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Physics of Control Water
Sep. 22nd, 2016 02:14 pmWe were talking last Sunday about how the spell control water would affect crossing running water. The spell description talks about raising or lowering a "volume" of water, which makes a ton of sense in an area of deep standing water like a lake or an ocean, but is a lot harder to grok when it comes to shallow water flowing down a gravity potential.
Does the spell affect a "volume" of water like a specific collection of water molecules, or like a region of space? Does the spell lift/lower the water, or change a physical property, or summon/banish water? If the raising or lowering of water causes a new gravity potential, will the water flow along that potential or along the original one?
The third question is pretty definitively answered in the spell itself. Lowering water will trap ships in the depression for the duration of the spell, and raising it will even cause water itself to "spill over onto dry land". The implication this has for crossing streams is that you definitely can't just lift the water above everyone's heads and peace it across - this will just result in everyone getting wet. Lowering the level of the water still could work though!
Addressing the first question, what would it look like to raise a specific collection of water molecules? Since we've established that the fluid follows the new potential, either the "raising" would have to be a rapid recycling of the same water, or the spell would last significantly less time than 70 minutes minimum as everything just kinda falls back down immediately. Both options seem counterintuitive? Conceivably the rapid recycling could work if the spell summons water at the height of the spell and banishes it at the bottom, but a) this would actually be a "region of space" interpretation, and b) we should prefer explanations that preserve the physical composition of the water, which this explanation either does NOT do or requires teleportation magic with significantly greater volume and duration than the similarly leveled spell dimension door. So, let's assume that the spell affects a region of space rather than a specific collection of water molecules. The implication for crossing rivers is very positive, because instead of an ephemeral displacement of the river we see a duration and stasis that could allow easy travel on foot.
This leaves the question of the mechanism for displacement. The conjuration interpretation has already been rejected, which I should point out is commensurate with control water's classification as a transmutation spell. If the water were merely moved around, as talked about before, it would either not work as described (water would immediately fall down or fill in) or it would take a lot of energy to keep the water cycling with itself. What I like most, and what frankly jives best with the classification of the spell as transmutation, is for the spell to locally change some law of physics. The thing that I threw out while we were discussing it during play time was reducing the forces that keep water from compressing, and while I have a B.A. in chemistry it has...been a while...so I'm just gonna handwave and say that would totally work without horrible repercussions. So! If the spell either compresses or forces the expansion of some volume in space of water, what does that imply for crossing a river? It means that you can clear nearly the entire riverbed of water without damming up the flow, because the water will just flow slightly faster and certainly denser through the affected region. You'd have to take care not to come into contact with that "third rail" of water, because I'm pretty sure something that dense but still fluid and moving at river speeds would basically be like a belt sander, but you could cross the river without drowning! The spell would also have environmental externalities (or intended effects, if you're trying to move a river à la mode de Machiavelli et da Vinci) by increasing erosion temporarily.
This explanation for how control water works does leave some things to be desired - if the compression/expansion is uniform, doesn't that totally mess with the buoyancy of ships in the affected area? - but it seems like the coolest and most magical explanation to my sensibilities. Feel free to comment with your thoughts!
Does the spell affect a "volume" of water like a specific collection of water molecules, or like a region of space? Does the spell lift/lower the water, or change a physical property, or summon/banish water? If the raising or lowering of water causes a new gravity potential, will the water flow along that potential or along the original one?
The third question is pretty definitively answered in the spell itself. Lowering water will trap ships in the depression for the duration of the spell, and raising it will even cause water itself to "spill over onto dry land". The implication this has for crossing streams is that you definitely can't just lift the water above everyone's heads and peace it across - this will just result in everyone getting wet. Lowering the level of the water still could work though!
Addressing the first question, what would it look like to raise a specific collection of water molecules? Since we've established that the fluid follows the new potential, either the "raising" would have to be a rapid recycling of the same water, or the spell would last significantly less time than 70 minutes minimum as everything just kinda falls back down immediately. Both options seem counterintuitive? Conceivably the rapid recycling could work if the spell summons water at the height of the spell and banishes it at the bottom, but a) this would actually be a "region of space" interpretation, and b) we should prefer explanations that preserve the physical composition of the water, which this explanation either does NOT do or requires teleportation magic with significantly greater volume and duration than the similarly leveled spell dimension door. So, let's assume that the spell affects a region of space rather than a specific collection of water molecules. The implication for crossing rivers is very positive, because instead of an ephemeral displacement of the river we see a duration and stasis that could allow easy travel on foot.
This leaves the question of the mechanism for displacement. The conjuration interpretation has already been rejected, which I should point out is commensurate with control water's classification as a transmutation spell. If the water were merely moved around, as talked about before, it would either not work as described (water would immediately fall down or fill in) or it would take a lot of energy to keep the water cycling with itself. What I like most, and what frankly jives best with the classification of the spell as transmutation, is for the spell to locally change some law of physics. The thing that I threw out while we were discussing it during play time was reducing the forces that keep water from compressing, and while I have a B.A. in chemistry it has...been a while...so I'm just gonna handwave and say that would totally work without horrible repercussions. So! If the spell either compresses or forces the expansion of some volume in space of water, what does that imply for crossing a river? It means that you can clear nearly the entire riverbed of water without damming up the flow, because the water will just flow slightly faster and certainly denser through the affected region. You'd have to take care not to come into contact with that "third rail" of water, because I'm pretty sure something that dense but still fluid and moving at river speeds would basically be like a belt sander, but you could cross the river without drowning! The spell would also have environmental externalities (or intended effects, if you're trying to move a river à la mode de Machiavelli et da Vinci) by increasing erosion temporarily.
This explanation for how control water works does leave some things to be desired - if the compression/expansion is uniform, doesn't that totally mess with the buoyancy of ships in the affected area? - but it seems like the coolest and most magical explanation to my sensibilities. Feel free to comment with your thoughts!
