The Science of Fiction Q&A

Magical Fire Extinguishers

Would shrinking a room with magic effectively put out a fire?

To answer this, I'm going to assume that the fire, itself, is mundane and follows normal fire behavior, which may or may not be true if we're throwing around magic.

Fire needs three things to burn: oxygen, fuel, and heat. Remove any one of those and the fire goes out. Most fire extinguishers work primarily by separating fuel from air with a non-flammable substance. Throwing water on a fire works so well because it both cools it off, and creates a barrier between the oxygen and the fuel.

Shrinking a room would not immediately do any of those things. The heat, fuel, and air would still be in there. They would just be smaller. Depending on the details of the magic, that could either be somewhat effective, or disastrous.

When something shrinks, we have to ask where all the extra matter and energy goes. Magic lets us ignore physical limits like the laws of conservation of mass and energy, but it's still really important to know what happens to the matter for this question. There are basically two possibilities: the matter vanishes, or the matter compresses.

If the matter vanishes, then the density of the room and its contents stay constant. That would turn a full sized room into a little toy room. This method would be an effective way to mitigate the damage. The room would still burn, but it would be a much smaller, shorter fire. There's less fuel, less oxygen, and ultimately less energy released. It changes a building-sized problem, into a campfire sized problem.

However, if your magic uses this method, it raises another question. Your shrinking magic is capable of eliminating both matter and energy. If that's the case, why do you not have magic that just eliminates the energy? Why can't you just make the room colder? That would actually put the fire out.

Let's look at the other method, where we don't magically vanish the matter, we just make it take up less space.

Chemical reaction speeds are dictated by the amount of energy available, and how easily the reactants can interact with each other. We take advantage of this property in our car engines. We mix the air and gasoline in a cylinder, then compress the mixture before igniting it so the reaction is faster.

If you crush all of the energy, air, and fuel in a room into something the size of a dollhouse, the speed of the reactions would increase enormously. It would burn hotter and faster, possibly explosively.

Probably not ideal.