The Earth-Sun Relationship: A Matter of Perspective

[jackoblacko]

I am under the impression that the Earth orbiting the sun is really a matter of perspective and we can actually take a perspective in which the Earth is stationary and the sun and everything else move in relation to it. And we can take any perspective we want but we just have to use fictional forces etc.

I am under the understanding that the Earth orbits the sun no more then the sun orbits the earth. And I'm not talking about a Barycentre, that would be a third perspective, none of which is more true than the other.

Is this wrong?

 

[tom.stoer]

With a star of infinite mass there is a unique perspective: the star is at rest (or is defining an inertial frame of reference), and the planet is orbiting the star. With a star of finite mass both the star and the plant are orbiting the center of mass (which again defines an inertial frame of reference in the absence of other forces)

 

[jackoblacko]

forgive me for feeling like you didn't address my question

 

[tom.stoer]

I think I did

 

[jackoblacko]

I think I did

so is that a yes or no?

 

[tom.stoer]

think about it

 

[jackoblacko]

think about it

What are you assuming that I did when I read the answer?

 

[tom.stoer]

OK, let's try again.

What I am saying is that indeed the two bodies are both orbiting the c.o.m.; therefore there is no preferred perspective "earth" or "sun", but there is a preferred perspective i.e. frame of reference defined by the c.o.m. which defines an inertial system (whereas Earth and sun don't).

It's different if the sun would have infinite mass b/c then the rest frame of the sun is identical with the c.o.m. inertial frame.

The question with fictitious forces is difficult. In the most general sense (in general relativity) there is a class of equivalent frames, namely all frames defined by free falling bodies (in our case c.o.m., Earth and sun and infinily many more). All description of the motion of Earth and sun w.r.t. to these frames are equivalent. But you don't need any fictitous forces.

Then there is a different class af reference frames, namely all frames defined by accelerated bodies (i.e. not free falling bodies). Of course you can describe the motion of c.o.m., sun and Earth w.r.t. to these frames, but now this description is physically different b/c in these frames you can feel acceleration, i.e. a force.

So I would say that we can indeed take any perspective we want (and transform all equations accordingly, using fictitious forces etc.), but there are two "classes of perspectives", one defined by free falling bodies (c.o.m, sun, earth, ...), and one defined by accelerated bodies (starting rockets, ...). These two are different!

 

[pervect]

It really rather depends on what you mean by "perspective". If we interpret "perspective" as "a description using generalized coordinates", I believe the answer would be yes, since one can assign generalized coordinates in any way one likes, though I haven't tried to construct any such description.

Such a "perspective" will be somewhat artificial. If you for instance use Fermi Normal coordinates, you'll find that your coordinate system with the "stationary Earth" covers only a small region of space-time. A coordinate system with a "stationary sun" will cover a greater region, one with a stationary barycenter will be even better.

 

[jackoblacko]

OK, let's try again.

Thank you! Way more helpful than your last couple tries ;p

What I am saying is that indeed the two bodies are both orbiting the c.o.m.; therefore there is no preferred perspective "earth" or "sun", but there is a preferred perspective i.e. frame of reference defined by the c.o.m. which defines an inertial system (whereas Earth and sun don't).

I'm bright but uneducated, I don't think I understand this paragraph.

It's different if the sun would have infinite mass b/c then the rest frame of the sun is identical with the c.o.m. inertial frame.

Now is mass relative because it changes when we move, or that's silly?

The question with fictitious forces is difficult. In the most general sense (in general relativity) there is a class of equivalent frames, namely all frames defined by free falling bodies (in our case c.o.m., Earth and sun and infinily many more). All description of the motion of Earth and sun w.r.t. to these frames are equivalent. But you don't need any fictitous forces.

Does this paragraph support what I'm saying or go against it?

Then there is a different class af reference frames, namely all frames defined by accelerated bodies (i.e. not free falling bodies). Of course you can describe the motion of c.o.m., sun and Earth w.r.t. to these frames, but now this description is physically different b/c in these frames you can feel acceleration, i.e. a force.

which is reality?

So I would say that we can indeed take any perspective we want (and transform all equations accordingly, using fictitious forces etc.),

see this is why I tell people I am right, because you clearly state it so...

but there are two "classes of perspectives", one defined by free falling bodies (c.o.m, sun, earth, ...), and one defined by accelerated bodies (starting rockets, ...). These two are different!

Now here are you suggesting I am wrong? Very unclear.


I feel like you are saying that it depends on how you look at it.

Isn't my question "Does it depend how you look at it or do we know for sure in reality the Earth orbits the sun?"

I'm not be sarcastic or anything, its hard for me to understand your answers.

thx for your time

 

[jackoblacko]

It really rather depends on what you mean by "perspective". If we interpret "perspective" as "a description using generalized coordinates", I believe the answer would be yes, since one can assign generalized coordinates in any way one likes,

How else might we define perspective.

though I haven't tried to construct any such description.

No of course not, there's no real world application according to schools.

Such a "perspective" will be somewhat artificial. If you for instance use Fermi Normal coordinates, you'll find that your coordinate system with the "stationary Earth" covers only a small region of space-time. A coordinate system with a "stationary sun" will cover a greater region, one with a stationary barycenter will be even better.

Is this true and something I can't understand, or is it that Fermi Coo's, when applied to stationary Earth would make the stars spin etc.


I realize some of my question are going to make sense because I don't understand what I am talking about but by asking them perhaps you can extract my level of knowledge

Thx for your time!

 

[tom.stoer]

The way looking at it depends on how we define 'preferred frames of reference'; and there's a big differenc between Newtonian mechanics and general relativity. So I think in order to proceed we first have to define the context, therefore my question is what is the context: 1) Newtonian mechanics or 2) general relativity?

 

[Mentz114]

The Earth really is orbiting the sun in the sense that any observer who correctly measures the motions of the sun and Earth ( obviously using his own clocks and rulers) will conclude that the Earth is bound to the sun and traveling in an orbit around it.

Depends what you call 'real'.

 

[jackoblacko]

The way looking at it depends on how we define 'preferred frames of reference'; and there's a big differenc between Newtonian mechanics and general relativity. So I think in order to proceed we first have to define the context, therefore my question is what is the context: 1) Newtonian mechanics or 2) general relativity?

I may be confused but my understanding is if it depends on definition X or Y, then my belief that the truth is relative to perspective is correct.

I mean which one am I referring to? I am asking about reality, I don't know which that points too.

This relates to the observations of the double slit experiment, but I'll admit my understanding is 'cartoonish'.

 

[jackoblacko]

The Earth really is orbiting the sun in the sense that any observer who correctly measures the motions of the sun and Earth ( obviously using his own clocks and rulers) will conclude that the Earth is bound to the sun and traveling in an orbit around it.

Depends what you call 'real'.

But if this observers perspective is changed his observed results will change.

So if I look to the sky with a clock and a ruler, will I not conclude the Earth is stationary and the sun is moving around it?

 

[Mentz114]

...then my belief that the truth is relative to perspective is correct

In physics, theories are formulated so that the 'truths' are not dependent on perspective.

If you see a cow in the distance, does it bother you that it looks smaller than the one in the front yard ?

 

[jackoblacko]

In physics, theories are formulated so that the 'truths' are not dependent on perspective.

If you see a cow in the distance, does it bother you that it looks smaller than the one in the front yard ?

Does this exclude relativity and quantum physics?

 

[Mentz114]

Does this exclude relativity and quantum physics?

I don't know about QM, but the whole purpose of relativity theory is to enable theories to make the correct predictions independent of frame of reference. So, just the opposite of being excluded on those grounds.

One of the strongest motivations for SR was to resolve the apparent failure of Maxwell's equations in Gallilean relativity, for instance.

A charged particle moving in an electric field follows a certain path, and without using the relativistic version of EM, a moving frame would predict a different path, possibly resulting in a paradox ( did the electron go through the hole ?). With rhe relativistic version this cannot happen. So the truth ( eg the electron did go through the hole) is preserved in all frames.

I've been using sloppy language to avoid a wordstorm, but this is the nub of it.

 

[jackoblacko]

I don't know about QM, but the whole purpose of relativity theory is to enable theories to make the correct predictions independent of frame of reference. So, just the opposite of being excluded on those grounds.

One of the strongest motivations for SR was to resolve the apparent failure of Maxwell's equations in Gallilean relativity, for instance.

A charged particle moving in an electric field follows a certain path, and without using the relativistic version of EM, a moving frame would predict a different path, possibly resulting in a paradox ( did the electron go through the hole ?). With rhe relativistic version this cannot happen. So the truth ( eg the electron did go through the hole) is preserved in all frames.

I've been using sloppy language to avoid a wordstorm, but this is the nub of it.

I thought relativity meant: Car A travels at 20mph, car b at 30mph both relative to a stationary tree. Car b is going 10mph relative to car a. Relativity says car b is going both 30mph, and 10mph, and Xmph because the Earth is moving, and its stationary etc.

This can be said with its size to as it shrinks relate to the tree as it moves etc.

Hopefully that wasn't complete gibberish to you.

 

[Mentz114]

I thought relativity meant: Car A travels at 20mph, car b at 30mph both relative to a stationary tree. Car b is going 10mph relative to car a. Relativity says car b is going both 30mph, and 10mph, and Xmph because the Earth is moving, and its stationary etc.

You need to find out more about relativity from a good source. I'm sure someone can recommend a suitable text or online resource.

 

[jackoblacko]

You need to find out more about relativity from a good source. I'm sure someone can recommend a suitable text or online resource.

Does that mean my understanding of it is wrong? partial? something else?

thx.

Edit also you seem to disagree with the others is that correct?

 

[tom.stoer]

When asking regarding Newtonian mechanics or general relativity it's not about the underlying truth but about the level of explanation and the interpretation.

Regarding 'truth': I don't think that currently available theories tell us the 'truth' about nature or 'reality', they tell us about experimentally testable predictions derived from models of reality. But experiments and observations are observer-dependent. So there is a 'observer-relative truth' that the Earth is orbiting the sun, and there is an 'observer-dependent truth' that the Earth is on a cloverleaf orbit (the shape of the orbit always depends on the observer).

Of course there are observer-independent statements like 'there is a planet of rest-mass M in free fall = on a geodesic', but as soon as one introduced observers and observations statements become observer dependent. 'Reality' is not related to one specific perspective, but to the complete set of observer-dependent perspectives. I think this is the lesson to be drawn from relativity.

 

[jackoblacko]

When asking regarding Newtonian mechanics or general relativity it's not about the underlying truth but about the level of explanation and the interpretation.

Regarding 'truth': I don't think that currently available theories tell us the 'truth' about nature or 'reality', they tell us about experimentally testable predictions derived from models of reality. But experiments and observations are observer-dependent. So there is a 'observer-relative truth' that the Earth is orbiting the sun, and there is an 'observer-dependent truth' that the Earth is on a cloverleaf orbit (the shape of the orbit always depends on the observer).

Of course there are observer-independent statements like 'there is a planet of rest-mass M in free fall = on a geodesic', but as soon as one introduced observers and observations statements become observer dependent. 'Reality' is not related to one specific perspective, but to the complete set of observer-dependent perspectives. I think this is the lesson to be drawn from relativity.

That seems to be exactly how I explained it, and it seems to be the exact representation of what I am pointing at in whether the sun orbits the Earth or vice, or clover shape or which ever.

 

[tom.stoer]

That seems to be exactly how I explained it, and it seems to be the exact representation of what I am pointing at in whether the sun orbits the Earth or vice, or clover shape or which ever.

But I am sure you remember that I explained how the usual orbits are "preferred orbits" as they are observed from free-falling frames, not from accelerated frames (like clover shape orbits). Now you can argue that it's not the orbit which is preferred but the reference frame. OK. However it should be clear that in order to introduce such a strange orbit you must do something with the reference frame of the observer, you must accelerate it. And this acceleration is a physical property of the reference frame, not of the orbiting objects; they don't feel any force, whereas the accelerated observer does!

That's why I disagree with the sentence from your 1st post "and we can take any perspective we want but we just have to use fictional forces." That's not compatible with the spirit of GR.

 

[marty1]

Orbiting involves acceleration and I don't believe it depends on the observer.

If I see a spaceship getting closer to me at an ever slower rate, I cannot conclude that I am the one decelerating because I do not feel the force.

 

[tom.stoer]

Orbiting involves acceleration and I don't believe it depends on the observer.

No, it doesn't ;-)

The orbiting object is in free-fall, so there's no force and therefore no acceleration which could be observed. In Newtonian mechanics you may think that the object is accelerated, but this is only true w.r.t. 'absolute spacetime' which plays the role of a coordinate system and which has no physical significance.

If you are accelerated you will feel it. So either there is an external force (not gravity which is no force according to GR) or you feel the acceleration due to the propulsion.

 

[marty1]

No, it doesn't ;-)

The orbiting object is in free-fall, so there's no force and therefore no acceleration which could be observed. In Newtonian mechanics you may think that the object is accelerated, but this is only true w.r.t. 'absolute spacetime' which plays the role of a coordinate system and which has no physical significance.

If you are accelerated you will feel it. So either there is an external force (not gravity which is no force according to GR) or you feel the acceleration due to the propulsion.

Eactly, you don't feel a force when on the Earth orbiting around the sun, but I think if you took the position that you are at the center and worked out how the sun was moving you would conclude that you should be feeling a force.

 

[tom.stoer]

... I think if you took the position that you are at the center and worked out how the sun was moving you would conclude that you should be feeling a force.

which center?

 

[marty1]

You are at the center if you are the one experience less time dilation due to your velocity, I suppose.

 

[tom.stoer]

I don't understand. I am sitting on the earth. I use coordinates where I am located at r=0 (at rest w.r.t. to these coordinates). I am in free fall and do not feel any acceleration. Fine. I can use any geodesic C', C'', ... I like, use a co-moving frame S', S'', such that I am located at r'=r''=...=0. Fine (I will no longer be at the center of the orbit, the orbit of the sun will look differently ...).

What I can do in any reference frame is to calculate the orbit of sun and earth. I will always find that they are in free fall and that an observer sitting on the Earth wil not feel an acceleration. And I do not need any 'fictitious forces'.

 

[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.