I did not want to make a lone topic for this, but digging around I could not find one more suitable. Hopefully this sparks a bit of discussion so my simple question isn't so selfishly taking up boardspace.

Q. Specifically in FR, does energy (whether it be Positive, Negative, Elemental, Chi ... etc) have volume or mass? Does it stretch thin as it expands, or become more dense as it is contracts? Can one, if they manage the power and means to do so, store an infinite amount of energy in an open space?

Depends on if E=MC^2 in the Realms. No. Really. A sufficient accumulation of energy is eventually forced to adopt mass-like qualities. Asking for a concrete answer on a very open ended question like this has the potential to set a "ruling" on the very fundamentals of the world's physics, which has the potential to break the in-setting verisimilitude of a magic system entirely. A better answer could probably be given with a description of what you're trying to do, so that the Rule of Cool and storytelling potential can be assessed.

If I were making a ruling, however, I would say that no, an open space cannot contain a theoretically unlimited amount of "energy". There is some precedent for energy requiring a battery of sorts, typically a crystal. The larger the crystal, the better the battery. Or a magic circle perhaps, the dimensions and complexity of which dictate the amount of power it can store. This seems to indicate that that least -some- form of energy to mass ratio exists in Forgotten Realms, even if it's a fluid, arbitrary thing.

Thank you for that; I like to make sure I'm not treading on pre-formed rulings or themes often found in novels or handbooks that I may have simply missed in my dealings. It's pretty common for magical powers of all sort to 'overload' whatever storage or catalyst they're being put into, causing adverse or otherwise damaging effects-- this may explain why higher quality gems (and gems of different types) may hold more potent magics. There seems to be a lot of material for suffusing elemental energies into materials such as metals, gems, crystals, and other enchantables, but open spaces? Less so unless dealing with basic physical properties.

For instance: a measure of elemental fire may be best set within a Fire Opal or Ruby of graduating qualities depending on the power of the spell / energy the creator is working with, choosing the most efficient vessel for the intended effect. However, what if that same person wanted to put that same elemental fire into a box? Elemental Fire I would assume would be less tangible in a holdable, moldable aspect than Elemental Earth or Water. Essences taken from living Elementals by Alchemists are kept within containers, but would containers meant to house larger amounts of essence need to increase in size or only quality, as a gemstone or garment might?

On a side note, Mythals seems to be stronger by virtue of being larger and less flawed. But that simply goes back to the whole crystal/gemstone schtick, which is less appropriate when working with other objects. Rings can hold just as much, if not typically more magical energies than say... Boots of much larger size.

I care entirely too much about semantics and very minor details.

Energy in open spaces I would assume to eventually leach back into their respective planes without some physical thing to bind them, or the continued focus of a spellcaster. An array of mirrors and lenses, for instance, could coalesce elemental energy in open air for a period of time, such as the gathering phase of a larger ritual, but only so long as the casting is ongoing.

Your box of (element), for instance, could come in two flavors. One, is a box with elemental (whatever) infused into the materials itself. Basically, a very eccentric magic item.

Or, it could be a box, enchanted and designed in such a way as to hold elemental (whatever) that's -actively- put there by a spell caster. The size and quality of the box, and the strength of the enchantments would hypothetically dictate how much (whatever) the box could hold, and how fast it leaks once the spellcaster removes her focus. My opinion is that "open air" storage isn't possible permanently, at least as presented. They would work more like magical capacitors, almost.

No, it's perfectly fine to think about those details, because it's something a character would. A character would have to, I imagine, or understand it innately I guess, like how we understand air fills up all available space, water fits to the bottom of its container, etc.

The laws of FR are nothing if not mutable to the beliefs people impose upon them. If fire being less tangible than the other elements makes sense to your PC, then maybe whatever essence your PC happens to extract has those properties. But maybe for a character who's studying efreeti, creatures said to be made of "solid flames" among other things, maybe not so much. I know there would be great want for something as fundamental as elemental essences being the same across the board, everywhere, and with everyone like our own periodic elements, but most of what FR shows us speaks to the contrary. That same mote of elemental fire may double in size near places closer to the Plane of Fire, half in size near places near the Plane of Water, or all together cease to exist in places like the Plane of Vacuum. I wouldn't put it past the realms to have some sort of Sozin's Comet deal based on the alignment of planets and spheres, either.

The bit about mythals and stored magical energy in general is indeed interesting. I believe that you can have a larger form of energy stored in a smaller physical form that it would normally be found, but it's easier to stabilize and create with more material. A boulder the size of a tire launching Fireball once per day is likely going to be a bit more trivial that a cats-eye marble that summons a Meteor Swarm every two hours. A similar thing happens with mythals and crafting. It is possible to have more solidified magic in smaller space (a few items in plots have even demonstrated this), but mythals may follow a more (super)natural pattern. You're more likely to find things in their lowest energy state i.e. a large boulder having rolled down a hill, rather than being precariously balanced on the point of it. So you're more likely to find mythals in a size respective and accommodating of their energies, because the smaller, more densely packed but unstable ones would have already broken apart at the smallest impetus.

That's how I've imagined as the explanation for Large Weapons having more slots than other weapons or items. Of course it's purely a mechanical decision, but we can imagine that it's easier to safely shove 5 MacGuffins of voodoo into a greatsword than it is to shove them into a dagger. Rings are a bit tricky, though. Because the first thing everyone says is "why can I only wear 2 rings?". Though PnP bluntly states that if you put on a third magic ring, it just doesn't work, my own explanation when PnP DMing and is a"ring" can actually represent a set of rings. You can have a single Ring of Dhoom, but unless its specially crafted to work with other sets on the same hand, the magics average out (degrading the overall effects of both) or cancel out, a side effect of things so small storing so much magical energy, un-shielded from each other. Sure you can wear your Nasher +3 STR ring on the same hand as your Ring of Universal Saves +3, but the end effect will be something like +1 STR and +1 Uni, equivalent to the powers that could be hosted on a single "crafted" ring.

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Here is something about mythals and masses