So, bit, zero-one, yes-no, head or tails, true
or false,

the distinction between two different
possibilities.

Now, it's also interesting to note that the
bit is a physical object, so, for instance,

in a computer, we have transistors which
are switches,

and a switch could be opened, so that current
cannot flow through, or the switch can be closed,

so current can flow through.

And we could call this zero, and we could also
call this one.

And if we were to close the switch, so that
zero then becomes one, and we can also

open this one, so that one becomes zero, then 
we're transforming and processing that information.

Now there's a famous saying, really, a slogan
from Rolf Landauer at IBM,

and the slogan says, "Information is physical."

What does this mean, "information is physical"?

Well it's a slogan so it's not clear what it means.

It's kind of like the slogan "Information is the difference
that makes a difference".

But there are two ways to interpret this, both
equally valid.

One is that all bits that we're familiar with
are actually physical systems.

So switches open and closed,

when I get information in my eye, so when I
see light, it means a particle of light - photon -

comes into my eye, excites a retinal molecule
on my eye which causes a cascade of

neurotransmitters which then allows me to
physically see what's going on.

So we could have a photon, which we'll call one.

Or no photon, which we'll call zero.

Actually, it's interesting that photons come
in two kinds.

We can have a photon that's polarized like this,

which was the kind that gets let through your
polarizing Ray-Ban sunglasses.

We get a photon that's polarized like this,
and we could just call photon polarized

like this one and we could call photon
polarized like this zero.

So whenever a bit exists in the world, it is
actually represented by a physical object

that has two different possible states, like
a coin, which can be heads or tails.

So information is physical in the sense that
all physical bits are physical,

any bit that we're familiar with, a bit in
our computer, a bit that we see from seeing

a particle of light, a flash of light,

is actually some physical system that represents
information through us.