Physics Buffs, I have questions on Hawkins shape of the universe

[Melodybe]

There's a black hole in the Milky Way?!?! Physics buffs come talk to me

So there is a blackhole in the Mily Way!?!

Why hasn't anyone been talking about this?

Sure it just means that in a few million years our solar system will eventually be crushed into darkness, but it's cool nonetheless.

So since I only read the first hundred pages of Hawkins' Brief History of Time, can someone give me in laymens terms what black holes mean to the shape of the universe.

I was always very fond of Hawkin's hourglass shape of the universe, he stopped using it because he forgot to factor time into the equation. I personally would leave time out of the equation, since it is a manmade construct, but are there any other reasons why that shape has been debunked?

With the proof that the universe is expanding does that support the hourglass shape?

Since I have never taken any physics classes, I am out in the cold on this one. I am very interested to see what DU phyics buffs have to say about all this.

[salvorhardin]

1. Reposting from your other thread

Black holes seem essential in galaxy formation.
http://hubblesite.org/newscenter/newsdesk/archive/releases/2000/22/
http://www.pitsco.com/the_cause/cause3aero.htm
http://www.physorg.com/news3014.html

[Melodybe]

3. Thanks yo!

Any clue about my shape of the universe question?

[flakey_foont]

2. A black hole

is a star which has become so dense that it's mass has collapsed,generally at the end of the stars' life,,the force of gravity is so great that nothing, not even light can escape
theoretically, at the center of a black hole is the 'singularity point' .referred to as the point in 'space-time' where the known laws of physics break down...how it affects the shape of the universe..is that it's immense gravity sucks in anything and everything in the vacinity, and squeezed to infinite density...if the fabric of the universe is like a smooth sheet on your bed, and any object with mass sitting on it is going to create a small depression causing anything nearby to fall towards it - imagine a bed sheet with a super Oreck XL vaccuum at the center sucking in anything it can.....

[punpirate]

4. Well, as far as time goes...

... it's not a man-made construct. It exists. What is man-made about it is that man has applied a scale to it which is based on our own experience of it. We have years and months and days because of the rotation of the earth around the sun and the moon around the earth and the rotation of the earth itself, and finer and finer divisions of those as we have required them.

But, time simply is. Without it, we could not make basic assumptions about all other forces in the universe.

Think of it this way--if time did not exist, then all events in the universe would have to exist simultaneously, which, from our observations, has not happened. Therefore, time exists.

... Or, something like that. :)

Cheers.

[Ready4Change]

5. Our galaxies black hole isn't that immediate a threat.

We're orbiting the galaxies center fast enough we won't be drawn in so soon. (Think of water going down a drain.) It takes use 200 million years to go around once, and we'll do that a LOT before dropping in.

Long before then, odds are, our sun will burn out. For a bunch of reasons, as it burns out, the Earth will first be burnt to a cinder then maybe absorbed into the suns mass. If not, the sun will cool and Earth will become an icey, fried out tomb.

But that's not for BILLIONS of years.

I've not read Hawkins book, so I'm not informed about any hourglass shape to the universe. Sounds more like a metaphor for what some think the shape of space around a black hole might be.

Space is wide, but is drawn down into the black hole. Some think, on the "other side" of the black hole may be another space, like the bottom of an hourglass.

The hope there is that matter falling into a black hole might not be lost. It might instead be falling out somewhere else.

Without evidence, I don't buy it. I think it's more likely matter is crushed into an as yet state of matter/energy we don't yet understand. Otherwise, supermassive blackholes would grow less massive.

It is currently thought that blackholes do lose mass. But it seems to be slowly, through a radiation like effect called evaporation. Nothing close to the rapid mass loss should that matter be falling out elsewhere.

Ok, blabbed far too much for someone who's not an actual physicist. (D*mn my poor math scores!)

[SteppingRazor]

15. True, it might be falling out somewhere, but...

In another of his books, Black Holes and Baby Universes, Hawking (Yes, that's Hawking, not Hawkins) points out that, even if you were able to travel through a black hole to some other place, the gravitational forces involved would tear you into all of your separate molecules.

So, don't look to "wormholes" for soace travel or extradimensional travel. You may pass through to somewhere else, but in the end, you'll just be a subatomic cloud of particles when you arrive in your new home.

[Ready2Snap]

6. Consider this

the only structural shape seen all over the universe is the sphere.
This is because it models forces being equally distributed from the outside in or the inside out.
In the case of stars these inward forces (gravity) and outward forces (energy released from core thru nuclear fusion)
balance themselves out, hence a spherical shape.
It's called hydrostatic equilibrium.

Now, though there is some variation in the density of areas of the universe, on the scale of it these don't amount to much. So, if the universe has been expanding since the Big Bang simultaneously in all directions, it's a pretty safe bet the universe is spherical, too.

Here's something to chew on -

The universe we see is gauged to be 14 or so billion years old(Big Bang, expansion and like that.) Now since we haven't seen anything past what we assume is the edge of our universe, what's to say that there aren't other universes next to and probably similar to ours, kind of like bubbles in a soda?

And what if two universes expand to the point where they start to affect the shape of each other, or even contact and interact.
Wouldn't our universe eventually merge into a new and bigger universe, and wouldn't it most likely have an hourglass shape?

Also, the area where the universes intersect would cause the destruction and formation of tons of black holes and new galaxies. And when we detect the light from this great plane of formation, from the release of energy that it would cause, we could speculate that, having slammed up against another universe, our universe is now shaped like an hourglass.

But, what if three or more universes all expanded into each other? What would the resultant shapes look like?

Oh, by the way, Hawkins also theorizes that black holes will eventually evaporate, that is, their matter will bleed back into our time/space and will, at some point in time in the future, disappear, expelling all the stuff it sucked in.

[BillZBubb]

7. A few points

First, time is real as has already been pointed out. If nothing else, we can base that fact on the laws of Thermodynamics. Specifically, the law of entropy where all closed systems tend to "decay" in time towards randomness.

Second, space-time can have three possible shapes, curved out (saddle,hourglass), curved in (spherical), or flat. Most data indicates a closed, curved in universe. In particular, the Cosmic Background Radiation data appears to be uniform in all spacial directions with only random variation.

Cosmic background radiation is the heat from the origination event (Big Bang or whatever). We see this heat radiated to us from the edge of the universe in the form of microwave noise. It's the same principle as why we see star light from stars many light years away. We are looking backward in time when we see starts. Same with the Cosmic background radiation.

The fact the the Cosmic background is somewhat uniform in all direction implies a closed, curved space-time.

[Melodybe]

8. Interesting.

So even if the universe appears flat, it could be like middle age scientists assuming that the Earth is flat, only to find out that the Earth was so large that their flatness was really a sphere.

I like the hourglass shape cause it makes the most sense to me personally. I like the idea of a universe that expands and contracts into itself, made of big bangs and big crunches. It seems very fluid and infinite to me.

[hecate77]

9. Actually, it is no longer believed to be closed....

The recent observations of the increasing expansion rate of the universe seem to indicate that we have a negative cosmological constant, which means, in the above discussions, that we have a saddle-shaped universe.

What does a negative cosmological constant imply in terms of physics? It means there is some uniform background of repulsive gravity everywhere. Now, this repulsive force is too small to be measured on any local scale so far, but when you consider larger and larger scales, this very small value begins to matter (no pun intended).

As far as some other observations in this thread, that somehow a black hole at the center of the galaxy means that we will someday be sucked in, albeit a long time in the future, this is not necessarily the case. Galaxies don't suck, nor do black holes. They simply accrete. There is fundamentally no difference between the earth orbiting the sun and the sun orbiting the center of our galaxy. It does not matter one whit whether the mass at the center is in the form of a black hole or just a bunch of stars. If our sun suddenly collapsed (it cannot do that), we would stay in our current orbit. We would not be suddenly sucked in. What determines our orbit is our current velocity, our mass and the mass of all matter in our neighborhood, but most particularly, that inside our orbit. (mass outside our orbit affects us primarily by perturbing our orbit. Even though the black hole at the center of our galaxy is gaining mass by accreting from nearby gas and stars, it still is not changing the total mass at the center of our galaxy, but merely rearranging it, so to speak. So, other than some possible perturbations in our orbit, but overall energy of the orbit is not really significantly affected.

One issue that would come up if our sun became a black hole is that the current solar winds would no longer be there, and so we would lose some small outward pressure on our orbit. However, this is small in comparison to the total energy of our orbit, so it would not amount to much. Of course, our planet slows down anytime it runs into anything (meteoroids, dust, etc.) and whenever those silly rocket scientists use us to slingshot some silly spacecraft, but we won't worry too much about that unless the spacecraft get really, really big. In the end, the earth will maintaim orbit long after the sun is no longer so useful to us.

So, what could cause our sun to move towards the center of our galaxy? Anything that decreased its velocity in orbit, which is the same as decreasing its energy in the orbit. Changing the direction of the orbit is pretty much the same thing, so I just include that here. How could this happen? Hitting dust clouds is one way, this could slow down the sun in its path. Another would be close encounters with other stars, which could just as easily put us on an orbit further out. However, nothing that happens at the center of our galaxy, in the neighborhood of our billions of years old black hole is likely to do much to change our orbit. You might consider that the matter falling into the black hole might even push us out a little bit because of the intense radiation given off as matter nears the horizon (due to collisions with other matter in the rapidly rotating accretion disc, for instance). Is this enough to counter the loss of outward pressure due to the radiation from the stars that are being or have been accreted? Probably not, but the overall determinining factor is more in the orbital mechanics than in the increase or decrease of radiation and particle pressure from the center. Again, losing the pressure does not suck us in, but simply doesnt' push us out as much.

Anyway, just some points about the black hole at the center of our galaxy and how it is no threat whatsoever to our continued existence. Our sun will cease to support life on earth long before the end of our galaxy by any means that we can forsee (no galactic collisions on the near horizon, etc.).

[jayctravis]

10. I'm not a physicist, but I have a theory...

That a black hole is the "backside" of a star. "Backside" being relative, because for this to happen the "frontside" has to be in an alternate location outside of our 3-dimensional space. Alternate universe, or different dimension. (I mean, there are whacky behaviors in subatomic particles causing coordinance over long distances, existing in two places at once...etc.) A star would then be a "white hole" since matter can neither be created nor destroyed.

All the material sucked up is expelled as fuel for a star that's burning somewhere. Which could possibly suggest not a single "big bang" but simultaneous wormholes constantly dropping matter out of the universe in a black hole and bringing it back in as a star. Thus it would make sense...if a star dies out, the "fabric" of the universe springs from positive star (burning "out") to negative black hole(sucking "in") like a cosmic respiration. Imagine dropping a rock into water...that's like a black hole sucking a great amount of matter inside, then when the surface springs back, there is a splash which ejects water back out, which in turn falls back and causes another "gravitational" depression. The ripples caused in space time might result in what we see as supernovas or the colorful cloud formations of very old burnt-out stars that have over millenia run out of energy to cross this spacetime barrier...essentially the amount of energy it would take to travel in "hyperspace" or essentially teleport.

I'm probably a nut, but I've just always thought that.

[moggie]

11. Sorry for picking on a minor point

but it's Stephen Hawking, not Hawkins. Call me picky, but I see that mistake a lot, and it bugs me. I'm surprised to see someone who's read (part of) Brief History making that mistake.

Now, about Eisenstein's General Theory of Relativity...

[evermind]

12. About time...

One of the interesting bits of this thread is the discussion about time - a "man made" concept, a thing that exists, etc.

Time, like space, mass, and charge is part of the mathematical framework that physics uses to describe events - so by that token it's as "real" as any of those. In fact the "shape of the universe" as a physical concept, is discussed in Hawking's book as a shape of spacetime, a four-dimensional manifold with three space dimensions and one time dimension. This time dimension is built into Einstein's relativity, which deals with locations in spacetime via a 4-D co-ordinate system with special rules (the time dimension has a negative "signature", meaning that Pythagoras' theorem for finding a distance in spacetime works slightly differently than it would for a normal "euclidian" 4-D space - rather than the root of a^2 + b^2 + c^2 + d^2, you have the root of a^2 + b^2 + c^2 - d^2, where d is the time dimension!)

So anyhow, from that point of view, time in physics is as real as space.

However, it's interesting to note that there's nothing special about the present moment in physics using this kind of framework, any more than there is about a particular point in space - that is, the picture one gets is of a static, 4-D object, not a 3-D object that evolves and changes.

A particle (say) has a "world-line", which is a curve embedded in that 4-D manifold. The world-line curve doesn't move or change. Nothing in the equations picks out a particular point on the curve as special.

Yet if you were that particle, you would experience each point on the curve as your position at a particular time. You wouldn't experience the curve as a single static object at all. At any particular time, half of the curve is your invisible future position, and half is your invisible past.

Which is simply to say we don't experience time as a dimension in the same way we experience spatial dimensions: whereas we can look out across space and take in a great deal of it at once, we can only see one instant (a near-infinitessimal point) of the time dimension at any one time.

So relativistic theories don't include a description of the passage of time, as we experience it, as such. Nothing in them picks out the present moment as in any way special (which is why a poster above - and physicists generally - have to resort to thermodynamics or something like it to show that time flows from the past to the future.)

In quantum mechanics, there's a notion of an "event" - often conceived as a measurement event, where the idea of "now" can creep in, but the formulations used, as far as I know, don't provide precise descriptions of these measurement events; rather, they give probabilities for the results of such measurements, should they occur. What actually is going on in the measurements is rather left out of the picture.

So there is some sense in saying that, in a way, the passage of time as we experience it - a succession of events, seen point by point as we move along our world-line, is sort of left out of physical descriptions of the world.

Or that's how it seems to me right now, anyway.. ;-)

[moggie]

13. Photon time

Yet if you were that particle, you would experience each point on the curve as your position at a particular time. You wouldn't experience the curve as a single static object at all. At any particular time, half of the curve is your invisible future position, and half is your invisible past.

Unless you're a massless particle? Since a photon travels at c, doesn't time dilation effectively freeze subjective time for the particle? Eternal now, big bang to heat death in no "time" at all!

[evermind]

14. Yep, as far as I understand it, for photons it's over as soon

as it started.. they would experience their emission and absorption simultaneously no matter how long they'd travelled (from our point of view).

[Kraklen]

16. I think you've seriously misinterpreted A Brief History of Time.

I haven't read it since high school, so I may be mistaking this for someting else. But Hawking never describes an hourglass-shaped universe. There are hourglass shaped diagrams in the book, but these have nothing to do with the shape of the universe. I believe he calls them "light cones" or some such thing. They're diagrams describing the propagation of light across the xy plane with time on the z-axis.

I suggest rereading the book a bit more carefully.

As for the black hole in the center of the milky way, old news and completely irrelevant to this discussion in Hawking's book. It doesn't mean the milky way is going to get sucked into itself in a few million years. In fact it makes no difference. In fact, given the nature of the spin, there's a lot more stuff around the outside of the galaxy than at the center.

[benburch]

17. Don't worry about the hole...

Our galaxy will collide with the Andromeda Galaxy long before we have to worry about the black hole at the center.

We "only" have a few hundred million years left!

[TroubleMan]

18. And then after that...our whole supercluster of galaxies is heading toward

The Great Attractor.....whatever that is. Nobody's 100% sure yet.

The current theory is that it's a huge supercluster of galaxies.