499 — LIGHT DELAY

The astronomical unit — the mean Earth-Sun distance — was fixed by the International Astronomical Union in 2012 at exactly 149,597,870,700 metres. The speed of light has been exactly 299,792,458 metres per second since 1983, when the metre was defined from it. Divide one by the other and the light-time for one astronomical unit is 499.004783836 seconds. Neither number carries an error bar, so neither does the answer: 8 minutes and 19 seconds.

The consequence is literal rather than poetic. If the Sun went out now, nobody would find out for eight minutes and nineteen seconds. The delay scales with distance and with nothing else: the Moon is 1.3 seconds away, Mars is between roughly 3 and 22 minutes away depending on where the two planets have got to in their orbits, which is why a rover is driven by scripts and not by joystick. Proxima Centauri is 4.2 years away. The Andromeda galaxy is about 2.5 million years away — the smudge visible to the naked eye on a dark night left before Homo sapiens existed.

There is no exception to this and no way around it. Every observation is a report from the past, and how far in the past depends only on how far away the thing is. The sky is not a picture of the universe as it stands; it is a picture of the universe at a range of different dates, all arriving at the same instant. 'Now' is not something the universe delivers. It is something an observer reconstructs afterwards, by subtracting a delay it can calculate.

'Now' is not something the universe delivers.

The same limit applies at desk scale, where it stops being astronomy and becomes engineering. Light covers about 30 centimetres in a nanosecond, and a signal in fibre or copper travels at roughly two-thirds of that. It is why a processor clocked in gigahertz has to be physically small to keep its own ends in agreement, and why a call across an ocean has a floor under its lag that no amount of bandwidth will lift. 499 is simply the smallest of these delays that anyone actually lives inside.