1 January 2022 | New Scientist | 49We know that whenever we measure
something, we affect it, but the nitty-gritty
of this process is often ignored in quantum
mechanics. According to Huber, the act of
measuring should create a flow of energy
that may be best described by the laws of
thermodynamics. “I think the measurement
postulate is the second law in disguise,” he
says. Perhaps quantum measurements, like
clocks, create rising entropy and hence an
emergent arrow of time.
Erker, on the other hand, points out the
research could also help to test ideas that
combine the notoriously clashing theories
of quantum mechanics and general relativity
into a quantum theory of gravity. Such tests
are extremely hard. Because gravity is so
weak, you either need to put massive objects
in a quantum superposition state to probe
gravitational effects,which is tricky and has
only been done in molecules of up to 2000
atoms. Or you need to be able to make
incredibly precise measurements – and
quantum clocks could help with that. “If we
could build clocks that are accurate on very
short timescales, we could actually build
tabletop quantum experiments that test
for gravitational effects,” says Erker.
Any theory that explains gravity and
quantum mechanics needs to describe how
clocks work on the quantum scale. “All this
research on understanding what clocks
really are and how they kind of interact with
quantum mechanics and with relativity is
probably an important step to understanding
how those theories fit together,” says Adlam.
Bergson and Einstein’s debate cost the
physicist the Nobel prize for general relativity.
The president of the Nobel committee said that,
while it was complex, “it will be no secret that
the famous philosopher Bergson in Paris has
challenged this theory”. Instead, Einstein won
for the less-glamorous photoelectric effect.
But a century on, Einstein now seems the real
winner of the debate. The next question is
whether there will ever be a way to merge his
theory of general relativity with quantum
mechanics. On that, only time will tell. ❚long-standing obsession with it,” he says.
Unlike relativity, in which time is local and
relative, quantum mechanics assumes there is
a universal background time. Time in quantum
mechanics doesn’t have an arrow: equations
work equally well forwards as backwards in
time. But sometimes this reversibility can be
broken. When we measure a quantum system,
the act of measuring causes the system to
collapse from a superposition, a mix of
different possible states, into a specific
outcome. This cannot be reversed, creating
an arrow of time. How time manages both
to have, and not have, an arrow is just one of
quantum mechanics’ many puzzles. But if
the thermodynamic arrow can explain our
perceptual time arrow, maybe it can explain
the quantum one too.
This is what Huber wants to tackle next.G.E.^ M
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AMiriam Frankel is a science
journalist based in London“I’m not able to prove this statement yet,”
says Erker. “But my hunch definitely goes in
this direction.”
If he’s right, it would have meaning beyond
proving an arrow of time exists outside of our
consciousness. The link between clocks and
thermodynamics may also reflect time on
a smaller scale. If there is a limit on how
accurately we can resolve time, could this be
a sign that time itself isn’t perfectly smooth,
but instead lumpy – packed into tiny units in
the same way that light comes in photons?
Answering this could be tricky. To probe
space-time at this tiniest of scales, below those
we can currently reach with our best particle
accelerators, would require vast amounts
of energy. At a certain level of energy, you
would expect to create a black hole that would
swallow the entire experiment, suggesting it
is impossible to resolve time perfectly. “You
end up with a sort of fundamental limit on the
sensitivity to which you can measure a time
interval,” says Adlam. This might be related to
the limit caused by thermodynamics, she says,
but the link isn’t clear yet.
Probing time at minuscule scales is
exciting, but what Huber is most thrilled about
relates to quantum mechanics and a mystery
called the measurement problem. “I have a
“ Could this be a sign that
time is not perfectly
smooth, but lumpy -
packed into tiny units?”
The most accurate
atomic clocks are
powered by lasers