Gravity Analogy: Why Does Moon Orbit Earth?

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Discussion Overview

The discussion revolves around the analogy of gravity and its implications for understanding why the moon orbits the Earth. Participants explore the effectiveness of visual representations, such as the 'bowl' analogy, and the limitations of these models in explaining gravitational interactions, particularly in the context of General Relativity.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the Earth's mass creates a 'dip' in space that the moon orbits, likening it to a marble in a bowl, but expresses confusion about the analogy's applicability when dropping an object.
  • Another participant questions the analogy's validity in the context of General Relativity, indicating that it does not apply to objects falling from rest and suggests further reading.
  • A participant states that if the moon were to stop in its orbit, it would fall to Earth, implying a relationship between orbital motion and gravitational pull.
  • One participant critiques a common visual representation of gravity, arguing that it oversimplifies the four-dimensional nature of spacetime and misrepresents the relationship between the Earth and the 'bowl' analogy.
  • Another participant agrees with the critique of the 2D representations, emphasizing that such models do not fully capture the complexities of gravitational interactions.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of the gravity analogy and its visual representations. There is no consensus on the validity of the analogy, particularly in relation to General Relativity, and the discussion remains unresolved regarding the best way to conceptualize gravity.

Contextual Notes

Participants highlight limitations in the analogies used to explain gravity, particularly regarding their dimensionality and the assumptions made in visual representations. The discussion reflects a need for clarity in how these models relate to actual gravitational phenomena.

oneamp
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An intuitive gravity analogy that explains why the moon orbits the Earth is, the Earth's mass' effect on space creates a 'dip' that the moon circles around in a frictionless orbit, kind of like like a marble in a bowl. Am I understanding this right? If so, I am confused about something. If I drop something out of my hand, it hits the earth. The 'bowl' shouldn't be anywhere near my hand... at least it's not in the pictures that I've seen, that try to explain gravity. What up with that?

Thanks
 
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oneamp said:
If I drop something out of my hand, it hits the earth. The 'bowl' shouldn't be anywhere near my hand..

Not sure if a get what you mean. But you should read this regarding this analogy:

http://en.wikipedia.org/wiki/Gravity_well#Gravity_wells_and_general_relativity

As far General Relativity is concerned, that analogy doesn't work for objects that you let fall from rest. You need the time dimension like shown here:

https://www.youtube.com/watch?v=DdC0QN6f3G4
 
If the moon stopped in its orbit it would fall to earth.
 
You're talking about a picture like this, I take it: http://en.wikipedia.org/wiki/File:GPB_circling_earth.jpg.

That kind of picture is astonishingly bad as soon as you start to ask questions. The bowl that is shown is a two-dimensional surface; spacetime is four-dimensional. What they've actually drawn is a 2d slice through spacetime, and a 3d slice through Earth. The result is contradictory, to say the least, although it looks pretty.

If they were being consistent, they'd also draw a 2d slice through the Earth. Then it would appear as a circle embedded in the curved surface, rather than as a sphere on top of it. Then you could imagine yourself as a little stick figure on the edge of the circle, and you would see that your hand is in the bowl.

You still have no clear explanation why the ball falls, but AT's video might help with that.
 
Thank you
 
Ibix said:
If they were being consistent, they'd also draw a 2d slice through the Earth. Then it would appear as a circle embedded in the curved surface, rather than as a sphere on top of it. Then you could imagine yourself as a little stick figure on the edge of the circle, and you would see that your hand is in the bowl.
Right. In all those analogies that reduce dimensions to 2D everything actually happens within those 2D dimensions. Things shown outside of the 2D sheet are not part of the model, and just there to show the relation to the real world situation. What matters is their projections onto the 2D sheet.
 

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