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Regarding the second law of thermodynamics

שו”תCategory: generalRegarding the second law of thermodynamics
asked 10 months ago

Hello Rabbi, as I understand it, this is less of a question style than is customary in this forum (a purely scientific question), but I will ask it here anyway. I was exposed to the concept itself from your books.
As far as I understand (and I may not have understood it correctly), according to the second law of thermodynamics, entropy will inevitably increase over time, this is a one-way and irreversible process. But I also remember hearing that if we put any substance in an isolated box for an infinite amount of time, it will take on any possible form – including returning to its original form. Doesn’t this contradict the second law of thermodynamics?
It’s certainly possible that this question is stupid and stems from my almost complete lack of understanding in the field, but I’d be happy to answer it nonetheless.
Thank you, Sheila
Let’s say we put an apple in a sealed, heat-proof box.


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מיכי Staff answered 10 months ago
I will try to explain as clearly as I can. First of all, it’s not like what you put in a box will take any shape. It can take many shapes. To answer your question, consider a closed box with gas inside. 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 disperse in the box. What does it mean to disperse? To move between all possible states (each particle can be anywhere and at any speed). When you look at such a situation over time, you will see a uniformly distributed gas throughout the entire box all the time. Therefore, there is a direction to the process: from a concentrated gas to a gas dispersed in the box. The reverse process will not happen. What does it mean not to happen? Think about a situation where you start with a gas that is dispersed throughout the entire box and follow it. There may be one moment when it is all concentrated in one corner (as with the bag). But it will be for one moment and this special state will disappear immediately. Thermodynamics deals with constant states (what is called equilibrium). Such a state has a direction: from the concentrated state to the dispersed state and not vice versa. This is even though each microscopic state has similar weight and you can reach from any state to any other state symmetrically. To understand how this happens, entropy calculations must be done, but intuitively it is completely clear.

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שילה replied 10 months ago

Thank you very much, are you actually saying that in infinite time matter will not take on any possible form? Isn't that a bit paradoxical? After all, that's also quite intuitive.

מיכי Staff replied 10 months ago

The apple will not turn into a flying elephant. Sometimes there is a limit cycle, meaning a cyclical transition between a certain set of states without exiting them (think of a movement that gets caught in a closed circle).

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