Can thermodynamics macrostates hide relevant quantum information ?

[mirko1]

I’m a chemistry student, I’ve been thinking about the connection between quantum mechanics and thermodynamics.
In thermodynamics we describe a system through macroscopic quantities such as T, P, Energy…
Quantum mechanics on the other hand, gives us more detailed description on the microscopic state.
This made me wonder what exactly is lost when we go from the quantum description to a thermodynamics macrostate.
In particular:
Suppose 2 quantum states are indistinguishable with respect to the macroscopic variables we use to define the thermodynamic state, but have different “microscopic correlations” or different “internal quantum structure”.
Could those differences still affect thermodynamic quantities such as extractable work, free energy or entropy production ?
Or does fixing the thermodynamic macro state necessarily make all the remaining microscopic information thermodynamically irrilevant?
I’m asking because I initially arrived at this idea by thinking of the wave function not only as a probability amplitude, but also as a possible description of the information and dynamics of the whole system.
Thanks for reading, I’m curious to hear what you think.

 

[jeffn1]

This may be a dumb question (if so, I apologize in advance), but have you looked into quantum thermodynamics?

 

[mirko1]

Yes I have, I recently started looking into quantum thermodynamics, especially the connection between quantum information, correlation and coarse-graining.
That’s what actually led me to this question. I’m trying to understand if some information contained in the full quantum state could be lost when we describe the system only through its thermodynamic macrostate.

 

[PeterDonis]

I recently started looking into quantum thermodynamics, especially the connection between quantum information, correlation and coarse-graining

What references have you looked at?

 

[PeterDonis]

I’m trying to understand if some information contained in the full quantum state could be lost when we describe the system only through its thermodynamic macrostate.

The answer to this is easy: "of course". The whole point of thermodynamics, and more generally of statistical mechanics, is to not try to work with information about the microstate because there's too much of it to usefully handle, and work instead with the coarse-grained information in the macrostate. Indeed, in contexts where we use thermodynamics, we don't even know what the microstate is to begin with; the thermodynamic variables in the macrostate are all that we actually know about the system.

This is not limited to QM, by the way; the same is true of classical statistical mechanics and thermodynamics based on underlying classical microstates.

 

[PeterDonis]

does fixing the thermodynamic macro state necessarily make all the remaining microscopic information thermodynamically irrilevant?

Yes. As per my previous post just now, that's the whole point of thermodynamics.

 

[mirko1]

I agree that in ordinary thermodynamic situations, most microscopic information can be safely discarded. What I’m wondering is whether there could be situations where the opposite becomes important:
where we normally go from the microscopic description to the macroscopic one, but under extreme, strongly interacting or far from equilibrium conditions we may need to go back to the microscopic description to understand the macroscopic.

Maybe there are regimes where the microscopic information that thermodynamics normally ignores becomes relevant again.

 

[PeterDonis]

I agree that in ordinary thermodynamic situations, most microscopic information can be safely discarded. W

It's not a matter of it being "discarded". We don't even have it in the first place. That's why we use thermodynamics--because we don't know any more information than what the thermodynamic variables tell us.'

under extreme, strongly interacting or far from equilibrium conditions we may need to go back to the microscopic description to understand the macroscopic.

If we have a microscopic description and can use it to make predictions, we don't need a macroscopic description. In such a situation we would not even be doing thermodynamics in the first place.

 

[mirko1]

I think I explained my point badly. I’m not saying that we should replace thermodynamics with a microscopic description whenever we can use one.
What I’m wondering is whether the macroscopic variables we use are always enough to predict how the system will evolve.
In other words, could two systems be indistinguishable with respect to the macroscopic variables we use, but still evolve differently because of microscopic information that was lost in the coarse-graining?
So my question is less about whether we need a microscopic description instead of thermodynamics, and more about when a particular coarse-grained description ceases to be dynamically sufficient.

-Rubino, Brukner & Manzano 2026, Coarse-grained quantum thermodynamics: observation-dependent quantities, observation-independent laws.
studies how losing microscopic information through coarse graining can affect quantities such as work and dissipation.
-Rignon-Bret & Elouard 2026, Qquantum stochastic thermodynamics of macroscopic systems: an algebric approach.
looks at macroscopic quantum thermodynamics and at nonequilibrium resources that can be hidden by coarse graining.
I’m still exploring the literature

 

[Matterwave]

Are you asking about the domain of validity of thermodynamics?

I still have a hard time parsing your question into a concrete one.