← Theory of RelativityPart 0 — Foundations · 05

Seeing the Past

▶ DRILL

George waves from far away. You don't see the wave itself — you see the lightthat bounced off him, and that light takes time to travel. So Liza sees the wave a moment after it happened: she's seeing him in the past. Push him farther and the lag grows — the Sun you see is 8 minutes old, the nearest star 4 years, distant galaxies millions. Looking out is looking back in time. Use the Explain it for a kid / a grown-up switch to set the depth.

EXPLAIN IT FORsimple words, names & pictures

HOW FAR IS GEORGEthe Sun

SLOW-MOnormal

So — George is about as far as the Sun. The light from his wave takes 8.3 minutes to reach Liza, so she sees him that far in the past.

The on-screen gap is squished to fit, but the idea is exactly real: the Moon you see is 1.3 seconds old, the Sun 8 minutes, the nearest star 4 years. Looking far away is looking into the past. And this only works because light has a speed — which the next drill makes strange.

# formuladelay = d / c — how long the light takes to cross the distance d at the speed of light

# real delaysMoon 1.28 s · Sun 8.3 min (499 s) · Proxima Centauri 4.24 yr · distant galaxies millions of years

# scalethe on-screen gap is compressed to fit; light's speed is constant, so distance alone sets the delay — no relativity yet

# nextthis works because light has a speed — and everyone measures that speed the same