Expanding/Collapsing Universe vs Bouncing Ball: Kinetic & Potential Energy?

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Discussion Overview

The discussion explores the analogy between the physics of an expanding and collapsing universe and the mechanics of a bouncing ball, particularly focusing on kinetic and potential energy. Participants consider whether concepts from General Relativity can be mapped onto these classical mechanics ideas, and they examine the nature of gravitational and electromagnetic waves in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the physics of an expanding and collapsing universe is analogous to that of a bouncing ball, with potential and kinetic energy corresponding to the maximum and minimum values of radius and height, respectively.
  • There is a question about whether gravitational waves cause space-time to "vibrate" and if parts of their energy can be identified as kinetic and potential.
  • Another participant notes that gravitational and electromagnetic waves are similar but argues that the analogy of energy oscillating between kinetic and potential does not hold for electromagnetic waves, as the electric and magnetic fields are in phase.
  • A later reply questions the nature of standing electromagnetic waves and whether they differ from traveling waves, suggesting that energy distribution might vary in standing waves.
  • One participant challenges another's assertion about standing waves, indicating a disagreement about the phase relationship of electric and magnetic fields in such waves.

Areas of Agreement / Disagreement

Participants express differing views on the analogy between mechanical and wave energy, particularly regarding gravitational and electromagnetic waves. There is no consensus on how these concepts relate to standing waves or the nature of energy distribution in these contexts.

Contextual Notes

The discussion includes assumptions about the nature of energy in waves and the applicability of classical mechanics concepts to cosmological models, which may not be universally accepted or fully resolved.

Spinnor
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I think in a piece by John Baez we learn that the physics of an expanding and collapsing dust filled universe is the same as the physics of a bouncing ball,
Radius, R, verses time and height, h, verses time?

If so are there counterparts to the kinetic and potential energy of a bouncing ball and an expanding and colapsing dust filled universe? It seems that when R and h are maximum we have max potential energy and when R and h are near zero we have max kinetic energy, if General Relativity has "things" like potential and kinetic energy?

If you and I were nearby dust particles, in an expanding and collapsing universe, in the expansion stage say, we could estimate our potential and kinetic energies of each other by observation? I can "measure" your distance and velocity and thus estimate kinetic and potential energy? If

When a gravitational wave goes by does space-time kind of "vibrate"? If so can we identify parts of the energy of a gravitational wave as being part "kinetic" and part "potential"?

Could an oscillating dust filled universe be considered a "standing wave" because the potential and kinetic energy are 90 degrees out of phase?

Thanks for any help!
 
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When a gravitational wave goes by does space-time kind of "vibrate"? If so can we identify parts of the energy of a gravitational wave as being part "kinetic" and part "potential"?
Gravitational and electromagnetic waves are closely similar. Can you do this for an electromagnetic wave?

Think - the electromagnetic energy density is (E2 + B2)/2. But in an electromagnetic wave, E and B are in phase. They reach maximum at the same time, and fall to zero at the same time. So the analogy with a mechanical wave, where energy goes back and forth between a "kinetic" part and a "potential" part simply does not hold. Like for gravitational waves.
 
Bill_K said:
Gravitational and electromagnetic waves are closely similar. Can you do this for an electromagnetic wave?

Think - the electromagnetic energy density is (E2 + B2)/2. But in an electromagnetic wave, E and B are in phase. They reach maximum at the same time, and fall to zero at the same time. So the analogy with a mechanical wave, where energy goes back and forth between a "kinetic" part and a "potential" part simply does not hold. Like for gravitational waves.

Unless we have a standing E and M wave?
 
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Unless we have a standing E and M wave?
How is that different? E and B are still in phase.
 
Bill_K said:
How is that different? E and B are still in phase.

In a standing wave its all E and then all B? I would normally defer to your expertise but I think you are wrong.
 

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