In 1977, the physicist Edward Purcell
calculated that if you push a bacteria and then let go,
it will stop in about a millionth of a second.
In that time, it will have traveled less than the width of a single atom.
The same holds true for a sperm and many other microbes.
It all has to do with being really small.
Microscopic creatures inhabit a world alien to us,
where making it through an inch of water is an incredible endeavor.
But why does size matter so much for a swimmer?
What makes the world of a sperm so fundamentally different
from that of a sperm whale?
To find out, we need to dive into the physics of fluids.
Here's a way to think about it.
Imagine you are swimming in a pool.
It's you and a whole bunch of water molecules.
Water molecules outnumber you a thousand trillion trillion to one.
So, pushing past them with your gigantic body is easy,
but if you were really small,
say you were about the size of a water molecule,
all of a sudden, it's like you're swimming
in a pool of people.
Rather than simply swishing by all the teeny, tiny molecules,
now every single water molecule
is like another person you have to push past
to get anywhere.
In 1883, the physicist Osborne Reynolds
figured out that there is one simple number
that can predict how a fluid will behave.
It's called the Reynolds number,
and it depends on simple properties like the size of the swimmer,
its speed, the density of the fluid,
and the stickiness, or the viscosity, of the fluid.
What this means is that creatures of very different sizes
inhabit vastly different worlds.
For example, because of its huge size,
a sperm whale inhabits the large Reynolds number world.
If it flaps its tail once,
it can coast ahead for an incredible distance.
Meanwhile, sperm live in a low Reynolds number world.
If a sperm were to stop flapping its tail,
it wouldn't even coast past a single atom.
To imagine what it would feel like to be a sperm,
you need to bring yourself down to its Reynolds number.
Picture yourself in a tub of molasses with your arms moving
about as slow as the minute hand of a clock,
and you'd have a pretty good idea of what a sperm is up against.
So, how do microbes manage to get anywhere?
Well, many don't bother swimming at all.
They just let the food drift to them.
This is somewhat like a lazy cow
that waits for the grass under its mouth to grow back.
But many microbes do swim,
and this is where those incredible adaptations come in.
One trick they can use is to deform the shape of their paddle.
By cleverly flexing their paddle
to create more drag on the power stroke than on the recovery stroke,
single-celled organisms like paramecia
manage to inch their way through the crowd of water molecules.
But there's an even more ingenious solution
arrived at by bacteria and sperm.
Instead of wagging their paddles back and forth,
they wind them like a cork screw.
Just as a cork screw on a wine bottle
converts winding motion into forward motion,
these tiny creatures spin their helical tails
to push themselves forward
in a world where water feels as thick as cork.
Other strategies are even stranger.
Some bacteria take Batman's approach.
They use grappling hooks to pull themselves along.
They can even use this grappling hook
like a sling shot and fling themselves forward.
Others use chemical engineering.
H. pylori lives only in the slimy, acidic mucus
inside our stomachs.
It releases a chemical that thins out the surrounding mucus,
allowing it to glide through slime.
Maybe it's no surprise
that these guys are also responsible for stomach ulcers.
So, when you look really closely at our bodies and the world around us,
you can see all sorts of tiny creatures
finding clever ways to get around in a sticky situation.
Without these adaptations, bacteria would never find their hosts,
and sperms would never make it to their eggs,
which means you would never get stomach ulcers,
but you would also never be born in the first place.
(Pop)