General Relativity- the Sun revolves around the Earth?

[1MileCrash]

I understand that the way it seems can vary depending on where one is experiencing the moving objects, but both statements can't actually be true, can they? The physical reality is that the Earth is revolving around the Sun...right?

Well, not to veer off from the topic, but to answer this question in a general sense, yes, both statements can be true, and no, there is no "physical reality."

Lets have an example. Say you're traveling extremely fast, toward and relative to someone holding a plank of wood to where you would pass next to them, at a constant speed. You are facing perpendicular to them.

They drop the plank of wood straight down to their feet as you begin to pass them, so that both ends hit at the same time from their frame of reference.

However, from your frame of reference, the end of the board closer to you hits the ground first, and the other end hits second.

Which one is true? Which is the physical reality? Both, and neither. Relative to you you're still, they are moving extremely fast towards you, and they drop the board one end at a time. Relative to them, they are still, you are moving extremely fast towards them, and they drop the board both ends at the same time.

You can't say who's "right" because you can't say who's "absolutely" moving. So even some of the things that seem like they have a physical reality are still just relative.

I could be wrong, but I would say that there is no "absolute" way to show that the Earth revolves around the sun and not vice-versa. My guess for why we say "the Earth revolves around the sun" is because if we established Earth as a frame of reference, the sun orbits the earth, BUT every other planet in the solar system orbits the Earth while orbiting the sun. If we establish the sun as the frame of reference, all planets are revolving around it.

 

[DaveC426913]

This thread is all over the map.

The discussion can't even settle on what the OP means. It has, at times, answered with reference to:
- the barycenter of the Earth-Sun system (sun revolves around Earth - check),
- relativity (all frames of reference are relative - check)
- relativity of simultaneity (two events are observed to occur at differnt times depending on FoR - check)

Perhaps we should (re)start by clearly defining what the OP was asking (two years ago).

 

[Cleonis]

That would prove that one physical interpretation is correct and the other is not.

Your disagreement is not with me. The point of view you are arguing against isn't mine.


It goes without saying that as physicists we're not claiming to know absolute truths.

Still, we are confident that knowledge is possible. For instance, we rely on the principle of relativity of inertial motion. As physicists we work with a range of degrees of certainty. Some theories are tentative, some are regarded as beyond reasonable doubt.

Part of our set of physical interpretations is that such a thing as gravitational potential exists, and that there are gravitational time dilation effects.

As physicists we're not in the business of delivering absolute proofs, as those don't exist. As physicists our measures to go by are weight of evidence, and reasonable doubt.

 

[K^2]

Cleonis, you seem to also be confused on the purpose of the theory.

Is Heliocentric system more convenient, and therefore, more useful? Absolutely. Does it give you any predictions you could not acquire in Geocentric system? Absolutely not.

And when I map coordinates on Earth, is it more convenient to keep inertial frame of reference, or one that is fixed to rotating Earth. The later, of course. But does that mean that the Earth suddenly stopped rotating? No.

Convenience of one model over the other does not imply any sort of physical truth. It's just that, a convenience. In order for one thing to be true and for another to be false, the two models must provide disagreement in predictions.

Are there any disagreements in predictions? No. Then it is no more wrong to say that Earth is the center of the universe around which all else rotates, than it is to say that Earth rotates around the Sun.

 

[1MileCrash]

Right, right, I agree with you K^2.

However, please help me out with the mental picture.

From Earth's frame of reference, it's completely still, and therefore the sun orbits around it once per year. However, the Earth is also rotating. If we also "transfer" that movement as to be earth-relative, the sun is also orbiting around the Earth once per 24 hours.

I'm just having trouble picturing it. Or is the Earth still rotating from it's own reference frame?

What exactly is the sun doing from Earth's frame of reference, taking into account both orbit and rotation?

 

[Gabe911]

Right, right, I agree with you K^2.

However, please help me out with the mental picture.

From Earth's frame of reference, it's completely still, and therefore the sun orbits around it once per year. However, the Earth is also rotating. If we also "transfer" that movement as to be earth-relative, the sun is also orbiting around the Earth once per 24 hours.

I'm just having trouble picturing it. Or is the Earth still rotating from it's own reference frame?

What exactly is the sun doing from Earth's frame of reference, taking into account both orbit and rotation?

i second this question

 

[Dale]

I'm just having trouble picturing it. Or is the Earth still rotating from it's own reference frame?

What exactly is the sun doing from Earth's frame of reference, taking into account both orbit and rotation?

You can make either kind of coordinate system.

You can make an earth-centered non-rotating frame where the center of the Earth is always at the origin and the surface of the Earth revolves once per sidereal day. In this coordinate system the stars wobble a little over the course of the year but they don't orbit the earth. The sun orbits the Earth once per year.

You can also make an earth-centered rotating frame where the center and the surface of the Earth are always at rest. In this coordinate system the stars orbit the Earth once per sidreal day. The sun orbits the Earth once per day.

 

[1MileCrash]

You can also make an earth-centered rotating frame where the center and the surface of the Earth are always at rest. In this coordinate system the stars orbit the Earth once per sidreal day. The sun orbits the Earth once per day.

But what happens to the yearly orbit?

 

[Dale]

But what happens to the yearly orbit?

That is the difference between the orbit of the stars once every sidereal day and the orbit of the sun once every day.

 

[D H]

But what happens to the yearly orbit?

The sun's altitude cycles over the course of a year.

 

[Dale]

Oh yeah, I forgot about that. And it drifts from south to north and back south again. And the distant stars wobble a little over the year.

 

[K^2]

From Earth's frame of reference, it's completely still, and therefore the sun orbits around it once per year. However, the Earth is also rotating. If we also "transfer" that movement as to be earth-relative, the sun is also orbiting around the Earth once per 24 hours.

I'm just having trouble picturing it. Or is the Earth still rotating from it's own reference frame?

What exactly is the sun doing from Earth's frame of reference, taking into account both orbit and rotation?

How many solar days are there in a year?

But how many turns does Earth make in that time?

Try to think about it, and the answer to your question should become clear.

 

[Canes]

First of all, I think some of you are abusing what GR says.

"There is no preferred reference frame" means that the mathematics work in any reference frame, but that does not imply that any reference frame is an accurate description of reality...

The job of a physicist is to find mathematical equations which describe the world, and then to interpret them in a meaningful way. You can not just look at an equation and read it literally.

Second, I believe that one can find an absolute frame of reference - the CMBR. You can define an absolute reference frame as one in which the CMBR is isotropic - that is has zero redshift in all directions. Such a frame would be the same anywhere in the Universe.

In fact, you can measure our Galaxies velocity through space by measuring the redshift in the cosmic background in one directions versus another, something that is actually pretty easy to do.

 

[sophiecentaur]

First of all, I think some of you are abusing what GR says.

. . . .
Second, I believe that one can find an absolute frame of reference - the CMBR. You can define an absolute reference frame as one in which the CMBR is isotropic - that is has zero redshift in all directions. Such a frame would be the same anywhere in the Universe.

In fact, you can measure our Galaxies velocity through space by measuring the redshift in the cosmic background in one directions versus another, something that is actually pretty easy to do.

This is interesting. Has it been done? What was the result?

i.e are we going at 10000m per second thataway?

 

[Canes]

This is interesting. Has it been done? What was the result?

i.e are we going at 10000m per second thataway?

This is a paper on it:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...419...1K

Looks like they find that we are moving at 622 km/s in relation to the cosmic microwave background.

 

[Dale]

First of all, I think some of you are abusing what GR says.

"There is no preferred reference frame" means that the mathematics work in any reference frame, but that does not imply that any reference frame is an accurate description of reality...

As long as a given reference frame produces correct predictions about the outcome of any given experiment then it is perfectly valid. There is no abuse of GR involved. That is the whole intention of the tensor formulation of the laws of physics.

Second, I believe that one can find an absolute frame of reference - the CMBR. You can define an absolute reference frame as one in which the CMBR is isotropic - that is has zero redshift in all directions. Such a frame would be the same anywhere in the Universe.

You believe wrong. First, such a frame is not "the same anywhere in the universe" since two distant objects which are each locally at rest wrt the CMBR would not be at rest wrt each other. Second, none of the laws of physics are different in a frame where the CMBR is at rest than in any other frame, this is what is meant by "absolute reference frame".

 

[Cleonis]

[...] two distant objects which are each locally at rest wrt the CMBR would not be at rest wrt each other.

Elaborating on that:

We have that the Universe is expanding. The very concept of space itself expanding has implications for the concept of 'being at rest wrt to each other'.

It does seem that the CMBR can be used as a reference for velocity that is available, and consistent, throughout the Universe.
But the reference isn't stationary, the Universe is expanding - a moving target. Also, astronomers report that the expansion is accelerating.

 

[Cleonis]

Convenience of one model over the other does not imply any sort of physical truth.

Sure, I agree with that.

But as I said, what you are discussing and what I am discussing are different subjects.

The Sun is more massive than the Earth; that is the decisive factor.
The Sun/Earth mass ratio and convenience of models are two distinct subjects.


Putting the discussion in a wider context: what is the purpose of GR?
First, let me rephrase that: What is the achievement of GR?
(A purpose is usually a preconceived notion, and the achievement is what you actually end up with.)


Gravitational mass (as a physical property) is like a coupling constant. The analogy: the electric charge of a particle is a measure of its coupling to an electric field. The gravitational mass of an object is a measure of how strongly it couples to the gravitational field that is present.

In terms of GR we have that as a matter of principle the coupling that is involved in gravitation and the coupling that is involved in inertia are one and the same coupling.

Newtonian dynamics used two separate laws: F=ma for describing inertia, and Newton's universal law of gravity. Two independent theories, enforcing a distinction between inertial and gravitational mass. (But experiments aimed at finding any difference between inertial and gravitational mass give null results.) In terms of GR there is a single theory, with a single concept of mass; this unification is the achievement of GR. It's the one thing that GR has and that Newtonian theory doesn't have.

John Wheeler summed up GR as follows: Matter/energy is telling spacetime how to curve, curved spacetime is telling matter/energy how to move. GR introduced a fully fledged reciprocity.
By contrast, in terms of Newtonian mechanics the story is one-sided: space is acting upon matter, but matter isn't acting upon space. This one-sided state of affairs was unsatisfactory to Einstein.
In terms of GR: spacetime is a full participant in the physics taking place. GR-spacetime is acting upon inertial mass and is being acted upon by inertial mass.

Both SR and GR have ill-fitting names. Einstein indicated that in retrospect a better name for special relativity would haven been 'Invariance theory'. For GR I suppose a technical name such as 'Tensor Gravitation' would have been better.
Anyway, the names are what they are, the names can't be changed. But they're not particularly descriptive as to what the theories achieve.

 

[espen180]

@Cleonis
The purpose of GR, as you put it, is to consolidate SR with Newtonian gravitation, from what I've read. It's achievement is being a good physical theory with accurate predictions.

In Newtonian gravitation, you have mass acting on mass. I can't recall ever reading or hearing that there is any significant mass-space coupling in Newtonian gravitation.

You seem to be discussing the philosophy of GR rather than its physics.

Whether you choose to believe that there exists a curved 4-D entity called spacetime which couples to matter, or whether you believe spacetime is a convenient mathematical tool for GR is completely up to the individual phycisist.

 

[Cleonis]

In Newtonian gravitation, you have mass acting on mass. I can't recall ever reading or hearing that there is any significant mass-space coupling in Newtonian gravitation.

To describe the difference between GR and Newtonian dynamics it's necessary to adopt a GR perspective. That's what I did in my previous post; I described Newtonian dynamics from a GR perspective.

I take the GR perspective to be the one summerized by John Wheeler: "Matter/energy is telling spacetime how to curve, curved spacetime is telling matter/energy how to move."

The part 'spacetime is telling matter/energy how to move' does not distinguish between gravitation and inertia. There is just a generic 'telling matter/energy how to move'. From a GR perspective inertia is spacetime telling matter/energy how to move.

Of course, in vintage Newtonian thinking inertia is considered to be an irreducible phenomenon. In Newtonian thinking F=ma is simply F=ma.So yes, in vintage Newtonian thinking space is not considered, but as we know: in terms of GR spacetime is key.

 

[espen180]

If you want to express Newtonian gravity as a spacetime theory, in order to compare it to GR, there is nothing stopping you from doing that. You'll get the same kind of matter->space and space->matter coupling as in GR, but the field equations will look different.

 

[Dale]

The Sun is more massive than the Earth; that is the decisive factor.
The Sun/Earth mass ratio and convenience of models are two distinct subjects.
...

What does their relative mass have to do with the possible coordinate charts that can be used?

 

[K^2]

The Sun is more massive than the Earth; that is the decisive factor.
The Sun/Earth mass ratio and convenience of models are two distinct subjects.

Sun is more massive. Yes. Why does that make any difference? Tell me a way to measure which one is actually moving, and then you have a point. Saying that the heaviest thing ought to be in the center is on the same shelf as Aristotelian Mechanics. Seems intuitively correct, but otherwise meaningless.

 

[Cleonis]

Tell me a way to measure which one is actually moving, and then you have a point.

Once again I will move to a wider context.

A comparison:
If you ask a particle physicist whether neutrino's have mass, what will he answer? He will answer that the weight of evidence is towards neutrino's having mass. The accumulated evidence is that the rates at which the various flavors of neutrinos are detected are consistent with the theoretically predicted phenomenon of neutrino oscillation. Neutrino oscillation will occur if and only if the neutrino's have non-zero mass.

OK, my point is that neutrino mass has not been measured directly. The existence of neutrino mass is inferred from the neutrino oscillation. In turn the neutrino oscillation is inferred from neutrino detection rates (starting with the observation of the solar neutrino deficit.)

Growth of knowledge

There is rarely - if ever - a direct measurement. The growth of knowledge is in the form of an intricate web of inferences. Scientists do not demand absolute proof; the criterium is that the evidence settles the issue beyond reasonable doubt.

For the motion of the Earth and the Sun: it is unwarrented and unrealistic to insist on direct measurement for that case. It is sufficient to argue the case on general considerations. Rephrasing the question by the original poster:
"Is the motion of the Earth and the Sun relative to each other just as relative as the motion of the magnet and the coil of the SR-inspiring magnet-and-coil case?"

- For the magnet-and-coil case SR asserts as a matter of principle that no experiment will indicate such a thing as the velocity of either the magnet or the coil with respect to some absolute reference of motion. As a scientist you must gather all the information that you can, but no matter how much information you gather, you won't be able to infer such a thing as a velocity vector for either the coil or the magnet with respect to some absolute referene of motion.

- For orbital motion (a planet orbiting a sun) information is abundant. And around the time of the Copernican revolution it was inferred that the Earth is orbiting the Sun.

It's the difference between no existence of information in the magnet-and-coil case, and abundant information in the case of orbital motion.Summerizing:
There is rarely - if ever - a direct measurement.
For the motion of the Earth and the Sun: it is unwarrented and unrealistic to insist on direct measurement for that case. It is sufficient to argue the case on general considerations.

 

[K^2]

You are missing the point. Any kind of measurement, even an indirect one, would be a violation of GR postulates. If you say that you can tell that Earth revolves around the Sun, you are saying that General Relativity does not work. It is as simple as that.

 

[Cleonis]

You are missing the point. Any kind of measurement, even an indirect one, would be a violation of GR postulates.

So let's examine the principle that is unique to GR.

There is no experiment, under any circumstance, that will indicate a difference between inertial mass and gravitational mass. The distinction inertial/gravitational mass is not inherent to the phenomena. .

I will refer to this principle by its usual name, the Principle of Equivalence.


http://physics.bu.edu/people/show/stachel" , a physicist and historian of physics, has written a story to illustrate the nature of the transition between SR and GR. It's called 'The story of Newstein'. In this story history takes another course, and the principle of equivalence is implemented before the year 1900. Stachel argues that many of the elements necessary for such a development were available by that time, and that the principle of equivalence being implemented first is in fact a plausible course of events. The fictional physicist developing that theory is called 'Newstein'.

Four theoretical framework can be arranged on four corners of a square.
- From left to right the transition is the introduction of the (-,-,-,+) signature metric
- From top to bottom the transition is the implementation of the principle of equivalence


Classical dynamics | Special relativity
--------------------------------------------
Newstein theory | General Relativity


This diagram illustrates that the two transitions are independent. Neither is an extension of the other.

The purpose of the Newstein story is to illustrate that the transition from SR to GR is unrelated to any relativity concept. The principle of equivalence could have been implemented without any awareness of the (-,-,-,+) signature metric.

 

[K^2]

What the hell are you talking about? What does any of that have to do with the discussion?

Again, can you produce an experiment that would detect Earth's rotation around the Sun? No, because that would violate GR. Not JUST the equivalence principle, but it's one of the possible violations. I don't know in what way you propose to make a measurement, so I don't know specifically which principle you are going to violate.

 

[D H]

Again, can you produce an experiment that would detect Earth's rotation around the Sun?

http://improbable.com/airchives/paperair/volume7/v7i3/angels-7-3.htm

I would agree with you K^2 if the Sun and Earth were the only things in the universe. They aren't. We can see other planets, other stars, quasars, ... Observations of these objects coupled with parsimony says that the Earth orbits the Sun rather than the other way around.

 

[espen180]

Taking the risk of entering a semantical debate, the two are, strictly speaking, orbiting each other. The centre of mass, and the point about which the planet and star orbit just happens to be inside the sun.

If you want to say that "one orbits the other", you have to define explisitly what this means. Once a mathematical definition is in place, you can talk about measuring whether one orbits the other or vice versa according to that definition.

 

[D H]

Taking the risk of entering a semantical debate, the two are, strictly speaking, orbiting each other. The centre of mass, and the point about which the planet and star orbit just happens to be inside the sun.

If you want to say that "one orbits the other", you have to define explisitly what this means. Once a mathematical definition is in place, you can talk about measuring whether one orbits the other or vice versa according to that definition.

I'll propose one, although slightly problematic: Is the center of mass inside of one of the objects or between them? The IAU considered but rejected this back in 2006 to determine whether a pair of co-orbiting objects are a planet and moon versus a double planet. A couple of issues with this definition: (1) In a few billion years, our Moon will magically become our Double Planet. (2) The Sun-Jupiter barycenter is outside of the Sun. Everyone still thinks of Jupiter as a planet and Jupiter as orbiting the Sun (more or less).

Note that the Sun-Earth barycenter is so deep inside the Sun that neither of these concerns is in play.