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Post by mrsonde on Mar 18, 2015 9:03:55 GMT 1
The evidence for dark matter - the reason it's been hypothesised, more or less. Spiral galaxy: A cyclone:
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Post by mrsonde on Mar 20, 2015 8:24:18 GMT 1
Is no one going to say: ah, but these are two very different phenomena? And you can explain the shape of a cyclone because of such and such, that doesn't exist in the case of a spiral galaxy, so in that case we need to hypothesise this entirely novel form of matter which no one has ever seen and in fact has the mysterious property that it can'tbe seen in order to explain this otherwise totally inexplicable shape?
What a shame. I'm very curious to know what this such-and-such might be.
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Post by fascinating on Mar 20, 2015 10:08:22 GMT 1
The spiral itself is not exactly a mystery. In the case of the cyclone, I guess that thermal energy has caused the air to spin and drag clouds along with it, and as the rate of spin is slower away from the centre, the clouds there lag behind so that instead of concentric rings of clouds you get the spiral pattern. The spiral galaxy is a lump of matter in (what appears to be) a vacuum. The mystery is, how is it held together? Why don't the stars on the outer limbs just fly off into space. From what we can directly observe, there isn't enough mass in the galaxy as whole to keep it together, hence the theory that there is a halo of dark matter which provides the necessary mass.
What's your point?
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Post by mrsonde on Mar 20, 2015 20:35:22 GMT 1
The spiral itself is not exactly a mystery. In the case of the cyclone, I guess that thermal energy has caused the air to spin and drag clouds along with it, and as the rate of spin is slower away from the centre, the clouds there lag behind so that instead of concentric rings of clouds you get the spiral pattern. The spiral galaxy is a lump of matter in (what appears to be) a vacuum. Never mind your explanation of a cyclone, there's no point in quibbling about that: I assume we all agree that the spiral shape of a cyclone is not a mystery (though that is of course by no means a safe assumption: but let's just take that as a given, for the sake.) I'm more interested in what you mean by "vacuum" - an apparent one or not. You don't wish to include fields of force in that, do you? It's a mystery if you consider the space between stars a vacuum, and the only effective force between them to be gravitation. Errrr...no, that's not the mystery! The mystery is not that there should be a centrifugal force dispersing the spirtal structure - on the contrary, the mystery is that according to standard physics, the further you go from the centre, the slower the stars should rotate. The mystery is how they manage to increase their speed to maintain the overall structure. Yes, we all understand that, thanks. My point is that this is obviously a conclusion produced by not taking into account other factors other than gravitational attraction. In the same way that a cyclone in the atmosphere is explained. We don't call upon gravtational calculations to explain that phenomenon - the effects would be infinitesimal. We call upon calculations of angular momentum and fluid dynamics. Right? Your facile "appears to be a vacuum" is a patent nonsense. There is clearly a plasma throughout a galaxy, as anyone who has studied plasma physics since the mid-60s is fully aware (this does not include the majority of theoretical physicists - cosmologists - quite evidently.) As for the physics of angular momentum - I've said this before on this board, I believe (through amply demonstrated experience in interviewing Physics experts, from graduate students to fully tenured professors) that this is the least understood field there is in science. No one understands it - some of the more brainy and aged professors are honest and humble to admit as much, that's the only differentia. (I'm hoping to lure Alan into the discussion here - he lectures on the subject, he says.)
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Post by fascinating on Mar 21, 2015 9:15:35 GMT 1
"Errrr...no, that's not the mystery! ..... The mystery is how they manage to increase their speed to maintain the overall structure." I see that as the other side of the same coin. My understanding of the way gravitation acts on celestial objects is that only those objects with the exactly right amount of momentum are held in orbit. Those that didn't have enough fell into the nearest major mass, those that had more than enough went off into the distance. (So, in the Solar System, lots of mass has been lost and gone to the Oort clound, and 99% of the rest has fallen into the Sun). Those outer stars appear to have enough momentum to escape from the galaxy, but they don't.
There is a plasma throughout the galaxy is there? But not one outside the galaxy? How would that help to explain the structure of a spiral galaxy?
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Post by mrsonde on Mar 21, 2015 11:00:35 GMT 1
"Errrr...no, that's not the mystery! ..... The mystery is how they manage to increase their speed to maintain the overall structure." I see that as the other side of the same coin. How is that the "same coin"? So on your understanding the puzzle is not why the arms don't fly off into space, but why they don't fall into the centre. No - you've misunderstood the situation. Clearly they have all the momentum they need to be in the orbital positions that we see. The puzzle is how the outer stars in a spiral galaxy get enough momentum to hold those relative positions - the further out they are, the slower they should be moving. The proposed extra mass of dark matter is held to be necessary not to hold them from flying into space, but to provide that extra momentum - something like 75% more mass is required to maintain this shape than we can observe. Yes. There's a lot of dust too - but not enough to provide that missing gravitational impetus. Probably. If we ever probe outside of the galaxy we'll find out. It's only sixty years or so since we discovered the one in our solar system (decades after Zwicky's proposal of dark matter), and only thirty since it was taken seriously by the scientific establishment, as opposed to people like Parker or Arp or Alfven being mocked and marginalised as cranks. It's still virtually ignored in astrophysics and as far as I can tell cosmologists don't think about it at all. I refer you to the two photos I posted above. To explain the shape of a cyclone requires fearsome maths and no doubt has its own still to be solved conundrums, but is not as far as I'm aware so mysterious that it demands the invention of a whole new form of matter to account for - matter that totally dominates the whole universe but that no one has ever observed or seen the slightest sign of, despite decades and billions being spent trying to do so. I would suggest that should a spiral galaxy be analysed as a spinning object in a unitary fluid electromagnetic field it would turn out to be no more puzzling - other than the difficulties I've already mentioned of understanding exactly how angular momentum works in such a field, which as I've said is probably the least understood field in science. You might think you know that this is how a simple bar magnet works, for example - the spinning electric charges on electrons lining up their generated magnetic field lines, or some such. But for electrons to generate a magnetic field with the strength that they do they would have to either be the size footballs or else be spinning at several times the speed of light. Hence the invention of "spin".
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Post by mrsonde on Mar 21, 2015 11:46:34 GMT 1
Here you are look - those musings set me off on a little google, with the first reward this little beauty: "The heliospheric current sheet is a three-dimensional form of a Parker spiral that results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium"
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Post by fascinating on Mar 21, 2015 18:03:10 GMT 1
I suggest that you look at things in a different way. As to "how they get enough momentum", well, they have got it, maybe inherited billions of years previously from the momentum of the gas clouds in which they formed. They have enough momentum to hold those relative positions, yes, but they have MORE than enough, enough to make them fly off into space away from the galaxy (if, that is, the galaxy has the gravitational force which we calculate it exerts from the mass that we surmise from the structures we observe): surely the puzzle is, given the momentum they have, they don't. I'm not talking about centrifugal force here, I am simply talking about momentum, the relative motion of a body, the origin of which is unknown, and does not need to be known.
Say several comets, all of the same mass, have left the Oort cloud (after some disturbance by a distant body) and gone in the general direction of the Sun, one has some momentum but when it approaches the Sun the gravity makes it spiral in to impact with it, another has more momentum, it is diverted toward the Sun by the gravity, but it never reaches it, its momentum carries it forward so that it eventually moves away from the Sun. A third comet, it just so happens, has enough momentum so that it equals the acceleration due to the Sun's gravity; this comet should be kept in orbit (it's momentum becomes angular as it is constantly "falling" toward the Sun). Then a fourth comet, with more momentum than all of them, and coming from the same direction as the rest, mysteriously stays in orbit - that would be a puzzle.
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Post by mrsonde on Mar 22, 2015 6:54:17 GMT 1
I suggest that you look at things in a different way. As to "how they get enough momentum", well, they have got it, maybe inherited billions of years previously from the momentum of the gas clouds in which they formed. It's not rectilinear motion. These stars are in orbit. The problem is that if the situation is as you describe it, the outer stars should have either lost their orbital speed billions of years ago or, moving at the speeds that they do, have spiralled into the centre of the galaxy - taking up closer orbits given their relative speeds - billions of years ago. I don't seem to be getting through to you. You've got it the wrong way around. The momentum that we observe that they have is MORE than enough to collapse the whole structure. Instead of this glorious spiral shape, persisting through time, all we should see is an inchoate globular form, or a disc of rings like those of Saturn's. Yes. Therefore there must be an additional source of energy other than gravitational to sustain that momentum (or the gravitational force is a lot greater than we can observe.) When I asked for the differentia with the clearly parallel case of a cyclone, you suggested thermal energy, remember? The origin is irrelevant. The question is a matter of orbital position. Considered purely as a gravitational sysyem, the further from the centre the orbit the slower the orbiting body must move. Therefore there must be an additional source of accelerating impetus to account for the persistence of this shape. Now, you can suppose there must be a vast amount of a new form of matter, that has mass but does not absorb or radiate light in the normal manner, providing this additional impetus (in the same way we use the gravitation of the moon or planets or sun to accelerate space vehicles), or the whole system must be looked at as a single object, rotating as one because it's held together by the actions of another force altogether that is either unknown or has as yet been neglected or not fully understood. It would be if the orbit it stayed in was further out than a body with the same relative speed but whose orbit was closer into the sun! Instead of Pluto moving a lot slower than Mercury, it would have to be inexplicably moving very much faster! That would require an additional source of energy; or, put in another but equivalent way, an additional force other than gravitation between those bodies allowing them to retain their relative synchrony. Imagine this force to be analogous to the spokes of a bicycle wheel, the different orbits different radii from the hub. You agree that the end of the spoke fixed at the rim of the wheel is moving faster than the end fixed at the hub, don't you? The distance moved through space in the same amount of time is further? Hence they maintain their wheel shape as they turn together? Now - what is the equivalent of the spoke in this situation?
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Post by fascinating on Mar 22, 2015 10:44:09 GMT 1
Is it the centripetal force? (I have never understood that, is it any different to gravity itself?) Would that apply to spiral galaxies, or spiral cyclones?
You seem to be trying to determine what is giving those outer stars of a spiral galaxy the additional momentum that makes them move at those unexpectedly high speeds. But I thought that the problem was: at those speeds the stars, at that distance from the centre of mass of the galaxy, should leave the galaxy and go into space.
If a probe was sent to Venus and they wanted it to stay in orbit around the planet (without using additional fuel), they would choose the distance they want it to be above the planet, then calculate what speed they would need to accelerate the probe to so that it stayed there; that calculation would be using the assumed gravitational force of Venus at that point. Now suppose they got the probe there, but for some reason changed their decision, now wanting to move it on, they increase the speed of the probe, giving it more momentum. If the probe nevertheless stayed in the originally-chosen orbit, that would be a puzzle, it would imply that the gravity of Venus is bigger than they thought.
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Post by mrsonde on Mar 22, 2015 16:38:01 GMT 1
Is it the centripetal force? (I have never understood that, is it any different to gravity itself?) Would that apply to spiral galaxies, or spiral cyclones? That's actually a very deep question ma'am. Newton himself wondered very much the same thing, and could never decide...you know his pale of water thought experiment? But seriously, no. The standard run-of-the-mill question of angular momentum caused by rotation of mass will have been taken into account in these calculations, I presume. I am far less confident about the issue of how it affects a magnetohydrodynamic field, however, and vice versa. This is a particular field of interest for me, trying to understand the physical mechanism of how astrology (cosmobiology, rather than fortune-telling) works; so I know full well that it's still a huge mystery, a spanking new field of physics, that we've hardly even begun to comprehend. By "we" I mean the few scientists who are studying it - and that doesn't include any cosmologists or your standard theoretical physicist. But what is known is that it's effects have been and are consistently under-estimated. When we could measure the strength of the sun's magnetic field not so many years ago, in its interactions here with our own, it was found to be a hundredfold stronger than had been theoretically calculated - no one had taken into account the action of the interplanetary plasma in conveying that energy. I strongly suspect it hasn't been in these back-of-the-envelope (what else could it be?!) calculations about the contents of spiral galaxies either. To put it in its most simple and basic terms, dark matter has been proposed because there is, according to such calculations, an energy deficit - too little gravitational mass for too much motion. The energy within a galaxy's field, generated by a rotating black hole at its centre, interacting with stars of all varieties, with magnetic fields of their own, has not been taken into account - or if it has been to some extent, it can only be pure guesswork and, as with the plasma interctions of our own star, very likely underestimated by a factor of a hundred or more. Then again - no one knows whether there's a reciprocal dynamo effect produced by this spinning plasma, interacting with itself - theoretically, there should be, but no one knows how to measure or calculate it. It's the same enigma of how certain aspects of the solar wind manage to end up here travelling at near speed-of-light velocities, when they left at a few hundred or thousand mph - or how charged particles in the aurorae are accelerated in a similar manner, hundreds and thousands of times the speed they arrive at, or how ionspheric electrons ("whistlers") bouncing in our magnetic field from pole to pole, back and forth, manage to attain such incredible energies. Not really, I wouldn;t even begin to try - I'm not a theoretical physicist. I'm merely extremely sceptical (my default position, generally) about this dark matter and dark energy hypotheses - I've studied the history of science for too long not to be, and I'm afraid the fragmented compartmentalised nature of physics is growing worse and worse every year, and the strength of peer pressure and standardisation of approved theory more compelling, and the barriers against objections and dissenting theories ever higher - the virulence of approved "memes" in science gets all the more irresistable, if you like. My educated guess is that the majority of experimental physicists, and observational astonomers, have no confidence in the dark matter theory whatsoever - but they can't or won't say so, not only from fear of humiliation from what they must assume are more highly qualified experts, but from the real danger of damaging their future career prospects. The problem is that at that distance they shouldn't have those speeds - they should be much closer to the centre. Well - depending on the vector, of course. Accelerate without changing it would indeed be equivalent to the gravitational attraction between the probe and Venus increasing, and it would take up a closer orbit.
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Post by mrsonde on Mar 22, 2015 17:22:03 GMT 1
Oh - unless your acceleration was in a counter direction to the orbit, that is! That is, the probe effectively slowed down its orbital speed. Then it would move away from the planet, as the Moon is moving away from us by a few centimetres every year due to the tidal pull of its oblateness slowing it down.
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Post by fascinating on Mar 23, 2015 9:07:13 GMT 1
It occurs to me that I do not understand the mechanics of an orbiting system. Musing on the Earth and Moon, think that if the Earth suddenly disappeared then the Moon, having it's own momentum, would continue moving at the same relative speed, in a path tangential to its motion now. I consider the Moon to be going around the Earth by virtue of its own momentum, which is basically balanced (I'll ignore elliptical orbits for the present) by the acceleration due to gravity from the Earth. I consider that the energy of the momentum is kinetic energy, and it is slowly being lost by tidal friction, which you referred to earlier. That means the Moon will slow down. By my way of thinking, that should mean that the Moon will slowly spiral in toward the Earth, as gravity overcomes the Moon's momentum. But I am obviously wrong in that because the Moon is going further from the Earth as we speak/write.
So I ask you to explain to me why the stars at the outer parts of the galaxy should not have those speeds?
I am skeptical about Dark Matter too.
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Post by mrsonde on Mar 23, 2015 17:46:00 GMT 1
It occurs to me that I do not understand the mechanics of an orbiting system. Well, that's pretty much what I'm saying - no one does. Hmmm...well, I suspect you've been confused by this "kinetic energy" notion. Mybe, I don;t know. I never took it on board, being taught it as a given at A Level but when I asked my teacher (a vastly over-qualified scientist, in between better posts) what this meant, in comparison to "potential energy", was yold, quite correctly with hindisght, that it was purely mathematical. The analytical understanding of Einstein's GR is far superior, and generates completelt different intuitive understanding - correct ones, as far as my experience goes, so far. Oh - I can't do that! Quite clearly, they should have! They're not breaking any laws! Just the ones we've managed to figure out so far, that's all. I'm reading at the moment the very entertaining and informative memoirs of Les Woods, who was perhaps the world's leading expert in fluid dynamics and plasma physics - or, at least, he was acknowledged to be, until he started to point out that the mathematics at the heart of the subject were entirely wrong (incomplete, which amounts to the same thing.) Then of course the whole weight of the Establishment came to bear. He recounts how at the Atomic Research Est. at Harwell he investigated why in all attempts to confine plasma it escaped across the containing magnetic field at many thousands of times the rate predicted. His conlcuison was that the maths fusion physicists used was utterly inapplicable to this situation, we didn;t understand a thing about the complexity of such fluids, and that therefre the multi-billion multi-decade Tokamak program (the most expensive sceinctific research program ever undertaken in Europe) would forever be doomed to failure. This was 40 years ago, and as far as I've ever heard, no further progress has been made on that project. Pleased to hear it. I posted this because I'd watched the Horizon on the subject the night before. It would have been nice to see some of that scepticism represented, but sadly Horizon is not at all what it was in its heyday of the 70s and 80s, and as I've suggested, it would be a brave (or old, or retired) physicist who so exposed him or herself (Jocelyn Bell excepted, blessed be her name.) My overall impression, from this and other associated indications, is that physics in general and cosmology in particular is deep in one of those periodic crises, that are eventually solved by a complete and drastic revision. I hope to see such progress in my lifetime.
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