What's the energy-spread of the quantum Universe state?

[PeterDonis]

ated quantum system in a mixed state

How can an isolated quantum system be in a mixed state? Again, I understand that you can write down such a state mathematically. But how does it make sense physically?

 

[Robert Shaw]

Mathematically, yes, as I have already said in response to @kimbyd. Math is not the same as physics. Physically, I do not think these solutions are reasonable for closed systems, for reasons I have already explained.

Ok let's take the simplest quantum Hamiltonian, zero potential energy everywhere.

Then form a wavepacket.

Wavepackets can exist in isolated closed systems.

Yet they are superpositions of energy levels

 

[Robert Shaw]

Ok let's take the simplest quantum Hamiltonian, zero potential energy everywhere.

Then form a wavepacket.

Wavepackets can exist in isolated closed systems.

Yet they are superpositions of energy levels

Reality is too complex for physics to model.

Football, a game of football, obeys the laws of physics. Physicists cannot model it however, not quantum nor classical.

Knowledge is the problem. We don't know enough about almost everything to write equations.

Look around you. Mostly complex molecules. Air is mostly molecules.

That's for engineers who do a great job using approximations of physics.

 

[Robert Shaw]

Reality is too complex for physics to model.

Football, a game of football, obeys the laws of physics. Physicists cannot model it however, not quantum nor classical.

Knowledge is the problem. We don't know enough about almost everything to write equations.

Look around you. Mostly complex molecules. Air is mostly molecules.

That's for engineers who do a great job using approximations of physics.

Closed systems are idealisations.

Reality seldom approximates to a closed system.

On rare occasions real systems can be created that approximate to an idealised closed system.

Our knowledge of such systems is from the mathematics of the idealised closed system, for which we can write down equations.

Quantum physics textbooks are mostly concerned with closed systems. They tend to focus on eigenstates because they are mathematically simple and good for teaching purposes. They say a little about superpositions - the wavepacket, the Gaussian superposition of oscillator states, etc. but these are more difficult mathematically so get limited coverage.

 

[weirdoguy]

Why don't you write it all in ONE post? There is edit button.

 

[PeterDonis]

@Robert Shaw you are simply repeating your position without responding to what anyone else has said. That is not going to lead to a productive discussion.

Thread closed.