The Earth-Sun Relationship: A Matter of Perspective

[marty1]

How about this ... If the sun is orbiting the Earth at its current distance (which is impossible, but for the sake of an impossible mind experiment like most of them) and the Earth is not in freefall around it but the sun is magically going around, then there would necessarily be net forces pulling the Earth apart as the sun went around. Conclusion is that we must be accelerating toward it to cancel that force.

 

[tom.stoer]

You change the whole setup by introducing "no freefall" and "magically going around".

Yes, whenever an object is not in freefall there are some forces (besides gravity) responsible for it. Is there any implication besides this trivial one?

 

[marty1]

but for the sun to be orbiting the Earth it would have to be a magically contrived change of the whole setup.

There is no relativistic ambiguity. If anything our situation is ambiguous with flying through space in a straight line, not ambiguous with the sun orbiting us.

 

[tom.stoer]

but for the sun to be orbiting the Earth it would have to be a magically contrived change of the whole setup.

Why?

Both objects are orbiting the common c.o.m.; both objects are in freefall; the only difference is that the c.o.m. is located within the sun, so we do not distinguish between the Earth orbiting the sun and the Earth orbiting the c.o.m; think about a similar problem with two bodies of (nearly) equal mass.

 

[marty1]

The original post asked why it is not equally correct to treat the physics as if the sun is orbiting us, not that there is confusion between us orbiting the sun or both of us orbiting the c.o.m.

The question of WHO is orbiting WHO is some kind of topological problem. If one of them continued in a straight line without the other and intersects the path of the other, then it is the one that was being orbited.

In that test there is no symmetry.

 

[tom.stoer]

The original post was about whether "we can take any perspective we want but we just have to use fictional forces". I think I explained in detail why this does not really make sense in general relativity.

Your idea

If one of them continued in a straight line without the other and intersects the path of the other, then it is the one that was being orbited.

completely misses the fundamental idea of general relativity; both objects following geodesics, there is no physical way to distinguish between them (except for the simple observation that the Earth is orbiting the c.o.m located within the sun ;-)

 

[marty1]

So are you suggesting I lock myself in a box (with a cat) and perform an experiment that determines whether I am on a planet orbiting a sun or on a planet with a sun orbiting it?

 

[tom.stoer]

You cannot distinguish between the following scenarios
- sitting in a box on a planet w/o any sun
- sitting in a box on a small moon orbiting a planet
- sitting in a box on a large planet with a moon orbiting the planet
In all cases the planets, moons and suns are in free fall; they do not feel any force.

 

[jackoblacko]

i appreciate the discussion even if its slightly different than what I'm asking (not even sure if it is or its the same), cause its help me understand more.

Can we start with the simplest example...

an object say a ball in an infinite expanse. It can't move etc.

Then 2 balls in an infinite expanse, say a blue and yellow for our understanding. If we say the blue ball moves how can we suggest that its not the yellow ball moving and/or both. Or they aren't moving just getting smaller etc.

And finally the yellow is in the centre orbited by the blue, how can we know its not the other way around.

thx.

 

[marty1]

Put a clock on each one and see which clock ticks faster?

 

[Mentz114]

i appreciate the discussion even if its slightly different than what I'm asking (not even sure if it is or its the same), cause its help me understand more.

Can we start with the simplest example...

an object say a ball in an infinite expanse. It can't move etc.

Then 2 balls in an infinite expanse, say a blue and yellow for our understanding. If we say the blue ball moves how can we suggest that its not the yellow ball moving and/or both. Or they aren't moving just getting smaller etc.

And finally the yellow is in the centre orbited by the blue, how can we know its not the other way around.

thx.

You are beginning to sound like an absolutist. The whole point is that it makes no difference whether you say blue is moving and yellow is stationary or vice-versa. The physics remains the same. The physics will not change when the viewpoint changes. There is no answer to 'is yellow orbiting blue or vice-versa'. You were told this several times, but you are now asking exactly the same questions.

 

[jackoblacko]

You are beginning to sound like an absolutist. The whole point is that it makes no difference whether you say blue is moving and yellow is stationary or vice-versa. The physics remains the same. The physics will not change when the viewpoint changes. There is no answer to 'is yellow orbiting blue or vice-versa'. You were told this several times, but you are now asking exactly the same questions.

I was told this in words I couldn't understand.

So what you are saying is "there is no answer to" whether the sun is orbiting the Earth or vice versa? Or does this model lose its meaning eventually?

 

[jackoblacko]

Put a clock on each one and see which clock ticks faster?

Is this a level?

 

[A.T.]

Is there an ananalytical solution for the metric created by two large masses?

 

[tom.stoer]

Is there an ananalytical solution for the metric created by two large masses?

afaik - no

 

[tom.stoer]

Mentz114 is right; I tried several times to explain the basic concepts of relativity; my question is: am I not able to explain this? are you not able to understand? or are you not willing to understand and/or to accept?

 

[chill_factor]

I was told this in words I couldn't understand.

So what you are saying is "there is no answer to" whether the sun is orbiting the Earth or vice versa? Or does this model lose its meaning eventually?

The Earth is orbiting the sun. However that's because they are not the only 2 objects that exist. Is it a coincidence that Venus, Mars, Jupiter... etc. all just *happen* to look like they're orbiting the sun, but actually everything is orbiting Earth and the other planets are moving in weird cycles that don't follow any laws of mechanics... or that everything orbits the sun?

So everything orbits the sun, end of story.

However, if you had an imaginary universe with exactly 2 things it is not possible to tell which one was orbiting the other. If you had 3 things in an orbit, and one of them was very big, then its easily distinguishable that one thing is orbiting the other.

 

[marty1]

here's is another variation ... 2 objects in co-geosynchronous orbit. They are held apart but you can't tell if either of them are moving.

 

[jackoblacko]

Mentz114 is right; I tried several times to explain the basic concepts of relativity; my question is: am I not able to explain this? are you not able to understand? or are you not willing to understand and/or to accept?

Its because I'm not able to understand, I think that's a given and I shouldn't have to repeat it over and over.

I re read the whole thread a few times, have a better understanding (still not solid).

So I want to make sure me adding 'we just have to use fictional forces' didn't mess this up.

I think what we got is Newtonian puts a static 3d grid over everything and we can measure everything in relation to that grid.

If that's try I was asked if that's the context or general relativity. To me Newtonian is an old school limited representation, although its 100% useful.

I don't understand 'frames' and 'inertial' (is that just accelerated?).

But when we talk about accelerated perspectives for example: our perspective goes mach 2 left. Isn't it that our perspective moves in one way (left) and it feels the force of acceleration, but in another way everything else moves at mach 2 right and the perspective is stationary and force the perspective feels is the new 0?

 

[jackoblacko]

However, if you had an imaginary universe with exactly 2 things it is not possible to tell which one was orbiting the other. If you had 3 things in an orbit, and one of them was very big, then its easily distinguishable that one thing is orbiting the other.

I just wanted to clarify that observation with 2 things first. If we can do that we can talk about three. If we are going to talk about size, then we have to establish that with 2 balls only, there sizes are not static whatsoever. Is that true?

 

[tom.stoer]

It seems that you start with some introductory lessons on the basic ideas of general relativity

 

[jackoblacko]

It seems that you start with some introductory lessons on the basic ideas of general relativity

are you telling me to go learn some intro gr, or are you pointing out that's what I started with?

 

[tom.stoer]

I want to say that it's a bit problematic to talk about that "the Earth orbiting the sun is really a matter of perspective" and that "we can take any perspective we want but we just have to use fictional forces" if there are such basic questions like "frames", "inertial" and "is that just accelerated?"

I think we tried to explain a couple of things several times:
1) when two bodies A and B orbiting each other due to the gravitational field they are both in free fall (which means they are not accelerated in the sense that nobody would feel a force)
2) when two bodies A and B orbiting each other neither A's rest frame nor B's rest frame is special; so it is safe to say that A is orbiting B, B is orbiting A, or A and B are orbiting the center of mass
3) looking at two bodies like the sun and the Earth there is no qualitzative difference; what's special about the sun is that the common center of mass of the system earth-sun is located inside the sun
4) the situation changes when there are not only two bodies but n small bodies and one large one (like in the solar system); in that case the statement that n small bodies orbiting the large one is correct ('end odf story' as chill_factor said)
5) it does not make sense "to take any perspective" and "just use fictional forces" b/c any perspective means that the frame of reference may no longer define an (unaccelerated) inertial frame; that means that the observer may no longer be in free fall. But this is an effect she introduces due to her perspective, and therefore it has nothing to do with the two bodies, but with the observer. And this is no longer about fictitious forces but real forces she can feel.

OK, hope this summary helps.

 

[jackoblacko]

yes it helps ill be back later tonight to comment thx!

 

[tom.stoer]

fine

 

[jackoblacko]

i dont' understand 5 very well, 'frame of reference' or 'inertial frames'.

 

[Mentz114]

i dont' understand 5 very well, 'frame of reference' or 'inertial frames'.

Post #5 doesn't mention those things.

This is a quote from the Wiki article http://en.wikipedia.org/wiki/Inertial_frame_of_reference

All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not accelerating in the sense that an accelerometer at rest in one would detect zero acceleration. Measurements in one inertial frame can be converted to measurements in another by a simple transformation (the Galilean transformation in Newtonian physics and the Lorentz transformation in special relativity). In general relativity, in any region small enough for the curvature of spacetime to be negligible one can find a set of inertial frames that approximately describe that region.[2][3]

Physical laws take the same form in all inertial frames.[4] By contrast, in a non-inertial reference frame the laws of physics vary depending on the acceleration of that frame with respect to an inertial frame, and the usual physical forces must be supplemented by fictitious forces.[5][6] For example, a ball dropped towards the ground does not go exactly straight down because the Earth is rotating. Someone rotating with the Earth must include the Coriolis force to predict the horizontal motion. Another example of a fictitious force associated with rotating reference frames is the centrifugal force.

 

[jackoblacko]

can i tease you for referencing post 5 instead of the numbered point in the post above mine?

I found this http://archive.org/details/frames_of_reference i have to read the whole thread again, i kept assuming inertial meant accelerated.

 

[Mentz114]

can i tease you for referencing post 5 instead of the numbered point in the post above mine?

I found this http://archive.org/details/frames_of_reference i have to read the whole thread again, i kept assuming inertial meant accelerated.

Yes, you may. I didn't see that. I hope you understand what inertial means now. Are you clear about point 5 ?

 

[bahamagreen]

Your example of the cars demonstrates some grasp of Galilean relativity. Modern relativity takes account of some other factors that become important at high speeds and long distances which have not been brought up, yet.

With sufficiently complicated fictitious forces, it is possible to allow any arbitrary position, inertial or accelerated, or changing by any rate or derivative, to be the center of rotation of any arbitrary collection of objects.

Early solar system models had lots of these complicated things... the history has been to remove as much as possible, simplify, and retain only what is necessary to preserve the observed result.

It is often the case that early theories have "too much" in them, and as more is learned, less is needed... fewer assumptions is typically a good sign that one is on the path forward. Likewise with choosing a perspective; some are heavy with complex and peculiar assumptions, requirements, stipulations, and limiting conditions; others make way with only a couple of very simple assumptions or postulates.

I think you should read a history of physics to get a sense of this process in action over the centuries before diving into a Relativity textbook...