Was looking through some of my old works and found this:

By putting Portal A1 on the right side and Portal B1 on the left, we have a tunnel to two places. If we place Portal A2 at a high elevation and Portal B2 at a lower elevation, we will create airflow form B1 to A1 as the pressure attempts to equalize. By increasing the difference in height, we can control the flow of air...

Was years-ago-me correct about this? I feel like physics doesn't work for this, and yet it makes sense in an odd way.

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    $\begingroup$ For how much power, do the portal thing with water. As for whether it will work, that depends of how the portals work, whatever they are. Basically, the question is asking whether magic works; and nobody can know whether magic works except the Almighty Author. $\endgroup$
    – AlexP
    Commented Oct 26, 2021 at 12:00
  • $\begingroup$ For the reasons outlined above I'm voting to close this question as POB. $\endgroup$
    – sphennings
    Commented Oct 26, 2021 at 12:23
  • $\begingroup$ Two remarks. 1. Forget about the portals, place whatever your turbine is in a wind tunnel. The input and output of the wind tunnel are your portals. In this sense, it becomes a pure physics question with no magic involved. Even to such an extend that you should ask it on the physics forum. 2. You haven't described your turbine and how transforms wind into rotation. How do you expect any numbers without describing the key components? Even "standard" wind turbines vary in efficiency a lot. $\endgroup$
    – D.J. Klomp
    Commented Oct 26, 2021 at 12:27
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    $\begingroup$ Apart from being OB, it seems to cover many open-ended issues, can it also be cut down to a single focussed question? $\endgroup$ Commented Oct 26, 2021 at 12:37
  • $\begingroup$ OP didn't ask about magic portals. Remember out job is to consider this as an object beyond our current understanding of science and answer with respect to that ultra-science accordingly. $\endgroup$
    – elemtilas
    Commented Oct 26, 2021 at 20:32

2 Answers 2


Yes, it can work. No, it doesn't obey physics. (Maybe.)

You've created a Perpetual Motion Machine. Congratulations! Obviously you are either violating physics something fierce, or your portal consumes a lot of energy (enough, probably, that any energy you could collect from such a setup is more than negated by how much the portal(s) consume).

...but we are, presumably, talking about magic. That, or as above, the energy costs of maintaining such portals would make such a setup stupidly wasteful.

Ignoring that, however, the only relevant question would seem to be whether your portals allow air to pass. Some (SG1 comes to mind) resist small forces, such that a person can walk through (or perhaps push through), but air on either side won't mix.

We'll assume you aren't talking about those. We'll also assume your portal is a magical discontinuity in spacetime, and not a magical teleporter (i.e. SG1, again).

Looked at that way... your portal should act (almost¹) exactly like a hole between two huge containers made of infinitely strong material. That is, if you imagine two huge boxes, touching on one side, one with sea level atmosphere and one with Everest atmosphere, with the portal representing a hole in the shared side... then yes, air is going to flow through just as you'd expect.

(¹ I'm ignoring effects of the shape of the containers, since, technically, the hole isn't in the side of the container, but in its middle. This will probably have small-scale implications, such as the air on the "back" of the portal — if they are one-sided — wanting to flow around the lip of the portal and make a U turn, but that's getting overly pedantic and a typical observer probably won't notice the difference.)

  • $\begingroup$ I'm not sure this would count as a perpetual motion machine. It only works so long as there's a gradient, so in other words, it's really no different than colder upper atmosphere air falling. As we know, that depends on energy input from the Sun. And it probably wouldn't work at all on Venus where T & P are so high that, practically speaking, there may not be sufficient difference to get any flow. And it definitely wouldn't work on the Moon, where there's no atmosphere to speak of. $\endgroup$
    – elemtilas
    Commented Oct 26, 2021 at 20:28
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    $\begingroup$ @elemtilas, gravity is still acting on the higher-density air causing it to fall again. Mind AlexP's comment on the question, and imagine sticking the portals at either end of an inclined aqueduct. Basically, the portal is imparting potential energy to whatever passes through it. If the portal also consumes a bunch of energy, fine and good; otherwise, perpetual motion. $\endgroup$
    – Matthew
    Commented Oct 26, 2021 at 21:17

Any connection between high pressure area and low pressure area will work thus.

If you are leaving an inflated dome you will feel wind pushing you out the door: high pressure inside, low outside. Wind Cave on a hot day has high pressure outside, low inside and wind blows inward thru the cave mouth. Air pressure equalizes. If you have a portal instead of a cave mouth or Metrodome exit, air will still equalize. If you put a windmill in a place with wind, it can convert wind energy to work. You are tapping whatever energy made the difference in air pressure. If it is Wind Cave, it is the sun. If it is the Metrodome it is some big blowers.

Portals would usually be windy I think. Unless for your fiction you don’t like that because as your pretty actor stands pensively in front of the portal, pondering the mysteries, it looks like he has a leaf blower in his face. I do like that. Quit your pondering, Fabio! Get your messy-ass wig through that portal!


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