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.