Light covers 299,792 km every second, fast enough to circle Earth seven times. Yet space is so big that even light has to wait. Send a pulse and feel how long the universe makes it travel.
The pulse to the Moon runs in real time: 1.28 seconds, every time. Pick a farther destination and the speed jumps up so you don't wait all day. Set it back to Real time to feel the full wait.
Jupiter's moon Io slips into Jupiter's shadow every 42.5 hours, regular as a clock. Ole Rømer noticed the eclipses ran late when Earth was far from Jupiter and early when it was near. The extra time is light crossing the extra distance.
Rømer predicted the eclipse of 9 November 1676 would come 10 minutes late, and it did. His numbers implied about 22 minutes for light to cross Earth's orbit; the true value is 16.6.
Steer the rover by radio and stop it before the boulder. Your camera picture is old by the light time, and your stop command takes just as long to get there. Time runs 60× fast here: one second is one minute.
Light time to Mars right now: 12.5 minutes each way. Press Send "drive".
Each trip is drawn to scale between Earth on the left and the destination on the right, and the pulse moves at exactly the speed of light, sped up by the playback factor. The ruler at the top uses a log scale so a 1-second trip and a 4-year trip fit side by side.
Mars is the odd one out: its distance changes by a factor of seven as Earth and Mars swing around the Sun, so radio messages take between 3 and 22 minutes.
Every observer, however they move, measures the same speed for light. That one fact forces time and space to bend to keep it constant, which is Einstein's special relativity.
It also means there is no universal "now". You see the Moon as it was a second ago, the Sun 8 minutes ago and Proxima Centauri over 4 years ago. Looking out is always looking back.
Since 1983 the metre has been defined as the distance light travels in 1/299,792,458 of a second, so the speed of light is now exact by definition.
Light is fast enough to feel instant and slow enough to measure. Much of modern technology lives in that gap.
Apollo 11, 14 and 15 and two Soviet rovers left reflectors on the Moon. Observatories fire laser pulses at them and time the echo, measuring the distance to millimetres. It shows the Moon drifting away by 3.8 cm a year and tests general relativity.
In the demo: Moon with the echo on, 2.56 seconds there and back.Your phone times signals from several satellites about 20,000 km up. Each nanosecond of timing error is 30 cm of position error, so the satellites carry atomic clocks and even correct for relativity.
In the demo: distance = speed of light × travel time, with times in billionths of a second.Trading firms built straight microwave links between Chicago and New York because light in glass fibre travels about a third slower than through air. Firms spent hundreds of millions of dollars on new routes to shave a few milliseconds off the 1,200 km trip.
In the demo: at 1 ms per 300 km, only the route and medium are left to improve.London to Sydney is about 17,000 km. Even at the speed of light in fibre, a round trip takes well over 100 milliseconds. That is why video calls lag across oceans and why companies copy websites to servers near you.
In the demo: turn on the echo: every question waits for a round trip.NASA's big dishes in California, Spain and Australia keep every deep-space probe in view as Earth turns. Commands to Voyager 1 take nearly a day to arrive, and the reply takes nearly a day to return, at a data rate of about 160 bits per second.
In the demo: choose Voyager 1 and wait for the echo.Nobody can joystick a rover across a 20-minute delay. Engineers send a day's plan at once, and Curiosity and Perseverance use their own hazard cameras and AutoNav software to pick safe paths between waypoints.
In the demo: switch on the hazard cameras and the rover stops itself.