The Earth-Sun Relationship: A Matter of Perspective

[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...

 

[jackoblacko]

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.

What if there are three things (balls) and they are equal?

 

[jackoblacko]

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

I am going to brush up on relativity and all that jazz again, I read some stuff on it a few years ago but I guess I forget the words. Maybe I'll relearn a bunch on the basics and come back.

As for point 5, so if this perspective (she?) is an accelerated one, she observers certain things in respect to our 2 balls. She feels the force of the acceleration and they don't.

I thought we can see that as she is at rest, and they are accelerated but they don't 'feel' the force because there is a fictional sort of negative force. So in other words using our understanding (glasses) from our Earth orbits sun model, if you are at rest in that universe you feel a force.

 

[Mentz114]

I am going to brush up on relativity and all that jazz again, I read some stuff on it a few years ago but I guess I forget the words. Maybe I'll relearn a bunch on the basics and come back.

As for point 5, so if this perspective (she?) is an accelerated one, she observers certain things in respect to our 2 balls. She feels the force of the acceleration and they don't.

I thought we can see that as she is at rest, and they are accelerated but they don't 'feel' the force because there is a fictional sort of negative force. So in other words using our understanding (glasses) from our Earth orbits sun model, if you are at rest in that universe you feel a force.

I think you should start with Newton's laws.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

The tricky thing about gravity is that it can change the speed of a body without the kind of force you can feel.

 

[chill_factor]

What if there are three things (balls) and they are equal?

the reason that the Earth orbiting the sun is not a matter of perspective is this:

any system of particles has a center of mass. the entire system will behave as if its mass was concentrated at the center of mass if we "zoom out" far enough such that the particles themselves have no size. For the purposes of astrophysics, both the Earth and the sun are particles since their extended size is puny compared to the distances between them.

so we define the center of mass of the system to be stationary. it so happens that the center of the mass of the solar system is inside the sun. Therefore, the dynamics of the planets is almost indistinguishable from them just orbiting the sun. You can't say the sun orbits the Earth and the planets move in weird ways because the Earth is a non-inertial frame. It simply does not work that way.

if we had 3 equal balls orbiting each other then we can look at it from the rest frame of the center of mass and it will be distinguishable which ball is orbiting the other. however do note that the 3 body gravitating system in general is not stable so one of the balls will either crash into another or get tossed out of the system.

in general you can look at N particles through the rest frame of their center of mass.

If you have any more questions about this, pick up a book on engineering dynamics and orbital mechanics. This is all classical stuff from 300 years ago. Asking us is not useful because online guys can't teach an entire course on classical mechanics. The only way to learn this is to get in a classroom, sit down for 3 hours per week listening to a teacher talk, then going home and doing problems.

 

[jackoblacko]

The only way to learn this is to get in a classroom, sit down for 3 hours per week listening to a teacher talk, then going home and doing problems.

Or sit in your bedroom, fire up some youtube lecture from top universities around the world lulz.

Thx, all I obv got to study up before I can ask such a question.

I wish I made a poll though in the meantime:

A) Earth orbits suns
B) Barycentric
C) Either Earth orbits sun, or Sun orbits Earth are fine.
D) Depends on which branch of physics
E) Questions is invalid because of lack of understand of the term 'orbit' etc.
F) Leave our forum.



to me this was a 4th perspective not sure how you all feel about this one if its correct or accepted or what ever.

It'll take me a month or two at least before I dig back into relativity and all that stuff. Maybe I'll bump this again when I do.

 

[jackoblacko]

I think you should start with Newton's laws.

I watched physics overview and i understand why you suggest this thanks.

 

[cosmik debris]

Wittgenstein said to some guy "why did people think that the sun went around the Earth"? The guy said "that's the way it looks". Wittgenstein said "how would it look if it were the other way around"?

 

[jackoblacko]

Is this not legit:

Einstein and Infeld, The Evolution of Physics, p. 212 (248 in 1938 ed)):

"The struggle, so violent in the early days of science, between the views of Ptolemy and Copernicus would then be quite meaningless. Either coordinate system could be used with equal justification. The two sentences, 'the sun is at rest and the Earth moves,' or 'the sun moves and the Earth is at rest,' would simply mean two different conventions concerning two different coordinate systems." Major figures in cosmology, physics, and astronomy (Albert Einstein, Max Born, Stephen Hawking, George Ellis, and Fred Hoyle, among others) are in agreement that there is no scientific way to prove either model over the other, nor any non-philosophical/theological justification for doing so, SPECIFICALLY in the context of General Relativity.

 

[Dale]

Yes, it is legit. Copernicus was no more nor less correct than Ptolemy. The main advantage of Copernicus' approach was not that it was uniquely "true"; the main advantage is that it was much easier to calculate.

Sometimes I have the feeling that we need another Copernicus more than anything else in theoretical physics. Just a much more elegant and easy way to calculate things that we already can compute laboriously.

 

[jackoblacko]

Yes, it is legit. Copernicus was no more nor less correct than Ptolemy. The main advantage of Copernicus' approach was not that it was uniquely "true"; the main advantage is that it was much easier to calculate.

Sometimes I have the feeling that we need another Copernicus more than anything else in theoretical physics. Just a much more elegant and easy way to calculate things that we already can compute laboriously.

ok but everyone in here argued that I was correct in this...am i cleary right or do i still not understand this? not sure if you are familiar with this thread. thanks for your time.

 

[Dale]

ok but everyone in here argued that I was correct in this...am i cleary right or do i still not understand this? not sure if you are familiar with this thread. thanks for your time.

You took 5 months off. I have forgotten both your position and the arguments that you are referring to, but I am disinclined to re read it all when you cannot even be bothered to write a clear post. Please be specific in your question.

 

[jackoblacko]

You took 5 months off. I have forgotten both your position and the arguments that you are referring to, but I am disinclined to re read it all when you cannot even be bothered to write a clear post. Please be specific in your question.

Apologies, I can certainly be bothered however, I am not good with words. My question was simply doesn't the Earth orbit the sun equal to the sun orbiting the earth. In other words i was under the impression that its just a perspective we take for our general math that the sun is the center.

Again I am bad with words but it seems to me that the quote I posted cleary shows I am correct, yet I've been essentially laughed at on three forums now including this one for suggesting it

thx.

 

[1977ub]

Or sit in your bedroom, fire up some youtube lecture from top universities around the world lulz.

Thx, all I obv got to study up before I can ask such a question.

I wish I made a poll though in the meantime:

A) Earth orbits suns
B) Barycentric
C) Either Earth orbits sun, or Sun orbits Earth are fine.
D) Depends on which branch of physics
E) Questions is invalid because of lack of understand of the term 'orbit' etc.
F) Leave our forum.



to me this was a 4th perspective not sure how you all feel about this one if its correct or accepted or what ever.

It'll take me a month or two at least before I dig back into relativity and all that stuff. Maybe I'll bump this again when I do.

I think the best response integrates parts of B, C, D, and E.

 

[Dale]

Apologies, I can certainly be bothered however, I am not good with words. My question was simply doesn't the Earth orbit the sun equal to the sun orbiting the earth. In other words i was under the impression that its just a perspective we take for our general math that the sun is the center.

Again I am bad with words but it seems to me that the quote I posted cleary shows I am correct, yet I've been essentially laughed at on three forums now including this one for suggesting it

You can calculate all of the physical observables using any coordinate system you like, provided you do the math correctly. The math is simplest in what is called an inertial frame, but other coordinates are not wrong and can often be useful.

 

[jackoblacko]

You can calculate all of the physical observables using any coordinate system you like, provided you do the math correctly. The math is simplest in what is called an inertial frame, but other coordinates are not wrong and can often be useful.

Thank you I was sure that was the case, it seems to me that is C.

 

[jackoblacko]

I think the best response integrates parts of B, C, D, and E.

I'm curious how you left out A but included B. And E doesn't seem to fit here either.

 

[1977ub]

I'm curious how you left out A but included B. And E doesn't seem to fit here either.

C includes A. Better grasp of definitions might render question unnecessary... ? "E"

 

[jackoblacko]

C includes A.

ah right!

Better grasp of definitions might render question unnecessary... ? "E"

Thx!

 

[jackoblacko]

Does this mean in reality Earth orbits sun or sun orbits Earth are both correct?

 

[1977ub]

Does this mean in reality Earth orbits sun or sun orbits Earth are both correct?

You could make either argument. The sun-orbiting-earth argument would be more sophisticated, and has less practical application.

 

[jackoblacko]

You could make either argument. The sun-orbiting-earth argument would be more sophisticated, and has less practical application.

thank you.

 

[pervect]

Does this mean in reality Earth orbits sun or sun orbits Earth are both correct?

I would first say that I agree with the reference you cited that there isn't any observation that definitively settles the question - as far as I know. (I haven't attempted to check other references on the point, but the point seems valid to me.)

However, I would also say that I couldn't produce a theory in which the Sun orbits the Earth that's valid for all of space-time - at least not easily. The details and underlying issues are a bit technical, basically though locally there exists a fermi normal frame in which the Earth as it rest, but this frame is only valid locally, if you try to extend it to far it, one runs into mathematical difficulties.

So while it's true that GR allows one to determine a "local viewpoint" for any observer, in which they are at rest, you need to read the fine print about the view only being local. If you want your theory to cover all of space-time, it's harder.

I can't prove, at the moment, that it's utterly impossible to find a theory covering all of space-time with some "relaxed" notion of a viewpoint (i.e. perhaps a viewpoint is not NECESSARILY equal to a fermi-walker coordinate system), so I'll make a lesser claim. I don't think you'll find a _published theory_ that starts out assuming the Sun orbits the Earth.

Furthermore, saying that you do sounds a bit - odd. So I'd avoid saying it.

If you consider Newtonian theory, it will tell you that the Earth and Sun orbit a common barycenter.

If you consider general relativity, it will not single out such a point directly. Instead it will say something along the lines of ""Mass tells space-time how to curve, and space-time tells mass how to move."" (a quote from Wheeler, http://astronomyandspace.blogspot.com/2010/06/quote-john-wheeler.html).

In GR, the idea of an orbit is an approximation anyway. It's a pretty good approximation, but you'll see things like "orbits decaying" that don't have any Newtonian counterpart. And if you try to take your thesis really seriously, you'd have to settle issues like "what is the center of a decaying orbit".

So, I suppose, my best answer is that the basis of GR is not really built around orbits, orbits come about as a good approximation but aren't a fundamental part of the theory - orbits come about when the system exhibits repetitive, or nearly-repetitive, motion.

 

[1977ub]

Actually it is only in very casual conversation is one body said to "orbit around" another. It is simplest to say that the two bodies orbit the barycenter. I believe that is the closest thing to an inertial frame and that the GR metric is simplest... Thus, Earth and Moon "orbit around" their barycenter, which rests inside the surface of the Earth. The Earth/Moon and the Sun orbit a barycenter which is inside the surface of the Sun. And the Sun of course is "orbiting around" a galactic barycenter near the galactic core. Then there is a supragalactic core and other structures. Pluto and Charon "orbit around" a center of mass in space between them. "Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycenter with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's center. " http://en.wikipedia.org/wiki/Jupiter
http://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy)
http://en.wikipedia.org/wiki/Orbit

 

[Dale]

Does this mean in reality Earth orbits sun or sun orbits Earth are both correct?

I have a fundamental aversion to questions about "reality". Every time someone asks that I ask them to define "reality" or to describe an experiment to test the "reality" of something. Very few people even attempt to answer.

 

[Chronos]

Science makes no attempt to define reality, it is content with modeling things in a way that is consistent with observational evidence.

 

[jackoblacko]

I would first say that I agree with the reference you cited that there isn't any observation that definitively settles the question - as far as I know. (I haven't attempted to check other references on the point, but the point seems valid to me.)

However, I would also say that I couldn't produce a theory in which the Sun orbits the Earth that's valid for all of space-time - at least not easily. The details and underlying issues are a bit technical, basically though locally there exists a fermi normal frame in which the Earth as it rest, but this frame is only valid locally, if you try to extend it to far it, one runs into mathematical difficulties.

So while it's true that GR allows one to determine a "local viewpoint" for any observer, in which they are at rest, you need to read the fine print about the view only being local. If you want your theory to cover all of space-time, it's harder.

I can't prove, at the moment, that it's utterly impossible to find a theory covering all of space-time with some "relaxed" notion of a viewpoint (i.e. perhaps a viewpoint is not NECESSARILY equal to a fermi-walker coordinate system), so I'll make a lesser claim. I don't think you'll find a _published theory_ that starts out assuming the Sun orbits the Earth.

Furthermore, saying that you do sounds a bit - odd. So I'd avoid saying it.

If you consider Newtonian theory, it will tell you that the Earth and Sun orbit a common barycenter.

If you consider general relativity, it will not single out such a point directly. Instead it will say something along the lines of ""Mass tells space-time how to curve, and space-time tells mass how to move."" (a quote from Wheeler, http://astronomyandspace.blogspot.com/2010/06/quote-john-wheeler.html).

In GR, the idea of an orbit is an approximation anyway. It's a pretty good approximation, but you'll see things like "orbits decaying" that don't have any Newtonian counterpart. And if you try to take your thesis really seriously, you'd have to settle issues like "what is the center of a decaying orbit".

So, I suppose, my best answer is that the basis of GR is not really built around orbits, orbits come about as a good approximation but aren't a fundamental part of the theory - orbits come about when the system exhibits repetitive, or nearly-repetitive, motion.

obviously I can't fully understand this but I really appreciate the tone.

 

[jackoblacko]

Actually it is only in very casual conversation is one body said to "orbit around" another. It is simplest to say that the two bodies orbit the barycenter. I believe that is the closest thing to an inertial frame and that the GR metric is simplest... Thus, Earth and Moon "orbit around" their barycenter, which rests inside the surface of the Earth. The Earth/Moon and the Sun orbit a barycenter which is inside the surface of the Sun. And the Sun of course is "orbiting around" a galactic barycenter near the galactic core. Then there is a supragalactic core and other structures. Pluto and Charon "orbit around" a center of mass in space between them. "Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycenter with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's center. " http://en.wikipedia.org/wiki/Jupiter
http://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy)
http://en.wikipedia.org/wiki/Orbit

so Earth and the sun orbit each other around a point inside the sun, but Jupiter and the sun orbit each other around a point above the suns surface. Doesn't this describe two different patterns of the sun? That the sun is orbiting around a spot inside itself and outside itself?

 

[jackoblacko]

I have a fundamental aversion to questions about "reality". Every time someone asks that I ask them to define "reality" or to describe an experiment to test the "reality" of something. Very few people even attempt to answer.

Agreed this is what I was under the impression of which was why its just perspective to say one certain way

 

[jackoblacko]

Science makes no attempt to define reality, it is content with modeling things in a way that is consistent with observational evidence.

I could see this too, haven't hear it said this way. The observation I think of is watching the sun go across the sky, and we are unable to tell if it is the sun moving or the earth. Of course there are other 'observations' than visual but some of the people with physics degrees told me to look outside

 

[1977ub]

so Earth and the sun orbit each other around a point inside the sun, but Jupiter and the sun orbit each other around a point above the suns surface. Doesn't this describe two different patterns of the sun? That the sun is orbiting around a spot inside itself and outside itself?

An orbit is a relationship between two bodies. Any pair of bodies has it's own relationship.

As some person in their infinite wisdom has typed into wikipedia: "In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System.[1][2] Orbits of planets are typically elliptical."

 

[BobG]

Is this not legit:

Einstein and Infeld, The Evolution of Physics, p. 212 (248 in 1938 ed)):

"The struggle, so violent in the early days of science, between the views of Ptolemy and Copernicus would then be quite meaningless. Either coordinate system could be used with equal justification. The two sentences, 'the sun is at rest and the Earth moves,' or 'the sun moves and the Earth is at rest,' would simply mean two different conventions concerning two different coordinate systems." Major figures in cosmology, physics, and astronomy (Albert Einstein, Max Born, Stephen Hawking, George Ellis, and Fred Hoyle, among others) are in agreement that there is no scientific way to prove either model over the other, nor any non-philosophical/theological justification for doing so, SPECIFICALLY in the context of General Relativity.

You can make the bookkeeping work out in any reference frame you want, so I'd say it's impossible to prove mathematically which model is correct. And the laws of physics apply regardless of the reference frame you're in.

I don't think that should be confused with what's actually happening.

Instead, that's a reason you need some actual observations of what's happening instead of 'proving' a model solely with mathematics (although the laws of physics and mathematics could certainly prove a model can't possibly be true).

And calculations in the Copernicus model was no easier than the Ptolemaic model, since a solar system filled with circular orbits required just as many epicenters and deferents as the Ptolemaic model. You didn't get an easier model to work with until the Keplerian model with elliptical orbits.

But it is a perfect example of being unable to prove the reality of either model solely by the math, since the laws of physics worked in both. It takes some other observations such as "can you detect any change in the angles between stars as you orbit the Sun?" to prove which is correct, except if don't know how far away the stars are and have no way to measure their difference, in fact, don't even have the capability to measure really small changes in their angles (it wasn't until Galileo that telescopes even started being used for observations) you don't know if the answer to that question really proves your point or not.

Not only did people not know how far away the stars were, they didn't even know how far away the Sun was (hence the long tradition of measuring planetary distances in "astronomical units" instead of a unit of known length, such as meters). In fact, the world's first international science project was an attempt to use the transit of Venus in the 1770's to finally, once and for all, detrmine just how far away the Sun was from the Earth.

That doesn't mean both models can accurately describe reality. It just means there's no way to determine which model describes reality.

 

[jackoblacko]

That doesn't mean both models can accurately describe reality. It just means there's no way to determine which model describes reality.

Much appreciated from all. I think I might understand, and although your explanation is good I'm afraid I wouldn't without Feynmans lecture series here

By our understanding of models or representations or whatever (I don't want to be to specific in the wording) things could work any of these ways I described. The sciences basically admit that.

But that's not to say they are equal, or that one is not more useful or even plausible than the other.

The best example I just thought of would be that the model of the solar system can function backwards, but we would be hard pressed to show that it does.

Science or physics isn't interested in exploring those thoughts, but rather seeks to explain the things that help our understanding of the tangible application to reality?