Zašto bi tvoja karta do ručka bila pogrešna

Tvoj telefon zna gdje se nalaziš zato što, u biti, prisluškuje satove u svemiru. Sateliti neprestano odašilju vrijeme, a tvoj telefon izračunava svoj položaj na temelju toga koliko je dugo svakom signalu trebalo da stigne. U svojoj srži, to je problem mjerenja vremena prerušen u kartu.

Your phone knows where you stand because it is, in effect, eavesdropping on clocks in space. Satellites broadcast the time, relentlessly, and your phone calculates its position from how long each signal took to arrive. It is, at bottom, a timing problem masquerading as a map.

Evo neugodnog detalja. Ti satovi ne kucaju istom brzinom kao onaj na tvojoj ruci. Dalje od mase planeta vrijeme teče nešto brže; kretanje velikom brzinom u orbiti vuče tu brzinu u suprotnom smjeru. Ta se dva učinka ne poništavaju, pa sateliti naposljetku dobiju otprilike trideset osam milijuntinki sekunde svakog dana.

Here is the inconvenient detail. Those clocks do not tick at the same rate as the one on your wrist. Further from the planet's mass, time runs slightly faster; moving at speed in orbit pulls the rate the other way. The two effects fail to cancel out, and the satellites end up gaining roughly thirty eight millionths of a second every day.

To zvuči kao ništa. Nije. Svjetlost prijeđe otprilike trideset centimetara u milijarditinki sekunde, pa se nekorigirana pogreška te veličine nakuplja u otprilike deset kilometara pogreške položaja dnevno. Tvoja bi plava točka odlutala s ulice, zatim iz grada, prije nego što padne večer. Zato su satovi namjerno napravljeni tako da na tlu kucaju pogrešnom brzinom, kako bi u orbiti stigli do prave.

That sounds like nothing. It is not. Light travels about thirty centimetres in a billionth of a second, so an uncorrected error of that magnitude compounds into roughly ten kilometres of position error per day. Your blue dot would wander off the street, then off the town, before dinner. The clocks are therefore built, deliberately, to run at the wrong rate on the ground, so that they arrive at the right one once they reach orbit.