Comment by the_mitsuhiko
Comment by the_mitsuhiko 4 days ago
> What about entropy?
Look at 3.1. "spontaneous recombination of an unstable particle" for how this works.
Comment by the_mitsuhiko 4 days ago
> What about entropy?
Look at 3.1. "spontaneous recombination of an unstable particle" for how this works.
It's a poorly worded way of saying, if heat doesn't exist, and objects don't gain or lose energy, but only change configuration in a controlled way, physics is reversible. This is traditional time-symmetric Newtownian physics before thermodynamics. "What does down bounces back up."
Entropy is measure of information. If an object changes state according to some law or data that you don't know, then you can't predict the result, and you can't set up conditions to undo the changes. Thus you can't reverse it, because you don't know the original state. If you can explicitly model every particle, you can reverse it.
You can't uncrack an egg, because you don't know an egg works. But you can unopen a door, because you know how a door works. All you need is a perfectly elastic mirror, carefully placed to bounce the swinging door off of.
An expert egg mechanic can uncrack an egg with a precise arrangement of mirrors.
Yes but mirror boxes are not real, except maybe the case of quark color confinement inside a proton?
Thanks, and LOL:
we will model the spaceship as an idealized box with perfectly reflecting walls. This is necessary, because the second law of thermodynamics applies only to thermally isolated systems, to which we can assign a Hamiltonian [5, section 11] [Landau L and Lifshitz E 1980 Statistical Physics vol 5, 3 edn (Pergamon)].
Chasing down that source: https://ia802908.us.archive.org/31/items/ost-physics-landaul...
§11. Adiabatic processes
Among the various kinds of external interactions to which a body is subject, those which consist in a change in the external conditions form a special group. By "external conditions" we mean in a wide sense various external fields. In practice the external conditions are most often determined by the fact that the body must have a prescribed volume. In one sense this case may also be regarded as a particular type of external field, since the walls which limit the volume are equivalent in effect to a potential barrier which prevents the molecules in the body from escaping.
If the body is subject to no interactions other than changes in external conditions, it is said to be thermally isolated. It must be emphasized that, although a thermally isolated body does not interact directly with any other bodies, it is not in general a closed system, and its energy may vary with time.
In a purely mechanical way, a thermally isolated body differs from a closed system only in that its Hamiltonian (the energy) depends explicitly on the time: E = E(p, q, t), because of the variable external field. If the body also interacted directly with other bodies, it would have no Hamiltonian of its own, since the interaction would depend not only on the co-ordinates of the molecules of the body in question but also on those of the molecules in the other bodies.
This leads to the result that the law of increase of entropy is valid not only for closed systems but also for a thermally isolated body, since here we regard the external field as a completely specified function of co-ordinates and time, and in particular neglect the reaction of the body on the field. That is, the field is a purely mechanical and not a statistical object, whose entropy can in this sense be taken as zero. This proves the foregoing statement.
Let us suppose that a body is thermally isolated, and is subject to external conditions which vary sufficiently slowly. Such a process is said to be adiabatic. We shall show that, in an adiabatic process, the entropy of the body remains unchanged, i.e. the process is reversible.
I am unable to make these statements coherent