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Could energy be extracted from spacetime expansion?

  1. Jun 10, 2015 #1
    Purely theoretically, could it be possible to extract energy from spacetime's expansion?
    Like, say, imagine two celestial bodies far away enough so that they are not bound enough by gravity, they get farther away from each other due to spacetime expansion. We tie them to each other with a rope. As they get further apart the stretching of the rope gets converted to energy. Or would the rope itself get stretched in the same scale? Would a rope 1 km long become 100,000 km long due to space expansion without any gain in energy?
  2. jcsd
  3. Jun 10, 2015 #2


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    The rope would either stay 1km or break, because the molecular bonds holding it together don't expand with the universe. As far as I know, the expansion of space can't actually exert any forces on any objects. Objects just follow geodesics, or you can think of it as inertia carrying them apart. So, if you throw one end of the rope east and the other end of the rope west, and let the rope ends travel apart under their own inertia, eventually, the rope will snap it back or break if you threw them hard enough.

    I don't know about the energy question. I'm not sure if the universe's expansion conserves energy, since it keeps creating more dark energy. Since the expansion is presumably determined by some laws of physics, once we understand those physics, we can probably come up with a definition of energy that is conserved.
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