Read the first for a summary by SLAC (Stanford Linear Accelerator Center). The second link is if you wanna read the whole article initially released on the discovery; well not article, more like scientific paper/report, with all the nifty rules that they need to follow in writing it. Posts: 2705 | Registered: Sep 2006
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Naw, I was extremely excited when I heard, as well. This is massive news! Not as what will happen when the Large Hadron Collider is complete so we can prove SuperSymmetry, but still pretty darn big. Don't you love living in this day and age?
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quote:Originally posted by Telperion the Silver: What I don't understand is why/how this matter is dark... why doesn't it reflect any light?
Because if it did, we'd be able to see it. We'd have known about it all along, and it wouldn't be called dark matter.
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quote:Originally posted by Telperion the Silver: What I don't understand is why/how this matter is dark... why doesn't it reflect any light?
Two reasons : First, what light is there for it to reflect? Starlight is incredibly thin over interstellar distances. Second, dark matter apparently doesn't interact electromagnetically anyway, so photons pass right through it as though it weren't there. As for why it doesn't interact electromagnetically, who knows?
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Originally posted by Telperion the Silver: What I don't understand is why/how this matter is dark... why doesn't it reflect any light? --------------------------------------------------------------------------------
Because if it did, we'd be able to see it. We'd have known about it all along, and it wouldn't be called dark matter. --------------------------------------------------------------------------------
This is going in my portfolio of ridiculous questions I have read on forums.
Oh good lord. The article had said planets and other stuff was considered normal matter, but I guess it's because they are near stars.
quote:Two reasons : First, what light is there for it to reflect? Starlight is incredibly thin over interstellar distances. Second, dark matter apparently doesn't interact electromagnetically anyway, so photons pass right through it as though it weren't there. As for why it doesn't interact electromagnetically, who knows?
quote:The article had said planets and other stuff was considered normal matter, but I guess it's because they are near stars.
Umm, I think you still don't understand the definition. It doesn't matter whether or not it is near light. Matter is matter, and dark matter is dark matter and invisible, regardless of being near a star or not.
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What I don't understand is why the regular matter, which interacts both radiatively and gravitationally slows down more than the dark matter which only interacts only gravitationally. I can kind of see how radiative interactions could repell two objects which would slow the regular matter as the two galaxies approached each other, but at some point as the galaxies are moving through each other, that repulsive force should cause the regular matter to accelerate more than the dark matter accelerates so that the regular matter would catch up to the dark matter. Maybe the two galaxies just haven't crossed that point yet.
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I don't think it was radiative interactions that significantly slowed down or accelerated the regular matter parts of the 2 colliding galaxies. I would guess it's "friction" with dust clouds that slowed down the regular matter. With this explanation, there would be no counteracting acceleration, and it would also not affect the dark matter.
I tried to google a firmer guess, but stuff I saw was either over my head or too basic, no middle ground.
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Um, friction is an electromagnetic interaction. Morbo's got it right. There's no overall repulsive force between the galaxies; on average, they are neutral. But individual dust grains, or whatever, will lose speed as they meet grains travelling the other way.
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Jon, that is a good summary of dark matter. Sometimes I google with wiki in the search, sometimes not.
Wiki has already updated for the recent Bullet Cluster dark matter discoveries, but it's vauge about the seperation mechanism.
quote: The hot gases interacted during the collision and remain closer to the center. The individual galaxies and the dark matter did not interact and are further from the center.
A NASA page said the ordinary matter had "drag."
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So why is the discovery of dark matter so important. I mean it doens't sound like it has any real bad or good effects on anything. It is just kind of there. Or is that the beauty of it, that it does nothing?
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Umm, it does do something sort of good. It means that the universe won't collapse in a fiery inferno of pain and suffering .
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How so. I am not trying to pretend that I know what this stuff is but according to the little bit on the Wiki page is dark matter what stabalizes all gravity? It counter balances everything else in the Universe or something like that? If so then it is good but the Wiki page just made it sound like it is just there out in space not really doing anything and from the rest of the posts in this thread it sounds like it only reacts to gravity so that is why I asked the other questions and wonder why the universe would turn into a fireball if it did not exist.
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B34N, I only have a vauge understanding of it myself, but it is crucial to galaxy and galaxy cluster formation and dynamics, and probably star formation in general. With only normal matter, it's possible the Universe would have remained a diffuse gas cloud.
quote: Likewise, a significant amount of non-baryonic, cold matter [ie dark matter--Morbo] is necessary to explain the large-scale structure of the universe.
Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures collapsing first and followed by galaxies and then clusters of galaxies. As the structures collapse in the evolving universe, they begin to "light up" as the baryonic matter heats up through gravitational contraction and the object approaches hydrostatic pressure balance. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the big bang to collapse and form smaller structures, such as stars, via the Jeans instability.
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Cool, then it is important and does something good beyond my level of comprehension. I take back my earlier post then.
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quote:Originally posted by King of Men: Um, friction is an electromagnetic interaction. Morbo's got it right. There's no overall repulsive force between the galaxies; on average, they are neutral. But individual dust grains, or whatever, will lose speed as they meet grains travelling the other way.
I still don't clearly understand why dust grains slow each other down as they collide anymore than anti-matter particles would slow each other down as they collide. Even neutrinos exchange momentum when they collide.
Electostatic interactions between uncharged dustparticles are extremely short range. They drop to zero with a separation of only a nanometers. Since the galaxies are overall charge neutral, the longer range electrostatic interactions between charged species should be balanced between attractive and repulsive forces.
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Well, yes, the collision cross-section for dust grains is indeed rather small, but the point is that for dark matter it is essentially zero. On the scale of galaxies, the tiny difference adds up rather quickly. I think perhaps you may be attaching too much importance to the word 'colliding'. Neutrinos, basically, do not collide. They can occupy the same bit of space and still not exchange momentum; their interaction is extremely weak. Dark matter, in fact, has this quality to an even larger extent, since gravity is even weaker than the weak force. Dark matter, even a large chunk of it (well, actually, there can't be any chunks, since there's nothing keeping it together... but let that pass) is quite capable of passing right through another chunk of dark (or ordinary) matter. I'm not talking about one gas cloud sliding through another; dark matter would happily pass right through steel. That's the difference; and when you're trying to pass through an entire galaxy, the dust grains do add up. I doubt there are many straight-line paths through the Milky Way that don't intersect at least one dust grain on the way!
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quote:Originally posted by King of Men: I doubt there are many straight-line paths through the Milky Way that don't intersect at least one dust grain on the way!
You had me up until here, KoM. The rest of your argument is what I was thinking of. However, I'd venture to guess that there are an infinite number of straight-line paths through the Milky Way that don't intersect a dust grain.
If you have to pass through the core it might get dicier, but still.
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quote:However, I'd venture to guess that there are an infinite number of straight-line paths through the Milky Way that don't intersect a dust grain.
If this is true (and please note, I'm including hydrogen atoms, at 1 per cubic meter, in my definition of 'dust grains'), you are left with a local version of Olber's paradox : Why don't we see the stars on the other side of the Milky Way?
In fact, though, we don't have to argue, we can run the numbers. Consider hydrogen atoms; they have a radius of about 10^-10 meters. Run a tube of that thickness through the 1000 light years of our galaxy's thickness. (You'll note that I am taking the shortest path.) The total volume, then, is
\pi*(10^{-10})^2*1000*365*24*60*60*300*10^8 m^3, or about 30 cubic meters. In this volume you would expect to encounter about 30 atoms; it's quite unlikely for this to Poisson-fluctuate down to zero!
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On second thought, I suppose the actual number of paths may indeed be infinite; I think I've demonstrated, however, that they are vanishingly rare.
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quote:However, I'd venture to guess that there are an infinite number of straight-line paths through the Milky Way that don't intersect a dust grain.
If this is true (and please note, I'm including hydrogen atoms, at 1 per cubic meter, in my definition of 'dust grains'), you are left with a local version of Olber's paradox : Why don't we see the stars on the other side of the Milky Way?
In fact, though, we don't have to argue, we can run the numbers. Consider hydrogen atoms; they have a radius of about 10^-10 meters. Run a tube of that thickness through the 1000 light years of our galaxy's thickness. (You'll note that I am taking the shortest path.) The total volume, then, is
\pi*(10^{-10})^2*1000*365*24*60*60*300*10^8 m^3, or about 30 cubic meters. In this volume you would expect to encounter about 30 atoms; it's quite unlikely for this to Poisson-fluctuate down to zero!
Don't mean to be the mathematician or anything, but this calculation is wrong.
The speed of light is 3 * 10^8 m/s, not 300 * 10^8 m/s. This changes your answer to something closer to .3, not 30.
Just to be picky, the radius of the hydrogen atom (according to Wiki, and in the ground state - which most of these will be in) is closer to half of what you quoted, which corresponds to 1/4 of your actual answer (once squaring), and so in the end, you'll actually get something closer to .075 as an answer. (Plugging in actual values, I got .0832)
quote:Originally posted by King of Men: On second thought, I suppose the actual number of paths may indeed be infinite; I think I've demonstrated, however, that they are vanishingly rare.
This, of course, changes with the new calculation with the correct speed of light and more accurate radius for hydrogen. In fact, what you demonstrated is that if you pick a path at random, it's quite likely to not hit any hydrogen (at least, head on).
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Oopsie. That's what happens when you calculate before dinner. I'm going to disagree about the hydrogen atom, though; you have to consider the effective, not the geometric, cross-section. Then you must consider that this was only hydrogen atoms; add in the dust grains and whatnot, and the density goes up again.
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Based on the kinetic theory of gases, the mean free path of an atom in a gas (i.e. the average distance an atom travels before colliding with another atom) = 1/(sqrt(2) pi d^2 n), where d is the diameter of the atom and n is the number of atoms per unit volume.
Of course this assumes that the atom is a rigid sphere, which of course it is not. But if we set d, the diameter of the hydrogen ion equal to the 99% probability for the electron, we get a very liberal estimate of the diameter of hydrogen of 8.4 Å (or 8.4 x 10^-10 m). This yields a mean free path of hydrogen in the interstellar media of 3.2 x 10^14 km, or approximately 1 million times the distance across the galaxy.
In other words, the average atom in the interstellar gas will travel 1 million times the distance across the galaxy before it collides with another hydrogen ion. In other words, collisions between atoms in the interstellar gas are vanishingly small.
99% of the mass of the interstellar media is in the hydrogen and helium atoms, so it is reasable to assume that since collisions between hydrogen atoms are extraordinarily unlikely, collision between a hydrogen atom and a dust particle is far more unlikely.
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That's division by 3E8, by 3600, by 24, and by 365 respectively. This means that the shortest distance across the galaxy is about thirty mean free paths, using your admittedly liberal estimate for the radius. With half that, you would get 134 light-years; that still makes the galaxy ten mean free paths across. Going down to the estimate of 10^-10 gives something in excess of two thousand light-years; which is still to say that about 40% of the incoming particles will collide with something. (This from the equation I(x) = I_0*e^{-x/l), with I the intensity, I_0 the incoming intensity, x the distance travelled, and l the mean free path. Setting x = 0.5l gives I = 0.6 I_0, so the other 40% have collided.)
As a side note, I think you are using the wrong equation for the mean free path; its derivation relies on a Maxwell distribution of velocities, which is not the case in a collision. However, the difference is only in the root-two, so it should not matter for these order-of-magnitude calculations.
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You are right KoM, I don't know how I could have screwed up the size of the galaxy so badly. I do disagree about the correct equation for mean free path. If you remove the root-two, you are assuming that all the other atoms are stationary, which is a far worse assumption than a Maxwell distribution of velocities.
I will also note that the 8.4Å diameter for hydrogen is very very liberal. The Bohr radius for hydrogen (most probable) is 0.54 Å. If you use the Bohr radius, you get a mean free path of 1.9x 10^16 km, which would make the galaxy 1.5 mean free paths across.
I choose the very liberal estimate of 99% probability limit because this is a reasonable estimate of the maximum separation distance for any electrostatic interaction between two hydrogen atoms.
[ September 26, 2006, 03:52 PM: Message edited by: The Rabbit ]
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