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Subject: Philip Ball: A common misunderstanding about wave-particle duality
Date: Wed, 19 Jun 2024 18:27:00 +0000
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https://www.chemistryworld.com/opinion/a-common-misunderstanding-about-wave-particle-duality/4019585.article


In the unlikely case it gets stuck behind a pay wall:

‘Particles caught morphing into waves’ was how a recent preprint from
researchers in France was widely reported. The timing could not have
been better, for this year is the centenary of Louis de Broglie’s
remarkable and bold thesis – presented at the Sorbonne in Paris, where
some of the team responsible for the new work are based – proposing that
matter can behave like waves. De Broglie’s idea was dismissed at first
by many of his contemporaries, but was verified three years later when
Clinton Davisson and Lester Germer at Bell Laboratories in New York, US,
observed diffraction of electrons – an unambiguously wave-like
phenomenon – from crystalline nickel. Such waviness became enthroned as
a central concept in the newly emergent quantum mechanics under the now
famous rubric of ‘wave-particle duality’.

Except… None of this is so simple. The meaning and the significance of
wave-particle duality is widely misunderstood, as some of the reporting
of the latest work shows. The common perception is that quantum
particles really are shape-changers: sometimes little balls of matter,
other times smeared-out waves. But physicists have generally been
dismissive of that idea. The notion of wave-particle duality was coined
by researchers centred around Danish physicist Niels Bohr in Copenhagen,
who together devised the so-called Copenhagen interpretation of quantum
mechanics that is widely regarded as the ‘orthodox’ view today. But in
fact the Copenhagen interpretation was never either consistent or
entirely coherent, and wave-particle duality was one of the points of
contention among its adherents.

For Bohr’s young colleague Werner Heisenberg, ‘light and matter are
single entities and the apparent duality arises in the limitations of
our language.’ Richard Feynman agreed: the electron, he said, ‘is like
neither’ a wave nor a particle. Even Bohr himself, for whom
wave-particle duality validated his concept of ‘complementarity’ –
loosely, the necessity of accepting contradictory truths in quantum
mechanics – did not say that quantum entities are sometimes waves and
sometimes particles. Both, he said, are classical concepts, which are
indispensable for interpreting quantum experiments but which say nothing
about the ‘reality’ of the quantum world. Some consider that Bohr denied
that there is any meaningful ‘quantum reality’ – how things are – at
all. (It’s contentious, largely because Bohr’s statements are themselves
so vague and inconsistent.)

    There is no reason to say that quantum entities are ever really waves

At any rate, historian of science Mara Beller says that wave-particle
duality is ‘neither unambiguous nor necessary in theoretical research.’
She’s right: there is no reason to say that quantum entities are ever
really waves. Rather, the probabilities of where we will observe them in
an experiment can be conveniently determined by the calculus of the
Schrödinger equation, proposed in 1926 in response to de Broglie, which
is formally analogous to a kind of wave equation. But a wave of what?
Not of a physical thing – a density or field – but of a probability. The
distribution of these probabilities, when observed over many repeated
experiments (or a single experiment with many identical particles),
echoes the amplitude distribution of classical waves, showing for
example the interference effects of the famous double-slit experiment.

Which brings us to the latest findings, reported by Joris Verstraten and
his coworkers.1 The experiments are rather beautiful. The researchers
trap ultracold lithium atoms in an optical lattice: interfering laser
beams that create a two-dimensional eggbox array of potential wells,
each of which can hold an atom. They image individual atoms by detecting
fluorescence from excited atomic states. Confined in a well, an atom’s
wavefunction is tightly confined: it looks like a discrete particle.

When the optical lattice is turned off, the atoms are free to wander –
and the Schrödinger equation predicts that their wavefunctions spread in
all directions. This doesn’t mean that the atoms themselves are smeared
out like waves; rather, what spreads is the probability distribution of
them being found subsequently in a given location. That is just what the
researchers see: they image the atoms’ later positions, and find that
the histograms of these positions over many repeated experiments on the
same system reveal a distribution that evolves in time just as the
Schrödinger equation predicts.

So the atoms themselves are only ever observed in a given experimental
run as particles – just as quantum mechanics says they should be. The
wavelike behaviour – which is to say, the smeared-out probability
distribution – is reconstructed from many particle-like observations. It
is in some ways analogous to reconstructing the classical probability
distribution of a microscopic particle moving diffusively. We’re not
directly seeing ‘matter waves’ as such.

The work thus offers a reminder of what Bohr – for all his
inconsistencies – was implying. When we talk about ‘how things are’ in
quantum mechanics, we are probably going to end up using classical
concepts to describe something they do not fit. Wave-particle duality is
not a property of the quantum world, but a flawed classical analogy for it.

References
[1] J Verstraten et al., arXiv, 2024, DOI: 10.48550/arXiv.2404.05699
https://doi.org/10.48550/arXiv.2404.05699