6 November 2021 | New Scientist | 39the future. Instead, quantum mechanics adds
uncertainty and superpositions, mixtures of
many possible identities that collapse into
one value when a measurement is taken.
Realism in macroscopic objects is called,
you guessed it, macrorealism. When you look
at the moon or measure how far away it is
with a laser, you don’t change it – at least, not
according to our common-sense view of the
world. “Macrorealism is the fullest expression
of classical reality,” says Halliwell. And, like
Bell’s inequality, there is a test for it.Testing reality
The Leggett-Garg inequality, devised in 1985 by
Anthony Leggett and Anupam Garg, also looks
for correlations between measurements to see
whether quantum or classical rules are being
followed. But instead of two particles separated
in space, like Bell’s inequality, this deals with a
single object over time. Because of this, Leggett
and Garg realised they could, in theory, test
the quantumness of very big objects. In other
words, their inequality could tell us whether
realism holds true in the everyday world.
In recent years, the first Leggett-Garg
experiments have been carried out on simple
quantum systems from superconducting
fluids and photons to atomic nuclei and tiny
crystals. These have demonstrated once again
that the microscopic world is non-real. The
trick with Leggett-Garg experiments is to
make sure they are non-invasive, which
means there needs to be a way of measuring
a particle without disturbing it. That isn’t
easy, but it can be done. And in each case, the
researchers found that for every non-invasive
measurement they could make, the system
was in a superposition of states.
Now, it is time to test something bigger.
“It all boils down to seeing how far we can push
this,” says Urbasi Sinha at the Raman Research
Institute in India. “We don’t really know.”
The largest things currently known to behave
in a quantum way were observed by Markus
Arndt and his colleagues at the University
of Vienna, Austria, who were performing a
different kind of experiment. In 2020, they
used a double-slit set-up, passing objectsT
HERE is an old philosophy question
about a tree in a forest. If it falls with
nobody there to hear it, does it make
a sound? Ask a quantum physicist and they
might say the sound was there – but you
couldn’t be sure the tree was.
Quantum mechanics has long pushed the
boundaries of our understanding of reality at its
tiniest. Countless experiments have shown that
particles spread out like waves, for instance,
or seem be in more than one place at once.
In the quantum world, we can only know the
likelihood that something will appear in one
place or another – until we look, at which point
it assumes a definite position. This troubled
Albert Einstein. “I like to think that the moon
is there even if I am not looking at it,” he said.
Now, a new class of experiments is putting
Einstein’s conviction to the test, seeing if
quantum weirdness stretches beyond the
tiny world of quarks, atoms and qubits into
the everyday world of tables, chairs and, well,
moons. “If you can go from one atom to two
atoms to three to four to five to a thousand, is
there any reason why it stops?” says Jonathan
Halliwell at Imperial College London.
These experiments are not just investigating
whether there is a hard boundary between the
quantum and classical worlds, but also probing
the true nature of reality. If the work goes as
some theorists expect, it might just kick the
legs out from under one of our most firmly
held beliefs: that things exist regardless of
whether we are looking at them.
In 1935, Einstein came up with a thought
experiment designed to reveal that quantum
mechanics was an incomplete theory of reality
that must, sooner or later, be replaced. Together
with his colleagues Boris Podolsky and Nathan
Rosen, he imagined a pair of particles entangled
with each other so that whatever you do to one
instantly affects the other. Measure, say, the
position or velocity of one particle and it will
reveal the position or velocity of the other
without having to measure it. Now imagine
placing these particles at opposite ends
of the universe and performing the same
measurement. At first glance, it seems that
information is being transmitted between
the particles faster than the speed of light.Einstein argued that this “spooky action at
a distance” was so absurd that the outcome
of any entanglement experiments must be
predetermined. Physicist John Bell was also
uncomfortable with the ill-defined nature
of quantum mechanics. In 1964, he devised
a mathematical way to put this paradox to
the test, called Bell’s inequality. If Einstein
and his colleagues were right, then Bell’s
inequality would be fulfilled.
Again and again, experiments have found
that Bell’s inequality is violated. If you insist
that reality behaves classically, as opposed
to in a quantum way, then to account for
entanglement and the violation of Bell’s
inequality, “you have to assume that
something happens faster than the speed
of light”, says Vlatko Vedral at the University
of Oxford. Take your pick, Einstein: quantum
weirdness is a reality or information breaks
the universal cosmic speed limit.
But that is only part of the story. Bell’s
inequality addresses locality, the idea that
the space between objects matters. It doesn’t
answer the question of whether the moon
is there when you aren’t looking. Realism
says the position, speed, energy and other
properties of particles can be reasonably well
defined and performing a measurement on an
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