- [Narrator] The signal fire
is no doubt one of the oldest technologies for transmitting information. Perhaps dating back to the
first controlled use of fire. It allows one person to
influence another's belief state across a distance. Because, with the ability to
notice either the presence or absence of something, we
are able to switch between one of two belief states. One difference, two states. And if we look back in
history, we find that this was of great importance to military powers, which all rely on
effective communications. And a great place to begin
is with the Greek myth of Cadmus, a Phoenician prince
who introduced the phonetic letters to Greece. The Greek alphabet borrowed
from the Phoenician letters, along with light and cheap papyrus, affected the transfer of power from the priestly
to the military class. And Greek military history
provides clear evidence of the first advancements
in communications stemming from the use of signal torches. Polybius was a Greek
historian born in 200 BC. He wrote the histories,
which is a treasure trove of detail related to the
communication technologies of the time. He writes, "The power of
acting at the right time "contributes very much to
the success of enterprises. "And fire signals are the
most efficient of all devices "which aid us to do this." However, the limitation of a
signal fire was clear to him. He writes, "It was possible
for those who had agreed "on this to convey information that, say, "a fleet had arrived, but
when it came to some citizens "having been guilty of
treachery, or a massacre having "taken place in town, things
that often happen but cannot "all be foreseen, all such
matters defied communication "by fire signal." A fire signal is great
when the space of possible messages is small. Such as enemy has arrived or not arrived. However, when the message
space, which is the total number of possible messages,
grows there was a need to communicate many differences. And in the histories, Polybius
describes a technology developed by Aeneas Tacticus,
one of the earliest Greek writers on the art of war
from the fourth century, BC. And his technology was
described as follows. "Those who are about to
communicate urgent news to "each other by fire signal
should procure two vessels "of exactly the same width and depth. "And through the middle should
pass a rod graduated into "equal sections, each
clearly marked off from the "next, denoted with a Greek letter." Each letter would correspond
to a single message in a look-up table which contained
the most common events that occur in war. To communicate they
would proceed as follows. "First the sender would
raise his torch to signal "he had a message. "Then receiver would then
raise his torch signalling "he was ready to receive it." Then the sender would lower
his torch and they would both begin to drain their
vessels from a board hole of equal size at the bottom. Now, when the event is
reached, the sender raises his torch to signal that they
should both stop the flow of water. This results in equal water
levels, denoting a single shared message. This ingenious method
used differences in time to signal messages. However it's expressive
capability was limited, mainly due to it's speed. Polybius then writes of
a newer method originally devised by Democritus,
which he claims was, "Perfected by myself and
quite definite and capable "of dispatching with
accuracy every kind of urgent "message." His method, now known
as the Polybius square, works as follows. Two people separated by a
distance each have 10 torches separated into two groups of five. To begin, the sender raises
a torch and waits for the receiver to respond. Then the sender lights a
certain number from each group of torches and raises them. The receiver then counts
the number of torches lit in the first group. This number defines the row
position in an alphabetic grid they share. And the second group of
torches signifies the column position in this grid. The intersection of the row
and column number defines the letter sent. Realize this method can be
thought of as the exchange of two symbols. Each group of five torches is a symbol, which was limited to five differences, from one to five torches. Together these two symbols
multiply to give five times five equals 25 differences. Not five plus five. This multiplication
demonstrates an important combinatorial understanding in our story. It was explained clearly
in a sixth century BC Indian medical text
attributed to Sushruta, an ancient Indian sage, as follows. "Given six different spices
how many possible different "tastes can you make?" Well, the process of making
a mixture can be broken down into six questions. Do you add A, yes or no? Do you add B? C? D? E? And F? Realize this multiplies
into a tree of possible answer sequences. Two, times two, times two, times two, times two, times two equals 64. 64 different sequences of
answers are therefore possible. Realize that given N yes or no questions, there are two to the
power of N possible answer sequences. Now in 1605 Francis Bacon
clearly explained how this idea could allow one to send
all letters of the alphabet using only a single difference. With his bilateral cipher,
Bacon wrote famously, "The transposition of two
letters by five placings "will be sufficient for 32 differences. "For by this art a way
is opened whereby man may "express and signify the
intentions of his mind "at any distance of place
with objects which are "capable of a two fold difference only." This simple idea of using
a single difference to communicate the alphabet
really took flight in the 17th century due to the
invention of the telescope by Lippershey in 1608 and Galileo in 1609. Because quickly the magnification
power of the human eye jumped from three to eight
to 33 times and beyond. So the observation of a single
difference could be made at a much greater distance. Robert Hooke, an English
polymath interested in improving the capability of
human vision using lenses, ignited progress when he told
the Royal Society in 1684 that suddenly quote,
"With a little practice, "the same character may
be seen at Paris within a "minute after it hath
been exposed at London." This was followed by a
flood of inventions to pass differences more effectively
across greater distances. One technology from 1795
perfectly demonstrates the use of a single difference to
communicate all things. Lord George Murray's shutter
telegraph was Britain's reaction to the Bonaparte's
threat to England. It was composed of six
rotating shutters which could be oriented as either open or closed. Here each shutter can be thought
of as a single difference. With six shutters we have six
questions, open or closed? Providing us with two to the power of six, or 64 differences. Enough for all letters, digits, and more. Now realize that each
observation of the shutter telegraph can also be thought
of as the observation of one of 64 different paths
through a decision tree. And with a telescope it was
now possible to send letters at an incredible distance between beacons. However, an observation in
1820 lead to a revolutionary technology, which forever
changed how far these differences could travel
between signalling beacons. This ushered in new ideas
which launched us into the information age.