Misconceptions concerning
the “quantum measurement problem,” have
been around for decades, as when one read, e.g. in Richard Schlegel's monograph * Superposition and Interaction:Coherence in
Physics* (1980, University of Chicago Press, p.
178,) about the opinion
expressed once by Prof. Eugene Wigner (at a QM conference).

To wit,

"** the consciousness of a dog would effect the
projection of a quantum system into a single state whereas that of an amoeba
would not**."

So based on Prof. Wigner's take, if a dog (like a French Poodle, say) could "*effect the projection into a single state*" why not a human who observes Stephen Curry closely enough in an NBA game to make his jump shot bounce off the hoop at the
last moment? Game to Celtics! Again, the reason is that basketballs are macro- sized
objects, as opposed to electrons, protons, etc. But leave aside macro-size objects, can a quantum physicist observing an atomic scale system actually cause an electron to change its state and jump to another orbital? Therein lies the crux of what we refer to as the quantum measurement problem: the quantum system observer bringing about the 'collapse' of a superposition of distinct quantum states into a new, single state.

In the same text, Schlegel points out (p. 143):

“An irremovable problem element enters into quantum theory with the observation of
the system given the prediction that it will be in some state y can only be made with the probability
measure [a_{n
}] ^{2}_{ } *where*: a_{n} = (y_{o}, y )

*and the defined superposition
of states is given by*

y = å _{a} [a_{n}]y_{o}

Then:

“*something happens which
is not within the purview of the exact equation for the wave function which can
do no more than give a probability estimate for the likelihood of finding the
system in a particular state*. “

In his monumental text ‘* Quantum
Theory’* (1951) the English physicist Daid Bohm noted that (p. 587
):

“*If we wish to observe the position
and momentum of the electron at a quantum level of accuracy we must regard the
electron – plus the light quanta used in making the observation – as part of an
indivisible, combined system*.”

In this case the thought experiment making use of a "Heisenberg microscope" is useful to fix ideas.

Consider a measurement
made to determine the *instantaneous position of an electron* by
means of a microscope. In such a measurement the electron must be illuminated,
because it is actually the light quanta (photon) scattered by the electron that
the observer sees. The resolving power of the microscope determines the
ultimate accuracy with which the electron can be located. This
resolving power is known to be approximately: l/
2 sin q

Δx = l/ 2 sin q

In order to be
collected by the lens, the photon must be scattered through any range of angles from **- ****q to q**. In effect, the electron’s momentum values range
from:

+ h sin q/ l to - h sin q/ l

Then
the uncertainty in the momentum p _{x } is given by:

D p _{x } = [ h
sin q/ l - (- h
sin q/ l)] = 2 h sin q/ l

The Heisenberg Uncertainty Principle product is:

D p _{x } Δx = (2 h sin q/ l ) l/ 2 sin q = h

In terms of how far
the interaction between observer and the system observed can be pushed he writes (*ibid*.):

“*The question is completely irrelevant as far as the theory of measurements is
concerned since it is only necessary to
carry the analysis to some classically describable stage of the
apparatus.” In other words there is no 'quantum measurement problem' in terms of an observer causing sudden
“collapses” of the observed electron – say – which renders its properties
inaccessible*."

__Physics Today__, June, p. 62, '

*There Is No Quantum Measurement Problem'*):

Paul Dirac, in his book * Quantum Mechanics,* basically agreed with this in terms of defining the “principle of superposition”:

**“A
state of a system may be defined as a state of undisturbed motion that is
restricted by as many conditions or data as are theoretically possible without
mutual interference or contradiction"**

The last sentence is most critical, "*as many conditions or data as are theoretically possible without mutual interference or contradiction*." Again mutual interference of the observables x and p occurs if:

[x, p]
= -i h = -i h/ 2p

Mermin goes on to elaborate further in his essay,

"The measurement problem stems from two ways of viewing a measurement: the system alone or the system + apparatus. If the system alone is measured its state collapses. But the state of the composite system does not collapse until the apparatus is examined. Which description is correct?"

Well, the answer as he notes depends on the physicists making said measurements. In Mermin's words: "What one chooses to regard as the physical system and what state one chooses to assign to it depend on the judgment of the particular physicist who questions the system and who uses quantum mechanics to calculate the probabilities of the answers."

One thing we can say then is that a dog - French poodle or other- will never be a quantum physicist so that we need not worry it will effect the projection of a wave function (superposition of states) into a single state. Nor will I - merely on observation of a Brewer hitter (e.g. Christian Yelich) during a live ball game - ensure he clobbers a homer at every at bat. There will only be genuine "measurement problems" when we interject metaphysics.

**See Also**:

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