Gravity, pressure and death portals to the water plane

[akma]

Edit: surprisingly, those loopholes are closed in source materials. So for the first question, please imagine that the plane of water has a directional, earth like gravity; For the second, imagine that water falls through the portal in the same speed it would fall if water was dropped from the sky in Earth.
And the plane somehow won't collapse into a black hole.

A common D&D trope is a plane of infinite water.
I did some math, and according to the formula of liquid flow speed, with an earth like gravity, it would take less than 0.5 light years of depth for the supposed speed to be faster than light, which is supposed to be impossible. So what would scientifically happen if you open a portal to an infinite plane of water with earth like gravity?

Additionally, I had another, related thought: maybe the water plane doesn't really have gravity, since matter is roughly equal in each direction, so it evens out. Maybe the reason why people feel like there is one, is because of a massive portal to another plane with earthlike gravity. Would creatures swimming above it be crushed by infinite pressure, or would the pressure be more uniform and life could possibly survive?

[hamishspence]

In 3.5, the Plane of Water is like the Plane of Air- there, gravity is subjective:

https://www.d20srd.org/srd/planes.htm#elementalPlaneOfAir
https://www.d20srd.org/srd/planes.htm#elementalPlaneOfWater

objects don't move at all, and inhabitants choose what is "down".

That's why "crushing pressure" isn't really a thing on the plane, the way it is in a regular ocean (according to Stormwrack).

So the liquid flow thing might not apply, because, regardless of where a portal is opened, the plane itself, is effectively all at 1 atmosphere or so of pressure.

[Rydiro]

In 3.5, the Plane of Water is like the Plane of Air- there, gravity is subjective:

https://www.d20srd.org/srd/planes.htm#elementalPlaneOfAir
https://www.d20srd.org/srd/planes.htm#elementalPlaneOfWater

objects don't move at all, and inhabitants choose what is "down".

That's why "crushing pressure" isn't really a thing on the plane, the way it is in a regular ocean (according to Stormwrack).

So the liquid flow thing might not apply, because, regardless of where a portal is opened, the plane itself, is effectively all at 1 atmosphere or so of pressure.
So I imagine that water doesn't actually flow out of the plane of water portal. It just forms a water surface regardless of how you orient it. Like the Stargate.
At least thats how i imagine it.

[Batcathat]

So I imagine that water doesn't actually flow out of the plane of water portal. It just forms a water surface regardless of how you orient it. Like the Stargate.
At least thats how i imagine it.

I kinda hope you're wrong, if only because I find the mental image of a wizard coming home and realizing he's accidentally flooded his dungeon quite funny.

[Aeson]

Additionally, I had another, related thought: maybe the water plane doesn't really have gravity, since matter is roughly equal in each direction, so it evens out. Maybe the reason why people feel like there is one, is because of a massive portal to another plane with earthlike gravity. Would creatures swimming above it be crushed by infinite pressure, or would the pressure be more uniform and life could possibly survive?
If it's a truly-infinite space filled with water and having a reasonable approximation of uniform mass distribution throughout, then no, there would not be any net gravitational force in any direction or at any location in the plane, and as a result there would not be any natural pressure in whatever water 'column' you cared to look at within the plane unless perhaps gravity can pass through your magic portal.


I kinda hope you're wrong, if only because I find the mental image of a wizard coming home and realizing he's accidentally flooded his dungeon quite funny.
The water in the plane isn't (well, shouldn't be) under pressure. What's pushing (or pulling) it out of the portal?

[Batcathat]

The water in the plane isn't (well, shouldn't be) under pressure. What's pushing (or pulling) it out of the portal?

Good question. I wonder if there's any consensus on whether or not gravity could affect matter through a portal like that? With our tragic lack of magical portals in reality, I'm guessing the answer will be very theoretical at best.

[Lord Torath]

Good question. I wonder if there's any consensus on whether or not gravity could affect matter through a portal like that? With our tragic lack of magical portals in reality, I'm guessing the answer will be very theoretical at best.Nonsense! There is a very firm answer: Whatever the DM says! :smallamused:

[Batcathat]

Nonsense! There is a very firm answer: Whatever the DM says! :smallamused:

I pity the poor in-universe scientists trying to understand the laws of nature and why they seem to arbitrarily change at a moment's notice ("Sire! It seems someone brought the GM pizza and now our enemies' weapons are twice as effective!").

[factotum]

Let's be honest, if the D&D universe ran according to the laws of physics, *any* infinite plane made up of a single material would just collapse into a black hole immediately, so not sure it's worth really thinking things through to that depth!

[Radar]

Let's be honest, if the D&D universe ran according to the laws of physics, *any* infinite plane made up of a single material would just collapse into a black hole immediately, so not sure it's worth really thinking things through to that depth!
This can be compensated by an appropriate value of the cosmological constant, so you can have an infinite, static space with homogeneous matter distribution that does not collapse into a black hole.

edit: and if you wonder, where that cosmological constant comes from and how there are no instabilities building up, the answer is very straightforward - Magic! :smallsmile:

[akma]

Since I didn't get a clear answer, I'll be more specific:

Assuming portals would allow water to flow freely.
First question: if there was a stable, infinite plane with an infinite amount of water, and a directed gravity in the same strength as Earth's gravity, how fast would water pour out of a portal to it?

Second question: if there was a stable, infinite plane filled with infinite amount of water, without any gravity, if someone opened a giant portal into it that sucked water with the same gravitational pull it would have in Earth, what would the water pressure be 20, 200, 2000 or a million meters above the portal?


Good question. I wonder if there's any consensus on whether or not gravity could affect matter through a portal like that? With our tragic lack of magical portals in reality, I'm guessing the answer will be very theoretical at best.

I imagine portals like holes in reality. Imagine that two planes are like airtight boxes glued to each other, with their own internal conditions. If suddenly a hole would appear and connect those two, there would be a mutual effect. Portals wouldn't work if they were very good at blocking stuff moving from one side to another.


Let's be honest, if the D&D universe ran according to the laws of physics, *any* infinite plane made up of a single material would just collapse into a black hole immediately, so not sure it's worth really thinking things through to that depth!

I'm asking because it's absurd; regardless, I often find that thinking about the consequences of different laws of nature can lead to very interesting results in fictional settings.

[tomandtish]

I kinda hope you're wrong, if only because I find the mental image of a wizard coming home and realizing he's accidentally flooded his dungeon quite funny.

But think of their poor neighbors!

A few months ago our neighbors had a pipe burst inside their home while they were gone. The flooding was severe enough that OUR backyard had flooded along with it, and a stream of water was running through our front yard to the street.

So if someone left an open portal to the plane of water, how long before they flood the planet?

[halfeye]

Not a plane, and not by D&D rules:

Had several lakes emptied onto armies with highly lethal consequences.

[gomipile]

Okay, I'm assuming normal physics on "this" side of the portal and no significant forces on the Plane of Water side. Water should still start flowing through, unless the portal is oriented horizontally facing up like the surface of the contents of a cauldron.

Here's why I think so: any small air currents should cause small ripples in the surface. As soon as the peak of a ripple pokes into "our" side, it's subject to regular forces. So, gravity will pull it down, and these will start collecting at the bottom edge of the portal. Surface tension will allow gravity to use this small bulge to "lever" more water out of the portal at the bottom.

The ultimate limit of how fast water could pour out should be the same as an infinitely long horizontal pipe full of water with one end cut to match the shape and orientation of the portal.

In (fantasy) practice, it should stabilize to something a bit less than that. But still should be a similar constant flow rate to the instantaneous flow you'd get right after knocking one end off of a long trough full of water.

[Yora]

If we have a plane of water with no gravity, but gravity from the material plane affects water through te portal, then my intuition would be that the water comes through the portal with the force of water spilling over an edge of a container.
If you make a square portal 2x2 meters wide, I think the water coming through would be like having a 1x4 meter wide hole in the side of a 1 meter deep pool. But that's all just intuition with no calculation.
In such a case, the water wouldn't be under a terribly high amount of pressure, but the sheer amount of water would probably push back anything standing in front of it. Maybe like getting into the water on a beach and getting hit by a decently large wave, except the wave never ends.
Or as another image, a ship getting hit by a torpedo and having a 2x2 meter hole in the side just below the water line. That would actually be pretty bad. It wouldn't pulverize or crush anything it hits, but sweep away everything in its path. Especially in closed corridors.

[NichG]

Since I didn't get a clear answer, I'll be more specific:

Assuming portals would allow water to flow freely.
First question: if there was a stable, infinite plane with an infinite amount of water, and a directed gravity in the same strength as Earth's gravity, how fast would water pour out of a portal to it?


Literally 'arbitrarily fast'. As in, there is not a unique answer to this question that is correct.

Pressure in water on Earth occurs because while gravity pulls water downwards, there is a bottom that pushes back up against the water. On a truly infinite plane with no bottom, the entire plane of water would effectively be in free-fall, accelerating at 1g (or whatever you set gravity to). That means that things like 'what frame of reference does the portal mouth adhere to?' start to matter. If we're going for relativistic physics, there should not be a special reference frame that a portal prefers to 'anchor' to, so one probably needs to provide a target piece of matter to anchor the portal - like if one were to open a portal on an airship, it shouldn't just scythe through the airship at whatever its' airspeed is.

So in that case, the most likely answer is that water wouldn't be flowing through the portal at all, because it'd be falling at the same rate as the surrounding water.

But if you did define a special 'static' rest frame, the speed of the water could be literally anything, depending on how long it's been since the plane of water formed, what that static frame is, etc...



Second question: if there was a stable, infinite plane filled with infinite amount of water, without any gravity, if someone opened a giant portal into it that sucked water with the same gravitational pull it would have in Earth, what would the water pressure be 20, 200, 2000 or a million meters above the portal?


Slightly negative compared to the resting pressure in the absence of the portal. See Bernoulli's Principle.

[Imbalance]

The usual formulae could possibly be applied, but how does one determine the roughness coefficient of a wormhole?

I went back and forth on this for a one-shot, ultimately throwing my hands up and blaming a water genie for the deluge that came through the portal.

[LibraryOgre]

My knee-jerk was "Water comes out of such a portal at a reasonable pressure, as if flowing from a freshly broken dam... high pressure, but not physics-breaking (any more than a hole in reality to another reality is, at least).

A slightly more nuanced answer might be that parts of the plane are "deeper" than others... either globes of water that are higher pressure, or water closer to Earth being higher pressure (as if one were near the ocean floor) and water near air being lower pressure (as if you were near the surface of the ocean).

I don't think the intention is for the portal to be a firehose (https://www.youtube.com/watch?v=pgPgsvxxxKE), unless you're talking a specific effect of the Decanter of Endless Water.

[Quizatzhaderac]

Nobody asked this, but liquid water also has a maximum pressure it can exist under. There is more than one kind of ice (https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg) and extreme pressure will cause liquid water to become ice. At standard gravity and room temperature, an ocean could be about 90km deep before it froze at the bottom.

The water in the plane isn't (well, shouldn't be) under pressure.You do need a certain amount of pressure to have liquid water. But it could easily be the same on both sides or the portal.


Good question. I wonder if there's any consensus on whether or not gravity could affect matter through a portal like that? With our tragic lack of magical portals in reality, I'm guessing the answer will be very theoretical at best.Gravity would probably be the same as the planet at the portal, pulling into the planet. Away from the portal on the water side gravity would be inversely proportional to the square of the distance to the portal.

Second question: if there was a stable, infinite plane filled with infinite amount of water, without any gravity, if someone opened a giant portal into it that sucked water with the same gravitational pull it would have in Earth, what would the water pressure be 20, 200, 2000 or a million meters above the portal?There would be some pressure near the portal as not all of the water affected by gravity could leave. But as you get far away from the portal gravity would become negligible and pressure would go to whatever it was when the plane was created (let's say 1 atm)

[Aeson]

You do need a certain amount of pressure to have liquid water. But it could easily be the same on both sides or the portal.
This is, supposedly, an infinite space full of liquid water; I don't see why there being no pressure in the water 'column' should be a deal-breaker.
- If you had any sort of normal physics going on, this really ought to be something more like a lot of void space with a few globes of water here and there.
- If there's a gravitational field pulling in a consistent direction, then you're either dealing with a water 'column' in freefall (in which case the water isn't under pressure beyond whatever the equivalent of local atmospheric pressure is in the Elemental Plane of Water) or your infinite space is only an infinite liquid-water-filled space in certain directions.
- If vapor pressure is keeping the majority of the water liquid, this implies that the 'infinite' space actually has some finite volume and thus that all this water is actually creating a(t least one) net gravitational field, which in turn says that something interesting is happening with temperatures and pressures to keep everything liquid.

Perhaps the right question to ask isn't "why aren't there massive flooding events every time someone opens a portal to the Elemental Plane of Water" but rather "why aren't there massive steam explosions every time someone opens a portal to the Elemental Plane of Water."


Gravity would probably be the same as the planet at the portal, pulling into the planet. Away from the portal on the water side gravity would be inversely proportional to the square of the distance to the portal.
If there was a risk of water being pulled through the portal by gravity on the side of the portal that isn't the Elemental Plane of Water, then portals to the Elemental Plane of Water would be horizontally- rather than vertically-aligned on the side with gravity and it wouldn't be a problem, unless some idiot put the portal on the ceiling rather than in the floor.

Also, the nominal gravitational field vector should be in the same plane as the 'surface' of a conventional vertically-aligned portal. As such, gravity shouldn't be able to pass through the portal unless the portal somehow masks the mass of whatever is 'behind' it (e.g. something on the Elemental Plane of Water looking out of a north-facing equatorial portal would feel the gravitational pull of the Northern Hemisphere but not the Southern Hemisphere)... and if the portal does that, then perhaps you should be rather less concerned about water from the Elemental Plane of Water flowing out of your portal than about things on your side of the portal being pulled in.

[NichG]

If you're doing stuff with infinite domains, then you could specify for example the average density of water molecules over the entire space. Barring Hilbert Hotel arguments (which if e.g. you consider a periodic unit cell that gets asymptotically bigger towards infinity, you avoid), that average density would have to be a conserved quantity. Given that you fix the density, there would be a sort of innate pressure due to the fact the water has nowhere to boil to without shoving other water out of the way. Basically that pressure would correspond almost exactly to the average boiling point pressure of water over whatever temperature variations the plane possesses.

[Aeson]

If you're doing stuff with infinite domains, then you could specify for example the average density of water molecules over the entire space. Barring Hilbert Hotel arguments (which if e.g. you consider a periodic unit cell that gets asymptotically bigger towards infinity, you avoid), that average density would have to be a conserved quantity. Given that you fix the density, there would be a sort of innate pressure due to the fact the water has nowhere to boil to without shoving other water out of the way. Basically that pressure would correspond almost exactly to the average boiling point pressure of water over whatever temperature variations the plane possesses.
If that were going on, there'd probably be enough instability to allow gas pockets to form, which either makes things very uncomfortable when those gas pockets subsequently collapse or pushes the system towards the "lots of void space with some globes of water" model.

Depending on exactly where the equilibrium point is, it may also not do you any favors with regards to avoiding steam explosions every time someone opens a portal to the Elemental Plane of Water.

[Radar]

If that were going on, there'd probably be enough instability to allow gas pockets to form, which either makes things very uncomfortable when those gas pockets subsequently collapse or pushes the system towards the "lots of void space with some globes of water" model.

Depending on exactly where the equilibrium point is, it may also not do you any favors with regards to avoiding steam explosions every time someone opens a portal to the Elemental Plane of Water.

While on an infinite plane even the most unlikely situation will occur somewhere, those gas pockets can be very unlikely - let alone of any size visible with a naked eye. It will suffice to pick pressure and temperature away from the liquid/vapor transition and the probability of spontaneous creation of vapor bubbles drops exponentially with the "distance" on the phase diagram. Additionally, if you pick 1atm pressure and let's say 300K temperature, the conditions will be very comfortable for anyone from an earthlike planet. Since there will be no noticeable pressure difference, there should be no steam explosion on portal opening. I'm all for climatic clouds to slowly reveal who is going out of a portal or what is on the other side though.

[Bohandas]

It may be relevant that in 3e most portals canonically function basically as teleporters - you can't step just halfway through one and some of them even only allow travel in one direction - and generally require some kind of intelligence to activate (althought not a command thought mind you, any intelligence) (Manual of the Planes pg 21-22). So even if the plane of water was under pressure water would only cross when somebody used the portal