WEBVTT

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In 1977, the physicist Edward Purcell

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calculated that if you push
a bacteria and then let go,

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it will stop in about
a millionth of a second.

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In that time, it will have traveled less
than the width of a single atom.

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The same holds true for a sperm
and many other microbes.

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It all has to do with being really small.

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Microscopic creatures inhabit
a world alien to us,

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where making it through an inch of water
is an incredible endeavor.

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But why does size matter
so much for a swimmer?

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What makes the world of a sperm
so fundamentally different

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from that of a sperm whale?

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To find out, we need to dive
into the physics of fluids.

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Here's a way to think about it.

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Imagine you are swimming in a pool.

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It's you and a whole bunch
of water molecules.

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Water molecules outnumber you
a thousand trillion trillion to one.

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So, pushing past them
with your gigantic body is easy,

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but if you were really small,

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say you were about the size
of a water molecule,

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all of a sudden, it's like you're swimming

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in a pool of people.

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Rather than simply swishing by
all the teeny, tiny molecules,

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now every single water molecule

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is like another person
you have to push past

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to get anywhere.

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In 1883, the physicist Osborne Reynolds

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figured out that there is
one simple number

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that can predict how a fluid will behave.

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It's called the Reynolds number,

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and it depends on simple properties
like the size of the swimmer,

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its speed, the density of the fluid,

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and the stickiness,
or the viscosity, of the fluid.

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What this means is that creatures
of very different sizes

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inhabit vastly different worlds.

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For example, because of its huge size,

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a sperm whale inhabits
the large Reynolds number world.

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If it flaps its tail once,

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it can coast ahead
for an incredible distance.

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Meanwhile, sperm live
in a low Reynolds number world.

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If a sperm were to stop flapping its tail,

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it wouldn't even coast past a single atom.

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To imagine what it would
feel like to be a sperm,

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you need to bring yourself down
to its Reynolds number.

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Picture yourself in a tub of molasses
with your arms moving

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about as slow as the minute
hand of a clock,

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and you'd have a pretty good idea
of what a sperm is up against.

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So, how do microbes
manage to get anywhere?

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Well, many don't bother swimming at all.

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They just let the food drift to them.

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This is somewhat like a lazy cow

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that waits for the grass
under its mouth to grow back.

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But many microbes do swim,

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and this is where those
incredible adaptations come in.

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One trick they can use
is to deform the shape of their paddle.

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By cleverly flexing their paddle

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to create more drag on the power stroke
than on the recovery stroke,

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single-celled organisms like paramecia

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manage to inch their way
through the crowd of water molecules.

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But there's an even more
ingenious solution

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arrived at by bacteria and sperm.

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Instead of wagging
their paddles back and forth,

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they wind them like a cork screw.

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Just as a cork screw on a wine bottle

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converts winding motion
into forward motion,

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these tiny creatures
spin their helical tails

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to push themselves forward

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in a world where water
feels as thick as cork.

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Other strategies are even stranger.

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Some bacteria take Batman's approach.

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They use grappling hooks
to pull themselves along.

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They can even use this grappling hook

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like a sling shot
and fling themselves forward.

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Others use chemical engineering.

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H. pylori lives only
in the slimy, acidic mucus

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inside our stomachs.

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It releases a chemical
that thins out the surrounding mucus,

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allowing it to glide through slime.

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Maybe it's no surprise

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that these guys are also responsible
for stomach ulcers.

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So, when you look really closely
at our bodies and the world around us,

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you can see all sorts of tiny creatures

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finding clever ways to get around
in a sticky situation.

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Without these adaptations,
bacteria would never find their hosts,

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and sperms would never
make it to their eggs,

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which means you would never
get stomach ulcers,

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but you would also never be born
in the first place.

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(Pop)