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Second Law of Thermodynamics (view source)
Revision as of 02:45, August 29, 2018
, 02:45, August 29, 2018New section on falsifiability
*While the probabilities of certain outcomes can be mathematically calculated, they are so small that, as a practical matter, we can say that '''they do not occur'''. People sometimes like to say things like "The Second Law of Thermodynamics means that it is very unlikely that heat will travel from a colder object to a warmer one." That's a fallacious way of thinking about it. It is a [[statistical impossibility]]—it just doesn't occur.
*While the probabilities of certain outcomes can be mathematically calculated, they are so small that, as a practical matter, we can say that '''they do not occur'''. People sometimes like to say things like "The Second Law of Thermodynamics means that it is very unlikely that heat will travel from a colder object to a warmer one." That's a fallacious way of thinking about it. It is a [[statistical impossibility]]—it just doesn't occur.
*An example is the question of how likely it is that all the air molecules in a room will move to one corner, asphyxiating everyone.<ref>Actually, conservation of momentum requires that we consider half the molecules going to one corner and the other half to the opposite corner.</ref> This is sometimes worked out in physics classes. But the conclusion has to be that this occurrence, or anything remotely resembling it, might have a probability on the order of 1 in 10<sup>10<sup>23</sup></sup>—it just doesn't happen.
*An example is the question of how likely it is that all the air molecules in a room will move to one corner, asphyxiating everyone.<ref>Actually, conservation of momentum requires that we consider half the molecules going to one corner and the other half to the opposite corner.</ref> This is sometimes worked out in physics classes. But the conclusion has to be that this occurrence, or anything remotely resembling it, might have a probability on the order of 1 in 10<sup>10<sup>23</sup></sup>—it just doesn't happen.
==Aren't we cheating when we say that heat flows from a warmer body to a colder one, when all we really know is that it "probably" flows that way?==
Well, yes, and no. The laws of probability and statistical mechanics are precisely true, since they are mathematical theorems. They are as precise as that 2+2=4 exactly. It's their application to the real world of molecules and such that is merely "probably" correct. When we say that heat flows downward in macroscopic objects (ice cubes, teapots, Carnot engines<ref>[https://en.wikiversity.org/wiki/Carnot_engine Carnot Engine]</ref>), what we really mean is that it flows downward with a probability of 99.999<put in Avogadro's number of 9's here>999 percent.
Everyone, scientists and lay people alike, accept this as true. It is the basis for just about everything, including breathing. No one ever questions the reading on the pressure gauge of a gas canister on the grounds that it is only probably correct, or is surprised when one blows into a balloon and it expands.
So the fine philosophical point about it being only probably correct is just that—a fine philosophical point.
==So is the second law falsifiable?==
Absolutely. The laws of statistics and probability are mathematical theorems and are absolutely true. The only thing for which falsifiability is an interesting question is the application of those theorems to the real world. Statements like "if you flip a coin 10 times it will never come up heads each time" is both falsifiable and false. One can easily imagine an experiment that would refute it—flip the coin and have it come up heads 10 times. One could even do the experiment, getting that result, if one had the patience for it.
But when we go from 10 coins to macroscopic objects like ice cubes or teapots, one can still imagine an experiment. If all the air molecules in one's house spontaneously moved to the East wall or the West wall, the pressure on those walls would make the house blow up, probably taking several nearby houses with it. So an observation of this phenomenon would refute the second law, whether it's ever been observed or not. (It hasn't.)
==Application to molecular behavior==
==Application to molecular behavior==