Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Soul Surfer on 27/09/2010 11:47:21

I don't often ask questions on these pages but concentrate on answering them however this is a question that this been exercising my mind for some time and maybe one of the naked scientists themselves or their friends might like to have a go at answering it.
A lot of cosmology and astronomy is based on mathematical models. Now the validity of these models is only dependant on the quality of the thinking involved in setting these up. It is therefore quite possible to miss a trick if you exclude some details that could be significant. I have been very familiar with these problems in my work as a scientific innovator over many years. Looking at the way models are often presented in the original papers and in the more popular texts I am wondering if a significant effect has been ignored that could produce some innovative physical situations.
Consider first some basic gravitational physics. The gravitational effect of the following structures
1 an infinite gravitating cylinder
2 an infinite thin gravitating planar sheet
I appreciate that these are purely theoretical structures but you will understand what I am getting at later. We are all familiar with the inverse square law associated with a simple point or spherical lump of material that produces all the models of which we are familiar however the infinite cylinder displays a gravitational field that is a simple inverse first power law, Not a square law, and an infinite sheet demonstrates a constant field irrespective of distance. This effectively modifies the law of gravity and matter could behave very differently under these conditions.
The question is could any structures that approximate infinite cylinders or infinite planes ever happen in our universe?
Consider now the gravitational collapse of a large rotating cloud of gas etc. As it collapses conservation of angular momentum around its main rotation axis causes its rotation to speed up and the usual model of collapse is that the cloud tends eventually to form a disc and eventually a sphere however it will also still possess some net angular momentum around other axes that could possibly result in a toroidal structure (like a smoke ring).
The main question therefore is a stable toroidal gravitational collapse possible? and What would the properties of objects like this be like? Bearing in mind that a thin toroidal loop could approximate to an infinite cylinder and that a thin toroidal surface could approximate to an infinite plane.
I know of only one object where gravitating loops or toroids appear in the standard literature and that is in the ring "singularity" described in the model of the rotating (Kerr) type of black hole but that does indicate that a toroid must be considered as a possible result of gravitational collapse.
I have several ideas worked out for what could happen and how these relate to the universe that we observe but to talk about these would consign this thought to the new theories area whereas this is clearly a simple question based on current science.

I'm not sure if that kind of gravitational collapse is possible, but I'm curious about how the gravitational field of a cylinder or plane come in to play. Do they have to do with the collapse itself, or are they results of what the gravitational field might be like nearby this object once it's formed?

The question is could any structures that approximate infinite cylinders or infinite planes ever happen in our universe?
The "dark matter scaffolding" can be likened to infinite cylinders, see http://www.cosmosmagazine.com/news/1785/darkmatterscaffoldinggalacticsuperclustermapped. And a spiral galaxy is a little like an infinite plane if you're above the disk. Note though that dark matter is only one hypothesis that attempts to explain gravitational anomalies, another is MOND, and neither of them address inhomogeneous space which both Newton and Einstein talked about.
The main question therefore is a stable toroidal gravitational collapse possible?
I don't think so. Galaxies don't actually collapse into ring singularities, the orbital velocity halts the collapse.
What would the properties of objects like this be like? Bearing in mind that a thin toroidal loop could approximate to an infinite cylinder and that a thin toroidal surface could approximate to an infinite plane.
These are hypothetical objects, but look to electromagnetism. The electric field of a long charged wire reduces in line with 1/r rather than 1/rē.

Farsight re i/r field of cylinder THAT IS PRECISELY WHAT I WAS SAYING!

Yep, Soul Surfer, it's all bog standard stuff. But I wouldn't say this modifies the law of gravity, and IMHO there are issues with a ring singularity (http://en.wikipedia.org/wiki/Ring_singularity) because there are issues with ordinary singularities. I should mention I'm John Duffield by the way. We've talked about black holes before, and I guess we'll have to agree to differ.

Thanks for reminding me of out previous contacts. We both dislike anything that requires mathematical points and lines to form but disagree on the existence of event horizons. I agree what I am talking about does not modify the law of gravity globally and should not nullify the results of a very well designed model but it shows how the presence of particular structures can create effects that look like the laws have been modified locally allowing different options to be stable in a highly complex many body situation.
Clearly the collapse to oblate spheroids is the most probable but I was interested in the possible structures for the collapse of large quantities of material to form the first stars or quasars that had too much angular momentum to collapse inside an event horizon. Also what might happen en route to the formation of the classic Kerr black hole ring "singularity"