Comment by derefr | Hacker News Reader

Comment by derefr | Hacker News Reader

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> You could "lift" matter outside the gravity well without spending energy.

How is that? Wouldn't the gravitational force just propagate across the wormhole, making for a continuous gravitational gradient?

Maybe we're talking about separate conceptions of what a "wormhole" is here. For one example of what I'm picturing, you could have a one-wormhole universe as a three-dimensional hologram along the surface of a 4D hypertorus. You can go "through the doughnut hole" to get somewhere quicker than you could by going "around the doughnut", but that doesn't mean that there's a discontinuity anywhere on the surface of the hypertorus. Gravitational waves from one point-source would be propagating both "into" the wormhole (along the inside of the doughnut) and "through regular space" (along the outside of the doughnut) and eventually encountering themselves at the destination and maybe constructively or destructively interfering, but they wouldn't be forced into a square wave at any point.

> How is that? Wouldn't the gravitational force just propagate across the wormhole, making for a continuous gravitational gradient?

That would be even worse. Let a strong gravitational force go through the wormhole and drop something near it (not through it).

Then move the wormhole (and the gravitational force) elsewhere, lift the object, and bring the wormhole back.

Normally the thing that generates gravity has inertial mass, so it costs energy to move. Not with a wormhole. If you speculate that it should have inertial mass, then how much?

I'm kind of lost by your contruction. Is the something we drop the same as the object we "lift"? What does it mean to lift the object? Try removing the pronouns, and being more specific with your steps.

Is this a thought experiment you came up with on your own, or do you have a paper you're citing? If the latter, do you have a link?

> Try removing the pronouns, and being more specific with your steps.

That would be even worse. Let the strong gravitational force of the Earth go through a wormhole located on a small platform in deep space, and drop a rock near the wormhole (not through the wormhole).

Then move the wormhole (and the gravitational force) elsewhere, lift the rock off the platform, and bring the wormhole back.

Your thought experiment clearly implies energy is spent moving the wormhole. That would be the energy needed to spend to move your example rock.

The solution to this is almost certainly "moving the wormhole takes energy and solves this 'paradox'". The mass-energy of the wormhole could be from gravitational field energy, for example.

How much energy? The wormhole doesn't have an inherent mass, so how much energy does it take? After all you can put an arbitrary amount of mass through it.

I would guess that that would depend on the situation.

I would think that whatever method you have of moving a mouth of the wormhole, would have some interactions with the masses providing the gravitation.

Also, I'm not 100% sure that a wormhole would not have something like an inherent mass, because iirc there are solutions where a black hole mass is entirely because of the way spacetime is set up, without any particles needed? I'm not sure about that, but I think that is what I read..

You're looking at it the wrong way. If you can extract energy from dropping the rock through the wormhole, it takes energy to pull the rock away from the wormhole (and, by symmetry, it takes that much energy to move the wormhole away from the rock).

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