Gravity and curvature in spacetime

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Discussion Overview

The discussion revolves around the conceptual understanding of gravity and its representation in spacetime diagrams. Participants explore the nature of gravity as it relates to objects on Earth and in orbit, as well as the implications of spacetime curvature.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the "spacetime fabric" touches Earth at specific points or uniformly around its surface.
  • Another participant clarifies that the diagrams are simplified metaphors and that the "fabric" is a representation of four-dimensional spacetime.
  • It is proposed that the spacetime fabric touches the Earth everywhere and acts like a web that moves with the Earth.
  • Participants discuss whether the gravity that keeps us grounded is the same as that which governs satellite orbits, with some asserting that it is indeed the same concept.
  • One participant introduces the idea that gravity is not a classical force but rather a topological anomaly, suggesting a misunderstanding among scientists regarding this point.
  • Another participant draws an analogy between escaping Earth's gravity and trying to run up a wall, emphasizing the limitations imposed by spacetime curvature.

Areas of Agreement / Disagreement

There is no consensus on the nature of gravity and its representation in spacetime. Participants express differing views on whether gravity should be considered a force and how it interacts with objects in various contexts.

Contextual Notes

Participants express uncertainty regarding the dimensionality of spacetime representations and the implications of gravity at different scales, including classical and quantum levels.

jal3308
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I'm am new to the forums and have a quick question. When I see diagrams of objects in the universe on the spacetime fabric, the bottom of the object seems to be touching the fabric. My question is, does the fabric touch Earth for example on the south pole like presented in the pictures, or on all 360 degrees of the Earth. Another question is, is the concept of gravity and the orbits of sattelites different than the concept of gravity that keeps us on the ground.
 
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Welcome to PF!

The diagram you would have seen do not represent any physical theory. They are just a simplified metaphor to try and convey the idea that gravity is caused by curvature. For starters, the 'fabric' in the pictures is just 2D, when it is representing 4D spacetime.

As for the second question, yes, it is the same concept of gravity that does both of those things.
 
Answers

a) The "space-time fabric" touches the Earth everywhere. It is more like an all encompassing "blanket" or "web" that has four (maybe more) dimensions. As the Earth rotates it drags this "space-time" web with it also. Those pictures in books are only simplified two-dimensional models but are not totally accurate.


b) The answer to your second question is yes... gravity is the same no matter what. As long as any object has "mass" it will be affected by gravity's "force". Gravity's "force" is due to space-time curvatures that cause inertially moving objects to accelerate towards each other.

The reason satellites don't come crashing down directly into Earth due to this "curvature" is because the satellites are rotating at a certain speed that allows them to escape accelerating directly into the Earth. Thus they will always be rotating but never "touch" the earth.
 
Thanks for the replies. I had trouble trying to understand how there could be a flat sheet spread across the universe if all the objects are in so many directions. So is the tension between Earth and the fabric, the force that makes objects fall to to the center of the Earth, for instance an apple falling from a tree?
 
Bear in mind that gravity is not a force [in the classical sense]. Einstein viewed gravity as a topological anomaly. Many scientists have misunderstood this point. It looks, smells, and behaves like a classical force, until you push it to the quantum level.
 
Well you may not be able to tell, but the space around is bent. It's bent because of how large and massive the Earth is. So yes, as you try to "escape it" (by jumping), you can only get so high up before you come back down.

It's similar to trying to run up a wall. You can only go up it so far before you fall on your head.
 

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