How fast can a signal go from Addis Ababa to Shanghai?
No faster than 29.38 ms one way and 58.76 ms there and back. That is light in a vacuum along the 8,808 km great circle between the two cities. Optical fibre slows light by its group index, 1.4682, so through glass the same round trip takes at least 86.27 ms.
| Medium | One way | Round trip |
|---|---|---|
| Light in a vacuum | 29.38 ms | 58.76 ms |
| Light in glass, group index 1.4682 | 43.14 ms | 86.27 ms |
| Great-circle distance, WGS84 | 8,808 km | |
What does a real line add?
Cables leave the great circle, and every router and queue on the way adds time, so a measured round trip from Addis Ababa to Shanghai lands above 86.27 ms whenever the whole path is glass. A line that only just reaches 86.27 ms runs at 68.1 % of light, the most fibre allows.
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How are these floors computed?
The distance is Lambert's formula on the WGS84 ellipsoid between the population-weighted centres UN DESA gives for the two cities in World Urbanization Prospects 2025, checked against GeographicLib to within 100 m on ten pairs. Light covers 299,792.458 km each second. The glass figure uses Corning's group index for SMF-28e+ fibre at 1550 nm, 1.4682.