Terra revolving round Sol question - Page 2

sol invictus · 24th April 2012, 05:21 AM

Originally Posted by OnlyTellsTruths

We would know that because, in this hypothetical, the Earth came from the disc section, which by it's very nature forms around the center. Most importantly because the disc starts out as a whole and ends up breaking apart, while the center always remains the center. Therefore a product from the disc section cannot be the center, otherwise it wouldn't have been from the disc section.

This assumes the center of the disk cannot rotate around a point near its edge. That isn't the case.

Merton · 24th April 2012, 07:41 AM

Originally Posted by OnlyTellsTruths

Again to clairify.

In my hypothetical, proof is found that the Earth came from the disc section, and the Sun came from the center section, both from the same protoplanetary disc. With no outside interference.

It seems you're describing a very simple solar system and that's where the confusion lies. In this example, both the planet and star comprise a bound system whose center of gravity lies nearly at the center of mass of the star. But think of a binary system and you'll begin to see the problem with this scenario. Two stars of relatively similar mass (compared with the planetary objects) will have a center of gravity close to the center of these stars (with minor variation due to massive planets). So, in this case, which star is rotating around which? It's still a bound system, like the single-star system in your example, but its center of gravity isn't near the center of mass of the star.

Hopefully that makes some sense. Essentially, in a bound system, all objects are rotating around a common center of gravity. In a single-star system, it appears that the star is the center, but it too is orbiting around the center of gravity and, to be accurate, the Earth is still not rotating around the sun, but the center of gravity. Of course, for practical reasons and because the center of gravity is very nearly the center of mass of the star, one can say the Earth is orbiting the sun.

Beerina · 24th April 2012, 07:52 AM

If the Earth were the only thing in the universe you could determine it was rotating.

sol invictus · 24th April 2012, 08:55 AM

Originally Posted by Beerina

If the Earth were the only thing in the universe you could determine it was rotating.

Assuming gravity gradients aren't really large at your location, you can always do an experiment and find a locally inertial frame. Something not rotating in that frame won't experience "fictitious forces"; it won't feel centrifugal stress for example. So it can be said to be "not rotating".

Most of the subtlety here arises because

(a) A rotating shell with very, very large radius can change the state of rotation of the locally inertial frames inside it. Similarly, two things each of which is "not rotating" by the above definition can be rotating with respect to each other.

(b) You are always perfectly free to use "rotating coordinates", in which a rotating object is static. Of course the presence or absence of centrifugal stresses is independent of coordinate choice (it's a real physical effect, so it had better be), but see (a).

Perpetual Student · 24th April 2012, 12:25 PM

This has been debated again and again on many threads here. For example: General Relativity. I'm not sure there exists a real final answer given our current tools and understanding -- but I hope I'm wrong!

1. On the one hand, we have GR totalitarianism, which holds that any sense we have of some notion of reality involving rotation -- like the earth actually does revolve around the sun or there is a top really spinning on my table -- is an illusion. All frames are equal; there is no real frame of reference to tell us what rotates or revolves around what -- according to this doctrine.

2. On the other hand we have our knowledge of history: I spun the top on my table so it is really spinning; the earth sun system was formed from a primordial cloud as described above, so the earth is really revolving around the sun. So GR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening.

I have been told by some here that option 2 above is wrong and that the key is in understanding the mathematics of GR. Maybe so and maybe not -- I'm working on it.

ynot · 24th April 2012, 12:54 PM

Originally Posted by Perpetual Student

This has been debated again and again on many threads here. For example: General Relativity. I'm not sure there exists a real final answer given our current tools and understanding -- but I hope I'm wrong!

1. On the one hand, we have GR totalitarianism, which holds that any sense we have of some notion of reality involving rotation -- like the earth actually does revolve around the sun or there is a top really spinning on my table -- is an illusion. All frames are equal; there is no real frame of reference to tell us what rotates or revolves around what -- according to this doctrine.

2. On the other hand we have our knowledge of history: I spun the top on my table so it is really spinning; the earth sun system was formed from a primordial cloud as described above, so the earth is really revolving around the sun. So GR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening.

I have been told by some here that option 2 above is wrong and that the key is in understanding the mathematics of GR. Maybe so and maybe not -- I'm working on it.

I conclude that the top is spinning for two obvious reasons . . .

(1) A spinning top “stands” a non-spinning top doesn’t. You don’t have to remember you spun it to know it’s spinning.

(2) Either you spun the top or you spun the entire Universe. Pick the only one we know to be possible.

Merton · 24th April 2012, 01:04 PM

Perpetual Student, #2 is indeed incorrect. Or, at least, it's not as accurate as #1. The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect.

To put it concisely, the center of gravity around which other massive objects orbit is not necessarily the center of a star's mass. This is made clear when discussing multi-star systems, whose masses also share a common history. In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system).

So history doesn't determine that the Earth revolves around the sun, just as history wouldn't be able to determine which star of a multi-star system a planet orbits. It's not orbiting a star at all... it's orbiting their common center of gravity. It's merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star.

Merton · 24th April 2012, 01:12 PM

The spinning top isn't a great analogy. Namely, the rotation of the top is slowed by forces that wouldn't affect massive objects like planets and stars; forces that exist within the frame of reference of the Earth and the top-spinner. If the top were spinning out in space, with or without the knowledge of who put it there, the frame of reference for the top would be equally as valid as any other frame of reference. Knowing the history of the object is an entirely separate question.

ynot · 24th April 2012, 01:19 PM

Originally Posted by Merton

Perpetual Student, #2 is indeed incorrect. Or, at least, it's not as accurate as #1. The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect.

To put it concisely, the center of gravity around which other massive objects orbit is not necessarily the center of a star's mass. This is made clear when discussing multi-star systems, whose masses also share a common history. In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system).

So history doesn't determine that the Earth revolves around the sun, just as history wouldn't be able to determine which star of a multi-star system a planet orbits. It's not orbiting a star at all... it's orbiting their common center of gravity. It's merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star.

Are you claiming that the gravity of the Sun has nothing to do with the mass of the Sun? If the Sun disappeared are you claiming the gravity would remain? Are you serious?

ynot · 24th April 2012, 01:23 PM

Originally Posted by Merton

The spinning top isn't a great analogy. Namely, the rotation of the top is slowed by forces that wouldn't affect massive objects like planets and stars; forces that exist within the frame of reference of the Earth and the top-spinner. If the top were spinning out in space, with or without the knowledge of who put it there, the frame of reference for the top would be equally as valid as any other frame of reference. Knowing the history of the object is an entirely separate question.

Nobody tell the Space Station and all the satellites orbiting Earth that gyroscopes don’t work in space.

Merton · 24th April 2012, 01:31 PM

Originally Posted by ynot

Are you claiming that the gravity of the Sun has nothing to do with the mass of the Sun?

No, not at all. Mass is the reason for gravity.

Originally Posted by ynot

If the Sun disappeared are you claiming the gravity would remain? Are you serious?

Some gravity would remain because there are other massive objects in the solar system, but the sun is such a large object compared to these others that removing the sun would remove most of the gravity we experience. It's important that you consider all of the mass within a bound system for its net gravitational force though, and that's why it's "merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star."

Originally Posted by ynot

Nobody tell the Space Station and all the satellites orbiting Earth that gyroscopes don’t work in space.

Either your reading comprehension isn't that great, or you're just looking for an argument.

Perpetual Student · 24th April 2012, 01:39 PM

Originally Posted by Merton

Perpetual Student, #2 is indeed incorrect. Or, at least, it's not as accurate as #1. The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect.

To put it concisely, the center of gravity around which other massive objects orbit is not necessarily the center of a star's mass. This is made clear when discussing multi-star systems, whose masses also share a common history. In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system).

So history doesn't determine that the Earth revolves around the sun, just as history wouldn't be able to determine which star of a multi-star system a planet orbits. It's not orbiting a star at all... it's orbiting their common center of gravity. It's merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star.

Everyone one here knows about rotation about the center of gravity -- using the sun as the center is merely a simplification for discussion purposes. You missed the point entirely. Under GR, it is claimed that "(your comment) In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system)." is also not a description of reality. Any frame other than the center of gravity is equally valid as a frame of reference.

ben m · 24th April 2012, 01:41 PM

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?

Originally Posted by modified PS

On the other hand we have our knowledge of history: I used to have the ball at rest, then I kicked it, so it's really moving. The ball was really in my hand a minute ago, and the only force that acted on it was that of my kick, so I know how much the ball accelerated. So SR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening.

Does that help?

Merton · 24th April 2012, 01:44 PM

Perpetual Student, I've apparently missed the point again. And I also don't see how my quoted statement contradicts the fact that any frame of reference is as valid as the next. Could you elaborate?

sol invictus · 24th April 2012, 02:00 PM

Originally Posted by Perpetual Student

This has been debated again and again on many threads here.
...
2. On the other hand we have our knowledge of history: I spun the top on my table so it is really spinning

We have indeed discussed this many times, and many times I've tried to explain to you that causality has absolutely nothing to do with this. Changing to a rotating frame is not in any way, shape, or form equivalent to physically rotating the entire universe. They are two distinct and different things.

Originally Posted by ben m

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've tried a very similar argument several times, to no avail. But PS has been studying GR (and I think you're better at explaining these things), so perhaps that will work.

Originally Posted by ynot

I conclude that the top is spinning for two obvious reasons . . .

(1) A spinning top “stands” a non-spinning top doesn’t. You don’t have to remember you spun it to know it’s spinning.

That indeed tells you that the top is spinning with respect to the local inertial frame. What it doesn't tell you is whether it's spinning with respect to another top somewhere else that won't stand up.

Quote:

(2) Either you spun the top or you spun the entire Universe.

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

ynot · 24th April 2012, 02:06 PM

Originally Posted by Merton

No, not at all. Mass is the reason for gravity.

So why do you say it’s a “coincidence” that the centre of gravity of the solar system is near the centre of the greatest mass in the solar system (Sun = more than 99.8%)?

Originally Posted by Merton

Some gravity would remain because there are other massive objects in the solar system, but the sun is such a large object compared to these others that removing the sun would remove most of the gravity we experience. It's important that you consider all of the mass within a bound system for its net gravitational force though, and that's why it's "merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star.".

The Sun and Earth both have independent mass that creates independent gravity. If the Sun disappeared the Earth would retain it’s gravity but would tend to travel in a straight path rather than circular. Without the Sun the solar system wouldn’t be a “bound system”.

Originally Posted by Merton

Either your reading comprehension isn't that great, or you're just looking for an argument.

I’m looking for clarification and understanding. I’m happy to argue in the form of debate in the process.

Merton · 24th April 2012, 02:17 PM

ynot, I agree with everything you've just said. What I mean by "coincidence" is that all the massive objects are orbiting the center of gravity, not any particular massive object.

ynot · 24th April 2012, 02:51 PM

Originally Posted by Merton

ynot, I agree with everything you've just said. What I mean by "coincidence" is that all the massive objects are orbiting the center of gravity, not any particular massive object.

Seems an inappropriate use of the word to me. Are you saying the Solar System has a discrete centre of gravity that the Sun orbits?

ynot · 24th April 2012, 03:08 PM

Originally Posted by sol invictus

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

If it was already spinning it would “stand”. If it was then stopped it wouldn’t “stand”. That it wasn’t standing initially and was after seems pretty conclusive evidence that it wasn’t previously spinning and now is.

WhatRoughBeast · 24th April 2012, 03:56 PM

Originally Posted by Merton

Perpetual The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect.

Actually, the center of mass of the solar system lies outside the surface of the sun a majority of the time, at least durning the current time period. Depending on the alignment of the planets, primarily Jupiter and Saturn, the seperation between the center of mass of the sun and the COM of the solar system varies from about .134 solar radii to 1.98. During the 21st cetury the solar system COM will lie outside the sun about 63% of the time.

Perpetual Student · 24th April 2012, 04:21 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?

Very challenging comments. Thinking...

OnlyTellsTruths · 24th April 2012, 04:31 PM

I misspoke earlier in the thread.

I did not mean that real world evidence makes other models physically invalid.

I only meant that it is possible to have real world evidence that shows which, of a certain set of perfectly valid physical models, is the one occurring in reality. That does not make the other models any less physically valid.

Merton · 24th April 2012, 04:41 PM

WhatRoughBeast, that's really awesome! I didn't think there was enough mass outside of the sun to move the COM that much. Thanks for the info!

WhatRoughBeast · 24th April 2012, 07:17 PM

Originally Posted by Merton

WhatRoughBeast, that's really awesome! I didn't think there was enough mass outside of the sun to move the COM that much. Thanks for the info!

De nada. And, just as a bonus, when you add the fact that earth's orbit is elliptical rather than circular, if counter-earth existed it would sometimes be visible. Since no such observation has occurred, this directly disproves the counter-earth myth.

sol invictus · 24th April 2012, 07:34 PM

Originally Posted by ynot

If it was already spinning it would “stand”. If it was then stopped it wouldn’t “stand”. That it wasn’t standing initially and was after seems pretty conclusive evidence that it wasn’t previously spinning and now is.

That's your item (1), not your item (2), which is what I was responding to.

theprestige · 24th April 2012, 07:57 PM

Originally Posted by ynot

If it was already spinning it would “stand”. If it was then stopped it wouldn’t “stand”. That it wasn’t standing initially and was after seems pretty conclusive evidence that it wasn’t previously spinning and now is.

Spinning relative to what? Standing relative to what? Relative to the CMB, the top is performing all sorts of fantastical gyrations, regardless of whether it's standing relative to the tabletop, or spinning relative to you.

It seems like all these examples actually boil down to, "first, pick an arbitrary frame of reference, then describe the history of an arbitrary object subject to arbitrary forces, relative to that arbitrary frame of reference."

We don't know that the top is spinning. We think of it as spinning because we've already selected an arbitrary frame of reference in which it is convenient to think of the top as spinning.

OnlyTellsTruths · 24th April 2012, 08:25 PM

Originally Posted by ynot

I conclude that the top is spinning for two obvious reasons . . .

(1) A spinning top “stands” a non-spinning top doesn’t. You don’t have to remember you spun it to know it’s spinning.

(2) Either you spun the top or you spun the entire Universe. Pick the only one we know to be possible.

I would like to point out that my case was not what I will call #2a, which I believe is just Occam's razor; but #2b which is that it is possible to have real world evidence that has the ability to exclude certain particular models from being one of the possible ones that is occurring in reality. Not that it excludes it from being a valid physical model.

OnlyTellsTruths · 24th April 2012, 08:36 PM

I have a new question (or a new wrinkle to the question).

First of all we are referring to the interesting model that appears when you have the Earth be the center and the Sun orbiting it (the Earth).

I assume that it is just as complicated, but not much more, to have the Earth be the center and the black hole from the center of our galaxy orbiting it (the Earth). (Or perhaps I should replace the Earth with the Sun in this scenario to make it even more like the Earth/Sun model from Page 1...)

My question is, how much more complicated would a model be that has the Earth at the center and a black hole from the center of another galaxy orbiting it (the Earth). I.E. objects from separate galaxies.

Or perhaps even even more complicated, the Earth is the center and a star from another galaxy is orbiting it?

Is this even possible since galaxies don't really normally orbit each other (or anything?)?

Even if it is possible, I assume the black hole at the center of our galaxy would make both of those models very complicated?

Instead of just Star - Planet - Moon you would have: other galaxy - black hole from the center of our galaxy - star - planet - moon.

OnlyTellsTruths · 24th April 2012, 08:42 PM

I just realized typing that post that typing out "the black hole at the center of our galaxy" is really annoying to type out!

So I googled it to try and find the correct name.... Is it called Sagittarius A?

Farsight · 25th April 2012, 12:21 AM

Originally Posted by OnlyTellsTruths

...Even if it is possible, I assume the black hole at the center of our galaxy would make both of those models very complicated? ...

Sure. They'd be perversely complicated. And we know quite enough about general relativity and gravity and cosmology to know that in this real universe, the little things go round the big things. And the big things go round the even bigger things. Et cetera. Anybody who tries to suggest to you that GR says otherwise is talking bollocks.

BowlOfRed · 25th April 2012, 12:53 AM

Originally Posted by OnlyTellsTruths

I just realized typing that post that typing out "the black hole at the center of our galaxy" is really annoying to type out!

So I googled it to try and find the correct name.... Is it called Sagittarius A?

I think Sagittarius A* is slightly better. Sgr A* is one part of Sgr A.

H'ethetheth · 25th April 2012, 01:52 AM

Originally Posted by OnlyTellsTruths

I only meant that it is possible to have real world evidence that shows which, of a certain set of perfectly valid physical models, is the one occurring in reality.

I understand that this is tricky, but what you said here doesn't actually mean anything.
Some phenomenon occurs in reality, and we can model it in any number of ways. If we do the modelling correctly these ways are all equivalent; none of them can be said to be more true, or to be the one occurring in reality, because they all describe the occurring phenomenon with the exact same accuracy.

OnlyTellsTruths · 25th April 2012, 03:22 AM

Originally Posted by H'ethetheth

I understand that this is tricky, but what you said here doesn't actually mean anything.
Some phenomenon occurs in reality, and we can model it in any number of ways. If we do the modelling correctly these ways are all equivalent; none of them can be said to be more true, or to be the one occurring in reality, because they all describe the occurring phenomenon with the exact same accuracy.

I'm going to have to disagree.

I'm sorry if I gave the impression that we are determining "the one occurring in reality". What I am proposing is excluding perfectly valid models after real world evidence is gathered.

In certain situations, if you are choosing between a specific number of perfectly valid models you can use real world evidence to exclude any number of them from actually being a contender for one that is occurring in reality as we know it.

That is not the same thing as saying that any particular model is definitely the correct one or even the only correct one. I'm talking about excluding models that are otherwise valid because of evidence that is gathered.

So if we are discussing two perfectly valid models of the same objects, and one of the models has legitimate real world problems, it can be excluded in favor of the other. There's no reason to ignore evidence and history about any particular situation.

H'ethetheth · 25th April 2012, 03:44 AM

Originally Posted by OnlyTellsTruths

I'm going to have to disagree.

I'm sorry if I gave the impression that we are determining "the one occurring in reality". What I am proposing is excluding perfectly valid models after real world evidence is gathered.

In certain situations, if you are choosing between a specific number of perfectly valid models you can use real world evidence to exclude any number of them from actually being a contender for one that is occurring in reality as we know it.

This assertion is internally inconsistent. If the models are all perfectly valid, then they all have the exact same predictive power, and will predict the exact same outcome.
In fact, we're discussing different formulations of one single model. It is impossible to say which of these formulations describe the real occurrence best, because the descriptions are exactly equivalent.

Originally Posted by OnlyTellsTruths

That is not the same thing as saying that any particular model is definitely the correct one or even the only correct one. I'm talking about excluding models that are otherwise valid because of evidence that is gathered.

And I'm telling you that this is not something that is logically possible.
It's like trying to gather evidence to decide the question whether I am sitting under a tree, or if the tree is standing over me.

Originally Posted by OnlyTellsTruths

So if we are discussing two perfectly valid models of the same objects, and one of the models has legitimate real world problems, it can be excluded in favor of the other. There's no reason to ignore evidence and history about any particular situation.

Yes, so now consider this: Not only are they not two models, but neither formulation of the one model can possibly have "legitimate real world problems" that the other doesn't, because they both predict the exact same outcomes for any given input.

OnlyTellsTruths · 25th April 2012, 03:54 AM

Originally Posted by H'ethetheth

This assertion is internally inconsistent. If the models are all perfectly valid, then they all have the exact same predictive power, and will predict the exact same outcome.

I'm sorry but that is where we disagree.

The models can all be perfectly valid in a physics sense. Real world evidence and context information can then exclude one of the models.

I have a feeling you are just going to repeat yourself again, and then I will have to repeat myself. So be it.

Originally Posted by H'ethetheth

And I'm telling you that this is not something that is logically possible.

Then you must not be understanding what I am saying at all. I'm sorry.

Models can accurately describe a situation. That does not mean you cannot then apply evidence and context information to exclude one of them.

Roboramma · 25th April 2012, 03:58 AM

OTT, because ben m said it so much better than I'm likely to, I'd like to highlight this bit from upthread and ask you to respond to it:

Originally Posted by ben m

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?

Originally Posted by modified PS

On the other hand we have our knowledge of history: I used to have the ball at rest, then I kicked it, so it's really moving. The ball was really in my hand a minute ago, and the only force that acted on it was that of my kick, so I know how much the ball accelerated. So SR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening.

Would you say that your viewpoint is in agreement with the "modified PS" quote in there?

OnlyTellsTruths · 25th April 2012, 03:59 AM

Originally Posted by sol invictus

Originally Posted by OnlyTellsTruths

We would know that because, in this hypothetical, the Earth came from the disc section, which by it's very nature forms around the center. Most importantly because the disc starts out as a whole and ends up breaking apart, while the center always remains the center. Therefore a product from the disc section cannot be the center, otherwise it wouldn't have been from the disc section.

This assumes the center of the disk cannot rotate around a point near its edge. That isn't the case.

Since we aren't talking about the center of any old disk, and we are in fact talking about a protoplanetary disk, correct me if I am wrong, but are you proposing that a protoplanetary disk can rotate around a point that is not the nebula??

H'ethetheth · 25th April 2012, 04:02 AM

Originally Posted by OnlyTellsTruths

I'm sorry but that is where we disagree.

The models can all be perfectly valid in a physics sense. Real world evidence and context information can then exclude one of the models.

I have a feeling you are just going to repeat yourself again, and then I will have to repeat myself. So be it.

Then you must not be understanding what I am saying at all. I'm sorry.

Models can accurately describe a situation. That does not mean you cannot then apply evidence and context information to exclude one of them.

In my world, that's exactly what that means*, but since you seem convinced that it doesn't, I'd be curious about an example.
For instance what kind of evidence or context would be needed to decide whether I'm sitting under a tree, or the tree is standing over me?

*Unless convenience is the "context information" you're talking about, because then we completely agree.

OnlyTellsTruths · 25th April 2012, 04:02 AM

Originally Posted by Roboramma

Would you say that your viewpoint is in agreement with the "modified PS" quote in there?

That is exactly the problem I am talking about. There is no reason for us to exclude evidence, historical information, and context. That is one of the benefits of being a sentient species.

OnlyTellsTruths · 25th April 2012, 04:04 AM

Originally Posted by H'ethetheth

For instance what kind of evidence or context would be needed to decide whether I'm sitting under a tree, or the tree is standing over me?

I can only say that that is a very poor analogy to what we are discussing? It doesn't even make sense.

Perhaps form an analogy where you knock the tree over.

24th April 2012, 07:52 AM	#43
Beerina Sarcastic Conqueror of Notions Join Date: Mar 2004 Location: A floating island above the clouds Posts: 23,835	If the Earth were the only thing in the universe you could determine it was rotating.
	__________________ "Great innovations should not be forced [by way of] slender majorities." - Thomas Jefferson The government should nationalize it! Socialized, single-payer video game development and sales *now!* More, cheaper, better games, right? Right?

24th April 2012, 08:55 AM	#44
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by Beerina If the Earth were the only thing in the universe you could determine it was rotating. Assuming gravity gradients aren't really large at your location, you can always do an experiment and find a locally inertial frame. Something not rotating in that frame won't experience "fictitious forces"; it won't feel centrifugal stress for example. So it can be said to be "not rotating". Most of the subtlety here arises because (a) A rotating shell with very, very large radius can change the state of rotation of the locally inertial frames inside it. Similarly, two things each of which is "not rotating" by the above definition can be rotating with respect to each other. (b) You are always perfectly free to use "rotating coordinates", in which a rotating object is static. Of course the presence or absence of centrifugal stresses is independent of coordinate choice (it's a real physical effect, so it had better be), but see (a).
	Last edited by sol invictus; 24th April 2012 at 08:57 AM.

24th April 2012, 12:25 PM	#45
Perpetual Student Illuminator Join Date: Jul 2008 Location: USA Posts: 3,707	This has been debated again and again on many threads here. For example: General Relativity. I'm not sure there exists a real final answer given our current tools and understanding -- but I hope I'm wrong! 1. On the one hand, we have GR totalitarianism, which holds that any sense we have of some notion of reality involving rotation -- like the earth actually does revolve around the sun or there is a top really spinning on my table -- is an illusion. All frames are equal; there is no real frame of reference to tell us what rotates or revolves around what -- according to this doctrine. 2. On the other hand we have our knowledge of history: I spun the top on my table so it is really spinning; the earth sun system was formed from a primordial cloud as described above, so the earth is really revolving around the sun. So GR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening. I have been told by some here that option 2 above is wrong and that the key is in understanding the mathematics of GR. Maybe so and maybe not -- I'm working on it.
	__________________ _{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 Last edited by Perpetual Student; 24th April 2012 at 12:26 PM.

24th April 2012, 12:54 PM	#46
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,653	Originally Posted by Perpetual Student This has been debated again and again on many threads here. For example: General Relativity. I'm not sure there exists a real final answer given our current tools and understanding -- but I hope I'm wrong! 1. On the one hand, we have GR totalitarianism, which holds that any sense we have of some notion of reality involving rotation -- like the earth actually does revolve around the sun or there is a top really spinning on my table -- is an illusion. All frames are equal; there is no real frame of reference to tell us what rotates or revolves around what -- according to this doctrine. 2. On the other hand we have our knowledge of history: I spun the top on my table so it is really spinning; the earth sun system was formed from a primordial cloud as described above, so the earth is really revolving around the sun. So GR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening. I have been told by some here that option 2 above is wrong and that the key is in understanding the mathematics of GR. Maybe so and maybe not -- I'm working on it. I conclude that the top is spinning for two obvious reasons . . . (1) A spinning top “stands” a non-spinning top doesn’t. You don’t have to remember you spun it to know it’s spinning. (2) Either you spun the top or you spun the entire Universe. Pick the only one we know to be possible.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

24th April 2012, 01:19 PM	#49
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,653	Originally Posted by Merton Perpetual Student, #2 is indeed incorrect. Or, at least, it's not as accurate as #1. The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect. To put it concisely, the center of gravity around which other massive objects orbit is not necessarily the center of a star's mass. This is made clear when discussing multi-star systems, whose masses also share a common history. In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system). So history doesn't determine that the Earth revolves around the sun, just as history wouldn't be able to determine which star of a multi-star system a planet orbits. It's not orbiting a star at all... it's orbiting their common center of gravity. It's merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star. Are you claiming that the gravity of the Sun has nothing to do with the mass of the Sun? If the Sun disappeared are you claiming the gravity would remain? Are you serious?
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise. Last edited by ynot; 24th April 2012 at 01:24 PM.

24th April 2012, 05:21 AM	#41
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by OnlyTellsTruths We would know that because, in this hypothetical, the Earth came from the disc section, which by it's very nature forms around the center. Most importantly because the disc starts out as a whole and ends up breaking apart, while the center always remains the center. Therefore a product from the disc section cannot be the center, otherwise it wouldn't have been from the disc section. This assumes the center of the disk cannot rotate around a point near its edge. That isn't the case.

24th April 2012, 07:41 AM	#42
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	Originally Posted by OnlyTellsTruths Again to clairify. In my hypothetical, proof is found that the Earth came from the disc section, and the Sun came from the center section, both from the same protoplanetary disc. With no outside interference. It seems you're describing a very simple solar system and that's where the confusion lies. In this example, both the planet and star comprise a bound system whose center of gravity lies nearly at the center of mass of the star. But think of a binary system and you'll begin to see the problem with this scenario. Two stars of relatively similar mass (compared with the planetary objects) will have a center of gravity close to the center of these stars (with minor variation due to massive planets). So, in this case, which star is rotating around which? It's still a bound system, like the single-star system in your example, but its center of gravity isn't near the center of mass of the star. Hopefully that makes some sense. Essentially, in a bound system, all objects are rotating around a common center of gravity. In a single-star system, it appears that the star is the center, but it too is orbiting around the center of gravity and, to be accurate, the Earth is still not rotating around the sun, but the center of gravity. Of course, for practical reasons and because the center of gravity is very nearly the center of mass of the star, one can say the Earth is orbiting the sun.

24th April 2012, 01:04 PM	#47
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	Perpetual Student, #2 is indeed incorrect. Or, at least, it's not as accurate as #1. The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect. To put it concisely, the center of gravity around which other massive objects orbit is not necessarily the center of a star's mass. This is made clear when discussing multi-star systems, whose masses also share a common history. In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system). So history doesn't determine that the Earth revolves around the sun, just as history wouldn't be able to determine which star of a multi-star system a planet orbits. It's not orbiting a star at all... it's orbiting their common center of gravity. It's merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star.

24th April 2012, 01:12 PM	#48
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	The spinning top isn't a great analogy. Namely, the rotation of the top is slowed by forces that wouldn't affect massive objects like planets and stars; forces that exist within the frame of reference of the Earth and the top-spinner. If the top were spinning out in space, with or without the knowledge of who put it there, the frame of reference for the top would be equally as valid as any other frame of reference. Knowing the history of the object is an entirely separate question.

24th April 2012, 01:23 PM	#50
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,653	Originally Posted by Merton The spinning top isn't a great analogy. Namely, the rotation of the top is slowed by forces that wouldn't affect massive objects like planets and stars; forces that exist within the frame of reference of the Earth and the top-spinner. If the top were spinning out in space, with or without the knowledge of who put it there, the frame of reference for the top would be equally as valid as any other frame of reference. Knowing the history of the object is an entirely separate question. Nobody tell the Space Station and all the satellites orbiting Earth that gyroscopes don’t work in space.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

24th April 2012, 01:31 PM	#51
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	Originally Posted by ynot Are you claiming that the gravity of the Sun has nothing to do with the mass of the Sun? No, not at all. Mass is the reason for gravity. Originally Posted by ynot If the Sun disappeared are you claiming the gravity would remain? Are you serious? Some gravity would remain because there are other massive objects in the solar system, but the sun is such a large object compared to these others that removing the sun would remove most of the gravity we experience. It's important that you consider all of the mass within a bound system for its net gravitational force though, and that's why it's "merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star." Originally Posted by ynot Nobody tell the Space Station and all the satellites orbiting Earth that gyroscopes don’t work in space. Either your reading comprehension isn't that great, or you're just looking for an argument.

24th April 2012, 01:39 PM	#52
Perpetual Student Illuminator Join Date: Jul 2008 Location: USA Posts: 3,707	Originally Posted by Merton Perpetual Student, #2 is indeed incorrect. Or, at least, it's not as accurate as #1. The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect. To put it concisely, the center of gravity around which other massive objects orbit is not necessarily the center of a star's mass. This is made clear when discussing multi-star systems, whose masses also share a common history. In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system). So history doesn't determine that the Earth revolves around the sun, just as history wouldn't be able to determine which star of a multi-star system a planet orbits. It's not orbiting a star at all... it's orbiting their common center of gravity. It's merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star. Everyone one here knows about rotation about the center of gravity -- using the sun as the center is merely a simplification for discussion purposes. You missed the point entirely. Under GR, it is claimed that "(your comment) In a multi-star system, even though everything within that system condensed from the same nebula, the planets and other stars do not orbit a central star, but a point somewhere between the stars (because stars have the majority of the mass within a system)." is also not a description of *reality*. Any frame other than the center of gravity is equally valid as a frame of reference.
	__________________ _{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

24th April 2012, 01:41 PM	#53
ben m Illuminator Join Date: Jul 2006 Posts: 4,653	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? Originally Posted by modified PS On the other hand we have our knowledge of history: I used to have the ball at rest, then I kicked it, so it's really moving. The ball was really in my hand a minute ago, and the only force that acted on it was that of my kick, so I know how much the ball accelerated. So SR may very well give us the powerful tool of looking at it from any frame of reference but is incapable of telling us anything about that history and so it cannot tell us what is actually currently happening. Does that help?
ben m Illuminator Join Date: Jul 2006 Posts: 4,653	Last edited by ben m; 24th April 2012 at 03:02 PM.

24th April 2012, 01:44 PM	#54
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	Perpetual Student, I've apparently missed the point again. And I also don't see how my quoted statement contradicts the fact that any frame of reference is as valid as the next. Could you elaborate?

24th April 2012, 02:00 PM	#55
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by Perpetual Student This has been debated again and again on many threads here. ... 2. On the other hand we have our knowledge of history: I spun the top on my table so it is really spinning We have indeed discussed this many times, and many times I've tried to explain to you that causality has absolutely nothing to do with this. Changing to a rotating frame is not in any way, shape, or form equivalent to physically rotating the entire universe. They are two distinct and different things. Originally Posted by ben m 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've tried a very similar argument several times, to no avail. But PS has been studying GR (and I think you're better at explaining these things), so perhaps that will work. Originally Posted by ynot I conclude that the top is spinning for two obvious reasons . . . (1) A spinning top “stands” a non-spinning top doesn’t. You don’t have to remember you spun it to know it’s spinning. That indeed tells you that the top is spinning with respect to the local inertial frame. What it doesn't tell you is whether it's spinning with respect to another top somewhere else that won't stand up. Quote: (2) Either you spun the top or you spun the entire Universe. No, either you spun the top or you stopped it from spinning.

24th April 2012, 02:06 PM	#56
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,653	Originally Posted by Merton No, not at all. Mass is the reason for gravity. So why do you say it’s a “coincidence” that the centre of gravity of the solar system is near the centre of the greatest mass in the solar system (Sun = more than 99.8%)? Originally Posted by Merton Some gravity would remain because there are other massive objects in the solar system, but the sun is such a large object compared to these others that removing the sun would remove most of the gravity we experience. It's important that you consider all of the mass within a bound system for its net gravitational force though, and that's why it's "merely coincidence that the center of gravity in a single-star system is very nearly the center of mass of the star.". The Sun and Earth both have independent mass that creates independent gravity. If the Sun disappeared the Earth would retain it’s gravity but would tend to travel in a straight path rather than circular. Without the Sun the solar system wouldn’t be a “bound system”. Originally Posted by Merton Either your reading comprehension isn't that great, or you're just looking for an argument. I’m looking for clarification and understanding. I’m happy to argue in the form of debate in the process.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise. Last edited by ynot; 24th April 2012 at 02:12 PM.

24th April 2012, 02:17 PM	#57
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	ynot, I agree with everything you've just said. What I mean by "coincidence" is that all the massive objects are orbiting the center of gravity, not any particular massive object.

24th April 2012, 02:51 PM	#58
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,653	Originally Posted by Merton ynot, I agree with everything you've just said. What I mean by "coincidence" is that all the massive objects are orbiting the center of gravity, not any particular massive object. Seems an inappropriate use of the word to me. Are you saying the Solar System has a discrete centre of gravity that the Sun orbits?
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

24th April 2012, 03:08 PM	#59
ynot Illuminator Join Date: Jan 2006 Location: New Zealand Posts: 4,653	Originally Posted by sol invictus No, either you spun the top or you stopped it from spinning. If it was already spinning it would “stand”. If it was then stopped it wouldn’t “stand”. That it wasn’t standing initially and was after seems pretty conclusive evidence that it wasn’t previously spinning and now is.
	__________________ Rumours of a god’s existence have been greatly exaggerated. My post are all (IMO) unless stated otherwise.

24th April 2012, 03:56 PM	#60
WhatRoughBeast Muse Join Date: Apr 2011 Posts: 818	Originally Posted by Merton Perpetual The Earth is not revolving around the sun, but both the Earth and sun are revolving around a common center of gravity (along with all the other planets, asteroids, etc.). Because this center of gravity is within the mass of the sun, it's practical to say that the planets orbit the sun, though it is theoretically incorrect. Actually, the center of mass of the solar system lies outside the surface of the sun a majority of the time, at least durning the current time period. Depending on the alignment of the planets, primarily Jupiter and Saturn, the seperation between the center of mass of the sun and the COM of the solar system varies from about .134 solar radii to 1.98. During the 21st cetury the solar system COM will lie outside the sun about 63% of the time.
WhatRoughBeast Muse Join Date: Apr 2011 Posts: 818	Last edited by WhatRoughBeast; 24th April 2012 at 03:57 PM.

24th April 2012, 04:21 PM	#61
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? Very challenging comments. Thinking...
	__________________ _{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

24th April 2012, 04:31 PM	#62
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	I misspoke earlier in the thread. I did not mean that real world evidence makes other models physically invalid. I only meant that it is possible to have real world evidence that shows which, of a certain set of perfectly valid physical models, is the one occurring in reality. That does not make the other models any less physically valid.
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	__________________ ________________________

24th April 2012, 04:41 PM	#63
Merton Muse Join Date: Apr 2012 Location: Montana, USA Posts: 576	WhatRoughBeast, that's really awesome! I didn't think there was enough mass outside of the sun to move the COM that much. Thanks for the info!

24th April 2012, 07:17 PM	#64
WhatRoughBeast Muse Join Date: Apr 2011 Posts: 818	Originally Posted by Merton WhatRoughBeast, that's really awesome! I didn't think there was enough mass outside of the sun to move the COM that much. Thanks for the info! De nada. And, just as a bonus, when you add the fact that earth's orbit is elliptical rather than circular, if counter-earth existed it would sometimes be visible. Since no such observation has occurred, this directly disproves the counter-earth myth.
WhatRoughBeast Muse Join Date: Apr 2011 Posts: 818

24th April 2012, 07:34 PM	#65
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 8,419	Originally Posted by ynot If it was already spinning it would “stand”. If it was then stopped it wouldn’t “stand”. That it wasn’t standing initially and was after seems pretty conclusive evidence that it wasn’t previously spinning and now is. That's your item (1), not your item (2), which is what I was responding to.

24th April 2012, 07:57 PM	#66
theprestige Philosopher Join Date: Aug 2007 Posts: 8,613	Originally Posted by ynot If it was already spinning it would “stand”. If it was then stopped it wouldn’t “stand”. That it wasn’t standing initially and was after seems pretty conclusive evidence that it wasn’t previously spinning and now is. Spinning relative to what? Standing relative to what? Relative to the CMB, the top is performing all sorts of fantastical gyrations, regardless of whether it's standing relative to the tabletop, or spinning relative to you. It seems like all these examples actually boil down to, "first, pick an arbitrary frame of reference, then describe the history of an arbitrary object subject to arbitrary forces, relative to that arbitrary frame of reference." We don't know that the top is spinning. We think of it as spinning because we've already selected an arbitrary frame of reference in which it is convenient to think of the top as spinning.
theprestige Philosopher Join Date: Aug 2007 Posts: 8,613

24th April 2012, 08:25 PM	#67
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	Originally Posted by ynot I conclude that the top is spinning for two obvious reasons . . . (1) A spinning top “stands” a non-spinning top doesn’t. You don’t have to remember you spun it to know it’s spinning. (2) Either you spun the top or you spun the entire Universe. Pick the only one we know to be possible. I would like to point out that my case was not what I will call #2a, which I believe is just Occam's razor; but #2b which is that it is possible to have real world evidence that has the ability to exclude certain particular models from being one of the possible ones that is occurring in reality. Not that it excludes it from being a valid physical model.
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	__________________ ________________________ Last edited by OnlyTellsTruths; 24th April 2012 at 08:38 PM.

24th April 2012, 08:36 PM	#68
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	I have a new question (or a new wrinkle to the question). First of all we are referring to the interesting model that appears when you have the Earth be the center and the Sun orbiting it (the Earth). I assume that it is just as complicated, but not much more, to have the Earth be the center and the black hole from the center of our galaxy orbiting it (the Earth). (Or perhaps I should replace the Earth with the Sun in this scenario to make it even more like the Earth/Sun model from Page 1...) My question is, how much more complicated would a model be that has the Earth at the center and a black hole from the center of another galaxy orbiting it (the Earth). I.E. objects from separate galaxies. Or perhaps even even more complicated, the Earth is the center and a star from another galaxy is orbiting it? Is this even possible since galaxies don't really normally orbit each other (or anything?)? Even if it is possible, I assume the black hole at the center of our galaxy would make both of those models very complicated? Instead of just Star - Planet - Moon you would have: other galaxy - black hole from the center of our galaxy - star - planet - moon.
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	__________________ ________________________

24th April 2012, 08:42 PM	#69
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	I just realized typing that post that typing out "the black hole at the center of our galaxy" is really annoying to type out! So I googled it to try and find the correct name.... Is it called Sagittarius A?
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	__________________ ________________________

25th April 2012, 12:21 AM	#70
Farsight Suspended Join Date: Mar 2008 Location: Poole, UK Posts: 1,956	Originally Posted by OnlyTellsTruths ...Even if it is possible, I assume the black hole at the center of our galaxy would make both of those models very complicated? ... Sure. They'd be perversely complicated. And we know quite enough about general relativity and gravity and cosmology to know that in this real universe, the little things go round the big things. And the big things go round the even bigger things. Et cetera. Anybody who tries to suggest to you that GR says otherwise is talking bollocks.

25th April 2012, 12:53 AM	#71
BowlOfRed Muse Join Date: Jul 2010 Location: Silicon Valley Posts: 535	Originally Posted by OnlyTellsTruths I just realized typing that post that typing out "the black hole at the center of our galaxy" is really annoying to type out! So I googled it to try and find the correct name.... Is it called Sagittarius A? I think Sagittarius A* is slightly better. Sgr A* is one part of Sgr A.

25th April 2012, 01:52 AM	#72
H'ethetheth fishy rocket scientist Join Date: Aug 2004 Location: among the machines Posts: 2,340	Originally Posted by OnlyTellsTruths I only meant that it is possible to have real world evidence that shows which, of a certain set of perfectly valid physical models, is the one occurring in reality. I understand that this is tricky, but what you said here doesn't actually mean anything. Some phenomenon occurs in reality, and we can model it in any number of ways. If we do the modelling correctly these ways are all equivalent; none of them can be said to be more true, or to be the one occurring in reality, because they all describe the occurring phenomenon with the exact same accuracy.

25th April 2012, 03:22 AM	#73
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	Originally Posted by H'ethetheth I understand that this is tricky, but what you said here doesn't actually mean anything. Some phenomenon occurs in reality, and we can model it in any number of ways. If we do the modelling correctly these ways are all equivalent; none of them can be said to be more true, or to be the one occurring in reality, because they all describe the occurring phenomenon with the exact same accuracy. I'm going to have to disagree. I'm sorry if I gave the impression that we are determining "the one occurring in reality". What I am proposing is excluding perfectly valid models after real world evidence is gathered. In certain situations, if you are choosing between a specific number of perfectly valid models you can use real world evidence to exclude any number of them from actually being a contender for one that is occurring in reality as we know it. That is not the same thing as saying that any particular model is definitely the correct one or even the only correct one. I'm talking about excluding models that are otherwise valid because of evidence that is gathered. So if we are discussing two perfectly valid models of the same objects, and one of the models has legitimate real world problems, it can be excluded in favor of the other. There's no reason to ignore evidence and history about any particular situation.
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	__________________ ________________________

25th April 2012, 03:44 AM	#74
H'ethetheth fishy rocket scientist Join Date: Aug 2004 Location: among the machines Posts: 2,340	Originally Posted by OnlyTellsTruths I'm going to have to disagree. I'm sorry if I gave the impression that we are determining "the one occurring in reality". What I am proposing is excluding perfectly valid models after real world evidence is gathered. In certain situations, if you are choosing between a specific number of perfectly valid models you can use real world evidence to exclude any number of them from actually being a contender for one that is occurring in reality as we know it. This assertion is internally inconsistent. If the models are all perfectly valid, then they all have the exact same predictive power, and will predict the exact same outcome. In fact, we're discussing different formulations of one single model. It is impossible to say which of these formulations describe the real occurrence best, because the descriptions are exactly equivalent. Originally Posted by OnlyTellsTruths That is not the same thing as saying that any particular model is definitely the correct one or even the only correct one. I'm talking about excluding models that are otherwise valid because of evidence that is gathered. And I'm telling you that this is not something that is logically possible. It's like trying to gather evidence to decide the question whether I am sitting under a tree, or if the tree is standing over me. Originally Posted by OnlyTellsTruths So if we are discussing two perfectly valid models of the same objects, and one of the models has legitimate real world problems, it can be excluded in favor of the other. There's no reason to ignore evidence and history about any particular situation. Yes, so now consider this: Not only are they not two models, but neither formulation of the one model can possibly have "legitimate real world problems" that the other doesn't, because they both predict the exact same outcomes for any given input.
	Last edited by H'ethetheth; 25th April 2012 at 03:50 AM.

25th April 2012, 03:54 AM	#75
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	Originally Posted by H'ethetheth This assertion is internally inconsistent. If the models are all perfectly valid, then they all have the exact same predictive power, and will predict the exact same outcome. I'm sorry but that is where we disagree. The models can all be perfectly valid in a physics sense. Real world evidence and context information can then exclude one of the models. I have a feeling you are just going to repeat yourself again, and then I will have to repeat myself. So be it. Originally Posted by H'ethetheth And I'm telling you that this is not something that is logically possible. Then you must not be understanding what I am saying at all. I'm sorry. Models can accurately describe a situation. That does not mean you cannot then apply evidence and context information to exclude one of them.
OnlyTellsTruths Join Date: Sep 2007 Posts: 5,797	__________________ ________________________