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.

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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.

Post a reply to this message

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.

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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.

Post a reply to this message

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.

Post a reply to this message

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

Post a reply to this message