Understanding Gravitational Waves and Their Impact on Entropy

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Discussion Overview

The discussion revolves around gravitational waves, their energy properties, and their potential impact on entropy. Participants explore the detectability of these waves, the implications of their energy transmission, and the conditions under which matter might absorb gravitational radiation. The conversation includes theoretical considerations and references to current detection efforts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that gravitational waves carry energy and travel through space unabsorbed, raising questions about their detectability and contribution to entropy.
  • Others argue that Einstein's equations suggest time reversal symmetry, implying that if gravitational waves can be emitted, they can also be absorbed.
  • There is mention of indirect evidence for gravitational waves, with some participants noting that while they have not been directly detected, current and future detectors like LIGO and LISA are being developed.
  • One participant highlights the challenges in detecting gravitational waves due to their weak nature and interference from environmental vibrations.
  • Questions are raised about whether matter can absorb gravitational radiation and under what conditions this might occur, with some suggesting that gravitational waves could do work on matter as they pass through.

Areas of Agreement / Disagreement

Participants express a mix of views regarding the detectability of gravitational waves and their energy properties. While there is some agreement on the existence of detection efforts, uncertainty remains about the absorption of gravitational waves and their implications for entropy.

Contextual Notes

Limitations include the unresolved nature of gravitational wave absorption, the dependence on theoretical models, and the challenges in current detection methods that may affect the understanding of gravitational waves' energy and entropy contributions.

Denton
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Ok so there are gravitational waves traveling through as constantly, and these waves carry energy. Since they travel through space unabsorbed, does this mean they are undetectable (by direct observation) and that the energy they contain cannot be restored?

Lastly then does this mean gravitational waves contribute to entropy: constantly radiate energy without the possibility to create work from them?
 
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Einstein's equations look like they have time reversal symmetry to me. So if something can emit gravitational radiation, then something can indeed absorb it.

There is a lot of indirect evidence for gravitational waves, but they have not yet been directly detected. But if GR is correct, they should be in principle detectable. There are some ground based detectors already built, and a detector (which will split into three parts) will be launched into space soon (hopefully).

If you'd like to read on their detection methods, you can check out wikipedia or the actual project websites. The only acronyms I can think of at the moment are LIGO and LISA.
 
yes..LIGO and LISA are the current detection efforts..
 
LISA is a future experiment. VIRGO is probably the one you are thinking of.
 
I should have mentioned that the difficulty in detecting gravitational waves is that they are very weak; gravity is weak enough; gravity waves weaker still hence very hard to detect. When you read about current experiments you'll note problems isolating detecting masses from vibrations such as passing traffic, Earth tremors,etc.
 
So if something can emit gravitational radiation, then something can indeed absorb it.

Has this been observed? And or how is this possible? Does matter absorb the energy currently or only under certain conditions?
 
Denton said:
Has this been observed? And or how is this possible? Does matter absorb the energy currently or only under certain conditions?

No, it has not been observed; that is what LIGO is for. The lack of observation (presumably) has nothing to to with its impossibility, but rather with the fact that gravitational radiation contains undetectably low amounts of energy to begin with.

It is possible because as a gravitational wave passes, the distances between objects change due to the changing curvature of spacetime as the wave passes. As the wave passes a piston, for instance, it compresses the gas inside. As the wave passes the Earth, for instance, it compresses the Earth and generates frictional heat. Voila: work is done. An undetectably teeny amount of work, but work nonetheless.
 

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