Corpus
In 1824, Sadi Carnot publishes Reflections on the Motive Power of Fire. He demonstrates that no heat engine can integrally convert heat into work. This limit is fundamental, independent of material, design, engineer's skill. It depends solely on the absolute temperatures between which the engine operates, expressed in Kelvin:
$$\eta = 1 - \frac{T_{\text{froid}}}{T_{\text{chaud}}}$$
An engine operating between 500K and 300K reaches at most 40% efficiency. The remaining 60% is dissipated. Not lost through negligence, dissipated by necessity. Absolute zero, 0K, is unattainable by the third law of thermodynamics. The denominator can never be null. Perfect efficiency is thus physically impossible, not as practical limit but as structural impossibility.
A reversible process, one that would theoretically achieve Carnot efficiency, is a succession of equilibrium states infinitely close to each other. Its duration would be infinite. For a transformation to occur at finite speed, disequilibrium is required. This disequilibrium creates entropy. A process without entropy production is a process that did not take place in real time. Inefficiency is the signature of transformation.
Clausius (1850) formalizes what Carnot had sensed: in every real process, the total entropy of the system and its environment increases. Engineers have worked for two centuries to improve efficiencies. Watt's steam engines reached 3%. Modern gas turbines reach 40%. Carnot's limit remains intact. We approach it. We do not attain it.
What remains on the other side of the limit is not waste to eliminate. It is proof that the process took place.
Doctrine
What is dissipated is not lost. It is what remains when one has subtracted what one wanted to do from what one had to traverse to do it. Efficiency tells what was obtained. The gap tells what it cost to exist.
Vecteur ouvert
Increasing entropy is not merely an efficiency limit. It is the physical definition of time's direction. Physics' fundamental equations are reversible, they function equally well in either direction. What distinguishes past from future is not inscribed in the laws of motion. It is inscribed in entropy's growth. One cannot rewind a real process because it has produced entropy, and this production is irreversible.
Landauer (1961) showed that erasing one bit of information necessarily dissipates $E = k_B T \ln 2$ joules. What information processing shares with the heat engine: every real transformation leaves a trace in what it could not convert.
The question is whether this trace, the entropy produced, the information dissipated, the missing efficiency, is simply what the universe deposits behind itself in advancing, or whether it constitutes a readable archive of what took place. And if readable, by whom.
