The situation described by Zeilinger is indeed a good example of the
ability of science to negate possible models. One can't prove that a
model is "correct" but one can often prove that a model is wrong. That
is, its predictions and explanations ("postdictions") are in large
disagreement with observations. What I understand Zeilinger to be
saying is that in the new wave of deep experimental probes of quantum
mechanics theory, the view that a bunch of particles is already in
some state before the observation, and that the observation simply
determines which state the particles were in at the time of the
observation, is false. That is not how the world behaves, despite our
notions (and those of Einstein) that "obviously" the system had to be
in the state we find it in, before we made the measurement.
However, I'll take the opportunity to point out that one occasionally
encounters an overly simplistic view of the power of negation in
science, the view that "one experiment is sufficient to overturn a
theory." It's more complicated than that. There's a particularly nice
example in a paper by Feynman and Gell-Mann in the late 1950s. In
their paper they proposed a particular structure for the weak
interactions (which had recently been shown not to conserve "parity"
-- handedness), called "V-A", which implied maximal violation of the
principle of conservation of parity. In their paper they reviewed all
the existing experiments that were relevant tests of their theory. One
after another they said essentially, "This experiment contradicts our
theory, and we think the experiment is wrong." They were correct. I
believe the problem was that multiple scattering of particles in thick
pieces of material washed out the relevant effects. Eventually better
experiments were performed, and V-A was confirmed as a good
description of the weak interaction.
For both negation AND for gathering evidence in support of a theory,
it is a complex web of diverse observations that is required to reject
or support a theory.
I'll give an example concerning negation in our own college-level
intro physics curriculum. We discuss spark formation in air. An
obvious model is that you have to apply a big enough electric field to
yank electrons out of air molecules in order to form a conducting
plasma of positive ions and free (unbound) electrons ("field
ionization"). It is easy to calculate the approximate electric field
value required to ionize an air molecule, by considering how large is
the electric field of the rest of an atom on an outer electron, and it
is about 1e11 volts/meter. However, the observed threshold for spark
formation is about 3e6 volts/meter. At first, students are inclined to
look for various corrections to the model, but eventually they come to
agree that you really can't nickel and dime a model into agreement
against a difference of 3e4; the model itself must simply be wrong.
A model that works well starts with the observation that there are
always a few free electrons in the air, because muons (heavy
electrons), created in impacts of high-energy cosmic-ray protons with
the nuclei in air molecules, penetrate the atmosphere and eject
electrons from air molecules along the muon's path. The critical
electric field only needs to accelerate a free electron sufficiently
in one mean free path to have enough kinetic energy to knock an
electron out of an air molecule. Now you have 2 free electrons, then
4, 8, 16, .... a chain reaction. In this model one estimates a
threshold value of electric field in rather good agreement with the
observed 3e6 volts/meter, which looks promising.
Then comes the kicker. The first model (field ionization) predicts
that the density of the air doesn't matter, since the effect is on
individual atoms. The second model (chain reaction) predicts that if
you double the air density the mean free path of electrons is cut in
half, so the energy imparted by the applied electric field in one mean
free path is cut in half, so the kinetic energy of the electron when
it hits a molecule is cut in half, so you need twice the applied field
strength in order to make the chain reaction happen. When you measure
the threshold field for different air densities, you find that it is
in fact proportional to the density, in agreement with the chain
reaction model and in disagreement with the field ionization model (in
fact, high-density gas is sometimes used as an insulator). One
characteristic of a good model is that it explains/predicts more
aspects of a phenomenon than those for which it was originally
developed.
Bruce
P.S. I'll advertise that on my home page (www4.ncsu.edu/~basherwo)
there's a video of a presentation I made to Santa Fe city government
staff on "Electric Fields, Cell Towers, and Wi-Fi", motivated by my
observations in public meetings that few people, even many with
technical backgrounds, have much conception of what an electric or
magnetic field is.
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