What is Dark Matter?

[Kleonaptra]

Hi Guys,
Tonight I caught a doco on SBS2 called 'is everything we know about the universe wrong?' I came in after it had started and hadnt planned on watching it, but I was utterly fascinated by the idea of dark matter.

Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory, and I understand there is plenty of evidence to suggest the existance of dark matter and dark energy. I just want to understand it. If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

Im really bad at maths, so if anyone wants to explain it with numbers, Im going to get lost quickly.

Any thoughts are welcome - I know only what I saw on this doco and openly admit my ignorance. Tell me what you know!:cool:

[Pixel42]

If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?
I think of it as being a bit like the nitrogen in the atmosphere, which makes up the bulk of it but is mostly inert.

[Zeuzzz]

Dark matter is basically something we have invented so established gravitational theories can still work, as they fall short of explaining the rotation curves of galaxies. We have no real way to experimentally test its existence, its all inferred, and for current theories to work it has to be there.

Its called a fudge factor.

[Pixel42]

There's certainly a whiff of epicycles about it.

[edd]

Hi Guys,
Tonight I caught a doco on SBS2 called 'is everything we know about the universe wrong?' I came in after it had started and hadnt planned on watching it, but I was utterly fascinated by the idea of dark matter.

Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory, and I understand there is plenty of evidence to suggest the existance of dark matter and dark energy. I just want to understand it. If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

There's no particular reason to associate mass with tangibility.


I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

There's a number of possible theoretical candidates for dark matter, and although they all have to interact with light very weakly and not bounce off matter very often, it doesn't preclude some very occasional interaction - much as neutrinos flood through the earth in enormous quantities but still very rarely interact. It's quite reasonable to try to detect it, although obviously it's very difficult.

[Dancing David]

Hi Guys,
Tonight I caught a doco on SBS2 called 'is everything we know about the universe wrong?' I came in after it had started and hadnt planned on watching it, but I was utterly fascinated by the idea of dark matter.

Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory, and I understand there is plenty of evidence to suggest the existance of dark matter and dark energy. I just want to understand it. If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

Im really bad at maths, so if anyone wants to explain it with numbers, Im going to get lost quickly.

Any thoughts are welcome - I know only what I saw on this doco and openly admit my ignorance. Tell me what you know!:cool:


dark matter is hypothesized to explain the observed speed of rotation of galaxies.

It is like neutrinos but more massive in theory.

[ben m]

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

Good question.

The astronomical data---which comes from multiple, independent sources which all seem to tell the same story---tell us only that dark matter is pretty weakly interacting. If it were something with a collision-cross-section as large as that of an ordinary atom, it would have behaved differently in galaxies and in the CMB plasma.

The astrophysics data, with a handful of exceptions, would be perfectly happy with strongly interacting dark matter. Imagine something that looked almost exactly like a neutron, only (a) stable and (b) immune to bound states. (Real neutrons, if not tied up in an appropriate nucleus, beta-decay with t1/2 = 12 minutes). Let's call it a Neutralon. Neutralons would be very "dark" and immune to most photon emission/absorption. Neutralons would fly straight through galaxies and galaxy clusters. Neutralons would decouple from the Cosmic Microwave Background plasma and undergo dark-matter-like acoustic oscillations. They'd be a fine dark matter candidate.

But they'd be easy to detect, if they existed; if you launch a dedicated neutron-sensitive detector into space, or into the upper atmosphere, you'd see this population of neutron-like particles colliding with your detector.

(This experiment has been done, see review at http://arxiv.org/abs/astro-ph/0105567 and it looks unlikely that this sort of particle exists.)

But yeah, there's no conflict between "weak enough to agree with cosmology data" and "strong enough to discover in an appropriate detector". But Nature doesn't necessarily care how good our detectors are---she could have pushed that "strong" threshhold way, way, way down. Dark matter made of GeV-scale neutrinos would have evaded discovery until the early 2000's. (It's now basically ruled out.) Dark matter made of axions might be discovered (or ruled out) in the next 5 years. Dark matter made of SUSY WIMPs might be discovered (or ruled out) any time in the next ~15 years. There are some reports of "discoveries" (but not very confident ones) of a lightweight dark matter particle colliding with the CoGeNT detector. To reiterate: any of the above would be consistent with the cosmology data.

[Kleonaptra]

Great ideas, wow!

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different?

Im not trolling, and Im not here to spout my beliefs at you, I just want to know why something so intangible is ultimately believed in while other intangibles are not. I know dark matter has plenty of other reasons other than faith to support it, but to the person seeing the ghost, they do too.

Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions.

And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

[Kleonaptra]

There's no particular reason to associate mass with tangibility.
There's a number of possible theoretical candidates for dark matter, and although they all have to interact with light very weakly and not bounce off matter very often, it doesn't preclude some very occasional interaction - much as neutrinos flood through the earth in enormous quantities but still very rarely interact. It's quite reasonable to try to detect it, although obviously it's very difficult.

Why? Dont we usually do so? In reality, to have mass, it must be tangible. Please explain if this isnt so, its just my understanding of it.

'Tangible - perceptable by touch; definite, clearly intelligible,not illusive or visionary. Corporeal.'

From the dictionary.

[ben m]

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different?

It's not that it has to exist "because it's in an equation". There's obviously something out there which behaves differently than normal matter. We see it causing gravitational lensing. We see galaxies and stars accelerate under its gravitational pull. We see it compressing and flowing in the waves of the Cosmic Microwave Background. It's obviously something that gravitates.

It's true that we wouldn't be able to be so precise about it if we weren't using equations. If you had shown a Cosmic Microwave Background map to a medieval astronomer, they would have been able to say "huh, it's got some waves" and that's it. A modern astronomer can say "... and the waves show increased power at a multipole of l=200" and other precise (and interesting, and useful-to-follow-up-on) statements. But you wouldn't say that they're 'just in the equation'.

Likewise: do you believe that the Earth has an iron core and a molten mantle? You've never seen them, and neither has anyone else. All you see are equations which say "This mathematical feature of the data would not be there if the mantle were solid, or if the core were silicate".

Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions.

No. Dark matter is meant to be ordinary "stuff" obeying ordinary physical laws---no more exotic, and no more mysterious, than the neutrino or the proton or the tau lepton. Just harder to experiment on for technical reasons, i.e. hard enough that we haven't discovered it directly.


And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

The most serious hypothesis is called MOND. This is the hypothesis that General Relativity, which is a well-tested theory of gravity under many circumstances, is invalid under some other (not terribly well understood) circumstances, and maybe this means that ordinary matter's gravity just so happens to add up to explain the data. It's worth pursuing, but no one has quite made it work yet and there are data (the Bullet Cluster) that make it look very unlikely.

[Roboramma]

Great ideas, wow!

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different? Everything we observe is observed based on it's interactions with other things. For instance, when I look out my window at the building across the street I don't have some mystical direct interaction with the building: light that interacted with ("bounced off) the building hits my eyes and interacts with them, and it's the pattern of that light that I use to deduce the presence of the building.

Dark matter doesn't interact with light (hence "dark"), but it does interact with gravity. So in much the same way that I deduce the presence of the building by the way it affects the light that strikes it, we can deduce the presence of dark matter by the way it affects the other matter in it's neighborhood (through gravitation).

Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions. What's similar in their descriptions? They may be similar in what they aren't (ie. interacting with electromagnetism), but I don't see any similarity in what they are (for instance, dark matter can't think).

But it's odd to consider things to be similar because they are lacking a particular quality: both the planet Jupiter and I don't have red hair, for instance, but that doesn't make me particularly similar to Jupiter.

And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

If there is no dark matter then general relativity needs to be revised.

[ben m]

Why? Dont we usually do so? In reality, to have mass, it must be tangible. Please explain if this isnt so, its just my understanding of it.

'Tangible - perceptable by touch; definite, clearly intelligible,not illusive or visionary. Corporeal.'

From the dictionary.

Nope. Counterexample: the neutrino is intangible (10^6 of them pass through every centimeter of your body every second, but you've only got a 50-50 shot of having one interact with you during your lifetime) but it's a real thing with mass.

"Tangible" has to do with the forces that allow matter to interact with your senses. It has nothing to do with whether or not something has mass, or can be called matter, or exists.

[Kleonaptra]

It's not that it has to exist "because it's in an equation". There's obviously something out there which behaves differently than normal matter. We see it causing gravitational lensing. We see galaxies and stars accelerate under its gravitational pull. We see it compressing and flowing in the waves of the Cosmic Microwave Background. It's obviously something that gravitates.

It's true that we wouldn't be able to be so precise about it if we weren't using equations. If you had shown a Cosmic Microwave Background map to a medieval astronomer, they would have been able to say "huh, it's got some waves" and that's it. A modern astronomer can say "... and the waves show increased power at a multipole of l=200" and other precise (and interesting, and useful-to-follow-up-on) statements. But you wouldn't say that they're 'just in the equation'.

Likewise: do you believe that the Earth has an iron core and a molten mantle? You've never seen them, and neither has anyone else. All you see are equations which say "This mathematical feature of the data would not be there if the mantle were solid, or if the core were silicate".



No. Dark matter is meant to be ordinary "stuff" obeying ordinary physical laws---no more exotic, and no more mysterious, than the neutrino or the proton or the tau lepton. Just harder to experiment on for technical reasons, i.e. hard enough that we haven't discovered it directly.



The most serious hypothesis is called MOND. This is the hypothesis that General Relativity, which is a well-tested theory of gravity under many circumstances, is invalid under some other (not terribly well understood) circumstances, and maybe this means that ordinary matter's gravity just so happens to add up to explain the data. It's worth pursuing, but no one has quite made it work yet and there are data (the Bullet Cluster) that make it look very unlikely.

I can only speak in broad terms - ie, in the equation - because my knowledge is limited to the doco I watched last night, and I dont know any of the equations. Im counting on you guys to know them.

I really like that statement Ive highlighted, a lot of the scientists in the show used similar words.

I believe the earth has an iron core, but I understand its represented in different ways. We dont agree on its specifics. Dark matter strikes me as the same thing - way into the grey area.

[Kleonaptra]

Everything we observe is observed based on it's interactions with other things. For instance, when I look out my window at the building across the street I don't have some mystical direct interaction with the building: light that interacted with ("bounced off) the building hits my eyes and interacts with them, and it's the pattern of that light that I use to deduce the presence of the building.

Dark matter doesn't interact with light (hence "dark"), but it does interact with gravity. So in much the same way that I deduce the presence of the building by the way it affects the light that strikes it, we can deduce the presence of dark matter by the way it affects the other matter in it's neighborhood (through gravitation).

What's similar in their descriptions? They may be similar in what they aren't (ie. interacting with electromagnetism), but I don't see any similarity in what they are (for instance, dark matter can't think).

But it's odd to consider things to be similar because they are lacking a particular quality: both the planet Jupiter and I don't have red hair, for instance, but that doesn't make me particularly similar to Jupiter.



If there is no dark matter then general relativity needs to be revised.

How do you know dark matter doesnt think?;)

The soul energy or counciousness doesnt think either - humans think, imagine, create. This creates the thought energy. I take your point, I get what you are saying, but I think we have different definitions of thought energy, or akasha.

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

[Kleonaptra]

Nope. Counterexample: the neutrino is intangible (10^6 of them pass through every centimeter of your body every second, but you've only got a 50-50 shot of having one interact with you during your lifetime) but it's a real thing with mass.

"Tangible" has to do with the forces that allow matter to interact with your senses. It has nothing to do with whether or not something has mass, or can be called matter, or exists.

My dictionary is from 1930....So we have a new definition of 'tangible'? I thought it meant literally 'what you can feel and touch' not just 'what you could interact with without knowing you are interacting with it'

Wow, I really dont want to debate the english language, there are other forums for that, but Im lost.

[Roboramma]

How do you know dark matter doesnt think?;) Thought is a type of information processing. The only way that we know of to do information processing is the complex physical interactions: ie. the set-up in the brain, the mechanisms in your computer, etc. Dark matter, because it doesn't interact electro-magnetically, is unlikely to be capable of forming the type of complex parts necessary to form an information processing system, thus thought is ruled out.

Of course, it's concievable that there are some other forces through which dark matter interacts that we're not aware of, but we so far have no reason to posit such. Further, even if there were, it's likely that like normal matter the vast majority of dark matter would not be in the form of thinking systems.

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?
No, because dark matter is simply made up of particles that have different properties than regular matter. It doesn't form "an exact replica of everything physical", and its distributions and interactions are very very different from that of regular matter, because the forces acting on it are different (ie. basically just gravity), and because it's mass, velocity, etc. are also different.

[Roboramma]

My dictionary is from 1930....So we have a new definition of 'tangible'? I thought it meant literally 'what you can feel and touch' not just 'what you could interact with without knowing you are interacting with it'

Wow, I really dont want to debate the english language, there are other forums for that, but Im lost.

He's not debating the english language: you said: "In reality, to have mass, it must be tangible. Please explain if this isnt so, its just my understanding of it."

But a neutrino has mass and yet doesn't fit your definition of tangible:
'Tangible - perceptable by touch; definite, clearly intelligible,not illusive or visionary. Corporeal.'

So it's not true that "to have mass it must be tangible".

[Cheetah]

Kleonaptra,
I can see these posters are just confusing you because they are throwing you in at the deep end and telling you how things are, talking about weird sounding particles and things, without explaining why. Once you understand why, it will make a lot more sense. So let me give you a quick and dirty rundown on why dark matter is 'dark' and difficult to see.

There are 4 fundamental forces of nature: gravity, weak nuclear, strong nuclear and electromagnetic.
Matter particles interact with each other through these forces and each have a sort of 'strength of interaction' property for each of these (and a force particle mediating it).
For gravity we call it 'mass' and for the electromagnetic force 'charge'.
Matter particles differ in their 'strength of interaction' for each of these forces, and if the gravity property is zero, the particle has no mass and if it's strong, its heavy (and the same principal holds for the other forces).
Photons (light) are force particles of zero mass (but are easy to see :D) and the mediators of the electromagnetic force (EF from now).

Almost everything that happens around us on a day to day basis happens due to the EF and gravity, but we observe things exclusively using only one of these, the EF, literally EVERYTHING (Hope I'm correct, or else I'm gonna get nailed :)).
Touch something, look at something, taste, hear etc. they are all mediated via the EF. Even weighing something and measuring gravity our observations are mediated via the EF, because it is what makes things able to touch each other.

Since our observations are mediated via the EF, if a particle has a EF 'strength of interaction' of zero, it is extremely hard to notice and we have to use indirect methods and, once again, do it via the EF.

Since 'normal matter' is made of atoms which are mostly empty space with a tiny little nucleus in there, and held together via the EF, a particle with a charge of zero will just fly right through it and we will never know, unless it happens to hit that tiny little nucleus and causes an effect we can observe, via the EF.

So we are effectively 'blind' to all but the good old EF. That means there can be (and are) plenty of particles flying around everywhere, even really heavy ones, and we are all but oblivious to them.

I hope that makes it somewhat clearer for you.

Cheers
Cheetah

(This is just my layman's understanding, off the top of my head, so if I have made any errors, please help out/correct/add/clarify)

[Ziggurat]

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

That would not fit at all. The whole point about dark matter is that its distribution does NOT match the distribution of ordinary matter.

Oh, and "though energy" is meaningless.

[Dancing David]

How do you know dark matter doesnt think?;)

The soul energy or counciousness doesnt think either - humans think, imagine, create. This creates the thought energy. I take your point, I get what you are saying, but I think we have different definitions of thought energy, or akasha.

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

No, thoughts are NOT energy they are biochemical exchanges.

:)

Repeat THOUGHTS ARE NOT ENERGY they are complex cascades of biochemical events.

:) :)

Neurons are comparable to biochemical switches, not to electrical components.

Now all matter and energy are transferable.

[Dancing David]

My dictionary is from 1930....So we have a new definition of 'tangible'? I thought it meant literally 'what you can feel and touch' not just 'what you could interact with without knowing you are interacting with it'

Wow, I really dont want to debate the english language, there are other forums for that, but Im lost.

Except in the case of neutrinos, they do not interact with you through the EM force, which is that force which leads to most sensations. (And things like the impermeability of 'solid objects;)

They do however interact gravitationaly.

So while they pass through you, because they do not interact with your EM fields, they are effected by and effect other masses.

:)

[Cuddles]

I can only speak in broad terms - ie, in the equation - because my knowledge is limited to the doco I watched last night, and I dont know any of the equations. Im counting on you guys to know them.

But the point is that it's not about equations at all. Dark matter is, at its most fundamental, simply an observation. The equations only come later when we try to figure out what could be causing that observation. Or in this case, as already mentioned, a whole set of independent observations.

I believe the earth has an iron core, but I understand its represented in different ways. We dont agree on its specifics. Dark matter strikes me as the same thing - way into the grey area.

That's actually a pretty good example. To start with, we make a variety of observations. First we notice that the Earth has a global magnetic field. Then, completely separately, we look at the way different waves from Earthquakes spread across the planet. Both these observation, and others, lead us to the conclusion that there is a region inside the planet with properties different from the rocks we can actually see, and we decide to call it the core. We then start trying to figure out from all these observations exactly what the core is and how and why it results in these particular observations and not something else.

It's exactly the same for dark matter. We have a set of observations, including things such as the details of how things orbit within galaxies, what the cosmic background radiation looks like, how fast galaxies themselves are moving, and so on. All these separate observations taken together lead us to the conclusion that there is some gravitational influence that we hadn't previously noticed. We call that dark matter. Just as with the Earth's core, that's nothing more than a label for something that starts off being almost entirely unknown. And just like the core, we then use all those observations to try to work out exactly what it is and how it works.

Ultimately, there's nothing special about dark matter. The reasoning behind it is exactly the same as the reasoning behind every scientific discovery - notice something that doesn't quite fit with existing ideas, give it a name and try to figure out what's going on. The only real difference is the difficulty in first discovering and then later investigating it. And the reason for that is quite simple - we've already discovered and investigated the easy stuff. If you can see and pick up something with your human eyes and hands, then so could people thousands of years ago. So most (I won't say all since there can always be stuff we've missed) of the things left to discover are things that unassisted human senses can't detect themselves. That doesn't make them any less real, and it doesn't mean it's all just silly people playing with meaningless equations, but it does mean it can be hard for the layperson to accept since there's just no way to understand them without first understanding all the equations and complex equipment needed to detect them.

[sol invictus]

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different?

If you saw a guy straining and struggling to carry a bulging canvas bag, would you regard it as a leap of faith that there is something heavy inside the bag? That's more or less the situation we're in, although in this case we have many more measurements than just one (let's say the guy with the bag is also standing on a scale that reads "500kg").


Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions.

What?

And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

There are alternate explanations, yes. But the interesting ones are mathematical theories that make specific predictions that can be compared to what we actually observe, and so far none of them have been able to account for all the observations.

[Marshall]

Just to amplify Sol's point, it's my impression from what I've seen on it that clouds of dark matter filling and surrounding clusters of galaxies can be observed and mapped by their (weak) lensing effect on the images of more distant stuff in the background.

It is not uncommon to plot "contour maps" showing the variation of density of the clouds. It's like how you would see a clean transparent lens in front of the wall of your livingroom---by its effect on the light passing thru, distorting the images of familiar stuff. (except the familiar stuff is distant galaxies)

So we actually CAN see these large irregular shaped clouds of dark matter and they are very interesting to study.

[Marshall]

Here is a key 2006 "Bullet Cluster" paper. It has some pictures and although technical is not especially hard to read.
http://arxiv.org/abs/astro-ph/0611496
DM interacts with light, but only gravitationally. It bend rays of light the way any concentration of mass does. So it "lenses".
...
Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory,...

Before they started using gravitational lensing to observe and study and map the clouds of DM, it was constructive and legitimate to propose modifications of the law of gravity as alternative explanations for some of the observed effects (galaxies holding together in spite of revolving too fast for their apparent mass.)

Mordehai Milgrom did that in 1983, Jacob Bekenstein in 2004, John Moffat in 2006. As it happens these guys are all rather old now FWIW. Perhaps it shouldn't matter. It is getting harder to find qualified people who will deny the existence of DM. You hear less and less about these alternative versions of gravity which avoid the need for it.

They don't work, partly because DM has been discovered to participate in many different observable things at many different scales. The alternatives devised between 1980-2006 were focused on explaining one or two phenomena and wouldn't explain the other evidence that was accumulating.

You can still find a few diehards, but it seems to be a closed chapter.

A significant historical corner was turned with the report on "the Bullet Cluster" which used gravitational lensing to study the DM participation in the collision of two clusters of galaxies, together with their two accompanying clouds of DM.

SBS2 Australian television sounds like it makes an effort to be provocative :D

[Roboramma]

Thanks for that post Marshall. :)

[Kleonaptra]

Okay Ive gotta go get ready for work but I just want to say ThankYou Cheetah, I'll be back to read this tomorrow cos Im not processing properly right now and I really want to understand this.

Observing dark matter, that would be cool.

[Marshall]

Thanks for that post Marshall. :)

My pleasure, Roboramma :) I'm glad to find others interested in details like this!

[Cheetah]

ThankYou Cheetah, I'll be back to read this tomorrow cos Im not processing properly right now and I really want to understand this.

My pleasure.

You might be wondering why we don't use gravity to detect these individual particles. The reason is that gravity is actually very weak, verrrry weak, despite what it feels like some days. :)
We only notice gravity on a day to day basis because normal matter tends to clump together in lumps (and gravity adds up), something dark matter cannot do.
A single particle zipping around is just too light (even a pretty heavy one, as particles go) for us to detect using gravity.
In fact gravity is about 4200000000000000000000000000 times weaker than the EF, and that's a LOT!

That is why you can tie a piece of string around a brick and pick it up, because the string is held together by the EF, but all the gravity combined of all the particles constituting the whole earth (about 6000000000000000000000000 kilograms worth of particles) are not enough to break the piece of string. Unless you yank it!

(Sorry for explaining it using a few numbers, hope I didn't lose you there :D)

[Kleonaptra]

Cheers again Cheetah, numbers did throw me, but I still got the point.

I was wrong to imply that the spirit world is un-moving. I meant that a walker usually views it as a replica of the physical world, but it is under constant change. Its ok, Im so not going to go there.

Thankyou all so much for this, I hope you dont mind me using you as my tutors? I think I mentioned in my first post the 'doco' was purposefully inflammatory/provocative. Cant make good TV without that I suppose.

Ok, thoughts arent energy, but am I wrong to think that they produce energy? Doesnt a biochemical reaction produce energy?

Is dark matter affected by gravity, or anything in the EF spectrum at all? Or havnt we got that far yet?

If it sounds like Im talking crap, I apologise and reiterate - I ADMIT MY IGNORANCE. The total of my knowledge is this doco, some years fascinated by a telescope and a high school science class that was for dummies but had a talented professor.

[Cheetah]

I was wrong to imply that the spirit world is un-moving. I meant that a walker usually views it as a replica of the physical world, but it is under constant change. Its ok, Im so not going to go there.

Huh?? Thought we were talking about reality, so 'not going there' is a great idea.


Is dark matter affected by gravity, or anything in the EF spectrum at all?

Since, as I explained, the particles do have mass, yes absolutely. That is the reason we know it's there, as a bunch of previous posters have explained, with examples and everything.
As far as the EF, no (the spectrum is irrelevant to this point and doesn't matter). I thought I explained that, try reading my posts again, both of them, carefully. :)

[Dancing David]

Ok, thoughts arent energy, but am I wrong to think that they produce energy? Doesnt a biochemical reaction produce energy?

That depends, some absorb and some release. But by and large energy will go in and be dissipate , metabolism and all that.

When a neuron 'fires', it open channels in the cell membrane, some flow out of the cell due to osmotic pressure, some flow in. This causes a change in polarity in the difference across the cell membrane, it travels down the axon causing a release of neurotransmitter.
Action potential

Is dark matter affected by gravity, or anything in the EF spectrum at all? Or havnt we got that far yet?

[/QUOTE]

Yes, dark matter in theory is effected by gravity, but not by the electromagnetic force.

[edd]

Just to point out that dark matter doesn't have to have zero interaction with electromagnetism. Axions are a notable candidate particle that would turn into photons in the presence of a very strong magnetic field, for example.

[Dancing David]

Just to point out that dark matter doesn't have to have zero interaction with electromagnetism. Axions are a notable candidate particle that would turn into photons in the presence of a very strong magnetic field, for example.

Sweet, so weakly interacting, extremely weakly.

:)