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This is assuming the wizard in question has access to the spells Shadow conjuration or Greater Shadow Conjuration and uses it to duplicate the effects of major creation

Let's say you're in trapped room based on balancing weight and any movement will cause it to shift (thus trapping those in the room to until they can balance the weight or dropping them into a pit below), could one create an illusion of weight to balance out the weight of the PCs in the room, allowing them to escape safely? Because an illusion is only at most 20% real(shadow conjuration) - 60% real (greater shadow conjuration) due to shadow-stuff, would that mean that whatever the image is would X times as large as it would normally be? That is assuming the room is able to disbelieve. Let's say the illusionist is outside the trapped room, how could s/he find out how large of an illusion that needed to be made?

According to this question: Can an illusion hold weight? it was opined that it does. If it can hold weight, why wouldn't it have some sort of mass?

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    \$\begingroup\$ Where is the "illusions are 50% real" located in the rules? \$\endgroup\$ – Reibello Jul 23 '16 at 20:55
  • \$\begingroup\$ @Reibello They typically do half the normal damage, and 20% against disbelievers (tho the greater shadow conjuration does 60%) \$\endgroup\$ – Jesse Cohoon Jul 23 '16 at 21:05
  • \$\begingroup\$ @JesseCohoon: "Shadow conjurations are actually one-fifth (20%) as strong as the real things, though creatures who believe the shadow conjurations to be real are affected by them at full strength." "Spells that deal damage have normal effects unless an affected creature succeeds on a Will save. Each disbelieving creature takes only one-fifth damage from the attack." (Shadow evocation) \$\endgroup\$ – user17995 Jul 23 '16 at 21:09
  • \$\begingroup\$ @TuggyNE so "between 20% and 60% real" would be a better edit instead of the 50% I have listed? \$\endgroup\$ – Jesse Cohoon Jul 23 '16 at 21:12
  • \$\begingroup\$ @JesseCohoon: If you're assuming that the trapped room effectively disbelieves, yes, that would make sense. (Otherwise they are 100%.) \$\endgroup\$ – user17995 Jul 23 '16 at 21:15
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Based on the information available to me in the SRD, it appears that only Shadow illusions would be able to create any weight against such a trap. Other illusions would require the trap to be able to "perceive" the illusion. So let's look at some excerpts from Shadow Conjuration.

You use material from the Plane of Shadow to shape quasi-real illusions of one or more creatures, objects, or forces.

Shadow conjuration can mimic any sorcerer or wizard conjuration (summoning) or conjuration (creation) spell of 3rd level or lower.

Shadow conjurations are actually one-fifth (20%) as strong as the real things, though creatures who believe the shadow conjurations to be real are affected by them at full strength. Any creature that interacts with the conjured object, force, or creature can make a Will save to recognize its true nature.

Shadow objects or substances have normal effects except against those who disbelieve them.

Objects automatically succeed on their Will saves against this spell.

So it looks like the trap just "knows" that it's not a completely real object. For the purposes of SCIENCE! let's assume that we're using Greater Shadow Conjuration, with a 60% effect ratio, and usable up to 6th level Conjuration Spells. My suggestion is a Greater Shadow Conjuration Wall of Iron, which creates an 1" thick 5' x 5' wall per level. Since we're clearly at least level 13 if we can cast this, we get a 1" thick 65' x 5' wall of iron. Per square inch, iron weighs around .26 lbs. According to some quick and dirty math, this means that the wall is 46,800 sq. inches, multiplied by 0.26, which leaves us with 12,168 lbs of fake iron. Which we need to reduce, because it's not all real. It turns out, that we end up with only 7,300 lbs of fake iron, which will probably take your party's balancing act, and turn it upside down.

Unless you took that wall of iron, and placed it symmetrically across the axis of room rotation, which would make your weight pretty negligible. It also might just collapse the room as well though.

Of course, the spell does require Line of Effect to the destination, so you'd have to get some way to be able to see into the room. If you're just on the other side of the door or wall, Stone Shape would work nicely.

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Yes, a Shadow Conjuration would have some mass.

Since they are suggested to be about 20% real if the subject does not believe it, then they would have about 20% mass, compared to a fully real object.

It should be argued that the room itself, as an unintelligent object, is incapable of belief in the illusion.

But if you have a wizard with far too much free time, an intelligent room is not entirely impossible...

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  • \$\begingroup\$ I appreciated the "unintelligent objects are incapable of belief in the illusion" interpretation. \$\endgroup\$ – Tristian Jul 7 '17 at 5:54
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Quasi Real is Real Enough for Me

The Spell descriptions sometimes say "quasi-real", which can implicitly lead one to conclude that if the thing imitated is intended to have mass, it does. Explicitly, it is not a necessary condition for shadow spells to produce mass, as force spells do not produce mass and can imitate effects like gravity (Reverse Gravity) or a Wall (Wall of Force). We could think of the shadow effects as a play of forces. I once proposed shadow magic could be like a 4th dimensional object's shadow, with the force acting on objects in the third dimension at imperceptible angles.

I once asked my physics professor about how much force was produced by an impervious force field colliding at 100 meters per second, and he asked what the mass was. I said zero, and he said the answer was also zero.

One of the shadow spells uses electricity as its example. This is where it gets tricky. Things like electricity and acid inflict a kind of damage that comes from a substance (electrons) interacting because of the structure of that substance. A force field by itself would have to be complex on some subatomic level in order to imitate, but not literally be electrons. Which, Occam's Razor leads us to believe the simpler solution is the best - that some of shadow magic is real, and has mass, because it would be harder (and unnecessarily complex) for the spell to replicate so many effects without it.

So while we could easily imagine force fields and static and magnetism, etc. as source of shadow damage being partially real, it stops making sense when you replicate a Black Dragon that stomps on the Barbarian and Spews Acid all over the Paladin's Armor.

Quantum Shadow Stuff? The one tricky line is this:

Nondamaging effects, such as Gust of Wind, have normal effects except against those who disbelieve them. Against disbelievers, they have no effect. (PHB, p.277)

No effect? That's directly inconsistent with the 20%-80% rule for spells like Shadow Invocation and Shades. Shades is a 9th level spell that makes things 80% real. If you disbelieve a bridge that is 80% real, can you cross it or do you pass through? If a Wall is 80% real, and you successfully disbelieve it, can you walk through it?

This appears to imply that the Shadow-stuff is semi-intelligent, and reacts, withdrawing itself from disbelievers, sort of like a shadow version of the Double Slit experiment: the Shadow-stuff is only where you need it to be when you don't look to see if its really there, but if you are skeptical and focus on whether it is there, it isn't.

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  • \$\begingroup\$ this begs the question: can a shadow mage cross their own shadow bridge? \$\endgroup\$ – Tristian Jul 7 '17 at 6:27
  • \$\begingroup\$ Note that the 'Nondamaging effects..." line is only in shadow evocation and not shadow conjuration, making conjuration, somewhat predictably, better than evocation. \$\endgroup\$ – Chemus Jul 7 '17 at 6:27
  • \$\begingroup\$ Chemus - nice catch. That means a shadow bridge that uses an evocation spell for its creation probably can't be crossed as easily as one that used conjuration. If you froze a part of a lake with shadow cone of cold, you would fall through? But if you shadow summoned a dragon turtle and floated across on its back, you wouldn't? \$\endgroup\$ – Tristian Jul 7 '17 at 6:32
  • \$\begingroup\$ If by 'froze part of a lake' you're referring to wall of ice versus ... a summon spell, then yes, you have to fail your save versus the wall of ice to walk on it. But if the lake froze from the spell's effect ... well I'm not sure that it can, as objects always make their saves vs shadow conj/evoc. But a lake is unattended, so gets no save, so that's probably a question of its own. Regarding the wall of stone being walkable, it's got an answer here \$\endgroup\$ – Chemus Jul 7 '17 at 6:35
  • \$\begingroup\$ define "4th dimensional object's shadow" please \$\endgroup\$ – Jesse Cohoon Jul 8 '17 at 1:08
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Illusions do not have any mass and therefore create no weight. For instance, you can't create an illusion of a wall to block a volley of arrows—they'll go right through.

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  • \$\begingroup\$ In this question rpg.stackexchange.com/questions/83715/…: it was opined they can hold weight. If this is true, how can they not have mass? \$\endgroup\$ – Jesse Cohoon Jul 23 '16 at 21:02
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    \$\begingroup\$ @JesseCohoon: You may want to add that to the question. \$\endgroup\$ – user17995 Jul 23 '16 at 21:05
  • \$\begingroup\$ You're thinking of figments and glamors, which are the prototypical illusions. Not everything in the illusion school is one of those two types though. \$\endgroup\$ – fectin Jul 24 '16 at 4:48

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