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“clamped”. But following Woolley and Sutcliffe (
1977
), Hendry argues that in the
process of applying the Born-Oppenheimer approximation, the symmetry proper-
ties of the molecular wavefunction are removed (Hendry
2010a
,
b
). The idea here is
that quantum mechanics cannot recover the structure (and the lower symmetry) of
real molecules. For example, in the case of isomers, quantum mechanics cannot
distinguish between two different molecules, for it assigns the same wavefunction
to two distinct molecular structures - a wavefunction which is in fact a superposi-
tion of the wavefunctions corresponding to the two definite molecular structures
(and has thus a higher symmetry). Hendry suggests that just as the measurement
problem in quantum mechanics cannot be solved by a “superposition approxima-
tion” (i.e., simply discarding the part of the wavefunction that does not correspond
to what is observed), it is just as much a mistake to invoke the Born-Oppenheimer
approximation to argue that the structure of molecules is determined by resultant,
albeit hard to obtain Hamiltonians.
In reply to Hendry
s arguments, Scerri (
2012
) has pointed out that the lower
symmetry of the molecules can be accounted for by their quantum-mechanical
interaction with the environment (decoherence). Molecules are never in isolation;
they are always surrounded by other molecules, to which they interact. Conse-
quently, the wavefunction of a given molecule will not be for a long time in a
superposition of states corresponding to two different molecular structures. The
idea here is that pretty fast, the superposition will collapse and the molecule will
assume the observed structure. Scerri claims that “taking account of quantum
decoherence allows one to tame the effect of entanglement and appears to alleviate
the concern that ontological entities such as molecules with particular structures
might not exist in their own right” (Scerri
2012
, p. 20).
The appeal to decoherence is an interesting move, but it is not without its
problems. First, it should be mentioned that decoherence does not solve the problem
of definite outcomes, which together with the problem of the preferred basis forms
the so-called measurement problem in the foundations of quantum mechanics
(Adler
2003
; Zeh
2003
). Decoherence just passes the entanglement on to the
environment. In fact, decoherence exacerbates the measurement problem. Scerri
recognizes that decoherence does not allow one to predict any particular outcomes.
But he claims that this concern can be addressed by assuming that the collapse is
ubiquitous, and it happens even in the absence of observers. He claims that this
intuition is supported by the fact that the classical world is populated by definite
outcomes (i.e., definite outcomes are not just an effect of conscious observers). But
these remarks essentially amount to taking a stand on the interpretation of quantum
mechanics; of course, they do not by themselves amount to an interpretation of
quantum mechanics, but they favour a set of interpretations over others. So it looks
like that the debate about configurational Hamiltonians and molecular structure has
become entangled with the problem of interpreting quantum mechanics. Thus, it
seems that to elucidate the hard problem of emergence in chemistry one needs to
elucidate a perhaps even harder problem. Since there is the risk that this debate
could degenerate into a debate about the proper interpretation of quantum mechan-
ics or even turn into a stalemate, perhaps it is worth considering a different
approach to emergence in chemistry.
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