In 458 BCE, Aeschylus staged Agamemnon before an Athenian audience. The play opens with a watchman who has spent a year atop the palace of Atreus in Argos, waiting for a fire signal from Troy. When the flame appears, he cries out in recognition. The signal has traveled approximately 550 kilometers from Mount Ida to Mycenae through eight relay stations, crossing the Aegean Sea and passing over the peaks of Lemnos, Mount Athos, through Boeotia and over Mount Cithaeron. According to the account given by Clytemnestra in the play, the journey took a single night.

The information transmitted: one binary choice. Troy has fallen, or it has not. Fire, or darkness.

What Aeschylus describes was contemporary practice, not fantasy. Fifth-century Greek armies used systems called phryctoria, fire-signal towers positioned on mountaintops within line-of-sight of each other. Thucydides records that during the Peloponnesian War, Salaminians used beacon fires to alert Athens when Cnemus attacked their island. The limitation was absolute: these systems could transmit only pre-agreed messages. The fire resolved a question that both parties had established in advance.

This constraint proved useful. The Troy beacons worked because Clytemnestra and her watchmen had agreed beforehand on what the fire would mean. The transmitted signal was minimal. The meaning it triggered was not. One fire meant: Troy has fallen, the war is over, Agamemnon returns, prepare the household.

The Greek Experiments

The Greeks recognized that pure binary signaling had limits and developed systems to overcome them. In the fourth century BCE, Aeneas Tacticus of Stymphalus invented the hydraulic telegraph.

The system used two identical earthenware vessels on separate hills, each containing a cork float with a vertical rod marked in equal sections. Different messages were inscribed at different heights on the rods: "Cavalry arrived in the country," "Heavy infantry," "Ships," "Corn." Both vessels had identically sized drain holes at the bottom. When the transmitter raised a torch, both operators opened their drain spigots simultaneously. Water drained at identical rates. When the desired message reached the vessel's rim, the transmitter raised the torch again. Both operators read the message at the water level.

Modern experiments have demonstrated approximately 151 letters per hour were achievable with this technology.

In the second century BCE, the historian Polybius developed an alphabetic torch system. He arranged the 24 letters of the Greek alphabet in a 5×5 grid across five tablets. Two walls at each beacon station held five torch holders each. The left wall indicated which tablet (row), the right wall indicated which letter position (column). To signal the letter Π (pi), an operator displayed five torches on the left wall and three on the right.

Polybius had created coordinate-based encoding. Any message could now be transmitted letter by letter, not just pre-arranged codes. But transmission was slow. For urgent military alerts, civilizations continued to rely on binary signals.

Rome's Frontier Network

The Roman Empire maintained the largest binary signaling infrastructure in the ancient Western world. Along the Germanic Limes, 568 kilometers of frontier from the Rhine to the Danube, more than 900 watchtowers stood within visual signaling range of each other. Spacing ranged from 500 meters to 2.5 kilometers. Each tower measured approximately 5 meters square at the base and rose 10 to 12 meters high. Four to eight soldiers staffed each tower, watching for signals from neighboring stations.

Trajan's Column, erected in the early second century CE, depicts these signal towers: two-story wooden structures with projecting torches and adjacent platforms piled with logs and straw for sustained burning. The system operated on binary logic. Fire at night, smoke during day. A lit beacon meant trouble on the frontier. The signal cascaded tower to tower until it reached the legionary fortresses miles behind the lines.

The frontier's purpose lay in granting delay or warning that could be used to summon concentrated Roman forces to the site. A single binary signal, transmitted at approximately 600 kilometers per hour through the relay chain, compared to perhaps 100 kilometers per day for a mounted messenger.

Hadrian's Wall stretched 117.5 kilometers across northern Britain with milecastles at approximately one Roman mile intervals. Modern GIS analysis has confirmed that the positioning of these structures prioritized inter-visibility for signaling over optimal views into hostile territory.

The Great Wall Beacons

The Chinese beacon system (烽火台, fenghuo tai) operated from the Western Zhou Dynasty (eleventh century BCE) through the Ming Dynasty (1368-1644 CE). Unlike Western systems that used simple fire or no-fire signals, Chinese beacon towers developed graduated codes that transmitted multiple levels of information simultaneously.

The basic system used smoke during day and fire at night. The smoke was called langyan, meaning "wolf smoke," because wolf dung was mixed into the fuel to produce smoke that rose straight without dispersing. The Ming Dynasty codified a protocol that encoded enemy count:

One beacon fire plus one cannon salvo indicated fewer than 100 enemies approaching. Two fires plus two salvos meant approximately 500 enemies. Three fires plus three salvos signaled over 1,000 attackers. Four fires plus four salvos warned of 5,000 enemies. Five fires plus five salvos indicated 10,000 or more.

Towers were positioned every 10 li (approximately 5 kilometers) along the Great Wall. Some sections had towers every 10 to 100 footsteps. Each tower was a solid platform 4 to 10 meters high, with top platforms housing watchmen's quarters, signaling equipment, and storage for fuel and grain.

The legend of King You of Zhou (771 BCE) illustrates an early understanding of signal degradation. According to the account, King You repeatedly lit beacon fires as entertainment to amuse his concubine Baosi, who enjoyed watching nobles rush to defend against nonexistent threats. When the Quan Rong tribe invaded, the beacons were lit again. No armies came. King You was killed and the Western Zhou dynasty collapsed.

The Talking Drums

While Mediterranean and East Asian civilizations developed visual signaling, the peoples of West and Central Africa created acoustic networks. The drums produced only two tones: a high tone called limiki lya otolome ("voice of the male") and a low tone called limiki lya otomali ("voice of the female"). This was binary output mapped to tonal patterns of spoken words.

Many words in tonal African languages share identical tonal patterns. English missionary John F. Carrington, who conducted fieldwork in the Belgian Congo from 1938 to 1964, documented the solution: stereotyped phrases that added redundancy. The single word for "moon" became the extended phrase "the moon looks down at the earth" (songe li tange la manga). The additional words provided context that disambiguated the tonal pattern.

The results: a single drum was audible for 3 to 7 miles under ideal conditions, up to 15 miles at night for the Bulu people of Cameroon. Through relay transmission, messages could travel 100 miles in approximately one hour. In 1881, news of the wreck of the steamer Ethiopia was transmitted 60 to 70 miles in under two hours through drum relay.

The Byzantine Time-Code

The most technically complex beacon system of the ancient world was devised by Leo the Mathematician for Byzantine Emperor Theophilos around 829-842 CE, during the Arab-Byzantine wars. Leo used time as an additional information channel.

Synchronized water clocks were maintained at both terminal stations: Loulon near the Arab frontier and the Lighthouse of the Great Palace in Constantinople. The day was divided into 12 hours. Each hour was assigned a different pre-arranged message. When a fire was lit at Loulon at a specific hour, the operator in Constantinople checked his water clock and read the corresponding message.

A fire at the third hour might mean "Arab cavalry raid in progress." At the seventh hour, "garrison under siege." The binary signal (fire or no fire) now carried approximately 3.6 bits of information (log₂ of 12 possible messages) rather than the single bit of a simple beacon.

The network stretched 720 kilometers across Anatolia through nine stations: Loulon, Mount Argaios, Mount Samos, Aigilon, Mount Mamas, Mount Kyrizos, Mount Mokilos, Mount Saint Auxentius, Constantinople. Some stations were 97 kilometers apart in open terrain. The entire transmission took approximately one hour.

The Spanish Armada Warning

On July 19, 1588, the Spanish Armada of 130 ships was spotted off the Lizard in Cornwall. Within 30 minutes, the warning reached London through a network of beacon sites.

The system had been established in 1586, two years before the invasion, by order of the Privy Council. The beacons used pitch-soaked rope in iron fire baskets mounted on wooden or iron poles. The material ensured ignition in wet weather, sustained burning, and dense black smoke visible during daylight. Devon County alone maintained 89 documented beacon sites.

The operational protocol: beacons were grouped in sets of three on coasts and pairs inland. One beacon meant ships sighted. Two beacons meant enemy confirmed. Three beacons meant invasion underway. Local militias monitored neighboring beacons around the clock. The standing orders required "two people by day, three by night, each an honest householder above thirty years of age."

The warning gave Sir Francis Drake and the English fleet time to organize the pursuit that led to the Battle of Gravelines on July 29.

For the 400th anniversary in 1988, 461 beacons were lit across England, replicating the original network.

One If by Land

On the night of April 18, 1775, sexton Robert Newman climbed the steeple of Boston's Old North Church, the tallest point in the city at 191 feet, and hung two lanterns in the window facing Charlestown. The signal was visible for less than 60 seconds before Newman extinguished the lights to avoid British detection.

The common understanding of this event, shaped by Henry Wadsworth Longfellow's 1861 poem "Paul Revere's Ride," is backward. The lanterns were not a signal to Revere. They were a signal from Revere, a backup plan he had arranged in case he was captured crossing the Charles River, which was banned at that hour and patrolled by HMS Somerset. Two lanterns told patriot contacts in Charlestown that British troops were taking the water route to Cambridge rather than marching over Boston Neck.

One lantern meant the longer land route, giving colonials more time to prepare. Two lanterns meant the shorter water route, requiring immediate action. The entire strategic situation, British troop movements, their chosen route, the timing of the operation, was compressed into a single binary choice.

Revere crossed the Charles by rowboat, evading the Somerset. He landed in Charlestown, where contacts confirmed they had seen the lanterns. He rode to Lexington to warn Samuel Adams and John Hancock. Hours later, the first shots of the American Revolution were fired at Lexington and Concord.

The Chappe System

The optical telegraph developed by Claude Chappe in revolutionary France in the 1790s moved beyond binary signaling. Chappe's semaphore towers could transmit approximately 92 distinct signals through combinations of a central regulator bar (4 positions) and two indicator arms (7 positions each), yielding 196 combinations, of which 92 were used for data and 6 for control signals.

The system used two-stage encoding: the first signal indicated a page number (1-92) in a codebook, and the second indicated a line number on that page, yielding 8,464 possible coded phrases in the initial implementation, later expanded to over 40,000.

The first official message was transmitted on August 15, 1794, announcing the recapture of Le Quesnoy from the Austrians. The message arrived in Paris approximately one hour after the battle, ten or more hours before a courier could have delivered the news.

At its peak, the French network encompassed 556 stations spanning 5,000 kilometers, reaching Venice, Amsterdam, Brussels, and Mainz. Messages traveled from Paris to Calais in 3 minutes, Paris to Toulon in 20 minutes.

The system could not operate at night (experiments with lanterns on the indicators failed) or in fog, rain, or snow. Operators at intermediate stations did not know the content of what they transmitted. They repeated patterns. The electrical telegraph, first demonstrated in 1837, eventually replaced the optical system.

Shannon's Framework

In 1948, Claude Shannon of Bell Telephone Laboratories published "A Mathematical Theory of Communication". The paper established the theoretical foundation for understanding information transmission.

Shannon defined the bit as the amount of information required to specify a choice between two equally likely possibilities. The mathematical relationship is logarithmic: information content equals log₂ of the number of possible messages. A beacon with one possible message (fire equals invasion) versus its absence carries exactly 1 bit.

Shannon entropy, expressed as H = -Σ pi log pi, measures the uncertainty of a message source. Maximum entropy occurs when all possible messages are equally likely. A beacon system achieves maximum efficiency when fires are lit approximately 50% of the time.

The Shannon-Hartley theorem establishes that every communication channel has a maximum capacity C = W log₂(1 + S/N), where W is bandwidth and S/N is signal-to-noise ratio. Ancient beacon systems operated in a specific regime: very low bandwidth (one bit per transmission) but relatively high signal-to-noise ratio (a mountain fire is visible for miles and hard to mistake). Binary signaling maximizes the distance between valid signal states, making errors easier to detect.

Shannon noted that "the semantic aspects of communication are irrelevant to the engineering problem." What matters is that the receiver can reconstruct the message. But when sender and receiver share context (pre-arranged codes), the transmitted message can be far shorter than its semantic content. This is why "one if by land, two if by sea" conveyed complex military intelligence with a single bit. The context was pre-arranged.

The Poke

Facebook launched the poke feature in 2004. Mark Zuckerberg described the design intention: "We thought it would be fun to make a feature that has no specific purpose... So we created the poke." The deliberate ambiguity, that a poke could mean friendly greeting, flirtatious interest, or simple acknowledgment, recapitulated the pre-arranged meaning structure of ancient beacons. The poke's meaning emerged from relationship context, not from the signal itself.

The Like button was introduced by Facebook on February 9, 2009. Development had begun in July 2007 under the code name "Awesome Button." Zuckerberg initially opposed it for nearly two years, concerned it would replace meaningful comments with low-effort clicks.

Researchers have identified at least seven distinct types of Likes: content-based (genuine appreciation), friendship likes (supporting a friend), acknowledgment likes (noting a post), presence likes (indicating activity), obligatory likes (social duty to acknowledge profile pictures), conversation-regulation likes (signaling end of exchange), and conspicuous non-likes (pointed withholding of expected approval). A single binary action carries multiple levels of social information through context.

Read Receipts and Typing Indicators

Read receipts were introduced with iMessage in 2011. WhatsApp added blue checkmarks. The signal transmits exactly one bit: message was read or not. Research has found that 35% of people feel ignored when messages are marked read but not responded to.

Typing indicators, the pulsing ellipsis that appears when someone is composing a message, were developed at IBM in 1997 and implemented in Microsoft's MSN Messenger around 1999. The signal transmits one bit: actively composing or not. The inventor, Jerry Cuomo, noted an unexpected use case: seeing the dots when checking on elderly relatives means "they're okay". A single bit of activity confirms continued existence.

Machine Heartbeats

In distributed computing systems, nodes send periodic signals called heartbeats. The information content: "I'm alive." The absence of a heartbeat within a timeout period communicates the opposite: "Something is wrong." Nodes typically send heartbeats at intervals of one to ten seconds. Monitors mark nodes as failed after missing two or three consecutive signals.

MQTT heartbeat protocols use PINGREQ/PINGRESP packets that are only 2 bytes in size. This is ancient beacon logic in digital form. The Roman watchtower fire meant "we are here, we are watching." The Kubernetes heartbeat means the same: this node exists, this service is running, this process has not crashed.

Dead man's switches extend this logic. The term originated with railroad safety mechanisms, switches that must be actively held closed, automatically activating brakes if the operator becomes incapacitated. Digital equivalents include Google's Inactive Account Manager, which transfers account access if the user fails to log in for a configurable period.

The Safety Check

On October 15, 2014, Facebook launched Safety Check, originally named "Disaster Message Board." The feature was developed after observing how people used social media during the 2011 Tōhoku earthquake and tsunami in Japan.

When Facebook detects a disaster event, users in the affected area receive a notification: "Are you safe?" The user taps either "I'm safe" or "I'm not in the area." The status is shared with friends.

The first major deployment came during the Nepal earthquake on April 25, 2015. Seven million people marked themselves safe, generating notifications to 150 million friends. A single binary action per user propagated through the social graph.

The Equation

Across the history of single-bit communication, a pattern holds. The value of a binary signal can be expressed as:

Value = Pre-arranged meaning × Speed advantage × Decision clarity

The Spanish Armada beacon scored high on all three dimensions. Pre-arranged meaning was established through years of preparation. Speed was approximately 20 times faster than horseback messengers. Decision clarity was absolute: fire meant invasion, no fire meant peace.

A Like button follows the same structure. Pre-arranged meaning is culturally established (positive acknowledgment). Speed is instantaneous compared to composing a response. Decision clarity is binary (clicked or not clicked).

Systems that fail violate one or more of these. King You's false beacon fires destroyed pre-arranged meaning through signal degradation. Complex semaphore systems sacrificed speed for bandwidth. Ambiguous signals sacrifice decision clarity for flexibility.

Binary Persists

Contemporary technology continues to generate single-bit channels. Apple's Crash Detection, introduced in 2022, uses accelerometers capable of measuring 256g of force combined with machine learning to detect automobile accidents. If a crash is detected and the user doesn't respond within 20 seconds, the device contacts emergency services. The signal is binary: crash detected or not.

Emergency SOS via satellite extends single-bit emergency signaling to locations without cellular coverage. The constraint of satellite bandwidth forces a return to the compression strategies of ancient signaling: maximum meaning in minimum bandwidth.

Wearable health devices transmit continuous streams of biometric data, but the most critical signals remain binary: heart stopped or beating, user fallen or upright, emergency detected or normal operation.

The beacon towers of the Great Wall still stand. The infrastructure for modern signaling is invisible: server farms, fiber optic cables, cellular towers. The signal structure is the same. A single bit. A choice between two states. Fire or darkness. Safe or not safe. Alive or silent.