Ancient Roman telegraph

Question

Communicating using the sun:

Frontinus, while studying the Greek classics, invented a system of communication based on towers, mirrors, and shutting aparatuses that could communicate along Roman empire by means of light rays (day only). Using something similar to a morse code, operators positioned in towers along a route, distant 20 by 20 km, could deliver messages from all over the empire in a lightning quick speed.

Question:

What would be the consequences for the empire? Could this help alleviate the incoming barbarian invasions? Could this free legions to be used in more massive attacks against Rome enemies? How this could effect Roman commerce?

Answer 1

A semaphore system would have great logistical advantages, in that supplies could be ordered with a reduced lead time, meaning that with vital supplies that cannot be allowed to run out, less stock need be kept on hand, as the reduced lead time reduces the safety levels that must be maintained. As a consequence, warehouses could be smaller.

However, the military implications, while not negligible, would not be all that great either. It would allow the military to maintain fewer, larger garrisons since there would be some additional lead time to respond to an incursion, and would also free up some soldiers as the number of men required to respond to an incursion would remain the same, while the garrison force would be effectively reduced. However, given the low travel speeds of a roman army on land, the differences would be that garrisons would need to be placed , say, 8 hours march apart rather than 6, as the communication lag is much lower. The difference between 8 hours march and 6 is around 10km at most.

Answer 2

The empire was definitely well spread out enough that this could have had a decent impact in a couple aspects...

Military:

The Western Roman Empire that broke off from the Eastern Empire was pretty well doomed and this invention wouldn't have saved them. That said...it would have helped due to their setup. The Roman army was divided into two types of forces. The first was the Limitanei. These were Roman citizens that lived in Frontier towns for their lives. The would frequently marry women from these frontier towns and become part of the town and people...raising their families et al. They weren't exactly the best equipped, but they could defend their town as required. Once a frontier town was attacked, the Roman garrison would send word to the second line of Roman defence. The second line of defence were Comitatenses...this was the professional Roman army that was designed for quick deployment using their well created road systems. The Limitanei would hold their ground defending a towns walls while they waited for the Comitatenses to ride in and break the seige. Quite effective in that one army could effectively garrison multiple frontier towns.

The impact in this sense would be extremely beneficial...the time that it took a Limitanei force to report the attack on them and the Comitatenses to start on their way would be minimal compared to what it took to dispatch a messenger instead...it would really make this tactic that much more effective.

Would it have stopped the Barbarian invasion? Unfortunately it's highly doubtful, the empire was heavily dependent on disappearing farming income and was pretty close to completely broke. Facing this broke military was entire tribes of people that uprooted themselves and marched their entire towns into Roman territory, in some cases sacking Rome a long the way. The Vandals marched through Europe and into spain, across into Africa, before settling into former Carthage (and sacked Rome from the south before being defeated by Byzantine). The Visigoths took up shop in italy after sacking Rome...the Franks moved into whats now France...the Lombardi people moved into Milan...the Huns came a long with. It's a long list, the Romans, even with near instant communication, couldn't resist this onslaught. If they had held onto France and the former Celtic territories, odds were they'd be facing a viking invasion as well. Roman power struggles didn't help much...an Eastern empire under Constantine started making deals where he just gave Western empire land away to the new tribes (franks in particular) and the will to maintain a Western empire pretty much disappeared.

Economics:

The Western Roman Empire was cash strapped...and unfortunately this setup would cost a significant amount of money to build and maintain. Nearing the end of their demise, they were actively disbanding troops they couldn't afford to pay.

The Eastern empire would have had a decent go with the technology...messages from Constantinople over to Egypt would be quite quick. I'm not sure how this would translate into economic advantages though.

Answer 3

There would be some military advantage, but primarily the advantage would be in economics and possibly in some sense social cohesion. Having news of what was happening in Spain or Constantinople or Alexandria or anywhere else in the empire from anywhere else in the empire would make those places seem closer and would allow decisions to made with better information.

Knowing about a ship that didn't make port as soon as it didn't make port gives one information that would otherwise take quite some time to get back so that deciding if and where pirates may be located becomes easier, allowing for more reliable commerce to occur, as well as shipping to be routed in alternative directions.

This certainly doesn't solve the problems of the empire; it wouldn't prevent the barbarian invasion; it could possibly offset the cost of building and maintaining itself, but without social and governmental reforms in the end it just changes how much the Eastern Empire holds onto vs. how much the barbarians take.