Does matter spontaneously move through a gate?

Question

A gate spell is cast, let's say to the elemental plane of water. Now I have a portal with air on one side and water on the other.

Gate says:

and anyone or anything that moves through is shunted instantly to the other side.

Does this imply the water rushes in through the gate, or does it have to be willfully pushed through?

The same reasoning can be done between areas of different air pressure, like the top of a mountain and a seaside beach. Does it create airflow?

Does medium matter freely cross gate boundaries without being pushed/moved through?

Answer 1

The effect of a gate probably shouldn't let through freestanding material

When the description of the 9th-level spell gate [conj] (PH 234) says that "anyone or anything that moves through is shunted instantly to the other side" (emphasis mine), this DM has always read that as rhetorical, the spell's author offering a tacit warning that what's on the other side of the gate spell's effect may not be something the gate effect's creator particularly wants nearby, like a horrible extradimensional monster rather than, for example, the pure flame from the Elemental Plane of Fire or highly pressurized water from the bottom of the ocean. That is, the gate spell is already powerful enough to allow an unchecked caster to pretty much win D&D; there's no real reason to allow the caster to win D&D even harder.

This DM bases the idea that freestanding material does not move through a gate spell's partially on the spell's author's casual tone as well as the spell's lack of details. Spells typically do what they say they do, and while there is that pregnant or anything in the spell's description, it's not expanded upon to include specifically freestanding environmental material.

Further, this DM tends to compare the effect of the spell gate to another effect that's close to mimicking the spell gate: magic portals, architecture that typically serves as two-way conduits between a pair of architectural features (as described in the Forgotten Realms Campaign Setting 59-61 and elsewhere for that setting specifically and touched on in the Stronghold Builder's Guide 49-50 for other settings generally). Specifically, Underdark on Portal Seepage says

A newly created portal functions well and sustains a solid barrier between its origin and destination points. As centuries or millennia pass, however, a portal can decay or malfunction…. In addition to malfunctions, portal seepage may occur in older portals. When this phenomenon occurs, qualities of the portal’s destination side start to soak into its origin side.

When a portal seeps, the planar traits described in Chapter 5 in the Dungeon Master’s Guide begin to affect the surrounding area. The rate can vary, but the area covered by the seepage averages a 5-foot radius around the portal per 100 years of age. (53)

So this DM assumes that a magic item costing potentially hundreds of thousands of gp and that mandates as one of its possible creation requirements access to the spell gate is at least equal to just a lone casting of the spell gate and its 1 round/level duration, and a portal must decay for hundreds of years to affect even a small area with its planar traits. So, for example, even portal seepage from a portal to the Elemental Plane of Water doesn't make the surrounding area damp.

That is, since it takes hundreds of years of a portal decaying to even get portal seepage, this DM views the 15 or 100 or whatever rounds of a caster's gate spell as insufficient to breech whatever force keeps the environment on the other side of a gate effect from passing over to the side of the gate effect's creator.

Answer 2

There's not really a difference between "moved by pressure" and "pushed".

If a gas is "under high pressure", what this means is that there's a lot of material, usually on top of it, which is pushing downward on it.

Likewise, if you push something, the way that works is by applying pressure to it with your hand.

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Imagine someone creates a gate spell that is horizontal. If a pebble falls onto the gate, will it pass through? Does the answer depend on whether a sentient creature pushed the pebble before it fell?

Let's keep our horizontal gate, and now suppose that some water falls onto it. Does the water pass through? Does the answer depend on whether a sentient creature poured the water from a bucket?

Now the gate is vertical, and a leaf runs into it, blown by the wind. Does the leaf pass through? Does the answer depend on whether a sentient creature threw the leaf?

If you wish to draw a distinction between objects which are pushed by sentient creatures and objects which are merely pushed by gravity, I suppose you could do that if you wanted. It's magic, after all.

If you really want to draw this distinction, I think you would get a clearer result if you argue that liquids and gases should not pass through the gate.

Answer 3

D&D Physics prevent this from happening

Pressure isn't really a thing in baseline D&D 3.5. The Water on the Plane of Water is part of the Plane's [Water] trait, and the Gate spell doesn't change Planar traits.

Four basic elements and two types of energy together make up everything. The elements are earth, air, fire, and water. The types of energy are positive and negative.

The Material Plane reflects a balancing of those elements and energies; all are found there. Each of the Inner Planes is dominated by one element or type of energy. Other planes may show off various aspects of these elemental traits. Many planes have no elemental or energy traits; these traits are noted in a plane’s description only when they are present.

The Material Plane seeks to balance all the elements (and, well, pretty much everything else, actually), so it's possible a Gate might let stuff from the outside in, there, in moderation. The Material Plane doesn't allow elemental excesses, though, normally, so the flow would have to stop at some point. A Gate to the Plane of Water might pool water around it on the Material Plane side, drip, or do nothing at all, at the GM's discretion, whichever best reflects the Plane's careful balancing of the elements.

The Plane of Water side, however, is strongly [Water] dominant, and certainly won't lose any water/become less water domininant, even if water appears to pass through to the other side.

In my games, I allow characters a Knowledge(Planes), Spellcraft, or Sense Motive check to identify the planes linked by a Gate spell and how they feel about eachother, just the planes linked, or just the relationship respectively. It's probably important to remember that Planes have alignments and stuff, even if they aren't fully sentient (which some of them are). Furthermore, "Except for rare linking points, each plane is effectively its own universe with its own natural laws."

Answer 4

It appears from the wording that physical contact with the Gate surface causes the transportation, if and only if the object is also moving toward the Gate. Thus, it seems reasonable a strong wind heading toward the Gate would pass through, as would a rushing river, or thrown stone, while a sedentary pool of magma, or depths of the ocean would not pass through. A school of fish swimming into the edge of the gate would pass through as well.

If there were an Abyssal army marching on a highway to hell, and you stuck your Gate on that highway, it is reasonable to assume many demons would march through involuntarily before the rest stepped back.