Tvoja karta bi bila pogrešna do ručka

Tvoj telefon zna gdje se nalaziš jer osluškuje satove u svemiru. Sateliti odašilju vrijeme, iznova i iznova, a tvoj telefon izračunava svoj položaj prema tome koliko je vremena svakom signalu trebalo da stigne. To je zapravo problem mjerenja vremena, obučen u kartu.

Your phone knows where you are because it is listening to clocks in space. Satellites broadcast the time, over and over, and your phone works out its position from how long each signal took to arrive. It is really a timing problem wearing a map.

Evo neugodnog dijela. Ti satovi ne kucaju istom brzinom kao sat na tvom zapešću. Gore, dalje od mase planeta, vrijeme teče nešto brže. Brzo kretanje u orbiti vuče u suprotnom smjeru i malo ih usporava. Ta dva učinka se ne poništavaju, i sateliti na kraju dobiju otprilike trideset i osam milijunitih dijelova sekunde svaki dan.

Here is the awkward part. Those clocks do not tick at the same rate as the clock on your wrist. Up there, further from the mass of the planet, time runs slightly faster. Moving fast in orbit pulls the other way and slows them slightly. The two effects do not cancel, and the satellites end up gaining about thirty eight millionths of a second every day.

To zvuči kao ništa. Nije. Svjetlost putuje otprilike trideset centimetara u milijarditom dijelu sekunde, pa neispravljena pogreška te veličine naraste na otprilike deset kilometara pogreške u položaju dnevno. Tvoja bi plava točka skrenula s ulice, zatim s grada, prije večere. Satovi su namjerno napravljeni da kucaju pogrešnom brzinom na tlu, tako da kucaju pravom brzinom kad se nađu gore.

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