now ask yourself (or the flat people on Earth) where the "edge" of the surface of the balloon or Earth lies? Surely something must be beyond that "edge," but in reality there is only more balloon/Earth surface. Keep going, and you do indeed wind up where you started.

These easy to visualize analogies are two dimensional surfaces occupying a three dimensional volume. In the "classical" Einsteinian universe, "space" is a three dimensional "surface" occupying a four dimensional "volume" (our concepts of surface and volume are a bit limited, but close enough for the simplistic explanation I'm giving). Just as the Earth's surface has no center, space (the three dimensions we are familiar with) has no center. And just with the balloon, if you go far enough in one direction in space you may well end up where you started (there are other possibilities, but this is the simplest to visualize and relate to something familiar). And just as all points on the balloon's surface expand when you inflate it, all points in our space expand as the universe's size grows. No point is preferred as a center.

Ahh, but you might point out that the Earth and the balloon do have a center to their volume, so the universe should as well. Quite so. The center does not lie on the surface, however, but instead is found by moving in the "third" dimension. The balloon's center is, for our purposes, where the balloon was before you started inflating it. Similarly, to find the "center" of our four dimensional universe, you must move through the "fourth" dimension, which is conveniently represented as time. This provides the intuitive concept that the only "place" you can find the center of the universe is actually not a place at all, but a "time" - the beginning of the universe.

This answers most of the questions you asked in a fairly simplistic (and probably not completely consistent or rigorously correct, but nevertheless understandable) way.

Earth/balloon surfaces are torus/videogame like scenarios, so in essence you're saying that's the way the universe is said to be. How did they arrive at this conclusion? How do we know the vacuum wasn't there always, and it just popped out of it? Especially given that from what I've heard given enough time anything can pop out of the vacuum, and with enough time it is certainty every possible thing will pop out.

edit: Besides, many an article/journal/documentary/site has been saying that it is unknown if the universe is finite or infinite in size(while the matter content is definitely finite.). One of the examples for a possible finite universe would be torus/videogame esque edgelessness, yet even in recent mags this is only hinted as possible but as they themselves say this is not yet a definitive conclusion(giving the ship passing through the origin of its journey if it goes far enough example. Even hinting that some of the galaxies we're seeing might be the result of light doing this very thing, creating a hall of mirrors like effect).

All the while they've been saying this, they've also been saying the universe has no center. Yet if the universe is/was considered infinite in space with finite mass and number of elements, even if you invoke a torus/videogame scenario at infinite distance, the finite amount of matter cannot possibly uniformly fill an infinite sized space. So if we based ourselves simply on the area with matter, that area should've both an edge and a center with regards to the area it occupies within this infinite space, if the universe is infinite(again, they've been even recently hinting at the torus/videogame solution as just a way the universe 'might' be but that's it's not proven, and an infinite universe stands and stood as a possibility, and favored scenario.).

So how exactly can you say you've an infinite area with finite number of elements within, as a possible scenario(even if it was in the past), yet say there is neither an edge nor a center? As far as I see it, It is impossible for a finite number of elements to entirely fill an infinite space(especially when torus/videogame solutions are said to be alternate examples of finite universes giving the illusion of vastness/infinity, and not true infinites). So while the infinite space does not obviously've a center or edge, my view in such a scenario that was considered and favored to be so for the universe, would be for the finite space, occupied by the finite matter within, to itself have a center and edge within the centerless/edgeless infinity with regards to the finite space it occupies not the total infinity.

You assume that time and space are linear, which they're not.

NO I assume the only way you're getting out of that conclusion is if you arrange it in a weird way/geometry, such that things like edges don't exist, since you'd have basically all possible sticked together, to each other possible edge, forming a contiguous surface without edges.

Even so a universal civilization, could still choose an arbitrary point and define it as the center with arbitrary imaginary edges(that just happen to be sticked together, in a videogame like way). While it'd not be a real center, as any other point could've just as well be choosen, it could be the center of their universal map.

Chalnoth, nice info on dark matter, but what about its concentration in a halo like fashion around the galaxy? It's all over the galaxy, but I've heard it's concentrated speficically at the edges, and I don't know what causes that. Or does it not occur?

As for the blackhole, I'm not talking about driving a ship into a blackhole, I'm talking about building a gigantic matter shell around it and collapsing it around the ship, in a controlled fashion, in such a way that a blackhole bigger than the ship(the shell is very very very massive and large) forms with the ship right at the center, so it should be right in the singularity just as the blackhole forms.(That is there being enough matter for the collapsing matter to turn into a blackhole before reaching the area occupied by the ship. aka, a small tiny ship within a massive multistar mass shell.)

 

[Diplo]

Interesting stuff, guys!

 

[arjan de lumens]

The problem of setting up a blackhole shell around a space-ship is that the sections of the shell that are on opposite sides of the space-ship still gravitationally attract each other, so they wil continue to move towards each other, crushing the space-ship inbetween them. You may have a chance if your blackhole shell is rotating at an extremely high speed; in this case, angular momentum apparently prevents the blackhole singularity from collapsing into a point, instead collapsing into a small ring around your spaceship. There are some unanswered questions around the physics of this ring, though, so you should do some testing before trying to protect spaceships in this way.

 

[KimB]

edit: Besides, many an article/journal/documentary/site has been saying that it is unknown if the universe is finite or infinite in size(while the matter content is definitely finite.). One of the examples for a possible finite universe would be torus/videogame esque edgelessness, yet even in recent mags this is only hinted as possible but as they themselves say this is not yet a definitive conclusion(giving the ship passing through the origin of its journey if it goes far enough example. Even hinting that some of the galaxies we're seeing might be the result of light doing this very thing, creating a hall of mirrors like effect).

Okay, there's a finite amount of stuff we can see an interact with. But there's some large amount outside of what we can see and interact with (we know this from the properties of homogeneity and isotropy). What we don't know is how much. Yes, the torus/videogame scenarios are a possibility, but I'm not sure if we could ever test that possibility (doesn't stop people from trying, of course...there's a supposition that very long-wavelength photons may be affected by such boundary conditions).

All the while they've been saying this, they've also been saying the universe has no center. Yet if the universe is/was considered infinite in space with finite mass and number of elements, even if you invoke a torus/videogame scenario at infinite distance, the finite amount of matter cannot possibly uniformly fill an infinite sized space.

Well, it doesn't really matter. Within our universe, the universe we can interact with, there's no concept of the center. So, even if you were around at the time of inflation and could pinpoint the location it all started, it'd have no meaning to us today.

Chalnoth, nice info on dark matter, but what about its concentration in a halo like fashion around the galaxy? It's all over the galaxy, but I've heard it's concentrated speficically at the edges, and I don't know what causes that. Or does it not occur?

Ah, you see, because there's about 10 times as much dark matter, it's galaxies which sit inside overdensities in dark matter, not dark matter which sits around galaxies. And it's not concentrated at the edges: most of the dark matter is in the center, tapering off as you go further and further. It's just that it goes out really far, and there's a whole lot of it.

As for the blackhole, I'm not talking about driving a ship into a blackhole, I'm talking about building a gigantic matter shell around it and collapsing it around the ship, in a controlled fashion, in such a way that a blackhole bigger than the ship(the shell is very very very massive and large) forms with the ship right at the center, so it should be right in the singularity just as the blackhole forms.(That is there being enough matter for the collapsing matter to turn into a blackhole before reaching the area occupied by the ship. aka, a small tiny ship within a massive multistar mass shell.)

Right, that's exactly what I described (and is described in Arjan's post above).

 

[dizietsma]

It would be quite interesting to put an egg sized blackhole at the surface of the earth and watch it go. I assume it oscillates to a standstill at the earth's centre and then starts eating lunch.

How much does an egg sized black hole weigh at the earths surface ?

 

[Bigus Dickus]

Well, not really. You actually need many more extra dimensions if you want to attempt to describe the curvature of spacetime as some surface embedded in a higher-dimensional volume.

So it's not done. Instead we just develop a mathematical formalism for describing curvature without worrying about some sort of surface embedded in higher dimensions. It may be harder to visualize this way, but it's far easier mathematically.

Yes, I made a lot of simplifications and "end runs" around proper and formal definitions and usage, but I think my "layman's" way of describing things helps the "layman" to actually understand them. The surface/volume analogy is an extremely powerful one, and is probably the most widely used one to introduce someone to these concepts. Maybe it didn't grab your attention, but my use of "surface" and "volume" were in "" for a reason, with a disclaimer in that very sentence, and another more general one at the bottom... "probably not completely consistent or rigorously correct, but nevertheless understandable...."

If you look at the terminology he used to phrase his question, it's obvious he didn't need an answer steeped in heavy mathematical formalism. But such answers are interesting nonetheless.

 

[zidane1strife]

The problem of setting up a blackhole shell around a space-ship is that the sections of the shell that are on opposite sides of the space-ship still gravitationally attract each other, so they wil continue to move towards each other, crushing the space-ship inbetween them. You may have a chance if your blackhole shell is rotating at an extremely high speed; in this case, angular momentum apparently prevents the blackhole singularity from collapsing into a point, instead collapsing into a small ring around your spaceship. There are some unanswered questions around the physics of this ring, though, so you should do some testing before trying to protect spaceships in this way.

Ok found it

In Newtonian physics, the shell theorem states that the gravity due to a uniform spherical shell is zero on an object inside the shell, and acts on an object outside the shell as if the entire mass of the shell were at its center. By extension, the same can be said of a spherically symmetric object (and all relatively large celestial bodies are spherically symmetric to a good approximation).-wiki

A controlled collapse should make it so at all times during the collapse it is a uniform spherical mass falling in around the ship. So that the ship should experience zero gravitational force until it transitions into a blackhole at least, and then it'll be right in the singularity, if this somehow continued to apply(I've heard it's still beyond our understanding what happens there.) it shouldn't experience gravitational force either.

 

[Fred]

Yea, its an open question whether there is a 'center' to the universe w.r.t the Big bang.

For instance if the universe is topologically like a flat plane, the Big bang would happen everywhere (between all points), there would be no 'center' to anything, except that the proper length between any two points is identically zero (even for pts that are arbitrarily far apart... yes I realize its a little confusing)

Otoh, if the Universe is say something like a four dimensional sphere, the whole thing would be homotopically contractable to a pt.

Observationally this can lead to the same identical metric (the same physics), so the only way to discern the two cases would be to look at very distant stars/clusters on opposite sides of our horizon, and look to see for mirror images. This has been done, and we don't see any recurences, but that could just mean we aren't looking far enough, or that light hasn't had time to make a 360 around the sphere yet.

Dark matter is believed to be filamentary in nature (state of the art numerical simulations and observations of Lyman alpha forests), but it has resisted proper fundamental theoretical treatment for some time now. We don't know the entire dynamical spectrum yet, so lets just say its very much a work in progress (what I learned 7 years ago is quite different than what I would learn today for instance), and there are still many open issues at smaller scales.

 

[Fred]

Zidane, drop two balls with a small seperation from one another, from outer space. They will both feel a gravitational pull towards the center of the earth. Yet b/c they are seperated from one another, they will have some small vector components pointing towards one another (eg they will want to get closer and closer as they fall). Contrast that with a uniform gravitational field, where both objects move in straight lines forever.

This is essentially what happens with a black hole, there are 'tidal' forces on any objects that are not pointlike (for instance a human being will feel his arms being wrenched into his chest

 

[zidane1strife]

Zidane, drop two balls with a small seperation from one another, from outer space. They will both feel a gravitational pull towards the center of the earth. Yet b/c they are seperated from one another, they will have some small vector components pointing towards one another (eg they will want to get closer and closer as they fall). Contrast that with a uniform gravitational field, where both objects move in straight lines forever.

This is essentially what happens with a black hole, there are 'tidal' forces on any objects that are not pointlike (for instance a human being will feel his arms being wrenched into his chest

That's what happens to an object outside the blackhole falling in. Obviously whatever's inside would probably be annihilated someway(would turn into some spaghetti, according to some, if m/s theory is correct.). But if the shell theorem applies the object should not be destroyed as mass collapses around it in a hollow shell shape until it turns into a blackhole at which point it'd be at the singularity, which should make it unpredictable to know what actually happens to it, but that theorem most likely does not apply to blackholes, so I dunnoh. BTW, what causes internal tidal forces in the blackhole?

There's a new article on newscientists online saying tidal forces would destroy something inside a blackhole(do those apply at the singularity?), but if the blackhole was 5d it would be quite calm, and that this universe is likely inside one of such. Now I've no subscription so I've not read the article, that's only the intro.

 

[arjan de lumens]

Ok found it

A controlled collapse should make it so at all times during the collapse it is a uniform spherical mass falling in around the ship. So that the ship should experience zero gravitational force until it transitions into a blackhole at least, and then it'll be right in the singularity, if this somehow continued to apply(I've heard it's still beyond our understanding what happens there.) it shouldn't experience gravitational force either.

The ship itself may not have gravitationall pull working directly on it, but as the interior walls of the shell cave in, they will eventually reach the ship, and the pressures that drive them inwards will be applied to the ship too, eventually crushing it.

The outermost layer of your shell will be pulled inwards by all the interior layers of your shell, and the pressure of this pull will propagate inwards, applying everywhere within the interior. If you view your shell as a bunch of concentric layers, each layer will in this way apply additional pressure to all layers beneath it too. It is this pressure rather than gravity directly that causes the interior of the shell to cave in and destroy your spaceship.

It may be possible that you can have an intact spaceship in the center of the blackhole for a very short while, after the exterior of your shell has fallen into the event horizon but before it has fallen all the way into the center, but for a solar-mass blackhole, this period of time presumably amounts to only a few nanoseconds at most.

 

[KimB]

It would be quite interesting to put an egg sized blackhole at the surface of the earth and watch it go. I assume it oscillates to a standstill at the earth's centre and then starts eating lunch.

How much does an egg sized black hole weigh at the earths surface ?

Ah, now that would be fun

Well, let's see. First of all, an egg-sized black hole would be a little bit more massive than the Earth. So you would see the Earth and the black hole mutually fall into one another.

Then, most of the matter in the Earth would miss the black hole, with a tiny amount entering it, causing a massive explosion. As most of the matter is expelled into space at high velocity, what remains would be a slightly larger black hole with an accretion disk. Slowly the black hole will eat up the accretion disk, expelling some portion of the matter at extremely high velocity from the poles (note: this is what happens with the black holes in the centers of galaxies).

 

[blakjedi]

If a star that is arbitrarily larger (in mass and volume) than a given black hole "collides" with that black hole what happens?

can black hole ever be on the same plane as the observer? IE, does it really have a height/depth? or does it always appear "flat"

If two black holes meet do the form a super or do they release the sum of energy that they have absorbed?

Are black holes volatile? If black holes have do have volatility can they be "harvested" to recreate big bang like conditions and recycle mass and energy within a given area of space?

What I read in a some journal is the Universe is saddle shaped. The explanation as to why is well beyond my understanding.

Cheers

 

[KimB]

If a star that is arbitrarily larger (in mass and volume) than a given black hole "collides" with that black hole what happens?

Well, it's really no different than what I outlined above. The majority of the mass gets ejected, with some fraction falling into the black hole, and some other fraction forming an accretion disk which gets eaten up.

can black hole ever be on the same plane as the observer? IE, does it really have a height/depth? or does it always appear "flat"

Well, a black hole is defined by its event horizon (the horizon from which no light can escape). You can clearly never "see" a black hole, but light is bent around it pretty spectacularly. So a black hole looks like a distortion in space surrounding a black disc.

If two black holes meet do the form a super or do they release the sum of energy that they have absorbed?

Well, that's an interesting field of research going on right now, actually, as black hole-black hole mergers could possibly be an excellent means of measuring distances. The basic idea is that since you can't eject matter from the reaction, but there's a similar loss in gravitational potential energy from the collapse, there must clearly be some release in energy. This energy is released in gravitational waves, and these waves have a very specific signature. It may be possible, once our gravitational wave detectors get sensitive enough, to detect this signature of black hole-black hole mergers. I would personally expect to hear about a detection sometime in the next 3-5 years.

Are black holes volatile? If black holes have do have volatility can they be "harvested" to recreate big bang like conditions and recycle mass and energy within a given area of space?

Volatile in what sense?

A rotating black hole is extremely violent: if you drop matter into it, you get a portion of that matter spewed out from the poles in beams, with energy equal to the mass energy of the matter that went into the black hole. So one might use a rotating black hole as a power source.

Also, all black holes radiate some amount of energy through hawking radiation. Basically, black holes have a temperature, and this temperature is related to the mass of the black hole: smaller-mass black holes have higher temperatures. So what happens is that for a large black hole, it radiates away energy, losing mass, while its temperature increases.

What I read in a some journal is the Universe is saddle shaped. The explanation as to why is well beyond my understanding.

Okay, a saddle-shaped universe would be an open universe. But we really don't know right now whether the universe is open, closed, or flat. Current experiments are completely consistent with a flat universe, with a bias towards open or closed depending upon what experiments you look at. So I would say that if the author of the article you were reading said that with certainty, he was pulling stuff out of his ass.

Anyway, the basic idea here is what happens with parallel lines. If you have a flat universe, then parallel lines never intersect, and never change their distance from one another as you move along them. If you are in an open (saddle-shaped) universe, then parallel lines get further and further apart. If you are in a closed universe, then parallel lines get closer and closer together, eventually intersecting.

The curvature of the Earth, for example, can be described as closed. Parallel lines on the Earth get closer and closer, eventually intersecting. Some parallel lines on the Earth's surface, for example, are the lines of longitude, which intersect at the poles.