It's all about energy, thermal
energy. In thermodynamics, there's no such thing as "cold." It's all "heat": varying degrees of it. The cold water for cooling has less heat than the hot wort that you're trying to chill, and you're trying to move the heat from the wort to the coolant.
(As an aside, there's a trick question which demonstrates this: Which has more heat, a burning match or an iceberg? It's actually the iceberg. The heat
present in an object is the sum total of the motion of all its constituent atoms and molecules, while its temperature
is a measure of the average motion of the atoms and molecules.)
Originally Posted by TerraNova
If there was a way to keep the rocks sterile, placing frozen rocks in the wort would cool it just the same way. However, you would likely need to replace the rocks with fresh cold ones as they can only get so cold. Hmmmm.
Strictly speaking, any solid rock (or any solid substance, for that matter) is already
But leaving pedantry aside, you wouldn't be adding the "cold" of the rocks to the wort; you're transferring heat from the wort into the rocks, from which you have previously removed
more heat than they would have at room temperature by sticking them in the freezer first.
If I haven't convinced you yet, think about a cooler with a grain inside it being mashed. The cooler is resistant to the flow of heat, so it feels cool on the outside. This coldness doesn't get into the mash taking place inside. Otherwise, you couldn't maintain the temperature inside the cooler, and insulators would make the best wort chillers. It's the parts of the cooler that feel warmer to the touch than others (e.g., ball valve, lid) that cause the temperature to drop; this is because those are the parts where the system is losing heat.