Terra revolving round Sol question - Page 4

sol invictus · 27th April 2012, 10:20 AM

Originally Posted by Merton

sol invictus, thank you very much, that's making sense now. One more question, if you would indulge me.

Where do these patches of inflation occur, or are they a yet-unobserved prediction of the inflationary model?

If our universe is undergoing eternal inflation, then the part of the universe we can see is part of a patch that at some point exited from the eternally inflating phase. That patch is surrounded, probably on all sides, by eternally inflating volume.

That volume hasn't been observed, but that could just be because our patch is very large.

ynot · 27th April 2012, 02:10 PM

Originally Posted by sol invictus

No, either you spun the top or you stopped it from spinning.

Please explain how a top lying down on a surface can be spinning about it’s central axis. Also explain how an otherwise unsupported top standing on it’s tip can do so without spinning.

ynot · 27th April 2012, 02:12 PM

Originally Posted by theprestige

Spinning relative to what?

Relative to not spinning.

epepke · 27th April 2012, 02:28 PM

Originally Posted by ynot

Please explain how a top lying down on a surface can be spinning about it’s central axis.

The whole surface could be spinning.

Anyway, it's kind of funny how people get this way. I think that we evolved for this slow-moving stuff. So Aristotelian stationary, absolute physics makes a lot of sense. When we learn more, we want it to make sense again. So we insist on these things like inertial, non-rotating frames of reference. Then we get to think, "Ah, I can sort of understand. This is just like a big forest, only with weird time." This works pretty well up through Special Relativity.

Then General Relativity comes along and smacks us in the face and says, "Sucker! You think it gets simple? It doesn't. There are no inertial frames! HAH! You think you are going to get out of solving ten coupled differential equations? Sucks to be you."

At least, though, the compensation is that you get to make the coordinate system whatever the hell you want. You want to see the universe from the point of view of a power ball in the Addams Family pinball machine? Go ahead.

ynot · 27th April 2012, 02:43 PM

Originally Posted by epepke

The whole surface could be spinning.

It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Same applies to everything else in the room and the entire Universe that would also have to be spinning about the central axis of the top.

What about a second top on the same surface? Can they both be spinning about their own axis and also spinning about the axis of the other? How does the surface spin about the axis of both tops at the same time?

Merton · 27th April 2012, 03:02 PM

Originally Posted by ynot

It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Same applies to everything else in the room and the entire Universe that would also have to be spinning about the central axis of the top.

What about a second top on the same surface? Can they both be spinning about their own axis and also spinning about the axis of the other? How does the surface spin about the axis of both tops at the same time?

Because, as I said earlier, the top analogy isn't an accurate one. The person, table, room, top, Earth, etc. are all within the same reference frame when the top is at rest. You apply force to it and the top spins for a while, but slows down due to the friction created due to the table and air (both in the Earth's reference frame). The top will feel centrifugal force because it's running into all this other stuff in the Earth's reference frame. The person, table, etc. does not feel this force because they're all in the same reference frame.

Merton · 27th April 2012, 03:03 PM

Originally Posted by sol invictus

If our universe is undergoing eternal inflation, then the part of the universe we can see is part of a patch that at some point exited from the eternally inflating phase. That patch is surrounded, probably on all sides, by eternally inflating volume.

That volume hasn't been observed, but that could just be because our patch is very large.

Thanks for the great info! Much appreciated!

epepke · 27th April 2012, 03:09 PM

Originally Posted by ynot

It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force?

Who are you to say they aren't? Why did you think their tie was stuck to the window anyway?

ynot · 27th April 2012, 03:09 PM

Originally Posted by Merton

Because, as I said earlier, the top analogy isn't an accurate one. The person, table, room, top, Earth, etc. are all within the same reference frame when the top is at rest. You apply force to it and the top spins for a while, but slows down due to the friction created due to the table and air (both in the Earth's reference frame). The top will feel centrifugal force because it's running into all this other stuff in the Earth's reference frame. The person, table, etc. does not feel this force because they're all in the same reference frame.

Don’t know what your discussing but I’m discussing the question whether we can know from observation whether a top is spinning about it‘s central axis or not. The top isn’t an analogy it‘s a real thing.

ynot · 27th April 2012, 03:14 PM

Originally Posted by epepke

Who are you to say they aren't? Why did you think their tie was stuck to the window anyway?

Humour (if that's what it's meant to be) is always good but in this case I would prefer and appreciate a serious answer if you can provide one.

ben m · 27th April 2012, 03:38 PM

Originally Posted by ynot

It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Same applies to everything else in the room and the entire Universe that would also have to be spinning about the central axis of the top.

What about a second top on the same surface? Can they both be spinning about their own axis and also spinning about the axis of the other? How does the surface spin about the axis of both tops at the same time?

Because the actual laws of motion in the frame with the top at rest are not the same as the laws of motion in the frame where the top is spinning.

a) In the frame where the top is spinning and the ground is at rest, the observer says "the top is spinning, and I'm standing here next to a wall".

b) In the frame where the top is at rest and the ground is spinning, the observer says "the top is at rest, I am standing on the spinning ground, and there's an equivalence-principle-obeying, gravity-like force pulling me inwards so I don't crash into that wall."

If you forget to include the 2nd part---the coordinate-dependent forces---then you're not doing GR, and it's not valid to treat all coordinate systems as equal.

ben m · 27th April 2012, 03:41 PM

(But it's the same thing that happens in "familiar" curved spacetime. Take an astronaut on the space station. He's going around and around the Earth at a rate such that, if you were using the laws of flat space, you'd expect him to be plastered against the back wall of the space station, like a passenger in a fairground ride, by an 9.8g centripetal force. But he's not, for reasons that appear different from different perspectives. An observer in the space station says "Of course I'm not plastered against the wall, I'm sitting still with respect to the wall and obeying Newton's Laws." An observer on the ground says "Of course he's not plastered against the wall, he's being kept away from it by an equivalence-principle-obeying force I will call 'Earth's gravity'".)

ynot · 27th April 2012, 04:07 PM

Originally Posted by ben m

Because the actual laws of motion in the frame with the top at rest are not the same as the laws of motion in the frame where the top is spinning.

Two different laws of motion apply to the same event at the same time? Different frames have different observational perspectives of the same event but I don’t see how perception anomalies can change the reality of the event (or the actual laws of motion).

Originally Posted by ben m

a) In the frame where the top is spinning and the ground is at rest, the observer says "the top is spinning, and I'm standing here next to a wall".
b) In the frame where the top is at rest and the ground is spinning, the observer says "the top is at rest, I am standing on the spinning ground, and there's an equivalence-principle-obeying, gravity-like force pulling me inwards so I don't crash into that wall."
If you forget to include the 2nd part---the coordinate-dependent forces---then you're not doing GR, and it's not valid to treat all coordinate systems as equal.

So the laws of motion can be broken but an equivalence principle must be obeyed? Convenience principle?

Where does this "gravity-like force " come from and go to?

ben m · 27th April 2012, 04:27 PM

Originally Posted by ynot

Two different laws of motion apply to the same event at the same time? Different frames have different observational perspectives of the same event but I don’t see how perception anomalies can change the reality of the event (or the actual laws of motion).

Remember, "the laws of motion" are merely things like "if I know the x,y,z coordinates of the ball at time t=0, what will the x,y,z coordinates be at time t=1". Think of how complicated it is to make the same statement in spherical coordinates---"I know the r, theta, phi of the ball at t=0, what will the r,theta,phi be at t=1?". All we're talking about is your ability to do that with a rotating collection of coordinate labels. Or a distorted collection. Or a time-dependent collection.

The point of this discussion is: there are certain oddities---free-falling objects follow weird paths!---that show up in coordinate systems distorted by gravity. The surprising-ish thing, and the counterintuitive thing, is that exactly the same oddities---free-falling objects follow weird paths!---show up in rotating (or otherwise accelerating) coordinate systems.

The laws of physics are perfectly happy predicting the coordinates of a baseball (Or a photon, or a wave in an atomic clock, or whatever) as it follows a weird path through the familiar distorted coordinate-system we use near Earth. It turns out that the laws of physics are equally happy predicting the coordinates of a baseball (or a photon, or an atomic clock, etc.) if you drop it while sitting on a merry-go-round, using distorted merry-go-round coordinates and all of the centripetal/Coriolis stuff that is attached.

Quote:

Where does this "gravity-like force " come from and go to?

Where does Newtonian gravity "go" when for an observer in freefall? Nowhere. Newtonian gravity---9.8m/s/s downwards---is something you have to include in your equation-of-motion if your coordinate system is glued down at a fixed radius from the Earth. It's not something you have to include if your coordinate system is inertial. It's a coordinate-system-dependent pseudoforce.

ynot · 27th April 2012, 04:51 PM

Originally Posted by ben m

Where does Newtonian gravity "go" when for an observer in freefall? Nowhere. Newtonian gravity---9.8m/s/s downwards---is something you have to include in your equation-of-motion if your coordinate system is glued down at a fixed radius from the Earth. It's not something you have to include if your coordinate system is inertial. It's a coordinate-system-dependent pseudoforce.

Gravity (it's not Newton's) doesn't "go" anywhere for an observer in freefall. Merely the ability to "feel" it is "gone". It remains in place and it's force remains fully active as is evident by the falling. That the freefalling observer doesn't experience that force doesn't mean the force has gone away (actually does experience it but does so equally on every atom). I've never been able to understand how or why the ability to observe or experience an event can have any effect on the actual event. Illusions don't make illusion fact.

Given I'm not freefalling toward the top in your a) scenario why don't I experience this equivalence-principle-obeying, gravity-like force pulling me toward the top? So I repeat - Where does this mysterious force come from in b) and where does it go to in a)?

ynot · 27th April 2012, 05:04 PM

Originally Posted by ben m

The point of this discussion is: there are certain oddities---free-falling objects follow weird paths!---that show up in coordinate systems distorted by gravity. The surprising-ish thing, and the counterintuitive thing, is that exactly the same oddities---free-falling objects follow weird paths!---show up in rotating (or otherwise accelerating) coordinate systems.

I may be derailing this thread but my point of this dicusion is whether or not it can be known whether a thing is spinning or not (specifically a top).

theprestige · 27th April 2012, 05:10 PM

Originally Posted by ynot

Relative to not spinning.

Not spinning in what frame of reference?

ynot · 27th April 2012, 05:16 PM

Originally Posted by theprestige

Not spinning in what frame of reference?

In the reference frame of reality.

ben m · 27th April 2012, 05:20 PM

Originally Posted by ynot

Gravity (it's not Newton's) doesn't "go" anywhere for an observer in freefall. Merely the ability to "feel" it is "gone". It remains in place and it's force remains fully active as is evident by the falling.

OK. So now suppose you and Bob are arguing about whether the top is spinning. Bob says it's not. You say it is, and to prove it you say: "And I know I'm in a non-rotating frame, because I do experiments which don't feel any centripetal forces." Bob's reply would sound just like yours above:

Originally Posted by Bob as ynot

Centripetal forces don't "go" anywhere for an observer in freefall, i.e. in circular an orbit around the centripetal force center. Merely the ability to "feel" centripetal forces is "gone". The centripetal force remains in place and it's force remains fully active as is evident by your trajectory, which is going around and around the stationary top.

ynot · 27th April 2012, 05:31 PM

Originally Posted by ben m

OK. So now suppose you and Bob are arguing about whether the top is spinning. Bob says it's not. You say it is, and to prove it you say: "And I know I'm in a non-rotating frame, because I do experiments which don't feel any centripetal forces." Bob's reply would sound just like yours above:

But I'm not freefalling toward the top and I'm not doing anything I know of which could or would remove my ability to feel any centripetal forces. We know and can explain why freefall doesn't feel gravity (or isn't aware it does) how would Bob (you) otherwise explain not feeling either centripetal forces or this "gravity-like force"? Also the top is displaying properties of having centripetal forces. How would Bob (you) explain it's ability to stand on it's point if it wasn't spinning?

DarbyII · 27th April 2012, 05:32 PM

Originally Posted by sol invictus

I don't understand your question.

It really wasn't a question.

There's a reason why Special Relativity is limited to inertial frames. In the case of inertial frames, yes, there is no preferred frame of reference. Motion between the bodies is relative in an inertial frame. The same isn't necessarily true in all accelerated frames. There are absolute motions in an accelerated frames.

A planet in orbit is in an accelerated frame. The planet may be moving at a more or less constant speed but speed is a scalar value. Velocity is a vector - magnitude and direction. The direction of the planet as it orbits is constantly changing thus the velocity is constantly changing, i.e. accelerated frame. If you are in an accelerated frame there are experiments that can be run to detect the acceleration. An laser accelerometer might be useful here.

If you can detect the acceleration then it is possible to determine in a two body experiment which is the major body and which is the minor orbiting body.

ben m · 27th April 2012, 05:52 PM

Originally Posted by ynot

But I'm not freefalling toward the top and I'm not doing anything which could or would remove my ability to feel any centripetal forces. We know and can explain why freefall doesn't feel gravity (or isn't aware it does) how would Bob (you) explain not feeling this "gravity-like force"? Also the top is displaying properties of centripetal forces.

According to Bob, and every experiment he can think of, the equations of motion of the Universe naturally include a centrifugal force that pulls you towards the axis of his coordinates. Bob says you are free-falling under that force---and, because it's an equivalence-principle-obeying force, Bob fully expects that free-falling bodies can't feel it---not relative to other free-falling bodies anyway.

If you insist, Bob can write down the equation-of-motion of your head, and the equations-of-motion of the ground underneath you, and he will *derive* that the centripetal/coriolis force is pulling you and the ground around in such a way that there's no differential acceleration between you.

There's a decent animation of this in on Wikipedia. http://en.wikipedia.org/wiki/Fictiti...ircular_motion

Toontown · 27th April 2012, 07:37 PM

On a related note: when I'm walking, is it OK to think of myself as stationary, rolling the earth beneath my feet?

epepke · 27th April 2012, 08:08 PM

Originally Posted by ynot

Humour (if that's what it's meant to be) is always good but in this case I would prefer and appreciate a serious answer if you can provide one.

Well, people have been trying. @ben_m is doing a particularly good job. However, you seem to exhibit a huge reluctance against this kind of thing.

See, you seem to be really, really convinced that there is something called "spinning" and something called "not spinning," and that this has some meaning. That works fine in Newtonian physics and up through Special Relativity. It doesn't work in General Relativity.

Let's say that you are on a rotating space station, like the one on 2001: A Space Odyssey. If you haven't seen it, go see it now. Like the scene where they meet the Russians in the funny-looking chairs and the whole set is curved, got it? Only there are no windows, but the chair still pushes up on your butt.

Well, you are going to conclude that this isn't real gravity. It's fake gravity, or pseudo-gravity. Because you're rotating, see?

But in General Relativity, there is no distinction. It is real gravity, just as real as if you were on a planet.

Trying to prove me wrong, you do a GR calculation assuming that your space station is rotating, and you find that it agrees with what you measure. Hah, you say!

Then I tell you to redo the calculation, this time assuming that the space station is stationary, and the entire universe is rotating in the opposite direction. That's funny, you say. The calculations predict exactly the same thing.

So you can have fifteen guys on fifteen little tops, and four on jackhammers, and two on potatoes, or something, and none of them can ever really say "I'm rotating, and you're not."

And that's because, unlike Special Relativity or Galilean Relativity, where the laws of physics are different for a rotating rather than a non-rotating frame, under General Relativity, the laws of physics are the same. This is probably why it's called General Relativity. It's relative in general, not in the special case of an inertial frame.

sol invictus · 28th April 2012, 07:15 AM

Originally Posted by ynot

Please explain how a top lying down on a surface can be spinning about it’s central axis. Also explain how an otherwise unsupported top standing on it’s tip can do so without spinning.

It stands or doesn't stand largely depending on whether or not it is rotating with respect to the locally inertial frame. So the question is, can the locally inertial frame itself be spinning?

The answer is certainly yes. I suggest reading the thread from the beginning if you haven't already.

sol invictus · 28th April 2012, 07:19 AM

Originally Posted by DarbyII

A planet in orbit is in an accelerated frame. The planet may be moving at a more or less constant speed but speed is a scalar value. Velocity is a vector - magnitude and direction. The direction of the planet as it orbits is constantly changing thus the velocity is constantly changing, i.e. accelerated frame. If you are in an accelerated frame there are experiments that can be run to detect the acceleration. An laser accelerometer might be useful here.

If you can detect the acceleration then it is possible to determine in a two body experiment which is the major body and which is the minor orbiting body.

That's true only if you assume something else (such as that your orbiting bodies are in a nearly flat spacetime, with no rotating shells out near infinity, standard boundary conditions, etc.).

Farsight · 28th April 2012, 10:35 AM

Originally Posted by DarbyII

If you can detect the acceleration then it is possible to determine in a two body experiment which is the major body and which is the minor orbiting body.

Quite right, Darby.

Originally Posted by Toontown

On a related note: when I'm walking, is it OK to think of myself as stationary, rolling the earth beneath my feet?

Nope. Because I'm walking past you the other way, and I'm rolling the earth beneath my feet. LOL! No I'm not, we're just walking.

Originally Posted by ynot

In the reference frame of reality.

Absolutely. There are some people on this forum who peddle some total woo, and try to get away with it by suggesting that's what GR tells us when it doesn't. All GR says is that your map is as valid as mine. It doesn't say that your map is the territory and mine is too.

You know it's the top spinning because you can change its rotation speed. When some woomeister clique tries to tell you that you can just as well assert that the universe is rotating around it, you point out that actually, you changed its rotation speed to zero, and now it's just sitting there. So now they're trying to tell you that your top is rotating in line with the universe, when there's no evidence whatsoever for any rotation at all. And across the room my top is whirling like a dervish. It's a load of old cobblers.

theprestige · 28th April 2012, 02:01 PM

Originally Posted by ynot

In the reference frame of reality.

And which frame would that be, exactly?

Perpetual Student · 28th April 2012, 04:13 PM

Originally Posted by ben m

Perpetual Student, perhaps part of the problem is the difference between:

a) "A coordinate system in which the top happens to be at rest" and
b) "A coordinate system glued to the top, with its Z-axis along the top's handle and its x-axis aligned with this particular spot of paint."

The former is the sort of thing GR normally talks about. (It can handle the latter, sort of, but, ugh.) You describe "I know I spun up the top". Well, in coordinate system (a), you started out with a spinning top on a spinning table, and a spinning person reached over and stopped the top from spinning. That can happen, right? If you went to the North Pole, sat down, and started playing with a very slow top, that's exactly what an outside observer would see. "A spinning top on a spinning planet, then the spinning person set it down and made it stop."

(system (b) changes with time. "for t<0 we choose one coordinate system, and at t=0, PS exerted a force on the top, and for t >0 we transform to a different system", such that the top is "at rest" both before and after you exert the force. This is a mess.)

Let me translate your "I know I spun the top" to another system, see if your intuition does something different. We're hovering in outer space watching a soccer ball fly away from us. I tell you that SR allows me to treat this soccer ball as being at rest. You just define a set of coordinates that are moving along with the ball, and in those coordinates we're moving away from the ball, right?

Does that help?

There remains the unanswerable question of the frame of reference of the whole universe, which very well might be demonstrated for us by the CMB. It may be clear that GR allows us to look at the universe from any frame we choose, but there has been no convincing argument made here that there is not a universal frame of reference. It seems to me that this tool (GR - which is only a model) helps us understand and analyze the universe -- but it is not the universe. Have physicists not gone to far in interpreting what GR is telling us?

ynot · 28th April 2012, 04:15 PM

Originally Posted by sol invictus

It stands or doesn't stand largely depending on whether or not it is rotating with respect to the locally inertial frame. So the question is, can the locally inertial frame itself be spinning?

A top spinning at the centre of a non-spinning disc will stand on it’s tip. It will do exactly the same thing regardless whether the disc is spinning in either direction. Ignoring the friction between the top and disc, the spinning disc will have no effect on the properties of the spinning top. A non-spinning top at the centre of a spinning disc wont stand. Calling the spinning disc a spinning local inertial frame doesn't change a thing.

Originally Posted by sol invictus

The answer is certainly yes. I suggest reading the thread from the beginning if you haven't already.

Not agreeing or accepting isn’t the same thing as not reading and understanding.

sol invictus · 28th April 2012, 06:58 PM

Originally Posted by ynot

A top spinning at the centre of a non-spinning disc will stand on it’s tip. It will do exactly the same thing regardless whether the disc is spinning in either direction. Ignoring the friction between the top and disc, the spinning disc will have no effect on the properties of the spinning top. A non-spinning top at the centre of a spinning disc wont stand. Calling the spinning disc a spinning local inertial frame doesn't change a thing.

To make sure we're clear, let's get rid of the top and disc and substitute a cetrifugoscope^*, a small device with arms that will extend out when it's spinning, but hang down limply when it isn't. OK?

Quote:

Not agreeing or accepting isn’t the same thing as not reading and understanding.

Do you realize that the device I described above will extend its arms if you put it inside a very large shell of matter, and spin the shell (but keep the centrifugoscope fixed in place)?

^*There's probably a name for such a thing, but embarrassingly I have no idea what it is.

ynot · 28th April 2012, 08:06 PM

Originally Posted by sol invictus

To make sure we're clear, let's get rid of the top and disc and substitute a cetrifugoscope^*, a small device with arms that will extend out when it's spinning, but hang down limply when it isn't. OK?

OK - But let’s retain the disc so we can have second frame.

Originally Posted by sol invictus

Do you realize that the device I described above will extend its arms if you put it inside a very large shell of matter, and spin the shell (but keep the centrifugoscope fixed in place)?

If that shell of matter has enough mass to create a gravity that acts on the centrifugoscope or it somehow creates some form of “induced“ gravity on it then I have no trouble accepting it.

I have these big problems with your shell of matter scenario however . . .

(1) There is no credible evidence that such a thing exists and no credible reason why it should.
(2) If such a thing existed surely it’s effects would be evident on more than merely the centrifugoscope.
(3) It would need to be exactly coaxially aligned with the centrifugoscope.
(4) Every other centrifugoscope in existence would have to have it’s own coaxially aligned shell of matter.
(5) All these shells of matter would have to have a gravitational effect only on a particular centrifugoscope but have no effect on any other centrifugoscope (or apparently anything else).

Solve these problems and you might gain my acceptance of your theory.

Originally Posted by sol invictus

^*There's probably a name for such a thing, but embarrassingly I have no idea what it is.

A whatever by any other name . . .

ETA - Just thought of another couple of valid problems . . .

(6) If a rotating shell of matter is causing the arms of the centrifugoscope to extend then anything loosely placed on the arms would fly off them for no apparent reason. And they would continue flying until they reached the rotating shell of matter.
(7) If a constantly rotating shell of matter exists and can force the arms of a centrifugoscope to extend against the force of the gravity of the earth, why can’t and don’t we use this force to produce free energy?

ynot · 29th April 2012, 12:31 AM

Isn't there a conflict between ben m's equivalence-principle-obeying, gravity-like force pulling inwards and sol invictus's very large shell of matter created gravity force pulling outwards at the same time in the same frame?

Farsight · 29th April 2012, 04:34 AM

Originally Posted by Perpetual Student

There remains the unanswerable question of the frame of reference of the whole universe, which very well might be demonstrated for us by the CMB. It may be clear that GR allows us to look at the universe from any frame we choose, but there has been no convincing argument made here that there is not a universal frame of reference. It seems to me that this tool (GR - which is only a model) helps us understand and analyze the universe -- but it is not the universe. Have physicists not gone to far in interpreting what GR is telling us?

No, because they aren't physicists, they're mathematicians pretending to be physicists. Real physicists stay connected to reality. They know that GR tells us that all maps are equally valid, but the map is not the territory, and the earth goes round the sun.

sol invictus · 29th April 2012, 06:22 AM

Originally Posted by ynot

I have these big problems with your shell of matter scenario however . . .

(1) There is no credible evidence that such a thing exists and no credible reason why it should.

Have you ever looked at the sky at night?

Quote:

(2) If such a thing existed surely it’s effects would be evident on more than merely the centrifugoscope.
(3) It would need to be exactly coaxially aligned with the centrifugoscope.
(4) Every other centrifugoscope in existence would have to have it’s own coaxially aligned shell of matter.

The shells don't need to be aligned. Even if the centrifugoscope is off center or not aligned with the rotation axis of the shell, its arms will still rise. As for effects on other things, sure - but one thing at a time. The main effect is to change what you mean by "not spinning".

Quote:

(5) All these shells of matter would have to have a gravitational effect only on a particular centrifugoscope but have no effect on any other centrifugoscope (or apparently anything else).

No, the shells affect everything both inside and outside them at least to some extent.

Quote:

(6) If a rotating shell of matter is causing the arms of the centrifugoscope to extend then anything loosely placed on the arms would fly off them for no apparent reason. And they would continue flying until they reached the rotating shell of matter.

Yes. And?

Quote:

(7) If a constantly rotating shell of matter exists and can force the arms of a centrifugoscope to extend against the force of the gravity of the earth, why can’t and don’t we use this force to produce free energy?

You can extract energy from the relative rotation of things, sure, but I don't know what you mean by "free".

Perpetual Student · 29th April 2012, 09:38 AM

Originally Posted by Farsight

No, because they aren't physicists, they're mathematicians pretending to be physicists.

Nonsense! You don't appear to have the faintest idea of what the word "physicist" means.

Quote:

Real physicists stay connected to reality. They know that GR tells us that all maps are equally valid, but the map is not the territory, and the earth goes round the sun.

Your map analogy is quite persuasive. If you have read some of my posts you can readily see that I am struggling with this question of what GR does and does not imply. However, unlike you, I cannot simply dismiss the accumulated knowledge and wisdom of thousands of physicists throughout the world because of my own semi-educated understanding of this very complex and deep subject.
I often find people being dead wrong by making uninformed judgements about areas in which I am knowledgeable. I know of no other way to change their opinions than through education. Then, if they still persist in their wrongness, so be it.
Similarly, it's my preference to try to learn why physicists universally interpret GR this way by actually studying their science. If I still can't get it, so be it. But -- I will not -- unlike you -- proclaim their science as bogus and prop myself up as some sort of physics savant who knows more than people who have actually mastered the subject.

ynot · 29th April 2012, 02:41 PM

Originally Posted by sol invictus

Have you ever looked at the sky at night?

Yes I frequently look at the night sky. It’s something I like to do. When I do it appears that the Universe is spinning. So you’re saying that the entire extraterrestrial Universe is this large shell of matter spinning about the axis of the Earth?

Originally Posted by sol invictus

The shells don't need to be aligned. Even if the centrifugoscope is off center or not aligned with the rotation axis of the shell, its arms will still rise.

Even when off center or not aligned by 90 degrees (at the equator)?
Are there are multiple shells? Or does the same shell simultaneously spin about infinite axis to suit infinite frames? Do all planets and all things have their own large shell of matter?
If you’re saying that this shell somehow creates a gravitational force that applies equally around the Earth, and that force is greater than gravity, then how does gravity remain active in effect?

Originally Posted by sol invictus

As for effects on other things, sure - but one thing at a time.

No - As much as possible at once. We will never see the trees if we only look at the wood.
Given gravity has an equal effect on all things, and all things free-fall at the same rate, then to be “equivalence-principle-obeying” shouldn’t all things “free-rise” equally as well? Why does this force created by the shell cause the arms of the centrifugoscope to rise and extend against the force of gravity but the body of the centrifugoscope doesn’t even get any lighter?

Originally Posted by sol invictus

The main effect is to change what you mean by "not spinning".

By “not spinning” I mean not displaying the known properties of spinning (evidence of spinning).

Originally Posted by sol invictus

No, the shells affect everything both inside and outside them at least to some extent.

See above.

Originally Posted by sol invictus

Yes. And?

The big “and” is that clearly this doesn’t happen in practice. Any loose material thrown from a spinning thing doesn’t rise to a distant shell, it falls to the Earth. Centrifugal force threw it off then gravity pulled it down. Obviously it therefore wasn’t pulled off by some distant spinning shell of matter.

Originally Posted by sol invictus

You can extract energy from the relative rotation of things, sure, but I don't know what you mean by "free".

Inserting “free” was a subtle tongue-in-cheek comment.

Perpetual Student · 29th April 2012, 03:27 PM

Do I understand correctly that some are claiming here that a top spinning on my table can be equally explained, under GR, by having the total universe spin around the top? If so, how would one explain the top next to it that is laying on its side because it is not spinning? Why would the spinning universe not effect the second top, which I could keep at rest or make it spin in the opposite direction?
Somehow, these considerations make it appear the top is really spinning and these GR equivalency insights are interesting and even useful, but are not reflective of the real world.

ynot · 29th April 2012, 04:13 PM

Originally Posted by Perpetual Student

Do I understand correctly that some are claiming here that a top spinning on my table can be equally explained, under GR, by having the total universe spin around the top? If so, how would one explain the top next to it that is laying on its side because it is not spinning? Why would the spinning universe not effect the second top, which I could keep at rest or make it spin in the opposite direction?
Somehow, these considerations make it appear the top is really spinning and these GR equivalency insights are interesting and even useful, but are not reflective of the real world.

If I understand it correctly then yes you understand it correctly.

Apparently it has to do with different tops being in different frames which creates different realities. Infinite frames create infinite realities for infinite things and everything happens concurrently but not at the same flow of time. But I could be wrong as I’m still trying to get it explained in a manner I can assess it.

ben m · 29th April 2012, 04:36 PM

Originally Posted by Perpetual Student

Do I understand correctly that some are claiming here that a top spinning on my table can be equally explained, under GR, by having the total universe spin around the top? If so, how would one explain the top next to it that is laying on its side because it is not spinning?

This is the thing that seems to be the barrier. In a rotating coordinate systems, Newton's Laws are not true. If you're a passenger on a merry-go-round, and you drop an apple, it does not fall in a straight line---it follows a weird curve. A body in motion does not necessarily tend to stay in motion. A body at rest does not necessarily tend to stay at rest.

In a rotating coordinate system, the laws of motion are NOT the same everywhere. A bullet fired through the coordinate-system axis has a different path than a bullet fired far from the coordinate-system axis.

In a rotating coordinate system, the laws of motion are NOT isotropic. A bullet fired in an axial, radial, or circumferential direction will have a path of a different shape.

These laws are complicated. In a nutshell, that's why you are unable to form a simple, Newton-like mental picture of "a second top, lying on its side off-axis, and orbiting the central nonrotating top" and get this top's equation-of-motion to make intuitive sense. Nonetheless, I assure you that when worked out in detail, the motion of the top-lying-on-its-side has an equation-of-motion that obeys the laws of physics of a spinning coordinate system, just as it obeys the laws of physics of a nonspinning system.

27th April 2012, 02:10 PM	#122
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by sol invictus No, either you spun the top or you stopped it from spinning. Please explain how a top lying down on a surface can be spinning about it’s central axis. Also explain how an otherwise unsupported top standing on it’s tip can do so without spinning.
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27th April 2012, 02:12 PM	#123
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by theprestige Spinning relative to what? Relative to not spinning.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

27th April 2012, 02:28 PM	#124
epepke Philosopher Join Date: Oct 2003 Posts: 7,950	Originally Posted by ynot Please explain how a top lying down on a surface can be spinning about it’s central axis. The whole surface could be spinning. Anyway, it's kind of funny how people get this way. I think that we evolved for this slow-moving stuff. So Aristotelian stationary, absolute physics makes a lot of sense. When we learn more, we want it to make sense again. So we insist on these things like inertial, non-rotating frames of reference. Then we get to think, "Ah, I can sort of understand. This is just like a big forest, only with weird time." This works pretty well up through Special Relativity. Then General Relativity comes along and smacks us in the face and says, "Sucker! You think it gets simple? It doesn't. There are no inertial frames! HAH! You think you are going to get out of solving ten coupled differential equations? Sucks to be you." At least, though, the compensation is that you get to make the coordinate system whatever the hell you want. You want to see the universe from the point of view of a power ball in the Addams Family pinball machine? Go ahead.
epepke Philosopher Join Date: Oct 2003 Posts: 7,950	__________________ "It probably came from a sticky dark planet far, far away." - Godzilla versus Hedora "There's no evidence that the 9-11 attacks (whoever did them) were deliberately attacking civilians. On the contrary the targets appear to have been chosen as military." -DavidByron

27th April 2012, 02:43 PM	#125
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by epepke The whole surface could be spinning. It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Same applies to everything else in the room and the entire Universe that would also have to be spinning about the central axis of the top. What about a second top on the same surface? Can they both be spinning about their own axis and also spinning about the axis of the other? How does the surface spin about the axis of both tops at the same time?
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

27th April 2012, 03:09 PM	#128
epepke Philosopher Join Date: Oct 2003 Posts: 7,950	Originally Posted by ynot It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Who are you to say they aren't? Why did you think their tie was stuck to the window anyway?
epepke Philosopher Join Date: Oct 2003 Posts: 7,950	__________________ "It probably came from a sticky dark planet far, far away." - Godzilla versus Hedora "There's no evidence that the 9-11 attacks (whoever did them) were deliberately attacking civilians. On the contrary the targets appear to have been chosen as military." -DavidByron

27th April 2012, 10:20 AM	#121
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by Merton sol invictus, thank you very much, that's making sense now. One more question, if you would indulge me. Where do these patches of inflation occur, or are they a yet-unobserved prediction of the inflationary model? If our universe is undergoing eternal inflation, then the part of the universe we can see is part of a patch that at some point exited from the eternally inflating phase. That patch is surrounded, probably on all sides, by eternally inflating volume. That volume hasn't been observed, but that could just be because our patch is very large.

27th April 2012, 03:02 PM	#126
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	Originally Posted by ynot It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Same applies to everything else in the room and the entire Universe that would also have to be spinning about the central axis of the top. What about a second top on the same surface? Can they both be spinning about their own axis and also spinning about the axis of the other? How does the surface spin about the axis of both tops at the same time? Because, as I said earlier, the top analogy isn't an accurate one. The person, table, room, top, Earth, etc. are all within the same reference frame when the top is at rest. You apply force to it and the top spins for a while, but slows down due to the friction created due to the table and air (both in the Earth's reference frame). The top will feel centrifugal force because it's running into all this other stuff in the Earth's reference frame. The person, table, etc. does not feel this force because they're all in the same reference frame.

27th April 2012, 03:03 PM	#127
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	Originally Posted by sol invictus If our universe is undergoing eternal inflation, then the part of the universe we can see is part of a patch that at some point exited from the eternally inflating phase. That patch is surrounded, probably on all sides, by eternally inflating volume. That volume hasn't been observed, but that could just be because our patch is very large. Thanks for the great info! Much appreciated!

27th April 2012, 03:09 PM	#129
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by Merton Because, as I said earlier, the top analogy isn't an accurate one. The person, table, room, top, Earth, etc. are all within the same reference frame when the top is at rest. You apply force to it and the top spins for a while, but slows down due to the friction created due to the table and air (both in the Earth's reference frame). The top will feel centrifugal force because it's running into all this other stuff in the Earth's reference frame. The person, table, etc. does not feel this force because they're all in the same reference frame. Don’t know what your discussing but I’m discussing the question whether we can know from observation whether a top is spinning about it‘s central axis or not. The top isn’t an analogy it‘s a real thing.
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27th April 2012, 03:14 PM	#130
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by epepke Who are you to say they aren't? Why did you think their tie was stuck to the window anyway? Humour (if that's what it's meant to be) is always good but in this case I would prefer and appreciate a serious answer if you can provide one.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise. Last edited by ynot; 27th April 2012 at 03:30 PM.

27th April 2012, 03:38 PM	#131
ben m Illuminator Join Date: Jul 2006 Posts: 4,647	Originally Posted by ynot It would have to be spinning about the central axis of the top. So would the person that “spun” the top. Why doesn’t this person experience the forces of this spinning and be thrown against the wall of the room by centrifugal force? Same applies to everything else in the room and the entire Universe that would also have to be spinning about the central axis of the top. What about a second top on the same surface? Can they both be spinning about their own axis and also spinning about the axis of the other? How does the surface spin about the axis of both tops at the same time? Because the actual laws of motion in the frame with the top at rest are not the same as the laws of motion in the frame where the top is spinning. a) In the frame where the top is spinning and the ground is at rest, the observer says "the top is spinning, and I'm standing here next to a wall". b) In the frame where the top is at rest and the ground is spinning, the observer says "the top is at rest, I am standing on the spinning ground, and there's an equivalence-principle-obeying, gravity-like force pulling me inwards so I don't crash into that wall." If you forget to include the 2nd part---the coordinate-dependent forces---then you're not doing GR, and it's not valid to treat all coordinate systems as equal.
ben m Illuminator Join Date: Jul 2006 Posts: 4,647

27th April 2012, 03:41 PM	#132
ben m Illuminator Join Date: Jul 2006 Posts: 4,647	(But it's the same thing that happens in "familiar" curved spacetime. Take an astronaut on the space station. He's going around and around the Earth at a rate such that, if you were using the laws of flat space, you'd expect him to be plastered against the back wall of the space station, like a passenger in a fairground ride, by an 9.8g centripetal force. But he's not, for reasons that appear different from different perspectives. An observer in the space station says "Of course I'm not plastered against the wall, I'm sitting still with respect to the wall and obeying Newton's Laws." An observer on the ground says "Of course he's not plastered against the wall, he's being kept away from it by an equivalence-principle-obeying force I will call 'Earth's gravity'".)
ben m Illuminator Join Date: Jul 2006 Posts: 4,647

27th April 2012, 04:07 PM	#133
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by ben m Because the actual laws of motion in the frame with the top at rest are not the same as the laws of motion in the frame where the top is spinning. Two different laws of motion apply to the same event at the same time? Different frames have different observational perspectives of the same event but I don’t see how perception anomalies can change the reality of the event (or the actual laws of motion). Originally Posted by ben m a) In the frame where the top is spinning and the ground is at rest, the observer says "the top is spinning, and I'm standing here next to a wall". b) In the frame where the top is at rest and the ground is spinning, the observer says "the top is at rest, I am standing on the spinning ground, and there's an equivalence-principle-obeying, gravity-like force pulling me inwards so I don't crash into that wall." If you forget to include the 2nd part---the coordinate-dependent forces---then you're not doing GR, and it's not valid to treat all coordinate systems as equal. So the laws of motion can be broken but an equivalence principle must be obeyed? Convenience principle? Where does this "gravity-like force " come from and go to?
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27th April 2012, 04:27 PM	#134
ben m Illuminator Join Date: Jul 2006 Posts: 4,647	Originally Posted by ynot Two different laws of motion apply to the same event at the same time? Different frames have different observational perspectives of the same event but I don’t see how perception anomalies can change the reality of the event (or the actual laws of motion). Remember, "the laws of motion" are merely things like "if I know the x,y,z coordinates of the ball at time t=0, what will the x,y,z coordinates be at time t=1". Think of how complicated it is to make the same statement in spherical coordinates---"I know the r, theta, phi of the ball at t=0, what will the r,theta,phi be at t=1?". All we're talking about is your ability to do that with a rotating collection of coordinate labels. Or a distorted collection. Or a time-dependent collection. The point of this discussion is: there are certain oddities---free-falling objects follow weird paths!---that show up in coordinate systems distorted by gravity. The surprising-ish thing, and the counterintuitive thing, is that exactly the same oddities---free-falling objects follow weird paths!---show up in rotating (or otherwise accelerating) coordinate systems. The laws of physics are perfectly happy predicting the coordinates of a baseball (Or a photon, or a wave in an atomic clock, or whatever) as it follows a weird path through the familiar distorted coordinate-system we use near Earth. It turns out that the laws of physics are equally happy predicting the coordinates of a baseball (or a photon, or an atomic clock, etc.) if you drop it while sitting on a merry-go-round, using distorted merry-go-round coordinates and all of the centripetal/Coriolis stuff that is attached. Quote: Where does this "gravity-like force " come from and go to? Where does Newtonian gravity "go" when for an observer in freefall? Nowhere. Newtonian gravity---9.8m/s/s downwards---is something you have to include in your equation-of-motion if your coordinate system is glued down at a fixed radius from the Earth. It's not something you have to include if your coordinate system is inertial. It's a coordinate-system-dependent pseudoforce.
ben m Illuminator Join Date: Jul 2006 Posts: 4,647	Last edited by ben m; 27th April 2012 at 04:32 PM.

27th April 2012, 04:51 PM	#135
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by ben m Where does Newtonian gravity "go" when for an observer in freefall? Nowhere. Newtonian gravity---9.8m/s/s downwards---is something you have to include in your equation-of-motion if your coordinate system is glued down at a fixed radius from the Earth. It's not something you have to include if your coordinate system is inertial. It's a coordinate-system-dependent pseudoforce. Gravity (it's not Newton's) doesn't "go" anywhere for an observer in freefall. Merely the ability to "feel" it is "gone". It remains in place and it's force remains fully active as is evident by the falling. That the freefalling observer doesn't experience that force doesn't mean the force has gone away (actually does experience it but does so equally on every atom). I've never been able to understand how or why the ability to observe or experience an event can have any effect on the actual event. Illusions don't make illusion fact. Given I'm not freefalling toward the top in your a) scenario why don't I experience this equivalence-principle-obeying, gravity-like force pulling me toward the top? So I repeat - Where does this mysterious force come from in b) and where does it go to in a)?
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27th April 2012, 05:04 PM	#136
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by ben m The point of this discussion is: there are certain oddities---free-falling objects follow weird paths!---that show up in coordinate systems distorted by gravity. The surprising-ish thing, and the counterintuitive thing, is that exactly the same oddities---free-falling objects follow weird paths!---show up in rotating (or otherwise accelerating) coordinate systems. I may be derailing this thread but my point of this dicusion is whether or not it can be known whether a thing is spinning or not (specifically a top).
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27th April 2012, 05:10 PM	#137
theprestige Philosopher Join Date: Aug 2007 Posts: 8,564	Originally Posted by ynot Relative to not spinning. Not spinning in what frame of reference?
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27th April 2012, 05:16 PM	#138
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by theprestige Not spinning in what frame of reference? In the reference frame of reality.
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27th April 2012, 05:20 PM	#139
ben m Illuminator Join Date: Jul 2006 Posts: 4,647	Originally Posted by ynot Gravity (it's not Newton's) doesn't "go" anywhere for an observer in freefall. Merely the ability to "feel" it is "gone". It remains in place and it's force remains fully active as is evident by the falling. OK. So now suppose you and Bob are arguing about whether the top is spinning. Bob says it's not. You say it is, and to prove it you say: "And I know I'm in a non-rotating frame, because I do experiments which don't feel any centripetal forces." Bob's reply would sound just like yours above: Originally Posted by Bob as ynot Centripetal forces don't "go" anywhere for an observer in freefall, i.e. in circular an orbit around the centripetal force center. Merely the ability to "feel" centripetal forces is "gone". The centripetal force remains in place and it's force remains fully active as is evident by your trajectory, which is going around and around the stationary top.
ben m Illuminator Join Date: Jul 2006 Posts: 4,647

27th April 2012, 05:31 PM	#140
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by ben m OK. So now suppose you and Bob are arguing about whether the top is spinning. Bob says it's not. You say it is, and to prove it you say: "And I know I'm in a non-rotating frame, because I do experiments which don't feel any centripetal forces." Bob's reply would sound just like yours above: But I'm not freefalling toward the top and I'm not doing anything I know of which could or would remove my ability to feel any centripetal forces. We know and can explain why freefall doesn't feel gravity (or isn't aware it does) how would Bob (you) otherwise explain not feeling either centripetal forces or this "gravity-like force"? Also the top is displaying properties of having centripetal forces. How would Bob (you) explain it's ability to stand on it's point if it wasn't spinning?
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise. Last edited by ynot; 27th April 2012 at 05:44 PM.

27th April 2012, 05:32 PM	#141
DarbyII Student Join Date: Jan 2008 Posts: 45	Originally Posted by sol invictus I don't understand your question. It really wasn't a question. There's a reason why Special Relativity is limited to inertial frames. In the case of inertial frames, yes, there is no preferred frame of reference. Motion between the bodies is relative in an inertial frame. The same isn't necessarily true in all accelerated frames. There are absolute motions in an accelerated frames. A planet in orbit is in an accelerated frame. The planet may be moving at a more or less constant speed but speed is a scalar value. Velocity is a vector - magnitude and direction. The direction of the planet as it orbits is constantly changing thus the velocity is constantly changing, i.e. accelerated frame. If you are in an accelerated frame there are experiments that can be run to detect the acceleration. An laser accelerometer might be useful here. If you can detect the acceleration then it is possible to determine in a two body experiment which is the major body and which is the minor orbiting body.
DarbyII Student Join Date: Jan 2008 Posts: 45	Last edited by DarbyII; 27th April 2012 at 05:42 PM.

27th April 2012, 05:52 PM	#142
ben m Illuminator Join Date: Jul 2006 Posts: 4,647	Originally Posted by ynot But I'm not freefalling toward the top and I'm not doing anything which could or would remove my ability to feel any centripetal forces. We know and can explain why freefall doesn't feel gravity (or isn't aware it does) how would Bob (you) explain not feeling this "gravity-like force"? Also the top is displaying properties of centripetal forces. According to Bob, and every experiment he can think of, the equations of motion of the Universe naturally include a centrifugal force that pulls you towards the axis of his coordinates. Bob says you are free-falling under that force---and, because it's an equivalence-principle-obeying force, Bob fully expects that free-falling bodies can't feel it---not relative to other free-falling bodies anyway. If you insist, Bob can write down the equation-of-motion of your head, and the equations-of-motion of the ground underneath you, and he will derive that the centripetal/coriolis force is pulling you and the ground around in such a way that there's no differential acceleration between you. There's a decent animation of this in on Wikipedia. http://en.wikipedia.org/wiki/Fictiti...ircular_motion
ben m Illuminator Join Date: Jul 2006 Posts: 4,647

27th April 2012, 07:37 PM	#143
Toontown Illuminator Join Date: Jun 2010 Posts: 3,555	On a related note: when I'm walking, is it OK to think of myself as stationary, rolling the earth beneath my feet?
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27th April 2012, 08:08 PM	#144
epepke Philosopher Join Date: Oct 2003 Posts: 7,950	Originally Posted by ynot Humour (if that's what it's meant to be) is always good but in this case I would prefer and appreciate a serious answer if you can provide one. Well, people have been trying. @ben_m is doing a particularly good job. However, you seem to exhibit a huge reluctance against this kind of thing. See, you seem to be really, really convinced that there is something called "spinning" and something called "not spinning," and that this has some meaning. That works fine in Newtonian physics and up through Special Relativity. It doesn't work in General Relativity. Let's say that you are on a rotating space station, like the one on 2001: A Space Odyssey. If you haven't seen it, go see it now. Like the scene where they meet the Russians in the funny-looking chairs and the whole set is curved, got it? Only there are no windows, but the chair still pushes up on your butt. Well, you are going to conclude that this isn't real gravity. It's fake gravity, or pseudo-gravity. Because you're rotating, see? But in General Relativity, there is no distinction. It is real gravity, just as real as if you were on a planet. Trying to prove me wrong, you do a GR calculation assuming that your space station is rotating, and you find that it agrees with what you measure. Hah, you say! Then I tell you to redo the calculation, this time assuming that the space station is stationary, and the entire universe is rotating in the opposite direction. That's funny, you say. The calculations predict exactly the same thing. So you can have fifteen guys on fifteen little tops, and four on jackhammers, and two on potatoes, or something, and none of them can ever really say "I'm rotating, and you're not." And that's because, unlike Special Relativity or Galilean Relativity, where the laws of physics are different for a rotating rather than a non-rotating frame, under General Relativity, the laws of physics are the same. This is probably why it's called General Relativity. It's relative in general, not in the special case of an inertial frame.
epepke Philosopher Join Date: Oct 2003 Posts: 7,950	__________________ "It probably came from a sticky dark planet far, far away." - Godzilla versus Hedora "There's no evidence that the 9-11 attacks (whoever did them) were deliberately attacking civilians. On the contrary the targets appear to have been chosen as military." -DavidByron

28th April 2012, 07:15 AM	#145
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by ynot Please explain how a top lying down on a surface can be spinning about it’s central axis. Also explain how an otherwise unsupported top standing on it’s tip can do so without spinning. It stands or doesn't stand largely depending on whether or not it is rotating with respect to the locally inertial frame. So the question is, can the locally inertial frame itself be spinning? The answer is certainly yes. I suggest reading the thread from the beginning if you haven't already.

28th April 2012, 07:19 AM	#146
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by DarbyII A planet in orbit is in an accelerated frame. The planet may be moving at a more or less constant speed but speed is a scalar value. Velocity is a vector - magnitude and direction. The direction of the planet as it orbits is constantly changing thus the velocity is constantly changing, i.e. accelerated frame. If you are in an accelerated frame there are experiments that can be run to detect the acceleration. An laser accelerometer might be useful here. If you can detect the acceleration then it is possible to determine in a two body experiment which is the major body and which is the minor orbiting body. That's true only if you assume something else (such as that your orbiting bodies are in a nearly flat spacetime, with no rotating shells out near infinity, standard boundary conditions, etc.).

28th April 2012, 10:35 AM	#147
Farsight Graduate Poster Join Date: Mar 2008 Location: Poole, UK Posts: 1,960	Originally Posted by DarbyII If you can detect the acceleration then it is possible to determine in a two body experiment which is the major body and which is the minor orbiting body. Quite right, Darby. Originally Posted by Toontown On a related note: when I'm walking, is it OK to think of myself as stationary, rolling the earth beneath my feet? Nope. Because I'm walking past you the other way, and I'm rolling the earth beneath my feet. LOL! No I'm not, we're just walking. Originally Posted by ynot In the reference frame of reality. Absolutely. There are some people on this forum who peddle some total woo, and try to get away with it by suggesting that's what GR tells us when it doesn't. All GR says is that your map is as valid as mine. It doesn't say that your map is the territory and mine is too. You know it's the top spinning because you can change its rotation speed. When some woomeister clique tries to tell you that you can just as well assert that the universe is rotating around it, you point out that actually, you changed its rotation speed to zero, and now it's just sitting there. So now they're trying to tell you that your top is rotating in line with the universe, when there's no evidence whatsoever for any rotation at all. And across the room my top is whirling like a dervish. It's a load of old cobblers.

28th April 2012, 02:01 PM	#148
theprestige Philosopher Join Date: Aug 2007 Posts: 8,564	Originally Posted by ynot In the reference frame of reality. And which frame would that be, exactly?
theprestige Philosopher Join Date: Aug 2007 Posts: 8,564

28th April 2012, 04:13 PM	#149
Perpetual Student Illuminator Join Date: Jul 2008 Location: USA Posts: 3,707	Originally Posted by ben m Perpetual Student, perhaps part of the problem is the difference between: a) "A coordinate system in which the top happens to be at rest" and b) "A coordinate system glued to the top, with its Z-axis along the top's handle and its x-axis aligned with this particular spot of paint." The former is the sort of thing GR normally talks about. (It can handle the latter, sort of, but, ugh.) You describe "I know I spun up the top". Well, in coordinate system (a), you started out with a spinning top on a spinning table, and a spinning person reached over and stopped the top from spinning. That can happen, right? If you went to the North Pole, sat down, and started playing with a very slow top, that's exactly what an outside observer would see. "A spinning top on a spinning planet, then the spinning person set it down and made it stop." (system (b) changes with time. "for t<0 we choose one coordinate system, and at t=0, PS exerted a force on the top, and for t >0 we transform to a different system", such that the top is "at rest" both before and after you exert the force. This is a mess.) Let me translate your "I know I spun the top" to another system, see if your intuition does something different. We're hovering in outer space watching a soccer ball fly away from us. I tell you that SR allows me to treat this soccer ball as being at rest. You just define a set of coordinates that are moving along with the ball, and in those coordinates we're moving away from the ball, right? Does that help? There remains the unanswerable question of the frame of reference of the whole universe, which very well might be demonstrated for us by the CMB. It may be clear that GR allows us to look at the universe from any frame we choose, but there has been no convincing argument made here that there is not a universal frame of reference. It seems to me that this tool (GR - which is only a model) helps us understand and analyze the universe -- but it is not *the* universe. Have physicists not gone to far in interpreting what GR is telling us?
	__________________ _{It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong. - Richard P. Feynman} ξ

28th April 2012, 04:15 PM	#150
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by sol invictus It stands or doesn't stand largely depending on whether or not it is rotating with respect to the locally inertial frame. So the question is, can the locally inertial frame itself be spinning? A top spinning at the centre of a non-spinning disc will stand on it’s tip. It will do exactly the same thing regardless whether the disc is spinning in either direction. Ignoring the friction between the top and disc, the spinning disc will have no effect on the properties of the spinning top. A non-spinning top at the centre of a spinning disc wont stand. Calling the spinning disc a spinning local inertial frame doesn't change a thing. Originally Posted by sol invictus The answer is certainly yes. I suggest reading the thread from the beginning if you haven't already. Not agreeing or accepting isn’t the same thing as not reading and understanding.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise. Last edited by ynot; 28th April 2012 at 04:33 PM.

28th April 2012, 06:58 PM	#151
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by ynot A top spinning at the centre of a non-spinning disc will stand on it’s tip. It will do exactly the same thing regardless whether the disc is spinning in either direction. Ignoring the friction between the top and disc, the spinning disc will have no effect on the properties of the spinning top. A non-spinning top at the centre of a spinning disc wont stand. Calling the spinning disc a spinning local inertial frame doesn't change a thing. To make sure we're clear, let's get rid of the top and disc and substitute a cetrifugoscope^, a small device with arms that will extend out when it's spinning, but hang down limply when it isn't. OK? Quote: Not agreeing or accepting isn’t the same thing as not reading and understanding. Do you realize that the device I described above will extend its arms if you put it inside a very large shell of matter, and spin the shell* (but keep the centrifugoscope fixed in place)? ^*There's probably a name for such a thing, but embarrassingly I have no idea what it is.

28th April 2012, 08:06 PM	#152
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Originally Posted by sol invictus To make sure we're clear, let's get rid of the top and disc and substitute a cetrifugoscope^, a small device with arms that will extend out when it's spinning, but hang down limply when it isn't. OK? OK - But let’s retain the disc so we can have second frame. Originally Posted by sol invictus* Do you realize that the device I described above will extend its arms if you put it inside a very large shell of matter, and spin the shell (but keep the centrifugoscope fixed in place)? If that shell of matter has enough mass to create a gravity that acts on the centrifugoscope or it somehow creates some form of “induced“ gravity on it then I have no trouble accepting it. I have these big problems with your shell of matter scenario however . . . (1) There is no credible evidence that such a thing exists and no credible reason why it should. (2) If such a thing existed surely it’s effects would be evident on more than merely the centrifugoscope. (3) It would need to be exactly coaxially aligned with the centrifugoscope. (4) Every other centrifugoscope in existence would have to have it’s own coaxially aligned shell of matter. (5) All these shells of matter would have to have a gravitational effect only on a particular centrifugoscope but have no effect on any other centrifugoscope (or apparently anything else). Solve these problems and you might gain my acceptance of your theory. Originally Posted by sol invictus ^*There's probably a name for such a thing, but embarrassingly I have no idea what it is. A whatever by any other name . . . ETA - Just thought of another couple of valid problems . . . (6) If a rotating shell of matter is causing the arms of the centrifugoscope to extend then anything loosely placed on the arms would fly off them for no apparent reason. And they would continue flying until they reached the rotating shell of matter. (7) If a constantly rotating shell of matter exists and can force the arms of a centrifugoscope to extend against the force of the gravity of the earth, why can’t and don’t we use this force to produce free energy?
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise. Last edited by ynot; 28th April 2012 at 08:25 PM.

29th April 2012, 12:31 AM	#153
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,641	Isn't there a conflict between ben m's equivalence-principle-obeying, gravity-like force pulling inwards and sol invictus's very large shell of matter created gravity force pulling outwards at the same time in the same frame?
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

29th April 2012, 04:34 AM	#154
Farsight Graduate Poster Join Date: Mar 2008 Location: Poole, UK Posts: 1,960	Originally Posted by Perpetual Student There remains the unanswerable question of the frame of reference of the whole universe, which very well might be demonstrated for us by the CMB. It may be clear that GR allows us to look at the universe from any frame we choose, but there has been no convincing argument made here that there is not a universal frame of reference. It seems to me that this tool (GR - which is only a model) helps us understand and analyze the universe -- but it is not *the* universe. Have physicists not gone to far in interpreting what GR is telling us? No, because they aren't physicists, they're mathematicians pretending to be physicists. Real physicists stay connected to reality. They know that GR tells us that all maps are equally valid, but the map is not the territory, and the earth goes round the sun.

29th April 2012, 06:22 AM	#155
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by ynot I have these big problems with your shell of matter scenario however . . . (1) There is no credible evidence that such a thing exists and no credible reason why it should. Have you ever looked at the sky at night? Quote: (2) If such a thing existed surely it’s effects would be evident on more than merely the centrifugoscope. (3) It would need to be exactly coaxially aligned with the centrifugoscope. (4) Every other centrifugoscope in existence would have to have it’s own coaxially aligned shell of matter. The shells don't need to be aligned. Even if the centrifugoscope is off center or not aligned with the rotation axis of the shell, its arms will still rise. As for effects on other things, sure - but one thing at a time. The main effect is to change what you mean by "not spinning". Quote: (5) All these shells of matter would have to have a gravitational effect only on a particular centrifugoscope but have no effect on any other centrifugoscope (or apparently anything else). No, the shells affect everything both inside and outside them at least to some extent. Quote: (6) If a rotating shell of matter is causing the arms of the centrifugoscope to extend then anything loosely placed on the arms would fly off them for no apparent reason. And they would continue flying until they reached the rotating shell of matter. Yes. And? Quote: (7) If a constantly rotating shell of matter exists and can force the arms of a centrifugoscope to extend against the force of the gravity of the earth, why can’t and don’t we use this force to produce free energy? You can extract energy from the relative rotation of things, sure, but I don't know what you mean by "free".