1 January 2022 | New Scientist | 47To produce their ticks, clocks need a source
of energy. A grandfather clock must be wound
up and a wall clock is powered by a battery. The
most accurate atomic clocks, with ticks that
correspond to electromagnetic signals given
off by atoms changing energy levels, are driven
by high-powered lasers.
This isn’t surprising. But rather than just
requiring energy to run each mechanical
part, new research suggests something
more might be at play. Clocks could be a type
of so-called thermodynamic machine, with
fundamental constraints on their performance
set by the underlying physics. If this is true, not
only will it mean there could be a limit to how
accurately we can measure time’s passing, it
“will have a huge impact on how philosophers
think about time”, says Gerard Milburn, a
quantum physicist at the University of
Queensland, Australia.
We know of two types of thermodynamic
machine. The first comprises heat
engines – things like fridges and combustion
engines – which have a maximum efficiency
set by thermodynamics. The second
group encompasses information storage
devices, like DNA and hard discs. In these,
thermodynamics tells us the cost of erasing
information. If clocks are a third, it would
mean there are limits on how accurately we
can tell the time, due to the involvement of
energy’s messy cousin, entropy.
The maximum efficiency of heat engines
was determined by engineer Sadi Carnot
in 1824, before entropy was defined. But his
calculation paved the way for the discovery of
the second law of thermodynamics, which says
any closed system – something that nothing
can enter or leave – will increase in entropy, a
measure of disorder or randomness, over time.
Low entropy means high order. If the atoms
in a box of gas clustered in one corner rather
than being spread out chaotically, entropy
would be low. But because there are fewer
ways for atoms to be ordered than disordered,
making the latter more likely, closed systems –
like the universe – tend towards disorder. A cup
of hot tea loses heat to its surroundings, raising
overall entropy, but never spontaneously heats
up. This creates an arrow of time.
The second law is the only law of physics in
which rules are irreversible in time. Because of
this, thermodynamics is used to explain the
arrow of time we perceive. But the second >