How fast can a signal go from Istanbul to London?
No faster than 8.36 ms one way and 16.72 ms there and back. That is light in a vacuum along the 2,507 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 24.55 ms.
| Medium | One way | Round trip |
|---|---|---|
| Light in a vacuum | 8.36 ms | 16.72 ms |
| Light in glass, group index 1.4682 | 12.28 ms | 24.55 ms |
| Great-circle distance, WGS84 | 2,507 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 Istanbul to London lands above 24.55 ms whenever the whole path is glass. A line that only just reaches 24.55 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.