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Old 3rd May 2012, 10:22 PM   #281
ben m
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Originally Posted by ynot View Post
Does the distant shell of rotating mass theory hold up when there's a stack of tops spinning in different directions at different speeds? By "stack" I mean tops on on top of each other so they all share the same axis. I was just in a toy store and saw a product called "Totem Tops" that does this. If two "shells" share the same axis and rotate in different directions why don't their opposing forces cancel each other out?
Ynot, I have no idea what mental picture you're analyzing here.

a) Start with empty space. Do you know how the laws of physics work? Is it OK to play with multiple tops, according to these laws of physics? Do you need gravity to help you spin them? No you don't.
b) Add a distant, spinning mass shell.
c) The space inside the shell is identical to case (a) in every possible way, except that if you point a telescope at the stars outside the shell, they are rotating. Other than that, all of the laws of physics are the same, so---can you still use the same tops? Can you spin them, stop them, etc., however you want? Of course you can.
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Old 3rd May 2012, 11:05 PM   #282
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Originally Posted by ben m View Post
Ynot, I have no idea what mental picture you're analyzing here.

a) Start with empty space. Do you know how the laws of physics work? Is it OK to play with multiple tops, according to these laws of physics? Do you need gravity to help you spin them? No you don't.
b) Add a distant, spinning mass shell.
c) The space inside the shell is identical to case (a) in every possible way, except that if you point a telescope at the stars outside the shell, they are rotating. Other than that, all of the laws of physics are the same, so---can you still use the same tops? Can you spin them, stop them, etc., however you want? Of course you can.
Itís not a matter if I can use and spin the top or not or whether thereís gravity or not and I have no idea why you mention such things. It seems to be a form of obfuscation. Itís purely whether I can tell if a top is spinning or not. The claim is that when I think Iím spinning a top I could actually be stopping it as it may have been previously spinning in unison with a very distant shell of spinning matter. Itís also claimed that when I think Iím observing properties created by a spinning top I could equally be observing identical properties caused by the spinning shell. I canít see in a practical sense how several tops could be concurrently spinning in two direction and at different speeds on the same axis and all be non-spinning let alone a shell or shells causing them to appear as if theyíre spinning when they arenĎt.
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Old 3rd May 2012, 11:36 PM   #283
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Originally Posted by ynot View Post
Itís purely whether I can tell if a top is spinning or not.
But that's easy enough to do: Simply pick a reference frame, and check to see if the top is spinning relative to that reference frame.

Do you have a table handy? So pick a reference frame in which the table is stationary. This shouldn't be difficult; most of us do it all the time without even thinking about it. Is the top spinning relative to the frame of the stationary table? The answer is probably obvious.

A different question is whether the frame of the stationary table is more important or "real" than other frames. The answer to that question depends on whether you're checking to see if tops are spinning relative to the table. If you are, then yes, the frame of the stationary table is extremely important. If you're not, then there's no reason to prefer that frame over any other frame.

In fact, the physics of spinning tops--along with the physics of everything else--are all the same regardless of which frame you choose.
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Old 3rd May 2012, 11:41 PM   #284
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Originally Posted by ynot View Post
Itís not a matter if I can use and spin the top or not or whether thereís gravity or not and I have no idea why you mention such things. It seems to be a form of obfuscation. Itís purely whether I can tell if a top is spinning or not. The claim is that when I think Iím spinning a top I could actually be stopping it as it may have been previously spinning in unison with a very distant shell of spinning matter. Itís also claimed that when I think Iím observing properties created by a spinning top I could equally be observing identical properties caused by the spinning shell. I canít see in a practical sense how several tops could be concurrently spinning in two direction and at different speeds on the same axis and all be non-spinning let alone a shell or shells causing them to appear as if theyíre spinning when they arenĎt.
The tops are obviously spinning with respect to each other---that's something you can measure, using local rulers, and no coordinate-system trick can obscure this. If they're spinning with respect to the table, no coordinate-system trick can obscure this. The only remaining question is: can you can find one solid reference which is absolutely not spinning?

You might think "Aha, using my centrifugoscope I have found that the table is really not spinning". You might think, "Aha, I will point my telescope at a distant star, and if the telescope is fixed on the star, and the star is not moving faster than light, then the telescope is not spinning." These are two *alternative* definitions for "what reference frame is not spinning". The point of the shells-discussion: gravitomagnetism, if present, might make these two things disagree with each other.
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Old 4th May 2012, 05:38 AM   #285
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Originally Posted by H'ethetheth View Post
Er, do you even remember what we are discussing, or why? Let me remind you:

"...imagine that the earth were standing still and that the rest of the universe were rotating around it: would its equator still bulge? Newton would have said "No".

So in this thought experiment the earth is not rotating, but universe is rotating around it. That is given. All I wanted to point out is that Newton would not have said "No", in the case that this rotating universe is anything like the one we have.
Fine. But remember this: "In the case of Gravity Probe B this is sometimes referred to as the 'missing inch' argument because space curvature shortens the circumference of the spacecraft's orbital path around the earth by 1.1 inches". Hypothetical distant shells of rotating mass twist space. The Earth's equator gets shorter, not longer. Rotating the whole of space is something different.
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Old 4th May 2012, 05:47 AM   #286
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Originally Posted by Farsight View Post
Fine. But remember this: "In the case of Gravity Probe B this is sometimes referred to as the 'missing inch' argument because space curvature shortens the circumference of the spacecraft's orbital path around the earth by 1.1 inches". Hypothetical distant shells of rotating mass twist space. The Earth's equator gets shorter, not longer. Rotating the whole of space is something different.
You've confused de Sitter precession and Lense-Thirring precession I think.
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Old 4th May 2012, 07:13 AM   #287
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Originally Posted by Farsight View Post
Fine. But remember this: "In the case of Gravity Probe B this is sometimes referred to as the 'missing inch' argument because space curvature shortens the circumference of the spacecraft's orbital path around the earth by 1.1 inches". Hypothetical distant shells of rotating mass twist space. The Earth's equator gets shorter, not longer. Rotating the whole of space is something different.
Yet rotating the whole of space is what NASA is talking about in that sentence. I can't say I really understand why, but there it is.
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Old 4th May 2012, 07:26 AM   #288
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Originally Posted by H'ethetheth View Post
Yet rotating the whole of space is what NASA is talking about in that sentence. I can't say I really understand why, but there it is.
It's worth rereading that page in full, as it's a remarkably clear description ( http://einstein.stanford.edu/SPACETIME/spacetime4.html to save you looking back for the link).

The missing inch bit is not related to the Earth's rotation - watch the video of Kip Thorne as well as that's helpful in understanding it.

The de Sitter (geodetic) precession comes about from the test particle moving round the Earth, not anything to do with the Earth rotating once every 24 hours.

The frame dragging (Lense-Thirring) effect is the bit under discussion in this thread. It's impressive that Farsight managed to confuse the two given how clearly separated they are in that text, how clear the explanation is, and given Farsight seems to think he's an expert on the subject.
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Old 4th May 2012, 07:37 AM   #289
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Originally Posted by ynot View Post
It’s also claimed that when I think I’m observing properties created by a spinning top I could equally be observing identical properties caused by the spinning shell.
No one has ever made any such claim. There are no "properties created by a spinning top".

The spinning or not-spinning top has essentially no effect on anything else, including the shell. because the top is very small and light.

The spinning or non-spinning shell of matter can have a very large effect on everything else, including the top, because the shell is huge and very massive.

Adding multiple tops to the story doesn't change it in the slightest, which is why none of us can understand the questions you keep asking about them.

Last edited by sol invictus; 4th May 2012 at 07:39 AM.
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Old 4th May 2012, 08:12 AM   #290
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Originally Posted by H'ethetheth View Post
Yet rotating the whole of space is what NASA is talking about in that sentence. I can't say I really understand why, but there it is.
It's like space is a thin flat rubber sheet. We drop a bowling ball in the middle, then you walk underneath and grab it through the rubber. Then you either twist it round, or hold it steady while I get hold of the rim of the sheet and twist that round instead. You get the same effect either way - gravitomagnetism. Space is frame-dragged, like the NASA picture. It is rotated, but not rotating. Rotating space is like me getting rid of the cannonball and getting out the marbles instead. Then I mount the rubber sheet on a turntable, press the on button, and we start flicking marbles across it. An object's path in this universe is now curved instead of linear. However inertia still resists your attempt to change that path. And it's the inertia of the object, not that of some distant mass that does it. When the earth is spinning, inertia makes it tough to bend the path, so the equator bulges. When the universe is spinning instead, inertia makes it tough to straighten the path, so the equator bulges. But there's no twist, it's different to gravitomagnetism. A gravitomagnetic field is like an electromagnetic field. The rotating universe is like a magnetic monopole.
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Old 4th May 2012, 12:36 PM   #291
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Originally Posted by Farsight View Post
The rotating universe is like a magnetic monopole.
I might regret this, but I'm very curious as to what you mean here.
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Old 5th May 2012, 05:45 AM   #292
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Originally Posted by ynot View Post
I canít see in a practical sense how several tops could be concurrently spinning in two direction and at different speeds on the same axis and all be non-spinning let alone a shell or shells causing them to appear as if theyíre spinning when they arenĎt.
Nobody has suggested that this is valid. In your scenario, two tops are stacked on a common axis and rotating in opposite directions at 1000 RPM each relative to the table. That's a valid statement in the reference frame where the table is non-rotating. If I understand correctly, all reference frames should describe Top A moving relative to top B at 2000 RPM. Nobody has proposed a single reference frame where both tops are non-rotating. You can define a reference frame where one top is non-rotating, but in that frame, the other is spinning twice as fast. Additionally, in that reference frame, the distant shell is spinning (distant galaxies revolving around the stationary top) at 1000 RPM.

Basically, you can define a reference frame where top A is not spinning, or define one where top B is not spinning, but there is not a reference frame where neither top is spinning.
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Old 5th May 2012, 04:47 PM   #293
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Originally Posted by ynot View Post
...
Itís also claimed that when I think Iím observing properties created by a spinning top I could equally be observing identical properties caused by the spinning shell. I canít see in a practical sense how several tops could be concurrently spinning in two direction and at different speeds on the same axis and all be non-spinning let alone a shell or shells causing them to appear as if theyíre spinning when they arenĎt.
I'm on very thin ice trying to say anything about this stuff, but I think I am beginning to imagine that a sliver of light might be breaking through.
It appears that the mathematics of GR allows one to regard the top as spinning or not depending on what one wants to examine and how. By having the top not spin, the GR equations provide the forces needed to maintain reality by having the rest of the universe act as a spinning shell -- either perspective will produce the same results -- specifically, the top will not fall over because of the resulting forces.
I think I have fallen into the same trap of trying to look at multiple tops spinning resulting in contradictory spinning shells. That will not work because you must choose only one frame of reference at a time. One cannot expect two different frames of reference to provide the same explanation simultaneously. Another spinning top (going in a different direction) cannot simultaneously be regarded as stationary at the same time the first one is stationary. Those result in two different frames of reference with different results in the GR equations.
GR is a mathematical tool (a model) that provides these perspectives, but I still don't understand how all this relates to reality -- but I'm working on it.
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Old 5th May 2012, 08:49 PM   #294
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Originally Posted by Perpetual Student View Post
I'm on very thin ice trying to say anything about this stuff, but I think I am beginning to imagine that a sliver of light might be breaking through.
It appears that the mathematics of GR allows one to regard the top as spinning or not depending on what one wants to examine and how. By having the top not spin, the GR equations provide the forces needed to maintain reality by having the rest of the universe act as a spinning shell -- either perspective will produce the same results -- specifically, the top will not fall over because of the resulting forces.
I think I have fallen into the same trap of trying to look at multiple tops spinning resulting in contradictory spinning shells. That will not work because you must choose only one frame of reference at a time. One cannot expect two different frames of reference to provide the same explanation simultaneously. Another spinning top (going in a different direction) cannot simultaneously be regarded as stationary at the same time the first one is stationary. Those result in two different frames of reference with different results in the GR equations.
GR is a mathematical tool (a model) that provides these perspectives, but I still don't understand how all this relates to reality -- but I'm working on it.
Hope you donít get blinded by the light . Perhaps the real ďtrapĒ is not looking at multiple tops spinning concurrently from a Universal frame. I donít understand how any limited, single perspective observation can correctly represent a complete actual reality
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Old 5th May 2012, 08:56 PM   #295
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Originally Posted by jadey View Post
Nobody has suggested that this is valid. In your scenario, two tops are stacked on a common axis and rotating in opposite directions at 1000 RPM each relative to the table. That's a valid statement in the reference frame where the table is non-rotating. If I understand correctly, all reference frames should describe Top A moving relative to top B at 2000 RPM. Nobody has proposed a single reference frame where both tops are non-rotating. You can define a reference frame where one top is non-rotating, but in that frame, the other is spinning twice as fast. Additionally, in that reference frame, the distant shell is spinning (distant galaxies revolving around the stationary top) at 1000 RPM.

Basically, you can define a reference frame where top A is not spinning, or define one where top B is not spinning, but there is not a reference frame where neither top is spinning.
I understood the claim to be that you canít tell if anything is spinning period, not just a particular top. So if two tops are spinning in opposite directions I take it you agree that we can say at least one top is spinning?
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Old 5th May 2012, 09:40 PM   #296
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Originally Posted by ynot View Post
I understood the claim to be that you canít tell if anything is spinning period, not just a particular top. So if two tops are spinning in opposite directions I take it you agree that we can say at least one top is spinning?
Yes.
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Old 6th May 2012, 05:34 AM   #297
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Originally Posted by ynot View Post
Hope you donít get blinded by the light . Perhaps the real ďtrapĒ is not looking at multiple tops spinning concurrently from a Universal frame. I donít understand how any limited, single perspective observation can correctly represent a complete actual reality
I'm sure they'll correct me if I'm wrong, but isn't that is exactly what they're trying to explain to us here. You get to choose any perspective that you find useful, because there is no single absolute perspective.

Originally Posted by sol invictus View Post
General relativity, our current best theory of gravity and a massive conceptual advance (one that has still not been fully appreciated by most people) allows us to choose any coordinate system we like. Its predictions for all physical experiments are independent of coordinate choice, and - unlike in Newtonian theories - there is nothing that prefers one coordinate system over another apart from convenience. There are no inertial frames, and all gravitational "forces" are "fictitious" in the sense of Newton.

As a result, you're perfectly free to choose coordinates in which the sun goes around the earth, or in which the earth isn't rotating and all the stars go around it once/24 hours. Do those coordinates "require many novel forces and assumptions"? Nope, not in the slightest - no assumptions are necessary, nor are there any new forces. Are they more complex? Not really - calculations can be done in whatever coordinates are the most convenient, and the results transformed to any other, so on that basis it's very hard to argue that one set of coordinates are more complex than another.

But come on - doesn't the earth really go around the sun? Well, people used to think that there was an absolute rest frame, and really things at rest with respect to the earth and air where at rest. Galileo and Newton overturned that notion by making it clear that "at rest" is a meaningless statement, one that only has relative meaning. General relativity to a large degree overturned even the notion that acceleration or rotation is absolute, because it allows one to use any coordinates whatsoever, and proves that all physical predictions are unchanged.
I'm only piping in because I'm going through the same process as you. Personally, I'll never understand the specifics, but I'm striving to understand the concepts. I'll appreciate if the experts will correct me where I will almost certainly go wrong.

I think the trap is that we want to hold on to that absolute reference frame. We abandoned our geocentric view of the world and took an "outside-looking-in" perspective and noticed that the planets were revolving around the sun. We zoomed out further and saw that the stars in our galaxy were revolving around a center. We want to keep zooming out to get the "big picture" view of reality.

Somehow GR tells us that this "absolute big picture" inertial perspective does not exist. I've been reading this wiki link on inertial frames (minus the math) and trying to really understand conceptually why there are no inertial frames. That seems to be due to the curvature of spacetime in the presence of massive objects. Apparently, massive objects distort and drag (and even rotate?) spacetime. Therefore, an inertial perspective of the universe is an inaccurate approximation.

Quote:
Einsteinís general theory modifies the distinction between nominally "inertial" and "noninertial" effects by replacing special relativity's "flat" Euclidean geometry with a curved metric. In general relativity, the principle of inertia is replaced with the principle of geodesic motion, whereby objects move in a way dictated by the curvature of spacetime. As a consequence of this curvature, it is not a given in general relativity that inertial objects moving at a particular rate with respect to each other will continue to do so. This phenomenon of geodesic deviation means that inertial frames of reference do not exist globally as they do in Newtonian mechanics and special relativity.
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Old 6th May 2012, 06:59 AM   #298
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I'm really confused about inertial vs non-inertial frames and fictitious forces.

Conceptually, I've been thinking of inertial reference frames as "approximations of reality", and non-inertial frames as "reality". It seems counterintuitive that if we choose a realistic model (non-inertial frame), we need to introduce fictitious forces to explain how objects behave. However, if we choose an unrealistic model (inertial frame), we don't.

Can someone please point out my logical failure(s) and correct my understanding?

1. GR states that there are no inertial frames.

2. GR recognizes gravity as a consequence of the curvature of spacetime rather than a "force" in the newtonian sense. Gravity is a fictitious force?

3. An inertial frame is an approximation of reality. To presume an inertial frame is to disregard (not recognize) the effects of massive objects on the geometry of spacetime.

4. A non-inertial frame is an accurate depiction of reality. Choosing a non-inertial frame is an attempt to recognize the effects of massive objects on the geometry of spacetime.

5. If we choose to recognize the effects of massive objects on the geometry of spacetime (= non-inertial frame), we need to invent fictitious forces.

If I'm understanding 1 thru 4 correctly, then number 5 makes no sense to me.
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Old 6th May 2012, 09:50 AM   #299
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Originally Posted by jadey View Post
I'm really confused about inertial vs non-inertial frames and fictitious forces. (...)
First of all, let me clarify something that Ynot had trouble with. GR can tell you what *would* happen in various counterfactual worlds. GR can tell you what an utterly-empty, finite-size Universe would be like---even though we don't happen to live in such a Universe.

By analogy, SR can tell you what *would* happen if you had a relativistic racecar on a relativistic spaceship full of identical twins, even though we don't have such a thing; and questions about "what does SR mean" can be answered in terms of what it says about the spaceship.

Quote:
1. GR states that there are no inertial frames.
No. In space with masses in it, there's no global inertial frame.

Quote:
2. GR recognizes gravity as a consequence of the curvature of spacetime rather than a "force" in the newtonian sense. Gravity is a fictitious force?
Yes. The funny laws-of-motion seen under gravity ("this cannonball's coordinates describe a parabola") come from the exact same GR terms as the funny laws-of-motion ("things get pulled right off the carousel") seen by accelerating observers. There is no identifiable theoretical or experimental distinction that lets you label one as a "force" and one as "fictitious".

Mass/energy causes spacetime curvature, that's a real thing, but feeling a force due to curved space is, in some sense, a fictitious force.

Quote:
3. An inertial frame is an approximation of reality. To presume an inertial frame is to disregard (not recognize) the effects of massive objects on the geometry of spacetime.

4. A non-inertial frame is an accurate depiction of reality. Choosing a non-inertial frame is an attempt to recognize the effects of massive objects on the geometry of spacetime.
No. If you're looking at a small, finite region of space, your choice of frame can fully take into account a nearby gravity/mass/curvature. That's true whether you pick an inertial or non-inertial frame.

The only inaccuracy that comes in is if you try to extend your frame over a large region. Then, the thing you're ignoring is tidal forces, i.e. gravity gradients, i.e. curvature.

Examples: near Earth, an astronaut in a free-falling space capsule will think of his walls as an inertial frame. A person in an elevator on the Earth's surface will think of his walls as a non-inertial frame. Both of these are perfectly "taking into account" the presence of gravity/curvature. Both are limited, in the same way, by the finite size of the space considered---if you have a hundred-mile-wide box, you might measure that gravity points in a different direction at the left edge of the box vs. the right edge, but that's a tidal force---there's no frame-transformation you can do, inertial or otherwise, to prevent that difference from being there.

A better place to start would be: free-falling observers are, by definition, in inertial frames. Gravity/curvature tells us that sometimes a free-falling observer doesn't move in a straight line (or, equivalently, that an observer moving in a straight line---like one standing on the Earth's surface---is not inertial). If you wish to represent all non-straight-line motion as though due to a force---as though straight-line motion was the default, and only F=ma could violate it---then you can invent an explanation along these lines.
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Old 6th May 2012, 11:15 AM   #300
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Originally Posted by ben m View Post
First of all, let me clarify something that Ynot had trouble with. GR can tell you what *would* happen in various counterfactual worlds. GR can tell you what an utterly-empty, finite-size Universe would be like---even though we don't happen to live in such a Universe.

By analogy, SR can tell you what *would* happen if you had a relativistic racecar on a relativistic spaceship full of identical twins, even though we don't have such a thing; and questions about "what does SR mean" can be answered in terms of what it says about the spaceship.



No. In space with masses in it, there's no global inertial frame.



Yes. The funny laws-of-motion seen under gravity ("this cannonball's coordinates describe a parabola") come from the exact same GR terms as the funny laws-of-motion ("things get pulled right off the carousel") seen by accelerating observers. There is no identifiable theoretical or experimental distinction that lets you label one as a "force" and one as "fictitious".

Mass/energy causes spacetime curvature, that's a real thing, but feeling a force due to curved space is, in some sense, a fictitious force.



No. If you're looking at a small, finite region of space, your choice of frame can fully take into account a nearby gravity/mass/curvature. That's true whether you pick an inertial or non-inertial frame.

The only inaccuracy that comes in is if you try to extend your frame over a large region. Then, the thing you're ignoring is tidal forces, i.e. gravity gradients, i.e. curvature.

Examples: near Earth, an astronaut in a free-falling space capsule will think of his walls as an inertial frame. A person in an elevator on the Earth's surface will think of his walls as a non-inertial frame. Both of these are perfectly "taking into account" the presence of gravity/curvature. Both are limited, in the same way, by the finite size of the space considered---if you have a hundred-mile-wide box, you might measure that gravity points in a different direction at the left edge of the box vs. the right edge, but that's a tidal force---there's no frame-transformation you can do, inertial or otherwise, to prevent that difference from being there.

A better place to start would be: free-falling observers are, by definition, in inertial frames. Gravity/curvature tells us that sometimes a free-falling observer doesn't move in a straight line (or, equivalently, that an observer moving in a straight line---like one standing on the Earth's surface---is not inertial). If you wish to represent all non-straight-line motion as though due to a force---as though straight-line motion was the default, and only F=ma could violate it---then you can invent an explanation along these lines.
Thanks for the response. I need to try to digest all of this. However, can you please rephrase the last sentence. I'm especially not understanding the highlighted portion.
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Old 6th May 2012, 11:49 AM   #301
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Originally Posted by jadey View Post
1. GR states that there are no inertial frames.
Right - although you can always choose a frame that's close to inertial in some finite region. The degree of "non-inertialness" depends on how curved the spacetime is, which in turn depends on how much matter and energy there is nearby, whether there are gravity waves passing through, etc.

Quote:
2. GR recognizes gravity as a consequence of the curvature of spacetime rather than a "force" in the newtonian sense. Gravity is a fictitious force?
Yes.

Quote:
3. An inertial frame is an approximation of reality. To presume an inertial frame is to disregard (not recognize) the effects of massive objects on the geometry of spacetime.
Yes.

Quote:
4. A non-inertial frame is an accurate depiction of reality. Choosing a non-inertial frame is an attempt to recognize the effects of massive objects on the geometry of spacetime.
It's more quantitative and specific than that, but yes.

Quote:
5. If we choose to recognize the effects of massive objects on the geometry of spacetime (= non-inertial frame), we need to invent fictitious forces.
Einstein's equations let us calculate the curvature of spacetime given the matter and energy content. Once we know the spacetime, we can describe it using various choices of coordinates - those are frames. Since the spacetime is curved, none of the coordinates will be inertial (inertial coordinates can't describe a curved space, any more than you can make a map of the surface of the earth that's both flat and not distorted).

One you have the spacetime and the coordinates, you can predict the motion of particles. Those equations of motion will look approximately like ma=0 unless either (a) the spacetime is curved, in which case you cannot use inertial coordinates, or (b) it's not (very) curved but you choose to use non-inertial coordinates, in which case there are "fictitious forces".
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Old 6th May 2012, 03:21 PM   #302
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Originally Posted by jadey View Post
I'm sure they'll correct me if I'm wrong, but isn't that is exactly what they're trying to explain to us here. You get to choose any perspective that you find useful, because there is no single absolute perspective.
Yes, and I think I understand what theyíre saying but I donít accept it. If someone can explain how limitations of observation from a limited perspective can have any effect on actual reality (or accurately represent total reality) them I might believe them. Observations from different perspectives provide different information but so what? Reality is what it is, itís not how itís perceived to be from limited observation. The invention of microscopes and telescopes didnít create a new reality they just created means by which we can better observe the reality that always existed. Observation has no magical qualities that create or destroy anything.
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Old 6th May 2012, 08:38 PM   #303
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Originally Posted by ynot View Post
If someone can explain how limitations of observation from a limited perspective can have any effect on actual reality (or accurately represent total reality) them I might believe them.
But they don't.

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Observations from different perspectives provide different information but so what? Reality is what it is, it’s not how it’s perceived to be from limited observation. The invention of microscopes and telescopes didn’t create a new reality they just created means by which we can better observe the reality that always existed. Observation has no magical qualities that create or destroy anything.
That's true. So what?

Quote:
Yes, and I think I understand what they’re saying but I don’t accept it.
No, I don't think you do.

In a sense, the whole point of relativity is to figure out what isn't relative. The results of physical experiments aren't relative, but statements like "object A is moving" are. People have great difficulty understanding that, because they're used to thinking of absolute motion as real - when in fact it's only relative motion that means anything.
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Old 6th May 2012, 11:31 PM   #304
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Originally Posted by ynot View Post
Yes, and I think I understand what theyíre saying but I donít accept it. If someone can explain how limitations of observation from a limited perspective can have any effect on actual reality (or accurately represent total reality) them I might believe them.
I don't understand what you mean by this.
Do you mean that no one has explained to your satisfaction how from a "limited perspective" the sun can be said to revolve around the earth, thereby "having an effect on the "actual reality" that the earth revolves around the sun?

Would that be a fair description?
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Old 7th May 2012, 06:06 AM   #305
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Originally Posted by edd View Post
I might regret this, but I'm very curious as to what you mean here.
See the NASA page on gravity probe B. The gravitomagnetic field is twisted space or a "twist field". The electromagnetic field is similar. If you move through it, you turn, and in that respect a magnetic field is a "turn field". That's why Maxwell talked about a screw mechanism, and don't forget the torque on a compass needle. For a pure magnetic field or "rotation field", a region of space has to be rotating like a rotor running free and disconnected from the surrounding space. Space doesn't let this happen, it makes as much sense as a disc of the rubber sheet in the bowling-ball analogy rotating like a turntable. But the nature of space doesn't stop you giving a rotation to the whole rubber sheet.

I thought this was interesting by the way.
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Old 7th May 2012, 07:09 AM   #306
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Originally Posted by Farsight View Post
See the NASA page on gravity probe B. The gravitomagnetic field is twisted space or a "twist field". The electromagnetic field is similar. If you move through it, you turn, and in that respect a magnetic field is a "turn field". That's why Maxwell talked about a screw mechanism, and don't forget the torque on a compass needle. For a pure magnetic field or "rotation field", a region of space has to be rotating like a rotor running free and disconnected from the surrounding space. Space doesn't let this happen, it makes as much sense as a disc of the rubber sheet in the bowling-ball analogy rotating like a turntable. But the nature of space doesn't stop you giving a rotation to the whole rubber sheet.

I thought this was interesting by the way.
Could you demonstrate using the equations for gravity and magnetic monopoles?
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Old 7th May 2012, 01:27 PM   #307
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Originally Posted by sol invictus View Post
But they don't.
That's true. So what?

No, I don't think you do.

In a sense, the whole point of relativity is to figure out what isn't relative. The results of physical experiments aren't relative, but statements like "object A is moving" are. People have great difficulty understanding that, because they're used to thinking of absolute motion as real - when in fact it's only relative motion that means anything.
I continue to think that absolute motion is real and that all things are constantly in Universal motion. Relative motion is different Universal motions and relatively stationary motion is same Universal motions. Inertial (non-accelerating) Universal motion can only be observed compared to a different Universal motion (relatively). Accelerating Universal motion can be observed by the observable effects of acceleration. Acceleration could be described as being non-relative or very locally relative. Accelerating can only be observed if there is a lag within the accelerating thing. Gravity accelerates everything equally so there is no lag to observe. Acceleration is a change in Universal speed or direction. Whether a thing can be observed or not doesnít establish whether a thing exists or not. Relative motion only ďmeans anythingĒ and Universal motion ďisnĎt realĒ only because The Theory of Relativity says so. If youíre only going to ask Relativity youíre only going to get Relativity answers.

ETA - All motion is relative in the sense that all motion is not Universally the same and all things are always moving relative to something else. This doesnít mean that motion per se is or has to be relative.

All this is merely my current opinions and conclusions.
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Old 7th May 2012, 01:35 PM   #308
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Yep. Regretting it.
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Old 7th May 2012, 02:30 PM   #309
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Originally Posted by sol invictus View Post
Right - although you can always choose a frame that's close to inertial in some finite region. The degree of "non-inertialness" depends on how curved the spacetime is, which in turn depends on how much matter and energy there is nearby, whether there are gravity waves passing through, etc.



Yes.



Yes.



It's more quantitative and specific than that, but yes.



Einstein's equations let us calculate the curvature of spacetime given the matter and energy content. Once we know the spacetime, we can describe it using various choices of coordinates - those are frames. Since the spacetime is curved, none of the coordinates will be inertial (inertial coordinates can't describe a curved space, any more than you can make a map of the surface of the earth that's both flat and not distorted).

One you have the spacetime and the coordinates, you can predict the motion of particles. Those equations of motion will look approximately like ma=0 unless either (a) the spacetime is curved, in which case you cannot use inertial coordinates, or (b) it's not (very) curved but you choose to use non-inertial coordinates, in which case there are "fictitious forces".
Thanks for the reply. I'll try to do some more reading on the subject. Did you mean ma=0 above or did you intend f=ma? If its not a typo then I'm confused.

Also, in (b) you specified that it wasn't very curved. What if (c) it was very curved and you chose non-inertial coordinates? Would you still need fictitious forces?

I'm really trying to appreciate what "causes" the fictitious forces. Are they similar to gravity and just a consequence of the geometry of spacetime as well? Or, are they more like a property of the chosen reference frame?
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Old 7th May 2012, 02:33 PM   #310
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Originally Posted by ynot View Post
Yes, and I think I understand what theyíre saying but I donít accept it. If someone can explain how limitations of observation from a limited perspective can have any effect on actual reality (or accurately represent total reality) them I might believe them. Observations from different perspectives provide different information but so what? Reality is what it is, itís not how itís perceived to be from limited observation. The invention of microscopes and telescopes didnít create a new reality they just created means by which we can better observe the reality that always existed. Observation has no magical qualities that create or destroy anything.
I'm not sure that you are understanding. As far as I can tell, you seem to be arguing against ideas that have not been suggested in this thread.
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Old 8th May 2012, 03:18 AM   #311
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Originally Posted by ynot View Post
ETA - All motion is relative in the sense that all motion is not Universally the same and all things are always moving relative to something else. This doesn’t mean that motion per se is or has to be relative.
How would one go about testing or falsifying this claim?
How does one test if something is absolutely moving or standing still?
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Old 8th May 2012, 04:49 AM   #312
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Originally Posted by H'ethetheth View Post
How would one go about testing or falsifying this claim?
How does one test if something is absolutely moving or standing still?
Let's see what James Clerk Maxwell said about this in Matter and Motion.

Originally Posted by Maxwell
Section 18. ABSOLUTE SPACE

Absolute space is conceived as remaining always similar to itself and immovable. The arrangement of the parts of space can no more be altered than the order of the portions of time. To conceive them to move from their places is to conceive a place to move away from itself.

But as there is nothing to distinguish one portion of time from another except the different events which occur in them, so there is nothing to distinguish one part of space from another except its relation to the place of material bodies. We cannot describe the time of an event except by reference to some other event, or the place of a body except by reference to some other body. All our knowledge, both of time and place, is essentially relative. When a man has acquired the habit of putting words together, without troubling himself to form the thoughts which ought to correspond to them, it is easy for him to frame an antithesis between this relative knowledge and a so-called absolute knowledge, and to point out our ignorance of the absolute position of a point as an instance of the limitation of our faculties. Any one, however, who will try to imagine the state of a mind conscious of knowing the absolute position of a point will ever after be content with our relative knowledge.
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Old 8th May 2012, 05:39 AM   #313
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Originally Posted by edd View Post
Yep. Regretting it.
Why? Go look at that NASA page again. See where it says Einstein was right again. There is a space-time vortex around Earth. And see where it says But if space is twisted, the direction of the gyroscope's axis should drift over time. Then take a look at On Physical Lines of Force where on page 53 Maxwell says this:

Originally Posted by Maxwell
"A motion of translation along an axis cannot produce a rotation about that axis unless it meets with some special mechanism, like that of a screw".
Also see the title. He's talking about vortices. Now go do your own research and start thinking for yourself. Stop being a smart-alec ignorant naysayer who dismisses Einstein and Maxwell and anything else that doesn't square with your piss-poor physics education.
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Old 8th May 2012, 05:46 AM   #314
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Originally Posted by H'ethetheth View Post
How would one go about testing or falsifying this claim? How does one test if something is absolutely moving or standing still?
You compare its motion with the CMBR dipole anisotropy. The CMBR allows you to gauge motion relative to the universe, and since the universe is absolutely everything, you say that this motion is absolute.

Note however that if you were the something, and if you were inside a black box, you wouldn't be able to detect this motion, so it isn't "abolute motion" in the relativity sense.
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Old 8th May 2012, 06:27 AM   #315
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Originally Posted by Farsight View Post
You compare its motion with the CMBR dipole anisotropy. The CMBR allows you to gauge motion relative to the universe, and since the universe is absolutely everything, you say that this motion is absolute.
Well it's the part we can see, anyway. More importantly, it's still as arbitrary as any, depending on where you put your origin and how space is curved there.
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Old 8th May 2012, 06:55 AM   #316
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Originally Posted by Farsight View Post
Now go do your own research and start thinking for yourself.

For quite some time now, edd and many others here have been thinking for ourselves instead of letting Farsight tell us how to think.

Originally Posted by Farsight View Post
Stop being a smart-alec ignorant naysayer who dismisses Einstein and Maxwell and anything else that doesn't square with your piss-poor physics education.

The person who wrote that sentence has been unable to follow discussions of Einstein and Maxwell because he does not understand first-year calculus and electromagnetism. He doesn't even understand Einstein's algebra: Farsight got lost at equation (3) of Einstein's Die Grundlage der allgemeinen Relativitštstheorie.

Although Farsight pretends to champion Einstein, he's been disagreeing with Einstein for years, notably by denying the two fundamental principles of Einstein's general theory of relativity: the equivalence principle, and the admissibility of all coordinate transformations allowed by differential geometry.
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Old 8th May 2012, 07:27 AM   #317
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Originally Posted by Farsight View Post
Why? Go look at that NASA page again. See where it says Einstein was right again. There is a space-time vortex around Earth. And see where it says But if space is twisted, the direction of the gyroscope's axis should drift over time. Then take a look at On Physical Lines of Force where on page 53 Maxwell says this:


Also see the title. He's talking about vortices. Now go do your own research and start thinking for yourself. Stop being a smart-alec ignorant naysayer who dismisses Einstein and Maxwell and anything else that doesn't square with your piss-poor physics education.
Funny, I was about to raise Maxwell here. What in Maxwell's equations (modified from the standard form to permit a magnetic monopole) suggests anything 'twisty' about magnetic monopoles?

edit to add: I don't think attacking the level of other peoples physics education is going to be a successful line of argument for you.
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Old 8th May 2012, 07:50 AM   #318
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Originally Posted by jadey View Post
Thanks for the reply. I'll try to do some more reading on the subject. Did you mean ma=0 above or did you intend f=ma? If its not a typo then I'm confused.
If by "f" you mean all forces other than gravity, then yes, that's what I meant. I was referring to a situation in which no external forces are acting except perhaps gravity - but in GR as you know, gravity is just geometry.

Quote:
Also, in (b) you specified that it wasn't very curved. What if (c) it was very curved and you chose non-inertial coordinates? Would you still need fictitious forces?
If it's very curved you must choose non-inertial coordinates, because inertial coordinates cannot describe curved space. Whether or not you regard the gravitational/non-inertial/curvature "forces" in such a situation as "fictitious" is a matter of taste. But what is clear is that in such a curved spacetime, gravity cannot be separated from centrifugal-type forces.

Quote:
I'm really trying to appreciate what "causes" the fictitious forces. Are they similar to gravity and just a consequence of the geometry of spacetime as well? Or, are they more like a property of the chosen reference frame?
They're a combination. In flat spacetime you can eliminate them by choosing an inertial frame. In a curved spacetime all frames necessarily have them.

Last edited by sol invictus; 8th May 2012 at 07:52 AM. Reason: F --> f
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Old 8th May 2012, 03:08 PM   #319
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Originally Posted by H'ethetheth View Post
How would one go about testing or falsifying this claim?
How does one test if something is absolutely moving or standing still?
It’s not a claim it’s part of “my current opinions and conclusions”.

As I see it there are two possibilities . . .

1) If there is a Universal stationary then everything that moves relative to that orientation has Universal motion.

2) If there is no Universal stationary then everything has Universal motion.

That there is no known evidence of a Universal stationary doesn’t mean it absolutely can’t and doesn’t exist. It does mean however that there is absolutely no reason to assume or conclude that it can or does exist (no more than a god). My default conclusion must be therefore that everything has Universal motion. You conclusion can be whatever you choose.
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Old 8th May 2012, 03:27 PM   #320
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Originally Posted by ynot View Post
It’s not a claim it’s part of “my current opinions and conclusions”.

As I see it there are two possibilities . . .

1) If there is a Universal stationary then everything that moves relative to that orientation has Universal motion.

2) If there is no Universal stationary then everything has Universal motion.

That there is no evidence of a Universal stationary doesn’t absolutely mean it can’t and doesn’t exist. It does mean however that there is absolutely no reason to assume it can or does exist (no more than a god). My default conclusion must be therefore that everything has Universal motion. You conclusion can be whatever you choose.
Does your idea of "Universal motion" have any physical significance? Does it make any physical predictions?

ETA: Because as far as I can tell, all your "Universal motion" really means is, "stuff is moving relative to other stuff, which may or may not be in absolute motion".

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