0:00:04.528,0:00:08.529
The signal fire is no doubt one of the oldest technologies

0:00:08.529,0:00:10.662
for transmitting information –

0:00:10.662,0:00:14.509
perhaps dating back to the first controlled use of fire.

0:00:14.509,0:00:19.142
It allows one person to influence another's belief state –

0:00:19.142,0:00:21.119
across a distance.

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Because with the ability to notice

0:00:22.775,0:00:25.762
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

0:00:45.382,0:00:47.796
is with the Greek myth of Cadmus –

0:00:47.796,0:00:50.035
a Phoenician prince who introduced

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the 'phonetic' letters to Greece.

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The Greek alphabet –

0:00:53.956,0:00:55.773
borrowed from the Phoenician letters –

0:00:55.773,0:00:58.471
along with light, and cheap, papyrus –

0:00:58.471,0:01:00.598
effected the transfer of power

0:01:00.598,0:01:03.982
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

0:03:40.638,0:03:44.920
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."

0:04:04.916,0:04:07.336
His method – now known as the 'Polybius Square' –

0:04:07.336,0:04:08.902
works as follows:

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Two people, seperated by a distance,

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each have 10 torches –

0:04:12.692,0:04:15.209
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

0:04:22.518,0:04:26.268
from each group of torches – and raises them.

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The receiver then counts

0:04:32.887,0:04:36.512
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

0:04:43.860,0:04:47.329
signifies the column position in this grid.

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The intersection of the row and column number

0:04:50.093,0:04:52.327
defines the letter sent.

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Realize, this method can be thought of

0:04:54.101,0:04:56.734
as the exchange of two symbols.

0:04:56.734,0:05:00.215
Each group of five torches is a symbol,

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which was limited to five differences –

0:05:02.828,0:05:05.327
from one to five torches.

0:05:05.327,0:05:07.644
Together, these two symbols multiply

0:05:07.644,0:05:12.512
to give 5 x 5 = 25 differences –

0:05:12.512,0:05:15.142
not 5 + 5.

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This multiplication demonstrates

0:05:17.272,0:05:21.417
an important combinatorial understanding in our story.

0:05:21.417,0:05:25.069
It was explained clearly in a 6th-century-BC

0:05:25.069,0:05:28.713
Indian medical text, attributed to Sushruta –

0:05:28.713,0:05:32.389
an ancient Indian sage – as follows:

0:05:32.389,0:05:34.619
"Given 6 different spices,

0:05:34.619,0:05:38.174
how many possible different tastes can you make?"

0:05:38.174,0:05:41.061
Well, the process of making a mixture

0:05:41.061,0:05:44.384
can be broken down into in six questions:

0:05:44.384,0:05:46.963
Do you add A? Yes or no?

0:05:46.963,0:05:48.885
Do you add B?

0:05:48.885,0:05:50.055
C?

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D?

0:05:51.142,0:05:52.180
E?

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and F?

0:05:53.508,0:05:55.898
Realize, this multiplies into

0:05:55.898,0:05:59.324
a tree of possible answer sequences –

0:05:59.324,0:06:05.074
2 x 2 x 2 x 2 x 2 x 2 = 64 ...

0:06:05.074,0:06:07.896
64 different sequences of answers

0:06:07.896,0:06:10.759
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

0:06:24.245,0:06:26.786
how this idea could allow one to send

0:06:26.786,0:06:28.849
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

0:06:51.067,0:06:53.219
of a two-fold difference only."

0:06:53.219,0:06:56.538
This simple idea of using a single difference

0:06:56.538,0:06:58.538
to communicate [all of the letters of] the alphabet

0:06:58.538,0:07:01.415
really took flight in the 17th century,

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due to the invention of the telescope

0:07:03.607,0:07:08.178
by Lippershey, in 1608, and Galileo, in 1609.

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Because quickly, the maginification power of the human eye

0:07:11.338,0:07:15.927
jumped from 3, to 8, to 33 times – and beyond.

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So the observation of a single difference

0:07:18.067,0:07:21.239
could be made at a much greater distance.

0:07:22.715,0:07:26.332
Robert Hooke, an English polymath interested in

0:07:26.332,0:07:30.130
improving the capability of human vision, using lenses,

0:07:30.130,0:07:34.856
ignited progress when he told the Royal Society, in 1684,

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that suddenly, "with a little practice,

0:07:37.917,0:07:41.012
the same character may be seen at Paris,

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

0:07:45.546,0:07:48.027
This was followed by a flood of inventions

0:07:48.027,0:07:50.969
to pass differences more effectively

0:07:50.969,0:07:54.149
across greater distances.

0:07:54.149,0:07:58.831
One technology, from 1795, perfectly demonstrates

0:07:58.831,0:08:02.352
the use of a single difference to communicate all things.

0:08:02.352,0:08:05.703
Lord George Murray's 'shutter telegraph'

0:08:05.703,0:08:09.926
was Britain's reaction to the Bonapartist threat to England.

0:08:09.926,0:08:12.778
It was composed of six rotating shutters,

0:08:12.778,0:08:16.579
which could be oriented as either 'open' or 'closed.'

0:08:16.579,0:08:19.884
Here, each shutter can be thought of as a single difference.

0:08:19.884,0:08:24.484
With six shutters, we have six questions: open or closed –

0:08:24.484,0:08:29.498
providing us with 2^6, or 64, differences –

0:08:29.498,0:08:33.662
enough for all letters, digits, and more.

0:08:33.662,0:08:37.655
Now realize that each observation of the shutter telegraph

0:08:37.655,0:08:39.893
can also be thought of as the observation

0:08:39.893,0:08:45.056
of one of 64 different paths through a decision tree.

0:08:51.688,0:08:55.242
And with a telescope, it was now possible to send letters

0:08:55.242,0:08:58.741
at an incredible distance between beacons.

0:08:58.741,0:09:01.156
However, an observation in 1820

0:09:01.156,0:09:03.795
led to a revolutionary technology,

0:09:03.795,0:09:07.279
which forever changed how far these differences

0:09:07.279,0:09:10.209
could travel between signaling beacons.

0:09:10.209,0:09:12.339
This ushered in new ideas

0:09:12.339,0:09:16.596
which launched us into the 'Information Age.'