Spiral Arms Round #2

[CraigS]

So here we go again ...
New theory of evolution for spiral galaxy arms

Since 1960s, the most widely accepted explanation has been that the spiral arm features move like a Mexican wave in a crowd, passing through a population of stars that then return to their original position. Instead, computer simulations run by Grand and his colleagues at University College London’s Mullard Space Science Laboratory (MSSL) suggest that the stars actually rotate with the arms. In addition, rather than being permanent features the arms are transient, breaking up and new arms forming over a period of about 80-100 million years.
"We have found it impossible to reproduce the traditional theory, but stars move with the spiral pattern in our simulations at the same speed. We simulated the evolution of spiral arms for a galaxy with five million stars over a period of 6 billion years. We found that stars are able to migrate much more efficiently than anyone previously thought. The stars are trapped and move along the arm by their gravitational influence, but we think that eventually the arm breaks up due to the shear forces," said Grand.

Can somebody please convince me that the nature of the spiral arm pattern in a spiral galaxy, is not caused by the interaction between simple behaviours of individual stars (gravitation mainly) which produces complex, yet organized group behaviour amongst a bunch of them ?

The behaviours/motions of the stars within the spiral arms seems inherently nonlinear so mixing them, gives the group behaviour a chaotic aspect. The negative feedback may be provided by the behavioural controllers (eg: inertia/momentum, centrifugal force, centripetal force, etc), which tends to keep the group dynamics ordered. The result is both chaotic behaviour (which has neither short nor long term predictability) .. like how long the arms last, how long a shape persists and; ordered (static or periodic) behaviour .. like the motion of stars moving towards or away from the galaxy centre ?

Looking at things this way, leads to 'no big surprises' when we find other than spiral galaxy shapes (elliptical, clusters, etc). Maybe even 'the hunt' for dark matter/energy disappears also (when considering the rotation rates) ?

I look forward to the day when someone important notices these aspects and applies the thinking to the galaxy and universe formation models.

Cheers

[Robh (Rob)]

Craig,

I agree that the motion of the stars will largely evolve chaotically. The result of this intermix as per theoretical modelling should match real observation.
I find the proposal given here fairly palatable. If stars did not rotate with the arms then there would be no arms! Inner stars migrating inwards, outer stars migrating outward. This is a lot like what happens in globular clusters but no arms there of course.
The stars move with the spiral pattern at the same speed. Meaning what? The speed of rotation of the arm? If this is the case, what causes the shearing that breaks up the spiral arms?

Regards, Rob

[CraigS]

Originally Posted by Robh

Craig,

I agree that the motion of the stars will largely evolve chaotically. The result of this intermix as per theoretical modelling should match real observation.
I find the proposal given here fairly palatable. If stars did not rotate with the arms then there would be no arms! Inner stars migrating inwards, outer stars migrating outward. This is a lot like what happens in globular clusters but no arms there of course.
The stars move with the spiral pattern at the same speed. Meaning what? The speed of rotation of the arm? If this is the case, what causes the shearing that breaks up the spiral arms?

Regards, Rob

Seems to me the speed of the stars (relative to the galactic centre, assuming zero relative speed), varies as the star moves either towards or away from the core anyway.

I think the forces causing the shear (or breakup) of the arms with Chaos Theory, would be exactly the same forces which cause the arms in the first place. This would be the chaotic phase of a 'cycle'. This is the unique thing about chaotic/complex systems .. the presence of order and chaos from exactly the same initial conditions.

By the way .. I apologise to all, if my original post/challenge seemed arrogant. More exasperated than anything. But the point remains .. if there was more thinking coming from complexity modelling area, in what these guys are attempting to do, I have a gut feel that the need for dark matter would disappear.

After all, we don't need dark matter to explain how a flock of geese form patterns whilst in flight, do we ? I just wish I knew more about complexity. Oh well, I've got a few years ahead of me ...

Cheers

[CraigS]

Ok .. done some reading. Found this paper .. I recommend it for those interested in understanding how Chaos Theory may fit into Astrophysical phenomena:

Chaos in Galaxies

It has some excellent words in the Introduction about the relevance of Chaos Theory in galactic astrophysics modelling. I particularly like the following:

Chaos is therefore important because it touches to an essential aspect of the scientific activity: to represent faithfully natural phenomena with formal models. At the heart of the scientific process is the determination of the domain of applicability and the limits of models and theories.

Ok .. so that's hit the nail on the head with me. Where the chaotic processes start and finish is the big issue. There's almost no question that Complexity and Chaos define how structures form in the universe. The big question is … 'Where are the boundaries' ?

This guy, (Daniel Pfenniger), goes on to talk about how the models themselves are subject to chaotic behaviours because of the inherent estimations built into them. The errors made in the initial estimates designed to replicate the real phenomena, themselves can all of a sudden behave chaotically. Perhaps this is why the arms break up in the model, Rob ?

The main areas where Chaos plays a role according to Pfenniger are:
- Newtonian dynamics
- Orbit description.
- Phase space fluid description.
- N-body description.
- The complex physics of baryons
- The dynamics of non-baryonic matter.

(Note the last aspect shows that they incorporate dark matter in the models. This is never questioned, so it seems).

Sensitive Dependence in Galaxy Models are:
- Collisionless Chaos
- N-body Chaos
- Sensitivity to Quantum Physics.

So, at the end of the day, I was probably wildly off in my own assumptions about how these guys go about making their models.

Yep … I was wrong .. this guy Pfenniger does take it all into consideration .. in far more detail than I gave anyone credit for. Whether or not Robert Grand does, (from the article which I used to start this thread), I have no idea.

There's always more knowledge to gain about a topic, eh ?

Cheers

[Robh (Rob)]

Originally Posted by CraigS

The main areas where Chaos plays a role according to Pfenniger are:
- Newtonian dynamics
- Orbit description.
- Phase space fluid description.
- N-body description.
- The complex physics of baryons
- The dynamics of non-baryonic matter.

Interesting. A chaos-based simulation has to evolve with dynamical interactions on many levels. In fact, any model that ignores key influences is going to lead to misleading results.
This is all going to take an extraordinary amount of computer memory and processing capability considering the N-body description alone involves around 10^12 stars over an enormous amount of simulated time. With so many interconnected processes, one would have to question the reliability of any conclusions deduced by current simulations. The effects of dark matter is yet surely uncertain.
My main bone of contention is that a simulation that reproduces what we see today is not necessarily based on an accurate hypothesis. If B is true and A implies B, it does not mean A is true. For example: "the numbers 3 and 4 are prime" implies "3 is prime", the latter is true but the former is not.
A simulation must not only show what we see to be true but predict something that is not yet seen to be true to add credibility to its assumptions. Example, some unknown follow on from newer galaxies through to older ones. A process akin to that of a classical scientific theory.

Regards, Rob

[CraigS]

Originally Posted by Robh

Interesting. A chaos-based simulation has to evolve with dynamical interactions on many levels. In fact, any model that ignores key influences is going to lead to misleading results.
This is all going to take an extraordinary amount of computer memory and processing capability considering the N-body description alone involves around 10^12 stars over an enormous amount of simulated time. With so many interconnected processes, one would have to question the reliability of any conclusions deduced by current simulations. The effects of dark matter is yet surely uncertain.
My main bone of contention is that a simulation that reproduces what we see today is not necessarily based on an accurate hypothesis. If B is true and A implies B, it does not mean A is true. For example: "the numbers 3 and 4 are prime" implies "3 is prime", the latter is true but the former is not.
A simulation must not only show what we see to be true but predict something that is not yet seen to be true to add credibility to its assumptions. Example, some unknown follow on from newer galaxies through to older ones. A process akin to that of a classical scientific theory.

Regards, Rob

Hmm … looks like you read the 'Introduction', Rob.
Its a good piece, I find it to be fairly balanced ...

AS Pfenniger says:

Since physical theories try to build an isomorphism between Nature and a subset of mathematics, it follows that no “theory of everything” (TOE) can summarize with provable derived theorems the implicit complexity of its content.

As an aside: Its kind of amusing too, that I started out this thread with a bone of contention … now Rob has one …

I guess Rob's bone might be the incorporation of Dark Matter ? Pfenniger talks about three types .. neutrinos, axions and CDM. He says:

The often adopted collisionless property of cold dark matter is just an assumption that may be acceptable in present day galaxy models, but may turn out to be invalid during perturbation sensitive events like pancake or filamentary collapses.

The presense or absence of DM is never questioned. Why would any self-respecting scientist question this aspect, anyway ?

As for predictions: models are the computer-age equivalent analogy to classical theories so, yes, they should be expected to make predictions which can be tested. In the case of Galaxy simulation, there is a lot that goes into them though. Dark Matter is only one part of it all. The sensitivity of the predictions compared against each hypotheses would also seem to be a testable outcome, though, eh ?

Robert Grand's predictions might be break-up of spiral arms and that leading and trailing edge stars migrate towards and away from the core of the galaxy, eh ? Their dependence, or sensitivity to the hypotheses (or assumptions), would be a very interesting topic. Perhaps we should look for this aspect in future reports about computer model predictions ?

Also, surely if our hypotheses about DM were completely off-track, then we would never end up with the complex, highly recognisable shapes which they do produce ?

Interesting (& complex ! )

Cheers