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Comment by digging | Hacker News Reader

Without being able to harvest dark energy, it's actually the exact opposite.

The extra energy comes from the increase in gravitational potential energy between objects. The accelerating increase in this potential energy implies the total energy of the universe is increasing. I’m not talking about harvesting dark energy directly.

I'm no physicist, but isn't the gravitational potential energy between two objects inversely proportional to the distance between them?

it is, in the negative direction though. So increasing the distance increases the potential energy by making -1/r closer to zero.

Heh, I had to go off and read some stuff to get my head around what "negative" potential energy would even mean (high school physics was decades ago).

So... the negative is just a convention to represent work done against the "field", and positive is work done by the field? ie more of a vector than actually being negative energy? I think I get that bit now.

So now I'm wondering if that still applies to an expanding universe vs eg a rocket leaving earth. If things aren't moving further apart by work (ie force x distance) being done against their gravitational fields so much as space time expanding, is there an increase in potential energy? And then if objects are moving apart faster than escape velocity, could that still be seen as increasing potential energy?

I think I'm confusing myself further...

What would a hypothetical dark energy engine look like?

two black holes in highly elliptical orbit, so they they nearly collide when they come together. at the center of mass is a cloud of iron atoms. When the black holes nearly collide tidal forces across iron nuclei rip them apart. These ripped apart atoms can then be re-harvested and used as fuel in a fusion engine. The energy loss from ripping the atoms apart is then recovered by the black holes by taking advantage of dark energy when they go apart, which increases the distance between them, increasing the gravitational potential energy.

I could definitely be wrong, but I don't see how dark energy would enter this scenario. DE doesn't seem to play a role in gravitationally bound systems like two black holes orbiting each other.

dark energy increases the distance between objects, adding to the system's energy by increasing the gravitational potential energy. Without the contribution from dark energy, after several cycles the pull from the iron atoms would cause the black hole orbit to decay.

It would harness dark energy. /s

We haven't detected dark matter or dark energy outside of their visible effects on the larger universe. Maybe we can't interact with it with baryonic matter. A dark energy engine made out of dark matter would be invisible.

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