Abstract
In 1929 P.A.M. Dirac, one of the fathers of quantum mechanics, wrote the following celebrated sentence, which is often quoted in discussions of the reduction of chemistry to physics
Physics is mathematical, not because we know so much about the physical world, but because we know so little: it is only its mathematical properties that we can discover. For the rest our knowledge is negative.
B. Russell.
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Notes
- 1.
Basically the solution of the Schrödinger equation for the hydrogen atom is the starting point of approximations for heavier atoms, e.g. using the Hartree-Fock or Thomas-Fermi methods.
- 2.
One can say that for Heisenberg, in the context of quantum mechanics, the classical mechanics is a Kantian “a priori”.
- 3.
Quite similar to the “metaphysical nomological pluralism” of N. Cartwright: different domains of nature are ruled by different systems of laws.
- 4.
Just for notation simplicity we write the Wigner function \(W(q,p,t)\) for one-dimensional systems:
$$\begin{aligned} W(q,p,t)={1 \over h} \int dq' \psi ^{*}(q+q',t) \psi (q-q',t) e^{i p q'/ \hbar } \end{aligned}$$where \(\psi (q,t)\) is the wave function which obeys to the Schrödinger equation. By integrating \(W(q,p,t)\) over either \(q\) or \(p\), one obtains the probability density for the other variable. Note that \(W(q,p,t)\) is not positive definite, sometimes it is called a “quasi-probability” distribution, however the expectation value of an operator \(G({\hat{q}}, {\hat{p}})\) can be written as
$$\begin{aligned} \langle \hat{G} \rangle =\int \, dq \,dp G( q, p) W(q,p,t) . \end{aligned}$$ - 5.
These are the exponential rates at which nearby trajctories separate, and at which information on the initial state is lost (which are strictly related concepts).
- 6.
For instance, if we consider the passive advection of a scalar field \(\theta (\mathbf{x}, t)\) in a given velocity field \(\mathbf{u}(\mathbf{x}, t)\)
$$\begin{aligned} \partial _t \theta +\mathbf{u}\cdot \nabla \theta = D \varDelta \theta , \end{aligned}$$it is easy to show that an initial uncertainty \(\delta \theta (\mathbf{x}, 0)\) cannot grow forever in time. On the other hand, the solution of this equation can be nontrivial (e.g. the spatial gradient of \(\theta (\mathbf{x}, t)\) can grow quickly) if the equation
$$\begin{aligned} {d\mathbf{x} \over dt}= \mathbf{u}(\mathbf{x}, t) \end{aligned}$$ - 7.
Cited by Schweber (1990).
- 8.
In a semi-serious fashion, we may claim that the symmetry breaking is somehow a solution of Buridan’s ass paradox. In a (large) magnetic system, the spins “choose” a preferred orientation, and, in an analogous way, the ass chooses one of the two haystacks, if they are large enough (formally infinite).
- 9.
For some aspects, DFT is similar to the Boltzmann equation, which describes the one-particle distribution.
- 10.
For instance, the Hohenberg-Kohn theorem states that the properties of the electronic ground state depend only on the density \(n(\mathbf{r})\) Jones and Gunnarsson (1989). But the determination of \(n(\mathbf{r})\) remains exceedingly difficult.
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Chibbaro, S., Rondoni, L., Vulpiani, A. (2014). Quantum Mechanics, Its Classical Limit and Its Relation to Chemistry. In: Reductionism, Emergence and Levels of Reality. Springer, Cham. https://doi.org/10.1007/978-3-319-06361-4_6
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