12 | New Scientist | 9 April 2022
News
WHEN a bubble pops in a liquid,
it can produce a flash of light,
which we now know is thanks
to quantum mechanics.
Sonoluminescence is a
phenomenon in which small
bubbles, produced and fixed in
place by an ultrasound wave in a
liquid, collapse and make particles
of light, or photons. Physicists
have known about this process
for decades, but the mechanisms
behind it weren’t fully known.
Now, Ebrahim Karimi at the
University of Ottawa, Canada,
and his colleagues have measured
the quantum nature of the
photons produced for the first
time, showing that the underlying
process is quantum mechanical.
Karimi and his team set up
a flask filled with water, which
contained a single air bubble,
and placed seven mirrors around
the flask to reflect photons onto a
photon-detecting camera. When
the bubble was made to collapse,
it produced a burst of photons,
which they measured sequentially.
In classical light sources, such
as a lamp or the sun, photons are
generated at random times andarrive at a destination – such as
a camera – with no discernible
pattern. But quantum processes
produce an effect called anti-
bunching, where photons arrive
with a certain regularity. It is
impossible to have anti-bunching
if a system is obeying classical
physics, says Karimi. “It requires
quantum [physics] to be involved.”
Karimi and his team used the
camera to calculate statisticalproperties of the photons that
are unique to quantum events.
“We did this for many different
samples during different days
with different timing,” says
Karimi. Doing so confirmed
the photons were correlated with
one another in a way consistent
with quantum processes
(arxiv.org/abs/2203.11337).
The experiment, which took
the researchers more than five
years to complete, was particularly
difficult because they didn’t know
when the incredibly short photon
pulse would be produced.“This is like standing alongside
a motorway and having one car
a day and trying to find it, not
knowing when the car will come,
and being there at the right
time,” says Claudia Eberlein at
Loughborough University, UK.
While the new information
doesn’t explain how the photons
were created, it certainly helps
narrow the possible sources.
Karimi speculates that they
could be the result of a form
of the dynamical Casimir effect,
a quantum mechanical process
where particles are produced
from a moving mirror, which
might be created at the surface
between the water and the bubble
when the bubble collapses.
Even if the photons’ origin is
never discovered, their quantum
nature could have practical
applications, says Karimi. To
produce purely quantum photons
for experiments, researchers
need expensive lasers and tools
that can cost millions of pounds.
It is possible that a much cheaper
set-up could be designed using
a flask of water exhibiting
sonoluminescence. ❚PhysicsAlex WilkinsSHUTTERST
OCK/OLEGRIBubbles give off weird light when
popped because of quantum physics
Bubbles sometimes
release flashes of
light when they popEnvironmentALMOST everyone on Earth lives
in areas with harmful levels of
air pollution that breach new
guidelines from the World
Health Organization (WHO).
The official figure is that
99 per cent of the world’s
population is affected, up from
90 per cent four years ago
under less stringent standards.
India has nine of the world’s
10 cities with the worst air pollutioncaused by tiny particles known
as PM2.5. Ahmedabad tops the list,
with Delhi in third, a new database
published this week by the WHO
shows. For a larger but still harmful
pollutant, PM10, the top 10 list
of the dirtiest places is more
diverse, including settlements in
Bahrain, India, Iran, Iraq, Pakistan,
South Africa and Saudi Arabia.
Both pollutants are caused
by a mix of fossil fuel burning in
cars and power plants, but also by
farming and natural sources such
as desert sand. Chinese cities,
which previously dominated lists
of the world’s most polluted urbanareas, have cleaned up their air
considerably. Beijing, famous for
its “airpocalypse” smog events in
the past, still has high annual levels
of PM2.5, but is now only the
76th most polluted city globally.
In a statement, WHO chief
Tedros Adhanom Ghebreyesus
said: “High fossil fuel prices,
energy security and the urgency
of addressing the twin health
challenges of air pollution andclimate change underscore the
pressing need to move faster
towards a world that is much
less dependent on fossil fuels.”
Low and middle-income countries
are worst affected by harmful levels
of particulate matter compared
with the global average. For a third
pollutant – nitrogen dioxide – the
economic split is less clear and
affluent countries are affected too.
The WHO updated its guidelines
for recommended air pollution
limits for the three pollutants
last September, although they
aren’t legally binding. ❚Harmful air pollution
now affects 99 per
cent of people“Addressing air pollution
underscores the pressing
need to be less dependent
on fossil fuels” Adam Vaughan