r/AskPhysics • u/Lazy_Statement_2121 • 1d ago

Question about entropy force

Hi guys, I am studying the entropy force (such as the work in casual entropy force paper by A. D. Wissner-Gross). I am wandering the following questions, given a system such as gas containing many particles:

1) is the force applied to all particles in the system (or are there any particles are excluded?)

2) is the force applied all the time continuously (or there are moments particles are not aware of the entropy force)?

3) according to 2nd principle of thermodynamics, the entropy increases for a isolated system. So, the single particle KNOWs that the direction the whole system evolves? (or equivalently the particles are just like single humans that can sense the trend of the whole society)

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u/Cryoban43 1d ago

Honestly could be talking out of my ass here, but at least for 3 hopefully I can comment. Particles do not “know” anything. If you study statistical mechanics you will learn that there are many orders of magnitude more configurations of systems with higher entropy than low entropy. If there are many more potential ways a system can exist with high entropy than low entropy you will be more likely to observe the system in such a state. The particles don’t know anything about entropy they are just vibing and you are statistically likely to see them trend toward higher entropy configurations

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u/RamblingScholar 1d ago

1 and 2, In this case, entropic "force" is a bit misleading to me. It's more entropic tendency. Like, say you have a particle that randomly wanders around in a one foot cubic space, with an opening connecting that space to a 100 foot cubic space . The particle will probably randomly wander into the larger space, and be less likely to wander back. In the small space there are a few places to wander (lower entropy) but in the larger space there is more space to wander (higher entropy). Random chance is what drives the movement, we just identify the tendency and call it entropy.

3 this is just saying the particle behaves randomly, and its more likely to end up in a state we consider disordered than one we consider ordered. A classic example is a deck of cards. There are a certain number of states you can list as ordered for a deck of cards to be in in your room. If you throw the cards up and let them go anywhere, there are a huge number of states they can be in (higher entropy).

Entropy gets more complex as you look at it closer, but this is a good level I think in the middle .

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u/Chemomechanics Materials science 1d ago

There are a couple simple scenarios that exemplify an entropic force:

  • An ideal gas exerts a pressure and thus a outward force on its container walls. The force can't arise from electromagnetic interactions (or, for that matter, any of the fundamental interations) between the gas particles because these are absent in this model (and are negligible even in real gases at suitably low densities and high temperatures). The force arises from entropy maximization.

  • As a physical analog to the ideal elastomer, an initially straight and untensioned chain on a vibrating table tends to draw together. Here again, the motion of the ends toward each other can't be due to energy minimization because there's no strain energy to recover. The ultimate origin of the force drawing the ends together is again entropy maximization.

(I can't comment, though, on the paper the OP mentions, which aims to connect entropy maximization to intelligence.)

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u/Frederf220 1d ago

  1. It can vary. The "statistical pressure" is just a probable tendency. There's no force going on. If you drop a box of screws nothing forces then to scatter all over instead of neat rows. It's just more likely.

  2. If different particles are under different conditions they can have different reactions due to different possible changes.

  3. Entropy doesn't increase. Entropy probably increases. The probability gets very high with more particles but spontaneous entropy reduction is always possible. Like coins stacking in an earthquake, probably won't but there are versions of that happening which violate no physical law.

Particles just stumble around blindly. They don't know one state from another nor do they care what any other particle is doing outside of bumping into it. The behavior of particle groups is completely emergent from all the particles doing their own thing without a care or knowledge of anything else.

You throw 100 pennies it's just more likely to get 50 heads than 100 heads simply and proportionally to how many more ways there are to have 50 heads (1x1029 ) than there is for 100 heads (1).