# I Confusion about the thermal interpretation's account of measurement

#### A. Neumaier

However, this is deducted from the calculation of a mathematical expected value !!!
The calculation of a q-expectation. This has in the thermal interpretation the meaning of a property of the individual system, not that of an ensemble expectation or a time average.

#### DarMM

Gold Member
However, this is deducted from the calculation of a mathematical expected value !!!
Yes, but this expectation is interpreted as a property itself not the long term average of observations.

#### microsansfil

Hi,

What is the reason for this interpretation? In quantum optics, a coherent state is called semi-classical because the expectation value of the quadrature (position, momentum) of the system oscillates like the position of a classical simple harmonic oscillator.

/Patrick

#### A. Neumaier

Hi,

What is the reason for this interpretation? In quantum optics, a coherent state is called semi-classical because the expectation value of the quadrature (position, momentum) of the system oscillates like the position of a classical simple harmonic oscillator.

/Patrick
The thermal interpretation is motivated from statistical thermodynamics (global and local equilibrium), where the observed macroscopic quantities are expressed microscopically as expectation values. There were several earlier discussions here on PF.

#### A. Neumaier

how important is it to your picture of measurement that we can't assign pure states to macroscopic objects? In a modified TI where the whole universe can always be assigned a pure state, does everything break in the story we're discussing?
That we can't assign pure states to macroscopic objects is a question of consistency.

The state of the universe determines every other state by taking partial traces. This doesn't preserve pureness; hence the state of a general quantum system is mixed. Indeed, in experimental practice one can make only those systems pure that have either very few degrees of freedom or widely separated energy levels. This is never the case for a macroscopic system.
Thus all large systems have mixed states. The only possible exception would be the whole universe, as its state is not a partial trace of something bigger. But why make an exception for the universe?

But why make an exception for the universe?
If there is no intrinsic randomness in nature then there can’t be any uncertainty about the state of the universe. So what would it mean for the universe to be in a mixed state?

#### A. Neumaier

If there is no intrinsic randomness in nature then there can’t be any uncertainty about the state of the universe. So what would it mean for the universe to be in a mixed state?
Subjective uncertainty about the state is not part of quantum theory, which is about what is objectively true, and not about which knowledge human beings have about what is objectively true.

In the thermal interpretation, a mixed state is therefore not a sign of uncertainty about the state. Indeed, the state of every system is determined by the state of the universe by taking the appropriate partial trace. Note that essentially every state is mixed. Even the states that we regard as pure are in fact only approximately pure. (For example, it is impossible to determine exactly the direction of a magnetic field in a Stern-Gerlach experiment, but it would be needed to prepare an exact spin up particle.)

Uncertainty about a state just means having an inaccurate state instead of the true one, just as uncertainty about the value of a measurement result (e.g., the number of atoms in a piece of matter).

#### DarMM

Gold Member
If there is no intrinsic randomness in nature then there can’t be any uncertainty about the state of the universe. So what would it mean for the universe to be in a mixed state?
Slightly related, it's actually an open question in QFT whether the global state can be a pure state. We already know states of matter in a finite volume must be mixed in QFT.

Subjective uncertainty about the state is not part of quantum theory, which is about what is objectively true, and not about which knowledge human beings have about what is objectively true.

In the thermal interpretation, a mixed state is therefore not a sign of uncertainty about the state. Indeed, the state of every system is determined by the state of the universe by taking the appropriate partial trace. Note that essentially every state is mixed. Even the states that we regard as pure are in fact only approximately pure. (For example, it is impossible to determine exactly the direction of a magnetic field in a Stern-Gerlach experiment, but it would be needed to prepare an exact spin up particle.)

Uncertainty about a state just means having an inaccurate state instead of the true one, just as uncertainty about the value of a measurement result (e.g., the number of atoms in a piece of matter).
Okay, but how does that apply to the universe? If it’s not uncertainty and there is nothing to trace over, how does one interpret the mixed state?

#### DarMM

Gold Member
Okay, but how does that apply to the universe? If it’s not uncertainty and there is nothing to trace over, how does one interpret the mixed state?
In the thermal interpretation what is denoted as a mixed state is just another possible state of the system no different than a pure state.

#### nicf

Thus all large systems have mixed states. The only possible exception would be the whole universe, as its state is not a partial trace of something bigger. But why make an exception for the universe?
Yes, I think I understood this part already; this motivation makes sense to me and I agree with it. I realize now that my last post was too long and not very clear, so I'll try to say it with fewer words :).

It is commonly argued that the measurement problem can't be solved with ordinary quantum mechanics. People who argue this find a situation where, by the linearity of time evolution, the universe has to end up in a superposition of both possible measurement outcomes, so the final state doesn't tell us "which one actually happened".

You reject this picture in favor of one where what's important is the q-expectation of the observable $X^E$ that records the macroscopic result of the experiment. The presence of mixed terms like (in our earlier notation) $\psi_1\psi_2^*\otimes\rho^E$ mean that we can't extract the result of measuring $\frac{1}{\sqrt{2}}(\psi_1+\psi_2)$ from the results of measuring $\psi_1$ and $\psi_2$.

This explanation depends on the fact that these mixed terms can contribute a large amount to the q-expectation of $X^E$. Furthermore, it seems like we would also like, in all three states, the q-variance of $X^E$ to be small. I'm worried these two desires might be incompatible, that requiring the q-variances to be small might also force the mixed terms to be small. The intuition I'm working from comes from pure states: in the extreme case where the final states are eigenstates of $X^E$, the mixed terms are zero, and for pure states that are merely close to being eigenstates we should be able to make the mixed terms small, which also seems bad.

The question I'm asking is more a mathematical one than a physical one; I realize we have good physical grounds to disallow pure states, but I'm interested in exactly how that prescription interacts with the picture of measurement you're advocating.
(a) If, hypothetically, the thermal interpretation did allow pure states, would the thing I just described be a problem, or did I make a mistake again?
(b) If the answer to (a) is yes, why does the problem go away when we disallow pure states?

I think what might help me is an explicit example. How hard is it to construct a $\rho^E$, $X^E$, $\psi_1$, and $\psi_2$ that behave this way after applying some unitary map?

#### A. Neumaier

Yes, I think I understood this part already; this motivation makes sense to me and I agree with it. I realize now that my last post was too long and not very clear, so I'll try to say it with fewer words :).

It is commonly argued that the measurement problem can't be solved with ordinary quantum mechanics. People who argue this find a situation where, by the linearity of time evolution, the universe has to end up in a superposition of both possible measurement outcomes, so the final state doesn't tell us "which one actually happened".

You reject this picture in favor of one where what's important is the q-expectation of the observable $X^E$ that records the macroscopic result of the experiment. The presence of mixed terms like (in our earlier notation) $\psi_1\psi_2^*\otimes\rho^E$ mean that we can't extract the result of measuring $\frac{1}{\sqrt{2}}(\psi_1+\psi_2)$ from the results of measuring $\psi_1$ and $\psi_2$.

This explanation depends on the fact that these mixed terms can contribute a large amount to the q-expectation of $X^E$. Furthermore, it seems like we would also like, in all three states, the q-variance of $X^E$ to be small. I'm worried these two desires might be incompatible, that requiring the q-variances to be small might also force the mixed terms to be small. The intuition I'm working from comes from pure states: in the extreme case where the final states are eigenstates of $X^E$, the mixed terms are zero, and for pure states that are merely close to being eigenstates we should be able to make the mixed terms small, which also seems bad.

The question I'm asking is more a mathematical one than a physical one; I realize we have good physical grounds to disallow pure states, but I'm interested in exactly how that prescription interacts with the picture of measurement you're advocating.
(a) If, hypothetically, the thermal interpretation did allow pure states, would the thing I just described be a problem, or did I make a mistake again?
(b) If the answer to (a) is yes, why does the problem go away when we disallow pure states?

I think what might help me is an explicit example. How hard is it to construct a $\rho^E$, $X^E$, $\psi_1$, and $\psi_2$ that behave this way after applying some unitary map?
All these problems are mathematically difficult. You conjecture a particular problem based on informal worries, I conjecture its absence based on seeing what the quantum formalism is known to achieve and the experimental record requires.

Note that by definition, the measured macroscopic value is the q-expectation of the pointer variable, independent of the value of its uncertainty. Whether the latter is small is secondary, though it seems to be automatically the case for macroscopic systems.

In the thermal interpretation what is denoted as a mixed state is just another possible state of the system no different than a pure state.
But doesn’t it mean it can be written as a mixture of several disconnected pure states? So you can consider each of those pure states as separate universes that evolve independently.

#### A. Neumaier

But doesn’t it mean it can be written as a mixture of several disconnected pure states? So you can consider each of those pure states as separate universes that evolve independently.
By the same argument you can postulate that the possibility of writing 5=2+3 can be considered to mean that there are different universes in which 5 is realized as 2 amd 3.

The thermal interpretation declares such statements to be nonsense.

#### vanhees71

Gold Member
That we can't assign pure states to macroscopic objects is a question of consistency.

The state of the universe determines every other state by taking partial traces. This doesn't preserve pureness; hence the state of a general quantum system is mixed. Indeed, in experimental practice one can make only those systems pure that have either very few degrees of freedom or widely separated energy levels. This is never the case for a macroscopic system.
Thus all large systems have mixed states. The only possible exception would be the whole universe, as its state is not a partial trace of something bigger. But why make an exception for the universe?
Well, you should be able to answer this within your thermal interpretation, because you claim to have a physically testable meaning about single systems. Within the standard statistical interpretation the idea of a "state of the entire universe" doesn't make sense, at least I've never understood the attempts to define such a mathematical fiction.

Maybe, one day there's an answer to that question, when we have a complete understanding of gravity and spacetime that is consistent with quantum theory.

As long as that, we have to take "cosmology" as what it is: an extrapolation of local observations and thereby derived natural laws, extrapolated to the "entire universe" (or to be more careful the piece of the universe that is at least in principle observable to us). This works indeed not too badly and there has been a lot of progress in the past few decades, but that's what it is: An extrapolation of locally discovered physical laws considered universal by hypothesis (cosmological principle).

#### DarMM

Gold Member
But doesn’t it mean it can be written as a mixture of several disconnected pure states? So you can consider each of those pure states as separate universes that evolve independently.
I think it would view this as no different to how in classical electromagnetism states of the electromagnetic field are sums of other states of the field.
Even within interpretations of QM modern forms of Copenhagen would be similar, viewing mixed states as simply states of higher entropy.

In QFT mixed states of finite volume systems aren't sums of pure states so viewing them like that is difficult to maintain.

#### vanhees71

Gold Member
The von Neumann-Shannon-Jaynes entropy is defined in QT (no matter whether it's formulated as a QFT or not) is given by (in SI units)
$$S=-k_{\text{B}} \mathrm{Tr}(\hat{\rho} \ln \hat{\rho}).$$
One can show that it is 0 if and only if $\hat{\rho}=|\psi \rangle \langle \psi|$, for some normalized vektor $|\psi \rangle$, and that's by definition representing a pure state.

I don't understand your last sentence.

Since $\hat{\rho}$ is a self-adjoint operator, it provides a (generalized) orthonormal eigenbasis and thus you can write it as sum (and/or integral) of the form
$$\hat{\rho}=\sum_{k} P_k |k \rangle \langle k|.$$
Again, it doesn't matter whether your QT is formulated as a QFT or not.

#### A. Neumaier

Well, you should be able to answer this within your thermal interpretation, because you claim to have a physically testable meaning about single systems.
Within the thermal interpretation it is considered to be in a mixed state. Indeed, the experimental record tells us that the universe has a positive temperature, whereas no temperature can be assigned to a pure state.

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#### DarMM

Gold Member
Again, it doesn't matter whether your QT is formulated as a QFT or not.
It does. In QFT the local observable algebra involves type $III_{1}$ factors which have no pure states, thus density matrices for finite volume systems do not decompose like this.

#### A. Neumaier

It does. In QFT the local observable algebra involves type $III_{1}$ factors which have no pure states, thus density matrices for finite volume systems do not decompose like this.
For type $III_{1}$ factors and how this, at first counterintuitive fact, may arise in examples, see this post and the surrounding discussion there.

#### vanhees71

Gold Member
Within the thermal interpretation it is considered to be in a mixed state. Indeed, the experimental record tells us that the universe has a positive temperature, whereas no temperature can be assigned to a pure state.
What is considered to be in a mixed state? The entire universe? Well, why not?

#### A. Neumaier

What is considered to be in a mixed state? The entire universe?
yes, its state was under discussion. Assigning to the universe a temperature distribution (microwave background) means that the state of the universe is approximately a local equilibrium state and hence a mixed state.

#### vanhees71

Gold Member
Well, the microwave background temperature is a very much coarse grained observable. It's about the large-scale "average" structure of the universe, described by an FLRW metric using the above considered assumption of the cosmological principle to extrapolate it to "the entire (observable) universe". In this sense it's a paradigmatic example for the concept to consider coarse-graining to understand the classical behavior of macroscopic observables of a many-body system.

"Confusion about the thermal interpretation's account of measurement"

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