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tommac
2008-Apr-29, 08:01 PM
If the universe was infinite in size wouldnt there be infinite gravity?

Gravity propogates at speeds in excess of the speed of light. ( how fast does gravity propogate)?

If gravity can propogate faster than the expansion of the universe.
Using the ant and the rope thing ...
and assuming that the universe is infinitely large
and assuming that a gravitational field extends forever

the gravitational pull at any point in space would be infinite???


Is that right?

speedfreek
2008-Apr-29, 08:12 PM
If the universe was infinite in size wouldnt there be infinite gravity?

Only if it contained infinite mass.



Gravity propogates at speeds in excess of the speed of light. ( how fast does gravity propogate)?

Gravity propagates at the speed of light and we think the amount of mass in the universe is finite, therefore the rest of your post doesn't really apply.

tommac
2008-Apr-29, 08:28 PM
Gravity propagates at the speed of light

Can you please show source of this? I have been reading differently.

cosmocrazy
2008-Apr-29, 08:29 PM
If the universe was infinite in size wouldnt there be infinite gravity?

If there was infinite gravity wouldn't we be back at the singularity?

tommac
2008-Apr-29, 08:31 PM
Ummmm ... wouldnt an orbit be unstable if gravity propogated at the speed of light?

I am pretty sure that gravity propogates at speeds much faster than the speed of light if not instantaneously.



Only if it contained infinite mass.




Gravity propagates at the speed of light and we think the amount of mass in the universe is finite, therefore the rest of your post doesn't really apply.

tommac
2008-Apr-29, 08:32 PM
If there was infinite gravity wouldn't we be back at the singularity?

Hah ...

This is more of a thought experiment that real life question.

But I think at the end it would prove that either gravity is not instantaneous or that the universe is not infinite.

tommac
2008-Apr-29, 08:34 PM
How can a black hole have gravity then?




Gravity propagates at the speed of light and we think the amount of mass in the universe is finite, therefore the rest of your post doesn't really apply.

cosmocrazy
2008-Apr-29, 08:36 PM
Can you please show source of this? I have been reading differently.

Where did you read about this? i raised this as my first question on this forum and most answers indicated that it propagates at the speed of light along with the other known forces. I am still unclear so would like to find out either way.:confused:

Amber Robot
2008-Apr-29, 08:36 PM
... we think the amount of mass in the universe is finite, therefore the rest of your post doesn't really apply.

If the universe is truly infinite in spatial extent there should be infinite mass as long as the mass density is non-zero, right?

cosmocrazy
2008-Apr-29, 08:40 PM
If the universe is truly infinite in spatial extent there should be infinite mass as long as the mass density is non-zero, right?

only if the all infinite spatial extent was filled with mass?

speedfreek
2008-Apr-29, 08:40 PM
Can you please show source of this? I have been reading differently.

Does Gravity Travel at the Speed of Light? (http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html)

First speed of gravity measurement revealed (http://www.newscientist.com/article/dn3232-first-speed-of-gravity-measurement-revealed.html)

cosmocrazy
2008-Apr-29, 08:42 PM
If the universe is truly infinite in spatial extent there should be infinite mass as long as the mass density is non-zero, right?

I guess in a way it does prove that the mass density is not infinite if it was then surely space would contract to zero?

tommac
2008-Apr-29, 08:48 PM
Where did you read about this? i raised this as my first question on this forum and most answers indicated that it propagates at the speed of light along with the other known forces. I am still unclear so would like to find out either way.:confused:


Heh ... nothing would work then ... Who told you that it was at the speed of light ... that is rediculous. That would mean a black hole could not have gravity. That would mean that nothing could be in orbit ... Think about the effects on the earth if there was a delay of gravity from the sun.

tommac
2008-Apr-29, 08:55 PM
Does Gravity Travel at the Speed of Light? (http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html)

First speed of gravity measurement revealed (http://www.newscientist.com/article/dn3232-first-speed-of-gravity-measurement-revealed.html)

OK ... in GR. ... yes speed of light ... but relative to us it is instantaneous.
That is what it is saying right?

speedfreek
2008-Apr-29, 09:01 PM
No, we think gravity travels at the speed of light relative to us, just like light does. Did you read the second link?

cosmocrazy
2008-Apr-29, 09:02 PM
Heh ... nothing would work then ... Who told you that it was at the speed of light ... that is ridiculous. That would mean a black hole could not have gravity. That would mean that nothing could be in orbit ... Think about the effects on the earth if there was a delay of gravity from the sun.

I have! and i have just read the 2 links provided by Speedfreek there is strong mathematical evidence to support either way, all depending on our definition of how we describe gravity, Newtonian or by GR. I wonder whether the underlining constant of C is actually the constant of space-time expansion and that the forces we known are instantaneous but are perceive to propagate at that speed because of this?:confused:

tommac
2008-Apr-29, 09:04 PM
Does Gravity Travel at the Speed of Light? (http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html)

First speed of gravity measurement revealed (http://www.newscientist.com/article/dn3232-first-speed-of-gravity-measurement-revealed.html)

Does GR ignore aberation?

alainprice
2008-Apr-29, 09:05 PM
Do me a favor and google "precession of mercury" . We will continue once you have consulted this test of GR.

tommac
2008-Apr-29, 09:07 PM
I think even in GR it propogates faster than the speed of light if you take abberration into account.


I have! and i have just read the 2 links provided by Speedfreek there is strong mathematical evidence to support either way, all depending on our definition of how we describe gravity, Newtonian or by GR. I wonder whether the underlining constant of C is actually the constant of space-time expansion and that the forces we known are instantaneous but are perceive to propagate at that speed because of this?:confused:

cosmocrazy
2008-Apr-29, 09:15 PM
Do me a favor and google "precession of mercury" . We will continue once you have consulted this test of GR.

Yes this proves to support very strongly that GR describes the true effect of gravity.

speedfreek
2008-Apr-29, 09:17 PM
And the underlying difference between Newton and GR is that Newton had gravity propagating instantly, whereas in GR it propagates at the speed of light.

cosmocrazy
2008-Apr-29, 09:19 PM
I think even in GR it propogates faster than the speed of light if you take abberration into account.

Explain this argument to me, please. :confused:

tommac
2008-Apr-29, 09:19 PM
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVM-40S7CT4-3W&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=cecd9673960235c949606db9eff773ea

tommac
2008-Apr-29, 09:20 PM
Explain this argument to me, please. :confused:

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVM-40S7CT4-3W&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=cecd9673960235c949606db9eff773ea


I am not sure if I have it right so I could tell you what I think but it is probably best if you read it yourself.

Amber Robot
2008-Apr-29, 09:32 PM
only if the all infinite spatial extent was filled with mass?

So is speedfreek implying that there is an infinitely large universe that is only partially filled with mass? If this were the case, what stops all that mass from gravitationally attracting itself? There would be an "edge" to matter and object on the edge would feel a net gravitational attraction toward the center of the mass distribution.

cosmocrazy
2008-Apr-29, 09:32 PM
I am not sure if I have it right so I could tell you what I think but it is probably best if you read it yourself.

I read the link and i,m not sure it is saying that in GR gravity propagates faster than C

"aberration in general relativity is almost exactly canceled by velocity-dependent interactions, permitting cg=c. This cancellation is dictated by conservation laws and the quadrupole nature of gravitational radiation"

:confused:

alainprice
2008-Apr-29, 09:34 PM
If I spill a gallon of beer on the floor and wait 10 minutes. Now I come back and walk a small distance away from where I dropped the beer. My feet are wet. Did the spill travel at the speed of beer or was it instantaneous since my socks are wet as soon as they touch the ground.

It's important to understand that once gravity has propagated to a certain place, the effect on a moving mass is instantaneous. The speed of gravity is still the speed of light.

Did you research the precession of mercury, bro?

cosmocrazy
2008-Apr-29, 09:38 PM
So is speedfreek implying that there is an infinitely large universe that is only partially filled with mass? If this were the case, what stops all that mass from gravitationally attracting itself? There would be an "edge" to matter and object on the edge would feel a net gravitational attraction toward the center of the mass distribution.

There could be a net gravitational effect towards the center of the mass distribution of the finite mass spread. But maybe expansion of space could counter act this for now. Prior to the discovery of accelerating spatial "expansion" was this not the theory of the big crunch?

cosmocrazy
2008-Apr-29, 09:41 PM
If I spill a gallon of beer on the floor and wait 10 minutes. Now I come back and walk a small distance away from where I dropped the beer. My feet are wet. Did the spill travel at the speed of beer or was it instantaneous since my socks are wet as soon as they touch the ground.

It's important to understand that once gravity has propagated to a certain place, the effect on a moving mass is instantaneous. The speed of gravity is still the speed of light.

Did you research the precession of mercury, bro?

Well put! this was the way i was thinking but could not put it into clear understanding to post.:)

speedfreek
2008-Apr-29, 09:47 PM
So is speedfreek implying that there is an infinitely large universe that is only partially filled with mass? If this were the case, what stops all that mass from gravitationally attracting itself? There would be an "edge" to matter and object on the edge would feel a net gravitational attraction toward the center of the mass distribution.

The original question was


If the universe was infinite in size wouldnt there be infinite gravity?

Define infinite gravity. In my definition, you need infinite mass-density to produce infinite gravity. What is your definition of infinite gravity? Am I barking up the wrong tree here?

Second point, who said the universe is infinite? I am certainly not implying that the universe is infinite, that's for sure. I tend to go with "finite but unbounded".

Amber Robot
2008-Apr-29, 09:57 PM
There could be a net gravitational effect towards the center of the mass distribution of the finite mass spread. But maybe expansion of space could counter act this for now. Prior to the discovery of accelerating spatial "expansion" was this not the theory of the big crunch?

Yes, this was not the theory of the big crunch. Prior to the accelerating universe, none of the three options, i.e., flat, positive, or negative curvature, had edges to the matter distribution. The big crunch, which results from a closed universe, is different from the scenario I was describing.

Jeff Root
2008-Apr-29, 10:02 PM
So is speedfreek implying that there is an infinitely large universe that is
only partially filled with mass? If this were the case, what stops all that
mass from gravitationally attracting itself? There would be an "edge" to
matter and object on the edge would feel a net gravitational attraction
toward the center of the mass distribution.
That is just one of several possible configurations of the Universe.
As far as I know there is no observational evidence that this particular
configuration is not the case. There is precious little evidence that it
is the case, though.

-- Jeff, in Minneapolis

grant hutchison
2008-Apr-29, 10:14 PM
If the universe is truly infinite in spatial extent there should be infinite mass as long as the mass density is non-zero, right?True, I think.
You can have an infinite universe of finite mass, if you distribute the mass fractally (in which case the mass density tends to zero as scale tends to infinity). This gets rid of Seeliger's Paradox (which actually long predates Seeliger): that in an infinitely old infinite universe of infinite mass, the gravitational force on any point is undefined, since it's the balance of infinite forces. Which would mean that the Earth's orbit around the Sun (for instance) would be catastrophically disrupted by any tiny variation in the mass distribution of the whole Universe.

Grant Hutchison

Addendum: Actually, if memory serves, I think some of these fractal cosmologies accommodate infinite mass while having only a finite gravitational force on any constituent. I think Charlier's original "hierarchical cosmos", for instance, had the mass increase in proportion to the radius of the enclosing sphere. Mass obviously goes to infinity as radius goes to infinity, but the mass density declines towards zero.

tommac
2008-Apr-30, 02:14 AM
I read the link and i,m not sure it is saying that in GR gravity propagates faster than C

"aberration in general relativity is almost exactly canceled by velocity-dependent interactions, permitting cg=c. This cancellation is dictated by conservation laws and the quadrupole nature of gravitational radiation"

:confused:

doesnt that mean that it can propogate faster than light when the aberration is taken into account?

tommac
2008-Apr-30, 02:19 AM
If I spill a gallon of beer on the floor and wait 10 minutes. Now I come back and walk a small distance away from where I dropped the beer. My feet are wet. Did the spill travel at the speed of beer or was it instantaneous since my socks are wet as soon as they touch the ground.

It's important to understand that once gravity has propagated to a certain place, the effect on a moving mass is instantaneous. The speed of gravity is still the speed of light.

Did you research the precession of mercury, bro?

Yeah I read it ... still digesting.

What I dont understand is why light and gravity do not work in parallel directions?


The Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes.

tommac
2008-Apr-30, 02:24 AM
Also again ... how do blackholes work if gravity is propogated at the speed of light?
Wouldnt that mean that a black hole couldnt have gravity?

alainprice
2008-Apr-30, 03:26 AM
Gravity is the bending of spacetime. Light is motion through spacetime.

They both share the same speed, but that doesn't mean that light declares the speed limit on space highways.

Remember that electrical charge(the field rather) escapes BHs as well. Dump a bunch of electrons in, and you can measure it from the outside.

tommac
2008-Apr-30, 02:04 PM
I am not claiming anything with this post. It was just a question of what ifs ...
it was leaning towards proving something was not true. Because if all of those were true then we would have infinite gravity. Hope nobody took it the wrong way.

phunk
2008-Apr-30, 03:47 PM
You can have an infinite universe with infinite mass that does not have 'infinite gravity' at any point, whatever infinite gravity means.

If the density is uniform at large scale, any point will have roughly the same amount of mass surrounding it in every direction, and the gravity will cancel out. This is pretty much what we see in the universe now, the distribution of mass is roughly the same in every direction, and we only really experience gravity from the local sources that aren't homogeneous.

Jeff Root
2008-Apr-30, 03:54 PM
He might have meant "infinite mass" instead of "infinite gravity".

However, the standard description of the cosmic expansion is that
it is/was/has been slowed by self-gravitation. If that is true, it is
natural to expect that in an infinite universe, with infinite mass, the
gravitational force slowing the expansion ought to be infinite.

-- Jeff, in Minneapolis

phunk
2008-Apr-30, 04:04 PM
Imagine an infinite universe with 1 proton per cubic light year. That's infinite mass, but very very low density. If it was expanding at the rate our universe is expanding, would you expect gravity to stop it?

Jeff Root
2008-Apr-30, 04:08 PM
You can have an infinite universe of finite mass, if you distribute the
mass fractally
Is there some reason you're ignoring the simplest possible case, of
infinite space with only a single, finite clump of matter? No special
distribution needed.

Of course, if the mass is infinite, the expansion and finite age of the
visible universe solve the infinite gravity problem the same way they
solve Olbers' paradox.

-- Jeff, in Minneapolis

Jeff Root
2008-Apr-30, 04:13 PM
Imagine an infinite universe with 1 proton per cubic light year. That's
infinite mass, but very very low density. If it was expanding at the rate
our universe is expanding, would you expect gravity to stop it?

I certainly wouldn't, no. tommac might expect anything.

The real Universe, though, appears to have a density indistinguishable
from the critical density.

-- Jeff, in Minneapolis

tommac
2008-Apr-30, 04:17 PM
Gravity is the bending of spacetime. Light is motion through spacetime.

They both share the same speed, but that doesn't mean that light declares the speed limit on space highways.

Remember that electrical charge(the field rather) escapes BHs as well. Dump a bunch of electrons in, and you can measure it from the outside.

But why doesnt gravity and light flow parallelly ?

tommac
2008-Apr-30, 04:20 PM
You can have an infinite universe with infinite mass that does not have 'infinite gravity' at any point, whatever infinite gravity means.

If the density is uniform at large scale, any point will have roughly the same amount of mass surrounding it in every direction, and the gravity will cancel out. This is pretty much what we see in the universe now, the distribution of mass is roughly the same in every direction, and we only really experience gravity from the local sources that aren't homogeneous.


Good point!!! Thanks! But this brings up some interesting thoughts for me ...
Maybe I need to post in a different thread ...

What I am thinking is that at any point if there is an equally infinite force in every direction acting on a point. I guess that would create a 0 force since all of the infinities cancel themselves out?

tommac
2008-Apr-30, 04:20 PM
I certainly wouldn't, no. tommac might expect anything.

The real Universe, though, appears to have a density indistinguishable
from the critical density.

-- Jeff, in Minneapolis


You are getting personal here.

phunk
2008-Apr-30, 04:23 PM
Is there some reason you're ignoring the simplest possible case, of
infinite space with only a single, finite clump of matter? No special
distribution needed.


In my opinion that would be a special distribution, not the one I described. Uniform density is easier to consider than an infinite universe only having a single clump of mass.

Besides, a single clump is a trivial case, gravity would be determined by the mass of the clump. The OP was asking about infinite gravity, so he must have been considering infinite mass.

phunk
2008-Apr-30, 04:28 PM
I certainly wouldn't, no. tommac might expect anything.

The real Universe, though, appears to have a density indistinguishable
from the critical density.

-- Jeff, in Minneapolis

My argument is that there is no critical density in an infinite universe. If it's truely infinite and uniform density (at the largest scale), there's no net gravity in any direction. Only local clumping of matter (galaxies, clusters, etc) leads to local deviations in gravity.

mugaliens
2008-Apr-30, 04:35 PM
True, I think.
You can have an infinite universe of finite mass, if you distribute the mass fractally (in which case the mass density tends to zero as scale tends to infinity). This gets rid of Seeliger's Paradox (which actually long predates Seeliger): that in an infinitely old infinite universe of infinite mass, the gravitational force on any point is undefined, since it's the balance of infinite forces. Which would mean that the Earth's orbit around the Sun (for instance) would be catastrophically disrupted by any tiny variation in the mass distribution of the whole Universe.

Grant Hutchison

Addendum: Actually, if memory serves, I think some of these fractal cosmologies accommodate infinite mass while having only a finite gravitational force on any constituent. I think Charlier's original "hierarchical cosmos", for instance, had the mass increase in proportion to the radius of the enclosing sphere. Mass obviously goes to infinity as radius goes to infinity, but the mass density declines towards zero.

Actually, not required! The universe may be infinate, but we can only observe part of it, known as the "observable universe. The very edge (most distant from us), known as the comoving distance, or cosmological boundary, space itself is expanding away from us at the speed of light. Beyond that is undoubtedly more of the universe, as there is no way we're at the exact center of the actual universe!

However, since gravity propogates at the speed of light, we're only affected by mass within the observable universe, not the entire universe. There may yet be an infinate universe with infinate mass. However, due to expansion, only what's in the observable universe affects us, and that's finite in both dimension and total mass.

Bogie
2008-Apr-30, 05:32 PM
Infinite gravity would only exist under such exotic conditions if the entire universe collapsed into one zero volume infinitely dense point. Otherwise the effect of distance on the effect of gravity would keep the average gravitation effect below infinite.

There are some very good posts in this thread and not to single any one out but the reference someone made to Olbersí is interesting. Olbersí is solved by the finite life of stars and the expansion of the universe. In Tommacís OP, with an infinite universe and I assume you mean infinite mass regardless of the distribution of the mass, then Olbersí analogy would apply if expansion was occurring infinitely (doubtfully in a spatially infinite universe with infinite mass) or if the life of mass that propagates gravity was finite, i.e. if the gravitation propagation ends under some conditions like inside a black hole or something IMHO.

Jeff Root
2008-Apr-30, 05:50 PM
You can have an infinite universe of finite mass, if you distribute the
mass fractally
Is there some reason you're ignoring the simplest possible case, of
infinite space with only a single, finite clump of matter? No special
distribution needed.
In my opinion that would be a special distribution, not the one I described.
Uniform density is easier to consider than an infinite universe only having a
single clump of mass.
The distribution I referred to as "special" was a fractal distribution.
I'd say that is vastly more special than a single clump.



Besides, a single clump is a trivial case, gravity would be determined by
the mass of the clump. The OP was asking about infinite gravity, so he
must have been considering infinite mass.
Yes, but Grant was talking about finite mass.

-- Jeff, in Minneapolis

Jeff Root
2008-Apr-30, 05:59 PM
My argument is that there is no critical density in an infinite universe.
If it's truely infinite and uniform density (at the largest scale), there's
no net gravity in any direction.
Really? In that case, you disagree with the standard view, and agree
with mine.

You're betting on a long shot if you side with me!

-- Jeff, in Minneapolis

phunk
2008-Apr-30, 06:02 PM
In my opinion (which is mostly worthless I know), uniform density is less unusual than an infinite universe having just one special point that's different than the rest (the clump).

I wasn't responding to grant though, I was responding to the OP. Infinite mass was implied in his question.

Michael Noonan
2008-Apr-30, 06:04 PM
There is also a solution for infinite gravity in an infinite universe with infinite matter providing gravity propagates at the speed of light.

In an infinite universe the expansion of space means that gravity is infinitely unconnected with any other part of the universe. That is because beyond the Hubble limit relative to any point in an infinite universe of infinite mass there is no connection.

However it plays merry heck with the notion of dark energy because as all of space is expanding the local gravity should in time exceed the disconnection from all other infinite gravity.

Noonan's Paradox

tommac
2008-Apr-30, 06:18 PM
My argument is that there is no critical density in an infinite universe. If it's truely infinite and uniform density (at the largest scale), there's no net gravity in any direction. Only local clumping of matter (galaxies, clusters, etc) leads to local deviations in gravity.

Couldnt a point then be mathmatically defined as the sum of an infinite set of infinitely strong scalers of gravity?

tommac
2008-Apr-30, 06:27 PM
If at the edge of our visible universe there is an extremely large web of gravitationally bound objects. it was positioned in a way ( maybe L shaped ) so that space between us and the furthest point is expanding at a different rate from us and its closest point. Yet it is gravitationally bound with itself so that locally the L doesnt change shape?

Can part of that object be moving away from us due to the expansion of the universe at faster than light speed while the other part is travelling at just under light speed?

lets assume that the L has a length and width millions of light years long.





There is also a solution for infinite gravity in an infinite universe with infinite matter providing gravity propagates at the speed of light.

In an infinite universe the expansion of space means that gravity is infinitely unconnected with any other part of the universe. That is because beyond the Hubble limit relative to any point in an infinite universe of infinite mass there is no connection.

However it plays merry heck with the notion of dark energy because as all of space is expanding the local gravity should in time exceed the disconnection from all other infinite gravity.

Noonan's Paradox

grant hutchison
2008-Apr-30, 06:33 PM
You can have an infinite universe of finite mass, if you distribute the mass fractally (in which case the mass density tends to zero as scale tends to infinity). This gets rid of Seeliger's Paradox (which actually long predates Seeliger): that in an infinitely old infinite universe of infinite mass, the gravitational force on any point is undefined, since it's the balance of infinite forces. Which would mean that the Earth's orbit around the Sun (for instance) would be catastrophically disrupted by any tiny variation in the mass distribution of the whole Universe.Actually, not required! The universe may be infinate, but we can only observe part of it, known as the "observable universe. The very edge (most distant from us), known as the comoving distance, or cosmological boundary, space itself is expanding away from us at the speed of light.If you check my post, you'll notice that I referred (carefully) to "an infinitely old infinite universe of infinite mass", which is the source of Seeliger's Paradox. The infinite age gives light and gravity time to reach everywhere from everywhere else. Seeliger isn't applicable to the real Universe as we understand it today, but like Olbers' Paradox, it was a worry in its day.

BTW: "comoving distance" is just a term used in setting up a coordinate system for the Universe, one which expands with time. It doesn't particularly apply to the edge of the observable Universe.
The distance at which comoving objects recede at the speed of light is the Hubble distance, and it doesn't mark the edge of the observable Universe either: there are many objects, currently observable, which are receding from us faster than light.

For a discussion of this terminology, try Davis and Lineweaver's Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe (http://arxiv.org/abs/astro-ph/0310808).
For more on fractal universes, try Grujic's The concept of fractal cosmos: II. Modern cosmology (http://adsabs.harvard.edu/abs/2002SerAJ.165...45G).

Grant Hutchison

tommac
2008-Apr-30, 06:37 PM
How does the expansion of space work when super cluster complexes are taken into account?

If space is being magically inserted in between us and other galaxies then something large and gravitationally bound would have space being inserted between us and sections of it at different rates. Would the furthest galaxies still be redshifted more than the closer ones? If so would that mean that the super cluster complexes itself is being pulled apart?

grant hutchison
2008-Apr-30, 06:39 PM
What I am thinking is that at any point if there is an equally infinite force in every direction acting on a point. I guess that would create a 0 force since all of the infinities cancel themselves out?This is the point of Seeliger's Paradox, that infinity minus infinity is not necessarily zero. If we add 12 to infinity, we still have infinity. Add a million to infinity, and we still have infinity. Add infinity to infinity, and we still have infinity. So infinity minus infinity could be any number at all: the net gravity at any location would be undefined, not zero.
It was a disconcerting intrusion of infinite arithmetic into the real world, and it wasn't at all welcome.

Grant Hutchison

phunk
2008-Apr-30, 07:10 PM
How does the expansion of space work when super cluster complexes are taken into account?

If space is being magically inserted in between us and other galaxies then something large and gravitationally bound would have space being inserted between us and sections of it at different rates. Would the furthest galaxies still be redshifted more than the closer ones? If so would that mean that the super cluster complexes itself is being pulled apart?

Yes, the furthest may be redshifted more (or blueshifted less). But it doesn't necessarily mean the cluster will be pulled apart, gravity could be pulling it back together more than expansion is pulling it apart, that's why things can remain gravitationally bound even in an expanding universe. You have to look at all of the forces together to determine the outcome.

John Mendenhall
2008-Apr-30, 07:20 PM
You are assuming ATM positions in the Q&A thread. You might be better off in ATM.

Try the article 'Speed of Gravity' in Wiki. The last time I read it, it was still good.

tommac
2008-Apr-30, 07:49 PM
You are assuming ATM positions in the Q&A thread. You might be better off in ATM.

Try the article 'Speed of Gravity' in Wiki. The last time I read it, it was still good.

Hope I havent made any ATM comments ... I am just asking questions on how gravity works. I realize that my current understanding my be different that MS but I am actually just trying to figure out how mainstream answers the questions that I am asking . I think these should be fairly easy to answer.

Michael Noonan
2008-Apr-30, 10:35 PM
Hope I havent made any ATM comments ... I am just asking questions on how gravity works. I realize that my current understanding my be different that MS but I am actually just trying to figure out how mainstream answers the questions that I am asking . I think these should be fairly easy to answer.

It might be meant for me because I brought in a dark energy reference and also because I was lazy and didn't check out Seeliger's Paradox. Questions are alright in question and answer. Answers have to be well thought out and provide information as taught to anyone who wishes to study science.

John Mendenhall
2008-May-01, 02:17 PM
"Nature is obliged to let reality determine its laws, whereas mathematics is under no such constraint."



My apologies in advance, this is way off topic, but Michael's quote sure is a good description of string theory.

Red Dawn
2008-May-01, 03:20 PM
I certainly wouldn't, no. tommac might expect anything.

If tommac did think that, and Newtonian physics applied, then I'd have to agree with him. The actual density has no bearing on the paradox grant hutchinson mentions. The force at any point would be a combination of infinite forces pulling in different directions.

Finite universe, different story. GR universe, I don't know enough to say.

With regards to other things people are talking about, at least under Newton's rules, a universe in which the density of mass is given by 1/(r^alpha), where r is the distance from some given point, and 1<alpha<=3, also has an infinite amount of mass but a finite force of gravity at each point.

mugaliens
2008-May-01, 05:09 PM
If you check my post, you'll notice that I referred (carefully) to "an infinitely old infinite universe of infinite mass", which is the source of Seeliger's Paradox. The infinite age gives light and gravity time to reach everywhere from everywhere else. Seeliger isn't applicable to the real Universe as we understand it today, but like Olbers' Paradox, it was a worry in its day.

BTW: "comoving distance" is just a term used in setting up a coordinate system for the Universe, one which expands with time. It doesn't particularly apply to the edge of the observable Universe.
The distance at which comoving objects recede at the speed of light is the Hubble distance, and it doesn't mark the edge of the observable Universe either: there are many objects, currently observable, which are receding from us faster than light.

I appreciate the clarification of terms. However, you're correct - we can see them, because back when they emitted their radiation, they were not moving faster than the speed of light. Thus, we see their radiation, and feel their gravity. As expansion pulled them beyond relative lightspeed, however, their radiation and gravity stopped being sent our way, relative to us. At a later date, we'll stop seeing that object, and stop feeling the effect of it's gravity.

Thus, even in an infinate universe with infinate mass, we only experience finite gravity from everything that was inside the Hubble limit 14.7 billion years ago.

grant hutchison
2008-May-01, 05:18 PM
However, you're correct - we can see them, because back when they emitted their radiation, they were not moving faster than the speed of light.In fact, they were moving faster than light when they emitted the light that is reaching us now, and they may well still be moving faster than light at the current cosmological time ("now"). For instance, the cosmic microwave background photons reaching us at the moment come from regions of space that were receding at 50 times the speed of light when these photons were emitted.

To quote Lineweaver & Davis (http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03&page=1):
One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.
The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light. Another way to put it is that the Hubble distance is not fixed and does not mark the edge of the observable universe.Grant Hutchison

tommac
2008-May-01, 07:59 PM
Would gravity overcome the effects of universal expansion?

A black hole still has gravity because gravity is the thing that is curving space-time back in on the BH itself. So the gravity still propogates out.

Now would the same be true for the expansion of space? That gravity would not be effected by the expansion of space at all? I guess there wouldnt be a big enough gravitational source to be able to determine this right?





I appreciate the clarification of terms. However, you're correct - we can see them, because back when they emitted their radiation, they were not moving faster than the speed of light. Thus, we see their radiation, and feel their gravity. As expansion pulled them beyond relative lightspeed, however, their radiation and gravity stopped being sent our way, relative to us. At a later date, we'll stop seeing that object, and stop feeling the effect of it's gravity.

Thus, even in an infinate universe with infinate mass, we only experience finite gravity from everything that was inside the Hubble limit 14.7 billion years ago.

NEOWatcher
2008-May-01, 08:07 PM
Would gravity overcome the effects of universal expansion?
The last I heard, NO.

But; I really don't get into it because that's been a major question of cosmology for a long time.
Just the simple fact that two theoretical physicists* had a bet going as to what the outcome would be, tells me that it's so far over my head that I just need to take modern science's word for it.

*IIRC, I think that was the big "magazine" bet described by Hawking.

phunk
2008-May-01, 08:11 PM
I thought that bet was over the black hole information paradox?

NEOWatcher
2008-May-02, 11:38 AM
I thought that bet was over the black hole information paradox?
Oops, my bad. :doh:
I'm gonna have to watch that series again, I know that was a major point made, and somehow I combined the two.

Michael Noonan
2008-May-02, 12:08 PM
Would gravity overcome the effects of universal expansion?

A black hole still has gravity because gravity is the thing that is curving space-time back in on the BH itself. So the gravity still propogates out.

Now would the same be true for the expansion of space? That gravity would not be effected by the expansion of space at all? I guess there wouldnt be a big enough gravitational source to be able to determine this right?
(my underline)
No is most certainly the correct answer.

Gravity acts back to a point. Universal expansion is equal across the universe and the rate of increase is uniform. The answer is the two shapes as they apply to space time curvature are different and so one can not cancel out the other.

Second the rate at which expansion propagates if that is a term that could be used is apparently also uniform in similar matter densities. Gravity bends more around matter density while expansion reduces significantly so the way in which each act is also different.

The answer is that we currently do not have a mainstream answer to the question of what is causing expansion. Because the way gravity is known to deform space time is known it is most certainly not going to be the solution to what is causing expansion.

By gravity propagating out of a black hole it is a force and not subject to the event horizon in the same way that light and matter are so it is not escaping as such as light escapes if it is not caught within the event horizon. That is why gravity is said to deform or pull on the fabric of space time rather than travel through it.

Quote:

Originally Posted by John Mendenhall

Originally Posted by Michael Noonan

"Nature is obliged to let reality determine its laws, whereas mathematics is under no such constraint."


My apologies in advance, this is way off topic, but Michael's quote sure is a good description of string theory.

Much appreciated John Mendenhall :)
I will try not to bring in string theory in this post.

tommac
2008-May-02, 12:51 PM
(my underline)
No is most certainly the correct answer.

Gravity acts back to a point. Universal expansion is equal across the universe and the rate of increase is uniform. The answer is the two shapes as they apply to space time curvature are different and so one can not cancel out the other.

.

Sorry if I am being stubborn here but I am trying to learn your answer.
If you have a point and say a large sphere. If you draw an arrow from the point ( lets assume it is in the exact center of the sphere ) out in all directions ( infinite number ) to the surface of the sphere ... wouldnt that be the direction of universal expansion? Now flip the direction of all of the arrows to point in to a point. Wouldnt that be gravity?

The problem is in the size of the sphere we are talking about and the fact that gravity decreases with distance while the expansion of the universe is constant throughout. Right? Am I off the mark here?