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Dark Matter/Energy

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Tas
54241.  Wed Feb 22, 2006 4:26 am Reply with quote

I am a little confused (again!) about how a particle can have no mass. Am I confusing weight with mass, or something?
If something is there, and affects the universe, then it must have some mass, surely?

:-)

Tas

 
grizzly
54270.  Wed Feb 22, 2006 6:11 am Reply with quote

Tas wrote:
So, all Dark Matter really boils down to is the amount of stuff that we do not know about, right?

So, we know have evidence to support the theory of super massive black holes, but cannot see them, and who knows just how much matter those buggers contain? We can't see them, so....that sounds like alot of matter that we theorise about. Has anyone done any serious calculations on how big these are?

:-)

Tas


Of course that assumes that just because we can't see something that it has not been included in the estimate of total known matter. Although we have not seen them that does not mean that we are not aware of their existence nor does it mean that they have not been included in the calculations.

However, black holes do somewhat fail to solve the problem even if they could account for all of the missing matter. The reason for believing that there is dark matter is that the stuff we see travelling around the edge of galaxies is travelling so fast that it should just flight out into space. It therefore requires more gravity to stay in its orbit around the galaxy. However, it also means that this gravity has to be in a certain place.

If this gravity were all centred in the middle of the galaxy, all this would mean is that everything in the galaxy would have to travel faster to maintain its orbit (just as if the sun had a stronger gravity then everything from Mercury to Pluto would need to orbit faster to maintain its current orbit). This therefore means that the problem of the stuff travelling around the edge of the galaxy at too high a speed will remain.

Thus dark matter needs to be something that is spread out accross the galaxy, which will explain why there is enough gravity for stars to orbit galaxies at the same speed as the stars that are orbiting at the centre of the galaxy, without flying off into deep space.

 
grizzly
54271.  Wed Feb 22, 2006 6:17 am Reply with quote

I don't like horizon all that much since it began dumbing down (or is it that I am getting alot smarter? :-) but they do have a decent explanation for dark matter on their website. If only they had the graphs to explain it as well:

Quote:
The accidental discovery

In 1974 the astronomer Vera Rubin, was working on a project investigating stars at the outer edges of galaxies. What she discovered was quite a surprise.

Shortly after the apple fell on his head, Newton famously declared that gravity was 'universal'. An apple falling on Earth obeys the same mathematical rules as an apple falling on the other side of the Universe. In the same way that the Sun controls the orbiting planets by exerting gravity on them, a spiral galaxy must be controlled by the gravity-giving black hole at its centre.

It has long been known that Pluto, at the edge of our solar system, travels much slower than Mercury, close to the Sun. In fact observations like these allowed Newton to pin down his laws in the 17th century. When Vera Rubin did her work on galaxies she expected to find that as you reach the edge of a galaxy the stars would be moving much slower than those close to the centre. But it didn't work out like that at all.

She found that almost all of the stars in spiral galaxies are racing around the centre at approximately the same speed. This was very strange. Could it be that Newton's laws weren't really universal and didn't apply in galaxies?

Questioning Newton seemed unthinkable, so the majority of scientists went down a different route altogether. Rather than variable gravity, they argued, there had to be something else in galaxies, something that was providing extra gravity. With extra gravity, the stars would be pulled harder, and would travel faster as Rubin's observations suggested. And the name they gave to this extra stuff? Dark matter.

 
Tas
54312.  Wed Feb 22, 2006 7:42 am Reply with quote

Couldn't it be the gravity of nearby stars that act like a daisy-chain of gravity, as each pulls the next one along?

I know that is probably overly simplistic, but, if one star is travelling so fast and it goes past another, then they would affect one another...the cumulative gravity of ALL the stars might be acting in concert, possibly, maybe?

:-)

Tas

 
dr.bob
54320.  Wed Feb 22, 2006 7:48 am Reply with quote

Quote:
Questioning Newton seemed unthinkable


I think that's a bit misleading as it paints scientist as unbending dogmatics. Sure, some of them are, but the fundamental principle of science is to constantly question things.

To explain the motion of stars in the outer reaches of galaxies, there are two solutions: either Newton was wrong and we need a new theory of gravity, or there's a bunch of mass that we can't see.

Certainly it's possible that Newton was wrong. Indeed, some of his theories have already been replaced by newer theories such as relativity. However, his laws of gravitation have been shown to work in almost every observation that we've made so far all around the universe. To come up with a theory that not only works that well but works a bit better is really hard.

By comparison, the only matter that we can actually see in the universe is stuff that radiates brightly. So it's not wholly unreasonable that there should be some stuff out there that we can't see.

So, while both explanation could be true, the current gut feeling is to go for the one that sounds easiest to explain. Except it's turning out to not be quite so easy. Who knew?! :)

 
grizzly
54375.  Wed Feb 22, 2006 9:09 am Reply with quote

dr.bob wrote:
Quote:
Questioning Newton seemed unthinkable


I think that's a bit misleading as it paints scientist as unbending dogmatics. Sure, some of them are, but the fundamental principle of science is to constantly question things.

To explain the motion of stars in the outer reaches of galaxies, there are two solutions: either Newton was wrong and we need a new theory of gravity, or there's a bunch of mass that we can't see.

Certainly it's possible that Newton was wrong. Indeed, some of his theories have already been replaced by newer theories such as relativity. However, his laws of gravitation have been shown to work in almost every observation that we've made so far all around the universe. To come up with a theory that not only works that well but works a bit better is really hard.

By comparison, the only matter that we can actually see in the universe is stuff that radiates brightly. So it's not wholly unreasonable that there should be some stuff out there that we can't see.

So, while both explanation could be true, the current gut feeling is to go for the one that sounds easiest to explain. Except it's turning out to not be quite so easy. Who knew?! :)


Yes I have to agree with you that they are not painting scientists in the correct light. Then again, as I say it has been drastically dumbed down over recent years so this is no surprise.

BTW in response to Tas, I believe that has been ruled out because the gravity provided by those stars is too weak over the large distances between them, remember that even in these crowded galaxies we are talking about light years of distance between these stars.

 
Tas
54387.  Wed Feb 22, 2006 9:36 am Reply with quote

Cheers Grizzly....it seems that the whole thing is too not-simple for my simple brane.

:-)

Tas

 
Quaintly Ignorant
54410.  Wed Feb 22, 2006 10:12 am Reply with quote

I suspect the answer to these riddles will suprise everyone, assuming an answer/answers are ever found. It is slightly comforting to know that everything isn't known, it gives us something to work towards.

 
Tas
54414.  Wed Feb 22, 2006 10:20 am Reply with quote

If we could just get people to stop killing each other for a decade or so, we might be able to concentrate on what is out there, and not waste resources on dreaming up wonderful new ways to maim and kill each other over silly disagreements over who's interpretation of G*d is more right, or over who gets to live on this or that bit of mud.



:-)

Tas

 
djgordy
54508.  Wed Feb 22, 2006 12:35 pm Reply with quote

Tas wrote:
If we could just get people to stop killing each other for a decade or so, we might be able to concentrate on what is out there,


Ok, I absolutely promise not to kill anyone for the next ten years. Except the ones already on my list of course.

 
Tas
54663.  Thu Feb 23, 2006 4:56 am Reply with quote

*Hides.....just in case....and arms himself with tactical nukes, lasers and all sorts of other offe...er defensive counter-measures*

:-)

tas

 
tetsabb
55606.  Sun Feb 26, 2006 3:13 pm Reply with quote

Well I reckon I know the answer to this cosmological conundrum.

Socks

Yup, socks.

It all seems blindingly obvious when you think about it.
We have all experienced the phenomeonon of the sock that goes into the washing machine but never comes out.
They disappear into one of those other dimensions covered in another thread, and gain incredible mass etc.
The unaccounted-for matter is clearly the accumulated mass of all those missing socks. So all we have to do to gain access to other dimensions is to build a very large washing machine, and fly into it in a sock-shaped spacecraft.

Sorted.

Nurse, I'll be grateful if you don't do up my sleeves too tight tonight.....

 
gerontius grumpus
55626.  Sun Feb 26, 2006 5:15 pm Reply with quote

What about all the biros and tupperware lids?

 
tetsabb
55650.  Sun Feb 26, 2006 7:56 pm Reply with quote

gerontius grumpus wrote:
What about all the biros and tupperware lids?

If you put them in the washing machine, you have misunderstood something at a very fundamental level, surely?

 
grizzly
55733.  Mon Feb 27, 2006 10:34 am Reply with quote

It's funny that you should introduce socks into the equation:

Quote:
Disappearing teaspoons

SEVERAL readers, among them Jenney Shepherd, Caroline Hauxwell and Jackie Leung, have alerted us to an intriguing paper in the British Medical Journal, "The case of the disappearing teaspoons" by Megan S. CLim, Margaret E. Hellard and Campbell K. Aitken of the Institute for Medical Research and Public Health, Melbourne, Australia (http://bmj.bmjjournals.com/cgi/reprint/331/7531/1498). This sets out to answer the age old question "Where have all the bloody teaspoons gone?" using a classic release-and-capture methodology.

The half-life of the 70 teaspoons studied in the institute's tearooms was 81 days, with 56 (80 per cent) disappearing during the five-month study. This led to the alarming conclusion that "an estimated 18 million teaspoons are going missing in Melbourne each year. Laid end to end, these lost teaspoons would cover over 2700 kilometres - the length of the entire coastline of Mozambique - and weigh over 360 metric tonnes - the approximate weight of four adult blue whales."

The authors say that "no plausible explanations were advanced for the high rate of teaspoon loss", though they did note the theory of Douglas Adams et al that Somewhere Out There is a teaspoon planet to which they repair.

However, while engaged in some essential stochastic blue-sky research (otherwise known as idly surfing on company time) Feedback came across a discussion of possible mechanisms on the www.urban75.com bulletin boards. This confirms, anecdotally, that the phenomenon is as common in domestic South London as it is in labs in Melbourne. By far the most popular explanation on the site was "house faeries", though we rather like the proposals that "odd socks are seducing the spoons with stories of The Life That's Waiting Outside" and the more materialist "there's a bloke down East Street market that sells 'em on".

One contributor insists: "It is well known that teaspoons and Biros are alternate manifestations of the same underlying quantum object. Proof: when you attempt to observe one, all you see is the other. Obviously, there's a reflection of the fundamental asymmetry of the universe here too, because you can stir your tea with a Biro."


From New Scientist's feedback section :-)

 

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