# Is objective reality possible under orthodox QM?

1. Oct 21, 2012

### cdog1350

One of the postulates of QM is that the physical state of any system (with any number of constituent particles) corresponds to a Hilbert state vector ψ. So an isolated system that includes an object to be observed, the observing apparatus, and also the human observer himself is also represented by some state vector ψ.

This state evolves in time according to the Schrodinger equation, no matter what the internal interactions of the objects in the system are. An observer performing a measurement on an object is just such an internal interaction (between the particles making up the object+apparatus+observer), and does not mark or signify anything special happening to the state vector - certainly not a 'collapse' of any kind.

But according to the orthodox interpretation of QM, after the measurement the state of the system collapses to an eigenvector A$_{n}$ of the observable operator.

Both the uncollapsed state ψ' and the collapsed A$_{n}$ are then valid states of the system according to this interpretation. Since ψ' and A$_{n}$ give different predictions for measurements, we must conclude that they are subjective states - i.e. states relative to some observer. For an observer outside the system, the state is ψ', but for the observer inside the system, the state is A$_{n}$.

But this means that the orthodox statement of QM theory explicitly postulates a special status for the human observer! In other words, the state describing the system depends on who will be doing the measurement (someone inside or someone on the outside), and so can not be talked about without reference to a specific observer. Not only that, but because the state is subjective, then reality (by "reality" I mean the description predicting results of measurements made in the future) will also be subjective. If we say that only one reality actually exists (multiple realities would be the MWI interpretation), then the conclusion is that only one observer actually exists. In other words, orthodox QM implies solipsism.

Is this the necessary conclusion, or is an objective reality somehow possible under that interpretation?

2. Oct 21, 2012

### joerng2003

I have always thought of objective reality as the point in space-time where the vectors of several measuring components meet all agreeing upon what is being measured and the results of those measurements. Although my ability to test this theory is often halted by my lack of mathematical understanding and formal training in the field of QM, I believe it accounts for many variables that must be dealt with in order to approach a unified theory on the construct of reality.
One of those obstacles is in my opinion something we call perception and can be explained absent math and syntactical language as "a measurement of an event absent data from the other members also measuring the event in the same space-time location."

3. Oct 21, 2012

### Staff: Mentor

You are drawing conclusions that are very interpretation dependent and jumping to conclusions that are not implied by the facts.

With decoherence taken into account one can legitimately hold the view that a system has a property prior to observation and the role of a conscious observer is not required:
http://arxiv.org/pdf/quant-ph/0312059v4.pdf
'The reduced density matrix looks like a mixed state density matrix because, if one actually measured an observable of the system, one would expect to get a deﬁnite outcome with a certain probability; in terms of measurement statistics, this is equivalent to the situation in which the system is in one of the states from the set of possible outcomes from the beginning, that is, before the measurement. As Pessoa (1998, p. 432) puts it, “taking a partial trace amounts to the statistical version of the projection postulate.”'

I will not hide from you that decoherence does not solve the issues explicitly - it is for all practical purposes - meaning there is no way experimentally of distinguishing it from an ensemble where it has the outcome prior to observation hence removing the need for a conscious observer. As Schlosshauer puts it decoherence transforms a pure state into an improper ensemble - here improper means it is not really an ensemble - just that it is experimentally indistinguishable from one. But that's the point - you do not need to assume a conscious observer - you can assume it 'objectively' exists with no contradiction of the rules of QM.

Wigner held views along the lines of what you posted but when he found out about the early work of Zurek on decoherence realised it no longer was required and abandoned it.

Thanks
Bill

Last edited: Oct 21, 2012
4. Oct 26, 2012

### Maui

It seems to be more subtle than that - a particle's wavefunction would collapse at contact with a barrier(due to decoherence - like charges repel each other) instead of tunneling through it(e.g. in a tunneling diode). Yet quantum tunneling exists and is widely utilized.

5. Oct 26, 2012

### lugita15

How would you respond to this thought experiment by Ghirardi, from his book "Sneaking a Look at God's Cards"? He claims to provide a means of empirically distinguishing between actual wavefunction collapse and decoherence. Basically, if A is the observable whose eigenstates form the pointer basis of an apparatus, Ghirardi proposes to perform a measurement on an observable Z of the apparatus which is incompatible with A. In practice our apparatus has a position pointer basis, because we have to read off the position of the pointer, so we would have to somehow perform a momentum measurement of the apparatus pointer or something.

6. Oct 26, 2012

### Naty1

7. Oct 26, 2012

### Staff: Mentor

For decoherence to occur it needs to interact with other particles like photons, atoms etc so you consider it a composite system. A potential barrier is not enough to cause decoherence as the standard exercise of calculating what occurs when it meets such a barrier shows, so tunneling can occur which can be detected later by an actual observation.

In interpretations based on decoherence the quantum state is considered a pre-probability and not an actual outcome until it is made real by an observation in which case you have to take into account the whole observational setup including decoherence.

Thanks
Bill

Last edited: Oct 26, 2012
8. Oct 26, 2012

### Staff: Mentor

It's more than in practice - its in fact. There is zero doubt the pointer is in some quantum state depending on its position. The system has become entangled with the apparatus so its an actual reflection of the property the system has. Being a macroscopic object the pointers quantum uncertainty in position is way way below its resolution. Conceptually it is probably better to view the measurement result as a digital output.

The real issue is why for all practical purposes can the system be considered to be in a mixed state and observation picks an outcome and the outcome can be considered to be there prior to observation. Decoherence does not explain why that is - it merely accepts that's what the math shows. To be exact we can interpret it in a sensible way and resolve the issues but it does not compel us to do so.

This is why I sometime make the comment its almost as if people want it to be more perverse than it actually is - yes one can claim, correctly, it does not explain how a particular result is selected or even why we get any result at all, but it is consistent with a much more common sense view of the world so why not simply choose that - its what I do. In fact that it is experimentally indistinguishable from our usual intuition is undoubtedly how our intuition developed - why fight it.

Thanks
Bill

Last edited: Oct 26, 2012
9. Oct 26, 2012

### lugita15

So bhobba, what are your thoughts on Ghirardi's thought experiment?

10. Oct 27, 2012

### Staff: Mentor

All the detail is not present for me to make a full assessment.

However it simply seems to be another analysis of an EPR type experiment and that has been done to death - from the decoherence viewpoint you will find many analysis such as that found in Consistent Quantum Theory by Griffiths. From that viewpoint he seems to be introducing an inconsistent history which you are not allowed to do (switching to plane polarization which is a different setup and hence a different history) - in Consistent Histories it is built in - in Decoherent Histories consistency is enforced by decoherence. Griffiths in the textbook mentioned previously (Consistent Histories) goes to great pains to explain you can't do that - if you do you run into consistency problems. Decoherence automatically enforces it for a given experimental setup - switching to measuring a different polarization enforces a different and mutually incompatible consistency condition.

I suggest you get a hold of that book and compare it against what your book says:
http://quantum.phys.cmu.edu/CHS/histories.html

Although I rather like Consistent Histories it is not the interpretation I personally hold to - I hold to the ensemble interpretation with decoherence.

Thanks
Bill

Last edited: Oct 27, 2012
11. Oct 29, 2012

### Ilja

Just to answer the original question: Yes, objective reality is compatible with standard QM, as was shown by Bohm. The de Broglie-Bohm interpretation is an example of a realistic interpretation of standard QM. There are also others like Nelsonian stochastics.

12. Oct 29, 2012

### cdog1350

What is this sensible way to interpret it? Can you please give more details?

Also, even after decoherence, isn't the whole system (i.e. observed object+apparatus+environment) still in a pure state? Even if it is locally experimentally indistinguishable from a statistical mixture, globally when you include the environment, etc. it is still pure. Given this, can we still just simply posit that the observed object "really" already had some definite property even before observation?

13. Oct 30, 2012

### Staff: Mentor

As posted previously:
With decoherence taken into account one can legitimately hold the view that a system has a property prior to observation and the role of a conscious observer is not required:
http://arxiv.org/pdf/quant-ph/0312059v4.pdf
'The reduced density matrix looks like a mixed state density matrix because, if one actually measured an observable of the system, one would expect to get a deﬁnite outcome with a certain probability; in terms of measurement statistics, this is equivalent to the situation in which the system is in one of the states from the set of possible outcomes from the beginning, that is, before the measurement. As Pessoa (1998, p. 432) puts it, “taking a partial trace amounts to the statistical version of the projection postulate.”'

Yes it can be interpreted that way and interpretations like Consistent Histories do.

The issue is, as has been explained many times, it only solves it for all practical purposes meaning no experiment can tell the difference - but it does not compel you do do so. This is what allows you to interpret it the way I do should you so desire.

To be specific the problem is the Kochen Sprecker Theorem that proves an observation can not be revealing a pre-existing state. But with decoherence you can because the pure state is transformed into an 'improper' mixed state by dechoerence - the phase is leaked out to the environment. For the details you should consult Schlosshauer's textbook where the full detail is spelled out:
https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

Thanks
Bill

Last edited by a moderator: May 6, 2017
14. Oct 30, 2012

### Ilja

You cannot do this for all properties together. Only for some properties you can do it.

For example, in de Broglie-Bohm theory the trajectory $q(t)\in\,Q$ in the configuration space is a well-defined property even without measurement. Instead, the momentum p is not. It is, in particular, not equal to $m\dot{q}$. The result of a "momentum measurement" depends not only on the trajectory of the "measured" object, but in general also on the trajectory of the "measurement device", so naming it "measurement" is misleading, and it would be better to name it "interaction".