3 ms·
> The enthalpy changes measured by a calorimeter are dependent on the design of the calorimeter, which could have been a different piece of equipment. Right, b
by kergonath 2y ago
> The enthalpy changes measured by a calorimeter are dependent on the design of the calorimeter, which could have been a different piece of equipment.
Right, but that is true of anything. Measuring devices need to be calibrated and maintained properly. It does not make something like a distance subjective, just because someone is measuring it in cm and someone else in km.
> If you introduced a new bit of macro information to the definition of an ensemble, you'd divide the number of microstates by some factor. That's the micro level equivalent of macroscopic entropy being undefined up to an additive constant.
It would change the entropy of your model. An ensemble in statistical Physics is not a physical object. It is a mental construct and a tool to calculate properties. An actual material would have whatever entropy it wants to have regardless of any assumptions we make. You would just find that the entropy of the material would match the entropy of one of the models better than the other one. If you change your mind and re-run the experiment, you’d still find the same entropy. This happens e.g. if we assume that the experiment is at a constant volume while it is actually under constant pressure, or the other way around.
> In a sense, that makes it dependent on the definition of enthalpy.
Not really. A joule is a joule, a kelvin is a kelvin, and the basic laws of thermodynamics are some of the most well tested in all of science. The entropy of a bunch of atoms is not more dependent on arbitrary definitions than the energy levels of the atoms.
> The measurables don't tell you S, they only tell you dS.
That’s true in itself, the laws of Thermodynamics are invariant if we add a constant term to the entropy. But it does not mean that entropy is subjective: two observers agreeing that the thing they are observing has an entropy of 0 at 0 K will always measure the same entropy in the same conditions. And it does not mean that actual entropy is dependent on specific assumptions about the state of the thing.
This is also true of energy, and electromagnetic potentials (and potentials in general). This is unrelated to entropy being something special or subjective.
- whatshisface 2y agoEntropy isn't subjective, but it is relative to what you can measure about a given system, or what measurements you are analyzing a system with respect to. In quantum mechanics there are degenerate ground states, and in classical mechanics there are cases where the individual objects (such as stars) are visible but averaged over. You should take a look at the Gibbs paradox.
- pixl97 2y agoI would say an even more limiting measure of entropy shows that entropy isn't subjective. That is entropy is the ability to extract useful work from a system by moving the system from a state of lower entropy to a state of higher entropy (in a closed system) No subjective measure of entropy can allow you to create a perpetual motion machine. The measurements of any two separate closed systems could be arbitrary, but when said systems are compared with each other units and measurements standardize.
- kgwgk 2y ago> entropy is the ability to extract useful work from a system https://www.damtp.cam.ac.uk/user/tong/statphys/jaynes.pdf https://www.damtp.cam.ac.uk/user/tong/statphys/jaynes.pdf The amount of useful work that we can extract from any system depends - obviously and necessarily - on how much “subjective” information we have about its microstate, because that tells us which interactions will extract energy and which will not; this is not a paradox, but a platitude. If the entropy we ascribe to a macrostate did not represent some kind of human information about the underlying microstates, it could not perform its thermodynamic function of determining the amount of work that can be extracted reproducibly from that macrostate. […] the rules of thermodynamics are valid and correctly describe the measurements that it is possible to make by manipulating the macro variables within the set that we have chosen to use. This useful versatility - a direct result of and illustration of the “anthropomorphic” nature of entropy - would not be apparent to, and perhaps not believed by, someone who thought that entropy was, like energy, a physical property of the microstate. Edit: I’ve just noticed that the article discussed links to this paper, and quotes the first sentence above, and details the whifnium example given in the […] above.
- tsimionescu 2y agoI think this is a very weird thought experiment, and one that is either missing a lot of context or mostly just wrong. In particular, if two people had access to the same gas tank, and one knew that there were two types of argon and the other didn't, so they would compute different entropies, one of them would still be wrong about how much work could be extracted out of the system. If whifnium did exist, but it was completely unobtainable, then both physicists would still not be able to extract any work out of the system. If the one that didn't know about whifnium was given some amount of it without being told what it was, and instructed in how to use it, they would still see the same amount of work being done with it as the one who did know. They would just find out that they were wrong in their calculation of the entropy of the system, even if they still didn't know how or why. And of course, this also proves that the system had the same entropy even before humanity existed, and so the knowledge of the existence of whifnium is irrelevant to the entropy of the system. It of course affects our measurement of that entropy, and it affects the amount of work we can get that system to perform, but it changes nothing about the system itself and its entropy (unless of course you tautologically define entropy as the amount of work the experimenter/humanity can extract from the system).