PDA

View Full Version : Spiral structure



edde
2009-Nov-17, 05:16 PM
What do people make of
http://en.wikipedia.org/wiki/Spiral_galaxy#Gravitationally_aligned_rosettes
?

Links to papers
http://arxiv.org/pdf/0901.3503
http://arxiv.org/pdf/0911.1594

I'm hesitant about it, and thought I'd ask around for more opinions.

bebe7
2009-Nov-18, 05:46 PM
What do people make of
http://en.wikipedia.org/wiki/Spiral_galaxy#Gravitationally_aligned_rosettes
?

Links to papers
http://arxiv.org/pdf/0901.3503
http://arxiv.org/pdf/0911.1594

I'm hesitant about it, and thought I'd ask around for more opinions.

Woderful, make sure you look at the links and references at the bottomof the wiki page.

John Jaksich
2009-Nov-18, 08:46 PM
Woderful, make sure you look at the links and references at the bottomof the wiki page.


That response seems a bit harsh-- don't you think so?

John Jaksich
2009-Nov-18, 09:15 PM
What do people make of
http://en.wikipedia.org/wiki/Spiral_galaxy#Gravitationally_aligned_rosettes
?

Links to papers
http://arxiv.org/pdf/0901.3503
http://arxiv.org/pdf/0911.1594

I'm hesitant about it, and thought I'd ask around for more opinions.

I looked at your links and -- to be quite frank about it-- they are beyond my immediate comprehension. This seems to be a good post, though. Maybe someone, else can provide the needed input that you seek?

AlexInOklahoma
2009-Nov-19, 09:57 PM
Are you asking about this as a 'proof-reading' purpose? Just asking to know what it is you are asking of the page :) I am no expert on technical parts at all, but would be happy to give opinion and link-check,etc should that be what you are asking... I might learn a bit more by doing so. Seems you want to be as accurate as possible, and happy to help that process (!).

Alex

edde
2009-Nov-19, 10:49 PM
No, not for proof reading purposes.

This seems to be an idea that has no widespread acceptance, but on the other hand it's a new idea, so maybe asking for widespread acceptance is too much. If it's not widely accepted though, should it be on a major wikipedia page?

It's an idea that's apparently been put on wikipedia by the originator, but that doesn't mean it's wrong. In a major article on a website that many people turn to first, it makes me want to look twice.

It's an idea that purports to explain the spiral structure in galaxies while referring only to the nearest stars in our own galaxy - only those within 300 parsecs, when the entire disc is what - tens of thousands of parsecs across? That makes me uncomfortable - extrapolation from a tiny part of just one galaxy.

As I said, I'm hesitant about it.

publiusr
2009-Nov-20, 09:26 PM
It is an interesting idea, but it is something for long lived superprobes to figure out. I would love to see a map of the universe with objects plotted where they actually are--no light delay. Q tells me that he is on that.

EricFD
2009-Nov-20, 10:19 PM
Sounds like very interesting stuff, edde. But, I'm hardly qualified to offer an intelligent opinion on the theory. It's a very interesting idea though.

Eric

Cougar
2009-Nov-20, 10:47 PM
This seems to be an idea that has no widespread acceptance, but on the other hand it's a new idea, so maybe asking for widespread acceptance is too much. If it's not widely accepted though, should it be on a major wikipedia page?

They seem to be saying that precession of elliptical orbits causes spiral structure. As you say, this seems new to me. And one paper does not necessarily overthrow a decades-old explanation, plausible though it may sound...




In a practical model for galactic spiral structure, stellar orbits are approximately elliptical
and are gravitationally aligned to a spiral arm. Unaligned orbits lying between the
arms will be drawn to one arm or the other, and orbits will precess due to the distributed
matter distribution of the galaxy, such that they become aligned. Once alignment of the
orbit with the spiral arm is achieved, it will be maintained by perturbations to the orbit
due to the gravity of the arm. A star close to apocentre will approach the inside of the
arm, on account of the pitch angle. If it has greater eccentricity than that of stars in the
arm, the gravity of the arm will draw it closer, causing a reduction in eccentricity. If it
has lower eccentricity than the arm stars, it will pass through the arm. Because of the
curve of its orbit, it will spend more time in the gravitational field on the outside of the
arm, and will be drawn back towards the arm, with a net increase in eccentricity.

I thought spiral galaxy orbital eccentricities were pretty small... :think:


It's an idea that purports to explain the spiral structure in galaxies while referring only to the nearest stars in our own galaxy - only those within 300 parsecs, when the entire disc is what - tens of thousands of parsecs across?

Yeah, a little over 30,000 parsecs. So they're looking at less than 1% of the galaxy.


That makes me uncomfortable - extrapolation from a tiny part of just one galaxy... As I said, I'm hesitant about it.

I think rightly so. On Wiki, then, it should at least be identified as a new possible explanation or some such. Then watch if anything else in the literature comes out in support or opposition...

frankuitaalst
2009-Nov-21, 11:42 AM
I haven't gone deeply through the article in the OP ( I keep this for the longer winter evenings :)) .
I can't really understand what the original question is while its vaguely defined , but I'm fascinated by the nice looking pictures.

I want to add as a response that much of what we see and observe mostly can be described by very basic equations of motion ( lets neglect the dark matter issue for a while ) if some good initial conditions are used .
Amazing looking patterns of motion can arise even using very simple initial conditions under Newtons Law . Some patterns can be found here :
http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1226781953

Note : in the link above : don't trust the title !

loglo
2009-Nov-22, 05:10 AM
Looking through the talk history (http://en.wikipedia.org/wiki/Talk:Spiral_galaxy) of the Wiki spiral galaxy page I see:-

I'm gravely concerned that a user whose name RQG matches rqgravity.net, the site of the author of a theory that is not widely accepted (despite having made it into a peer review journal). The material should not be stated as fact until this hypothesis is widely accepted, and ideally shouldn't be in the article at all. 87.194.227.22 (talk) 23:28, 28 August 2009 (UTC)

So looks like the author is just using Wiki to spruik his (largely unaccepted) ideas. The unknown critic tellingly says of the new theory:-

I'm aware of the potential for a misleading illusion but it's as plain as day that the outer stars are moving more slowly than the inner ones.

That contradicts pretty much all observations of spirals that I know of.

loglo
2009-Nov-22, 06:07 AM
His paper at http://arxiv.org/pdf/0911.1594 has an awesome Youtube link in it though. :)

Cougar
2009-Nov-22, 04:18 PM
I'm aware of the potential for a misleading illusion but it's as plain as day that the outer stars are moving more slowly than the inner ones.
That contradicts pretty much all observations of spirals that I know of.

Ah, good point. I couldn't quite figure out all of what the authors were claiming with their spiral model...




"Enlarging and rotating ovals aligned at the centre generates a two-armed spiral."

Well, of course it does. But so what? Galactic orbits are not ovals and not significantly enlarging within the age of the universe, to my knowledge.


I want to add as a response that much of what we see and observe mostly can be described by very basic equations of motion... Amazing looking patterns of motion can arise even using very simple initial conditions under Newtons Law.

I am a major fan of generating remarkably complex patterns by iterating remarkably simple formulas. Sometimes there's insight, as in predator-prey models. If I understand it correctly, these authors' idea may generate images that 'sorta look like' a spiral galaxy, but the mechanism doing the generating doesn't seem to match up with the "real world" of galactic orbital dynamics.

loglo
2009-Nov-23, 11:32 AM
Ah, good point. I couldn't quite figure out all of what the authors were claiming with their spiral model...




"Enlarging and rotating ovals aligned at the centre generates a two-armed spiral."

Well, of course it does. But so what? Galactic orbits are not ovals and not significantly enlarging within the age of the universe, to my knowledge.



I am a major fan of generating remarkably complex patterns by iterating remarkably simple formulas. Sometimes there's insight, as in predator-prey models. If I understand it correctly, these authors' idea may generate images that 'sorta look like' a spiral galaxy, but the mechanism doing the generating doesn't seem to match up with the "real world" of galactic orbital dynamics.

Yeah, his model makes me think of a spirograph (http://en.wikipedia.org/wiki/Spirograph).

transreality
2009-Nov-23, 10:42 PM
spiral can emerge from chaotic movement. (http://pandasthumb.org/archives/2009/11/order-in-chaos.html)

This is an image that shows emergence of the spiral morphology.

Cougar
2009-Nov-24, 02:04 AM
...spiral can emerge from chaotic movement.... This is an image that shows emergence of the spiral morphology.

That spiral has an awfully long tongue. :razz:

boom stick
2009-Nov-25, 09:14 PM
The author of the papers is not a paid scientist though he has a phd in mathematics. He has a home page where he has published papers about something called relational quantum gravity, a solution to the pioneer anomaly and more recently his stuff about galactic spirals. That + editing wikipedia. You can draw your own conclusions.

I don't know enough about density wave theory and our current understanding of the formation of spirals to comment on his criticisms on them. In one part of the first paper he seemed to pull two parameters k and tau out of nowhere to guesstimate the eccentricity of the sun's orbit, which I didn't really like. I'd give him some slack on using 'only' 20k stars in his analysis, because those are the closest stars for which we have the most accurate data. I wouldn't rush to say that current observations clearly contradict his research either, because we only know the proper motion of the few million brightest stars in our galaxy.

He managed to get it published in proceeding of the royal society though, so I guess the idea should still be looked in to. Imho the wikipedia page should be still about status quo density wave theory perhaps shortly mentioning some alternatives. The section about the formation of spirals is now 80% non-mainstream, which is completely unacceptable. Publishing your own research is against the rules of wikipedia.

Cougar
2009-Nov-26, 02:27 AM
Thanks for chiming in, boom stick.


In one part of the first paper he seemed to pull two parameters k and tau out of nowhere to guesstimate the eccentricity of the sun's orbit, which I didn't really like.

:lol:


I'd give him some slack on using 'only' 20k stars in his analysis...

Yeah, country-wide poll samples don't come close to polling the total population, and they're put forward as representative, plus or minus 'x'.


He managed to get it published in proceeding of the royal society though...

Apparently. Yeah, can't knock that. Who let it in, Penrose? :whistle:


Imho the wikipedia page should be still about status quo density wave theory perhaps shortly mentioning some alternatives. The section about the formation of spirals is now 80% non-mainstream, which is completely unacceptable. Publishing your own research is against the rules of wikipedia.

My opinion concurs.