Rubber sheet analogy - density change

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Discussion Overview

The discussion revolves around the rubber sheet analogy used to explain concepts in general relativity, particularly focusing on how changes in mass affect the structure of spacetime. Participants explore the implications of density changes in the analogy and whether spacetime can be thought of in terms of particles or a smooth manifold.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that as the rubber sheet stretches due to the bowling ball, the density of the rubber decreases, suggesting a parallel to spacetime being affected by mass changes.
  • Others argue against the idea of spacetime having "particles," stating that the rubber sheet analogy is misleading and that spacetime is better understood as a smooth manifold with intrinsic curvature.
  • A participant questions what is changing in spacetime when mass changes, seeking clarity on the representation of measurements and their implications in the analogy.
  • There is a suggestion that the distances along spacetime dimensions (the metric) change, rather than implying a particle-like structure within spacetime.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the rubber sheet analogy and the nature of spacetime, indicating that multiple competing perspectives remain without consensus.

Contextual Notes

Some limitations include the potential misunderstanding of the rubber sheet analogy as it relates to spacetime and the lack of clarity on what specific changes occur in spacetime metrics when mass varies.

Pjpic
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When the rubber sheet with the bowling ball in it stretches, the density of the rubber is lowered as the rubber molecules become more widely separated by empty space. Are there particles of space time(?) that are similarily separated by more empty space when when the amount of nearby mass(?) changes?
 
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Pjpic said:
When the rubber sheet with the bowling ball in it stretches, the density of the rubber is lowered as the rubber molecules become more widely separated by empty space. Are there particles of space time(?) that are similarily separated by more empty space when when the amount of nearby mass(?) changes?

No. You are stretching the rubber sheet analogy too far.

(:biggrin:)
 
DaveC426913 said:
No.

If the amount of mass changes space time, what is changing? If there is a measurement and the numbers are changing, what whole does each interger represent?
 
Pjpic said:
When the rubber sheet with the bowling ball in it stretches, the density of the rubber is lowered as the rubber molecules become more widely separated by empty space. Are there particles of space time(?)
The rubber sheet usually represents only space, and is therefore misleading. See post #4 in this thread:
https://www.physicsforums.com/showthread.php?t=286926
Pjpic said:
If the amount of mass changes space time, what is changing?
The distances along space-time dimensions (the metric) change. Space-time is "streched" but there are no "particles of space time" in the model. It is a smooth manifold with a intrinsic curvature, and free falling objects are moving on geodesic paths.
 

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