Wirebook › Lesson 6 of 18
How does a message cross the world in a blink?
Send a message from London to Sydney and it arrives before you've finished blinking. It's crossed about 17,000 km of planet. So how did it get there: through a tunnel under the sea, or bounced off a satellite in space? Mostly the sea. Let's race them and find out why.
Glass threads on the sea floor
Almost all the data that crosses an ocean travels through undersea cables. In deep water a cable is only about as thick as a garden hose. Inside are hair-thin strands of glass, and your message rides along them as flashes of laser light, billions of flashes every second.
- Light in glass is fast, but not the fastest. Glass slows light to about two-thirds of its top speed: roughly 200,000 km every second, or 200 km per millisecond.
- Cables don't go in straight lines. They hug coastlines, dodge undersea mountains and stop at landing stations in different countries, so a real route is often a third longer than the shortest path on a globe. The race above adds that extra third.
- Boosters on the sea floor. Light fades as it travels, so every 50 to 100 km a sealed box called a repeater boosts it again. They're powered through the cable itself.
- When they break, usually from ship anchors or fishing nets (and very occasionally a curious shark nibbling one), a special repair ship hooks the cable up from the sea floor and splices it back together. Meanwhile your data simply takes another cable.
Why not satellites?
Classic internet satellites sit in a geostationary orbit, about 35,786 km above the equator. Up there a satellite goes round once a day, exactly as fast as Earth spins, so it seems to hang still in the sky and a dish can stay pointed at it. The catch is the distance: even at the full speed of light, up and back down takes about a quarter of a second. Fine for TV, sluggish for a video call or a game.
Newer networks use thousands of low Earth orbit satellites only about 550 km up, passing messages between each other with lasers. In space light isn't slowed by glass, so on very long routes they can beat a cable, even after the trip up and down and the time the network spends sorting out its traffic (the race adds an estimated 15 ms for that). On shorter routes the cable usually wins. But a single cable can carry far more data than a whole fleet of satellites, which is why cables still do the heavy lifting.