# Different Strokes for Different Folks in Defining Entropy: Hypersonic Heat Shield

#### Atlan0001

I keep on finding different definitions, even opposing definitions, of entropy! But still something hopeful in hypersonic shielding:

#### billslugg

The simplest definition of entropy is disorder. In an isolated system, disorder cannot spontaneously go down, it must always go up.
Another definition is the amount of unusable heat in an isolated system. Say there is a hot glass of water and a cold glass of water next to each other in an insulated box. At the start, a heat engine could do work by moving heat from the hot one to the cold one. Eventually both glasses will be the same temperature. In each case there is exactly the same number of BTU's in the isolated system but in the early case, some of it usable, some is unusable. In the second case, none is usable, all is unusable. Entropy is at its max.

LKK

#### Atlan0001

The simplest definition of entropy is disorder. In an isolated system, disorder cannot spontaneously go down, it must always go up.
Another definition is the amount of unusable heat in an isolated system. Say there is a hot glass of water and a cold glass of water next to each other in an insulated box. At the start, a heat engine could do work by moving heat from the hot one to the cold one. Eventually both glasses will be the same temperature. In each case there is exactly the same number of BTU's in the isolated system but in the early case, some of it usable, some is unusable. In the second case, none is usable, all is unusable. Entropy is at its max.
You can't deal in such absolutes, Bill. I define higher energy weeds, the natural order of higher energy frontier (and my once-upon-a-time lawn), as "order" and the perfectly smooth lower energy lawn of cloned grass as the anarcho-tyranny natural disorder -- the Dystopia -- of Utopia. Equilibrium is an entropic run to cold, and an exactly equal but opposite entropic run to heat (the energetically expanding look of universe), the trojan third being always equilibrium.

It's almost too bad that we see it so differently, so opposed. That we then see the universe so differently, so opposed. You see the arrow of time a dead straight line from heat to cold . . . the END! I see the arrows circling the circle, both clockwise and counterclockwise, the "wheel(s) of time" incapable of end in dead still frozen absolute universe. I've described how it must work again and again, and that the universe in very large, and very small, looks to back me up in that description.

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billslugg

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