How Atomic Keep 原子鐘 Why Do We Need Perfect Time? Every time you launch Pokémon Go or other games that require the Global Position System (GPS), your phone’s GPS determines coordinates and shows your location. What if your avatar is teleporting across town or refusing to register the two kilometers you’ve walked to hatch an egg – frustrating, right? That’s exactly what happens when GPS timing drifts even for a small error. Radio signals travel at the speed of light, which is approximately 3 × 108 m/s. To determine the distance between you and a GPS satellite, the travel time of the radio signal emitted by the satellite to you is needed. By the simple velocity formula, we know that an uncertainty of just one nanosecond (10-9 seconds) in a clock corresponds to about 30 cm of range error [1], so a timing drift can misplace your avatar enough to miss a rare resource in game. Without atomic-level precision, our smartphones’ “blue dot” on the map could drift wildly from reality. Nature of Time Let’s start with a fundamental question: What is time? Philosophers have long debated whether time “flows” like a river – an ever-moving present that carries us from past to future – or whether all moments exist equally, with past, present, and future represented as slices in a four-dimensional space, and many other competing perspectives [2]. While no one knows what exactly time is, one pragmatic point of view is that we can define the length of time by counting some repeatable, periodic processes. The most common periodic processes are sunrise and sunset, caused by Earth’s rotation. We can also use gravity-driven swinging pendulums, which provide a near-constant oscillation period and form the basis of early mechanical clocks. Albeit less notably, even your body counts: You wake up refreshed and feel sleepy at night, which marks one full day (assuming your circadian rhythm stays on track). However, given that the Earth does not rotate at a perfectly uniform speed, and that the duration of one swing is slightly different from pendulum to pendulum due to manufacturing error [3], a new definition of time is needed. In 1927, a Canadian engineer, Warren Marrison, found that quartz crystals vibrate at a remarkably consistent frequency under an electric field [3]. When carefully cut into a proper shape and size, a standard quartz crystal in a clock vibrates at 32,768 Hz [3, 4]. By counting the duration for which 32,768 oscillations take, we can define that one second has passed. Core of Cesium-Beam Atomic Clock However, from 10 seconds per year for a mechanical clock to only one second in three
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