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Video transcript
- [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 Venetian prince who introduced the phonetic letters to Greece. The Greek alphabet borrowed from the Venetian letters, along with light and sheep pirate effected 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 Eneus Tacitus, 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 Democrtius, 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 int his 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 six 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. No 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.