My story has magic, and some of that magic involves manipulating pocket dimensions, hammerspace, whatever you'd like to call it. Whether it be a portal to a closet you can access with a thought, or the ability to make objects disappear and make them re-appear later, or a sleeping bag that when crawled inside leads to an entire house, a decent amount of powers, abilities and enchantments involve conjuring extra space out of nothing, in which to store things in some alternate dimension.

But this raises a rather awkward predicament I'm trying to work out how to avoid, revolving around the fact that when the person or thing a pocket dimension is tied to is destroyed, the pocket dimension associated with it is supposed to take everything from the real world that had been held inside of it and push it back out into the real world, and then disappear (anything that had been created by the magic as part of the pocket dimension, like the house in the sleeping bag example, would just vanish). This works just fine in most scenarios, but what would happen if there's no room?

Suppose a man using a pocket dimension power gets locked in a steel box that then gets encased in cement, causing him to suffocate and die. The man's pocket dimension then tries to "decompress", but the space it's emptying out into is completely airtight and won't let any more matter inside of it easily. As I see it, this can have two outcomes, both of them very, very bad for worldbuilding:

1: The pocket dimension gives up, and the matter kept inside of it is deleted and lost forever. This essentially means that anyone with one of these powers (powers which are reasonably common and can be tied to destructible objects rather than people) has the ability to completely and permanently delete matter from existence, which seems like something I really shouldn't make that easy.

2: The pocket dimension keeps pushing harder and harder, as hard as it needs to in order to get the "blockage" out of the way and dump everything inside of it out into the real world. This would introduce the concept of a hammerspace pressure bomb, which, while really, really cool, would also mean I'd never be able to do things as large as the "sleeping bag with a house inside of it" idea because the kind of explosion you could make by decompressing that kind of hammerspace would probably wind up being way more destructive force than I want to give to such commonplace magical abilities and objects.

I need an alternative method of emptying out the contents of these pocket dimensions. A method that would safely put everything inside them back into the real world within the reasonable vicinity of the source and within a maximum timeframe of about five minutes. A method that wouldn't, when met with resistance, give up and destroy the matter inside of it or apply blockage-destroying pressure.

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    $\begingroup$ This just has to be how drops in computer games work! Makes total sense. $\endgroup$ – gustafc Oct 4 '19 at 13:56
  • $\begingroup$ The "pressure" could increase gradually, breaking things but not explosively. (Unless you designed a vessel specially to have the slow pushing pressurize some fluid like air and store potential energy). Otherwise the concrete would eventually crack, and then the slow pushing could keep going slowly. edit: Starfish already posted this as an answer. $\endgroup$ – Peter Cordes Oct 4 '19 at 22:54

16 Answers 16


You can make the hammerspace open another portal nearby where there is empty space and disgorge its contents through there. Think of it like a very full bag of water springing a leak as you squeeze the bag smaller. The leak will open up where the fabric is weakest i.e. where there is empty space.

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    $\begingroup$ the Beauty of this is ‘weakest’ could mean anything depending on your system of magic. Thematically similar points, ‘nodes’ that things pop out at, different times... Hell, a clever thief could set up a ‘weak’ hammer space of their own so stuff from a destroyed space jumps sideways and straight into their pocket. $\endgroup$ – Joe Bloggs Oct 4 '19 at 6:56
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    $\begingroup$ On a similar note, maybe if they can't be disgorged near the deceased, they are disgorged according to some criteria (random, near strong ley lines, whatever) at places that person has been. This could make for interesting story-telling possibilities with objects suddenly appearing in "unexpected" places. $\endgroup$ – Matthew Oct 4 '19 at 16:15
  • $\begingroup$ @Matthew Ooooooooh! I like the sound of that. Within reason though. I don't want sharp objects falling on people's heads to just be a thing that can spontaneously happen at any time. $\endgroup$ – Jason Clyde Oct 5 '19 at 1:30
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    $\begingroup$ @JasonClyde, you can avoid falling objects by making the tears most likely at lowest free space (gravity weakens the space or something) and you can avoid most accidents by simply making the process of tearing the space take some time so people can notice and avoid it. $\endgroup$ – Jan Hudec Oct 6 '19 at 15:34
  • $\begingroup$ Strictly speaking, "lowest" is almost always going to mean "underground"... possibly very far underground. But, yeah, hand-waving something about objects not popping up (more than a few inches) above a surface would be typical. Also, the "where the owner has been" criteria would tend to preclude "midair" anyway, unless the owner is a bird... $\endgroup$ – Matthew Oct 7 '19 at 14:54

Hammerspace becomes unstable, but doesn't disappear completely until empty.

The bag's contents spills into the available space until it is filled (possibly under pressure), the rest remains in hammerspace. As soon as anyone opens the enclosure, he's met with an eruption (which can be as much or little violent as you want it) of stuff from the hammerspace, until it is empty and vanishes.

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    $\begingroup$ Buy jar of holding. Fill with nails and broken glass. Throw jar of holding at enemy. I see many opportunities for weaponisation. $\endgroup$ – Joe Bloggs Oct 4 '19 at 6:58
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    $\begingroup$ +1: This was the "blindingly obvious" 3rd case that jumped to mind as soon as I read the question. $\endgroup$ – Chronocidal Oct 4 '19 at 8:50
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    $\begingroup$ @JoeBloggs only when the stuff comes out quickly enough. if it just spills out like when you turn a jar upside down, the potential for havoc is small. but generally, yes, everything can be weaponized. (obligatory Kzinti lesson reference) $\endgroup$ – ths Oct 4 '19 at 9:18
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    $\begingroup$ This! Cue images of someone opening the cell door to check on their prisoner, cell spews out a seemingly never-ending stream of deceased prisoner's hoarded junk; highly cartoonish. $\endgroup$ – gustafc Oct 4 '19 at 10:05
  • $\begingroup$ I think the OP asked explicitly for 'within five minutes'. this proposal would gladly wait centuries. $\endgroup$ – Aganju Oct 7 '19 at 3:03

This would introduce the concept of a hammerspace pressure bomb

"Pressure" is a slightly misunderstood subject, and isn't necessarily as destructive as you might thing. High pressure can deform and rupture solid objects, but that deformation and rupturing doesn't imply an explosive release of energy.

Have a quick read up on hydroforming, for example, or hydrostatic testing of diving gas cylinders. In both cases, the working fluid is incompressible so if the pressure exceeds the ability of the container to hold it will rupture. Unlike a gas overpressure though, the fluid won't suddenly expand out in an explosion... it'll just spray, possibly forcefully, but it won't go boom, because the high-pressure fluid has almost the same volume as the low-pressure stuff.

Also have a look at stuff like this... a mushroom, made of relatively soft and delicate stuff, yet still capable of exeting sufficient pressure to punch through a layer of asphalt.

Slow expansion over a period of minutes can do what you need. It could still be highly destructive... a bulldozer isn't quite as exciting as a bomb, but you still wouldn't want it driving through a supporting wall for your house, for example.

A method that wouldn't, when met with resistance, give up and destroy the matter inside of it or apply blockage-destroying pressure.

So you don't want to damage a confined space, or damage an object too big to fit into that space, but you want to fit that object into that space anyway?

Well, you can't have your cake and eat it. Something has to give, unless your ruptured hammerspace region is going to manifest as a sort of warped region of space that's small on the outside and big on the inside but otherwise freely traversable.

If you didn't want external destruction at all, then maybe the hammerspace contents simply get pressed into a brick of superdense "hammerium" (hammerite already have been taken) such that they're not vanished, but do become dense enough to fit into the available space. A careful limit to the amount of stuff you can fit in a hammerspace region should prevent you ending up with problems like electron degeneracy.

Combine the two effects as required, and no-one need get blown up or vanished.

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    $\begingroup$ I actually really like the idea of the "warped region of space". Essentially a Container of Holding is a warped region of space with a wrapper to make it portable and reasonably safe to interact with. Without that, if you walked through such a region, you'd be effectively sucked inside on contact as your body attempts to map itself topologically onto a much larger space. How lethal this is depends very much on the size of the space relative to its exterior. Objects inside when it's destroyed would simply stay where they are, but become lethally dangerous to retrieve from a knot of spacetime. $\endgroup$ – Ruadhan Oct 4 '19 at 14:53
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    $\begingroup$ Actually, "container-of-holding" space/time minefields would be a horrifyingly lethal way to defend a location... You know, because everything can and must be weaponised :P $\endgroup$ – Ruadhan Oct 4 '19 at 14:54
  • $\begingroup$ The pressure could be applied through the things that were in hammerspace, potentially crushing them in the process of breaking the concrete. (Nice job writing this up as an answer; I was going to say the same thing. You only get an explosion if you let some fluid like air get compressed by the process, storing potential energy.) $\endgroup$ – Peter Cordes Oct 4 '19 at 23:00

Hammerspace is complicated

The first thing is that it doesn't strictly have a location in normal space, if it's opened then it has a connection to a specific location, but we're thinking in portals, the one is not necessarily anywhere "near" the other. Only the owner can open the hammerspace, theoretically anyway. It certainly can't be opened from the inside as it lacks the necessary anchor to know where to open.

So what happens when the owner dies? The space remains, until it decays.

An ownerless hammerspace detunes and can potentially be opened by someone with a similar signature, so if someone related to the deceased tries to open a new hammerspace it's possible they will open the abandoned one and find it's already full of stuff. Once opened in this way it will reattune to the new owner and stabilise.

When it decays, it collapses and scatters its contents across nearby hammerspaces. Sometimes smaller chunks, sometimes larger ones. You might find your hammerspace has a lot more dust in the bottom, you might find a hammer you didn't put in there.

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    $\begingroup$ I love it! This is just begging to be used as a plot device... Chapter 1, introduce the powerful Hero on some critical quest... and kill him. Chapter 2, some random schmuck (i.e. the involuntary protagonist) discovers that The MacGuffin has suddenly appeared in his hammerspace. $\endgroup$ – Matthew Oct 4 '19 at 16:27

There are a few spells and magic items in Dungeons & Dragons that have rules for when whatever they're trying to transport has nowhere to go. What happens as a result is usually one of three things:

  1. The objects take a load of force damage, and then land back where they were. This can happen when trying to teleport into solid rock. In your case, this would lead to the eventual disintegration of whatever's in the hammerspace dimension.
  2. The objects get scattered across the Astral Plane, where they will be difficult (but not, in principle, impossible) to recover. This happens when rupturing a Bag of Holding, and is similar to deleting the contents of of the hammerspace.
  3. The objects simply land in the nearest available open space. This can happen when using a spell like Etherealness to phase through a solid wall, but then the spell ends while you're in the middle of it.

Option 3 is probably the closest to what you're looking for. In this case, the hammerspace inventory of the guy buried in concrete would just spill out on the ground above where he's buried.


Objects don't get pushed out of Hammerspace. Hammerspace gets "pulled back".

Imagine squeezing several snooker balls into a balloon. The balloon itself is stretched, and under pressure.

When you take a pin and burst the balloon, it makes a tiny hole. This hole tears and gets larger, releasing the balls - but it doesn't "throw" them out under pressure. If you have a ring of sellotape or something around the puncture point, it stops expanding there.

Going back to your concrete coffin example - as soon as the coffin is "full", the hole can't expand any further. As soon as the coffin is sufficiently breached, the hole can expand through that can - retreating and leaving stored objects behind without momentum.


Lost Portal

If a person is tethered to the pocket dimension dies, then the portal is "released" in the sense that it is no longer tethered to that person. Perhaps the portal to the dimension is itself now wandering around, looking for a place to decompress. Because we're dealing with portals and dimensions, the portal is not stuck inside the metal box encased in cement - the person is. So when they die, the "tether" to that dimension is broken, and now the dimension can wander around 3d space.

This means the people killing the man could, through some magical means, detect the portal's wandering and catch it. Think of it like a balloon tied to someone's arm. If the string breaks, the balloon rises and flies around. So, do the same thing here: the "tether" to the portal is broken, so the portal wanders around, and can then be detected or caught.

Furthermore, you can have the portal seeking for a place to deposit it's contents. You could make this near or far away based on the rules of your universe.


Work it backwards in time: someone in a very compressed situation moving to a more confortable, non-compressed situation.

Seems like watching a tape of something being co.pressed by a garbage compactor, but on reverse.

Setting it forward again, I think expulsion from pocket space would do just that - objects coming out of the pokéball may become smashed inside some other container.


Here is something interesting you might use to keep the explosive results of Hammerspace dumping down somewhat

It has been theorized that most of what we consider as solid matter is, in fact, empty space. There is distance between the nucleus of an atom and it's electrons. There is space between the individual atoms of a molecule. There are gaps between molecules in a solid substance.

Those gaps are where the matter from you pocket dimensions are going to go if you seal the "door" in concrete inside a steel box.

Think of the matter the portal is surrounded by as a seive of sorts. Picture a filter cartridge filled with sand. You can pass water through it fairly easily. An RO filter has a much smaller weave and will pass less water.

Air is like a really open weave so objects come through intact with no problem. In a body of Water, the objects will easily displace the water upward and will therefore remain intact. Having the opening sealed in concrete forces all of the matter to get shoved into the gaps between atoms and molecules. It will result in the resulting concrete gain the mass of everything that gets ejected from the pocket dimension without displacing anything. It will make that concrete block gain mass exactly equal to the mass of objects returning to our dimension. Since the mass is returning from the dimension from which it came, the conservation of mass is maintained

Apologies if this is a kind of sludgy way to describe this, I know what I mean, but I'm not entirely certain it's totally clear.

Bonus. The reason for the matter returning to this dimension is the concept of dimensional inertia. Matter from this dimension really wants to remain in this dimension. The Magic User that moves stuff into a pocket dimension has to exert effort to keep the stuff in the pocket dimension. Once the magic user is dead, that effort ends causing the moved matter to snap back.

Edit A metaphor for the effect of hammerspace dumping back out into a concrete brick. Take a jar and fill it with sand and then weigh it. The sand represents the concrete. Now slowly add water until it barely covers the sand. That water represents the returning mass. Weigh the jar again. It will have a significant amount of mass more than it did before without appearing much different or gaining any volume.

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    $\begingroup$ So the net result is that the collapsing hammerspace forces the objects out if it can, but if it can't, it can cause them to form into degenerate and unstable matter similar to neutronium within the material blocking the exit? Essentially what you're describing is the classic "teleported into a wall" horror story. Your hammerspace objects would end up visibly fused to the concrete if they're even recognisable as objects anymore. $\endgroup$ – Ruadhan Oct 4 '19 at 15:00
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    $\begingroup$ That is exactly what I am talking about @Ruadhan. I am thinking the objects might even protrude in spots where they don't quite fit. or if you saw the block in half you could see traces of the objects inside.. $\endgroup$ – Paul TIKI -Monica come Home Oct 4 '19 at 17:43


Hammerspace is a core concept in videogaming, and making a player's inventory available after death is a problem that has been resolved. So let's take inspiration from Minecraft.

When suffering from death, a player spills out their inventory into the world. However, it doesn't spill out as solid blocks but rather in a pick-up form. In this form, they take no physical space and can therefore pile up on top of each other. You can shoot through them, and the game is nice enough to not throw items through walls, and if for any reason someone tried to place blocks on top of the pickable items, they just move to the nearest available free space.

You can't interact in a meaningful way with a pick-up, you can do only one thing: walk over it and pick it up. Effectively, they are incorporeal, but conveniently available to the player.

How does it translate to you?

When your hammerspace container is destroyed, hammerspace itself isn't destroyed, it simply isn't contained anymore. All the items magically bound to hammerspace become detached, and bubbles of hammerspace start forming into our own dimension, each holding one item.

Bubbles have no physical existence in the real world, but can only form in a free space (aka air), or overlap other hammerspace bubbles. Why? It's magic, don't ask so many questions.

Bubbles can compress and expand, and adapt to the volume of air they have available. They can float away, bump into walls and objects if you want. It's effectively like pockets of gas. They be visible to the naked eye, even displaying an image of the object inside, or they may only be visible through magical means.

The lifetime of these bubbles be can as long as you want. They can last forever, or you can put a hard timer on it. Then, either the items pops back into reality unceremoniously, or they disappear forever.

How do I reclaim items?

Option 1: items pop back into reality when you pop the bubble. By simple touch, by some kind of magical incantation, or like if you touch it and wish it to pop. You can imagine that bubbles will only respond to their owner, or someone closely related to them, or any mage-person, or anyone the owner whitelisted, or anybody at all, that's up to you. It's up to whomever to make sure there is enough space to safely pop the bubble though, and if need be to move it somewhere else or mine a space big enough around it.

Option 2: items pop back into hammerspace. When the original container is destroyed, its content spill out. You need to get those bubbles back into hammerspace, by building a new container and forcing bubbles to go into it. They may be naturally attracted by hammerspace containers, it's magic remember, or maybe you'll have to use a proton-pack to suck them into the container like a regular ghost buster. From there, you get your items back the same way you usually do.

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    $\begingroup$ I suggest that such bubbles will last up until the point where they're no longer intersecting solid matter, at which point they "pop" and release the object. They're lighter than the surrounding material (and semi-intangible) so they "float" in matter like bubbles in water. They should in principle be forced out of a concrete coffin, float up through the ground and dispense their contents on the surface. $\endgroup$ – Ruadhan Oct 4 '19 at 15:03
  • $\begingroup$ "you can do only one thing: walk over it and pick it up." Not quite true; you can also move dropped Minecraft items around (with flowing water) or destroy them without picking them up (with fire, cactus, explosions, what have you). $\endgroup$ – Sean Oct 4 '19 at 22:32

Your dimensional pocket spells have parameters, and it sounds common enough to be able to be adjusted for a myriad of purposes. Different people will have different ideas of what they want to happen to their stuff if they can't get to it. That combined with the mechanics behind your hammerspace magic will guide your failure effects.

You don't have to explain the mechanics of this to the people in your world, but you need to know them so you can be consistent with your consequences of failure.

In short …

How the spell is cast will determine the effects of premature failure

A tsundere that wants to have their Baka Basher 3000 always at hand will use their hammerspace differently than a somebody that wants what they are carrying lost on their death because it is possibly highly incriminating.

If the hammerspace needs to be consistently powered by the caster, then primary point of failure of the caster. It also likely means that the hammerspace is reliant on the caster to continue existing. This means that there are four main options here for what happens on failure in broad strokes.

  1. The hammerspace empties on or near the caster as the hammerspace shrinks upon the opening point
  2. The hammerspace empties at a predefined point the caster has pre-set as part of creating it in the first place
  3. Control of the hammerspace defaults to a predetermined beneficiary of it.
  4. The hammerspace breaks down and its contents are scattered in the alternate dimension as the entire hammerspace fails at once

Important to note that with the fourth option is that matter is not destroyed per se, it is just unavailable. Somebody close to the caster, say through the Law of Contagion, might be able to salvage the hammerspace and/or its contents. In fact, there could be an entire group of people that do that for a living … or one really (un)lucky person that is a magnet for dimensionally displaced items.

If the dimensional pocket is fully powered and defined at casting with no further power required from the caster, then your primary point of failure is the portal point where the non-dimensional space interacts with the material plane or the object/person that holds the entry point. Again, four broad failure options are presented:

  1. The hammerspace has a designated emptying spot as specified by the caster and will empty there on failure of the entry point. In essence, an emergency exit
  2. The hammerspace expels everything at the point of the opening
  3. The hammerspace becomes an untethered bubble floating in the alternate dimension and can be linked to by another caster or fall into another's control by happenstance (Read: Plot).
  4. Breaking the link breaks the hammerspace completely, and the contents are left floating in the alternate dimension as above -- lost but not destroyed.

Your Issue

Based on the bolded part of your question and the options presented above, the hammerspace spell by default has either a physical point where contents empty on the collapse of the hammerspace, or a secondary person/object that can only access it when the primary access method fails. If this is known or changeable is based on your world.


Two answers:

1) The caster configures the hammer space when it's created or set up. That configuration includes what it's bound to, of course. But perhaps explosive or problematic hammerspace issues happened ages ago, and now, anyone casting a hammerspace, attaches it to multiple objects - the second being a safe space the objects will be released to, if the main exit is blocked. And typically, that second exit is a wide open plain, the bottom of an ocean, the sun, space.....

2) Maybe hammerspace doesn't exert much pressure. So if there is easy space for things to come out, they will. But if they can't, or only partly can, the rest stays in hammerspace... but quickly either the portal decays, or the hammerspace pocket itself decays. In either case the object is unreachable after a few minutes, if it hasn't emerged.

  • $\begingroup$ If using option 1, and if it's possible to keep a living person in hammerspace, that could be used to set up a very interesting hostage situation, by imprisoning a person in a hammerspace with its alternate exit linked to (for instance) the center of the sun, the Chernobyl reactor core, the event horizon of a black hole, a running blast furnace, or somewhere else similarly lethal. $\endgroup$ – Sean Oct 4 '19 at 22:37

Mass == Energy

If you want to create a pocket dimension, you are going to need a bunch of energy to form and maintain it. This energy is tied to an anchor, which is some physical object that lets people interact with the pocket dimension. Sometimes it is a sleeping bag, sometimes it is a human brain, but there is always an anchor in "realspace" that is used to stabilize and interact with the hammerspace.

When that anchor is destroyed, the energy that maintains the pocket dimension is no longer stable and starts to collapse. This leads to the known explosive property where objects inside of the hammerspace are ejected back into realspace. So what happens when there is not enough realspace for objects to get ejected to?

The pocket dimension eats them.

More specifically objects inside of the pocket dimension are converted into more energy and used to fuel the rest of the collapse. Eventually one of two things will happen: Either enough energy is generated that the remaining objects are able to (explosively) exit the dimension and re-enter realspace, or all of the objects inside the pocket dimension are consumed and then the dimension quietly fizzles out due to being empty.

Basically you can think of your collapsing dimension as a bomb. Bombs are just what happens when you have more energy under pressure than the container they are in. Your hammerspace acts the same way. Too little pressure and it just shoves everything out without any issues. Way too much pressure and nothing noticeable happens. It is just that part in the middle, where there is a lot of pressure but not quite enough, that things get interesting.

  • $\begingroup$ Like how a bomb in a pressure cooker creates more damage than a bomb in its birthday suit, but a bomb encased in hundreds of meters of solid rock merely creates a self-contained spherical void in the rock? $\endgroup$ – Sean Oct 4 '19 at 22:40
  • $\begingroup$ Collapse a mine or sewer on a hammerspace full of sand or rocks and you potentially have a total conversion bomb under a city (vastly more powerful per mass than a nuke), depending on how much work you can extract from the expanding stuff before it's out. So it all depends on how fast the process is, how much force / pressure it will build up over the distance / volume of the stuff expanding. $\endgroup$ – Peter Cordes Oct 4 '19 at 23:13


When matter rematerializes into the world,it will generally try to bump stuff out of the way, but if this becomes impossible, the matter effectively turns on noclip mode (if you're familiar with videogame parlance) and blinks into existence, fuzing with any immmovable objects in the vicinity.

So in your steel and concrete box case, say the victim had a house in a purse, the house just pops back into being, but now it's a house with a sealed sarcophagus fuzed into the wood of the floor.

I like this angle, because it could be explored deeper, maybe wood fuzed with steel has interesting properties, or maybe fuzed stuff just crumbles to dust. It could be a cultural taboo to release material in such a way that it fuzes with someone else's house/property, and so on.

Alternatively, you could have the same behavior without the fuzing, so in the house in the box scenario, the house 'finds' the next chunk of space available for it to fit in. If the box is in an open field, the house pops back into being on top of the box. This angle is a little more shaky, because it implies some sort of intelligence or algorithm behind the scenes finding the empty space, where the first angle could be hand-waved with nature finding a balance, etc.


constant pressure Upon the destruction of the object/caster that maintains the hammer space, the dimensional pocket starts to contract and exerts a force on the items it contains. However, it can only exert a force up to a certain pressure. Think of a balloon, if you put some objects into the balloon, and the let go of the opening, if the object is small enough, or the pressure on the opening is low enough, the object will spill out, but if the object is large enough to not fit through the opening, then the object is stuck and the balloon simply stops contracting.

Should the opening to the hammer space is blocked when the hammer space is destroyed, it will initially spill out any objects that could fit inside the space with a constant pressure, but once a certain pressure is reached the object stops spilling out, and the collapsing pocket of space will stop collapsing, and would remain in this partially collapsed state indefinitely. Only after the blockage is cleared—the box is opened and the concrete is broken, so that the objects can be expelled under the set pressure, does the collapse and decompression continue until all the objects have been ejected (under mild pressure), or the exit is blocked again. The collapsing hammer space pocket will only disappear once fully empty.

This could give rise to one-time-use packaging and/or time capsules—- just put your important delivery inside a jar of holding, package the jar inside a strong enough sac and smash the jar from the outside. Should the sac be opened by someone in the middle of the delivery, the unstable pocket will collapse and disappear, leaving the important delivery item outside that is unable to be put back into the sac again.

You could also find relics of old hammer space pockets whose opening have been stuck by some oversized object when it’s owner dies, and upon removal of the stuck object, empties out it’s contents and disappears. A protagonist may find some important plot item when renovating his/her house, or during an archaeological dig, from a stalled collapsing dimensional pocket after unknowingly revealing it’s opening to normal space.


Hammerspace is like water

Boats float on the water. Rocks don't. Submarines have machines that control their density, so they can float or sink.

Hammerspace pockets are like underwater rooms. There is a tendency for leaks, but the forces involved are small, and constant, so if you get too ambitious, you just fail. Without ballast, stuff tends to get pushed out, but if there's a rock in the way, the stuff doesn't get pushed out; it just rests against the rock.

Everything in your universe could have a hammerspace density, which human bodies can't sense (like magnetism), but is measurable with the right instruments. Like magnets, the effect is obvious, but the math is difficult. In the industrial age, the mathy fellows weaponize the crap out of this, just like chemistry, but there's a long span of time before then.

Fun corollaries: 1) Some substances strongly resist going into hammerspaces. 2) Some substances tend to go into hammerspace as soon as they are made. 3) Hammerspace currents, residents, storms, turbulence. 4) Submarines. 5) The bends. 6) Light?


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