Dear Prof. Joos,Here is my response to your posting of 14 March.
>Facts - in the way experimentalists report them - are certainly=
part of the physical world.
In Zeh's theory there is the "real" reality represented by the=
wave function, and then there is the shadow-like reality of=
facts. Is the latter a part of the physical world?
>The task of theory is to invent a consistent model.
But thinking of correlations between shadow-like facts as=
constituting the "real" reality is inconsistent.
>Then, what is the difference between a single atom (which=
nowadays I can "see") and the
>tree I see in my garden? I never found one.
I suppose everyone agrees on the difference between seeing an=
atom and seeing a tree. After all, this is why you put quotes=
around "see". Seeing a thing means seeing its form. (Seeing an=
elephant doesn't mean seeing a trail in the jungle, and seeing=
an electron doesn't mean seeing a track in a cloud chamber.) The=
form of a thing consists of the forms of its parts and of the=
spatial relations between them. Take the form of a hydrogen atom=
in its ground state. If we ignore the forms of the proton and=
the electron, there remains the atom's one internal relative=
position. It's a spherically symmetric "cloud". Its density,=
integrated over any region R, gives the probability with which=
that position would be found in R if the corresponding test were=
made. If the test is actually made, the "1s-cloud" no longer=
represents the atom's internal relative position. Thus the=
"1s-cloud" represents the atom's ground state only if the test=
is not made, yet it represents it in terms of probability=
assignments that assume, counterfactually, that the test is=
made. The upshot: Making counterfactual probability assignments=
is the proper way to quantify the fuzziness that fluffs out=
matter, including trees.
>The (conceptual) wall between what is commonly called=
"microscopic" vs. "macroscopic"
>is demolished more and more every day by clever=
experimentalists.
Surely, and most excitingly. But this doesn't remove quantum=
fuzziness or its conceptualization in terms of probability=
assignments to counterfactual property-indicating events. Nor=
does it remove the difference between events and correlations=
(joint probability assignments to possible events), or the=
difference between actuality and possibility.
>Classical concepts proved wrong long ago; hence consistency=
requires that we use
>quantum (kinematical) concepts...
Of course. According to classical kinematics every particle=
follows a precise trajectory, which is wrong, while quantum=
kinematical concepts - probability assignments to possible=
events made on the basis of actual events - are appropriate for=
dealing with the fuzziness of the quantum world.
>...as building block of our world model. After all, we
>believe that macro-objects are made of atoms, don't we?
Saying that quantum kinematical concepts are building blocks of=
our world model is not quite the same as saying that=
macro-objects are made of atoms. But of course, counterfactual=
probability assignments are "building blocks" of any theory that=
adequately addresses the fuzziness in our world. Nor does the=
conceptualization of fuzziness through probability assignments=
to possible events contradict the classical, common-sense notion=
that macro-objects are made of atoms.
>Needless to say, there is a strong psychological barrier to=
overcome. In earlier times it
>was perhaps as difficult to accept that the earth is not flat=
(despite appearance), as it is
>nowadays to accept a description of the world in terms of wave=
functions. But wishful
>thinking does not help.
I fully agree.
>Talking about particles which go thru two slots at once is a=
contradiction in terms.
You are right when you take "the particle went through both=
slits" as a conjunction of two propositions ("the particle went=
through L and the particle went through R"). In the quantum=
world no property is a possessed property unless its possession=
is indicated by an actual event or state of affairs. That=
conjunction would be true only if the particle's passage through=
L and its passage through R were both indicated. This is never=
the case. By the same token, if nothing indicates the slit taken=
by a particle but something indicates that the particle went=
through L&R - the regions defined by the slits considered as one=
- then the proposition "the particle went through L&R" is true,=
while the propositions "the particle went through L" and "the=
particle went through L" lack truth values - they are neither=
true nor false. Note that "the particle went through L&R" is a=
single proposition with a single truth value, rather than a=
conjunction of two propositions with separate truth values.
But as you say, "there is a strong psychological barrier to=
overcome" - one whose neurobiological underpinnings I have=
discussed [1,2]. We tend to think of space as an intrinsically=
and maximally differentiated expanse. The spatial aspect of the=
quantum world is something else altogether. The spatial (as well=
as substantial) distinctions we make in our minds correspond to=
something in the real world only if, and only to the extend=
that, they are warranted by facts. Whenever it is correct to sum=
over amplitudes, the distinctions we make between the=
corresponding alternatives are distinctions that nature does not=
make.
>In the absence of new hints from experiments, the quest for=
conceptual consistency may
>be the best lead we have.
Surely.
Sincere regards,
Ulrich Mohrhoff
[1] U. Mohrhoff, Quantum mechanics and the cookie cutter=
paradigm, quant-ph/0009001.
[2] U. Mohrhoff, Making sense of a world of clicks,=
quant-ph/0202148.
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