The fate of the Earth?
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From: alien8er on 17 Sep 2009 15:59 On Sep 15, 7:55 pm, dow <williamsdavi...(a)gmail.com> wrote: > > > Put two objects in space, far from any others, so that the > distance > > > > between them is initially constant. Let them go and watch what > > > happens. If they move directly toward each other, attracted by their > > > gravity, and eventually collide exactly centrally, then they were > > > initially not revolving around each other. They were rotationally > > > stationary in an absolute sense. But if they go into orbit around > > > their common centre of gravity, they were initially revolving in an > > > absolute sense. It has nothing to do with fixed stars - maybe. > > > The second case is simply physically impossible. It requires one or > > both objects be given a lateral component of motion at release. > > The lateral component need not exist if the frame of reference is > rotating in an absolute sense. If the frame is rotating, both objects > can initially be stationary in it, but will still end up in orbit > after release. That's called "moving the goal posts". And no, they need not necessarily end up in mutual orbit. > > Now, one for you. Cause the objects to rotate around their own > > centers before release, on axes perpendicular to a line drawn between > > their centers. What happens? > > Are hou getting around to frame dragging? No, strictly Newtonian (slow) motion, nonrotating frame. > > > Have you heard of Mach's Principle? > > > Yes. It's completely irrelevant. > > No it isn't. It's an observed fact that a frame of reference in which > distant galaxies are not revolving is, at least very nearly, the same > as the frame that is absolutely rotationally stationary. What do you mean "very nearly"? Either it is, or it isn't. > Mach proposed > that somehow distant galaxies define the absolutely stationary frame, > but the mechanism by which they might do so is still highly > conjectural. Do you realize that you stated this as "an observed fact" in the above paragraph? Do you have a cite for this claim? Mark L. Fergerson
From: dow on 17 Sep 2009 17:20 On Sep 17, 3:59 pm, alien8er <alien8...(a)gmail.com> wrote: > > > No it isn't. It's an observed fact that a frame of reference in which > > distant galaxies are not revolving is, at least very nearly, the same > > as the frame that is absolutely rotationally stationary. > > What do you mean "very nearly"? Either it is, or it isn't. > > > Mach proposed > > that somehow distant galaxies define the absolutely stationary frame, > > but the mechanism by which they might do so is still highly > > conjectural. > > Do you realize that you stated this as "an observed fact" in the > above paragraph? > > Do you have a cite for this claim? > > Mark L. Fergerson- Hide quoted text - > > - Show quoted text - From: www.physics.adelaide.edu.au/itp/staff/pcwd/Guardian/1994/940922Mach.html Also See: http://www.jse.com/haisch/zpf.html -------------------------------------------------------------------------------- Further evidence in favour of Mach's principle comes from cosmology. If rotational motion is purely relative, then it is clearly nonsensical to talk about the rotation of the universe as a whole, for with respect to what would it rotate? In Newton's theory, it is entirely possible for the entire cosmos to spin about some axis. Given that almost all astronomical systems are observed to rotate to some extent, we might expect, if Newton is right, to observe a universal rotation too. Astronomers find no evidence for a systematic rotation of the universe. Their observations imply that the universe cannot have turned by even one degree since the big bang. If rotation is absolute, the absence of a universal rotation seems to be a very special and contrived state of affairs, but if as Mach claimed it is relative, then the observations are explained. (end of quotation) OBSERVATIONS imply that the universe has rotated by less than one degree since the Big Bang, but they do not rule out a smaller rotation.So a frame of reference in which the universe as a whole is not rotating is very nearly the same as an absolutely non-rotating frame, but they may not be identical. dow
From: G=EMC^2 Glazier on 17 Sep 2009 18:12 Sam your not thinking again. Fastest rate of rotation at the equator and going towards the poles its speed of rotation is slower. At the exact center of the axist pole it is not rotating. Think man Think Read book by Asminov called "PHYSICS" and when you read a page think on what it says. Bert
From: Peter Webb on 17 Sep 2009 21:26 Further evidence in favour of Mach's principle comes from cosmology. If rotational motion is purely relative, then it is clearly nonsensical to talk about the rotation of the universe as a whole, for with respect to what would it rotate? ________________________________________ No, that does not follow. You don't need an external non-rotating part of the Universe to use as a reference, as the effects of rotation are detectable without these. This is true in both Newton and GR. The equations of GR and Newton admit solutions in which the Universe as a whole rotates, and these are in principle detectable by measuring Coriolis forces within the Universe. No outside non-rotating frame is logically required. In Newton's theory, it is entirely possible for the entire cosmos to spin about some axis. Given that almost all astronomical systems are observed to rotate to some extent, we might expect, if Newton is right, to observe a universal rotation too. ___________________________________ Not neccesarily. If the Universe was born non-rotating, it will remain so, through conservation of angular momentum. A value of zero for the angular momentum of the Universe at the Big Bang is very plausible, but is not neccesarily the case. Astronomers find no evidence for a systematic rotation of the universe. Their observations imply that the universe cannot have turned by even one degree since the big bang. If rotation is absolute, the absence of a universal rotation seems to be a very special and contrived state of affairs, but if as Mach claimed it is relative, then the observations are explained. (end of quotation) OBSERVATIONS imply that the universe has rotated by less than one degree since the Big Bang, but they do not rule out a smaller rotation.So a frame of reference in which the universe as a whole is not rotating is very nearly the same as an absolutely non-rotating frame, but they may not be identical. dow ___________________________ You seem hung up on rotation implying there is some external frame of reference which does not rotate which can be used a reference. No such reference frame is necessary. If I was on that rotating children's roundabout, but it had walls all around it so I couldn't see the outside world, I could still determine that I was on a rotating platform using experiments entirely on the roundabout. No outside world is needed to provide a non-rotating frame for comparison purposes. I am not surprised that the measurements show that the Universe is not currently rotating to any significant degree. If it is rotating now, then by conservation of angular momentum it must always have rotated with that same angular momentum. When the Universe was very young and very small, even a small amount of angular momentum translates into a very high rotation speed indeed. This would seriously screw up models of the early Universe, a lot would have to be changed if the Universe was determined to have non-zero angular momentum. It is possible, but unlikely.
From: Sam Wormley on 17 Sep 2009 23:32
G=EMC^2 Glazier wrote: > Sam your not thinking again. Fastest rate of rotation at the equator > and going towards the poles its speed of rotation is slower. At the > exact center of the axist pole it is not rotating. Think man Think Read > book by Asminov called "PHYSICS" and when you read a page think on what > it says. Bert > Herb used the word "rotation". http://mathworld.wolfram.com/Rotation.html Rotation Rate is use defined in radians per unit time. |