tkdguy Posted April 8, 2012 Author Report Share Posted April 8, 2012 Re: More space news! Space Taxis by 2015? A new idea on how to remove space junk Plutonium Production May Avert Spaceship Fuel Shortage More bang for your buck at CERN Quote Link to comment Share on other sites More sharing options...
SteveZilla Posted April 13, 2012 Report Share Posted April 13, 2012 Re: More space news! Not exactly space, but it did get off the ground... http://video.nationalgeographic.com/video/national-geographic-channel/all-videos/av-10788-10988/ngc-moonshine-rocket-launch/ Quote Link to comment Share on other sites More sharing options...
tkdguy Posted April 15, 2012 Author Report Share Posted April 15, 2012 Re: More space news! Back to the drawing board! Quote Link to comment Share on other sites More sharing options...
tkdguy Posted April 18, 2012 Author Report Share Posted April 18, 2012 Re: More space news! Discovery's Final Flight Quote Link to comment Share on other sites More sharing options...
L. Marcus Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! No sight of dark matter in our part of the galaxy. Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! OK, they are MOND fans. Quote Link to comment Share on other sites More sharing options...
Christopher Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! OK' date=' they are MOND fans.[/quote'] That sound rather disaprooving. As I read the atricle they noted it - because it is the only existing alternate theory for the galaxy rotation problem - but they also noted it's flaws (that it unsolves most of what dark mater solves). They also clearly said that the current data only means the dark mater isn't where we expected it to be, not that it is not there at all. Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! Yeah, I admit it sounded disapproving. MOND has been around for quite a while (I remember some papers back while I was in grad school) and has never come up with enough results to overthrow (or become a reliable add-on to) standard orbital mechanics. I admit that there hasn't been enough do rule out MOND, either. I also admit that IMO their kinematic sample (1200 stars, and only ~500 beyond 1.3 kiloparsecs out of the plane, and finally winnowed down again to 412) is too small to say anything of statistical significance about the velocity distribution that far out in the thick Disk. That is perhaps just Old Stodge on my part. Quote Link to comment Share on other sites More sharing options...
McCoy Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! Yeah' date=' I admit it sounded disapproving. MOND has been around for quite a while (I remember some papers back while I was in grad school) and has never come up with enough results to overthrow (or become a reliable add-on to) standard orbital mechanics. I admit that there hasn't been enough do rule out MOND, either.[/quote'] Yeah, I'm older than you, I remember hearing about it and thinking it was a crackpot idea that would be easily disproven. Something that would come and go as quickly as cold fusion. Still waiting. As I understand it, it's now at the awkward stage. Still not proven false, but doesn't explain enough. What experiment would settle the question? I'm thinking tracking six probes leaving the solar system in different directions for the rest of the century? Probes that would last longer and give a stronger signal that the Pioneers or Voyagers. Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! The problem is that it's real hard to get direct hard-data tests of Newtonian orbital mechanics at distances much longer than a few thousand AU. The time needed to complete an orbit is longer than the duration of telescopic observations (400 years now, and about half that for solid positional measurements). You'd like to watch something complete an orbit that big, decide whether that orbit is or is not a real ellipse, and that the run of velocity around the orbit is or is not Keplerian. Given a few thousand years of high-quality data, it's duck soup. Quote Link to comment Share on other sites More sharing options...
McCoy Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! The problem is that it's real hard to get direct hard-data tests of Newtonian orbital mechanics at distances much longer than a few thousand AU. The time needed to complete an orbit is longer than the duration of telescopic observations (400 years now' date=' and about half that for solid positional measurements). You'd like to watch something complete an orbit that big, decide whether that orbit is or is not a real ellipse, and that the run of velocity around the orbit is or is not Keplerian. Given a few thousand years of high-quality data, it's duck soup.[/quote'] So all we need is to keep watching for five to ten times as long as we already have? Piece of cake. Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! Well, depends on what length scale the departure from the inverse square law happens. If it's on scales of kiloparsecs, then the problem is more complex. Ideally you'd watch two point masses separated by kiloparsecs for a couple hundred million years, but galaxies and star clusters aren't point masses (so gravity messes with the internal structure and therefore the magnitude and direction of the integrated gravity due to the ensemble). And finding an isolated binary star that wide ... probably much harder. Quote Link to comment Share on other sites More sharing options...
McCoy Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! Well' date=' depends on what length scale the departure from the inverse square law happens. If it's on scales of kiloparsecs, then the problem is more complex. Ideally you'd watch two point masses separated by kiloparsecs for a couple hundred million years, but galaxies and star clusters aren't point masses (so gravity messes with the internal structure and therefore the magnitude and direction of the integrated gravity due to the ensemble). And finding an isolated binary star that wide ... probably much harder.[/quote'] I was thinking it was on a scale of 100's of AU, less than a light year. Thought Pioneer and the Voyagers were far enough out they might be showing some effects if we could reliably track them. Quote Link to comment Share on other sites More sharing options...
SteveZilla Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! No sight of dark matter in our part of the galaxy. If this "new type of particle" *does* react normally with gravity, why is it there are no dark matter 'planets' or 'stars' where the stuff has coalesced into a 'solid' object? Quote Link to comment Share on other sites More sharing options...
Christopher Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! If this "new type of particle" *does* react normally with gravity' date=' why is it there are no dark matter 'planets' or 'stars' where the stuff has coalesced into a 'solid' object?[/quote'] A good question. As I understand it these particles move and also that there is no force capable of condensing it. It's kinda like trying to make any Noble Gas to ice: They simply can't bind to one another, so the best you get is a liquid that is sustained only by airpressure and very low temperatures. Quote Link to comment Share on other sites More sharing options...
L. Marcus Posted April 20, 2012 Report Share Posted April 20, 2012 Re: More space news! Yah -- the only reason you don't sink through your chair is because the chair's and your butt's electrons repelling each other. With just gravity and no other forces working on two particles, they'd attract each other but never "coalesce". Quote Link to comment Share on other sites More sharing options...
SteveZilla Posted April 21, 2012 Report Share Posted April 21, 2012 Re: More space news! Soooo, they attract each other though gravity, but only until they start to get really close together and then it just stops? o.0 Quote Link to comment Share on other sites More sharing options...
McCoy Posted April 21, 2012 Report Share Posted April 21, 2012 Re: More space news! A good question. As I understand it these particles move and also that there is no force capable of condensing it. It's kinda like trying to make any Noble Gas to ice: They simply can't bind to one another, so the best you get is a liquid that is sustained only by airpressure and very low temperatures. So what happens when dark matter falls into a black hole? Quote Link to comment Share on other sites More sharing options...
dmjalund Posted April 21, 2012 Report Share Posted April 21, 2012 Re: More space news! So what happens when dark matter falls into a black hole?the usual - the black hole gets bigger. As black holes are gravity based, and gravity is the main way we detect that dark matter exists, and event horizons are not made of matter. Quote Link to comment Share on other sites More sharing options...
Old Man Posted April 21, 2012 Report Share Posted April 21, 2012 Re: More space news! Soooo' date=' they attract each other though gravity, but only until they start to get really close together and then it just stops? o.0[/quote'] Yes. Because gravity falls off as the square of the distance. The electric field also falls off as the square of the distance, but your posterior carries no aggregate charge, being made up of positive and negatively charged particles. Only when you and the chair get into very close proximity does the electromagnetic force, which is overall stronger than the gravitational, take over. Quote Link to comment Share on other sites More sharing options...
Christopher Posted April 21, 2012 Report Share Posted April 21, 2012 Re: More space news! Soooo' date=' they attract each other though gravity, but only until they start to get really close together and then it just stops? o.0[/quote'] No, Gravity still works. But obviously gravity isn't the ideal force to make a solid body*, or you we could not build a chair. As I see it most solid mater even need enough other forces to counter it's own gravitational pull. If there wasn't any force preventing matter from collapsing on itself, the entire universe would be full of black holes. *at least not anything under a certian mass and sphere shaped Quote Link to comment Share on other sites More sharing options...
SteveZilla Posted April 21, 2012 Report Share Posted April 21, 2012 Re: More space news! Quote Link to comment Share on other sites More sharing options...
tkdguy Posted April 22, 2012 Author Report Share Posted April 22, 2012 Re: More space news! The space shuttle Discovery is now officially part of the Smithsonian. Quote Link to comment Share on other sites More sharing options...
SteveZilla Posted April 22, 2012 Report Share Posted April 22, 2012 Re: More space news! No, Gravity still works. But obviously gravity isn't the ideal force to make a solid body*, or you we could not build a chair. As I see it most solid mater even need enough other forces to counter it's own gravitational pull. If there wasn't any force preventing matter from collapsing on itself, the entire universe would be full of black holes. *at least not anything under a certain mass and sphere shaped (emphasis added) Which is kinda the point of my question. If Dark Matter attracts though gravity, and does not experience other forces that would oppose gravity, why don't we 'see' (potentially huge) globs of it? But instead it is spread around, unevenly at times, but often in a non-globby (is that a word?) 'halo' around galaxies. It is almost as if it attracts regular (Baryonic) matter via gravity, but not itself... Quote Link to comment Share on other sites More sharing options...
dmjalund Posted April 22, 2012 Report Share Posted April 22, 2012 Re: More space news! (emphasis added) Which is kinda the point of my question. If Dark Matter attracts though gravity' date=' and does not experience other forces that would oppose gravity, why don't we 'see' (potentially huge) globs of it? But instead it is spread around, unevenly at times, but often in a non-globby (is that a word?) 'halo' around galaxies. It is almost as if it attracts regular (Baryonic) matter via gravity, but not itself...[/quote']for a gas to collapse, there has to be a reduction of velocity of the particles that make up the gas. A normal gas can slow down via collision of the individual particles. Dark matter cannot collide with itself, so no collapse happens. Quote Link to comment Share on other sites More sharing options...
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