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In several episodes of SG1, Atlantis, and once in SGU a person is able to hold a gate open by sticking just their arm through. The show explains why the gate stays open (to prevent a person from being cutoff), but if matter can only travel one way through a gate how can the person pull their arm back through?

Also how would a gate know that the person is completely through, and not just their arm?

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People moving through the Stargate are stored in an energy buffer as they enter into it; the matter is transformed into energy and vice versa "on the fly" as it enters and exits the gate. Only when the object (or person) completely passes through the event horizon is the stream sent through to the distant gate.

It does raise some interesting questions, such as how the gate "simulates" someone's bloodflow and such (never mind their neural activity while their brain is partially-encoded!).

The gate has safety protocols in place to prevent it from being shut off under normal circumstances if there is something partially within the event horizon. There are a few instances of things being severed due to a gate being closed prematurely, such as when Teal'C holds the Goa'uld-infested Major Kawalsky's head in the gate in an early episode in order to partially decapitate him.

Williham Totland
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Asmor
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  • That explains how it stays open, and one can pull his arm through, but any ideas on how the gate would know the object being transported is complete and not just part of it? –  Apr 26 '11 at 04:38
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    @Joe: There are enough bonds in a complete object that it could simply look for the pull. I mean, if it couldn't somehow figure an object was complete, then how could it possibly put it back together right when if left the other side? – PearsonArtPhoto Apr 26 '11 at 04:40
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    Additionally, the event horizon has some level of "intelligence", for lack of a better term; it can discern between a solid object being pushed against it vs. the atmosphere, and in one episode there was even a submerged stargate, demonstrating that the stargate can tell the difference between liquids and solids. It's sufficient to say that an object which is no longer intersecting the plane of the event horizon is completely within the energy buffer. – Asmor Apr 26 '11 at 04:50
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    @Asmor: If I recall, the 'water' on that planet had some form of intelligence and was capable of some motion, including shaping itself. The question as to whether the gate held back the water or if the water pulled back from the gate wasn't clearly answered, to my knowledge. – Jeff Apr 26 '11 at 12:50
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    @Jeff; Gates can happily open into space without depressurising the entire room they're in, so there's clearly some restrictions there. – Phoshi Apr 26 '11 at 13:07
  • @Phoshi: Agreed, definitely. The event horizon does restrict atmosphere from moving through. It'd be interesting to see how much pressure would be needed before something DID go through. My comment was simply to point out that we can't be sure the event horizon was the only thing holding back the water - it could have been holding itself back (or at least restricting the pressure it put on the horizon). – Jeff Apr 26 '11 at 13:14
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    However the gates can be used to transport grain being poured down onto it. A pour grain makes it act as a liquid. – Jonathan. Apr 26 '11 at 14:35
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    I really think there should have been an episode where the gate malfunctions after dialling a water planet and sgc flooding – Jonathan. Apr 26 '11 at 14:37
  • or if you dialed a stargate that was floating in space... all the atmosphere in the gateroom gets vented... that would be a fun episode... – Jonathan Miller Jun 19 '12 at 14:12
  • @Jonathan Miller: Not impossible, but that would require some kind of catastrophic failure in the gate's failsafes. It's intelligent enough to prevent liquids and gases from passing through; we've seen gates opened both underwater and in space. – Asmor Jun 20 '12 at 14:31