48 | New Scientist | 1 January 2022law doesn’t tell the whole picture. There is still
a question of why we only ever experience
time moving forwards – many physicists
today argue that this is simply an illusion.
If the arrow of time in thermodynamics
could be linked to the practical reality of
measuring time, then thermodynamics could
help explain how we perceive time after all,
says Adlam. What is needed, she says, is a direct
link between thermodynamics and practical
timekeeping – something explaining why all
clocks run in the same direction as the entropy
increase of the universe. Find this link, and
we might just answer some of the questions
Einstein and Bergson were at odds over. In
search of this connection, a handful of
researchers are turning to clocks.
A few years ago, Paul Erker at the Institute for
Quantum Optics and Quantum Information in
Vienna, Austria, teamed up with Marcus Huber
at the Vienna University of Technology in an
attempt to understand what clocks really are.
They started off by modelling quantum clocks,
simple systems in which the flow of energy is
easy to track. In a 2017 paper, they and their
colleagues showed a clock made of just three
atoms – one hot, one cold and one “ticking”
thanks to the energy flow between the two
others – should dissipate more energy the
more accurate it is. This was a big step, butstill purely theoretical. By 2020, they were
ready to test it.
Teaming up with researchers including
Natalia Ares at the University of Oxford
and Edward Laird at Lancaster University,
both in the UK, the researchers built a simple
pendulum clock from a suspended membrane
of silicon nitride with a thickness of about
50 nanometres. “You could think of it more
like a drum than a pendulum,” says Laird.
They made their tiny drum vibrate, with each
vibration corresponding to one tick of the
clock. The strength of the vibrations could
be increased by applying an electric field. To
determine the clock’s accuracy – how regularly
the ticks occurred – they connected it to an
electrical circuit including a voltmeter. “It is
a beautiful experiment,” says Milburn.
The crux of that experiment was that the
clock became more accurate as more energywas supplied to the drum. And the more
accurate it was, the more entropy it produced.
This was the first result to explain why clocks
move forwards in time, because as they
measure time, they increase entropy, an
irreversible process. “This research gives
a very nice explicit link between the
thermodynamic arrow of time and
perceptual time,” says Adlam.
Carlo Rovelli at Aix-Marseille University
in France agrees the work sharpens our
understanding of the strict relationship
between time and heat. “Simply put, if there is
no heat involved, there is no way to distinguish
the past from the future,” he says. The research
strengthens his thermal time hypothesis,
which argues that time emerges from the
laws of thermodynamics on the macroscopic
scale of humans, regardless of what is going
on at a microscopic level.
Crucially, the research also shows that the
arrow of time isn’t something only humans
can experience. “It doesn’t really matter if it’s
a conscious agent who observes the clock or
a device, such as a detector,” says Huber. The
entropy still increases. “It’s true for anything.”
Rather than being a consequence of our
consciousness, this suggests the way we
perceive time may be physically built into
the process of timekeeping. If so, Bergson’s
argument falls apart and Einstein looks right
to have believed time is a physical entity.
This isn’t the first time a link between
energy cost and the accuracy of clocks has
been explored. A similar relationship between
accuracy and energy cost has been seen in the
biochemical clocks that operate inside ocean-
dwelling cyanobacteria, helping them generate
the chemicals needed for photosynthesis early
in the morning before the sun rises. This is
partly because they are living organisms,
not mechanical clocks. “Evolution probably
places additional constraints on what it means
for a clock to be good, beyond the energetic
constraints of precision,” says Jordan
Horowitz at the University of Michigan.
But not all clocks entirely follow the
rules, it would seem. The most accurate
atomic clocks appear more efficient than
the research predicts. These clocks involve
complex circuits, detectors and feedback,
making their energy flow difficult to model.
Both Erker and Huber are confident they
RIG will be shown to obey the same constraint.
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“ The way we perceive
time may be physically
built into the process
of timekeeping”
The efficiency of
engines is governed by
thermodynamic laws