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Darren New <dne### [at] san rrcom> wrote:
> Saying we're not inside a black hole implies we know what happens inside
> a black hole.
If we were inside a black hole, GR would be wrong, because GR predicts
certain things about the inside of a black hole (such as all space-time
geodesics going to the singularity), and this is clearly not the case
in our observable universe.
Given that it's GR which predicts the existence of black holes, and
saying that we may be inside a black hole, which means that GR doesn't
apply after all, it would be a statement that defeats itself: If GR doesn't
apply, then how can be was we are inside a black hole, which is a consequence
of GR?
> Plus, a sufficiently large black hole can have a
> surprising amount of livable space inside.
But according to the GR equations everything inside the event horizon
of the black hole would inevitably crunch into the singularity. There's
no avoiding it. Inside the event horizon moving in time is equivalent to
moving in space, and all the timelines go to the singularity. Even if a
force was applied to an object to try to keep it away from the singularity
it wouldn't help, because the object travels in time.
Source:
http://en.wikipedia.org/wiki/Schwarzschild_metric#Singularities_and_black_holes
Basically what you are saying is that GR applies inside the event
horizon as if there was no event horizon and no singularity, which is,
of course, contradictory.
Of course this is how I understand it. I wouldn't be surprised if I'm
wrong. There are many features of GR which I have had and still have
difficulties in understanding (the most recent case being the concept of
the cosmological horizon, which to me seemed impossible at first).
> How do you know everything doesn't go to a singularity in the future?
> That would be the "big crunch" theory, which I understand scientists
> believe is true if the universe contains enough mass to be finite in size.
If all geodesics and timelines point towards the singularity, how can
anything go away from the singularity?
Anyways, the current consensus, based on measurements, seems to be that
the universe is not only expanding, but is doing so at an accelerated rate.
It seems that there will not be a big crunch. The big rip seems more likely.
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > As far as I know, the universe is *expanding*, not going towards a single
> > point.
> If there's enough mass, then eventually gravity will pull everything
> into a "single point", just like everything came from a single point
> during the big bang. (At least, that's my lay understanding.)
Even if that was so, it still doesn't mean we are inside a black hole.
Anyways, it seems that this is not so after all.
> > A star is not a black hole even during its own collapse. Not until it
> > gets inside its own Schwarzschild radius. Collapse does not mean that
> > the thing which collapses is a black hole.
> I'm not sure "Schwarzschild radius" makes sense in the context of "all
> of space-time".
Why not? The Schwarzschild radius of a given mass is simply a distance
which is the product of the mass and a constant. The Schwarzschild radius
of the universe is the mass of the universe times a that constant.
If all the mass of the universe was compressed inside a volume smaller
than a sphere with that radius, it would form a black hole.
> I.e., what does "radius of the universe" mean?
It's not the "radius of the universe". It's the Schwarzschild radius.
Different thing.
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> > It's true that nobody dares to even guess what is happening inside the
> > event horizon of a black hole.
> Not true. They just won't guess what happens at the singularity. It's
> called a singularity because the math breaks down there.
No, I think it's inside the event horizon. While GR equations tell us
what happens if GR holds inside the event horizon in the same way as
outside it, it's still speculation.
> Calculate the gravitational curvature at the Schwartzchild radius for a
> galaxy-sized black hole. It's not that great. You likely wouldn't even
> notice you'd crossed any sort of boundary. (At least, that's what I've
> read. I am not sure how to do the math myself.)
I think you could clearly see the event horizon regardless of its size
because of the way it bends light.
Once inside, you are doomed. There's no escaping the singularity.
You'll go there no matter what you do.
> > However, if GR is right, everything inside
> > the event horizon inevitably goes to the singularity and there just isn't
> > escaping it. You just can't keep away from it no matter what you try.
> Sure. Let me know when you figure out how to escape the universe. :-)
Nothing is going towards a global singularity in the universe. On the
contrary, everything is going away from everything else.
> >> Surely you're not asking seriously? A sphere is a 2D finite surface with
> >> no border.
> >
> > The sphere is its own border.
> No, the sphere is a border for a ball, not a sphere.
A ball is a sphere.
Perhaps you are confusing it with a spherical surface?
--
- Warp
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Warp wrote:
> Of course this is how I understand it.
You probably understand it somewhat better than I do. :-)
> If all geodesics and timelines point towards the singularity, how can
> anything go away from the singularity?
But if it's a closed system that's (say) the 4D equivalent to the
surface of a sphere, all geodesics *do* point towards the singularity,
given enough time. I think.
> Anyways, the current consensus, based on measurements, seems to be that
> the universe is not only expanding, but is doing so at an accelerated rate.
Yep. I wouldn't be surprised if this changes again, tho. :-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Warp wrote:
> Why not? The Schwarzschild radius of a given mass is simply a distance
> which is the product of the mass and a constant.
What's the distance from the universe? Doesn't a distance have to be
between two events?
> If all the mass of the universe was compressed inside a volume smaller
> than a sphere with that radius, it would form a black hole.
>
>> I.e., what does "radius of the universe" mean?
>
> It's not the "radius of the universe". It's the Schwarzschild radius.
> Different thing.
How do you measure the "center" from which you take the radius of the
sphere? I.e., "distance" implies two space-time events between which
you're measuring a distance.
Certainly with enough mass, a finite universe *could* be within its
Schwartzchild radius. But I don't think I know enough about this to
really make any factual statements beyond that. :-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Warp wrote:
>> Not true. They just won't guess what happens at the singularity. It's
>> called a singularity because the math breaks down there.
>
> No, I think it's inside the event horizon. While GR equations tell us
> what happens if GR holds inside the event horizon in the same way as
> outside it, it's still speculation.
Fair enough. Altho people have spoken of time travel via rotating black
holes and such, but I suppose that's speculation until someone finds one
for sure. :-)
> I think you could clearly see the event horizon regardless of its size
> because of the way it bends light.
Yes, but all mass bends light. Even "dark matter". :-) It's kind of the
GR definition of "mass".
> Once inside, you are doomed. There's no escaping the singularity.
> You'll go there no matter what you do.
Yep. Big Crunch. :-) I think the math for charged rotating black holes,
or those taking into consideration quantum effects, are less obvious.
>> > However, if GR is right, everything inside
>>> the event horizon inevitably goes to the singularity and there just isn't
>>> escaping it. You just can't keep away from it no matter what you try.
>
>> Sure. Let me know when you figure out how to escape the universe. :-)
>
> Nothing is going towards a global singularity in the universe. On the
> contrary, everything is going away from everything else.
At the moment, yes.
>>>> Surely you're not asking seriously? A sphere is a 2D finite surface with
>>>> no border.
>>> The sphere is its own border.
>
>> No, the sphere is a border for a ball, not a sphere.
>
> A ball is a sphere.
Mathematically speaking, when mathematicians distinguish "sphere" from
"ball", the latter includes the points at distance < radius, while the
former includes only points at distance = radius.
http://mathworld.wolfram.com/Ball.html
(Occasionally I surprise myself with the useless information that sticks
in my head. Last week it was the reproductive cycle of jellyfish.)
The reason I made the distinction was to imply that the surface of a
sphere would be unbounded (as in, without any boundaries) to an entity
restricted to the surface. Sure, you couldn't change your radius, but
that's by definition of it being restricted to a sphere. An entity in a
ball, on the other hand, can move outwards from the center and hit a
boundary at the radius.
I believe Aristotle had an interesting insight (right or wrong). If
there is a boundary to the universe, and you throw a rock at the
boundary, what stops the rock? There can't be anything outside the
boundary to stop the rock.
> Perhaps you are confusing it with a spherical surface?
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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In article <477a9e54$1@news.povray.org>, dne### [at] sanrrcom says...
> Warp wrote:
> > As far as I know, the universe is *expanding*, not going towards a si
ngle
> > point.
>
> If there's enough mass, then eventually gravity will pull everything
> into a "single point", just like everything came from a single point
> during the big bang. (At least, that's my lay understanding.)
>
Actually, one aspect of at least one multi-universe system implies that
the big bang may have been a "white hole". Which is to say, something
that spews out matter, rather than condensing it. The math implies that
nearly any such structure though is unstable. I.e., it will only spew
matter for a short duration, from a singularity, before it stops doing
so. The question then becomes, "Did the white hole generate enough
matter, and with it, other forces, to prevent the whole thing falling
into a singularity again?" However, you are talking about GR. Quantum
mechanics, which GR doesn't apply to so well, implies that at the level
of a singularity its impossible to everything to fall into one, without
something getting back out again at all (i.e. Hawking's Radiation). Even
nastier is the implication that the structure of what comes out is
derivative of the material that goes in, such that information is never
lost in the system. Prior assumptions implied that the structure of a
thing, once in a black hole, would be randomized, such that it would
never come out in a mathematically predictable form.
--
void main () {
If Schrödingers_cat is alive or version > 98 {
if version = "Vista" {
call slow_by_half();
call DRM_everything();
}
call functional_code();
}
else
call crash_windows();
}
<A HREF='http://www.daz3d.com/index.php?refid=16130551'>Get 3D Models,
3D Content, and 3D Software at DAZ3D!</A>
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Darren New <dne### [at] sanrrcom> wrote:
> > If all geodesics and timelines point towards the singularity, how can
> > anything go away from the singularity?
> But if it's a closed system that's (say) the 4D equivalent to the
> surface of a sphere, all geodesics *do* point towards the singularity,
> given enough time. I think.
I must admit that I have no idea what the GR equations say about this,
but I got the impression that inside the event horizon all geodesics point
straight at the singularity, when going forward in time. I would expect
that no geodesic points away from the singularity at any point.
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >> Not true. They just won't guess what happens at the singularity. It's
> >> called a singularity because the math breaks down there.
> >
> > No, I think it's inside the event horizon. While GR equations tell us
> > what happens if GR holds inside the event horizon in the same way as
> > outside it, it's still speculation.
> Fair enough. Altho people have spoken of time travel via rotating black
> holes and such, but I suppose that's speculation until someone finds one
> for sure. :-)
I think the time travel is related to wormholes. Travelling inside a
wormhole does not imply going inside the event horizon of a black hole.
> > I think you could clearly see the event horizon regardless of its size
> > because of the way it bends light.
> Yes, but all mass bends light. Even "dark matter". :-) It's kind of the
> GR definition of "mass".
I said "they way it bends light", not "because it bends light".
No mass is dense enough to bend light like a black hole does because,
obviously, if the mass was dense enough, it would collapse into a black
hole...
> > Once inside, you are doomed. There's no escaping the singularity.
> > You'll go there no matter what you do.
> Yep. Big Crunch. :-)
I think the analogy is not correct.
The Big Crunch means that the universe, ie. space itself, contracts
until everything collapses into a singularity. Contraction of space doesn't
involve objects moving. (This is one of the hardest things to understand
about the expansion (and theoretical contraction) of the universe. One
counterintuitive result of the expansion of the universe is that the
distance between two objects can grow faster than c, which at first sounds
like it would break GR, but this is not so.)
A black hole doesn't contract. Instead, objects physically move towards
the singularity.
> > Nothing is going towards a global singularity in the universe. On the
> > contrary, everything is going away from everything else.
> At the moment, yes.
I can't say for sure, but I have the understanding that anything going
away from the singularity inside the event horizon of a black hole would
be against GR equations.
> (Occasionally I surprise myself with the useless information that sticks
> in my head. Last week it was the reproductive cycle of jellyfish.)
Whatever floats your boat. ;)
--
- Warp
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Warp wrote:
> I must admit that I have no idea what the GR equations say about this,
> but I got the impression that inside the event horizon all geodesics point
> straight at the singularity, when going forward in time.
I wouldn't think they point *straight* at the singularity, unless a
photon spiraling to its death is considered to be going in a "straight"
line. The fact that two particles can take different paths starting from
the same place would seem to imply that "straight at the singularity" is
either mistaken or so counter-intuitive that I don't understand what it
means. :-)
Take a photon that's halfway to the S-radius (for some meaning of
"halfway"). Fire it perpendicular to the radius (for some meaning of
"perpendicular"). I would think the photon would follow a spiraling
descent, rather than fall straight towards the singularity. Fire one in
the opposite direction. It'll take a different path. If photons always
follow geodesics, either you can have multiple geodesics pointing
"straight at" the singularity, or your impression is wrong. I don't know
which it would be, tho.
> I would expect
> that no geodesic points away from the singularity at any point.
Yeah. I'm just not sure the math works the same when you consider the
whole universe to be the black hole. If it were true, how could you
measure the "center" of the black hole? *I* sure don't understand the
subject well enough to be sure. I'm just saying what I've heard other
reputable GR scientists say.
Fun to babble about it, tho. ;-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Warp wrote:
> I think the time travel is related to wormholes.
Some is, yes. Negative density mass can do that.
> Travelling inside a
> wormhole does not imply going inside the event horizon of a black hole.
http://science.howstuffworks.com/time-travel2.htm
Right, but a rotating black hole might not have a singularity, and you
might be able to get out of the "event horizon" somewhere else.
>>> I think you could clearly see the event horizon regardless of its size
>>> because of the way it bends light.
>
>> Yes, but all mass bends light. Even "dark matter". :-) It's kind of the
>> GR definition of "mass".
>
> I said "they way it bends light", not "because it bends light".
Ah, I misinterpreted.
> No mass is dense enough to bend light like a black hole does because,
> obviously, if the mass was dense enough, it would collapse into a black
> hole...
Fair nuff.
> The Big Crunch means that the universe, ie. space itself, contracts
> until everything collapses into a singularity. Contraction of space doesn't
> involve objects moving.
I've never understood whether the expansion of space also implies the
expansion of the particles in that space. Actually, come to think of it,
that's a question relating GR to QM, so I'm not sure anyone knows. :-)
> A black hole doesn't contract. Instead, objects physically move towards
> the singularity.
Hmmmm.
> I can't say for sure, but I have the understanding that anything going
> away from the singularity inside the event horizon of a black hole would
> be against GR equations.
That certainly sounds intuitively correct, yes. But again, where would
the "singularity" of the universe be? How could you be "going away" from
it? Especially if your "going away" was caused by the expansion of
space, which as you say does not involve actual movement? :-)
> Whatever floats your boat. ;)
It came up because I was here last week:
http://www.youtube.com/watch?v=AtRHzcwjc1k
Very cool trip.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> Take a photon that's halfway to the S-radius (for some meaning of
> "halfway"). Fire it perpendicular to the radius (for some meaning of
> "perpendicular"). I would think the photon would follow a spiraling
> descent, rather than fall straight towards the singularity. Fire one in
> the opposite direction. It'll take a different path. If photons always
> follow geodesics, either you can have multiple geodesics pointing
> "straight at" the singularity, or your impression is wrong. I don't know
> which it would be, tho.
When the photon is outside the event horizon it follows some curve
(unless it's going directly towards the center of the black hole), but
once it passes the event horizon, things change.
> > I would expect
> > that no geodesic points away from the singularity at any point.
> Yeah. I'm just not sure the math works the same when you consider the
> whole universe to be the black hole.
Why not?
> If it were true, how could you
> measure the "center" of the black hole?
It's where the singularity is. A black hole always has a singularity
(if GR is right, that is).
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> I've never understood whether the expansion of space also implies the
> expansion of the particles in that space.
The current theory of the expansion of the universe is not a simple one.
This is how I have understood it after struggling with it for quite some
time (errors are possible):
Galaxies do not expand because their gravity keeps them in shape. In
other words, gravity inside (and near) galaxies is strong enough to
"resist" the expansion. Consequently nothing inside galaxies expands.
Dark matter halos surrounding galaxies might play a role in this too,
although this is only my own speculation.
In intergalactic space, far away from galaxies, the gravity caused by
galaxies is too weak to "resist" expansion and thus space expands there.
This is the reason why galaxies recede from us, but stars in our galaxy
don't.
The expansion of the universe doesn't imply movement of the galaxies.
The space itself expands, and distances get larger. (No, I don't know
for sure what is it exactly that causes this. I'm told that according
to GR this would happen even without dark energy.)
This is also the reason why galaxies can recede from us faster than c.
They are not actually moving, it's just that "new space" is forming between
us and those galaxies, making the distance between us and them larger.
There's nothing stopping this from making the distance grow faster than c,
not even in GR.
This causes, among other things, for it to be possible for there to be
a so-called cosmological horizon: From certain distance forward from us
there's universe which we can never see, no matter how long we wait.
Nothing from there ever arrives here, and we can never go there. This is
because those parts are receding from us faster than c. The actual size
of the universe is pure speculation (it's impossible for us to know it).
Conjectures range from the universe actually being much smaller than what
is observable to it being staggeringly larger.
Another slightly difficult thing to grasp is that a constant expansion
of the universe actually means that galaxies recede from us at an exponential
rate. At each certain unit of time space distances double, which means that
the distance between two galaxies grows exponentially with time. (If galaxies
receded at a constant velocity that would actually imply a logarithmic
expansion of the universe.)
Now, what baffled scientists was that the expansion is not constant nor
slowing down, but actually accelerating (which I suppose means that galaxies
are actually receding faster than exponentially from us).
Dark energy has been postulated as an explanation for this. It somehow
causes an inflationary effect on the universe, making it to expand in an
accelerated way.
I have understood this as being a bit like this: While galaxies "pull"
the geometry of space, contracting it, and keeping it static, dark energy
has the opposite effect and "pushes" the geometry of space, inflating it,
a bit like a gas. Close to galaxies this "pushing force" is not strong
enough to overcome the "pulling" effect of gravity, but far away from the
galaxies it overcomes it and causes intergalactic space to expand.
(All this is not based on professional literature, so don't take it
for granted. It's just how I have understood it with me extremely limited
understanding.)
--
- Warp
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Warp wrote:
> When the photon is outside the event horizon it follows some curve
> (unless it's going directly towards the center of the black hole), but
> once it passes the event horizon, things change.
I'll have to take your word for it.
>>> I would expect
>>> that no geodesic points away from the singularity at any point.
>
>> Yeah. I'm just not sure the math works the same when you consider the
>> whole universe to be the black hole.
>
> Why not?
Because the question of where the event horizon is, or where the
singularity is, doesn't make sense.
>> If it were true, how could you
>> measure the "center" of the black hole?
>
> It's where the singularity is. A black hole always has a singularity
> (if GR is right, that is).
Only if it's not rotating.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Warp wrote:
> Galaxies do not expand because their gravity keeps them in shape. In
> other words, gravity inside (and near) galaxies is strong enough to
> "resist" the expansion. Consequently nothing inside galaxies expands.
Hmmmm.... I'll take this idea with a grain of salt. :-)
> Another slightly difficult thing to grasp is that a constant expansion
> of the universe actually means that galaxies recede from us at an exponential
> rate. At each certain unit of time space distances double, which means that
> the distance between two galaxies grows exponentially with time.
Yet, funny enough, the number of galaxies we see in an area is constant
for a given volume, or close to it. This implies that perhaps the
galaxies aren't receding at all, and space isn't growing at all.
I saw an interesting paper that postulated that what we observe would
also be correct if further galaxies simply had a time axis pointing away
from us. I.e., plot the universe on the surface of a sphere, and have
time running perpendicular to the surface. The further apart two points
are, the more red-shifted they will see each other, even if the sphere
isn't expanding. It explains why we see a constant number of galaxies in
a given space, and a number of other problems as well (like the "dark
energy" expansion effect). It also explains the background radiation,
the fact that it's flat everywhere, and that
I'm eager to hear about it coming out in a peer-reviewed journal.
> Dark energy has been postulated as an explanation for this. It somehow
> causes an inflationary effect on the universe, making it to expand in an
> accelerated way.
Sounds ad hoc to me. :-) Fortunately, none of this affects my life in
even the most trivial way, until some scientist actually *does*
understand it well enough to make predictions and hence technology. :-)
> (All this is not based on professional literature, so don't take it
> for granted. It's just how I have understood it with me extremely limited
> understanding.)
Understood. Thanks for the lay interpretation.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> >> Yeah. I'm just not sure the math works the same when you consider the
> >> whole universe to be the black hole.
> >
> > Why not?
> Because the question of where the event horizon is, or where the
> singularity is, doesn't make sense.
Of course it makes sense. Ever heard of this thing called center of mass?
> >> If it were true, how could you
> >> measure the "center" of the black hole?
> >
> > It's where the singularity is. A black hole always has a singularity
> > (if GR is right, that is).
> Only if it's not rotating.
If I'm not mistaken, in a rotating black hole the singularity is a ring
(or, more precisely, a torus with minor radius 0). Still a singularity.
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > Galaxies do not expand because their gravity keeps them in shape. In
> > other words, gravity inside (and near) galaxies is strong enough to
> > "resist" the expansion. Consequently nothing inside galaxies expands.
> Hmmmm.... I'll take this idea with a grain of salt. :-)
Why?
> > Another slightly difficult thing to grasp is that a constant expansion
> > of the universe actually means that galaxies recede from us at an exponential
> > rate. At each certain unit of time space distances double, which means that
> > the distance between two galaxies grows exponentially with time.
> Yet, funny enough, the number of galaxies we see in an area is constant
> for a given volume, or close to it. This implies that perhaps the
> galaxies aren't receding at all, and space isn't growing at all.
Why? If the expansion is even throughout the universe and galaxies where
distributed about equally to begin with, they will be distributed equally
at all times regardless of the expansion.
> I saw an interesting paper that postulated that what we observe would
> also be correct if further galaxies simply had a time axis pointing away
> from us.
Are we the center of the universe?-)
> It explains why we see a constant number of galaxies in
> a given space
I don't understand why an expanding universe couldn't have a constant
number of galaxies in a given space.
--
- Warp
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>>>> Yeah. I'm just not sure the math works the same when you consider the
>>>> whole universe to be the black hole.
>>> Why not?
>
>> Because the question of where the event horizon is, or where the
>> singularity is, doesn't make sense.
>
> Of course it makes sense. Ever heard of this thing called center of mass?
Stop and think a bit. (Note that I'm no longer arguing whether the
universe *is* a black hole or not. :-)
Take a sphere (not a ball). Put dots of different sizes on it. Which is
the dot that's closest to the "center of gravity"? Where is the "center"
of the background radiation, if background radiation is literally
everywhere?
> If I'm not mistaken, in a rotating black hole the singularity is a ring
> (or, more precisely, a torus with minor radius 0). Still a singularity.
Could be. As I've said, you seem to have a better grip on this than I
do. In this case, note that the singularity isn't at the center of mass. ;-)
In any case, I think we've beat this dead horse enough. I'm going to
stop trying to figure it out. Interesting or amusing information is
still appreciated.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Warp wrote:
>>> Galaxies do not expand because their gravity keeps them in shape. In
>>> other words, gravity inside (and near) galaxies is strong enough to
>>> "resist" the expansion. Consequently nothing inside galaxies expands.
>
>> Hmmmm.... I'll take this idea with a grain of salt. :-)
>
> Why?
Because if space expands (say) 10%, and matter doesn't, then there's
something very funky going on. It would be like relativity saying
"clocks slow down, but not actual processes".
It doesn't seem logical to me that space expands 10% outside the galaxy
but not inside the galaxy. If there's a reason for "mass" to keep nearby
space from expanding at all, I don't know what it would be.
Now, if space expanded and gravity pulled everything back together, that
makes sense. But that still doesn't answer the question of whether (for
example) a brick floating in intergalactic space would be 10% bigger if
given enough time.
>>> Another slightly difficult thing to grasp is that a constant expansion
>>> of the universe actually means that galaxies recede from us at an exponential
>>> rate. At each certain unit of time space distances double, which means that
>>> the distance between two galaxies grows exponentially with time.
>
>> Yet, funny enough, the number of galaxies we see in an area is constant
>> for a given volume, or close to it. This implies that perhaps the
>> galaxies aren't receding at all, and space isn't growing at all.
>
> Why? If the expansion is even throughout the universe and galaxies where
> distributed about equally to begin with, they will be distributed equally
> at all times regardless of the expansion.
The distance between a galaxy 10 units away and one 20 units away should
be less than the distance between a galaxy 20 units and 30 units away,
because there will be more space added between us and 30 units than us
and 20 units. I think.
Like you said, more distant galaxies are moving away at an exponentially
faster rate. It was something to do with this, altho I might have
misremembered exactly the measurement. (Altho I remember it was a pretty
simple measurement.)
Hmmm... Maybe it was something like we should see more galaxies with a
5% redshift than a 10% redshift, or something.
>> I saw an interesting paper that postulated that what we observe would
>> also be correct if further galaxies simply had a time axis pointing away
>> from us.
>
> Are we the center of the universe?-)
Everyone is. :-) I thought maybe the reason SETI fails is that we
really *are* the first intelligent race because we're at a place where
time is running fastest. :-)
>> It explains why we see a constant number of galaxies in
>> a given space
>
> I don't understand why an expanding universe couldn't have a constant
> number of galaxies in a given space.
Hmmm... Maybe I'm misremembering. In any case, the paper claimed there
was an anomaly along those lines, in terms of "if space was expanding,
we'd see a distribution like ... but we don't". I wish I could find the
paper again. I tried searching a couple times and came up with nothing.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> > Of course it makes sense. Ever heard of this thing called center of mass?
> Stop and think a bit. (Note that I'm no longer arguing whether the
> universe *is* a black hole or not. :-)
> Take a sphere (not a ball). Put dots of different sizes on it. Which is
> the dot that's closest to the "center of gravity"?
Using spherical coordinates, you can calculate the point on the sphere
which has the smallest sum of distances to each of the points. The center
of mass differs from this only in that each point can have assigned a
different mass, and the distance is weighted by the squared inverse of
that mass.
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Now, if space expanded and gravity pulled everything back together, that
> makes sense. But that still doesn't answer the question of whether (for
> example) a brick floating in intergalactic space would be 10% bigger if
> given enough time.
I don't think the brick would expand because forces which are much
stronger than gravity are keeping it together.
--
- Warp
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Warp <war### [at] tagpovrayorg> wrote:
> This is also the reason why galaxies can recede from us faster than c.
Btw, there's an interesting paper on the subject here:
http://arxiv.org/pdf/astro-ph/0310808
--
- Warp
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Darren New nous apporta ses lumieres en ce 2008/01/02 21:48:
> Warp wrote:
>> Galaxies do not expand because their gravity keeps them in shape. In
>> other words, gravity inside (and near) galaxies is strong enough to
>> "resist" the expansion. Consequently nothing inside galaxies expands.
>
> Hmmmm.... I'll take this idea with a grain of salt. :-)
>
>> Another slightly difficult thing to grasp is that a constant expansion
>> of the universe actually means that galaxies recede from us at an
>> exponential
>> rate. At each certain unit of time space distances double, which means
>> that
>> the distance between two galaxies grows exponentially with time.
>
> Yet, funny enough, the number of galaxies we see in an area is constant
> for a given volume, or close to it. This implies that perhaps the
> galaxies aren't receding at all, and space isn't growing at all.
It is NOT constant! There are several regions in space that don't have any
galaxies for 100's of millions of light-years in any direction, and some where
galaxies are clustered toggether with average distances around a few 10000's
light-years. Our galaxy is around 27000 light years across.
You have galaxies clusters, then super clusters and mega clusters. You have
galaxies "filaments", literaly very long strings of galaxies and galaxies
clusters stretching for billions of light-years.
>
> I saw an interesting paper that postulated that what we observe would
> also be correct if further galaxies simply had a time axis pointing away
> from us. I.e., plot the universe on the surface of a sphere, and have
> time running perpendicular to the surface. The further apart two points
> are, the more red-shifted they will see each other, even if the sphere
> isn't expanding. It explains why we see a constant number of galaxies in
> a given space, and a number of other problems as well (like the "dark
> energy" expansion effect). It also explains the background radiation,
> the fact that it's flat everywhere, and that
Once again, the background radiation is realy not constant at all. It contains
both small scale and large scale "textures" with intensity variations of around
1 to 100.
>
> I'm eager to hear about it coming out in a peer-reviewed journal.
>
>> Dark energy has been postulated as an explanation for this. It somehow
>> causes an inflationary effect on the universe, making it to expand in an
>> accelerated way.
>
> Sounds ad hoc to me. :-) Fortunately, none of this affects my life in
> even the most trivial way, until some scientist actually *does*
> understand it well enough to make predictions and hence technology. :-)
>
>> (All this is not based on professional literature, so don't take it
>> for granted. It's just how I have understood it with me extremely limited
>> understanding.)
>
> Understood. Thanks for the lay interpretation.
>
It should be noted that some far galaxies are not receding fast enough and that
others, closer ones, are receding to fast.
--
Alain
-------------------------------------------------
Stay out of my head, its a bad neighborhood.
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Now, if space expanded and gravity pulled everything back together, that
>> makes sense. But that still doesn't answer the question of whether (for
>> example) a brick floating in intergalactic space would be 10% bigger if
>> given enough time.
>
> I don't think the brick would expand because forces which are much
> stronger than gravity are keeping it together.
In other words, space expands, but then the brick collapses again? Sure,
it's possible. It still doesn't explain why space expands but the matter
in it doesn't. It's like saying "time slows down, but the spring in the
clock keeps it running the same speed". :-)
I don't think this works for celestial objects. If you added space
between the sun and the earth without slowing the orbit of the earth (in
absolute velocity terms), you'd make an unstable system where the earth
would tend to move even farther from the sun.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> > I don't think the brick would expand because forces which are much
> > stronger than gravity are keeping it together.
> In other words, space expands, but then the brick collapses again? Sure,
> it's possible.
No. Space expands, the brick doesn't.
> It still doesn't explain why space expands but the matter
> in it doesn't. It's like saying "time slows down, but the spring in the
> clock keeps it running the same speed". :-)
Hey, I'm just repeating what they are saying. Don't blame me.
Things like expansion of the universe still make a lot of sense when
compared to the wackiness of quantum mechanics, which has all kinds of
things akin go magic. (For example particles being in two places at the
same time. This includes the possibility of a particle being inside and
outside the event horizon of a black hole at the same time. Go figure.)
> I don't think this works for celestial objects. If you added space
> between the sun and the earth without slowing the orbit of the earth (in
> absolute velocity terms), you'd make an unstable system where the earth
> would tend to move even farther from the sun.
AFAIK space doesn't expand inside galaxies.
--
- Warp
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In article <477d216a@news.povray.org>, war### [at] tagpovrayorg says...
> Darren New <dne### [at] sanrrcom> wrote:
> > > I don't think the brick would expand because forces which are much
> > > stronger than gravity are keeping it together.
>
> > In other words, space expands, but then the brick collapses again? Sure
,
> > it's possible.
>
> No. Space expands, the brick doesn't.
>
> > It still doesn't explain why space expands but the matter
> > in it doesn't. It's like saying "time slows down, but the spring in the
> > clock keeps it running the same speed". :-)
>
> Hey, I'm just repeating what they are saying. Don't blame me.
>
> Things like expansion of the universe still make a lot of sense when
> compared to the wackiness of quantum mechanics, which has all kinds of
> things akin go magic. (For example particles being in two places at the
> same time. This includes the possibility of a particle being inside and
> outside the event horizon of a black hole at the same time. Go figure.)
>
> > I don't think this works for celestial objects. If you added space
> > between the sun and the earth without slowing the orbit of the earth (i
n
> > absolute velocity terms), you'd make an unstable system where the earth
> > would tend to move even farther from the sun.
>
> AFAIK space doesn't expand inside galaxies.
>
You keep repeating this, but other theories imply that, yes it does, its
just that the force that makes the universe expand is *huge*, but weak.
I.e., its like 10 billion hand fans waving at a sail. That amount of
force *might* have the effect of making the boat move, but its effects
on a fly passing in front of any given fan isn't going to do much. By
the same token, gravity is sufficient, for now, to prevent any obvious
expansion of places where it dominates, but we are talking about an
expansion rate, where it dominates, of like 1 gazillionth of a
millimeter per billion years, or some absurdly small change. Small
enough that its not going to be detectable, using any known instrument,
until like 10,000 times to time its going to take for the sun to burn
out.
It rather depends on "which" of the current theories you look at, and
who is discussing them. There are a lot of attempts to fit the math
together to work, and none of them have a clear picture, so all we have
is approximate guesses. Some of what you say fits, some of it doesn't,
so its is possible that the universe will die heat death, by *very
slowly* being ripped apart, including atoms, but at a rate that will
leave 99.9% of all suns dead by the time it starts to happen anyway,
but, at this point, slightly less possible, according to some people in
the field, that matter will remain intact, but the universe will just
keep expanding at an accelerating rate.
--
void main () {
If Schrödingers_cat is alive or version > 98 {
if version = "Vista" {
call slow_by_half();
call DRM_everything();
}
call functional_code();
}
else
call crash_windows();
}
<A HREF='http://www.daz3d.com/index.php?refid=16130551'>Get 3D Models,
3D Content, and 3D Software at DAZ3D!</A>
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In article <477d1268$1@news.povray.org>, ele### [at] netscapenet
says...
> Darren New nous apporta ses lumieres en ce 2008/01/02 21:48:
> > Warp wrote:
> >> Galaxies do not expand because their gravity keeps them in shape. In
> >> other words, gravity inside (and near) galaxies is strong enough to
> >> "resist" the expansion. Consequently nothing inside galaxies expands.
> >
> > Hmmmm.... I'll take this idea with a grain of salt. :-)
> >
> >> Another slightly difficult thing to grasp is that a constant expansi
on
> >> of the universe actually means that galaxies recede from us at an
> >> exponential
> >> rate. At each certain unit of time space distances double, which means
> >> that
> >> the distance between two galaxies grows exponentially with time.
> >
> > Yet, funny enough, the number of galaxies we see in an area is constant
> > for a given volume, or close to it. This implies that perhaps the
> > galaxies aren't receding at all, and space isn't growing at all.
> It is NOT constant! There are several regions in space that don't have an
y
> galaxies for 100's of millions of light-years in any direction, and some
where
> galaxies are clustered toggether with average distances around a few 1000
0's
> light-years. Our galaxy is around 27000 light years across.
> You have galaxies clusters, then super clusters and mega clusters. You ha
ve
> galaxies "filaments", literaly very long strings of galaxies and galaxies
> clusters stretching for billions of light-years.
> >
And at least one recently discovered region that is completely "black"
in the sense that there are no stars, gases, etc. in it, but where light
seems to instead "slow down" as it passes through, rather than speeding
up, via gravitational shifts. So, yeah, hardly "constant". lol
--
void main () {
If Schrödingers_cat is alive or version > 98 {
if version = "Vista" {
call slow_by_half();
call DRM_everything();
}
call functional_code();
}
else
call crash_windows();
}
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3D Content, and 3D Software at DAZ3D!</A>
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Darren New wrote:
> stbenge wrote:
>> nemesis wrote:
>>>
>>> perhaps our own Universe is nothing but a side-effect of something
>>> bigger...
>>
>> Some of the new theories point an explanation regarding this.
>> According to the theories, only certain particles can pass between
>> dimensions, or branes. Two of these particles are tachyons and
>> gravitons (not sure if the latter have been identified yet though).
>
> And Dust!
>
Or grains of salt, depending on how much stock you put into these new
theories ;)
Sam
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>>> I don't think the brick would expand because forces which are much
>>> stronger than gravity are keeping it together.
>
>> In other words, space expands, but then the brick collapses again? Sure,
>> it's possible.
>
> No. Space expands, the brick doesn't.
That doesn't really make sense. "Everything gets longer, except your
rulers." It would seem to be changing fundamental physical constants.
> Hey, I'm just repeating what they are saying. Don't blame me.
Fair enough. :-)
> AFAIK space doesn't expand inside galaxies.
OK. More weirdness. Have you heard any postulates on why that would be
the case? Space only expands where the gravity is sufficiently low?
Sounds like "Fire Upon the Deep" to me, where the speed of light gets
faster the farther away from galaxies you get. :)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> > No. Space expands, the brick doesn't.
> That doesn't really make sense.
It makes sense when there's a force opposing the expansion.
Think about it this way: The distance between atoms in a molecule is
determined by the bounding forces between atoms. These are proportional
to the charge of the electrons, or whatever. It's a constant. It doesn't
change. If you tried to separate the atoms of a molecule, this force would
resist it.
I suppose it's not too much different than the cause why you don't
drop to the center of the Earth due to gravity: Because there's a force
stopping you from doing so.
> "Everything gets longer, except your
> rulers." It would seem to be changing fundamental physical constants.
Not everything gets longer. Only intergalactic space, far away from
the galaxies, gets longer.
> > AFAIK space doesn't expand inside galaxies.
> OK. More weirdness. Have you heard any postulates on why that would be
> the case? Space only expands where the gravity is sufficiently low?
http://en.wikipedia.org/wiki/Hubble%27s_law
"In using Hubble's law to determine distances, only the velocity due
to the expansion of the universe can be used. Since gravitationally
interacting galaxies move relative to each other independent of the
expansion of the universe, these relative velocities, called peculiar
velocities, need to be accounted for in the application of Hubble's
law. The Finger of God effect is one result of this phenomenon
discovered in 1938 by Benjamin Kenneally. In systems that are
gravitationally bound, such as galaxies or our planetary system, the
expansion of space is (more than) annihilated by the attractive force
of gravity."
--
- Warp
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>>> No. Space expands, the brick doesn't.
>
>> That doesn't really make sense.
>
> It makes sense when there's a force opposing the expansion.
Warning: Underinformed rambling ahead... :-)
Hmmm. Maybe it makes sense if you're assuming that the "expansion of
space" is caused by "adding space" between other bits of space. It
doesn't make as much sense if you think of "expansion of space" in the
same terms as you think of "contraction of things going near light speed".
I.e., it's like saying "time slows down, so you get more seconds per
minute, but the seconds are the same length." How would the universe
know to keep the seconds the same but make the minutes different?
> Think about it this way: The distance between atoms in a molecule is
> determined by the bounding forces between atoms.
It's determined by electrons exchanging photons, which gives rise to
what can be interpreted as forces, yes. It certainly depends on distance
(amongst other things) or you'd stick just as readily to glue on the
other side of the room as you do to the glue on your chair. :-) But the
underlying numbers stay the same ("n" and "j", basically, with "n" being
related to the mass of the electron (equal, if random spontaneous
"virtual particles" didn't interfere), and "j" being related to the
"charge" of the electron, which is proportional to that electron's
likelihood of absorbing or emitting a photon). But the formula using
those numbers is also based on distance, due to the inverse-square law.
(Or inverse-cube, I forget which.)
E.g., it's much more likely for an electron to wind up stuck to an
adjacent atom in a molecule than it is for that electron to wind up
thirty feet away.
> These are proportional to the charge of the electrons, or whatever.
Charge (which is the probability of an electron absorbing or emitting a
photon), mass (not sure if this includes relativistic mass), and distance.
> It's a constant. It doesn't change.
> If you tried to separate the atoms of a molecule, this force would
> resist it.
Right. But would the molecules themselves be getting bigger? Would the
protons and electrons be larger? Would the photons that mediate that
force be getting red-shifted in both directions?
Or would new space be getting inserted between the protons and the
electrons? If so, I'd think your chemistry would change, in just the
same way that chlorine is more chemically active than oxygen.
> Not everything gets longer. Only intergalactic space, far away from
> the galaxies, gets longer.
That's what has me curious. How does space far from galaxies "decide"
to get larger while space between stars doesn't?
>>> AFAIK space doesn't expand inside galaxies.
>
>> OK. More weirdness. Have you heard any postulates on why that would be
>> the case? Space only expands where the gravity is sufficiently low?
>
> http://en.wikipedia.org/wiki/Hubble%27s_law
>
> "In using Hubble's law to determine distances, only the velocity due
> to the expansion of the universe can be used. Since gravitationally
> interacting galaxies move relative to each other independent of the
> expansion of the universe, these relative velocities, called peculiar
> velocities, need to be accounted for in the application of Hubble's
> law. The Finger of God effect is one result of this phenomenon
> discovered in 1938 by Benjamin Kenneally. In systems that are
> gravitationally bound, such as galaxies or our planetary system, the
> expansion of space is (more than) annihilated by the attractive force
> of gravity."
OK. That still doesn't make sense to me. As space expands between two
stars within a galaxy, the gravity between those stars gets *weaker*,
meaning they orbit more slowly and are less likely to hold together.
Maybe that's why galaxies turn from globular clusters into spirals or
something, tho.
It makes sense if what it's saying is "the expansion of space isn't
rapid enough to keep a collapsing galaxy from collapsing", but if you
have a stable orbit and then start adding "space", the orbit won't
somehow adjust to take account of that - the sign of the change is the
wrong direction.
It also makes sense if what it's saying is "galaxies moving relative to
each other due to gravity are moving at a significant speed compared to
the speed caused by the expansion of space, and thus we must account for
that." That seems to be what the "finger of god" effect is talking
about, regardless of the quote.
(http://en.wikipedia.org/wiki/Fingers_of_God)
I'm not clear on two things: (a) why, if space expands, don't the things
in the space expand, and (b) why would gravity "overcome" the expansion
of space. There's no reason for the earth, for example, to have a
"preferred distance" from the sun, gravitationally-speaking. If extra
space gets added, either the earth will spiral away from the sun or
it'll slow down (as larger orbits are slower orbits).
Now, for (a), maybe stuff in the expanding space *does* get larger. A
red-shifted photon is, in many senses, "bigger" than a blue-shifted
photon, so maybe the expansion is small/slow enough that the expanding
things *do* get bigger. But if that was the case, atoms would also be
getting bigger in proportion to "space", and your rulers would show that
space isn't getting bigger after all, so the *only* way to notice might
be from red-shifts? But if "space expands" in the same sense that "time
slows down near the speed of light", then it doesn't make sense to say
"the galaxies will hold together", as everything will nevertheless
expand too - I expect the expansion of the matter in the galaxy (if it
leads to increased mass) might exactly balance (via increased gravity)
the increased distance between objects. (I'd have to do more thinking
about this than I want to at this hour to check.) Alternately, it's
possible that the "G" in "F=G m_1 m_2 / r^2" is fractionally higher than
thought, just enough to offset the expansion of space? Dunno.
To address (b), you could be inserting "new space" between other "bits
of space", but then I can't imagine the mechanism (other than "new
space" only appearing where curvature from gravity is flat enough) that
would create "new space" between galaxies but not inside galaxies.
I don't think saying "gravity holds them together" would keep galaxies
from expanding at the same rate as the rest of the universe, and indeed
I'd expect them to expand faster as the gravity holding things together
got weaker with increasing distance and stuff started "flying apart".
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> I'm not clear on two things: (a) why, if space expands, don't the things
> in the space expand, and (b) why would gravity "overcome" the expansion
> of space.
Gravity deforms space, as does the expansion of the universe. Gravity
resists the expansion.
Perhaps you are thinking about gravity as a force?
--
- Warp
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Darren New <dne### [at] sanrrcom> wrote:
> Space only expands where the gravity is sufficiently low?
Btw, this might have something to do with so-called "vacuum energy"
(in addition to the so-called "dark energy"). Existing vacuum energy
expands the universe, but can only do so if gravity is low enough
(becaues there's only a limite amount of such energy). But I'm not at
all sure of this.
--
- Warp
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Darren New nous apporta ses lumieres en ce 2008/01/03 11:59:
> Warp wrote:
>> Darren New <dne### [at] sanrrcom> wrote:
>>> Now, if space expanded and gravity pulled everything back together,
>>> that makes sense. But that still doesn't answer the question of
>>> whether (for example) a brick floating in intergalactic space would
>>> be 10% bigger if given enough time.
>>
>> I don't think the brick would expand because forces which are much
>> stronger than gravity are keeping it together.
>
> In other words, space expands, but then the brick collapses again? Sure,
> it's possible. It still doesn't explain why space expands but the matter
> in it doesn't. It's like saying "time slows down, but the spring in the
> clock keeps it running the same speed". :-)
>
> I don't think this works for celestial objects. If you added space
> between the sun and the earth without slowing the orbit of the earth (in
> absolute velocity terms), you'd make an unstable system where the earth
> would tend to move even farther from the sun.
>
Space expand where the mater density is under a treshold value. The very diffuse
gases and relative proximity from stars inside a galaxie prevent space from
expanding. Between galaxies, there is MUCH less gas and no stars, so, there is
not enough mater to "hold space in place".
The Andromeda galaxi is moving toward us as it is close enough for the mutual
gravitational pull to overcome the very weak spacial expantion in the area. The
mater density in the local group is high enough to, maby almost, maybe totaly,
cancel the expantion.
--
Alain
-------------------------------------------------
You know you've been raytracing too long when you downloaded and printed the
Renderman Interface documentation, so you'd have a little light reading to take
on holiday.
Alex McLeod a.k.a. Giant Robot Messiah
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Darren New nous apporta ses lumieres en ce 2008/01/03 23:51:
>> Not everything gets longer. Only intergalactic space, far away from
>> the galaxies, gets longer.
>
> That's what has me curious. How does space far from galaxies "decide"
> to get larger while space between stars doesn't?
It don't decide to get larger far from galaxies, galaxies prevent it to get larger.
>
>>>> AFAIK space doesn't expand inside galaxies.
>>
>>> OK. More weirdness. Have you heard any postulates on why that would
>>> be the case? Space only expands where the gravity is sufficiently low?
>>
>> http://en.wikipedia.org/wiki/Hubble%27s_law
>>
>> "In using Hubble's law to determine distances, only the velocity due
>> to the expansion of the universe can be used. Since gravitationally
>> interacting galaxies move relative to each other independent of the
>> expansion of the universe, these relative velocities, called peculiar
>> velocities, need to be accounted for in the application of Hubble's
>> law. The Finger of God effect is one result of this phenomenon
>> discovered in 1938 by Benjamin Kenneally. In systems that are
>> gravitationally bound, such as galaxies or our planetary system, the
>> expansion of space is (more than) annihilated by the attractive force
>> of gravity."
>
> OK. That still doesn't make sense to me. As space expands between two
> stars within a galaxy, the gravity between those stars gets *weaker*,
> meaning they orbit more slowly and are less likely to hold together.
> Maybe that's why galaxies turn from globular clusters into spirals or
> something, tho.
The concensus is rather that galaxies start as spiral and degenerate into
globular clusters.
Usualy, spiral galaxies are relatively young, with lots of forming stars and
interstellar gases. Globular galaxies are older, have very little or no new
stars forming and almost to totaly lack interstellar gases.
>
--
Alain
-------------------------------------------------
You know you've been raytracing too long when you've tried rendering hair with
each strand as an object.
Quietly Watching
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> I'm not clear on two things: (a) why, if space expands, don't the things
>> in the space expand, and (b) why would gravity "overcome" the expansion
>> of space.
>
> Gravity deforms space, as does the expansion of the universe. Gravity
> resists the expansion.
That was kind of what I was getting at with
"""
(other than "new space" only appearing where curvature from gravity is
flat enough)
"""
> Perhaps you are thinking about gravity as a force?
It *is* a force, depending on your definition of "force". :) As in
"F=ma". If you're going to argue gravity isn't a force, then magnetism
isn't a force either, nor is pushing on something.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Alain wrote:
>> That's what has me curious. How does space far from galaxies "decide"
>> to get larger while space between stars doesn't?
> It don't decide to get larger far from galaxies, galaxies prevent it to
> get larger.
Yeah. I was asking about the mechanism, tho. Is there an actual theory
as to *why* this happens?
> The concensus is rather that galaxies start as spiral and degenerate
> into globular clusters.
Ah. This is something new (or more correct at least) than when I last
looked at it.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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In article <477e656b$1@news.povray.org>, ele### [at] netscapenet
says...
> Darren New nous apporta ses lumieres en ce 2008/01/03 23:51:
>
> >> Not everything gets longer. Only intergalactic space, far away from
> >> the galaxies, gets longer.
> >
> > That's what has me curious. How does space far from galaxies "decide"
> > to get larger while space between stars doesn't?
> It don't decide to get larger far from galaxies, galaxies prevent it to g
et larger.
> >
> >>>> AFAIK space doesn't expand inside galaxies.
> >>
> >>> OK. More weirdness. Have you heard any postulates on why that would
> >>> be the case? Space only expands where the gravity is sufficiently low
?
> >>
> >> http://en.wikipedia.org/wiki/Hubble%27s_law
> >>
> >> "In using Hubble's law to determine distances, only the velocity due
> >> to the expansion of the universe can be used. Since gravitationally
> >> interacting galaxies move relative to each other independent of the
> >> expansion of the universe, these relative velocities, called peculiar
> >> velocities, need to be accounted for in the application of Hubble's
> >> law. The Finger of God effect is one result of this phenomenon
> >> discovered in 1938 by Benjamin Kenneally. In systems that are
> >> gravitationally bound, such as galaxies or our planetary system, the
> >> expansion of space is (more than) annihilated by the attractive force
> >> of gravity."
> >
> > OK. That still doesn't make sense to me. As space expands between two
> > stars within a galaxy, the gravity between those stars gets *weaker*,
> > meaning they orbit more slowly and are less likely to hold together.
> > Maybe that's why galaxies turn from globular clusters into spirals or
> > something, tho.
> The concensus is rather that galaxies start as spiral and degenerate into
> globular clusters.
> Usualy, spiral galaxies are relatively young, with lots of forming stars
and
> interstellar gases. Globular galaxies are older, have very little or no n
ew
> stars forming and almost to totaly lack interstellar gases.
> >
>
Was the consensus. The problem is, they have found old galaxies and new
that do *not* conform to those expectations. Its now thought that
globular systems may be missing the central black hole, or some other
factor, which would cause the tidal forces that make some galaxies spin,
and thus form spirals. Its kind of up in the air though.
--
void main () {
If Schrödingers_cat is alive or version > 98 {
if version = "Vista" {
call slow_by_half();
call DRM_everything();
}
call functional_code();
}
else
call crash_windows();
}
<A HREF='http://www.daz3d.com/index.php?refid=16130551'>Get 3D Models,
3D Content, and 3D Software at DAZ3D!</A>
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In article <477e8615$1@news.povray.org>, dne### [at] sanrrcom says...
> Alain wrote:
> >> That's what has me curious. How does space far from galaxies "decide"
> >> to get larger while space between stars doesn't?
> > It don't decide to get larger far from galaxies, galaxies prevent it to
> > get larger.
>
> Yeah. I was asking about the mechanism, tho. Is there an actual theory
> as to *why* this happens?
>
> > The concensus is rather that galaxies start as spiral and degenerate
> > into globular clusters.
>
> Ah. This is something new (or more correct at least) than when I last
> looked at it.
>
Actually, as I stating in the other post, its not really the consensus
any more, at least in new studies. And imho, it also makes no sense.
Tidal forces from a spinning black hole "could" pull stars into a
spiral. That would make sense. But... What kind of mechanism would
possibly make a galaxy start spinning, when it first formed *then* stop
spinning later? Either Alain has it backwards, or if just flat out
doesn't make any sense to me.
--
void main () {
If Schrödingers_cat is alive or version > 98 {
if version = "Vista" {
call slow_by_half();
call DRM_everything();
}
call functional_code();
}
else
call crash_windows();
}
<A HREF='http://www.daz3d.com/index.php?refid=16130551'>Get 3D Models,
3D Content, and 3D Software at DAZ3D!</A>
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Patrick Elliott wrote:
> Tidal forces from a spinning black hole "could" pull stars into a
> spiral. That would make sense.
Actually, I think the problem is that the calculations show that if you
start with a somewhat symetrical distribution of stars, you wind up with
a more symetrical rather than less symetrical distribution of stars
after a couple of orbits. I.e., gravity *won't* pull a galaxy into a
barred-spiral shape, as is so common out there.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> It *is* a force, depending on your definition of "force". :) As in
> "F=ma". If you're going to argue gravity isn't a force, then magnetism
> isn't a force either, nor is pushing on something.
According to GR gravity is not a force. A force causes acceleration and
gravity doesn't.
An object in free fall does not accelerate, it's simply travelling along
its geodesic. It might *look* to us like it was accelerating, but that's
only because we are seeing a 3D slice of space-time. (It's a similar effect
that railtracks in a photograph look like they are converging even though
in reality they are parallel and never converge.)
An object "resting" on the surface of the Earth, for example, is in
constant acceleration, caused by the force exerted by the surface of
the Earth. This force makes it constantly deviate from its geodesic.
What gravity is, is a deformation of space-time. Masses deform space-time
in such way that geodesics are not linear anymore (in the cartesian sense),
and thus objects in free fall which travel forward in time will travel
along these geodesics and will look like they are accelerating in our 3D
slice view of the world.
It's not even a question of "definition" of "force". According to GR
gravity is *not* a force because it doesn't cause an acceleration, and
a free-falling object is *not* accelerating.
--
- Warp
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Warp wrote:
> It's not even a question of "definition" of "force". According to GR
> gravity is *not* a force because it doesn't cause an acceleration, and
> a free-falling object is *not* accelerating.
I agree with everything you say.
Given that, some definitions of "acceleration" include "a change in
velocity". Clearly, gravity changes an object's velocity. That's the
sort of thing I meant. If you define gravity *not* to be a "force", then
you have to define "force" differently than Newton did (obviously), at
which point I don't know what the definition would be.
Interestingly enough, I understand that if you actually do the math, the
curvature of space by mass is surprisingly (to me) tiny. For example,
the circumference of the entire sun is only about a kilometer different
than you'd expect from multiplying the radius by Pi. (I don't know how
to do that math, but I read it in a text by someone you could safely
assume did.) It seems kind of odd that such a small change in the
curvature of space would lead to such a relatively large effect.
Oh, there it is. Wow, that was a bit of searching for the right google
terms.
"""
There is a simple formula for a spherical mass M - the change in the
radius (delta r) = MG/3c^2 For the earth this turn out to be about 1.5
millimeteres
"""
Also,
"""
If you consider the universe as a whole - it has the correct mass for a
black hole having a size equal to the Hubble radius.
"""
altho I can't say I know that's true.
<http://www.physicsforums.com/archive/index.php/t-7152.html>
Cool stuff. I guess light doesn't really bend very much going past the
sun, but it still seems like the earth has a pretty large gravitational
effect for something that's less than 2mm wrong. :-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> Clearly, gravity changes an object's velocity.
It only looks to us like it changes its velocity, again, because we
only see a 3D slice of space-time.
This is not much different as when we see a movie of a car driving
towards the horizon: On the 2D screen it looks like its velocity is
continually decreasing, until it gets to an almost halt, even though
in reality the velocity of the car could be constant. This is because
a 2D projection of the 3D world loses information.
Due to the principle of inertia, since there are no forces acting on
a free-falling object, its velocity doesn't change.
> If you define gravity *not* to be a "force", then
> you have to define "force" differently than Newton did (obviously), at
> which point I don't know what the definition would be.
If gravity does not change the velocity of an object (because it doesn't
cause any acceleration on it) then there's no need to redefine the meaning
of "force". The only difference with Newton is that Newton assumed that
gravity is a force even though it isn't (well, not according to GR anyways,
if we can assume it's correct).
--
- Warp
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Patrick Elliott nous apporta ses lumieres en ce 2008/01/04 14:41:
> In article <477e8615$1@news.povray.org>, dne### [at] sanrrcom says...
>> Alain wrote:
>>>> That's what has me curious. How does space far from galaxies "decide"
>>>> to get larger while space between stars doesn't?
>>> It don't decide to get larger far from galaxies, galaxies prevent it to
>>> get larger.
>> Yeah. I was asking about the mechanism, tho. Is there an actual theory
>> as to *why* this happens?
>>
>>> The concensus is rather that galaxies start as spiral and degenerate
>>> into globular clusters.
>> Ah. This is something new (or more correct at least) than when I last
>> looked at it.
>>
> Actually, as I stating in the other post, its not really the consensus
> any more, at least in new studies. And imho, it also makes no sense.
> Tidal forces from a spinning black hole "could" pull stars into a
> spiral. That would make sense. But... What kind of mechanism would
> possibly make a galaxy start spinning, when it first formed *then* stop
> spinning later? Either Alain has it backwards, or if just flat out
> doesn't make any sense to me.
>
Globular galaxies DO spin, otherwise they would totaly collapse unto themself,
all the stars merging toggether. This would cause the biggest, meanest,
mega-super-novae, if colapsing slow enough, or immediately collapse into a
super-massive black hole, if the collapse get a little faster. Both will cause
the formation of a huge black hole, the first way only add some mega firework.
Dopler shift analysis of all globular galaxies have shown that they are rotating.
Blobular galaxies are a lot like the nucleus of the spiral ones. The cenario is
that after the interstellar gases are all used up to form stars and planets, the
spiraling arm slowly fade away, leaving only the nucleus visible.
--
Alain
-------------------------------------------------
Phobophobia, n., The fear of fear. The fear of those who have phobia.
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Warp wrote:
> If gravity does not change the velocity of an object (because it doesn't
> cause any acceleration on it)
Except that newton assumed that gravity caused acceleration, and GR
isn't too far different from newton at low speeds.
Are you sure this doesn't apply only to things traveling at light speed?
I suspect I just don't understand the details of GR well enough to
understand the conflicts that seem inherent in saying that gravity
doesn't cause acceleration, etc. I'll stop now.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> Are you sure this doesn't apply only to things traveling at light speed?
Yes, I'm sure.
--
- Warp
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Warp <war### [at] tagpovrayorg> wrote:
> Darren New <dne### [at] sanrrcom> wrote:
> > Are you sure this doesn't apply only to things traveling at light speed?
> Yes, I'm sure.
Another way of thinking about it: When you drop a ball it looks to you
like it was accelerating because it's *you* who is accelerating, not the
ball. The ball is in inertial motion, you aren't.
--
- Warp
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Warp wrote:
> Another way of thinking about it: When you drop a ball it looks to you
> like it was accelerating because it's *you* who is accelerating, not the
> ball. The ball is in inertial motion, you aren't.
That's an interesting idea. Cool, thanks!
It still boggles my mind that a warp of space of < 2 mm can cause the
gravity on earth. :-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> It still boggles my mind that a warp of space of < 2 mm can cause the
> gravity on earth. :-)
I think there's too much experimental evidence of the GR equations being
right to not to believe it...
I suppose the geodesics "bend" quite a lot in the dimension we don't
"see" from our limited 3D perspective. Or, from another point of view,
we move in time quite "fast".
--
- Warp
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> It still boggles my mind that a warp of space of < 2 mm can cause the
>> gravity on earth. :-)
>
> I think there's too much experimental evidence of the GR equations being
> right to not to believe it...
(holds tongue... ;-)
But seriously, yes, I believe it. It's still mind boggling. :-) I think
it's more likely there's something more complex that's well understood
by people other than me that accounts for the apparently-large results
from apparently small distortions of space.
Maybe it's just that (a) we evolved with this gravity, so of course it
seems like a lot, (b) gravity isn't really that strong, as it takes the
entire sun just to keep the earth in an annual orbit, or something like
that.
> I suppose the geodesics "bend" quite a lot in the dimension we don't
> "see" from our limited 3D perspective. Or, from another point of view,
> we move in time quite "fast".
Since GR measures time and space in the same units (modulo "c"), I would
guess it's well-understood how fast we move in time. :-)
I think I need to dig up my feynman lectures on GR for people who aren't
physicists again. (aka "Six not-so-easy pieces")
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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