# Time reversal using entropy

We characterize the flow of time with respect to entropy where energy is going from an unstable state to a stable state. Basically from high energy to low energy. Would this mean any particle or anything that goes from a lower energy state to higher energy state experiences reverse entropy and thus time reversal? What would the particle experience as such if it's an isolated system and is going against the second law of thermodynamics and observing everything else?

• I think you should try to clarify your question and its starting point. Entropy increases in isolated systems. The energy of an isolated system is constant. It does not decrease. Energies of single particles contribute to the total internal energy of the system but are not the same thing. Mixing single-particle energies and internal energy is a common source of confusion in statistical mechanics. Apr 22, 2021 at 12:42
• I think the assumption that the second law of thermodynamics actually causes the arrow of time is disputed, to say the least. I mean, entropy decreases in lifeforms, compensated by an increase in the surroundings, if you zoom out and observe the whole system earth (which is still far from being isolated). Wouldn't the "arrow of time caused by second law of thermodynamics"-theory then propose that time runs backwards for lifeforms? Apr 22, 2021 at 13:52
• @Koschi What does it mean that "entropy decreases in lifeforms"? Apr 22, 2021 at 15:08
• @Quillo A biological process like photosynthesis uses energy from the sunlight for a chemical process/reaction where the results (sugar and O2) have lower entropy than the chemicals going in (CO2 and water). This is just a basic example of the complex processes found in biochemistry. I think the second law of thermodynamics does not really apply here, since sunlight was involved, so neither the plant, the ecosystem, nor the whole earth are a closed system. If you would look at the whole energetically closed system, let's say Sun and Earth, the total entropy still increases, as I understand it. Apr 22, 2021 at 17:21

Basically from high energy to low energy.

You seem to misunderstand what energy and entropy are. Let me try to explain.

The energy of any closed system is constant, and can never change unless it interacts with its surroundings. However, the energy can be distributed in different ways if there is more than one particle.

Entropy is, informally, a measure of how distributed energy is in a system of many particles. For example, a hot cup of tea standing on a cold table has low entropy, but the entropy increases as the heat energy spreads from the hot tea to the table until they are the same temperature.

There are more ways of distributing a fixed amount of energy among all the particles, than having it concentrated in a few particles. Therefore finding all the energy in a few particles is very unlikely; it is very likely that the energy will become more distributed as time goes on. This is the basis of entropy 'always' increasing with time (in human-size systems there are so many particles that the probability of entropy noticeably decreasing is astronomically small).

What would the particle experience as such if it's an isolated system and is going against the second law of thermodynamics and observing everything else?

The entropy of a single particle is undefined (at least in classical mechanics, I don't know how entropy is defined in quantum), since it is related to the distribution of energy.

Since the 2nd law of thermodynamics is a statistical law, though, it doesn't always have to hold. In other words, it is theoretically possible to carefully set up a closed system so that the entropy increases: for example, if you had really really tiny pincers, you could place all the air molecules in a room so that, when you let them go, they all fly towards one side of the room, and all collide with one single molecule, giving it all their kinetic energy for a brief moment. The entropy of only one molecule carrying all the energy is astronomically low, so the entropy of this closed system initially decreases. However, after that the particles will continue to move and collide chaotically, and the entropy will eventually increase again. The problem is this is practically impossible to do on that scale, and the motion of molecules in real life is practically random. Therefore the statistical laws of thermodynamics give an excellent description of the behaviour of gases, and other processes involving large amounts of matter.

• Thank you! So for a closed system, entropy can decrease(rarely)/increase but the overall energy can't change. Its the distribution of energy that changes and accounts for the change in entropy Apr 24, 2021 at 7:43
• @Ruchi excatly! And entropy is directly related to the probability of the energy being distributed the way it is. Apr 24, 2021 at 19:27