Juyongguan: How did ancient China “play” with light-speed communication and cryptography

Juyongguan is known as the most powerful pass in the world. You may know that it is part of the Great Wall, but have you ever thought that this pass, thousands of years ago, used the beacon towers that came with it to play with "light-speed communication" and "cryptography"? In an era without mobile phones and the Internet, the ancients had already invented a fast, accurate and effective information transmission system.

1. How beacon towers work: an ancient version of fiber optic networks

Imagine that there was no Internet in ancient times. If there was an enemy on the border, how could we notify all the defense lines at the speed of light? Beacon Tower: Watch me!

Once the enemy is spotted, the soldiers light a fire, and it is like pressing the "send button", and the message is relayed all the way to the command center. Juyongguan is an important "transfer station" in this "fire network". Is this like an ancient version of "fire optical fiber"? Fire and smoke can transmit signals very quickly in a vast space! The minimum speed of beacon transmission in the Han Dynasty was 30 kilometers per hour, but it was actually much faster.

2. The Morse Code of Beacon Towers: Ancient Cryptography

Do you think beacon towers only tell people that "enemies are coming"? Of course not only that! The ancients already had their own "coding system". Just like today we use 0 and 1 to form a complex Internet world, ancient beacon towers can also transmit more information through different signal combinations. The number of enemies, direction, and even the degree of urgency can all be conveyed through a combination of fire and smoke.

For example, if you heard of an invasion at night but were unsure of the situation, you would light a bundle of reeds; if you saw more than 500 people attacking during the day, you would raise three beacons and light a pile of firewood.

The combination of different numbers and types of fire and smoke is like an ancient version of "Morse code". The amount of information is so rich that you start calculating the permutations and combinations, and then have a headache because you are not good at math...

After receiving the signal, it can only be interpreted through the pre-agreed signal release rules, which is equivalent to a code table. That is a confidential document. Without it, outsiders can only stare blankly at the fire and smoke.

3. Multiple protections of "wireless network": ensuring that information is not lost

The beacon towers not only transmit information quickly, but also have high reliability. Each beacon tower is 0.5 to 2 kilometers apart, and generally no more than ten kilometers apart. Even if a beacon tower is "offline", the information can continue to be transmitted through other beacon towers. This design is a bit like the backup design in modern communication networks, which ensures the security and reliability of information transmission.

4. Reasonable building planning and design

Beacon towers usually occupy high ground to facilitate signal transmission. The daily life of soldiers should be taken into consideration when designing. Beacon towers are often distributed along rivers to facilitate water use and can also play a defensive role when the enemy attacks along the river.

Some beacon towers are mainly built of earth, while others are made of rammed earth and grass, and then painted with grass mud many times to prevent moisture. In the Han Dynasty, "Ma Shi Tu" was used, which was really a mixture of cow and horse dung, soil, and grass ash. This method is very scientific and can increase the viscosity of the "paint". It is said that the amount of cow and horse dung must be recorded for regular inspection and reinforcement. Science is science, but... you know the taste.

Beacon towers guarded the border for thousands of years. Later, in the Ming Dynasty, they also added the sound of cannons, hanging lanterns, colorful flags, blue shirts, and fur coats to convey messages.

Of course, beacon fires are not very adaptable. If the battle situation changes, it is difficult to adjust in time. Moreover, if the enemy is very cunning, they may send out false signals, "adding fuel to the fire" and causing a false alarm. Even our own people can be "piss teammates", otherwise there would not be the story of "playing with the princes with beacon fires".

However, the beacon tower system at Juyongguan shows us how the ancients relied on simple natural phenomena and scientific wisdom to achieve complex information transmission. The next time you stand at Juyongguan and look up at the ancient beacon tower, you may feel that it is really the "ancestor" of our high-tech communication system today!

Therefore, even ancient buildings are full of technological elements.

Author: Hikka Slate Restoration Group

Reviewer: Liu Jie, Associate Professor, School of Architecture and Art, Beijing Jiaotong University

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.