THERE 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 object shouldn’t
affect what the object does in the 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 objects through a slit one at a time to see if
they behave like waves by forming interference patterns, to show that
proteins obey quantum rules.
This approach has its problems. When you are working with big, complex
objects, their quantumness quickly disappears as a result of interaction
with the surrounding environment – a phenomenon called decoherence. Quantum
states are fragile. They easily break under bombardment from gas molecules,
stray photons of light and even delicate electric and magnetic fields. “Any
quantum object can behave classically if you’re not treating it properly,”
says Chiara Marletto at the University of Oxford. This is especially
troublesome for double-slit experiments because it takes a long time to
build up the double-slit interference pattern – time in which decoherence
can run riot.
Leggett-Garg experiments are just as tricky. They have their own sources of
decoherence, but researchers must also find ways to measure a system without
disturbing it. Only by doing this can you say for sure whether the object is
in a quantum superposition or not. “You have to do the measurement in a
clever way,” says Sinha. “You’re trying to measure something, but on the
other hand, you want to ensure that the act of measurement doesn’t leave any
invasive mark.”
It is impossible to drag most quantum systems – which move in discrete steps
– into the classical world, where movement is continuous. This makes it hard
to examine quantum objects and the stuff we would usually think of as
classical in the same experiment. But Sougato Bose, a theorist at University
College London, has a plan. He proposes using an experimental set-up that
can transcend the classical and quantum worlds.
The set-up he has in mind, a simple harmonic oscillator, comprises an object
trapped inside a well, moving back and forth like a swinging pendulum.
Precisely how it oscillates depends on whether it obeys quantum or classical
rules. And as there is theoretically no limit to how big a simple harmonic
oscillator can be, Bose and his collaborators hope to use it to take a leap
into the macroscopic world – using a nanocrystal 100,000 times more massive
than objects tested by Arndt’s team.
To do this, the researchers’ idea is to look for the swinging nanocrystal
when they expect it to be exactly in the middle of the oscillator, on the
border between left and right (see “Reality in the balance,”). “We don’t
observe, and then we suddenly take a snapshot observation,” says Bose. But
crucially, the detector will only be able to see one half of the oscillator.
If it sees the nanocrystal, the researchers know it is in that side. If the
detector doesn’t, they know it is in the other.
If the crystal is behaving in a classical way, it should be there half of the
time on this first measurement. Then, after waiting the time it takes to
complete half a swing and so return to the centre of the system, the
researchers would measure again and would expect to see it half the time. But
if the particle is quantum, the act of not seeing it in one half of the
oscillator would collapse its so-called wave function – the mathematical
description of the quantum state. Even though we don’t see the nanocrystal, we
now know its position, and because of quantum uncertainty, this injects the
particle with momentum and changes the way it is oscillating. By repeating
measurements at set intervals, the researchers hope to be able to build up
correlations that tell them whether the nanocrystal is behaving in a quantum
way or classically. The trick in all of this is to throw out the measurements
in which the nanocrystal is seen and only keep the ones in which it isn’t, so
that the measurements are non-invasive.
Since Bose and his collaborators proposed the experiment in 2018, advances
in the trapping and cooling of nanocrystals to avoid decoherence, alongside
new precision lasers, mean the idea can now be realised. Teaming up with
Hendrik Ulbricht, an experimentalist at the University of Southampton, UK,
Bose plans to carry out the test on a nanocrystal made of about a billion
atoms. “It’s a big jump,” says Ulbricht.
Only recently have lasers become sharp enough to determine which side of the
trap the nanocrystal is oscillating in. Bigger particles are described by
smaller waves and so for these nanocrystals, the laser must be able to
resolve widths about the size of a water molecule. Ulbricht and Bose expect
to have results within the next six months. If the experiment violates
Leggett-Garg inequalities, it will break the concept of realism in
macroscopic objects.
Still, even if this is the end result, it will be hard to convince everyone
that the quantum world extends this far. For one, Leggett-Garg experiments
actually test whether a system behaves classically; if it doesn’t, it is
assumed to be behaving quantum-mechanically, but that might not truly be the
case. Another stumbling block is the array of loopholes that might lead to
Leggett-Garg inequalities being violated even though the system is behaving
classically. Although measurements should be non-invasive, the
practicalities open up so-called clumsiness loopholes. “The stubborn
macrorealist cynic could always say: ‘Ah, the measurement itself disturbed
the system’,” says Halliwell, who is dreaming up new methods of measurement
to avoid such problems.
Brick walls
Then there is the collusion loophole, in which particles outside your
experiment make it look like macrorealism is violated when it isn’t. And
let’s not forget the detection loophole, where a detector’s inability to
register every particle that comes its way can distort the result.
Researchers like Sinha are busy trying to close all the possible loopholes
in Leggett-Garg experiments. Earlier this year, her lab carried out the most
watertight test of macrorealism yet, according to Halliwell, in a system
made of photons. “We have closed the remaining loopholes for now, but you
can never claim that it’s completely loophole-free,” says Sinha.
Loophole-free tests of Bell’s inequality were only published in 2015, half a
century after Bell’s original idea. Even now, eagle-eyed physicists keep
pointing out possible new cracks in the design of these experiments.
Ulbricht acknowledges that their experiment is likely to contain loopholes
too. “There will be a very healthy and long debate, I’m sure,” he says.
No experiment has ever contradicted quantum mechanics. And there is no reason
to think that quantum weirdness doesn’t apply to objects as big as the moon
and beyond, so long as you isolate your system from the environment’s
decoherence. “From a theoretical point of view, quantum theory doesn’t put any
limitation on how large an object you can put in a quantum superposition,”
says Marletto.
But Ulbricht hopes that these experiments might reveal a brick wall that no
quantum system can go beyond. Such a wall between the quantum and classical
worlds would save reality as we know it, offering a way for our common-sense
world to appear out of quantum weirdness. “There could be a universal
mechanism, which is turning all quantum systems into classical ones as soon
as they hit the brick wall,” he says.
One idea, suggested in 1987 by Lajos Diósi at the Wigner Research Centre for
Physics in Hungary and Roger Penrose at the University of Oxford, is that
our classical reality emerges through instabilities in the structure of
space-time.
By testing whether quantum mechanics applies to bigger and bigger objects,
Ulbricht says we can rule out some models of objective collapse theory, an
extension of quantum mechanics that makes predictions about where the brick
wall should be. Yet it will be impossible to be certain this wall exists, as
the collapse might be caused by run-of-the-mill decoherence. “Sometimes, the
environment may be very conspiratorial,” says Bose. Whether a brick wall or
something else entirely, “finding a deviation from the predictions of
quantum theory – whether or not you like quantum theory – is great, because
we would be able to then try to find a new theory”, says Marletto. “People
are frustrated that quantum theory is really good at being confirmed
experimentally.”
So is the moon there when you don’t look, or is that tree in the forest even
there to fall in the first place? As tests of Leggett-Garg inequalities
creep into the truly macroscopic world, the answer, increasingly, is no. “If
macroscopic realism is violated, then you can’t assume the moon is there,”
says Halliwell. Reality as we think of it might not be real after all.
It is even possible that future Leggett-Garg inequalities would not only
disagree with the rules of the classical world, but also break the so-far
unbreakable quantum ones. “This would give you a glimpse into some kind of
post-quantum world,” says Vedral. “It’s hard to imagine what this could be,
but I think we’re going to find something even weirder.”
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