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Author  Topic: Purple dosn't beleve in relativity  
RAZD Member Posts: 19816 From: the other end of the sidewalk Joined: Member Rating: 10.0 
enjoy. we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist
 
Sylas Member (Idle past 3368 days) Posts: 766 From: Newcastle, Australia Joined: 
This means that if you are inside a uniformly dense SPHERE of matter, then you only experience a force related to the amount of mass in the sphere that is closer to the center than you are. That is, drill a tunnel right through the Earth. Let g be the force you experience on the surface, and R be the radius of the earth. Then the force you feel when you are in the tunnel at a distance of r from the center of the Earth should be g*r/R. The effective mass is proportional to r^{3}, and the normal distance relation scales the force as r^{2}. However, I have been persuaded by comments from others that the gravitational time dilations at a certain depth are not directly related to the force you experience at that point. A thought experiment. A photon falling into a gravitational well will change wavelength; and this can be related to time dilation. But on passing through a shell of matter into an interior cavity, the photon should not suddenly chance frequency, but will remain at the frequency it had just prior to pentrating the shell. This implies that a clock at the center of the Earth will indeed run more slowly than one at a great distance. I think. Thanks all for the input on this; I've learned something. Cheers  Sylas This message has been edited by Sylas, 12142004 03:57 AM
 
contracycle Inactive Member 
quote: Actually I was kinda working from that  the sphere of the earth must exert gravity at the point of the wall you are passing, no? Nevertheless, you fall toward and through the centre of the sphere and oscillate around that.
 
Raymon Inactive Member 
Yes. And yes, presuming your hole goes through the center of the earth. It doesn't have to.
 
Raymon Inactive Member 
 
teratogenome Inactive Member 
The only other thing I can think of is some sort of dark matter crap. Even that would preclude a big bang unless it's repulsive force (per "mass" or "amount" ) diminishes over distance at a different rate than gravity and thereby overcomes it when normal matter becomes significantly diffused. Even then I don't see how it could come to be incorporated into the initial big bang material.
 
The Dread Dormammu Inactive Member 
First of all the milky way DOES appear to be in the center of our hubble volume (meaning observable universe) ALL the other galaxys seem to be moving away from US at increasing speed depending on their distance. This is due to the fact that space itself is expanding. I do not understand your question about their time being compressed relative to us, please elaborate.
 
PurpleYouko Member Posts: 713 From: Columbia Missouri Joined: 
I don't understand this statement. I have never really got this part of it at all. If space is made of nothing then how can it expand? Surely for something to expand, it would have to have something there which is actually doing the expansion. Vacuum can't exert a pressure to accelerate anything. It must take a lot of energy to accelerate a galaxy. Where is that energy coming from? I don't understand how exapnading nothingness can drag real things with it. If by an expanding universe you mean something like an equal increase in the the size of all space then wouldn't that mean that an observer would be expanding at the same rate as everything that he observed? Wouldn't this also make it impossible for him to actually observe the expansion? Or is the universe only expanding at the edges? Confused? I sure am. PY
 
Sylas Member (Idle past 3368 days) Posts: 766 From: Newcastle, Australia Joined: 
You can have different amounts of nothing. We call this "distance". Consider two objects, at rest, which are a certain distance from each other. If space expands, then the distance between them increases. This is what we observe galaxies doing. Of course, if the two objects are held together by, say, a piece of string, then they will remain at the same separation (although there will be some tension in the string). Same applies for objects held together by gravity. Hence the Milky Way galaxy itself is not expanding. It is gravitationally bound, and does not expand. Put another way; motions and forces in space can overcome the expansion of space to hold things together in a local association.
Curiously, a vacuum does exert pressure; though this has nothing to do with the expansion of space. It is called the Casimir Effect, predicted from quantum mechanics in 1948 and measured in 1996 to confirm the prediction. To measure this pressure, we need two plates very very close together in a vacuum. They experience an attractive force, because there is, in a sense, less vacuum between the plates, and the vacuum on the otherside pushes them together. Actually, the pressure is due to the existence of socalled virtual particles which pop in and out of existence continually in the vacuum. The very small space between conducting plates) limits the possibilities for virtual particles. The point of this example is that modern physics turns out to be rather unintuitive. Out intuitions about "nothing" and "space" and so on are frequently a poor guide to how the world actually works at the most fundamental levels of physical laws. The vacuum pressure has nothing to do with the expansion of space. (Actually, that may not quite be true; but the connection is indirect. Expansion of space is a consequence of general relativity, and the energy bound up in the vacuum, which is expressed as virtual particles, makes a difference to the equations and the rates of expansion or contraction of space.) Expansion of space is not about any material objects being accelerated by forces. A force accelerates objects in space. The expansion of space merely increases the distance between things at rest. Note that it makes perfect sense to speak of an increasing distance between things at rest, but only if you consider the space between things to be increasing. That is exactly what is predicted by general relativity. What we observe in the universe is consistent with those predictions.
I don't understand this paragraph. No media or aether is involved. One of the things which is inconsistent with the notion of an aether is that the expansion of space means that every point sees the rest of the universe moving away from it, at a speed that is proportional to distance. I'm using "speed" here advisedly; bearing in mind that this is not a rate of motion through space, but a rate at which separation distances are increasing. The Hubble constant, which measures the rate of expansion, is H_{0} = 71 km/sec/Mparsec This means that the distance between two objects that are a MegaParsec apart from each other is increasing at a rate of 71 kilometers every second. The speed of light is 3*10^{5} km/sec. Hence if two objects are about 4200 MegaParsecs apart, then the rate at which their separation distance is increasing is the speed of light. If objects are 5000 MegaParsecs apart, then the distance between them increases by more than the speed of light. This would be a violation of relativity, if expressed as a motion of objects in space. However, there is no violation, because the increasing separations is not motion, but expansion of space. I'm not kidding; this really is the basics of general relativity in an expanding space. A parsec, by the way, is about 3.26 light years, so 4200 MegaParsecs is 13.7 billion light years; the age of the universe. That is, the measured rate of expansion of space, extrapolated backwards, means that 13.7 billion years ago there was no space. This is the famous singularity at the start of the Big Bang.
This is a very good question. The short answer is that things can be held together over small regions of space, so that they remain the same volume. You are probably about 1.7 meters tall. A parsec is 3.1*10^{16} meters, so you are about 5.5*10^{23} MegaParces tall. The expansion of space means that the distance from head to toe will tend to increase by 71000*5.5*10^{23} = 3.9*10^{18} meters every second. That is not much, and the forces that hold your body together overwhelm this expansion to keep you from expanding. So no, the universe is not expanding only at the edges. It is expanding throughout all of space. The expansion of space means that the distance between objects at rest tends to increase. For objects that are close together, this increase in separation distance is very small, and is overwhelmed by forces and local motions in space. But for objects that are very far apart, the rate of increase in separation distance is far too great for any local motions or forces to overcome it. Cheers  Sylas This message has been edited by Sylas, 12152004 07:18 PM
 
JonF Member Posts: 4560 Joined: Member Rating: 4.4 
PurpleYouko initially thinks that there must be some "thing" expanding, and follows that thought to a logical conclusion which is suspiciously like aether, and is then puzzled because either the initial thought is wrong or there must be an aether ... neither possibility seems attractive.
 
PurpleYouko Member Posts: 713 From: Columbia Missouri Joined: 
Thanks Jon. That summed it up perfectly. I guess my difficulty in understanding all this is that (like most people) I think intuitively and that appears not to jazz with the explanations given by people who are obviously much more knowledgable about the actual mechanics and mathematics of the situation. I think my biggest nonunderstanding has to be the bit about local forces overcoming expansion of space. Does any of this logic make sense? Hopefully Sylas or someone else can shed some light here. PY
 
Sylas Member (Idle past 3368 days) Posts: 766 From: Newcastle, Australia Joined: 
No problem; I quite understand. It's the same for me as I continue to try learning about aspects of this. I am not a physicist, so I quickly get out of my depth when attempting to explain, and I'm still reading and trying to get to grips with various aspects of the matter. I have read fairly widely on the subject, both technical and popular literature; but I still don't really get the maths of general relativity; so I still need to deal with that qualitatively. This is all amateur explanation; and I will try to give a feel for where I am unsure and extrapolating beyond what I really know of the models.
My understanding is that local forces do not prevent expanding. They just act, as they have always done, to move things around. Don't think of the local force as preventing expansion of space, but of moving things through an expanding space so that they remain at about the same separation. The Earth itself is a ball of matter. Its size is balanced between the attraction of gravity holding it together, and the primarily electromagnetic forces at the levels of atoms to hold particles apart from each other. If somehow space expanded very rapidly, so that the space within which the Earth resides expanded by 2% over about a minute, then you would suddenly have Earth being 2% larger. But it would then very quickly collapse again back to about its present size, due to the forces of gravity. Hence the expansion of space does not change the size of the Earth. As a minor mathematical aside, the current cosmological expansion is roughly linear (measured as 71 km/sec/Mparsec). This means that as the universe gets large, it takes correspondingly longer to get a certain proportionate increase in size. At present, it takes 274 million years to get a 2% increase in size; but as time passes it will take longer to get a 2% increase. The kind of expansion in which space expands by a fixed proportion per unit time is very different; an exponential expansion. Most cosmologists believe that there was this kind of expansion very briefly and very early in the history of the universe; it is called inflation. The case for the orbit of the Earth is a bit more subtle; but it can be calculated. The expansion of space has effects analogous to a kind of pseudoforce acting to increase the radius of the Earth's orbit. The effect is much more complex than simply increasing orbit size by an amount relating to the amount of increased space, because the Earth is in constant motion. We have to calculate some rather hairy differential equations to combine the force of gravity with the expanding space. The calculations are available here: The influence of the cosmological expansion on local systems, Basically, the effect of expansion is to perturb an orbit to increase the radius and decrease the orbital period. The effect on the scale of the EarthSun system is small, to say the least. Over the life span of the solar system, the Earth orbit should be expected to increase by a fraction of about 10^{24}. This is less than than changes in orbit due to tidal effects. The effects of cosmic expansion can really only be detected at scales beyond that of our galaxy. On smaller scales, bodies are in constant motion under forces of gravity that hold them together against the expansion of the space in which they are embedded. The space still expands, but bodies move through space under the influence of gravitational forces to maintain the same separation to within the bounds of measurement. See also the usenet physics FAQ answer to this question: If the universe is expanding, does that mean atoms are getting bigger? Is the Solar System expanding? Here is the introduction. (I love the quotes from Annie Hall!)
This FAQ also points out that the actual metrics for expansion of space are fantastically complicated. The basic FRW solution for the universe as a whole is a very simple approximation, that gives a uniform rate of expansion through all of space. This same uniform expansion is assumed in the calculated orbit perturbations to which I allude above; but in a local mass concentration rates of expansion will vary; and calculating this completely is intractible. I do not know if the rate of expansion is locally greater or smaller; but in any case it is still bound to have insigificant consequences at the scale of our solar system. And finally, just for fun to really blow your minds. There is a speculative theoretical model for an accelerating expansion of the universe, in which the rate of acceleration increases. Let me introduce to you all the Big Rip; a model developed last year by Robert R. Caldwell, Marc Kamionkowski, and Nevin N. Weinberg. This has a singularity in the future; but not by a collapse back to infinite density. The future singularity involves an expansion so rapid that eventually atoms themselves cannot hold together against its effects. Here is a timeline from the formal paper at Phantom Energy and Cosmic Doomsday (astroph 0302506).
Cheers  Sylas PS. Fixed the first two links in edit. Thanks Nosy; I have removed your comment from the previous fixed, and applied the fix also the the second link. Also got rid of spacing problem. This message has been edited by Sylas, 12192004 04:30 AM
 
NosyNed Member Posts: 8842 From: Canada Joined: Member Rating: 7.4 
Btw Thanks to all for this, I had never thought to ask the question about local effects of cosmic expansion. I just arm waved it away as "small". ABE This message has been edited by NosyNed, 12162004 07:29 PM
 
JonF Member Posts: 4560 Joined: Member Rating: 4.4 
Happens to me, too, whenever I do a table. I bet it's a bug/feature.
 
sidelined Inactive Member 
Blame it on the expansion of space upon local computers.



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