# Closed Universe & entropy

## Main Question or Discussion Point

Entropy increases over time, which ultimately means total disorder and death or inactivity. However, this only applies to a fully closed system, of which it is presumed the Universe is the ultimate example.
Q) If the Universe is infinite in all dimensions including time, can it be accurately described as being fully closed? Therefore, can entropy ever reach maximum disorder?

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hellfire
I am not sure whether one can consider the universe to be infinite when arguing about thermodynamics. Note that a universe in which space expansion accelerates contains cosmological event horizons, which make it impossible to speak about the content of the whole universe as an ensemble due to the missing causal contact.

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Phobos
Staff Emeritus
Gold Member
You may need to carefully define "universe". If the universe is not a closed thermodynamic system, then it is receiving input from elsewhere. But the universe is often defined as "everything", so that "elsewhere" is included in the universe. Not sure we know enough about "everything" to make ultimate predictions about the universe. Anyway, I suspect that maximum disorder on the scale of the universe is an asymptotic condition.

Entropy in a closed system is constant. Both the Schrodinger wave equation and classical Newtonian mechanics are reversible. The conditions under which entropy increases are a source of great controversy. This is often glossed over in teaching of thermodynamics.

Phobos
Staff Emeritus
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charlesa said:
Entropy in a closed system is constant. Both the Schrodinger wave equation and classical Newtonian mechanics are reversible. The conditions under which entropy increases are a source of great controversy. This is often glossed over in teaching of thermodynamics.

The 2nd law of thermo states that the entropy in the final state of a closed/isolated system is never less than the original state (change greater than or equal to zero). If the change is zero, then the process is reversible. If the change is greater than zero, then the process is irreversible.

Chronos
Gold Member
If you accept the proposition the universe is finite in time, it is impossible to accept the proposition it is observationally infinite.

Phobos said:

The 2nd law of thermo states that the entropy in the final state of a closed/isolated system is never less than the original state (change greater than or equal to zero). If the change is zero, then the process is reversible. If the change is greater than zero, then the process is irreversible.
Yes, everyone is confused. No one has successfully shown how the Schrodinger wave equation leads to the second law of thermodynamics. The Schrodinger equation predicts that the entropy change for a closed system is ALWAYS zero. People have speculated that Quantum mechanics may need to be modified to introduce a time asymmetric component.

Nereid
Staff Emeritus
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charlesa said:
Yes, everyone is confused. No one has successfully shown how the Schrodinger wave equation leads to the second law of thermodynamics. The Schrodinger equation predicts that the entropy change for a closed system is ALWAYS zero. People have speculated that Quantum mechanics may need to be modified to introduce a time asymmetric component.
There's also the small (?) problem that we know QM can't be all there is (as a description of the 'rules of the universe') - it's incompatible with GR. So we need a theory of quantum gravity (at least); in that theory (and the successor theories that will likely embed and replace it), who knows how thermodynamics will work (or even if it will be a meaningful concept)?

Now thermodynamics works perfectly well for the tiny, tiny region of the universe (the parameter space of my statement includes things like energy density as well as space and time) that we have encountered so far.

turbo
Gold Member
All very interesting. Her approach, which if I understand the article correctly introduces causality as a necessary condition, may relate to the thermodynamics/QM mismatch as well. The reason for this is that causality is intricately related to reversibility - in fact there is arguably even a bidirectional implication between causality and reversibility.