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The signal fire is no doubt one of the oldest technologies

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for transmitting information –

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perhaps dating back to the first controlled use of fire.

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It allows one person to influence another's belief state –

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across a distance.

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Because with the ability to notice

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either the presence or absence of something,

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we are able to switch between one of two belief states.

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One difference. Two states.

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And ff we look back in history,

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we find that this was of great importance

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to military powers,

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which all rely on effective communications.

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And a great place to begin

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is with the Greek myth of Cadmus –

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a Phoenician prince who introduced

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the 'phonetic' letters to Greece.

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The Greek alphabet –

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borrowed from the Phoenician letters –

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along with light, and cheap, papyrus –

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effected the transfer of power

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from the priestly to the military class.

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And Greek military history provides clear evidence

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of the first advancements in communication,

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stemming from the use of signal torches.

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Polybius was a Greek historian born in 200 BC.

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He wrote 'The Histories,' which is

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a treasure trove of detail related to

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the communication technologies of the time.

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He writes: "The power of acting at the right time

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contributes very much to the success of enterprises.

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And fire signals are the most efficient of all devices

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which aid us to do this."

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However, the limitation of a signal fire was clear to him.

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He writes:

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"It was possible for those who had agreed on this

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to convey information that, say, a fleet had arrived.

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But when it came to some citizens

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having been guilty of treachery,

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or a massacre having taken place in town –

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things that often happen, but cannot all be foreseen –

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all such matters defied communication by fire signal."

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A fire signal is great when

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the space of possible messages is small –

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such as enemy has arrived or not arrived.

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However, when the message space – which is

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the total number of possible messages – grows,

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there was a need to communicate many differences.

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And in The Histories, Polybius describes a technology

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developed by Aeneas Tacticus –

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one of the earliest Greek writers on the art of war –

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from the 4th century BC.

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And his technology was described as follows:

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"Those who are about to communicate

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urgent news to each other by fire signal

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should procure two vessels

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of exactly the same width and depth.

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And through the middle should pass a rod,

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graduated into equal sections –

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each clearly marked off from the next,

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denoted with a Greek letter."

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Each letter would correspond to

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a single message in a look-up table which contain

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the most common events that occur in war.

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To communicate, they would proceed as follows:

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First, the sender would raise his torch

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to signal he had a message.

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The receiver would then raise his torch,

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signaling he was ready to receive it.

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Then, the sender would lower his torch,

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and they would both begin to drain their vessels

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from a bored hole of equal size at the bottom.

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Now, when the event is reached,

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the sender raises his torch

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to signal that they should both stop the flow of water.

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This results in equal water levels,

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denoting a single shared message.

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This ingenious method

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used differences in time to signal messages.

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However, its expressive capabilitiy was limited,

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mainly due to its speed.

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Polybius then writes of a newer method –

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originally devised by Democritus –

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which he claims was "perfected by myself,

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and quite definite and capable of dispatching –

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with accuracy –

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every kind of urgent message."

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His method – now known as the 'Polybius Square' –

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works as follows:

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Two people, seperated by a distance,

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each have 10 torches –

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separated into two groups of five.

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To begin, the sender raises a torch

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and waits for the receiver to respond.

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Then, the sender lights a certain number

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from each group of torches – and raises them.

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The receiver then counts

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the number of torches lit in the first group.

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This number defines the row position

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in an alphabetic grid they share.

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And the second group of torches

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signifies the column position in this grid.

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The intersection of the row and column number

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defines the letter sent.

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Realize, this method can be thought of

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as the exchange of two symbols.

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Each group of five torches is a symbol,

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which was limited to five differences –

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from one to five torches.

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Together, these two symbols multiply

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to give 5 x 5 = 25 differences –

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not 5 + 5.

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This multiplication demonstrates

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an important combinatorial understanding in our story.

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It was explained clearly in a 6th-century-BC

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Indian medical text, attributed to Sushruta –

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an ancient Indian sage – as follows:

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"Given 6 different spices,

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how many possible different tastes can you make?"

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Well, the process of making a mixture

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can be broken down into in six questions:

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Do you add A? Yes or no?

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Do you add B?

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C?

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D?

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E?

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and F?

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Realize, this multiplies into

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a tree of possible answer sequences –

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2 x 2 x 2 x 2 x 2 x 2 = 64 ...

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64 different sequences of answers

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are therefore possible.

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Realize that given n yes-or-no questions,

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there are 2 to the power of n possible answer sequences.

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Now in 1605, Francis Bacon clearly explained

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how this idea could allow one to send

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all letters of the alphabet,

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using only a single difference.

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[Regarding] his 'bilateral cipher,' Bacon wrote, famously:

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"The transposition of two letters by five placings

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will be sufficient for 32 differences.

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For by this art, a way is opened whereby a man

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may express and signify the intentions of his mind –

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at any distance of place – with objects which are capable

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of a two-fold difference only."

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This simple idea of using a single difference

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to communicate [all of the letters of] the alphabet

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really took flight in the 17th century,

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due to the invention of the telescope

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by Lippershey, in 1608, and Galileo, in 1609.

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Because quickly, the maginification power of the human eye

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jumped from 3, to 8, to 33 times – and beyond.

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So the observation of a single difference

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could be made at a much greater distance.

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Robert Hooke, an English polymath interested in

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improving the capability of human vision, using lenses,

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ignited progress when he told the Royal Society, in 1684,

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that suddenly, "with a little practice,

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the same character may be seen at Paris,

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within a minute after it hath been exposed at London."

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This was followed by a flood of inventions

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to pass differences more effectively

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across greater distances.

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One technology, from 1795, perfectly demonstrates

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the use of a single difference to communicate all things.

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Lord George Murray's 'shutter telegraph'

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was Britain's reaction to the Bonapartist threat to England.

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It was composed of six rotating shutters,

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which could be oriented as either 'open' or 'closed.'

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Here, each shutter can be thought of as a single difference.

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With six shutters, we have six questions: open or closed –

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providing us with 2^6, or 64, differences –

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enough for all letters, digits, and more.

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Now realize that each observation of the shutter telegraph

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can also be thought of as the observation

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of one of 64 different paths through a decision tree.

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And with a telescope, it was now possible to send letters

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at an incredible distance between beacons.

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However, an observation in 1820

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led to a revolutionary technology,

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which forever changed how far these differences

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could travel between signaling beacons.

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This ushered in new ideas

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which launched us into the 'Information Age.'