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Showing Revision 5 created 11/08/2013 by Mike Ridgway.

  1. The signal fire is no doubt one of the oldest technologies
  2. for transmitting information –
  3. perhaps dating back to the first controlled use of fire.
  4. It allows one person to influence another's belief state –
  5. across a distance.
  6. Because with the ability to notice
  7. either the presence or absence of something,
  8. we are able to switch between one of two belief states.
  9. One difference. Two states.
  10. And ff we look back in history,
  11. we find that this was of great importance
  12. to military powers,
  13. which all rely on effective communications.
  14. And a great place to begin
  15. is with the Greek myth of Cadmus –
  16. a Phoenician prince who introduced
  17. the 'phonetic' letters to Greece.
  18. The Greek alphabet –
  19. borrowed from the Phoenician letters –
  20. along with light, and cheap, papyrus –
  21. effected the transfer of power
  22. from the priestly to the military class.
  23. And Greek military history provides clear evidence
  24. of the first advancements in communication,
  25. stemming from the use of signal torches.
  26. Polybius was a Greek historian born in 200 BC.
  27. He wrote 'The Histories,' which is
  28. a treasure trove of detail related to
  29. the communication technologies of the time.
  30. He writes: "The power of acting at the right time
  31. contributes very much to the success of enterprises.
  32. And fire signals are the most efficient of all devices
  33. which aid us to do this."
  34. However, the limitation of a signal fire was clear to him.
  35. He writes:
  36. "It was possible for those who had agreed on this
  37. to convey information that, say, a fleet had arrived.
  38. But when it came to some citizens
  39. having been guilty of treachery,
  40. or a massacre having taken place in town –
  41. things that often happen, but cannot all be foreseen –
  42. all such matters defied communication by fire signal."
  43. A fire signal is great when
  44. the space of possible messages is small –
  45. such as enemy has arrived or not arrived.
  46. However, when the message space – which is
  47. the total number of possible messages – grows,
  48. there was a need to communicate many differences.
  49. And in The Histories, Polybius describes a technology
  50. developed by Aeneas Tacticus –
  51. one of the earliest Greek writers on the art of war –
  52. from the 4th century BC.
  53. And his technology was described as follows:
  54. "Those who are about to communicate
  55. urgent news to each other by fire signal
  56. should procure two vessels
  57. of exactly the same width and depth.
  58. And through the middle should pass a rod,
  59. graduated into equal sections –
  60. each clearly marked off from the next,
  61. denoted with a Greek letter."
  62. Each letter would correspond to
  63. a single message in a look-up table which contain
  64. the most common events that occur in war.
  65. To communicate, they would proceed as follows:
  66. First, the sender would raise his torch
  67. to signal he had a message.
  68. The receiver would then raise his torch,
  69. signaling he was ready to receive it.
  70. Then, the sender would lower his torch,
  71. and they would both begin to drain their vessels
  72. from a bored hole of equal size at the bottom.
  73. Now, when the event is reached,
  74. the sender raises his torch
  75. to signal that they should both stop the flow of water.
  76. This results in equal water levels,
  77. denoting a single shared message.
  78. This ingenious method
  79. used differences in time to signal messages.
  80. However, its expressive capabilitiy was limited,
  81. mainly due to its speed.
  82. Polybius then writes of a newer method –
  83. originally devised by Democritus –
  84. which he claims was "perfected by myself,
  85. and quite definite and capable of dispatching –
  86. with accuracy –
  87. every kind of urgent message."
  88. His method – now known as the 'Polybius Square' –
  89. works as follows:
  90. Two people, seperated by a distance,
  91. each have 10 torches –
  92. separated into two groups of five.
  93. To begin, the sender raises a torch
  94. and waits for the receiver to respond.
  95. Then, the sender lights a certain number
  96. from each group of torches – and raises them.
  97. The receiver then counts
  98. the number of torches lit in the first group.
  99. This number defines the row position
  100. in an alphabetic grid they share.
  101. And the second group of torches
  102. signifies the column position in this grid.
  103. The intersection of the row and column number
  104. defines the letter sent.
  105. Realize, this method can be thought of
  106. as the exchange of two symbols.
  107. Each group of five torches is a symbol,
  108. which was limited to five differences –
  109. from one to five torches.
  110. Together, these two symbols multiply
  111. to give 5 x 5 = 25 differences –
  112. not 5 + 5.
  113. This multiplication demonstrates
  114. an important combinatorial understanding in our story.
  115. It was explained clearly in a 6th-century-BC
  116. Indian medical text, attributed to Sushruta –
  117. an ancient Indian sage – as follows:
  118. "Given 6 different spices,
  119. how many possible different tastes can you make?"
  120. Well, the process of making a mixture
  121. can be broken down into in six questions:
  122. Do you add A? Yes or no?
  123. Do you add B?
  124. C?
  125. D?
  126. E?
  127. and F?
  128. Realize, this multiplies into
  129. a tree of possible answer sequences –
  130. 2 x 2 x 2 x 2 x 2 x 2 = 64 ...
  131. 64 different sequences of answers
  132. are therefore possible.
  133. Realize that given n yes-or-no questions,
  134. there are 2 to the power of n possible answer sequences.
  135. Now in 1605, Francis Bacon clearly explained
  136. how this idea could allow one to send
  137. all letters of the alphabet,
  138. using only a single difference.
  139. [Regarding] his 'bilateral cipher,' Bacon wrote, famously:
  140. "The transposition of two letters by five placings
  141. will be sufficient for 32 differences.
  142. For by this art, a way is opened whereby a man
  143. may express and signify the intentions of his mind –
  144. at any distance of place – with objects which are capable
  145. of a two-fold difference only."
  146. This simple idea of using a single difference
  147. to communicate [all of the letters of] the alphabet
  148. really took flight in the 17th century,
  149. due to the invention of the telescope
  150. by Lippershey, in 1608, and Galileo, in 1609.
  151. Because quickly, the maginification power of the human eye
  152. jumped from 3, to 8, to 33 times – and beyond.
  153. So the observation of a single difference
  154. could be made at a much greater distance.
  155. Robert Hooke, an English polymath interested in
  156. improving the capability of human vision, using lenses,
  157. ignited progress when he told the Royal Society, in 1684,
  158. that suddenly, "with a little practice,
  159. the same character may be seen at Paris,
  160. within a minute after it hath been exposed at London."
  161. This was followed by a flood of inventions
  162. to pass differences more effectively
  163. across greater distances.
  164. One technology, from 1795, perfectly demonstrates
  165. the use of a single difference to communicate all things.
  166. Lord George Murray's 'shutter telegraph'
  167. was Britain's reaction to the Bonapartist threat to England.
  168. It was composed of six rotating shutters,
  169. which could be oriented as either 'open' or 'closed.'
  170. Here, each shutter can be thought of as a single difference.
  171. With six shutters, we have six questions: open or closed –
  172. providing us with 2^6, or 64, differences –
  173. enough for all letters, digits, and more.
  174. Now realize that each observation of the shutter telegraph
  175. can also be thought of as the observation
  176. of one of 64 different paths through a decision tree.
  177. And with a telescope, it was now possible to send letters
  178. at an incredible distance between beacons.
  179. However, an observation in 1820
  180. led to a revolutionary technology,
  181. which forever changed how far these differences
  182. could travel between signaling beacons.
  183. This ushered in new ideas
  184. which launched us into the 'Information Age.'