Originally posted by twhitehead
My understanding was that it arose directly from basic mechanics and was a property of the direction of time.
So explain to me why a large number of particles in an ordered system evolves into a disordered system, but not the reverse.
If time is seen as flowing backwards, why is disordered to ordered now possible? Surely there is some difference between the two time directions that causes this?
I need to give it some more thought.
So explain to me why a large number of particles in an ordered system evolves into a disordered system, but not the reverse.
I am not an expert on this so please would someone correct me if I am wrong but, if I understand this correctly, the answer is that it only generally does but the reverse can happen! It is just that, the larger number of particles involved, the less probable it is to do it in reverse during any arbitrary chosen period of time T. Consider there being a continuum from a very 'small' number of particles to being a very 'large' number of particles and along that continuum the probability of 'reverse entropy' during period of time T steadily smoothly and seamlessly shifts its value.
Lets consider this just for thermal movements of particles:
Imagine a closed system containing, say, nothing but a simple gas.
Now imagine two distinct areas of gas distinct from each other only by a difference of temperature so that the gas in one half of of this closed system is at 0C and the other at 100C.
Now, if we ignore thermal convection and radiative movement of heat and only consider the movement of heat through the collisions of randomly moving particles, you can imagine a simulation where it makes sense that, at the boundary between the two areas of gas,
generally, the hotter faster-moving particles in the warm side would transfer more momentum to the colder slower-moving particles in the cold side than the other way around simply because the hotter particles are moving faster. So that explains why heat would generally tend to move from the hot area to the colder area in this case than the other way around.
BUT, this is only generally the case because of the way probabilities work out on a large scale.
Even if there are millions of gas molecules in this closed system, there must be a small probability ( albeit vanishingly small ) that, within a particular moment of time, the random motion of particles could for an instant be just exactly such that heat travels the other way causing a measurable greater temperature difference rather than a smaller temperature difference.
The larger the number of particles involved, the lower that probability. But, if you imagine, say, just four particles involved, and imagine the simulation of that, you should see it would happen very often. And, if you imagine just a few more particles involved, you should see it would happen less often. But, no matter how many more particles you add, you can never quite make it
never happen.