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shakran · 02-21-2005, 11:44 PM

that's one theory. . there's about a jillion of them actually. Here's the basic scoop, extremely condensed (although still freaking long) and with a lot of details left out.

Things move too fast. A good example is to look at galaxies. They rotate. One of Kepler's laws says that the orbital speed of an object relates to the radius of the orbit and the mass of the object in orbit. In other words, the earth's speed is determined by its mass and its distance from the sun - this makes sense when you remember that an orbit is just a perpetual fall. The earth is always falling toward the sun, but it's forward velocity is high enough to counteract the pull from the sun. Any faster and we'd fly away from the sun, any slower and we'd get up close and personal with the sun. And if we went the same speed but were closer to the sun where the gravity has more effect, we'd go toward the sun, AND if we were farther away from the sun than we are and kept the same speed there would be less gravity acting on us and we'd fly away from the sun. If the earth were any more or less massive that too would effect our orbit.

The trouble starts when you look at those galaxies. The outer rim of galaxies has less mass than the center (most of the matter in a galaxy is clustered in the center) so things on the outer rim should be orbiting the galactic center much slower than things near the center since they not only have less mass, but also are farther away and so feel less gravitational pull. But they don't. Instead, stuff on the outer rim rotates much faster than it should. Since the distance from the center to any other point in a galaxy is absolutely known for sure (we've measured them right) we must have the wrong idea about how much mass they contain. But we can't see any more matter that would account for the mass that they must have in order to rotate that fast, so scientists proposed the theory of dark matter, or matter that we can't observe. It's not just matter that doesn't reflect any light, it's matter that doesn't reflect ANYTHING (that we know of anyway.)

There's all sorts of weird shit this dark matter could be. One thing is the neutrino. Now, we know neutrinos exist, but we may be wrong about their nature. Right now the going theory is that they have NO mass at all, but some are starting to think they might have a very small mass. We're talkin' miniscule here, but neutrinos are incredibly numerous throughout the galaxy (hundreds of thousands go howling through your body every second at the speed of light, and by the time you finish this sentence, they'll be exiting the other side of the earth), so a boatload of small-mass particles adds up to a lot of extra matter, IF they have mass. (if you're interested in how we know these things exist when they have no measurable mass and therefore shouldn't be visible, check http://amanda.physics.wisc.edu/ which is a site about a fascinating neutrino detector buried deep under Antartica. The short answer, though, is that every GREAT once in awhile a neutrino will manage to hit a particle of regular matter, and when it does, a little light is given off. This detector has managed to see the trails of light caused by neutrino - matter collisions)

The really interesting theory that ranks about a 9.5 on the weird shit-o-meter is the WIMP, or Weakly Interacting Massive Particle. This is totally theoretical - we have no evidence they exist, but they would explain the extra mass in the universe, especially if the neutrino proved to be massless. Basically these things would be freaking heavy particles that barely interact with other matter. In other words, you couldn't see touch or smell them, and you could pass right through them and never know it, but their mass would still contribute to the mass of the universe and therefore account for the observed mass vs. observed effects of mass paradox.

Of course, another possibility is that all that stuff is wrong and that we just don't have a good grasp on how gravity really works. If that's the case, Kepler's laws aren't accurate, at least not on a galactic scale, so the galaxies could be rotating at a perfectly logical speed without needing any more mass than what we have already observed.

02-21-2005, 11:44 PM	#3 (permalink)
shakran Tone.	that's one theory. . there's about a jillion of them actually. Here's the basic scoop, extremely condensed (although still freaking long) and with a lot of details left out. Things move too fast. A good example is to look at galaxies. They rotate. One of Kepler's laws says that the orbital speed of an object relates to the radius of the orbit and the mass of the object in orbit. In other words, the earth's speed is determined by its mass and its distance from the sun - this makes sense when you remember that an orbit is just a perpetual fall. The earth is always falling toward the sun, but it's forward velocity is high enough to counteract the pull from the sun. Any faster and we'd fly away from the sun, any slower and we'd get up close and personal with the sun. And if we went the same speed but were closer to the sun where the gravity has more effect, we'd go toward the sun, AND if we were farther away from the sun than we are and kept the same speed there would be less gravity acting on us and we'd fly away from the sun. If the earth were any more or less massive that too would effect our orbit. The trouble starts when you look at those galaxies. The outer rim of galaxies has less mass than the center (most of the matter in a galaxy is clustered in the center) so things on the outer rim should be orbiting the galactic center much slower than things near the center since they not only have less mass, but also are farther away and so feel less gravitational pull. But they don't. Instead, stuff on the outer rim rotates much faster than it should. Since the distance from the center to any other point in a galaxy is absolutely known for sure (we've measured them right) we must have the wrong idea about how much mass they contain. But we can't see any more matter that would account for the mass that they must have in order to rotate that fast, so scientists proposed the theory of dark matter, or matter that we can't observe. It's not just matter that doesn't reflect any light, it's matter that doesn't reflect ANYTHING (that we know of anyway.) There's all sorts of weird shit this dark matter could be. One thing is the neutrino. Now, we know neutrinos exist, but we may be wrong about their nature. Right now the going theory is that they have NO mass at all, but some are starting to think they might have a very small mass. We're talkin' miniscule here, but neutrinos are incredibly numerous throughout the galaxy (hundreds of thousands go howling through your body every second at the speed of light, and by the time you finish this sentence, they'll be exiting the other side of the earth), so a boatload of small-mass particles adds up to a lot of extra matter, IF they have mass. (if you're interested in how we know these things exist when they have no measurable mass and therefore shouldn't be visible, check http://amanda.physics.wisc.edu/ which is a site about a fascinating neutrino detector buried deep under Antartica. The short answer, though, is that every GREAT once in awhile a neutrino will manage to hit a particle of regular matter, and when it does, a little light is given off. This detector has managed to see the trails of light caused by neutrino - matter collisions) The really interesting theory that ranks about a 9.5 on the weird shit-o-meter is the WIMP, or Weakly Interacting Massive Particle. This is totally theoretical - we have no evidence they exist, but they would explain the extra mass in the universe, especially if the neutrino proved to be massless. Basically these things would be freaking heavy particles that barely interact with other matter. In other words, you couldn't see touch or smell them, and you could pass right through them and never know it, but their mass would still contribute to the mass of the universe and therefore account for the observed mass vs. observed effects of mass paradox. Of course, another possibility is that all that stuff is wrong and that we just don't have a good grasp on how gravity really works. If that's the case, Kepler's laws aren't accurate, at least not on a galactic scale, so the galaxies could be rotating at a perfectly logical speed without needing any more mass than what we have already observed. Last edited by shakran; 02-21-2005 at 11:48 PM..