In S2E14 “Grace Under Pressure” how did Sheppard extend the shield around Mckays puddle jumper without killing him?

  • Why would it kill him?
    – Valorum
    Nov 20, 2020 at 8:07
  • @Valorum by at a minimum squishing him between the shield and inside of the rear compartment during expansion
    – polar
    Nov 20, 2020 at 8:08
  • 2
    @polar logically the same issue would be presented for the second jumper itself, why does the shield allow the jumper inside rather than pushing it away as it does the water? If we allow for the Jumper to be magically ok, then the same would apply to the content. Have you got a timestamp for the shield expansion, so I could take a look at the scene?
    – Jontia
    Nov 20, 2020 at 8:49

3 Answers 3


Shields in the Stargate universe are often able to selectively allow objects to pass through. For example, some Goa'uld shields allow slow-moving objects through and a submerged Stargate won't let water flood in, although that's not dematerialising it rather than necessarily a shield. Similarly, we see shields that extend to the ground (such as personal shields) but they never cut into the ground, suggesting they can stop at exactly the right point to not cause damage.

Since that's the established behaviour, it's reasonable that they could slowly extend an existing shield over a stationary object without cutting into it.

The shield extended between the two jumpers as McKay is rescued.

We don't see the shield extending, just the finished "corridor" between the two jumpers but that seems to be what happened. Sheppard and Zelenka extended their shield over the rear of McKay's jumper, pushing back the water but forming an airtight seal with the hull (and the seabed). Since the shield was passing over his jumper, McKay wasn't affected.

  • Slight nit-pick: gates keeping water from flowing through (or air, if the destination is a space gate) isn't a shield, but the gate itself simply not dematerializing the water/air. While we do see gates with shields (notably Atlantis's), they're a defensive mechanism to prevent hostiles from entering through an incoming wormhole; they don't stop air from flooding out through an outgoing wormhole to a space gate.
    – minnmass
    Nov 20, 2020 at 18:59
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    @minnmass Fair point. I did wonder about the exact mechanics. I just remembered the episode with the submerged gate and Carter making some comment on it. I'll tweak the phrasing, since I reckon an Ancient device being smart about dealing with water is still a useful example.
    – Withad
    Nov 20, 2020 at 19:24
  • @minnmass Slight nit-pick on your nit-pick: since there's no bidirectional movement of air molecules through the wormhole, there's no concept of a pressure differential, so you wouldn't expect an especially massive whoosh of air anyway. Things effectively have to be "pushed" through the gate by pressure. Of course, air at the source has its own pressure, so yes you would expect some leakage without the gate being clever about it, but not because there was a spacegate on the other end. Nov 20, 2020 at 20:49
  • @AsteroidsWithWings: point about the space gate not being needed. Air molecules that happen to bump into the event horizon should be flung to the other side; since nothing can come "back" through an outbound wormhole, the event horizon should be the equivalent to a hole in the hull of a shield-less space ship. That is, the event horizon should effectively have zero "back" pressure, creating a huge pressure differential. The gate itself provides this "back" pressure to prevent the cataclysm. We just don't think about it until the pressures are obviously different on either side of the wormhole.
    – minnmass
    Nov 20, 2020 at 21:58
  • @minnmass Yes I conceded that but what I'm saying is that this doesn't relate to the "real terms" difference in pressure between the environments on each side, so whether one end is a spacegate or a normal gate is immaterial, and the only relevant factor is how the gate mechanism itself deals with this (which it clearly must do!). Nov 20, 2020 at 22:20

In S5 E5 when the Vanir pass through Atlantis’s shield a gate technician confirm that Ancient ships can pass through the shields.


Also in S4 E1 Adrift when the city shiels shrinks to save power and oxygen it passes through a repair team, but they die a couple seconds later from suffocation and probably vacuum/hypothermia.


If I could draw a picture this would be easier but basically as the shield expands it expands around the other jumper.

The shield is a sphere around jumper 1 as it grows it encounters jumper 2 the shield actually opens to the exact dimension of jumper 2 and continues to expand pushing the water away from it when the shield reached the other side of jumper 2 it closes over it forming an area protected from the water.

Basically the shield never actually touches the other jumper or its inhabitant.

  • 1
    At what point are we made aware of this sort of intelligent shield capability
    – polar
    Nov 20, 2020 at 10:07
  • 1
    @polar S02E14, if not sooner.
    – Steve-O
    Nov 20, 2020 at 15:34
  • I think we see jumpers flying through Atlantis's shield in the opening shots of SGA: when Atlantis leaves Earth in the teaser, a puddle jumper flies towards it and, presumably, through its shield (which is clearly up at the time).
    – minnmass
    Nov 20, 2020 at 19:04
  • This specific mechanic has never been mentioned. Though it's hard to imagine how else it'd work... Nov 20, 2020 at 20:50
  • @minnmass Can confirm, the jumper flies straight through, with an accompanying effect. Nov 20, 2020 at 20:52

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