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Ring
2002-Jul-17, 02:19 AM
Yes, that's right I'm asking about bubbles.

In an infinite fluid filled universe there are two bubbles. Would these bubbles attract each other, repel each other, or neither? Don't ask what these bubbles are filled with, because I haven't a clue.

ZaphodBeeblebrox
2002-Jul-17, 10:44 AM
Well, that depends on a couple of Things:

What is the Difference between the Inner, versus the Outer Fluid, is it More Dense/Less Dense, More Charged/Less Charged/Differently Charged/Neutral or does it Respond to The Strong Nuclear Force?

And, Of Course, do our Four Fundamental Forces (Possibly Five), Even Apply?

John Kierein
2002-Jul-17, 12:39 PM
Well, I think that if the bubbles were "traditional", i.e., a spherical surface with a mass greater than the interior, these bubbles would gravitationally attract each other just as if their mass were at their centers (at the center of their masses. But if one bubble were inside the other, they'd ignore each other gravitationally, except that the outer bubble would be aware that it acted gravitationally on third objects more than if the inner bubble weren't there. Waddayathinkofthat?

<font size=-1>[ This Message was edited by: John Kierein on 2002-07-17 08:41 ]</font>

GrapesOfWrath
2002-Jul-17, 01:48 PM
John, I don't think Ring is asking about soap bubble-type bubbles. They seem to be more spherical holes in a fluid-filled universe. In other words, the fluid fills everything homogeneously so that there is no net attraction in any one direction--except there are two spherical voids, which Ring called bubbles.

I think we may have talked about this before, if not on the old BABB. Basically, you have to analyze all the forces produced by the material that would have filled the voids. Since those forces are "balanced" by all the fluid in the rest of the universe, when you subtract the material to create the void, the resultant is the opposite of those forces.

traztx
2002-Jul-17, 03:48 PM
Pretend the universe is this infinite homogenous fluid. Pretend you are a molecule. You have enough kinetic energy to be a fluid, so you are in motion. The matter far away from you on all sides is uniform so all effects from all sides are equal and cancel out. However, molecules nearby have a non-uniform effect on your journey, so locally the pressures fluctuate. But assuming the universe is well-tuned, all the fluctuations balance out and no clumping pressure zones develop.

Now, take the same scenario and add 2 bubbles (of lesser density and separated by a distance) and assume the fluid and bubbles are neutral charge.
1. If you are exactly between the bubbles, then you don't know they exist.
2. If you are along a line that passes through the centers of each bubble, then you feel acceleration towards the bubble farther away. Very few matter is exactly on the line and if so is probably passing thru it.
3. If you are not on the line, you will feel acceleration away from the line. More specifically, away from the center of the segment between the centers of the bubbles.
4. If you are on the line, but not between the bubbles, then you will also feel acceleration away from them. The acceleration is greatest near each bubble.

My take on it is this:
The pressure surrounding the bubbles will be slightly less, with the pressure between them being the least. The bubbles will then move towards each other.

There will be two pressure waves expanding away from the bubbles which will disturb the balance of the universe and make stars.
--Tommy

Ring
2002-Jul-17, 04:45 PM
I don't know it any of you guys remember (or ever knew of) a forum called Dr.Neutrino but this question was asked there and it generated an incredible number of responses. Unfortunately I can't remember the answer, however here is one response by a Ph.D. astrophysicist.




This turns out to be nothing more than Archimedes' idea - that overdense objects sink and underdense objects float. Check my post at Dr N for more details. I think my treatment works in a finite "universe" also - avoiding any "limiting case turnabouts". It is
simply obtained as the limiting case where you can make the bubble density less than any arbitrarily low local density (a definition of a bubble if ever I've heard one). It avoids the problem of having a gravitational force on a massless object by introducing the necessary concept of a pressure that acts on the bubble's rigid shell.

Oh... they "attract" by the way. Or should I say, a "test bubble" floats towards a big "cosmic bubble", against the (outwardly directed) net gravitational force at any radius.

GrapesOfWrath
2002-Jul-17, 04:58 PM
On 2002-07-17 11:48, traztx wrote:
3. If you are not on the line, you will feel acceleration away from the line. More specifically, away from the center of the segment between the centers of the bubbles.

The very center? I doubt that, even for bubbles of identical size.

Is that an inversion of the usual misconception that gravity acts towards the center of mass of a system?

traztx
2002-Jul-17, 05:16 PM
On 2002-07-17 12:58, GrapesOfWrath wrote:


On 2002-07-17 11:48, traztx wrote:
3. If you are not on the line, you will feel acceleration away from the line. More specifically, away from the center of the segment between the centers of the bubbles.

The very center? I doubt that, even for bubbles of identical size.

Is that an inversion of the usual misconception that gravity acts towards the center of mass of a system?


Good point! I didn't think of that.

If you take a point in space, subtract all the balancing mass around it...

This results in empty space with 2 bodies of same size as the bubbles, with density equal to the fluid minus the bubble density. From the selected point in space, the two bodies are located exactly opposite from where the bubbles were.

Now you can figure out the vector from 2 bodies and it will point more towards the closer body.

Which means the "tail" of the vector will be facing more towards the bubble closer to the point in space, but still on the line segment between the centers of the bubbles.

Thanks for correcting me /phpBB/images/smiles/icon_smile.gif
--Tommy

Chip
2002-Jul-17, 05:41 PM
Not really addressing your specific question but more in keeping with this topic in astronomy:
Book Recommendation: Bubbles, Voids and Bumps in Time - The New Cosmology, Edited by James Cornell
Amazon (http://www.amazon.com/exec/obidos/ASIN/0521426731/qid%3D1026926688/sr%3D11-1/ref%3Dsr%5F11%5F1/102-1117504-4804134)
Powells Used Copies (http://www.powells.com/search/DTSearch/search?kw=Bubbles%2C+Voids+and+Bumps+in+Time&pokey=skeptopotamus&Search.x=71&Search.y=10)
British Source (http://titles.cambridge.org/catalogue.asp?isbn=0521426731)

Chapter Recommendation from this book:
Geller, Margaret J. "Mapping the Universe: Slices and Bubbles." In Bubbles, Voids and Bumps in Time: The New Cosmology, edited by James Cornell, 50-72. Cambridge and New York: Cambridge University Press, 1989.
/phpBB/images/smiles/icon_wink.gif Chip

roidspop
2002-Jul-17, 06:31 PM
This is discussed in "Thinking Physics : Is Gedanken Physics; Practical Lessons in Critical Thinking" by Lewis C. Epstein. A really great book. It's out of print, but should be available from a dealer. My copy is in storage and I can't recall how the argument played out, so I should just shut up about that. I do think maybe it does have a bearing on astronomy, if we consider the giant voids that have been discovered in the cosmos. Are they attracted or repelled to each other?