Speed of light
~3×10⁸ m/s vacuum, ~2×10⁸ m/s in fibre.
Origin
The hard physical floor on inter-datacenter latency. Light through fibre is ~2/3 of c. New York → London ~28ms one-way minimum; nothing in software can beat it.
Where it shows up in production
- Cross-region replication Synchronous writes across continents are bounded below by 60-200ms regardless of compute speed.
- High-frequency trading The Hibernia Express transatlantic cable (2015) was justified by shaving ~6ms off NYSE-London latency.
On Semicolony
Sources & further reading
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