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Suekdccia
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- TL;DR Summary
- Mining the Universe's expansion?
There is an article written by astrophysicist Edward Harrison [1] which defends that energy could be extracted from attaching an imaginary cosmologically long string to a receding object from us in an expanding universe. He says that the energy extracted is potentially limited (in decelerating universes) or unlimited (in accelerating universes) due to the difficulty of defining conservation laws in cosmological scales.
However, I was discussing this with a physicist and he came out with a good point against the article: Even if our universe is accelerating, and even if there is perpetual tension in the string attached to the receding object, you cannot extract unlimited energy as you can't run a perpetual motion machine on perpetual tension. Even if Λ=0, there's a perpetual negative tension (pressure) in the tether due to gravitational attraction. You can extract work from it only until the tether's length reaches zero. If Λ>0, you can extract work until the tether's length reaches the Hubble length, then you hit the event horizon and the tether breaks. Reeling the mass back in before you hit the horizon costs as much energy as you extracted from letting it spool out. Is this argument right?
Moreover I have another question: in this article [2] it is indicated that if the attached object receding from us is stopped, at the right distance, it may be directed towards our galaxy, go through right it, and join the Hubble flow in the opposite side of us. Then, couldn't we mine some energy from the tension of the object being dragged by the Hubble flow, until putting it at rest (without reeling the object back to us), and then, after being directed towards our galaxy by gravity, going through it and being dragged by the Hubble flow in the opposite side, repeat the process again and again indefinetely...?
[1]: https://adsabs.harvard.edu/full/1995ApJ...446...63H
[2]: https://arxiv.org/abs/astro-ph/0104349
However, I was discussing this with a physicist and he came out with a good point against the article: Even if our universe is accelerating, and even if there is perpetual tension in the string attached to the receding object, you cannot extract unlimited energy as you can't run a perpetual motion machine on perpetual tension. Even if Λ=0, there's a perpetual negative tension (pressure) in the tether due to gravitational attraction. You can extract work from it only until the tether's length reaches zero. If Λ>0, you can extract work until the tether's length reaches the Hubble length, then you hit the event horizon and the tether breaks. Reeling the mass back in before you hit the horizon costs as much energy as you extracted from letting it spool out. Is this argument right?
Moreover I have another question: in this article [2] it is indicated that if the attached object receding from us is stopped, at the right distance, it may be directed towards our galaxy, go through right it, and join the Hubble flow in the opposite side of us. Then, couldn't we mine some energy from the tension of the object being dragged by the Hubble flow, until putting it at rest (without reeling the object back to us), and then, after being directed towards our galaxy by gravity, going through it and being dragged by the Hubble flow in the opposite side, repeat the process again and again indefinetely...?
[1]: https://adsabs.harvard.edu/full/1995ApJ...446...63H
[2]: https://arxiv.org/abs/astro-ph/0104349