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Imagine an airplane flying
one millimeter above the ground

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and circling the Earth
once every 25 seconds

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while counting every blade of grass.

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Shrink all that down so that it fits
in the palm of your hand,

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and you'd have something equivalent
to a modern hard drive,

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an object that can likely hold
more information than your local library.

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So how does it store so much information
in such a small space?

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At the heart of every hard drive
is a stack of high-speed spinning discs

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with a recording head 
flying over each surface.

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Each disc is coated with a film
of microscopic magnetised metal grains,

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and your data doesn't live there
in a form you can recognize.

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Instead, it is recorded 
as a magnetic pattern

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formed by groups of those tiny grains.

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In each group, also known as a bit,

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all of the grains have 
their magnetization's aligned

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in one of two possible states,

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which correspond to zeroes and ones.

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Data is written onto the disc

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by converting strings of bits 
into electrical current

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fed through an electromagnet.

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This magnet generates a field
strong enough to change the direction

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of the metal grain's magnetization.

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Once this information is written 
onto the disc,

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the drive uses a magnetic reader
to turn it back into a useful form,

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much like a phonograph needle
translates a record's grooves into music.

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But how can you get so much information
out of just zeroes and ones?

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Well, by putting lots of them together.

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For example, a letter is represented
in one byte, or eight bits,

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and your average photo 
takes up several megabytes,

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each of which is 8 million bits.

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Because each bit must be written onto
a physical area of the disc,

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we're always seeking to increase
the disc's areal density,

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or how many bits can be squeezed
into one square inch.

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The areal density of a modern hard drive
is about 600 gigabits per square inch,

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300 million times greater than that
of IBM's first hard drive from 1957.

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This amazing advance in storage capacity

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wasn't just a matter 
of making everything smaller,

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but involved multiple innovations.

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A technique called the thin film
lithography process

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allowed engineers 
to shrink the reader and writer.

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And despite its size, 
the reader became more sensitive

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by taking advantage of new discoveries in
magnetic and quantum properties of matter.

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Bits could also be packed closer together
thanks to mathematical algorithms

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that filter out noise
from magnetic interference,

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and find the most likely bit sequences
from each chunk of read-back signal.

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And thermal expansion control of the head,

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enabled by placing a heater 
under the magnetic writer,

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allowed it to fly less than
five nanometers above the disc's surface,

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about the width of two strands of DNA.

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For the past several decades,

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the exponential growth in computer
storage capacity and processing power

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has followed a pattern 
known as Moore's Law,

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which, in 1975, predicted that information
density would double every two years.

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But at around 100 gigabits 
per square inch,

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shrinking the magnetic grains further
or cramming them closer together

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posed a new risk
called the superparamagnetic effect.

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When a magnetic grain volume is too small,

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its magnetization is easily disturbed
by heat energy

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and can cause bits 
to switch unintentionally,

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leading to data loss.

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Scientists resolved this limitation
in a remarkably simple way:

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by changing the direction of recording
from longitudinal to perpendicular,

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allowing areal density to approach
one terabit per square inch.

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Recently, the potential limit has been
increased yet again

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through heat assisted magnetic recording.

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This uses an even more thermally
stable recording medium,

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whose magnetic resistance 
is momentarily reduced

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by heating up a particular spot 
with a laser

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and allowing data to be written.

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And while those drives are currently
in the prototype stage,

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scientists already have the next potential
trick up their sleeves:

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bit-patterned media,

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where bit locations are arranged 
in separate, nano-sized structures,

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potentially allowing for areal densities
of twenty terabits per square inch

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or more.

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So it's thanks to the combined efforts
of generations of engineers,

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material scientists,

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and quantum physicists

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that this tool of incredible power
and precision

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can spin in the palm of your hand.