Q&A: Regarding the Second Law of Thermodynamics
Regarding the Second Law of Thermodynamics
Question
Hello Rabbi, as I understand it this is less the style of questions usually asked on this forum (a purely scientific question), but I’ll ask it here anyway, since I was exposed to the concept itself through your books.
To the best of my understanding (and it’s entirely possible that I didn’t understand it properly), according to the second law of thermodynamics, entropy must necessarily increase over time; this is a one-way and irreversible process. But on the other hand, I also remember hearing that if we put some substance inside an isolated box for an infinite amount of time, it will take on every possible form—including returning to its original form. Doesn’t that contradict the second law of thermodynamics?
It is entirely possible that this question is foolish and stems from my almost complete lack of understanding of the subject, but I’d still be glad for an answer.
Thanks, Shila
Suppose we put an apple inside a sealed box that is impervious to decay
Answer
I’ll try to explain as clearly as I can.
First, it’s not that whatever you put in the box will take on any possible form. It can take on many forms.
As for your question, consider a closed box containing gas. If initially the gas is all concentrated in a bag in one corner, and now the bag is torn open, the gas will begin to spread throughout the box. What does it mean to spread? To pass through all the possible states (each particle can be in any place and at any speed). When you look at such a situation over time, you will see gas uniformly distributed throughout the entire box all the time. So the process has a direction: from concentrated gas to dispersed gas in the box. The reverse process will not happen. What does it mean that it will not happen? Think of a case where you start with gas dispersed throughout the whole box and track it. There could be a single moment when it is all concentrated in one corner (as with the bag). But that would last only for an instant, and that special state would disappear immediately. Thermodynamics deals with persistent states (what is called equilibrium). For such a state there is a direction: from the concentrated state to the dispersed state and not the other way around. This is despite the fact that every microscopic state has similar weight, and you can get from any state to any other state symmetrically.
To understand how this happens, you need to do entropy calculations, but intuitively it is completely clear.
Discussion on Answer
An apple will not turn into a flying elephant. Sometimes there is a limit cycle, meaning a cyclical transition among a certain set of states without leaving them (think of motion that gets trapped in a closed circle).
Thank you very much. So in effect you’re saying that over an infinite amount of time, matter will not take on every possible form? Isn’t that somewhat paradoxical? After all, that too seems fairly intuitive.