Francesca Agnesod: Around 1870, the Scottish physicist James Clerck Maxwell proposed a thought experiment which was supposed to refute the second law of
thermodynamics: the entropy of an enclosed system is a function growing along with time until it reaches a state of equilibrium. Let's imagine with him a box in which gas molecules are occupying uniformly the space. The molecules, moving about at different paces, mix spontaneously, thus creating a situation of thermodynamic equilibrium.
Let's imagine now a partition with a small door in the middle that separates the box in two parts, A and B. A microscopic being, a demon, stands guard over this door and has the faculty of opening or closing it. Always unfailingly, this demon is capable of following the course of the molecules and of detecting the ones running at a speed higher than the average speed; as soon as it perceives them, it opens the door and let them get into the B part of the box. Slow molecules also move to the A part.
After a while, the gas hold inside the B part will acquire a temperature higher than the one hold inside the A part. The demon will then have established a situation of non-disorder and non-uniformity. It will thus have succeeded in creating and maintaining a difference of temperature between the two parts, and would have avoid the natural dissipation of energy, which would imply that the hot part yields its heat to the cold one. The demon would then be able to deviate the spontaneous flow of phenomena and to reverse the arrow of time.
Delving deeper into Maxwell's experiment we could suppose that the demon, armed with its priceless trident, would be in a position to transfer heat from cold to hot bodies. In the same way we could presume that black ink spread into water could concentrate to form a drop, that the scattered fragments of a broken glass could spontaneously recompose their initial form, etc.
These phenomena, whose irreversibility seems so much obvious to the point it becomes ordinary, would have the same chance to be reversible. Maxwell's demon differs from any common experimenter only because of its dimensions: minuscule and eminently intelligent, it has the capacity for detecting at a very tiny scale the differences between particles that we are not likely to perceive. While we remain with our ignorance of imperfect observers, it disposes of a perception that enables it to manage matter at will. In the infinitesimal time of a microscopic wink it knows when a molecule is quick enough or when a drop is black enough to be selected. Its point of view is insomuch objective that it doesn't need any measuring device.
Even though the impossible existence of such a creature capable of refuting the entropic law within an enclosed space has been demonstrated, we could however imagine that in a distant future paradoxically becoming a deferred and reactivated past some phenomena will flow in a direction as well as another. In this scenario what will be from then on our perception of reality? To which extent the latter would be affected?

Édith Dekyndt: The philosophical concept of reality is used in particular to designate our real-life experiences of objects, temporalities, sensations. Insofar as it is doubtlessly impossible that two persons have the same experience of reality there would be much perceptions of reality as human beings having existed.
In a novel published in 1953, Childhood's End, Arthur C. Clarke imagines creatures coming from space, the Overlords. They have got a cerebral and scientific mastery extremely higher than the one men have. Nevertheless they are incapable of understanding what happens with humans when they play or listen to music. Music is just a noise for them. This expression which profoundly touches men of any culture and seems to be at the origin of our language affects nothing in them. They don't perceive.
Taking into account that the human scale is the meter, the phenomena occurring at our scale don't act in the same way as the ones acting at a nanometer or a zettameter scale. We can nowadays imagine these differences and observing them thanks to machines that give us an image, but this image is a computing calculation, not an optic experience. We can't see them because their scales are the ones of different worlds. And we can imagine that the passing of time as we know it exists in our perception, only there.
Some hours after having received your question by mail I was drawing while listening to the serial Numbers. (This is a sitcom where a mathematician is supposed to help the resolution of police investigations). In the episode that was broadcast that day the mathematician used the principle of Maxwell's demon to unravel the intrigue. I never heard about Maxwell's demon before reading your question.

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