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Interpretation of the phenomena.
Itinerary 1.3, page 1.
1.3.1 According to the unidirectional thermodynamics.
According to the thermodynamics, as defined to day, all the processes entail in the Universe a diminution of energy in a useful form, until all the energies will be completely degraded, and no process will be possible anymore.
Because of this reason, I call the current thermodynamics, as defined today, "one-way thermodynamics", or unidirectional thermodynamics. It is based on a key concept, that of entropy.
In a simple way to put the matter, it may be said that entropy indicates the probability of the condition of a system (which may be an animal, a plant), to procede from order to disorder, from usefulness to uselessness, towards a degraded condition, and eventually to the end.
A probability which tends to grow, but which, in a single system, may also decrease, thanks to exchanges with the environment, at the expense of it.
Each exchange with the environment, meant to decrease entropy in a system, in contrast with its trend, involves degradation of the energy, available in useful form, outside of that system.
The heat death.
Coherently with what has just been said, it follows that at each exchange, the energy stock in the universe tends to degrade to a condition called "heat death", when there will be no more energy in a useful form, necessary to decrease entropy, in favour of any system.
In the one way thermodynamics, each decrease of entropy is local, and takes place at the expense of the energy of elsewhere in the Universe. After a work has been done, in the Universe, there will be less available energy in a useful form.
In other words, there would be only one way ticket, which would lead us inexorably to the heat death.
It is on the base of this logical discourse that Einstein said that the second law of thermodynamics was to be considered eternal.
All the energies in the Universe would be in the course of degrading, in a continuous process of utilisation, without exception, until the terminal levelling out of the energy differential, down to zero.