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Talk:Perpetual motion machine (view source)
Revision as of 14:40, January 5, 2007
, 14:40, January 5, 2007Thoughts
:: That's an interesting point of linking observation to entropy. But I do think even a purely closed system would stop without observation. Don't you? Perhaps Newton would not be pleased, but the Second Law of Thermodynamics suggests that motion does eventually stop. --[[User:Aschlafly|Aschlafly]] 00:57, 5 January 2007 (EST)
:: That's an interesting point of linking observation to entropy. But I do think even a purely closed system would stop without observation. Don't you? Perhaps Newton would not be pleased, but the Second Law of Thermodynamics suggests that motion does eventually stop. --[[User:Aschlafly|Aschlafly]] 00:57, 5 January 2007 (EST)
::: No, I don't think it does. The question here is whether anything says ''how fast'' entropy increases... and what counts as "motion."
::: I think that the Second Law applies to large systems with many interacting particles or bodies and is some kind of statement about their statistical behavior and how easily that motion can be observed.
::: Imagine, say, an ideal, large, sealed box whose walls perfectly hard (do not flex or absorb energy), and imagine that it one ideal billiard ball. By an "ideal billiard ball" I mean, again, one that is perfectly hard and perfectly elastic. If you have a single billiard ball in the box and it is moving, I think it keeps bouncing off the walls and moves forever. After all, energy is conserved.
::: Now, suppose, instead, that you have twenty-one ideal billiard balls, twenty of them at the vertices of an icosahedron and one in the center, all connected to each other by ideal springs. The entire structure, which I'll call a "blob," resembles a '''non'''-ideal ball. Put one of these into the ideal box and set it in motion with a gentle and identical force on each of those billiard balls, so that they are not moving with respect to each other and the whole blob moves together.
::: Initially, the blob moves as a whole, and you can calculate the kinetic energy just by observing the blob; 1/2 mv<sup>2</sup> where m is the total mass of the blob and v is the velocity of the blob as a whole.
::: But when it strikes the walls, the billiard balls are going to hit it at more or less random times. The result is that the balls in the blob are going to start to acquire motion ''relative to each other,'' and soon there is going to be lots of relative motion ''within'' the blob.
::: This relative motion represents kinetic energy that belongs to individual billard balls within the blob, not to the blob as a whole, so because of conservation of energy, the energy we can ascribe to the blob as a whole is going to decrease, and so is the average velocity of the blob.
::: I '''think''' that what the Second Law is saying is that the way in which the blob hits the wall is essentially random, and that with each impact, statistically, more and more energy is going to end up in the form of billard balls oscillating with respect to each other within the blob, and less and less organized motion of the entire blob as a whole.
::: So that whereas the motion of the single billard ball "never stops," after a while the motion '''of the blob''' has stopped, and instead you just have a stationary blob with the billard balls within it oscillating on their springs.
::: In other words, the behavior of the system has ''degraded'' from observable motion of the blob as a whole to less-observable relative motion of the billiard balls within the blob. The system is in a less organized or "heat-like" state.
::: However, because in this case we're talking about fairly large particles and a fairly small number of them, it is clear that the system is still "in motion," just on a smaller scale, and since we posited that the box, the springs, and the billiard balls are all ideal (and don't absorb energy), by conservation of energy the blob also continues in motion forever.
::: Now, we go one step further and still keep the idealized, closed system with vacuum and perfect walls, but instead of a billard ball we use a real rubber ball. What the Second Law says is that the mechanical energy of the bouncing ball, 1/2 mv<sup>2</sup> where we can measure the "velocity" of the ball as a whole, inevitably and statistically degrades into heat; the ball "loses energy" with each impact with the wall, the measurable v decreases, and eventually it comes as close to "stopping" as we like. Conservation of energy says energy hasn't really been lost; it's been transformed into heat energy. The ball is warmer than before, meaning the molecules within it are moving, and since we've defined the system to be closed, it won't cool down. '''It''' has stopped moving, but there is still '''motion.'''
::: So, I think the whole thing becomes a sterile exercise in what we mean by "forever," and how close we can approximate ideal conditions with realizable machinery, and whether the motion of molecules due to heat counts as "motion."
::: I don't think Second Law has anything to say about ''how fast'' entropy increases, or how close we can come to an ideal situation where entropy doesn't decrease at all.
::: In a way the two are related, because "frictionless pivot," for example, means "no entropy increase in the form of heating at the pivot."
::: Probably the place where the Second Law comes into play is that it says that even if you have a perfectly idealized "closed system," within that system ''energy'' won't be lost, but nevertheless energy ''observable as macroscopic motion'' can still degrade into heat energy ''no longer observable as macroscopic motion.'' [[User:Dpbsmith|Dpbsmith]] 09:40, 5 January 2007 (EST)