I know what dark matter is(sort of), I know that it emits no radiation so cannot be seen, but can be detected because it has gravitational effects. I want to know WHY it can emit no radiation, and what the stuff is really made of, if it is known. Can someone help me out?

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Dark matter is only a theory. No one has ever detected it. It was postulated because some scientists believe in the Big Bang and think there should be some critical mass density that will slow and eventually reverse the expansion of the universe. There is no real evidence of dark matter outside of some scientists need it to exist to validate the models that they believe in. Be very careful of the mainsteam science that's in magazines that make thing sound more fantastic than it really is.

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TEAM
LL

Dark matter is only a theory. No one has ever detected it. It was postulated because some scientists believe in the Big Bang and think there should be some critical mass density that will slow and eventually reverse the expansion of the universe. There is no real evidence of dark matter outside of some scientists need it to exist to validate the models that they believe in. Be very careful of the mainsteam science that's in magazines that make thing sound more fantastic than it really is.

This is completely incorrect. Not only does the evidence for dark matter have nothing to do with the critical density of "mass" (really energy) in the universe, but the critical density has very little to do with what the expansion of the universe will do. Critical density refers to the density of all form of energy in the universe wuch that the overall geometry of space (not spacetime) is flat - that is, the geometry that works in the universe is that which we all learned in high school. That fact that the geometry of space is flat (or very close thereto) is well established by quite a few observations, not by models. In fact, given the amount of variation allowed by general relativity, it is quite surprising that the geometry should be flat. However, this has to do with more than just dark matter.

The evidence for dark matter comes from places that we can see gravitational effects, but cannot directly identify any mass that should be causing it. For example, the relation between the speed at which stars in a spiral galaxy rotate about its center and their distance from the center only depends on the mass profile of the galaxy. If the mass of the galaxy is concentrated where we observe light coming from, the speeds should drop off towards the outer edge of the galaxy. They don't. The shape of the rotation curves tells us that spiral galaxies seem to be embedded in a roughly spherical halo of mass. If this mass were able to interact through anything other than gravity, it would, and would end up falling into the disk. So, the fact that the halo is spherical, not disk -shaped, tells us that the extra mass is in the form of things which can only interact through gravitation. Similar considerations can be used to identify extra mass in galaxy clusters, often by looking at the gravitational lensing of light. The existance of dark matter is further supported by analysis of the cosmic microwave background radiation.

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I know what dark matter is(sort of), I know that it emits no radiation so cannot be seen, but can be detected because it has gravitational effects. I want to know WHY it can emit no radiation, and what the stuff is really made of, if it is known. Can someone help me out?

Unfortunately, no one knows exactly what it is, yet. It's not too hard to understand how it can fail to interact electromagnetically. Electromagnetic effects couple to electrical charge. It a particle has no charge and is not composed of charged particles, it will not interact electromagnetically, so it can't emit light. We do know of other particles like this, such as neutrinos; but none of them also fail to interact through both the strong and weak nuclear forces.

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I know what dark matter is(sort of), I know that it emits no radiation so cannot be seen, but can be detected because it has gravitational effects. I want to know WHY it can emit no radiation, and what the stuff is really made of, if it is known. Can someone help me out?

You seem very interested in science at a young age. I'll say this: Everything that you learn from now on,especially in school, should not be considered fact. You should listen to others and take everything with a grain of salt because everyone that comes before you will not be correct. Great scientific revolutions happen because scientists with new and novel thoughts broke through the main stream science which tells them what to think and they were able to see things in a new and better way. In short, for everything new thing that they learned, they questioned and asked "does this really seem right?". "Does this make sense?" It's not about memorizing what others have come up with but about looking at the data and drawing your own conclusions. Separate the data from peoples opinions and interpretations of that data.

This is completely incorrect.

In your opinion only.

A quote from WIKI just to start:

Scientific method or scientific process is fundamental to scientific investigation and to the acquisition of new knowledge based upon physical evidence by the scientific community.

It's "acquisition of new knowledge based upon physical evidence" not "aquistion of physical evidence base on new knowledge".

And "aquistion of physical evidence base on new knowledge" is what dark matter is. It was an assumption and then the data was sought and then it is assumed that the cause is dark matter.

This will often lead to misinterpretation of data. Data is data, models should constantly reinvented based on data. The data should not be constructed on models. Yes, the data is real.

The evidence for dark matter comes from places that we can see gravitational effects, but cannot directly identify any mass that should be causing it

Thus the theory is unverifiable. How convenient. And thus I say again, "Dark matter is only a theory. No one has ever detected it". This is were the scientific method comes in. We have a small amount of data and should not pollute peoples minds with conjecture.

Conjecture by www.m-w.com: 2 a : inference from defective or presumptive evidence b : a conclusion deduced by surmise or guesswork c : a proposition (as in mathematics) before it has been proved or disproved.

Electromagnetic effects couple to electrical charge. It a particle has no charge and is not composed of charged particles, it will not interact electromagnetically, so it can't emit light.

There are alternative models to this.

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TEAM
LL

This is completely incorrect.

In your opinion only.

No. It is completely incorrect because you miscategorize things as fundamenal as why it is dark matter was introduced in the first place. This is not a matter of opinion. It is a matter of what anomolous observations at least seem to require the presence of unseen matter.

A quote from WIKI just to start:

Scientific method or scientific process is fundamental to scientific investigation and to the acquisition of new knowledge based upon physical evidence by the scientific community.

It's "acquisition of new knowledge based upon physical evidence" not "aquistion of physical evidence base on new knowledge".

And "aquistion of physical evidence base on new knowledge" is what dark matter is. It was an assumption and then the data was sought and then it is assumed that the cause is dark matter.

Dark matter was never assumed. Frankly, it puts physics in a bit of a bind as to figure out what the heck it is. It was introduced, not because of some sort of aesthetic consideration, but because the observational data couldn't be explained without the presence of some sort of unseen mass. The distribution of that mass, as well as aspects of the theory of nucleosythesis after the big bang, have led people to conclude that it cannot simply be more normal matter; so, it must be something different, which does not interact other than by gravity.

This will often lead to misinterpretation of data. Data is data, models should constantly reinvented based on data. The data should not be constructed on models. Yes, the data is real.

The evidence for dark matter comes from places that we can see gravitational effects, but cannot directly identify any mass that should be causing it

Thus the theory is unverifiable. How convenient. And thus I say again, "Dark matter is only a theory. No one has ever detected it". This is were the scientific method comes in. We have a small amount of data and should not pollute peoples minds with conjecture.

The observation of the necessity for mass to be somewhere is a form of detection. I probably should have said "but cannot identify any mass that should be causing it, by any other means." Or do you deny that looking at gravitational effects is a way of identify the presence of mass?

An example: We identify the presence of planets around other stars by looking for a "wobble" in the stars motion, which we identify as the effect of a massive object orbiting it. Do you consider this an insufficient way of detecting a planet?

Conjecture by www.m-w.com: 2 a : inference from defective or presumptive evidence b : a conclusion deduced by surmise or guesswork c : a proposition (as in mathematics) before it has been proved or disproved.

Electromagnetic effects couple to electrical charge. It a particle has no charge and is not composed of charged particles, it will not interact electromagnetically, so it can't emit light.

There are alternative models to this.

The only alternative model I know to dark matter that can come close to reproducing its effects is the MOND theory, which has the convenient property that it cannot be made compatible with general relativy. So, if MOND is right, we can't explain things as simple as the precession of Mercury's orbit or the deflection of light by massive bodies (both of which are well verified observationally).

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I always wondered if black holes will "eat" dark matter. (I guess so)

This leads to an interesting question: What happens to the Hawking radiation of the black hole?

Will it be diminished by the percentage of dark matter that fell into the black hole,
or are black holes effectively converting dark matter into radiation?


Regards Hans

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For anyone following this thread and want current reading on the topic:

A Model of Dark Energy and Dark Matter, arXiv:hep-ph/0512282

Hung uses a false vacuum model which has been purposed to explain early expansion of the universe, if you belive in the Big Bang. He then introduces 3 particles to consider in the spin-rotation couplings. He eventually settles on the SU(2)z fermions Psi(z)1,2 as the top candidate for "cold dark matter".

OK Solomon, so we disagree. I'll start again by answering your question. Then I'll ask one or two.

An example: We identify the presence of planets around other stars by looking for a "wobble" in the stars motion, which we identify as the effect of a massive object orbiting it. Do you consider this an insufficient way of detecting a planet?

Of course I DO NOT cosider this an insufficient way of detecting a planet. We know that planets exist and their behavior is well defined. This allows us to make measurements such as that. I do understand your point, though.

I don't not believe that "dark matter" exists. I simply feel that much of science, physics specifically, is based on the hand waving arguments which then gets built upon. Yes, there are anomolous excellerations and we search for the mathmatical construct to describe it and then search for words to explain it. In such cases, the math could be correct but the words could be wrong. Explaining it fully in words sometimes requires us to effectively touch it and see it directly or at least to be able to compendently manipulate it.

The only real models that are prevailent are the ones that say "dark matter".

We know from the observations:

1. gravity interaction is strong
2. electromagnetic interaction is weak - One cannont say that it is non-existant. It would be comparible to saying gravity does not exist when one measures electrostatic potentials.

You can add to the list. I would really like to see a consolidated list of primary observations.

Possiblities:
1. Dark matter: Exotic matter which is unforseen or observed by us. Not unreasonable. It reconizes mans ignorance in an attempt to open up new possiblities.

2. Extremely concentrated matter: A possiblity but in some cases may break down to ineffective energy concetrations.

3. Regular matter experiencing novel effects yet directly observed by man.


Personally, I chose #3 because it builds on something we do know (matter) and it understands that we have not really been able to observe it in all of it's states. We are limited by what we can study closely in our solar system. This severely constrains us to particularly small groupings of low energy matter in a hydrogen dominated system. There are a lot of well defined variables that we cannot even begin to study well but we know they are there and they are tangible. Whereas, "dark matter" introduces too many concepts and requires too many leaps of faith. But if that's what you believe in, then that's what you should promote and examine. Outside of a personal belief, I feel that others should encouraged to listen to others,encouraged to form thei own opinions on subjects, and encouraged to understand an underlying scientific principle. When this is done, they might just find someting new that everyone else missed, but they might simply conclude that they agree. It's really a system of checks and balances.

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TEAM
LL

I intended to add that the novel effects might include the forces increasing to a point that overshadows the electromagnetic interactions. Such as in our knowledge, gravity is overshadowed by electromagnetic interactions.

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TEAM
LL

Thanks everyone!And I think dark matter is a theory as I read it on the web (no reference, i forgot where i got it from).Anyway, I read somewhere else that nothing can be "proven" with our current technology, as there will always be another variable, affecting the experiment, and we cannot cover ALL the variables. I will talk to Albert Einstein about this, and I'll tell you what he thinks.

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I don't not believe that "dark matter" exists. I simply feel that much of science, physics specifically, is based on the hand waving arguments which then gets built upon. Yes, there are anomolous excellerations and we search for the mathmatical construct to describe it and then search for words to explain it. In such cases, the math could be correct but the words could be wrong. Explaining it fully in words sometimes requires us to effectively touch it and see it directly or at least to be able to compendently manipulate it.

Possiblities:
1. Dark matter: Exotic matter which is unforseen or observed by us. Not unreasonable. It reconizes mans ignorance in an attempt to open up new possiblities.

2. Extremely concentrated matter: A possiblity but in some cases may break down to ineffective energy concetrations.

3. Regular matter experiencing novel effects yet directly observed by man.

Possibility 1 builds off of the your snippet above. Well before we sent rockets into space, the motions of the planets were observed and assumptions were made. Aside from questions of who orbited whom, the biggest assumption was that the planets were either fixed to "crystal spheres" or were traversing some medium. The discovery of comets disproved the "crystal spheres" model, so everyone more or less settled on the bodies traversing a medium theory.

The scientists of the day took observations of light and matter and used them to divine the characteristics of this medium, which came to be known as the ether. By the time all of the oddities were incorporated, they ended up describing a solid of infinite sniffness and zero density.

Eventually someone made the logical leap to consider a vacuum.

We know that matter and energy can be transmuted into one another under the right circumstances. Dark matter may not be matter at all, but a third transmutation of energy/matter. Or it could some aspect of spacetime curvature independant of what we currently consider to be mass and energy. Once properly framed and modeled, so-called dark matter might explain why "mass" changes at relativistic speeds and how light can posess momentum without any "mass."

As for possibility 2, I think that this is summed up by the MACHO theory of dark matter. Most candidates for MACHO deposits have been eliminated by careful observation.

Possibility 3 is the ether or epicycles (another early math trick used to explain planet orbits) of the current model. "We have observations, and the only explanation we have is that some mass needs to be there that we can't see. That's an awful lot of invisible matter, but my trusty abacus says it's there, so it must be there."

(edit for spelling)

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No animals were harmed in the making of the above post... much.

BBC article on CERN

Theory being tested is looking for the Higgs field that is thought that "The Higgs mechanism fills all of space with a field."

And "There's just a small problem with the idea, says John Ellis: it gives 120 orders of magnitude too much dark energy."

But if this Higgs field permeated all of space, that would reclaim the aether idea. I don't understand it, as it's constant energy would exceed the CMB (cosmic background radiation). With that alone it should be easy to find. But it is purposed to be neutral which I mentioned in another thread that truely neutral particles would have no magnetic moment, etc and thus not limited to the speed of light. Without a charge it would be hard for us to detect. At least this is predicted by something that works eslewhere, i.e. the SM (standard model).

Would the Higg's field have any intrinsic resistance?

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TEAM
LL

For anyone following this thread and want current reading on the topic:

A Model of Dark Energy and Dark Matter, arXiv:hep-ph/0512282

Hung uses a false vacuum model which has been purposed to explain early expansion of the universe, if you belive in the Big Bang. He then introduces 3 particles to consider in the spin-rotation couplings. He eventually settles on the SU(2)z fermions Psi(z)1,2 as the top candidate for "cold dark matter".

Interesting paper. I haven't read all the way through it yet, but I intend to give it a try.

OK Solomon, so we disagree. I'll start again by answering your question. Then I'll ask one or two.

An example: We identify the presence of planets around other stars by looking for a "wobble" in the stars motion, which we identify as the effect of a massive object orbiting it. Do you consider this an insufficient way of detecting a planet?

Of course I DO NOT cosider this an insufficient way of detecting a planet. We know that planets exist and their behavior is well defined. This allows us to make measurements such as that. I do understand your point, though.

I don't not believe that "dark matter" exists. I simply feel that much of science, physics specifically, is based on the hand waving arguments which then gets built upon. Yes, there are anomolous excellerations and we search for the mathmatical construct to describe it and then search for words to explain it. In such cases, the math could be correct but the words could be wrong. Explaining it fully in words sometimes requires us to effectively touch it and see it directly or at least to be able to compendently manipulate it.

The only real models that are prevailent are the ones that say "dark matter".

We know from the observations:

1. gravity interaction is strong
2. electromagnetic interaction is weak - One cannont say that it is non-existant. It would be comparible to saying gravity does not exist when one measures electrostatic potentials.

You can add to the list. I would really like to see a consolidated list of primary observations.

It's more than just saying that the electromagnetic interaction is weak. If the unseen matter could interact electromagnetically at all, it would be able to shed energy and angular momentum to radiation. But, if it could do this, we would expect to find it concentrated in the disk of the galaxy, just like the luminous matter, since it's the electromagnetic interactions that can lead to a relatively amorphous blob of matter organizing into a rotating disk in the first place.

Also, strength of the electromagnetic coupling is pretty much determined by the net charge of the particle and the charges of its constituents. Since charge is quantized, there's really not much of a way to create and electromagnetically interacting particle which couples more weakly than a neutron.

Since we already know of particles, like neutrinos, which do not interact electromagnetically, I'm not sure what your objection to the idea that dark matter is like this is.

Possiblities:
1. Dark matter: Exotic matter which is unforseen or observed by us. Not unreasonable. It reconizes mans ignorance in an attempt to open up new possiblities.

2. Extremely concentrated matter: A possiblity but in some cases may break down to ineffective energy concetrations.

3. Regular matter experiencing novel effects yet directly observed by man.


Personally, I chose #3 because it builds on something we do know (matter) and it understands that we have not really been able to observe it in all of it's states. We are limited by what we can study closely in our solar system. This severely constrains us to particularly small groupings of low energy matter in a hydrogen dominated system. There are a lot of well defined variables that we cannot even begin to study well but we know they are there and they are tangible. Whereas, "dark matter" introduces too many concepts and requires too many leaps of faith. But if that's what you believe in, then that's what you should promote and examine. Outside of a personal belief, I feel that others should encouraged to listen to others,encouraged to form thei own opinions on subjects, and encouraged to understand an underlying scientific principle. When this is done, they might just find someting new that everyone else missed, but they might simply conclude that they agree. It's really a system of checks and balances.

The problem with dark matter being just more normal matter is that we don't know of particles which don't couple to the EM field and which have significant mass.

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BBC article on CERN

Theory being tested is looking for the Higgs field that is thought that "The Higgs mechanism fills all of space with a field."

And "There's just a small problem with the idea, says John Ellis: it gives 120 orders of magnitude too much dark energy."

But if this Higgs field permeated all of space, that would reclaim the aether idea. I don't understand it, as it's constant energy would exceed the CMB (cosmic background radiation). With that alone it should be easy to find. But it is purposed to be neutral which I mentioned in another thread that truely neutral particles would have no magnetic moment, etc and thus not limited to the speed of light. Without a charge it would be hard for us to detect. At least this is predicted by something that works eslewhere, i.e. the SM (standard model).

Would the Higg's field have any intrinsic resistance?

As I replied to you in that same thread, the speed of light is not an inherently electromagnetic quantity. It shows up in E&M because light is massless. It should really be called the speed of massless particles.

The speed of light "speed limit" arises directly from special relativity, which makes no assumptions about the charges of particles involved, or their magnetic moments.

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As I replied to you in that same thread, the speed of light is not an inherently electromagnetic quantity. It shows up in E&M because light is massless. It should really be called the speed of massless particles.

The speed of light "speed limit" arises directly from special relativity, which makes no assumptions about the charges of particles involved, or their magnetic moments.

And it's just coincidence that the speed of light is where the magnectic engergy equals the electric energy and it's just coincidence that it is equal to the inverse sqrt of electric and magnetic constants.

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TEAM
LL

As I replied to you in that same thread, the speed of light is not an inherently electromagnetic quantity. It shows up in E&M because light is massless. It should really be called the speed of massless particles.

The speed of light "speed limit" arises directly from special relativity, which makes no assumptions about the charges of particles involved, or their magnetic moments.

And it's just coincidence that the speed of light is where the magnectic engergy equals the electric energy and it's just coincidence that it is equal to the inverse sqrt of electric and magnetic constants.

I'm not sure what you mean by your first statement, since electromagnetic energy is determined by the fields, and the fields don't have a speed.

As for the relation between the speed of light and the permittivity and permiability of free space, this is really just and awkward way of dividing up the constants which make Maxwell's equations invarient under Lorentz transformations. If you Loretz transform all of the equations, you'll find that their form is completely unchanged if and only if \\mu_0*\\epsilon_0 = 1/c^2, so this relation is actually just a statement of the relativistic invarience of this theory. I should probably also add that there are systems of units in which the only constant appearing in Maxwell's equations is the speed of light.

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""Maxwell developed equations showing that electric and magnetic fields can combine in ways that create waves. The equations also indicate that these electromagnetic waves travel at the speed of light. Maxwell said that light itself consists of electromagnetic waves -- a statement later proved to be true. He also said that other kinds of electromagnetic waves exist. The German physicist Heinrich Hertz discovered such waves -- now known as radio waves -- between 1886 and 1888. ""

from NASA

This discussion just seems insane to me. I cannot fathom the idea that the speed of light has nothing to do with E&M. They are inherently connected and it seems like for some reason you are intent on convincing me that I'm wrong. Maybe in some equations the relation has been hidden within the light itself but in a vacuum, it E&M says it moves at c. Go tell NASA they are wrong.

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TEAM
LL

""Maxwell developed equations showing that electric and magnetic fields can combine in ways that create waves. The equations also indicate that these electromagnetic waves travel at the speed of light. Maxwell said that light itself consists of electromagnetic waves -- a statement later proved to be true. He also said that other kinds of electromagnetic waves exist. The German physicist Heinrich Hertz discovered such waves -- now known as radio waves -- between 1886 and 1888. ""

from NASA

This discussion just seems insane to me. I cannot fathom the idea that the speed of light has nothing to do with E&M. They are inherently connected and it seems like for some reason you are intent on convincing me that I'm wrong. Maybe in some equations the relation has been hidden within the light itself but in a vacuum, it E&M says it moves at c. Go tell NASA they are wrong.

NASA's statement is perfectly correct; and, historically, the first place the speed of light was encountered as a physical constant was in Maxwell's work.

What I've been trying to explain is that the fact that light travels at the speed it is observed to, and which comes out of the Maxwell equations, comes from something more fundamental than E&M. The classical theory of electrodynamics is completely compatible with special relativity, but not with classical mechanics; and the fact the speed of light comes out of E&M is actually an indication of this.

It was a known problem before Einstein that the equations of E&M are not invarient under a Galilean transformation, which is the way you relate two relativily moving inertial frames of reference in classical mechanics. E&M is, instead, invarient under Lorentz transformations. Einstein took seriously the idea that E&M was, actually, the correct theory and that mechanics would have to be changed to be invarient under the same Lorentz transformations as E&M. Saying that these transformations are the fundamentally correct way to relate observations in two relatively moving inertial reference frames is what leads to all the strange features of special relativity; and, since the speed of light appears explicitly in the transformations, it elevates the speed of light from a "material" property of light waves to a fundamental property of the spacetime geometry of the universe. It is worth noting that the necessity of the transformations can be arrived at simply by postulating that a) the laws of physics should be the same in any two frames of reference moving relative to each other at constant velocity and b) the observed speed of light should not depend on the unaccelerated motion of an observer.

The rest of relativity can be derived from these considerations without any consideration of electric charge or magnetic moments. This includes the requirement that no particles move faster than light.

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OK, I find that response more palatable. That to me is equivelent to saying the way they came up with the constants is flawed because it does not tell the whole story. I'd jump on that ship in a second. Multi-unit constants bother me to my core because it's just a slope that may or may not be understood. I do believe that there might be some underlying law that determines c and determines the properties of E&M, which could bring us to common ground.

When it comes down to it, mass as we know it is made with the properties of E&M. Even neutrinos have magnetic moments. I personally find it hard to swallow massless particles there is another model to explain their non-interaction with matter.

You said:

It should really be called the speed of massless particles.

Is this to say that massless particles must always move at the speed of light? Or can they be at rest?

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TEAM
LL