An experiment showing that oil droplets can be propelled across
a fluid bath by the waves they generate has prompted physicists
to reconsider the idea that something similar allows particles
to behave like waves.
© Dan Harris / MIT
Owen Maroney worries that physicists
have spent the better part of a century engaging in fraud.
Ever since they invented quantum
theory in the early 1900s, explains Maroney, who is himself a
physicist at the University of Oxford, UK, they have been talking
about how strange it is - how it allows particles and atoms to
move in many directions at once, for example, or to spin clockwise
and anticlockwise simultaneously. But talk is not proof, says
Maroney. "If we tell the public that quantum theory is weird,
we better go out and test that's actually true," he says.
"Otherwise we're not doing science, we're just explaining
some funny squiggles on a blackboard."
It is this sentiment that has
led Maroney and others to develop a new series of experiments
to uncover the nature of the wavefunction - the mysterious entity
that lies at the heart of quantum weirdness. On paper, the wavefunction
is simply a mathematical object that physicists denote with the
Greek letter psi (Ψ) - one of Maroney's funny squiggles
- and use to describe a particle's quantum behaviour. Depending
on the experiment, the wavefunction allows them to calculate
the probability of observing an electron at any particular location,
or the chances that its spin is oriented up or down. But the
mathematics shed no light on what a wavefunction truly is. Is
it a physical thing? Or just a calculating tool for handling
an observer's ignorance about the world?
The tests being used to work
that out are extremely subtle, and have yet to produce a definitive
answer. But researchers are optimistic that a resolution is close.
If so, they will finally be able to answer questions that have
lingered for decades. Can a particle really be in many places
at the same time? Is the Universe continually dividing itself
into parallel worlds, each with an alternative version of ourselves? Is there such a thing as an objective reality at all?
"These are the kinds of
questions that everybody has asked at some point," says
Alessandro Fedrizzi, a physicist at the University of Queensland
in Brisbane, Australia. "What is it that is really real?"
Debates over the nature of reality
go back to physicists' realization in the early days of quantum
theory that particles and waves are two sides of the same coin.
A classic example is the double-slit experiment, in which individual
electrons are fired at a barrier with two openings: the electron
seems to pass through both slits in exactly the same way that
a light wave does, creating a banded interference pattern on
the other side (see 'Waveparticle weirdness'). In 1926,
the Austrian physicist Erwin Schrödinger invented the wavefunction
to describe such behaviour, and devised an equation that allowed
physicists to calculate it in any given situation.1 But neither
he nor anyone else could say anything about the wavefunction's
From a practical perspective,
its nature does not matter. The textbook Copenhagen interpretation
of quantum theory, developed in the 1920s mainly by physicists
Niels Bohr and Werner Heisenberg, treats the wavefunction as
nothing more than a tool for predicting the results of observations,
and cautions physicists not to concern themselves with what reality
looks like underneath. "You can't blame most physicists
for following this 'shut up and calculate' ethos because it has
led to tremendous developments in nuclear physics, atomic physics,
solid-state physics and particle physics," says Jean Bricmont,
a statistical physicist at the Catholic University of Louvain
in Belgium. "So people say, let's not worry about the big
But some physicists worried anyway.
By the 1930s, Albert Einstein had rejected the Copenhagen interpretation
- not least because it allowed two particles to entangle their
wavefunctions, producing a situation in which measurements on
one could instantaneously determine the state of the other even
if the particles were separated by vast distances. Rather than
accept such "spooky action at a distance", Einstein
preferred to believe that the particles' wavefunctions were incomplete.
Perhaps, he suggested, the particles have some kind of 'hidden
variables' that determine the outcome of the measurement, but
that quantum theories do not capture.
Experiments since then have shown
that this spooky action at a distance is quite real, which rules
out the particular version of hidden variables that Einstein
advocated. But that has not stopped other physicists from coming
up with interpretations of their own. These interpretations fall
into two broad camps. There are those that agree with Einstein
that the wavefunction represents our ignorance - what philosophers
call psi-epistemic models. And there are those that view the
wavefunction as a real entity - psi-ontic models.
To appreciate the difference,
consider a thought experiment that Schrödinger described
in a 1935 letter to Einstein. Imagine that a cat is enclosed
in a steel box. And imagine that the box also contains a sample
of radioactive material that has a 50% probability of emitting
a decay product in one hour, along with an apparatus that will
poison the cat if it detects such a decay. Because radioactive
decay is a quantum event, wrote Schrödinger, the rules of
quantum theory state that, at the end of the hour, the wavefunction
for the box's interior must be an equal mixture of live cat and
"Crudely speaking," says Fedrizzi, "in a psi-epistemic
model the cat in the box is either alive or it's dead and we
just don't know because the box is closed." But most psi-ontic
models agree with the Copenhagen interpretation: until an observer
opens the box and looks, the cat is both alive and dead.
But this is where the debate gets stuck. Which of quantum theory's
many interpretations - if any - is correct? That is a tough question
to answer experimentally, because the differences between the
models are subtle: to be viable, they have to predict essentially
the same quantum phenomena as the very successful Copenhagen
interpretation. Andrew White, a physicist at the University of
Queensland, says that for most of his 20-year career in quantum
technologies "the problem was like a giant smooth mountain
with no footholds, no way to attack it".
That changed in 2011, with the
publication of a theorem about quantum measurements that seemed
to rule out the wavefunction-as-ignorance models.2 On closer
inspection, however, the theorem turned out to leave enough wiggle
room for them to survive. Nonetheless, it inspired physicists
to think seriously about ways to settle the debate by actually
testing the reality of the wavefunction. Maroney had already
devised an experiment that should work in principle,3 and he
and others soon found ways to make it work in practice.4, 5, 6
The experiment was carried out last year by Fedrizzi, White and
To illustrate the idea behind
the test, imagine two stacks of playing cards. One contains only
red cards; the other contains only aces. "You're given a
card and asked to identify which deck it came from," says
Martin Ringbauer, a physicist also at the University of Queensland.
If it is a red ace, he says, "there's an overlap and you
won't be able to say where it came from". But if you know
how many of each type of card is in each deck, you can at least
calculate how often such ambiguous situations will arise.
Out on a limb
A similar ambiguity occurs in
quantum systems. It is not always possible for a single measurement
in the lab to distinguish how a photon is polarized, for example.
"In real life, it's pretty easy to tell west from slightly
south of west, but in quantum systems, it's not that simple,"
says White. According to the standard Copenhagen interpretation,
there is no point in asking what the polarization is because
the question does not have an answer - or at least, not until
another measurement can determine that answer precisely. But
according to the wavefunction-as-ignorance models, the question
is perfectly meaningful; it is just that the experimenters -
like the card-game player - do not have enough information from
that one measurement to answer. As with the cards, it is possible
to estimate how much ambiguity can be explained by such ignorance,
and compare it with the larger amount of ambiguity allowed by
That is essentially what Fedrizzi's
team tested. The group measured polarization and other features
in a beam of photons and found a level of overlap that could
not be explained by the ignorance models. The results support
the alternative view that, if objective reality exists, then
the wavefunction is real. "It's really impressive that the
team was able to address a profound issue, with what's actually
a very simple experiment," says Andrea Alberti, a physicist
at the University of Bonn in Germany.
The conclusion is still not ironclad, however: because the detectors
picked up only about one-fifth of the photons used in the test,
the team had to assume that the lost photons were behaving in
the same way.7 That is a big assumption, and the group is currently
working on closing the sampling gap to produce a definitive result.
In the meantime, Maroney's team at Oxford is collaborating with
a group at the University of New South Wales in Australia, to
perform similar tests with ions, which are easier to track than
photons. "Within the next six months we could have a watertight
version of this experiment," says Maroney.
But even if their efforts succeed
and the wavefunction-as-reality models are favoured, those models
come in a variety of flavours - and experimenters will still
have to pick them apart.
One of the earliest such interpretations
was set out in the 1920s by French physicist Louis de Broglie,8
and expanded in the 1950s by US physicist David Bohm.9, 10 According
to de BroglieBohm models, particles have definite locations
and properties, but are guided by some kind of 'pilot wave' that
is often identified with the wavefunction. This would explain
the double-slit experiment because the pilot wave would be able
to travel through both slits and produce an interference pattern
on the far side, even though the electron it guided would have
to pass through one slit or the other.
In 2005, de Broglie-Bohmian
mechanics received an experimental boost from an unexpected source.
Physicists Emmanuel Fort, now at the Langevin Institute in Paris,
and Yves Couder at the University of Paris Diderot gave the students
in an undergraduate laboratory class what they thought would
be a fairly straightforward task: build an experiment to see
how oil droplets falling into a tray filled with oil would coalesce
as the tray was vibrated. Much to everyone's surprise, ripples
began to form around the droplets when the tray hit a certain
vibration frequency. "The drops were self-propelled - surfing
or walking on their own waves," says Fort. "This was
a dual object we were seeing - a particle driven by a wave."
Since then, Fort and Couder have
shown that such waves can guide these 'walkers' through the double-slit
experiment as predicted by pilot-wave theory, and can mimic other
quantum effects, too.11 This does not prove that pilot waves
exist in the quantum realm, cautions Fort. But it does show how
an atomic-scale pilot wave might work. "We were told that
such effects cannot happen classically," he says, "and
here we are, showing that they do."
Another set of reality-based
models, devised in the 1980s, tries to explain the strikingly
different properties of small and large objects. "Why electrons
and atoms can be in two different places at the same time, but
tables, chairs, people and cats can't," says Angelo Bassi,
a physicist at the University of Trieste, Italy. Known as 'collapse
models', these theories postulate that the wavefunctions of individual
particles are real, but can spontaneously lose their quantum
properties and snap the particle into, say, a single location.
The models are set up so that the odds of this happening are
infinitesimal for a single particle, so that quantum effects
dominate at the atomic scale. But the probability of collapse
grows astronomically as particles clump together, so that macroscopic
objects lose their quantum features and behave classically.
One way to test this idea is to look for quantum behaviour in
larger and larger objects. If standard quantum theory is correct,
there is no limit. And physicists have already carried out double-slit
interference experiments with large molecules.12 But if collapse
models are correct, then quantum effects will not be apparent
above a certain mass. Various groups are planning to search for
such a cut-off using cold atoms, molecules, metal clusters and
nanoparticles. They hope to see results within a decade. "What's
great about all these kinds of experiments is that we'll be subjecting
quantum theory to high-precision tests, where it's never been
tested before," says Maroney.
Wave Particle weirdness
One wavefunction-as-reality model
is already famous and beloved by science-fiction writers: the
many-worlds interpretation developed in the 1950s by Hugh Everett,
who was then a graduate student at Princeton University in New
Jersey. In the many-worlds picture, the wavefunction governs
the evolution of reality so profoundly that whenever a quantum
measurement is made, the Universe splits into parallel copies.
Open the cat's box, in other words, and two parallel worlds will
branch out - one with a living cat and another containing a corpse.
Distinguishing Everett's many-worlds
interpretation from standard quantum theory is tough because
both make exactly the same predictions. But last year, Howard
Wiseman at Griffith University in Brisbane and his colleagues
proposed a testable multiverse model.13 Their framework does
not contain a wavefunction: particles obey classical rules such
as Newton's laws of motion. The weird effects seen in quantum
experiments arise because there is a repulsive force between
particles and their clones in parallel universes. "The repulsive
force between them sets up ripples that propagate through all
of these parallel worlds," Wiseman says.
Using computer simulations with as many as 41 interacting worlds,
they have shown that this model roughly reproduces a number of
quantum effects, including the trajectories of particles in the
double-slit experiment13. The interference pattern becomes closer
to that predicted by standard quantum theory as the number of
worlds increases. Because the theory predicts different results
depending on the number of universes, says Wiseman, it should
be possible to devise ways to check whether his multiverse model
is right - meaning that there is no wavefunction, and reality
is entirely classical.
Because Wiseman's model does
not need a wavefunction, it will remain viable even if future
experiments rule out the ignorance models. Also surviving would
be models, such as the Copenhagen interpretation, that maintain there is no objective reality - just measurements.
But then, says White, that is
the ultimate challenge. Although no one knows how to do it yet,
he says, "what would be really exciting is to devise a test
for whether there is in fact any objective reality out there
Nature 521, 278280 (21 May
Quantum physics: What is really
A wave of experiments is probing the root of quantum weirdness.
By Zeeya Merali, 20 May 2015
Zeeya Merali is a freelance writer
based in London.
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