# Information, Thermodynamics and Life: A Narrative Review
## 2 Information and Life
### The Old Problem of [PERSON]'s \"Demon\"  
To better understand the meaning of information, we need to refer to [PERSON]'s Demon. This is an imaginary experiment devised by [PERSON] and connects the concepts of information and entropy with energy. This hypothetical experiment was designed to better understand and possibly 'overturn' the second law of thermodynamics which forbids the production of energy from scratch in a closed system. [PERSON] first published his conception of this imaginary experiment in 1871 in his book _Theory of Heat_[23], in a section on the limits of the second law of thermodynamics. [PERSON]'s term, 'demon', is attributed to [PERSON]. In the device of the experiment, a \"being\" capable of knowing at any time the speed and position of each molecule of a gas, in a container divided into two parts, pulls and opens a door at will and allows the \"cold\" (low speed) and the \"hot\" (high speed) molecules to separate. The door is allowed to close immediately after the selective passage of each molecule and the spring returns to the demon the energy it expended to open it. In the end, without giving energy to the system, which is considered isolated from the environment, one side of the box appears with high-speed particles, thus hot gas, and the other with low-speed particles, thus cold gas, (and possibly different pressure in one container from the other). This leads to an increase in the energy of the system, since we can combine the two heat tanks (left and right), with different temperatures, with a heat engine in order to produce work (Figure 3). However, the existence of a \"being\" with such characteristics, capable of performing all this information processing without wasting energy, at least as much as it produces by separating particles to hot and cold, proves impossible. To transform a disordered system (mixed molecules) to an ordered system (the molecules are organized spatially according to their velocities), that is, to obtain information (which describes a more organized system) from the processing of the data of each molecule, requires energy expenditure. The increase in information, resulting from the description of a more organized system, is equivalent to a decrease in entropy, which is prohibited by the second law of thermodynamics for a closed system. This experiment was given many interpretations and finally it turned out that the second law of thermodynamics applies. To be more precise, the demon must waste energy to monitor and record all their molecules and velocities and decide which is slow and which is fast as they are constantly exchanging energy with each other. It needs to constantly send messages to each molecule in order to return the coded information of their position and speed, or to be able with some series of measurements to be able to receive this information. This information must be stored and processed in order to decide when to open and close the door, for example it must be careful not to be a \"fast\" molecule near the door and in the direction of the \"cold\" container when it opens. One of the most famous answers to the problem was proposed in 1929 by [PERSON] [24] and later by [PERSON] [25]. According to their interpretation, since the demon and the particles interact, we must consider that the total entropy of the system is the combination of the entropy of both. The increase in the entropy of the demon from the measurement process would ultimately be greater than the decrease in the entropy of the particles, so that the total entropy would increase. Assuming that information transfer by molecules was performed by a reversible thermodynamic processes, [PERSON] argued in 1960 that the measurement process was possible without increasing entropy [20]. This reasoning would apply, provided that the information collected and stored is not erased, as any reduction in information equates to an increase in entropy. The final proof came in 1982 from [PERSON].