Why GPS Would Not Work Without Relativity
Why GPS Would Not Work Without Relativity — Before this fact I fall deep into silence. All our lives we believed time flows the same for everyone. But the truth is that the time of the high and the low, the fast and the slow, differs minutely. If even time, which we took for absolute, differs by place, how much more must other things we firmly hold to be right be bound to their place? The same event flows differently for one watching from the mountain and one in the valley. Yet here is the deeper comfort: though time runs differently for each, only when we honestly admit the difference and correct for it do all the world's positions and times finally fall into one. Not by denying difference, but by measuring it exactly, do we keep from losing our way.
Even on an unfamiliar road, the map in your hand pins your position to within a few meters. This precision leans on time signals sent by satellites tens of thousands of kilometers overhead. But what if the clocks on those satellites do not run at the same rate as ours on the ground? What if our lifelong belief that time flows the same for everyone is, in fact, not exact?
In the early twentieth century a patent clerk overturned humanity's sense of time — Albert Einstein. He first showed that a fast-moving clock runs slow (special relativity), then offered something stranger: where gravity is weaker, high up, time runs faster (general relativity). A clock on a mountaintop runs ever so slightly faster than one in the valley — a difference so minute it cannot be felt in daily life. For decades this seemed a genius's beautiful theory, untouched by ordinary living.
GPS changed everything. The satellites sit in high orbit where gravity is weaker, so their clocks run about 45 microseconds faster per day than on the ground. At the same time they move fast, slowing them by about 7 microseconds. Together, a satellite clock gains about 38 microseconds a day over ours — thirty-eight millionths of a second. It sounds trivial, but GPS measures distance by the time light travels, so this small mismatch swells to a position error of about 10 kilometers a day. Uncorrected, navigation becomes useless within a single day. So every GPS satellite has Einstein's two relativities pre-calculated and built in. One person's thought experiment now works daily, a century later, in the palms of billions.
- Car navigation, delivery tracking, even the self-steering of farm machinery — nearly everything dealing with position stands on satellite clocks corrected by relativity.
- Banks and communication networks take precise time from GPS to align transactions and calls. Only by admitting that time differs from place to place do the world's clocks finally fall into one.
- That aircraft and ships do not lose their way over open sea and above the clouds is, too, thanks to handling the time of the satellites overhead with precision.
玄, 천자문 첫머리의 검고 깊은 하늘처럼, 시공간은 곳마다 시간이 다르게 흐르는 헤아리기 어려운 깊이를 품고 있다.
Meet this hanja in Cheonjamun →Before this fact I fall deep into silence. All our lives we believed time flows the same for everyone. But the truth is that the time of the high and the low, the fast and the slow, differs minutely. If even time, which we took for absolute, differs by place, how much more must other things we firmly hold to be right be bound to their place? The same event flows differently for one watching from the mountain and one in the valley. Yet here is the deeper comfort: though time runs differently for each, only when we honestly admit the difference and correct for it do all the world's positions and times finally fall into one. Not by denying difference, but by measuring it exactly, do we keep from losing our way.
