quote:Determinations of the rotation speed of stars in galaxies (galactic rotation curves) based on the assumption that Newtonian gravity is a good approximation have led to the inference that a large amount of dark matter must be present - more than can be accounted for by non-luminous baryonic matter. While there are plenty of attractive theoretical candidates for the additional dark matter, such as a lightest supersymmetric particle (LSP), it is also interesting to look into the details of the calculations that suggest the need for such exotica. Now F I Cooperstock and S Tieu of the University of Victoria have reworked the problem using general relativity in place of Newtonian gravity, and they find no need to assume the existence of a halo of exotic dark matter to fit the observed rotation curves. This is because even for weak fields and slow speeds, well-known nonlinearities change the character of the solution dramatically. The success of Newtonian mechanics in situations like our solar system can be traced to the fact that in this case the planets are basically "test particles", which do not contribute significantly to the overall field. However, in a galaxy this approximation is not a good one - all the rotating matter is also the source of the gravitational field in which everything rotates.
Anyone care to provide some additional insight to us physics laymen?
Posts: 26071 | Registered: Oct 2003
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Yeah, all of the people who've been called kooks over the past several decades for rejecting the "dark matter" voodoo explanation can now look up, bleary-eyed, and mutter, "I friggin' told you that already."
Personally, I'm waiting for the day that they finally get back to the electrical explanation for stars and for planet formation. The TMBG song may be cute, but I suspect the sun is not a mass of incandescent gas.
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That looks like English, but for someone with a pretty good vocabulary (who took 5 physics in college) I don't recognize but about 14 words in that quote. Including prepositions.
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I didn't take 5 physics in college and I get at least the general gist of it. Maybe it's because I have some interest in astronomy though.
Posts: 1960 | Registered: May 2005
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So basically the curves of rotations of stars (I assume that by this they mean their paths, not their spin) in our galaxy do not require any "extra" dark matter or unknown particles ("exotica") to make sense.
And this is because everything in the galaxy counts as sources of gravity, unlike our solar system in which only the sun really matters in the paths of the planets.
quote:Determinations of the rotation speed of stars in galaxies (galactic rotation curves) based on the assumption that Newtonian gravity is a good approximation have led to the inference that a large amount of dark matter must be present - more than can be accounted for by non-luminous baryonic matter.
The basic thing here is that the physics you learned in high school is based on Sir Isaac Newton and his laws of motion. But modern physics uses Einstein's stuff as well, and says (more or less) that Newton's conclusions were good, but only in a certain range. When you get into things that are too fast or two heavy or otherwise far away from the world we usually live in, Newton doesn't work any more.
Even so, since we rarely try to exceed those limits, except in science fiction, Newton is usually good enough.
"The rotation speed of stars in galaxies" is pretty simple. That's how fast stars rotate around the center of a galaxy.
"The assumption that Newtonian gravity is a good approximation" is what I just described.
"Non-luminous baryonic matter". Well, non-luminous means dark. Baryonic matter is neutrons and protons. Regular matter, in other words. So they're saying that regular matter that isn't luminous isn't enough to account for what they think they know. So there must be matter that's not regular and isn't luminous. They call that "dark matter".
quote:While there are plenty of attractive theoretical candidates for the additional dark matter, such as a lightest supersymmetric particle (LSP),
Jargon. Physicists have been looking for a particle they call the lightest supersymmetric particle. They haven't decided what it is, but they're sure it exists.
quote:it is also interesting to look into the details of the calculations that suggest the need for such exotica.
"Exotica" here meaning bizarre and way-out theories based on other theories, rather than anything that's ever been observed or detected. It's a case of the pot calling the kettle black, in my opinion.
quote:Now F I Cooperstock and S Tieu of the University of Victoria have reworked the problem using general relativity in place of Newtonian gravity, and they find no need to assume the existence of a halo of exotic dark matter to fit the observed rotation curves.
At this point, it starts to make sense. By using Einstein, instead of Newton, they claim to have an answer that doesn't require assuming that there's undetected and undetectable "dark matter" out there.
A lot of people have always thought that "dark matter" was a ridiculous deus ex machina solution that only came about because physicists were unable to say, "We don't know the answer just now, but we're working on it." It's nice to see that it's going the way of phlogiston.
quote:This is because even for weak fields and slow speeds, well-known nonlinearities change the character of the solution dramatically.
This means that there are differences between Einsteinian and Newtonian physics even without trying to approach the speed of light.
Kinda makes you wonder why it never occurred to anyone before to try using what they consider to be the real equations, rather than just approximations. There should be some serious embarrassment out there if this turns out to be correct.
quote:The success of Newtonian mechanics in situations like our solar system can be traced to the fact that in this case the planets are basically "test particles", which do not contribute significantly to the overall field.
In other words, our planets are a drop in the ocean of our galaxy, and aren't significant enough to make Newton's physics glitch.
quote:However, in a galaxy this approximation is not a good one - all the rotating matter is also the source of the gravitational field in which everything rotates.
Unlike our solar system, where the vast majority of the mass is in the sun, so that the planets are trivial in comparison, in a galaxy, it's the suns themselves that we're dealing with. They are the mass, and not just a bunch of rocks like our planets.
But you know, I don't think they intended that to be intelligible. Takes away from the mystique.
quote:Originally posted by Teshi: So basically the curves of rotations of stars (I assume that by this they mean their paths, not their spin) in our galaxy do not require any "extra" dark matter or unknown particles ("exotica") to make sense.
And this is because everything in the galaxy counts as sources of gravity, unlike our solar system in which only the sun really matters in the paths of the planets.
This whole "dark matter" thing is the modern equivalent of Einstein's "ether". It's essentially a placeholder in an equation so everything comes out neat.
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quote:Originally posted by Tante Shvester: Golly, Lisa. Do YOU get the CERN newsletter?
Gawd, no. That stuff makes my eyes water. And the religious awe given to CERN and the Church of the Peer Review just aren't my cup of tea.
I got a B and a C in freshman physics, and an A in intro to quantum. Mostly because that was all math. But it was like in one ear and out the other for most of it. I probably retained more from The Physics of Trek. <grin>
Posts: 12266 | Registered: Jul 2005
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What blows my mind is that with all the hubbub about dark matter, I guess I had always assumed they were using Einsteinian equations to determine galactic velocities.
The fact that this is news, is news.
That said:
quote:A lot of people have always thought that "dark matter" was a ridiculous deus ex machina solution that only came about because physicists were unable to say, "We don't know the answer just now, but we're working on it." It's nice to see that it's going the way of phlogiston.
There's an awful lot of science that comes from looking for things that ought to be there, based on the calculations. Black holes, for one, and Einstein's cosmological constant for another. Things go in and out of vogue, but it doesn't mean it's deus ex machina. Just a term that balances the equation.
Posts: 3735 | Registered: Mar 2002
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Check this out. Josh is a friend of mine. I sent him the CERN link (which he's almost certainly read already) and asked him what he thought. I'm not 100% sure I'll be able to fully parse his answer, since he has an exaggerated idea of what I'm able to understand, but it should be interesting.
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Glenn, I'm not sure I agree. This wasn't just a term. It was an assertion about a physical substance existing solely on the basis that making that assumption gave a term that would balance the equation.
See the difference?
Had they, instead, said, "Well, this can't be right", I can assure you that anyone suggesting "dark matter" would have been treated as though they were hypothesizing angels, and consigned to the crackpot heap.
Dark matter was not discussed as a hypothesis. It was not discussed as a theory. It was not discussed as a fudge factor. It was enthroned as scientific fact. That disturbs me. It should disturb you. How many other scientific facts are fudge factors?
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As a matter of fact, I do keep a link to the CERN Courier handy. Thanks for the heads up.
And like GlennArnold, the big surprise to me is that the Einsteinian calculations weren't done earlier; that all the "There must be dark matter cuz there ain't no other explanation." was based on Gallilean relativity and Newtonian gravity. Makes me wanna say, "Hey, Joe. Tell me it ain't so."
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Yea, I got the general gist, but then thought I must be missing something, because surely that had already been done.
Posts: 4655 | Registered: Jan 2002
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quote: Dark matter was not discussed as a hypothesis. It was not discussed as a theory. It was not discussed as a fudge factor. It was enthroned as scientific fact.
It was? Admittingly I don't follow all of the latest science news but every mention or discussion of dark matter I've seen treated it as a theory or hypothesis. Science in general does not seem to be in the habit of declaring things "scientific facts." It's more like "This is the best theory we have currently."
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Everything I read by scientists re: dark matter talked about trying to find evidence for the dark matter that had been predicted by theory.
Certainly lots of sensationalist journalism surrounded dark matter, but there's not much scientists can do about that.
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I don't think we should be so judgemental of previously held beliefs. Just because something is suddenly proved less right than wrong doesn't mean everyone who believed it is an idiot or an object of ridicule. And it doesn't mean that everyone who didn't believe it should be hailed as a genius or someone of great foresight.
Science changes all the time. In our journey to find out about our universe we are going to have to make assumptions and we are going to have to believe them until evidence to the contrary comes along.
We humans have believed all sorts of things- some of the greatest scientists in history have made incorrect assumptions and beliefs. Do not condemn people for daring to be wrong in the bid to be right.
quote:How many other scientific facts are fudge factors?
This is not the place to put your soapbox, Lisa.
Posts: 8473 | Registered: Apr 2003
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Dark matter was discussed precisely as a hypothesis. And looking for it is (or was) a matter of testing that hypothesis. It was never put forth as fact, because it was never found.
That said, I'd be cautious about jumping the gun based on this one article. As I said before, something is not right here if no one else noticed that these calculations had never been done with Einsteinian calculations.
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Glenn Arnold, you are right. This is indeed only one article and I'm seeing no report of it elsewhere- say, in the news.
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It is not necessarily suspect that these calculations have never been done before using Einsteinian physics. Einsteinian physics are highly non-linear which means that solving the equations is both mathematically and computationally challenging. Doing the calculations using Einsteinian assumption may only recently have become possible.
I read through the paper but I am not familiar enough with this field to know whether the equations became solvable as a result of some clever mathematical trick or simplifying assumptions or the solution became possible because of advances in computer power. At any rate, people should understand that knowing the equations that govern Einsteinian physics is not sufficient to be able to apply these equations to the solution of a particular problem. It is not really suprising that these calculations haven't been done before.
What is suprising is that including the non-linearities of Einsteinian physics makes such a huge difference.
Posts: 12591 | Registered: Jan 2000
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Good points, Rabbit. Thanks for reminding me.
However, in the last ~12years, computational application of Einsteinian physics to extreme relativistic modeling -- including for rotating self-gravitating fluids -- has become so common that I remain surprised that no one has applied Einstein's equations to the galactic rotation problem before now. But as you seem to imply, perhaps Cooperstock and Tieu are mathemagicians who tour de forced Bullwinkle's rabbit out of Einstein's hat. Even after nearly 90years, it still happens on occasion.
quote: I don't recognize but about 14 words in that quote. Including prepositions.
El JT, watch Firefly much???
::Wonders if she won't be able to sell off her Astronomy textbook next year because of new editions::
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I think it was your word choice, specifically, "but about", although I'm not sure that's necessarily Firefly-ish...
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I think that has more to do with my southern upbringing than my Firefly allegiance. I don't have much of an accent, but my word choice gives me away every time.
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I'll also say "might oughta". As in, "You might oughta think about changing your air filter while the hood's open."
The funny thing is one of my best friends in Cali caught himself saying that and told me about it. Apparently he picked it up from me, and I didn't even realize I said it.
And just when I was starting to think I was downright civilized.
Posts: 5462 | Registered: Apr 2005
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Back to dark matter. I was fairly certain its existence was not fully accepted as was claimed in this thread, but speculative. So I asked my father (a mathematical physicist) to clarify for me. He said,
quote: "Dark matter" is shorthand that according to established theory (mainly general relativity) the rate of Hubble expansion lets one conclude a certain matter density that is about 3 times (I think 3 is the number but maybe it's 4 or 2.5) what we can account for with the observed matter. Dark matter is shorthand for this puzzle. Any given explanation (including some that change general relativity to resolve the problem!) is currently speculative.
So, while interesting, this article is not really groundbreaking.
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