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When Ghenghis Khan was establishing his great empire, he felt that it was important to allow information to travel quickly. He created a postal system which allowed everyone from commoners to the military to send messages from one important location to another.

My question is, how long would it take to send a high-priority message from one major population center to another near one?

Now, let's table that answer. We have an interstellar empire highly resembling the Mongols. We don't have any ansibles or whatnot, so information has to travel on ships (FTL ships, luckily).

Assuming standard interstellar distances (3-10 ly), how quickly would the Empire's ships have to travel in order to have similar information lags?

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    $\begingroup$ The question title makes this sound like it's based on historical Earth, making this a question for History SE, not Worldbuilding. If this is meant for a fictional/alternate history world (which would make this on topic here), then you're going to need to provide some background information on your world: available technology, magic/lack of magic, geography if possible, etc. $\endgroup$
    – Palarran
    Commented Nov 14, 2017 at 1:49
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    $\begingroup$ High-priority messages usually did not travel by common post (20 mi/day) for several reasons. That's what couriers or messengers (30 mi/day) were for. $\endgroup$
    – user535733
    Commented Nov 14, 2017 at 2:30
  • $\begingroup$ Do you mean if the Mongol receive the message after 5 days, then you need the Empire Ships to arrive at 10 ly destination in 5 days? $\endgroup$
    – Vylix
    Commented Nov 14, 2017 at 7:23
  • $\begingroup$ You need to define how far apart the pop centers in Mongol, so we can work out what's the time lag. Without it, the time lag can vary from between 0 - 10 days, or even infinity, if it's a Mongol vassal in other side of universe. $\endgroup$
    – Vylix
    Commented Nov 14, 2017 at 7:27
  • $\begingroup$ I think you better decide an acceptable time range for your interstellar travel. 5 days in land travel on earth is not the same like 5 days interstellar travel. Instead you may want to be more lenient and make it 5 months instead. $\endgroup$
    – Vylix
    Commented Nov 14, 2017 at 7:29

2 Answers 2

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The Mongols established defined routes called a Yam much like the Pony Express routes of 19th century western America.

Relay stations were used to give food, shelter and spare horses for Mongol army messengers. Genghis Khan gave special attention to Yam because Mongol armies traveled very fast, so their messengers had to be even faster, covering 200–300 km per day.

(between 100 and 200 miles). Another entry gives an overall time:

When the great khan died in Karakorum, news reached the Mongol forces under Batu Khan in Central Europe within 4–6 weeks thanks to the Yam.

So 4-6 weeks to cross the earth based empire. You might assume similar time to cross your star empire, then divide by that distance to get your FTLs rate of travel to produce similar delays.

An example extrapolation (I'll make some assumptions):

Our Galactic Mongol Empire controls this arm of the galaxy, with Earth as its capital, and we need to send a message to a general near the galactic core, and this will take the same time (4-6 weeks) as the above mentioned message.

  • Earth to core distance = 8000 Kpc = 26,092 LY (round to 26100)
  • Time for Mongol message (pick 5 weeks) = 5 * 604800(seconds per week) = 3024000 seconds

gives us 26100/3024000 = 0.0086309523809524 LY/sec multiply by 60 gives us 0.5178571428571429 LY/minute

The last value we can round(if desired) to 1/2 LY per minute or 2 minutes / LY, so the OPs example of 3-10 LY separation means travel between typical worlds will take 6 to 20 minutes, to simulate an expansion of Mongol rates of travel to a galactic empire stretching from earth to the galactic core.

Note your mileage may vary.

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About the speed of a horse ( about One Horse power) with short breaks this speed produces about 30 to 50 miles a day depending on the horse . That's over flat terrain. Longer over mountains. Faster overseas or traveling along rivers it really depends on the terrain.

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