What is the coldest thing in the world? - Lina Marieth Hoyos
The coldest materials in the world
aren’t in Antarctica.
They’re not at the top of Mount Everest
or buried in a glacier.
They’re in physics labs:
clouds of gases held just fractions
of a degree above absolute zero.
That’s 395 million times colder
than your refrigerator,
100 million times colder
than liquid nitrogen,
and 4 million times colder
than outer space.
Temperatures this low give scientists a
window into the inner workings of matter,
and allow engineers to build
incredibly sensitive instruments
that tell us more about everything
from our exact position on the planet
to what’s happening in
the farthest reaches of the universe.
How do we create such
extreme temperatures?
In short, by slowing down
moving particles.
When we’re talking about temperature,
what we’re really talking about is motion.
The atoms that make up solids,
liquids,
and gases
are moving all the time.
When atoms are moving more rapidly,
we perceive that matter as hot.
When they’re moving more
slowly, we perceive it as cold.
To make a hot object
or gas cold in everyday life,
we place it in a colder environment,
like a refrigerator.
Some of the atomic motion in the hot
object is transferred to the surroundings,
and it cools down.
But there’s a limit to this:
even outer space is too warm
to create ultra-low temperatures.
So instead, scientists figured out a way
to slow the atoms down directly –
with a laser beam.
Under most circumstances,
the energy in a laser beam
heats things up.
But used in a very precise way,
the beam’s momentum can stall
moving atoms, cooling them down.
That’s what happens in a device
called a magneto-optical trap.
Atoms are injected into a vacuum chamber,
and a magnetic field
draws them towards the center.
A laser beam aimed
at the middle of the chamber
is tuned to just the right frequency
that an atom moving towards it will absorb
a photon of the laser beam and slow down.
The slow down effect comes from
the transfer of momentum
between the atom and the photon.
A total of six beams,
in a perpendicular arrangement,
ensure that atoms traveling
in all directions will be intercepted.
At the center, where the beams intersect,
the atoms move sluggishly,
as if trapped in a thick liquid —
an effect the researchers who invented it
described as “optical molasses.”
A magneto-optical trap like this
can cool atoms down
to just a few microkelvins —
about -273 degrees Celsius.
This technique was developed in the 1980s,
and the scientists
who'd contributed to it
won the Nobel Prize in Physics in 1997
for the discovery.
Since then, laser cooling has been
improved to reach even lower temperatures.
But why would you want
to cool atoms down that much?
First of all, cold atoms can make
very good detectors.
With so little energy,
they’re incredibly sensitive
to fluctuations in the environment.
So they’re used in devices that find
underground oil and mineral deposits,
and they also make
highly accurate atomic clocks,
like the ones used
in global positioning satellites.
Secondly, cold atoms hold
enormous potential
for probing the frontiers of physics.
Their extreme sensitivity
makes them candidates
to be used to detect gravitational waves
in future space-based detectors.
They’re also useful for the study
of atomic and subatomic phenomena,
which requires measuring incredibly
tiny fluctuations in the energy of atoms.
Those are drowned out
at normal temperatures,
when atoms speed around
at hundreds of meters per second.
Laser cooling can slow atoms to just
a few centimeters per second—
enough for the motion caused by
atomic quantum effects to become obvious.
Ultracold atoms have already
allowed scientists to study phenomena
like Bose-Einstein condensation,
in which atoms are cooled almost
to absolute zero
and become a rare new state of matter.
So as researchers continue in their quest
to understand the laws of physics
and unravel the mysteries of the universe,
they’ll do so with the help
of the very coldest atoms in it.
