Portals that don't allow you to create energy

[Thesh]

So I had an idea for a miniseries that I might (but probably won't) write one day. Basically the invention of a portal technology allows man to travel to distant worlds by sending these portals on rockets at .25c (well, nanobots that land on a planet and build the portal to launch into space). Now, the portals are created in pairs, and are always open. I would write this as hard science fiction and use that portal as my only liberty when it comes to physics.

So here is my problem. As soon as you create a portal, someone is going to wonder why you don't just use those for power generation. Place them apart, vertically, fill it with water and put a turbine in it and it generates energy indefinitely. I'm trying to come up with something that prevents that from happening. My first thought was that maybe there is resistance that slows down or stops any object trying to pass through, but if you can pass through it, you can imagine an object massive enough to put it on where it would work.

Does anyone have any ideas for something that is plausible, but makes it physically impossible to use portals to create energy?

[Charlie!]

So I had an idea for a miniseries that I might (but probably won't) write one day. Basically the invention of a portal technology allows man to travel to distant worlds by sending these portals on rockets at .25c (well, nanobots that land on a planet and build the portal to launch into space). Now, the portals are created in pairs, and are always open. I would write this as hard science fiction and use that portal as my only liberty when it comes to physics.

So here is my problem. As soon as you create a portal, someone is going to wonder why you don't just use those for power generation. Place them apart, vertically, fill it with water and put a turbine in it and it generates energy indefinitely. I'm trying to come up with something that prevents that from happening. My first thought was that maybe there is resistance that slows down or stops any object trying to pass through, but if you can pass through it, you can imagine an object massive enough to put it on where it would work.

Does anyone have any ideas for something that is plausible, but makes it physically impossible to use portals to create energy?

The brute force solution is to require the potential energy at the two portals to be equal before it works. Or you could equalize the potential energies by actually making the portals sinks or sources of gravitational force - in that case you get cool things like antigravity.

[idobox]

The solution I prefer is to allow gravity to pass through the portal.
For example, if you put one portal on Earth, and the other in orbit, the portal in orbit would start to attract stuff, because it would have a stronger gravitational flux.
That way, with two portal on top of each other, you would have almost no gravity between the two.

The main issue is that it would a pain in the ass to use that on long distances, since the gravitational pull for the local star, or the galaxy itself, would vary considerably.

An other solution would be a to have to face a force when going through the portal that equals the potential difference. To make crossing survivable even for large differences, it could be spread on a long distance, like comparing a slope to a cliff.

[SlyReaper]

To keep conservation of energy, then yes, gravitational force would have to be projected through the wormhole. It makes sense that that would be the default state of a wormhole though - if photons can pass through it, gravitons should too. This would of course cause tidal stresses on any object passing through it if the gravitational fields on either sides of the wormhole were not equal. You could get away with that if the "thickness" of a portal were non-zero. You could even write that into your story - tidal stresses causing "portal sickness", which makes the more sensitive folk feel nauseous as they pass through.

[eSOANEM]

Alternatively, you can have the portal take energy from its surroundings cooling them if energy would be created and have it dump energy as light (or other radiation if energy would have gone walkies). This would mean that there would be sharp changes in temperature of the portal frame (the stuff around the portal itself) whenever something large passed through presuming the potential energies were uneven.

Portal cooling would also be very useful from a military point of view. Enemy ships would be difficult to detect by conventional radar methods as you have an extra dimension to worry about and are at much further range. That said, it would be possible for a ship to have knowledge of how the CMB looks at a particular point at a particular time and look for hot patches which shouldn't be there where the enemy ships are. If the surface of the ship was a portal frame, objects could be made to gain energy using the acceleration of the ship and this could be used to cool the hull to the temperature of the CMB effectively camouflaging the ship.

[Yakk]

The portal requires energy to maintain and to set up. When you try to move matter through a gravitational gradient, one of them emits energy, the other one requires more energy to maintain.

The one that emits energy might emit black body radiation from the surface of the portal at an elevated temperature. Note that this means that when you cross the portal, it cooks your insides. So you don't want anything delicate to cross a portal that crosses a gravitational potential gradient.

This can be used as a generator (have water come into a portal and exit at a lower one. The exiting water is directly heated.) but not as a power source (If you use a portal low and send water high, the portal draws enough power to continue transmitting the matter, or it shuts down).

And naturally, in order to avoid time travel, all portals that can possibly exist need to somehow hook themselves to a universal frame of reference against which their travel is instant.

[Winter Man]

You could have it drain rest energy of the thing it's gravitationally attracted to, making the stuff that the planet or moon or whatever's made of horribly unstable.

[Meteorswarm]

You could have it drain rest energy of the thing it's gravitationally attracted to, making the stuff that the planet or moon or whatever's made of horribly unstable.

Adding instability sounds like adding energy to me.

[Tass]

I am also in the "let gravity act trough the portal" camp.

Make it simply a change in the universe's topology, maintained by some exotic matter. I also prefer such portals to have no front or back side, simply a ring of exotic matter where space is curled (actually not curved, the only curvature apart from local gravity is within the black box of the exotic matter) in such a way that you have to go two times around the edge before you are where you started. As a two-dimensional analogy imagine two sheets of paper on top of each other. These represent the local region of the two parts of space you want to connect. Now make a cut like this

O-------------O

trough both sheets and connect the bottom edge of one sheet to the top edge of the other and vice versa (requires the paper to cross over it self when embedded in a higher dimension, not otherwise).

I like the idea of a big hoop portal embedded in the ground so that one edge just looks like a big curved pillar, meeting the other one high above, and when you walk around this pillar you have to walk twice as far as normally before you meet you own foot-tracks, but there is no obvious point at which you have crossed the portal, no shimmering surface, no whoosh or anything.

The shape of the gravity field becomes a bitch to figure out if the two locations are at different potential though.... No... wait.... if they are far from each other and small compared to local gravity variations then I think it is tractable. The planes in the middle of the hoop would define a potential of the average of the two places V_inf, from there it is a relatively simple problem, equivalent to the electrostatic one of a metallic charged disc. If the potential difference is little then it will feel as though one portal stands on a hill top and the other in a valley, even though they are both on visually flat ground (I say both, really of course there is just one just twice as much of it). For big potential differences it will be a problem. Maybe just say that the planets are selected so that the gravity is manageable? There is after all a lot of planet to choose from.

[SlyReaper]

Well if what you want is a realistic wormhole (that is, one which is actually theoretically possible), the apertures would be spherical rather than circular, with the scene from the other side distorted appropriately.

Basically, something that looks like this:

Spoiler:

[Winter Man]

You could have it drain rest energy of the thing it's gravitationally attracted to, making the stuff that the planet or moon or whatever's made of horribly unstable.

Adding instability sounds like adding energy to me.

Point. What about simply having it drain energy from the fantastical device maintaining the wormhole? When the reserve's dry, the wormhole shuts.

[Copper Bezel]

Yeah, but that doesn't solve the physics problem, though. That makes it impractical as a way to get free power, but it doesn't negate the fact that the universe in which a system could exist has an "exploit" in the conservation of matter-energy.

The gravity thing does, though. If I'm understanding it correctly, no arrangement of the portals would get around it - a mass otherwise at rest but acted on by gravity would want to clump together across the portal and wouldn't "fall" through.

[PossibleSloth]

Alternatively, you can have the portal take energy from its surroundings cooling them if energy would be created and have it dump energy as light (or other radiation if energy would have gone walkies). This would mean that there would be sharp changes in temperature of the portal frame (the stuff around the portal itself) whenever something large passed through presuming the potential energies were uneven.

I'm pretty sure something like this was used in one of the Diskworld books In that case, though, it was used to cancel out a change in kinetic energy during teleportation.

[undecim]

I think the best you will be able to do is create a portal that has a short "tunnel", which requires as much force to pass through as there is difference in gravitational potential energy.

[Qwert]

Alternatively, you can have the portal take energy from its surroundings cooling them if energy would be created and have it dump energy as light (or other radiation if energy would have gone walkies). This would mean that there would be sharp changes in temperature of the portal frame (the stuff around the portal itself) whenever something large passed through presuming the potential energies were uneven.

Trouble with this idea is that it still can give you free energy by connecting a heat engine to the portal.

[eSOANEM]

I'm pretty sure it wouldn't be free. The portal itself is an internally energy conservative machine (that can, if the portals are rearranged correctly) arbitrary temperature gradients. Because it itself conserves energy, I'm pretty sure the usual thermodynamic arguments as to why you can't get free energy still apply.

[Qwert]

Free energy by using this system:

Spoiler:

So long as the object falls from high enough (and through a vacuum) for the bi-directional heat engine to equalize the temperature differential between the portals, you will get a useful amount of work every time it passes through the portal.

[Copper Bezel]

Am I misunderstanding something about Tass's version of the gravity solution? It still seems simpler to me.

It seems like if you're dropping a dude into the gravity well of a sun with a pair of portals and a flipper in the middle, you'd get exactly as much energy out of it as if you hadn't bothered.

Spoiler:

On the first pass he's been falling for a while before reaching the portal and is doing so at a pretty fair clip in the vacuum, so he gets the "turbine" going at a pretty fair clip, too. He's got quite a bit of inertia, but he's slowing with each pass, both from energy lost to the turbine and to the incline of passing through the portal itself, since he has to fork up the potential energy difference between the two portals to cross. Once he's lost the initial speed of his fall, he falls into equilibrium between the actual gravity well of the star below and the (localized) transposed one above.

And if they're proper, spherical portals like SlyReaper is saying, you could handwave any cheats having to do with changing the direction of the portals, too.

I don't even think that you're getting his deceleration for free. The portals have to be supported by something techy in an orbit around the star, and that something is affected by the dude's own mass.

[Yakk]

Stick one portal on the end of a stick (the "space" portal).

Hang a ship on the other end of the stick.

Place the other portal into low orbit of a sun.

The gravitational gradient towards the space portal will be huge. The ship will want to fall down towards it.

If you don't like the constant stick's free body diagram, do it in a pulsed sense. Fall towards the portal, then stop the portal and move it away again.

In essence, the portal needs to be pulled really, really, really hard by things falling towards it in order for this exploit to be avoided. It needs to be pulled as hard as a sun is by a near it, but it doesn't have nearly the mass of the sun, resulting in a ridiculous acceleration of the portal itself.

And if portals can be turned on and off, or destroyed, that admits an ability to generate a huge velocity by surfing a portal sling-shot.

At this point, the use of these portals for FTL travel seems like a relatively minor application.

[Copper Bezel]

Oh, wow. Fair enough. I guess I wasn't even considering the thought that having a portable gravity well you can take along wherever you go isn't necessarily an inconvenience.

[eSOANEM]

Free energy by using this system:

Spoiler:

So long as the object falls from high enough (and through a vacuum) for the bi-directional heat engine to equalize the temperature differential between the portals, you will get a useful amount of work every time it passes through the portal.

Oh, sorry, I misunderstood. I was thinking you meant free as in violating conservation of energy. Yeah, this would still allow you to create arbitrary amounts of useful energy, but does so whilst conserving energy. I guess that's worse as that means I've broken entropy.

[Tass]

Well if what you want is a realistic wormhole (that is, one which is actually theoretically possible), the apertures would be spherical rather than circular, with the scene from the other side distorted appropriately.

Basically, something that looks like this:

Spoiler:

Neat. I can see how that is actually the correct three dimensional analog to the two dimensional wormhole one often sees drawn.

Spoiler:

I wonder how it would feel to be warped in the way required to make it through such a hole. How big does it have to be to be survivable?

The throat is really a "tube" - space with one long dimension and two curled up, (in the picture on a meter scale). Can the throat be arbitrarily short or long? Or is the length defined by its width, maybe it is a hyper-catenoid (i suppose so)? If it was long the curvature in the tube would actually be zero, is that allowed? In such a case you could exist without problem in it, but you would only see you own neck in two of the dimensions. I guess it would look like a long tube made out of you. As you can no doubt tell I do not know general rel so I can't answer these questions my self.

[Copper Bezel]

Well, notice though that the length of the throat is on the axis perpendicular to the plane, and the plane represents three-dimensional space, so the length of the throat actually runs through the "rolled up" dimension that links the two points and not on any spatial axis. Unless I'm terribly misreading that graphic.