Nature of Gravity Itself

[Sunshine]

As far as I understand, planets orbit each other following the curvature (gravity well) in the fabric of space created by whatever larger body is creating said well. Hat experiment can be seen a million times, the bowling ball on the fabric with the marbles orbiting, every single documentary on the subject explains it to death. So, if gravity is not a force of attraction as once was thought, akin to a magnetic force, then why is it that a moon like Io for example can be so affected by Jupiter? suffering constant flexing and stretching. This in tern creating such friction that it melts Io from within, making it such a volcanically active body.

if Io is merely falling into Jupiter and, not being pulled or yanked along by a force, then why is there such flexing taking place? Jupiter is either physically pulling on it or it is not. One would think if gravity is not a force of exertion then it should not affect Io in thi way, this is where my confusion lies. Have I brought this up before? too lazy to scour my posts.

Edited September 27, 2020 by Sunshine

[wulfrun]

I'd suggest a visit to Wikipedia to look up gravity and the equivalence principle. Io is not "merely falling into Jupiter", which implies that it is doing so passively. Io is in a non-inertial frame of reference, meaning it's being accelerated* by Jupiter's presence. Io's orbit is also complicated by resonance with Europa and Ganymede so it's acceleration is changing, with means physical work is being done and that's the source of most of the heat. I'm not too sure I've explained it all that well and I'm not too sure I actually understand the concept all that well myself, never mind trying to explain it simply.

*Sorry if I'm patronising you but "accelerated" here is meant in the strict physics sense - a change in velocity - not the everyday sense of "going faster and faster"

Edited September 27, 2020 by wulfrun
changed wording

[andrew s]

In GR gravity is not a force.  As you say spacetime is curved and objects follow a geodesic (equivalent to a straight line in flat spacetime). This is what causes a moon to orbit a planet. It is in local free fall.

Tidal forces cause heating where the EM forces holding a moon together resist the individual particles following their individual geodesic.

Regards  Andrew 

[saac]

Im pretty sure something along these lines was discussed a few months ago.  Here is my take on it . The model of the gravity well would still hold and account for the tidal friction experienced by Io.  Think of a cube of Io, 1km by 1km by 1km.   The end of the cube reaching down deeper into the gravity well will experience a greater acceleration due to the increased warping of space than the end further up , in the shallow end so to speak.  So this km long wedge of Io will experience a differential acceleration along its length  - this affects every single atom so oriented along the wedge.  As Io rotates  in orbit it will experience the changing profile of the gravity well, in simple terms stretching and relaxing and hence it will be be subject to tidal forces and subsequent heating .  Not the best description without a diagram perhaps!

Jim

 

ps Andrew said what I was thinking in fewer words  

Edited September 27, 2020 by saac

[Sunshine]

29 minutes ago, wulfrun said:

Sorry if I'm patronising you but "accelerated" here is meant in the strict physics sense - a change in velocity - not the everyday sense of "going faster and faster"

Not at all patronizing, my friend, physics is very specific, thanks for the insight.

Edited September 27, 2020 by Sunshine

[saac]

Here you go , no guarantee it is technically correct but it is how I visualise what is happening.  Happy for it to be corrected. 

Jim 

[Sunshine]

3 hours ago, saac said:

Im pretty sure something along these lines was discussed a few months ago.  Here is my take on it . The model of the gravity well would still hold and account for the tidal friction experienced by Io.  Think of a cube of Io, 1km by 1km by 1km.   The end of the cube reaching down deeper into the gravity well will experience a greater acceleration due to the increased warping of space than the end further up , in the shallow end so to speak.  So this km long wedge of Io will experience a differential acceleration along its length  - this affects every single atom so oriented along the wedge.  As Io rotates  in orbit it will experience the changing profile of the gravity well, in simple terms stretching and relaxing and hence it will be be subject to tidal forces and subsequent heating .  Not the best description without a diagram perhaps!

Jim

 

ps Andrew said what I was thinking in fewer words  

Brilliant way to explain it, many thanks!

[ollypenrice]

Excellent replies both from Andrew and Jim.

I think it more than a little interesting that Newton himself, in a conversation with Halley, said that he did not wish the idea of a force operating instantaneously at a distance be attributed to him. This seems to anticipate the revision constructed by Einstein since it resolves the two points which troubled Newton.

Olly

[carastro]

Slightly off the main topic, but did any-one watch the Antiques Road Trip?  They often visit places of interest en route. 

Very recently en route they visited the home of Sir Isaac Newton, and showed the actual Apple tree that he observed the Apple fall from still alive and bearing fruit.

It was a surreal moment watching that.

Carole