Not sure I go along with this.
What exactly do you mean by "understandable" or "knowable?" Knowledge that is provable?
I'm not trying to put words in your mouth, I'm asking.
Posted on 01/15/2002 7:02:17 AM PST by PatrickHenry
Once upon a time -- a bit more than 100 years ago -- many scientists believed that seemingly empty space wasn't empty at all, but was filled with a substance called luminous ether. This mysterious stuff, never seen in any laboratory on Earth, was thought to explain how gravity from one celestial body could affect another.
By the end of the 19th century, though, luminous ether had gone the way of countless other scientific misconceptions. Today, another mysterious substance beguiles astronomers, and this one isn't going away. In fact, it's been at the forefront of cosmological theories for decades. It's called dark matter, and it is now widely accepted by astronomers as the stuff most of the universe is made of.
"We've known that it exists for more than 25 years," says astronomer Virginia Trimble of the University of California Irvine. "But we don't know what the hell it is."
How can astronomers be so certain of something they have never seen? The answer comes from observations of how stars and galaxies move, studies that have been going on for more than 50 years. Within spiral galaxies, individual stars and clouds of gas are orbiting faster than they should if they were only being affected by the gravity of the galaxy's visible matter. The same is true for clusters of galaxies: The motions of individual galaxies can't be explained by the gravity of what astronomers can see.
To explain these observations, astronomers have deduced that galaxies are surrounded by vast halos of a different, unseen kind of matter.
This so-called dark matter is invisible to us because it does not radiate energy. But it does have mass, and that means it can supply the extra gravity necessary to hold galaxies, and clusters of galaxies, together. Even in the bizarre world of cosmology, it's a strange proposition.
But is dark matter the only explanation?
Perhaps scientists don't entirely understand the way gravity works; perhaps Isaac Newton's famous law of gravitation needs some revising. But that idea, says the University of Arizona's Chris Impey, is not very popular.
"Definitely most astronomers are extremely unwilling to give up Newton's law," he says. "So it's essentially a choice of two evils: You either hypothesize that Newton's law is wrong, and that our knowledge of the gravity theory is incomplete. Or, you hypothesize a fundamental microscopic particle that has never been detected in any physics lab, whose properties are only constrained by these astronomical observations. Which is a pretty uncomfortable position for physicists to be in."
Still, as Trimble explains, dark matter is the lesser of the two evils, simply because it requires fewer departures from accepted physics.
To explain the observations by revising the theory of gravity, astronomers would have to identify a few different effects, each of which would operate at a different distance scale. But with dark matter as the explanation, Trimble says, "You only need one Tooth Fairy."
[The rest is omitted, but you can visit the source and read it all.]
Not sure I go along with this.
What exactly do you mean by "understandable" or "knowable?" Knowledge that is provable?
I'm not trying to put words in your mouth, I'm asking.
From the quantum standpoint, the situation of galactic spin rate profiles is suggestive to me of a sort of galactic-scaled quantum coherence. This suggests that information and energy generated by the galaxy in the past is being contained and recycled within the galaxy.
From the cosmological standpoint, I can't help wondering if energetically spinning black holes don't have a role in the picture, as the phyics of such objects is not completely understood. My guess is that information is not lost into black holes, but is instead carried "out of the plane" and into extended regions.
Shaky ground indeed.
I am always in awe with what physicists’ (indeed all of us) are trying to grapple with in the universe, considering how miniscule our knowledge is of our environment. In relation to the concept of God, some people say that if they had all the information, then we could explain everything. But that is a huge ‘if’.
We know about the seemingly impossibility of the accidental construction of the structure of the DNA by ‘natural’ phenomenon. (The analogy of the typhoon hitting an aircraft spare parts factory and blowing together a fully functioning 747 comes to mind.) Some people choose to believe that with time, this could be explained in a natural context whereas others believe that it can only be explained in a supernatural context. Each belief is valid – it’s just a question of faith.
Right. As theories are demonstrated to be inadequate, scientists will follow the data; and old theories will be junked in favor of better ones that can account for the new information. So in a century or so, we'll probably have a better understanding of the forces of nature, with better theories than we have today. They won't look back on today's scientists as idiots, any more than we look back on Isaac Newton that way. We're doing the best we can to understand the data we have. Also, 100 years hence, theology will be maintaining the same doctrines that it does today. The strength of science is its ability to learn and grow; while the strength of theology is the stability it provides.
Who would have considered Clint Eastwood such a heavy thinker when he said "A man's gotta know his limitations..."?
FReegards...
Not if they talked to a competent topologist.
Please. There's no topology in a flatland...it's FLAT. If it had topology, it would be three-dimensional, wouldn't it?
If you cheat in solving dilemmas, and subsequently convince yourself that you have solved it, where does that leave you? Perhaps in the same place as scientists who realize there must be additional mass at the outer layers of the galaxy, but cannot account for it?
Judging by the answers I get on these threads to difficult questions, I am starting to formulate a theory that the most intellectual-seeming among us are those who invent the most clever ways of avoiding the questions.
You mean like this?
Please. Of course there's topology in flatland. Having three dimensions have nothing to do with it. Read Abbot's "Flatland" available from Dover. Topology deals with transformation of spaces. We live in a three-dimensional space but we deal with much higher dimensional things regularly.
Please do not accuse me of avoiding questions just because you don't like the answers. If you didn't want to hear the answer, don't ask.
Even ignoring all the YEC stuff, it doesn't work as an explanation of the rotation of galaxies. If a galaxy is 100 million light-years away, the entire galaxy--center, edge, and points between--is that far off. We should percieve no relativistic distortions in the relative motions of the parts of the galaxy. The problem is the outer parts of spiral galaxies--any spiral galaxy at any distance from us--move faster than Kepler's Laws and the predicted mass of the luminous parts would allow.
Newton said there is an interaction between bodies, Einstien said space is distorted. Gavitation lens observations support Einstien, but classical mechanics still works real good, so, Newton view of the universe is valid until you get into the really heavy stuff.
But nothing exotic is needed for that to be the case. The closer you are to the center, the more light is being released from stars closer by, and the less intervening dust there is between each light source and you. Therefore, the closer you are to the center, the more likely you are to have received a photon recently, and thus the more likely you are to be shining. Perfectly ordinary gas clouds - or anything else not dense enough to support independent fusion reactions - should thus be darker the farther away from the galactic center you are.
From the articles, it appears they get frustrated with dark matter models because in their simulations, anything that doesn't interact with light (assumed to be just some special type that doesn't), but does attract gravitationally (and if it doesn't, it won't help the rotation story), automatically collects in the galactic centers. Which ought to be obvious enough, since by construction the *repelling* force, light pressure, has been removed, while the attracting force, gravity, has been retained. But the evidence is for spread out matter, not concentrated matter.
Therefore, whatever it is must be subject to some repealing force. They don't understand what that could be, because they are assuming it must be dark "in itself" (that is, weakly coupled to light), and therefore the source of the repelling force can't be light. It is not like there are lots of other candidates for repelling forces on galactic distance scales. The problem apparently is, if it is coupled enough to light to be "blown" away from the center, then why isn't it bright enough to see?
There is one other factor that may be involved, however. Nothing says that the velocity distribution we observe has to be an equilibrium case. It is possible the extra matter far from galactic centers has been "thrown" there by "transient" events, which in principle does not require an ongoing repelling force greater than the gravitational attractive one. So one may need to include "dynamics" as well as "statics".
For example, consider shockwaves from exploding stars. It is striking that the velocity distribution plateaus at a certain velocity value. What if that value is related to some average velocity of matter ejected from dying stars? Some goes in every direction, but the farther out you are, the more area of ejecta is pointed away from the galactic center (the angle subtended by the galactic center is smaller). Some portion of the ejecta then presumably flies out until it reaches an orbit distance at which its initial kinetic energy has translated into gravitational potential energy, slowing as it gets farther from the galactic center.
It may be as dark as you please (in terms of how much light is coming by) out in the regions reached "dynamically" by such ejecta, and therefore you would not see that (ordinary) matter in an estimate based on how bright a region is. And it doesn't need to (still) be coupled to light pressure to get it there. The energy that blew it there was initially created by fusion or what-not closer to the galactic center. But it moved outward afterward, just by ongoing momentum.
There may be other processes within galactic cores that also blown serious quantities of matter out to long distances from the core, with high kinetic energy to get them there. We see huge gas jets from the center of some galaxies, for instance (e.g. M87) - thought to be powered by core black holes. Overall, this represents a potentially enourmous flow of energy outward, not directly as light pressure, powered by events near the galactic center. Thus it represents another way the matter of the galaxy tends to become more spread.
It is possible that spiral galaxies that do not have such jets at their core now, once did. Indeed, it is one obvious potential source of spiral arm structure (especially "bar" spirals, with two main arms opposite one another). If the jet phenomenon is caused by an "active" core black hole, then the issue of having one or not would seem to turn on the existence or absence of fuel to power the core. Until the center was swept clear of dust and gas, you'd expect the conditions to exist for such jets. The velocity distributions we see may be a legacy effect from all spirals having gone through such a stage, while "young", perhaps.
As for why we see some such jets even in elliptical galaxies, that is another puzzle. M87 is not a young spiral but a very old, and huge, elliptical. However, it is also at the center of the local group, and thus in a sense at the "bottom" of the local gravity well. There may be special factors recreating the conditions for core jets there. We don't know half as much about such structures as we would need to to say.
The main point is that many factors in galactic mass distribution models are still very much in play, up in the air. It is not at all obvious that anything so exotic as special "cold dark matter" is needed to explain the galactic rotation data. It is at least as likely the whole subject is a sort of epicycle hunt on the part of those who can't imagine what is wrong or missing in their galactic models. They can't see what they've left out, so they make up some correction term and ascribe it to an unknown "x", and then go looking for (or speculating about) that "x".
It really only shows that there galactic models are incomplete, and it is just as likely they have left out some dynamic effect as that there is any strange whosits floating around galaxies. Indeed, the strange whosits explanation is fundamentally incoherent on its own, because a form of mass subject to gravitational attraction but not light pressure would be expected to concentrate at galactic cores, not remain spread out into galactic halos. The proposed epicycle does not fit the data any better than the pre-existing models. Either way they need spread out matter, and they are no closer to knowing why it would be spread out by imagining it is some strange whosits than by imagining it is diffuse ordinary gas clouds.
There almost has to be a catch, though. Here's my best guess: the concentration of ordinary dust-like stuff you're talking about would be too dense to be stable. It would condense readily to form more visible stars higher out and the galactic rotation wouldn't be mysterious, given what you can see. But--as I was doing with my black hole question--I'm only reasoning backwards from the fact that all the great minds of astronomy/cosmology/physics are pretty puzzled on this question just now.
I mean "re-emission." Electrons absorb photons and get excited, jumping up a level or two. Randomly, they calm down, emitting photons of the proper energy as they resettle. Both the absorption and the emission leave a spectral signature when you look at a star with dust or gas in front of it.
ThinkPlease (whom we should have pinged long before this); your thoughts on #113.....
I'm not exactly sure what this poster is getting at, it seems to be a stream of consciousness post, with some technical terms thrown in for good measure.
Let me respond with a few stream of consciousness points.
1. The current mass distribution has been mostly determined by 21 cm observations of neutral hydrogen clouds. This light is not reradiated light, but is a spin transition of the proton or electron in hydrogen. Since there is much more mass in neutral hydrogen than in any other source(until dark matter came along, that is), AND it is detectable to great distances, it's a great tracer for matter, among other things. With geometry, you can find the distances to other things inside the Solar Radius (the area inside the circle drawn by the sun in one orbit (about 8.5 kpc, or 33,000 light years).
2. The problem lies in that we cannot see dark matter at any wavelength, yet the dynamical observations say it is there in our galaxy, and the dynamical observations of other clusters say that it is there. At this point in Astronomy, we have had telescopes in orbit or on the ground that have surveyed the sky in every wavelength to a high degree of depth, and we haven't seen any widespread phenomenon that is indicative of dark matter, in any sort of distribution. It just isn't there.
3. Now, as you say, what about jets? Well, the good news is that people agree with you. The standard model is beginning to account for a black hole that is periodically fed by stars at the end of the evolutionary track, which then stimulations more star formation, with periods of quiescence. A good paper is here. There is something to be said for that idea. But the mass output by a jet isn't enough to account for a 10:1 ratio. There's just too much there. The black hole that would spit out that much mass would be much larger than the one we currently see.
The good thing is that the halo of our galaxy is more populated than we originally thought, with renegade stars and other detritus. There is a line of thought that I subscribe to that is beginning to grow wings: It is believed that the galaxy has gone through several merger events over the past billion or so years (Sagittarius galaxy), and several near misses (Magellanic Clouds). Those effects probably have had a significant effect on the rotation curve at large radii. It still isn't enough to account for 90% of the current mass, but it is a start. Things definitely aren't as simple as they look. As we get detectors that are more sensitive, we should get better and better at seeing things farther and farther away. So, the answer is: The answer will come.
I'm particularly interested in anything concerning why the plume would be coming from just one side of the center of the galaxy.
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