WEBVTT

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Many of the inanimate objects around you
probably seem perfectly still.

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But look deep into the atomic structure
of any of them,

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and you'll see a world in constant flux.

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Stretching,

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contracting,

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springing,

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jittering,

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drifting atoms everywhere.

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And though that movement may seem chaotic,
it's not random.

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Atoms that are bonded together,

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and that describes almost all substances,

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move according to a set of principles.

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For example, take molecules,
atoms held together by covalent bonds.

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There are three basic ways 
molecules can move:

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rotation,

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translation,

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and vibration.

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Rotation and translation 
move a molecule in space

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while its atoms stay 
the same distance apart.

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Vibration, on the other hand, 
changes those distances,

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actually altering the molecule's shape.

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For any molecule, you can count up
the number of different ways it can move.

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That corresponds to 
its degrees of freedom,

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which in the context of mechanics

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basically means the number of variables
we need to take into account

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to understand the full system.

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Three-dimensional space is defined by
x, y, and z axes.

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Translation allows the molecule to move
in the direction of any of them.

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That's three degrees of freedom.

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It can also rotate around 
any of these three axes.

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That's three more,

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unless it's a linear molecule, 
like carbon dioxide.

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There, one of the rotations just spins
the molecule around its own axis,

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which doesn't count because it doesn't
change the position of the atoms.

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Vibration is where it gets a bit tricky.

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Let's take a simple molecule,
like hydrogen.

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The length of the bond that holds the two
atoms together is constantly changing

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as if the atoms were connected 
by a spring.

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That change in distance is tiny,
less than a billionth of a meter.

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The more atoms and bonds a molecule has,
the more vibrational modes.

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For example, a water molecule 
has three atoms:

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one oxygen and two hydrogens,
and two bonds.

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That gives it three modes of vibration:

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symmetric stretching,

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asymmetric stretching,

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and bending.

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More complicated molecules have even
fancier vibrational modes,

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like rocking,

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wagging,

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and twisting.

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If you know how many atoms a molecule has,
you can count its vibrational modes.

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Start with the total degrees of freedom,

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which is three times the number
of atoms in the molecule.

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That's because each atom can move
in three different directions.

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Three of the total correspond 
to translation

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when all the atoms 
are going in the same direction.

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And three, or two for linear molecules,
correspond to rotations.

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All the rest, 3N-6
or 3N-5 for linear molecules,

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are vibrations.

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So what's causing all this motion?

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Molecules move because they absorb
energy from their surroundings,

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mainly in the form of heat
or electromagnetic radiation.

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When this energy gets transferred
to the molecules,

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they vibrate,

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rotate,

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or translate faster.

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Faster motion increases the kinetic energy
of the molecules and atoms.

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We define this as an increase 
in temperature and thermal energy.

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This is the phenomenon your microwave oven
uses to heat your food.

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The oven emits microwave radiation,
which is absorbed by the molecules,

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especially those of water.

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They move around faster and faster,

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bumping into each other and increasing
the food's temperature and thermal energy.

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The greenhouse effect is another example.

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Some of the solar radiation 
that hits the Earth's surface

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is reflected back to the atmosphere.

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Greenhouse gases, like water vapor
and carbon dioxide absorb this radiation

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and speed up.

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These hotter, faster-moving molecules
emit infrared radiation in all directions,

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including back to Earth, warming it.

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Does all this molecular motion ever stop?

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You might think that would happen
at absolute zero,

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the coldest possible temperature.

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No one's ever managed to cool
anything down that much,

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but even if we could,

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molecules would still move due to
a quantum mechanical principle

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called zero-point energy.

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In other words, everything has been moving
since the universe's very first moments,

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and will keep going long, 
long after we're gone.