Maxwell's demon challenged the second law of thermodynamics, thereby attracting much research attention for over a century. As this system does not exchange energy with circumstances, the energy is conserved. Consequently, it is useless to analyze this problem from an energy viewpoint. Therefore, numerous studies have been conducted to analyze the entropy change in this process. Further, a conclusion was reached that the irreversibility process of obtaining information necessarily induces the entropy production, so the whole system still satisfies the entropy increase principle in an isolated system. This study demonstrates that Maxwell's demon still follows the second law of thermodynamics from an energy conservation viewpoint in a macroscopic reversible process. Thermomass theory recognizes the mass nature of heat in the transfer process and proposes a new quantity called thermomass, which is the extra mass due to thermal vibration. Additionally, thermomass energy is a kind of relativistic energy that is carried by thermomass, and it is far smaller than that carried by the rest of the mass. To move the same amount of heat from a lower to a higher temperature region, the reversible thermodynamics analyses show that what is supposed to be input into the system can be thermomass energy and not necessarily external work or heat (conventional energy). Moreover, the negative entropy carried by heat corresponds to the positive thermomass energy and not the product of entropy and temperature. Inspired and motivated by these results, this study reconsiders the revised Maxwell's demon system following the viewpoint of thermomass energy and relativistic energy and explains their influences on the system from the introduction of information. There are two Maxwell's demons and two gates, and the chosen molecules can pass the gates one by one. Furthermore, when there is a molecule passing through gate 1 from left to right, another molecule concurrently passes through gate 2 from right to left with a different speed. Thus, the numbers of molecules at both sides remain unchanged. Thus, these two parts have constant density and heat capacity, thereby making quantitative and qualitative analyses easier. Moreover, the whole system is in equilibrium state at the beginning. Then, Maxwell's demons exchange the thermal energy and temperature difference arises between these two sides. The entropy in the gas system decreases, whereas the thermomass energy increases. No work or heat is input into this system, while the same amount of heat is carried from the low-temperature region to the high-temperature one. If the exchange process is slow enough, this process can be considered as a reversible one. Thus, the entropy and thermomass energy are supposed to be conserved, implying that the information obtained by the demons during the operation concurrently carries the negative entropy and positive thermomass energy. Furthermore, the energy carried by the information is called information energy, which is also a kind of relativistic energy. Therefore, it is the information energy that has been reversibly input into the system to promote heat transfer and not the external work or heat. In this way, the reversible energy conservation analyses demonstrate that Maxwell's demon problem still follows the second law of thermodynamics. Moreover, this process reveals that the negative entropy carried by the information corresponds to the positive thermomass energy and not the product of entropy and temperature. © 2021, Science Press. All right reserved.
