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The coldest materials in the world 
aren’t in Antarctica.

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They’re not at the top of Mount Everest

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or buried in a glacier.

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They’re in physics labs:

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clouds of gases held just fractions
of a degree above absolute zero.

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That’s 395 million times colder 
than your refrigerator,

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100 million times colder 
than liquid nitrogen,

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and 4 million times colder
than outer space.

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Temperatures this low give scientists a
window into the inner workings of matter,

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and allow engineers to build 
incredibly sensitive instruments

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that tell us more about everything

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from our exact position on the planet

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to what’s happening in 
the farthest reaches of the universe.

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How do we create such 
extreme temperatures?

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In short, by slowing down 
moving particles.

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When we’re talking about temperature,
what we’re really talking about is motion.

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The atoms that make up solids,

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liquids,

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and gases

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are moving all the time.

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When atoms are moving more rapidly,
we perceive that matter as hot.

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When they’re moving more
slowly, we perceive it as cold.

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To make a hot object 
or gas cold in everyday life,

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we place it in a colder environment, 
like a refrigerator.

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Some of the atomic motion in the hot
object is transferred to the surroundings,

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and it cools down.

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But there’s a limit to this:

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even outer space is too warm
to create ultra-low temperatures.

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So instead, scientists figured out a way 
to slow the atoms down directly –

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with a laser beam.

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Under most circumstances,

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the energy in a laser beam 
heats things up.

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But used in a very precise way,

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the beam’s momentum can stall 
moving atoms, cooling them down.

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That’s what happens in a device 
called a magneto-optical trap.

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Atoms are injected into a vacuum chamber,

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and a magnetic field 
draws them towards the center.

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A laser beam aimed 
at the middle of the chamber

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is tuned to just the right frequency

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that an atom moving towards it will absorb
a photon of the laser beam and slow down.

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The slow down effect comes from
the transfer of momentum

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between the atom and the photon.

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A total of six beams, 
in a perpendicular arrangement,

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ensure that atoms traveling 
in all directions will be intercepted.

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At the center, where the beams intersect,

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the atoms move sluggishly, 
as if trapped in a thick liquid —

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an effect the researchers who invented it 
described as “optical molasses.”

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A magneto-optical trap like this

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can cool atoms down 
to just a few microkelvins —

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about -273 degrees Celsius.

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This technique was developed in the 1980s,

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and the scientists 
who'd contributed to it

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won the Nobel Prize in Physics in 1997
for the discovery.

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Since then, laser cooling has been 
improved to reach even lower temperatures.

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But why would you want 
to cool atoms down that much?

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First of all, cold atoms can make
very good detectors.

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With so little energy,

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they’re incredibly sensitive 
to fluctuations in the environment.

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So they’re used in devices that find 
underground oil and mineral deposits,

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and they also make 
highly accurate atomic clocks,

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like the ones used 
in global positioning satellites.

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Secondly, cold atoms hold 
enormous potential

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for probing the frontiers of physics.

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Their extreme sensitivity 
makes them candidates

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to be used to detect gravitational waves 
in future space-based detectors.

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They’re also useful for the study 
of atomic and subatomic phenomena,

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which requires measuring incredibly 
tiny fluctuations in the energy of atoms.

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Those are drowned out 
at normal temperatures,

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when atoms speed around 
at hundreds of meters per second.

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Laser cooling can slow atoms to just 
a few centimeters per second—

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enough for the motion caused by
atomic quantum effects to become obvious.

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Ultracold atoms have already 
allowed scientists to study phenomena

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like Bose-Einstein condensation,

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in which atoms are cooled almost 
to absolute zero

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and become a rare new state of matter.

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So as researchers continue in their quest 
to understand the laws of physics

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and unravel the mysteries of the universe,

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they’ll do so with the help 
of the very coldest atoms in it.