# Is the gravitational center of the solar-system relative to planetary position?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by JEL, Feb 7, 2023.

1. ### DaveC426913Valued Senior Member

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It might be easier to say it's "already there".

(Assuming a two body system for simplicity) As long as the Sun and Saturn remain the same distance from each other , the curvature of spacetime between them does not change. IOW, it is static.
Only when there is motion that changes the distance between them - or a change in mass in the system - will the curvature change.

And yes, that change will propagate at the speed of light.

Yes, BUT -

- that "somehow" you mention is not trivial. Its cause must enter the system somehow, and it does so at a relatively slow speed. Therefore all changes that Saturn experiences in its local spacetime curvature happen at the same slow rate - even if they are some minutes late.

3. ### exchemistValued Senior Member

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I fear you still have a fundamental misunderstanding. The gravitational field of a body of a certain mass is static. There are no waves or particles "transmitting" it through space. It is just there.

If there were to be a sudden change in mass, that change would travel outward at the speed of light.

Think of the magnetic field from a DC electromagnet. That field is static, so long as the magnet is energised. But if you switch it off, the collapse of the field to zero propagates outward at the speed of light. It's the same with gravity.

5. ### James RJust this guy, you know?Staff Member

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That's not what "static" means. Static means that the magnitude and direction of the field is not changing around the object/system that is producing the field.

Using the Newtonian picture, the gravitational field strength anywhere in the solar system, due to the Sun, is
$g=\frac{GM}{r^2}$,

where G is a constant, M is the Sun's mass and r is the distance of an arbitrary point in space from the Sun.

If you consider a single planet - Saturn, let's say - then the gravitational field strength due to the Sun at Saturn is given by the above formula, where r is the distance of Saturn from the Sun (which doesn't change very much as Saturn orbits, and when it does change it changes much more slowly than the speed of light).

The only ways to change the Sun's field at Saturn are either to change the mass of the Sun or to change the distance of Saturn from the Sun. The Sun's mass, although decreasing over time, does not change significantly over time scales of hundreds of millions of years. Saturn's orbital distance is also quite stable. (Planetary orbits are elliptical, so the distance does change very slowly, in a cyclical way.)

If neither the Sun's mass or the gravitational field strength is changing, there's no concern about how fast a "message" notifying Saturn about a change in field strength would take to propagate from the Sun to Saturn. That concern would only become significant if either the Sun's mass or the orbital distance were to change very rapidly. In the case of a change in orbital distance, the distance would need to change at reasonable fraction of the speed of light in order for us to see any noticeable "lag" in the gravitational force.

7. ### JELRegistered Member

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It appears I'm not quite on the same page as you 3, but thanks anyway. Maybe a penny will drop later

(I suppose it's ok for these things to be complicated to understand. After all, people have been around for a long time and yet the understanding Einstein brought forward is barely 100 years old. I guess I'm just going to have to remain seated on the 'slow-train' for now

)

8. ### foghornValued Senior Member

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1,282
It's physics, so think models.
It's taken as a given, for the model, that the gravitational field of a body extends to infinity.
Remember the black hole merger giving rise to the first discovery of gravitational waves in 2015 at LIGO ?

It may be head scratching to realise that, even before the merger of the two black holes, we were and always was in the gravitational fields of both black holes.

What was detected was a GREAT change in that field. The energy of the merger distorted the field so much that it altered its field to such a distance that the detectors noticed the hiccup.

In other words, LIGO could ONLY detect the field of both black holes when the field was boosted or greatly distorted by the energy of the merger and not before.
Ps. I think.

9. ### JELRegistered Member

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My understanding is that the collision, which LIGO detected gravitational-waves from, did not happen at the same time as they detected the waves, but happened 1.3 billion years ago.

Currently I just can't match that with an understanding of the gravitational-field as being static (Instant)

I can understand how it could be perceived (Despite this perception effectively just being an illusion) as static as long as no acceleration influences the 'emitter/creator' of the gravitational-field (The mass)

But even mass itself appears not to be static:

https://www.fnal.gov/pub/today/arch...en if space had,can also appear and disappear.

And if mass is not static, then I just can't see how the effects derived from mass could possibly be static.

For something to be static it has to be a true singularity. A unit where distance or time has no meaning.

If we consider the wake extending behind a boat that travels steadily along the ocean, we observe that the wake always extends exactly opposite of the boat's direction-of-travel, in a straight line, despite being created over time as the boat moves.

Even with strong winds blowing perpendicular to the boat's traveling direction, this wake-line remains straight (Relative to the boat)

My understanding is that this is because both boat and wake-line are traveling 'side-ways' at the same relative velocity, and not because this combined 'system' (The boat and its wake) is somehow statically-connected.

And if the boat then accelerates (Making a left turn by 90 degrees, for example), then the wake-line from before the turn will not turn with the boat (Which, to me, disqualifies the notion of the system being somehow static)

But if it is indeed connected statically, as it would appear most in this thread believe, then that is an understanding I have yet to grasp myself (And given how slow humans are to change their perception of things, I fear I'm stuck on the slow-train toward further enlightenment, for the foreseeable future at least, despite your attempts at making your views clear to me. Attempts that are likely futile until further knowledge has been amassed. Life seems to be quite cruel that way; all the trees blocking the view of the forest to some, and so 'aha-moments' don't always come easily despite some seeing the forest painfully clearly)

A nice weekend to everybody here

10. ### DaveC426913Valued Senior Member

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Static is NOT synonymous with instant!

The field has always been there.

Only changes in the field by masses propagate at the speed of light.

exchemist likes this.
11. ### JELRegistered Member

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How do you define 'static'?

As something 'that is always there'?

If yes; then how do you define 'change'?

If something that 'was always there'... changes... is it then still the same thing that 'was always there' or has it then become something different?

A bronze-statue is 'static' (No temporal changes)

A photograph of the bronze-statue is 'static' (No temporal changes)

A movie can be a sequence of 'static' images, each taken a 24th of a second apart, of the bronze-statue while it is being taken down by politically-correct rioters.

The movie, the strip of celluloid with the collection of all the single still-frames printed onto it, while wound onto a reel, can be considered static while it lies on the table waiting for the projectionist to pick it up and play it.

But the INFORMATION... the movie when projected on the screen, frame by frame, in temporal coherence... is that still static?

And since you say yourself that changes in the field propagate over time... then you have also said that there's a beginning and a middle and an ending of this information-stream.

Don't get me wrong; I'm not saying I'm correct and you're wrong, but I am saying I can't match your view of 'static' with my view of 'static'.

I'm old enough to know that such understandings can't really be explained to people (Including myself), they have to be understood before they make sense.

And just as you clearly don't understand my view, I simply can't understand yours. Only time can move either you or me, depending on who is correct now, into a position where the 'aha-moment' arrives to the one of us who is currently (Perhaps) in-correct (I say 'perhaps', because there is also the chance we are both currently correct, but just use words/images/explanations that are perceived different to their intentions (If we are simply talking 'past each other', as frequently happens during conversations using text))

I posted this link in my previous post:

https://www.fnal.gov/pub/today/arch...en if space had,can also appear and disappear.

'Matter that pops in and out of the universe' (Apparently known as 'quantum foam')

Since you say gravity-fields are static... which one of the following would you say is more likely happening?

1: the particle-matter creates (It doesn't really matter if it's radiated 'gravitons' or if it's an 'imprint/dent' on the fabric of 'space-time'. The net-effect of gravitational influence is the same) gravity in the universe during the time in which it exists, and this gravity (Being statically connected to the matter) disappears again from the universe once the particle-matter disappears.

2: the particle-matter creates gravity in the universe during the time in which it exists, and this gravity-effect remains in the universe (Propagating through the universe 'forever', originating from the point where the particle briefly existed. Similar to the waves LIGO detected after they had propagated for 1.3 billion years) once the particle-matter disappears.

I gravitate (pun intended) toward #2, but obviously that option can't really be compatible with anything that is 'static'.

So assuming you go with #1 (Since that option is compatible with a 'static' field, as you seem to be saying is what exists in reality), how would you explain that the entire field could be statically connected to the particle-matter if information is limited to the speed-of-C?

Or in other words; how would the outer rim of the field 'know' when to disappear, if information from the center of the field (Where the particle-matter that created the field is located, and from where the particle-matter disappears again when it 'pops' out of the universe) can't reach it instantly because of the speed-limit of C?

These 'popping' particles with mass are said to have been experimentally verified (At least I can find several articles claiming this. But reading something in a book or online is of course not the same as actually seeing something (Most of us probably mostly know what other people tell us, which is of course a potential problem if the source was incorrect and nobody were able to detect it) So I obviously can't say if this 'shimmer' or 'quantum foam' is real or not), so to me they present an interesting 'problem'; they must either contribute to an ever-increasing accumulation of total gravity-effect in the universe... or not.

https://science.nasa.gov/science-news/science-at-nasa/2015/31dec_quantumfoam

https://en.wikipedia.org/wiki/Virtual_particle

And if the sun's gravity is not affecting Saturn instantly, but with 80 minutes delay... and if the LIGO-experiment was correct that gravity can exist and travel for 1.3 billion years... then, to my understanding at least, it should follow that gravity (Or the net-effect of gravity at least) can exist without mass as pure information (A bit like light, which can exist even if the emitter-star does not exist anymore. How many stars do we see on the sky that are not really there anymore? And yet we continue to see them every day. Because their emission, the information they broadcast millions or billions of years ago, are only reaching us now ('Now' as 'now' relates to our time and location in the universe, which is really their past))

Do we agree that gravity (The net-effect of gravity. The observed effect. The effect we can feel with our body) can exist independent of the mass that caused it?

12. ### exchemistValued Senior Member

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No. F=GmM/r². You recognise Newton's Law of Gravitation, right? Without M there is no F. Gravitation is due to the presence of mass.

Static is the opposite of dynamic and simply means unchanging, df(t)/dt = 0.

You seem to be overthinking this grotesquely. I have no idea why. In fact I begin to suspect some crank or nutcase agenda. I hope I am wrong.

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Ok.

14. ### DaveC426913Valued Senior Member

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science is not opinion-based.

I have come to the conclusion that your motive is not what I thought it was.

I thought you wanted the model of gravity explained to you. I now believe you are attached to your own view and are more interested in us understanding your view.

I don't think you will be satisfied until we collectively acknowledge your idea.

15. ### JELRegistered Member

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You choose to completely ignore my questions and post the above comment instead.
I obviously can't do anything about that.

16. ### DaveC426913Valued Senior Member

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I'd say you choose to completely ignore everything every one is telling you and post huge tangential ideas of your own instead.

At some point, a question is answered.

17. ### JELRegistered Member

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I grounded my question on information presented by Fermilab and NASA (And posted the relevant links to their articles), expecting those 2 sources to be considered both reputable and valid foundations for thoughtful discussion on this forum.

For some reason you ignore those sources.

I know some forums are haunted by social hierarchy and have their local 'king of the hill' types. Maybe I just ran into one of those...

18. ### exchemistValued Senior Member

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The information you presented does not seem relevant to the question you posed, or at least not as we have so far understood the question. So its reputation and validity are beside the point. I could post an authoritative study on the reactions of O2 ¹Δ, but everyone would ask what relevance it had.

Perhaps you are asking a different question from what we thought. If so, the remedy would be for you to explain it again, but more clearly than before. But please do not post any more long screeds or links to papers until we have all got clear what the issue is that are you are trying to address.

To start with, do you understand now what we mean by a gravitational field that is static relative to the mass giving rise to it? And, related to this, why did you say you cannot understand how anything created by mass can be static. Perhaps if we can sort this out the rest will become clearer.

Last edited: Feb 12, 2023
19. ### DaveC426913Valued Senior Member

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I am in no way 'King of the Hill'ing you.

But I do not seen to have much in the way that you see as constructive contribution to this discussion so I will step back.

20. ### James RJust this guy, you know?Staff Member

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JEL:
Yes. LIGO detected gravitational waves, which propagate outwards from a source at the speed of light. Waves are not "static". Waves move from one place to another. (Think water waves, for example.)
A field is quite different from a wave. A field is the complete set of values of some quantity. For instance, the "gravitational field" is the set of values of the acceleration due to gravity (with some directional information built in), with one value for every location (point) in space.

As an analogy, think about a "temperature field". The temperature field of your lounge room is just the complete set of the temperatures at every location in the room. The temperature field, in principle, could be static. It would be static, for instance, if the temperature was 20 degrees Celsius at every point in the room, and the temperature did not change over time (maintained by air conditioning or heating, perhaps).

Fields can also be dynamic, of course. If the room is at 30 degrees everywhere and then you turn on an air conditioner on one side of the room, then over time the "temperature field" in the room will adjust. When you first switch on the air conditioner, the temperature near the air conditioner might be 18 degrees, say. But on the other side of the room, the temperature will still be 30 degrees. Over time, convection of the air in the room will alter the "temperature field", until eventually the whole room will be at 18 degrees (ignoring other things that might affect the temperature at certain locations in the room).

A room at a constant temperature of 30 degrees, or 18 degrees, has a "static" temperature field. A room that is cooling down from 30 degrees to 18 degrees has a changing temperature field.

The gravitational field around the sun (viewed from the sun itself) is a static field (more or less). Everywhere in the solar system, the gravitational field strength due to the sun is not changing over time, which is what "static" means.

The signal detected by LIGO involved two massive black holes orbiting around one another - actually spiralling inwards and eventually merging into a single, larger black hole. The gravitational field around those spiralling holes was not static; it changed as the holes orbited one another. "Messages" about the changes propagated outwards from the holes as gravitational waves, and the waves are what LIGO detected.
The wake of a boat is actually quite a special type of wave. In effect, the wave you see moving forwards and outwards from a speedboat or a moving ship is a "shock wave", similar to the sonic boom of a jet travelling faster than the speed of sound in the air. What's going on there is that the boat's speed is faster than the speed of the waves it creates in the water.
The line you see is the shock front.
Yes. In general, wave phenomena are created by some kind of source. Usually there's some kind of motion of the source, which generates the wave. When the source stops moving, the wave stops being generated. (When you stop the boat in the water, it doesn't cause waves or a wake any more. The "water field" becomes static.)
Don't give up so soon!

Clearly, it is possible to explain concepts to people so that they can understand them. Nothing in what we're discussing here is impossible to understand. We know that, because people do understand it. Also, we know that people come to understand it, having not understood it previously.
That's more or less what happens. A gravitational field won't appear and disappear instantly, though. The fastest that messages about changes to the field can propagate from a source to an observation point is the speed of light.
That's true, too. Propagating waves do not require that the source continue to exist or operate, once the wave has been started. You can easily see this by starting a water wave in a pond by tossing a rock into it. The rock rapidly sinks into the water and disappears from view, but the ripples on the water surface that it causes last for a long time.
It "knows" when the information reaches it. At the speed of light, as you say.
The Sun's field is static, not changing.
Not "pure information" (I'm not sure what that is), but as information encoded in the gravitational field (roughly speaking).
Yes. We're seeing the "messages" put out by distant light sources, in the form of the light waves they emit. Those light waves represent changes in the electromagnetic field that exists everywhere in the universe. Note that in a lot of places, the electromagnetic field is "static" (more or less). Those places are where there's no light.
It can exist (as waves) in a place far removed from the mass that caused it, in the same way that light from a star can exist millions of light years away from the star that emitted it.

[As an aside, we have no direct sense of gravity. We cannot "feel" gravity directly, in any way. Any feeling of "weight" that you have is always a reaction force due to something physically pushing on your body (or due to one part of your body physically pushing on another part).]

21. ### JELRegistered Member

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Thank you, James. I will take some time to digest your post.

I feel like you gave me the dough for a bread, but it needs to bake for some time in my mind before becoming a bread

22. ### exchemistValued Senior Member

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Perhaps you could respond to post 35, too. That one needs little time to digest.

23. ### JELRegistered Member

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My main-question all along has been about whether gravity (The effect of gravity. The influence that gravity has) is limited by speed-C or instant (But of course I am also interested in the derived considerations, the consequences or 'problems/possibilities', the 2 different options present)

Gravity as a topic has been an interest of mine for a long time, but I have not been giving it much thought for a while so I had become 'rusty' regarding the whole process of it all (Including how to pose questions in the most relatable way)

It was the 2 'twin-quakes' in Turkey that spurred my curiosity in this again, and so the question was asked before I had really thought the whole thing through too deeply, and so was perhaps formulated too chaotically compared to if I had waited a bit to 'collect my thoughts' and think about how I could best present a meaningful question.

Being too hasty is rarely conducive to productive outcomes, as some of the confusion in this thread was obviously a result from.