Why does the speed of light limit how fast the internet can be?

Nothing can send a signal faster than light, and light in a glass fibre is slower still, so distance alone puts a floor under every delay. Latency is a consequence of the limited speed at which any physical interaction can travel, and that speed is never more than the speed of light. Any system with distance between cause and effect has some latency.

On the internet, delay is measured one way, or as a round trip, the there-and-back total that the ping tool reports. In optical fibre, the index of refraction is about 1.5, so light travels about 1.5 times slower than in a vacuum, which works out to about 5 microseconds of latency for every kilometre. So 1,000 km of fibre costs about 5 milliseconds one way, whatever you pay for your connection. Add the fact that fibre is rarely laid in a straight line, since it follows contours, roads and railways, and real routes are longer than the map distance.

Other delays pile on top. Packets queue when a gateway receives several at once for the same destination. A geostationary satellite adds about a quarter of a second for a trip up and down, which low Earth orbit cuts, at the price of needing more satellites.

Does a few milliseconds matter? In online games, fast responses are rewarded, so low-latency players have an edge. And in computerised trading, millisecond improvements in network speed offer a competitive advantage, which led one company to spend $300 million on a straighter cable.