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

In summary, the conversation discusses the similarities between the physics of an expanding and collapsing dust filled universe and a bouncing ball, specifically in terms of kinetic and potential energy. It also touches on the concept of gravitational and electromagnetic waves and their energy distribution. The possibility of a standing electromagnetic wave is also mentioned.
  • #1
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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  • #2
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.
 
  • #3
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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  • #4
Unless we have a standing E and M wave?
How is that different? E and B are still in phase.
 
  • #5
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.
 

1. What is the difference between an expanding universe and a bouncing ball?

The main difference between an expanding universe and a bouncing ball is the scale at which they occur. An expanding universe refers to the overall growth of the entire universe, while a bouncing ball is a localized event on a much smaller scale.

2. How does kinetic energy play a role in an expanding universe?

In an expanding universe, kinetic energy is responsible for the movement of galaxies and other celestial bodies. The expansion of the universe is driven by the collective kinetic energy of all the matter within it.

3. What is potential energy in relation to a bouncing ball?

Potential energy in a bouncing ball refers to the energy stored within the ball when it is at a certain height. As the ball falls, this potential energy is converted into kinetic energy, causing the ball to bounce back up.

4. Can the expanding universe be compared to a bouncing ball?

While both involve the transfer of energy, the expanding universe and a bouncing ball operate on different scales and have different mechanisms driving their movements. Therefore, it is not an accurate comparison to make.

5. How do these concepts relate to the laws of thermodynamics?

The expanding universe and bouncing ball both follow the laws of thermodynamics, specifically the first and second laws. The first law states that energy cannot be created or destroyed, only transferred, which is seen in both cases. The second law states that entropy (the measure of disorder) increases over time, which is also observed in the expansion of the universe and the decrease in height of a bouncing ball over time.

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