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> the idea that any system tries to minimize its energy is just nonsense. Very interesting, I feel somewhat misled if this is true.

by platz 8y ago

ktrask 8y ago
In a closed physical system energy is conserved. For example, if you have container that has Helium on one side and Oxygen on the other. Both gases will mix, i.e. maximize entropy. Minimizing energy would mean, the gas would reach absolute zero.
philipkglass 8y ago
I don’t know if it’s always true in the context he mentions - particle physics - but I’m pretty sure it is not always true in e.g. chemical physics. Both entropy and enthalpy matter. The reaction of potassium vapor with bromine vapor is spontaneous even though the solid crystalline potassium bromide product has lower entropy than the gas phase reactants. In the comments on the article Milkshake also gives an example of an entropically driven spontaneous reaction: ammonium nitrate dissolving in water. You need enthalpic and entropic terms to predict if a reaction is thermodynamically spontaneous. And if you want to calculate if a thermodynamically spontaneous reaction will appreciably take place, you also need to understand the actual intermediates between A + B -> C, and find reaction barrier heights for all of them, and weight them appropriately. This is still very hard to do so information of this sort is mostly determined by experiment rather than calculation.
lostmyoldone 8y ago
I'll try to answer as best as I can. While it is true that a system doesn't minimize the energy of the entire system, as energy IS conserved, except apparently for a miniscule amount due to the universe expanding, it is also true that locally it often looks like minimizing energy in some sense. If you have a vibrating spring on a table, after a while it will have stopped because it has created motions in the air and in the table, spreading out at the speed of sound in whatever material it encounters, and some of it also at the speed of light in the form of thermal radiation. So, looking only at the spring, it is minimizing it's energy by returning to a stable quiscent state. In the same way, if we look at a sphere with radius as "the speed of light times the time from the start of the experiment", all energy can still be accounted for within that volume. One system, several perspectives. I think the minimization model is often used because the statistical model of entropy can be hard to explain, as it simultaneously allows complex relatively ordered systems like us humans to exist, while at the same time explains the almost complete disorder of thermal energy and how it disperses. Thus, since the energy minimization of the local state can be used as a relatively truthful proxy, we learn that model first. With the possible bonus that it also allows one to temporarily avoiding some rather (afaict) gnarly math until you actually have the tools you need to understand it.
cygx 8y ago
Well... https://en.wikipedia.org/wiki/Minimum_total_potential_energy_principle https://en.wikipedia.org/wiki/Minimum_total_potential_energy... https://en.wikipedia.org/wiki/Principle_of_minimum_energy https://en.wikipedia.org/wiki/Principle_of_minimum_energy

> the idea that any system tries to minimize its energy is just nonsense. Very interesting, I feel somewhat misled if this is true.

by platz 8y ago

> the idea that any system tries to minimize its energy is just nonsense. Very interesting, I feel somewhat misled if this is true.

ktrask 8y ago

In a closed physical system energy is conserved. For example, if you have container that has Helium on one side and Oxygen on the other. Both gases will mix, i.e. maximize entropy. Minimizing energy would mean, the gas would reach absolute zero.

philipkglass 8y ago

I don’t know if it’s always true in the context he mentions - particle physics - but I’m pretty sure it is not always true in e.g. chemical physics. Both entropy and enthalpy matter. The reaction of potassium vapor with bromine vapor is spontaneous even though the solid crystalline potassium bromide product has lower entropy than the gas phase reactants. In the comments on the article Milkshake also gives an example of an entropically driven spontaneous reaction: ammonium nitrate dissolving in water. You need enthalpic and entropic terms to predict if a reaction is thermodynamically spontaneous. And if you want to calculate if a thermodynamically spontaneous reaction will appreciably take place, you also need to understand the actual intermediates between A + B -> C, and find reaction barrier heights for all of them, and weight them appropriately. This is still very hard to do so information of this sort is mostly determined by experiment rather than calculation.

lostmyoldone 8y ago

I'll try to answer as best as I can. While it is true that a system doesn't minimize the energy of the entire system, as energy IS conserved, except apparently for a miniscule amount due to the universe expanding, it is also true that locally it often looks like minimizing energy in some sense. If you have a vibrating spring on a table, after a while it will have stopped because it has created motions in the air and in the table, spreading out at the speed of sound in whatever material it encounters, and some of it also at the speed of light in the form of thermal radiation. So, looking only at the spring, it is minimizing it's energy by returning to a stable quiscent state. In the same way, if we look at a sphere with radius as "the speed of light times the time from the start of the experiment", all energy can still be accounted for within that volume. One system, several perspectives. I think the minimization model is often used because the statistical model of entropy can be hard to explain, as it simultaneously allows complex relatively ordered systems like us humans to exist, while at the same time explains the almost complete disorder of thermal energy and how it disperses. Thus, since the energy minimization of the local state can be used as a relatively truthful proxy, we learn that model first. With the possible bonus that it also allows one to temporarily avoiding some rather (afaict) gnarly math until you actually have the tools you need to understand it.

cygx 8y ago

Well... https://en.wikipedia.org/wiki/Minimum_total_potential_energy_principle https://en.wikipedia.org/wiki/Minimum_total_potential_energy... https://en.wikipedia.org/wiki/Principle_of_minimum_energy https://en.wikipedia.org/wiki/Principle_of_minimum_energy