Dark Matter sheds some light

The non amateur stuff. Hawking, black holes, that sort of thing

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KendalAstronomer
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Post by KendalAstronomer »

Dear Cliff

Yep, I hope I said that!

I think assessing he overall rotation problem without requiring three body plus mechanics could be analagous to your doctor measuring levels of stress on damaged muscles without having to know about the interatomic bonds. The sparsity of matter, at any level whether atomic, planetary or stellar, is such that any three body system interacting with other far off systems will do so as one body with the combined mass of the three original bodies. There will be distortions, gravitational waves etc that need a more sophisticated idea to probe it, but this system seems to work quite well. Once you have a system with a large number of bodies in it, such as all the particles in the Earth, then calculating gravitational effects becomes a statistical science and different complexities come in. Its the use of statistics that gets around the three body problem as cosmologists aren't looking at the individual effect of single stars on the galaxy, on the whole, rather the cumulative effect of a lot.

I saw recently a model of ring particles interacting as they orbited Saturn and were sheparded by moons. The activities of the (vastly reduced, but still statistically significant numbers of) particles were dependant on cumulative fields and the like. There were a lot of very interesting iterative methods created in the early days of computing that allow non-trivial solutions to be found using effectively trial and error techniques. These tend to be well used in all aspects of astronomy and help greatly in getting round many problems... provided big computers and lots of time.
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Dark debate..

Post by brian livesey »

Well,Joe,some of us did sit through the "dark matter" HORIZON programme last night and,as you said,the ball did seem to bounce repetitively from one end of the court to the other. Even so,I personally found the programme quite interesting,not being as familiar with cosmological debate (except with regard to Super-plughole theory and the shock and awe of a plumber's bill) as you and others seem to be on this thread.
One point I did find intriguing (which,surprisingly,I considered myself some time ago!) was that Newtonian mechanics might not be adequate to deal with motion on such a large scale. How do you feel about this?
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As I was saying..

Post by brian livesey »

There's something else I meant to add, what about the radio halo around a galaxy,that extends far further out than the visible image? Isn't that produced by invisible matter/energy of some sort?
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davep
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Post by davep »

From what I've read it's not fully understood what creates the radio halo.
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Post by jck100 »

I am interested in anything new about science but cannot see how some basic logic cannot be used to establish exactly what is being considered.

Dark matter is invisible?

What does that mean exactly?

It cannot mean it is invisble, something invisble cannot be seen at all.

So what exactly is being detected?

I am concerned that expected results play a part where there is an ifer that allows for discrepancies to dictate that the dark matter should be such and such a temperature and so many light years simply because what is detected will then agree with the results.

This is not saying the results are wrong only that with absolutely no previous knowledge or theory would the dark matter be detected.

Far from having found some dark matter to study in the lab we seem to feel setting parameters is actually establishing exactly what dark matter can and cannot do.

Personally I am happy that dark energy and dark matter are required for my own solution but this is fundamental for that.

john
If I have seen further than others it is because I moved the giants out of the way.
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Post by Cliff »

Dear Kendalastronomer
Thanks for your further comments.
However, what you suggest in some ways actually tends to confirm my worst fears.
In particulat I think the approximations that cosmologists use could be taking them more in the wrong direction than you suggest. If as I suspect the three body problem still causes problems, then I am not convinced that approximations you mention will be correct.
As one guy suggested on the recent horizon progamme just because a computer simulation shows something that looks like a galaxy it does not necessarily mean the inputted gen is necessarily the same as that which creates a real galaxy.
I may be doing cosmologists an injustice but a few years ago I bought my son a model train set. It was a Flying Scotsman. you could actually get a gadget to make the train pump out steam just like a the real thing. But of course the model was an electric train set.
I used to make model aeroplanes. They flew on similar principles to full size aircraft but there were scale effects. Now the aeroplanes I made were usually in the 1\6 to 1\4 scale sort of range.
I am not at all sure myself that on the scale of things that space is our cosmologists simulations get things right.
It could just be that even a small incorrect input could get things completely wrong. On the other hand just tinkering with many variables and producing the right apparent shape does not guarantee the inputted variables are right.
I was beginning to think that cosmologists new many if not most of the answers but now I am sometimes beginning to think they are only on the fringe of knowing very much at all.
Best wishes from Cliff
KendalAstronomer
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Post by KendalAstronomer »

Dear Cliff,

Perhaps the best analogy I could come up with is the universal gravitation equation for working out the gravitational acceleration at any point on a planet's surface. Its where we get our value for the gravitational acceleration on the surface of the Earth.

The Earth is not one body, it is a multiplicity of particles holding itself together by all kinds of forces including gravity. This is more evident in the Sun, where the gravity of a large ball of gas acts against radiation pressure. Yet, on the surface we can use a single equation to work out a pretty good guess at the cumulative effect of the acceleration due to particles both above and below the position of the observer.

I haven't done any comparisons between the size of a star compared to average seperation within the Galactic material and the size of atomic nuclei within the Sun compared to their average separation, or indeed the mass of stars/atomic nuclei compared to the mass of the galaxy/sun, but this is the kind of thing that would be used for a back of the envelope calculation of gravitational effects due to loads of particles.

The three body effect is still in evidence in all this - the gravitational constant is not known to great accuracy - but other effects acting together mean the cumulative gravitational laws have made good guesses on gravitation in the past. But the fact it is measured could illustrate that when the distance between two compound objects is far greater than the distance between the components of the objects, they have a similar effect to two singular objects, aside from libration etc. Otherwise, calculating planetary positions in the sky would be a job beyond us.

Though I agree, it is very easy to come to a defined answer using a vague concept with parameters limited only by your own assumptions and imagination, so models can be galaxy like. But in a published model, there is always more than the pretty pictures, there are the papers describing the physics behind them and comparing quantitavely observations and outputs. The trick is to get the model outputs operating over as many observational parameters as possible with inputs defined by known theory. Or even not, if your intention is to disprove a theory... The idea of tinkering with variables is those variables represent different arguments, so you're testing hypothesis.

I agree with both the comment on using smal scale models for simulating large scale events (but time and money prevail) and that Cosmologists are only on the fringes of finding whatever it is they are after. The whole point of Dark Matter and Dark Energy is to start looking for the big trends in Cosmology to give a foothold before refining the theory to smaller and smaller scales (the use of the word 'small' may have to be qualified, when it is in comparison with the age and size of the Universe...), in my humble opinion.
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Post by Cliff »

Dear Kendalastronomer
I think we are on the same general wavelength about all or certainly most of this but I think there are still some differences between us.
Whilst of course I agree that the the Earth is made up of many particles I am not sure we can consider gravitation in quite the way (I think) you suggest. Furthermore when we talk about the innards of the Sun I suspect that how gravitation affects thinks is far from understood (although I think gravitation does hold the Sun together).
I do not say that Dark Matter and Dark Energy do not exist, but let's say that I sit on the fence (I am something of an agnostic on the issue not a complete aetheist!). I must confess I would probably like MOND (Modified Newtonian Dynamics) theory or someting like it to be correct - although all the evidence does seem to point towards Dark Matter and Dark Energy.
However, I have problems accepting extra dimensions( although again a bit of an agnostic fence sitter). I tend to think OK from what they say if there are extra dimensions then supersymmetry or super strings or whatever will work. But I cannot help having a sneaky feeling that extra dimensions are mathematical devices that tweak things to get the "right" answers.
For similar reasons I am also a bit sceptical about computer models related to galaxy characteristics. I go along with concerns that it is possible to feed in values that produce computer simulations that look like galaxies, but I the values fed in may have no relation to real galaxies at all.
The Universe is so immense that as far as I am concerned not only newtonian mechanics but relativity might be limited to specific volumes of space. Amongst other things there now seems to be a suggestion that Dark Matter can only exist in volumes of a certain minimum size (about 1,000 light years across).
On a somewhat different tack and very much a simpler matter.
I wasa recently thinking about "the Titius -Bode Law" still one of my favourites although now I think largely considered obsolete - but I learnt it when I was a youngster and it was one of the few "technical" bits of astronomy I thought I could understand.
Until Neptune was discovered I think Bodes Law was pretty good.
I was always a bit naffed off that Bodes Law broke down then.
But I conjectured just suppose Bodes Law is right but at some point all our "supposed astronomical" laws break down at some point.
I know that really my conjecture is wishful thinking 'cos really I want Bodes Law to work. But I think it would be nice to think Bodes Law is just as good as any other law and all laws only work up to a point.
Of course I am sure my conjecture is complete nonsense but I do wish it was right.
Best wishes from Cliff
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Slip sliding away..

Post by brian livesey »

You're right, Cliff, to surmise that the "Laws" we impose on nature "break down" at some point,i.e. change into something else. This is bound to occur in an evolving universe, where the only "anchor" word has to be: change. :wink:
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davep
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Post by davep »

Is it not the case that we deduce, rather than "impose", such "laws"?
brian livesey
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Statistical averages..

Post by brian livesey »

Bertrand Russell described scientific laws as: "Statistical averages such as would emerge from the laws of chance".
The Dutch astro-physicist,the late Anton Pannekoek, described the laws as "human expectations",and that,when these expectations don't work (sometimes with disasterous results), we add "corrections". :wink:
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Post by davep »

Russell was, of course, often a witty fellow, but the quote simply tells us how witty he was (and don't you think that that quote has a faint whiff of Bierce about it?).

The quote you attribute to Pannekoek would seem to confirm "deduce" as the better choice of word.

But, quotes from dead third parties aside, don't you think it's closer to the truth to say deduced rather than imposed?
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Impostionable deductions..

Post by brian livesey »

I've never heard of Bierce,so I'm not sure what you mean. To expand a little on Pannekoek, he said that we observe certain "regularities" in nature and ascribe them to "laws" (of our own making),which we then
superimpose on nature: Impose,superimpose,deduce - does it matter? The idea of laws breaking down,in the sense that Cliff means it,might be analogous to two ticking clocks: eventually,the ticks of both clocks are in perfect unison,but,then they drift apart,so that a pattern (i.e. law) is no longer discernible. Well, summat like that,as we say in deepest Lancs. :wink:
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Re: Impostionable deductions..

Post by davep »

brian livesey wrote:I've never heard of Bierce
Ambrose Bierce. The Russell quote sounded like it could have come right out of The Devil's Dictionary.
brian livesey wrote:Impose,superimpose,deduce - does it matter?
I think it matters if communication and clarity matters. Impose implies that something is forced on something else whereas deduced implies observation. They imply opposite start and end points.
brian livesey wrote:The idea of laws breaking down,in the sense that Cliff means it,might be analogous to two ticking clocks: eventually,the ticks of both clocks are in perfect unison,but,then they drift apart,so that a pattern (i.e. law) is no longer discernible.
A good analogy -- although a pattern might still be discernable. The more data you have the more you can extrapolate.
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Fond farewell..

Post by brian livesey »

Mmm, bye.. . :wink:
brian
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