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Comment by Ma8ee | Hacker News Reader

Entropy is certainly a physical “thing”, in the sense that it affects the development of the system. You can equally well apply your argument that it isn’t a physical thing because it doesn’t exist on a microscopic scale to temperature. Temperature doesn’t exist when you zoom in on single particles either.

There’s no reason to involve our knowledge of the system. Entropy is a measure of the number of possible micro states for a given system, and that number exists independently of us.

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Exactly! Temperature isn't fundamental. It's a statistical measure made up by humans. It's certainly a very useful abstraction but it isn't a fundamental property. It describes an emergent pattern observed in larger systems. Same for entropy (and also AFAIK angular momentum).

It's entirely possible I'm wrong about any of the above but if so I've yet to encounter a convincing line of reasoning.

With the same line of reasoning you can argue (and some have) that numbers aren’t fundamental and are only made up by humans. In my view, the number of swans in the pond is three whether humans exist or not.

I think that's conflating distinct concepts. Numbers aren't a physical property. Whether or not they're a fundamental concept is largely a philosophical question.

Meanwhile temperature (for example), while physical, is a statistical property of a macroscopic system. It isn't fundamental - rather it's an abstraction over a (very) large set - but it is nonetheless a quantification of physical characteristics. Whereas a number, for example 2 rocks, is not quantifying any physical property of the individual rocks themselves.

The existence of a rock is definitely a physical property, and thus is the number of rocks in a collection a physical property, at least as much as temperature is it for a gas.

Numbers aren't "existence" they're a property of an abstract set. The observer draws a fairly arbitrary mental line around a cluster of objects. Whether the set numbers two, three, or four cannot be determined by observing one of the rocks in isolation.

Fundamental physical properties don't depend on other members of an arbitrary set as determined by an observer.

> at least as much as temperature is

But here I've been explicitly claiming that temperature isn't. It is a statistical abstraction over a physical property but it is not itself a physical property in any inherent sense. That's ... kind of the entire point I've been trying to make about an entire class of concepts.

Why would you need to observe anything in isolation for it to be a physical property? This is the proposition that I find absurd. It’s like saying that a hydrogen atom isn’t really a hydrogen atom, but just an arbitrary set of nucleons.

> Entropy is a measure of the number of possible micro states for a given system, and that number exists independently of us.

That number also exists independently of the system! I can imagine any system and calculate the corresponding number.

(And for an even more philosophical question, does the “system” really exist independently of us? What separates the “system” from anything else? Is every subset of the universe a “system”?)

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