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[OngBonga]

The "thermodynamic arrow of time" is directly related to the 2nd Law of Thermo.

Something I was watching about entropy yesterday seemed to suggest that it's the result of probability. This confused me, because they kept saying heat NEVER spontaneously moves from cold region to warm. Surely if probability governs this, then we should expet to see it happen rarely?

[MadMojoMonkey]

Something I was watching about entropy yesterday seemed to suggest that it's the result of probability. This confused me, because they kept saying heat NEVER spontaneously moves from cold region to warm. Surely if probability governs this, then we should expet to see it happen rarely?

Thermodynamics describes systems with large numbers (greater than Avogadro's number) of particles.
The statements of thermodynamics came about to explain and understand experimental results prior to the quantum description of nature.
The statistical statement that heat never flows spontaneously from cold to hot is experimentally verified for systems of particles.
Statistically speaking, the "never" is an overstatement, but not a terrible one. The equipartition theorem states, broadly, that systems of particles tend toward thermal equilibrium, and not toward isolated regions of high and low temperatures. "Tend toward" is a statistical statement, and we generally look at the large ensemble of particles as a dynamic equilibrium.

A moving pendulum is in dynamic equilibrium. At any given time, it's total energy is constant, but whether that energy is kinetic or potential varies.
This is similar for temperature. At any given time, the individual atoms of a molecule can have dramatically different thermal energy, but the overall energy of the molecule remains relatively constant and in thermal equilibrium with its environment.


However, Quantum Mechanically, there is no such thing as entropy. All QM processes are time-reversible. I.e. any process that is observed to happen forward in time is also observed to happen in another experiment, but in the opposite order, and with particles and anti-particles swapped.

So entropy is a breaking of symmetry at some scale of particle interactions?