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In the video game Portal, there are often puzzles which must be solved by gaining a large amount of momentum. Typically, this is accomplished by putting one portal on the ground and another directly above it on the ceiling, then jump into the portal in the ground, and the person would appear at the ceiling and started to fall off. When the person fall off, he/she gain kinetic energy and reach a certain speed and jump into the portal in the ground again. After that, he/she appear at the ceiling again. Thus you create an infinite loop of falling.

Although the idea of a portal is absurd to common sense, let's discuss its possibility to existence.

The portal would seem to be creating potential energy. Typically when an item falls, its potential energy is converted to kinetic energy by virtue of it losing height, and additional energy would need to be expended to raise it up again before it could fall again.

What is the possible source of the potential energy, if portal would exists? Does a portal provide potential energy in order to push people to higher position?

And this has lead me to wonder another point, though, whether the portals should also transmit gravity. This might remove the whole problem of conservation of energy; if the gravity is transmitted through the portal, then the object would be pulled back up with equal force, and would actually reach an equilibrium floating within the portal, rather than falling continuously.

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This actually seems more appropriate for the Physics SE - it's going to involve discussions of the "direction" of gravity, gravity as a force, etc. At first glance I'd say no, otherwise you'd be pulled toward a side-oriented portal with the same strength you're pulled toward the ground...but I am not a physicist. –  Ian Pugsley Aug 24 '11 at 15:51
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Given that portals are completely fictional, this isn't really physics; we can simply decide whether we want our fictional portal to transmit gravity or not. Maybe gaming.se? –  Colin K Aug 24 '11 at 16:25
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In terms of physics: the pair of portals changes the topology of space (making the room no longer simply connected). This has the side effect that the gravitational field, despite being curl free, is no longer a conservative force field. Hence you can circumvent the issue of conservation of energy. –  Willie Wong Aug 24 '11 at 16:32
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I'm with Colin on this: the behavior of these things is purely a matter of narrative convenience as they are not real. Attempting to form a consistent physics with something that isn't physically consistent is simple silly. –  dmckee Aug 24 '11 at 17:31
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I disagree -- I mean: I think the question is ill-posed as is, but to dismiss it out of hand because "portals aren't real!" is absurd and unimaginative. Willie has provided a provocative (in a good way!) answer, and in fact I think demonstrating how this fictional device interfaces with "real physics" in the questions context helps one sharpen their understanding of physics better than simply ignoring the question. –  wsc Aug 24 '11 at 18:39
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migrated from scifi.stackexchange.com Aug 24 '11 at 15:59

This question came from our site for science fiction and fantasy enthusiasts.

closed as off topic by dmckee Aug 24 '11 at 17:32

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