03.01 · Concept
Speed of Light Is a Floor
Compute the round-trip floor between two regions from distance alone, and say why no amount of engineering removes it.
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Network latency has a hard lower bound set by physical distance: a round trip must cross the path twice, and signals in fibre move well below vacuum light speed. Better routing, faster hosts and larger bandwidth can remove avoidable delay, but they cannot make inter-region communication beat propagation time.
What this lesson answers
- calculate minimum RTT between cloud regions
- why bandwidth does not fix network latency
- speed of light limit for distributed systems
Notes
Speed of Light Is a Floor — Network latency has an irreducible minimum because signals cannot travel faster than light, so without accounting for distance you will promise response times that no routing, caching, or hardware upgrade can achieve.
Key Concepts: - In vacuum, light speed is , so the absolute one-way floor is . - In optical fiber, signals travel at about , or roughly one-way.
Common questions
- How do I estimate the minimum RTT between two regions?
- Use the physical distance between the regions and assume the signal must travel there and back. In fibre, propagation is slower than in vacuum, so the practical floor is based on fibre speed, not ideal light speed. Real routes are usually longer than the map distance, so production latency sits above that floor.
- Can better hardware remove inter-region latency?
- Better hardware can reduce processing time, queuing, packet loss and serialisation overhead. It cannot remove the time required for a signal to cross physical distance. Once those avoidable costs are small, the remaining latency is dominated by propagation, which no instance type, NIC or bandwidth upgrade can eliminate.
- Why does this matter for database writes and handshakes?
- Protocols that require multiple round trips pay the distance cost repeatedly. Synchronous database commits, consensus protocols and connection handshakes all become slower when replicas or peers are far apart. Reducing the number of required round trips helps, but any remaining round trip still has the same propagation floor.
Short definition: what is Speed of Light Is a Floor?