Let's recapitulate and state both the Maxwell's demon and the official most-widely accepted solution to the paradox, namely Landauer's principle:

  1. The Maxwell demon paradox:

    A demon just by observing But Never Touching -hence no energy is transferred- the molecules of two chamber of gas separated by a door, opening and closing the door between the two sides at the right moments can create a temparature difference, therefore creating an entropy decrease, which can be used to do useful work, thus violating the Second Law of Thermodaynamics.

  2. Landauer's principle the officially accepted solution to the paradox:

    Not when the Demon acquires the information about the molecules of the gas, but when erases the accumulated information the Demon's memory offsets the created entropy decrease and amount of work extracted by an amount of dissipated heat-energy at least as much as much useful work was done by the action of the demon. Therefore the dissipated heat to the environment offsets the entropy decrease created by the action of the demon, thus the Second Law of Thermodynamics is preserved.

The problem: This is a so horribly shallow superficial explanation, that barely stands until said. Because let's see:

  • It is not mandatory to extract the created temperature difference into useful work, we can decide to stop the demon to do it's job, keep the information in the memory, and just to let the two sides to come back to thermal equilibrium, and because we settled on the conditions that the Demon doesn't exchange any energy with the molecules, the total energy is the same constant all the time, but if we now ask the Demon to erase it's memory, still has to dissipate heat-energy to the environment, because this is also settled in Landauer's erasure principle, but in this way energy is created on the top of already total constant energy, so the Conservation of Law of Energy (1st law of thermodynamics) is violated. Of course with this example can be seen that by applying Landauer's principle, and dissipate heat on erasure the 1st Law of Thermodyanics - the conservation of energy - is violated all the time, no matter what order the things are done.

I'm waiting for answers please. Thank you


1 Answer 1


I think, from the wording of the question, that you might not have understood the idea of Landauer's principle quite right. The principle asserts that if a system goes from one of two macroscopically distinct initial states to one and the same final state, by interacting with an environment which goes from its initial state to a final state described by the same set of macroscopic parameters (e.g. volume, temperature) in either case, then it must be that the entropy of the final state of the environment has gone up (because its macrostate now includes twice as many microstates as it did initially). Landauer then points out that in order to cause such a change in a thermal environment at finite temperature, one must supply not only entropy but also energy to the environment.

This principle does not assert that such an energy supplied to the environment is simply conjured up from nowhere, in violation of the first law. Rather, it has to be extracted from the system which is interacting with the environment. In the case of the Maxwell daemon, although no energy transfer is needed in order for the daemon to acquire information about microstates of a system, an energy transfer is required during the memory erasure. There is no contradiction between these two statements; they are two different processes.

  • $\begingroup$ So you want to say in order for the Demon to erase it's memory it has to extract energy from the "system" - which I think it means our experimental setup - But what if - for erasure - I take away the Demon's memory storage, far away, how it will extract energy from the "system" ?!? $\endgroup$
    – Gergo
    Sep 2, 2020 at 12:49
  • $\begingroup$ @Gergo No, any energy leaving the daemon is extracted from the daemon. If it can replenish the energy by transfer from the system, then that is the normal scenario. If it cannot then either we do not have a thermodynamic cycle, or else some further system is involved. $\endgroup$ Sep 2, 2020 at 13:31
  • $\begingroup$ @Gergo No, it means the hard disk wiper won't work unless it's plugged in. $\endgroup$
    – user253751
    Sep 2, 2020 at 14:10
  • $\begingroup$ @andrew-steane I guess doesn't make too much difference ... According to Clausius 2nd law of thermodynamics I'm not suppose to be able to create any thermodynamic cycle without first invest energy, but as we stated the Demon doesn't. $\endgroup$
    – Gergo
    Sep 2, 2020 at 14:13
  • $\begingroup$ @user253751 You want to say that an erasure operation is always more energy demanding than writing? And this would be the basic principle of Landauer's law?! $\endgroup$
    – Gergo
    Sep 2, 2020 at 17:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.