How does your smartphoneknow exactly where you are?
你的智能手机是怎么定位你的位置的？
The answer lies 12,000 miles over your head in an orbiting satellite
答案藏在你头顶19000多公里高空中的卫星上，
that keeps timeto the beat of an atomic clock powered by quantum mechanics.
卫星利用基于量子力学的原子钟来运行。
Phew. Let's break that down.
呼...来分解一下这句话。
First of all, why is it so important to know what time it is on a satellite when location is whatwe're concerned about?
首先，为什么知道卫星上的时间很重要，尤其当我们关心的是定位位置，而不是时间。
The first thing your phone needs to determine is how far it is from a satellite.
首先，你的手机要计算出你离卫星有多远。
Each satellite constantly broadcasts radio signals that travel from space to your phoneat the speed of light.
每个卫星都会间歇性地发射出无线电波信号，信号以光速从卫星传递到你的手机。
Your phone records the signal arrival time
手机会记录接受信号的时间间隔，
and uses it to calculate the distance to the satellite using the simple formula, distance = c*time,
然后计算出与卫星之间的距离，用这个简单的公式：距离=c*时间，
where c is the speed of lightand time is how long the signal traveled.
c就是光速，时间就是信号传递的时间间隔。
But there's a problem. Light is incredibly fast.
但是，有一个问题。光速是极快的。
If we were only able to calculate time to the nearest second,
如果我们只能粗略估计信号传递时间的长短，
every location on Earth, and far beyond, would seem to be the same distance from the satellite.
地球上的每一个地点，即便彼此相距很远，看起来也都会和卫星相隔同样的距离。
So in order to calculate that distanceto within a few dozen feet, we need the best clock ever invented.
所以为了计算出相隔只有几米远的距离，我们需要一个最精准的“钟”。
Enter atomic clocks, some of which are so precise
原子钟，是非常精准的钟，
that they would not gain or lose a second even if they ran for the next 300 million years.
它们不会多走或者少走一秒钟，即使走上3亿年也是这样。
Atomic clocks work because of quantum physics.
原子钟走的如此精准，是基于量子力学原理。
All clocks must have a constant frequency.
所有的钟都遵循固定的频率工作。
In other words, a clock must carry outsome repetitive action to mark off equivalent increments of time.
换句话说，也就是钟在固定时间内会重复同样的动作来标记相等时段的时间。
Just as a grandfather clock relies on the constant swinging back and forth of a pendulum under gravity,
就好像老式钟表依靠重力控制的钟摆会以固定频率的摆动来工作一样，
the tick tock of an atomic clock is maintained by the transition between two energy levels of an atom.
原子钟的走动依靠的是一个原子的两个能级间的跃迁。
This is where quantum physics comes into play.
这就是量子物理的实际应用。

Quantum mechanics says that atoms carry energy, but they can't take on just any arbitrary amount.
量子力学认为每个原子都带有能量，但不是带有任意数值的能量。
Instead, atomic energy is constrained to a precise set of levels.
实际上，原子的能量由一系列确定的等级决定。
We call these quanta. As a simple analogy, think about driving a car onto a freeway.
这些等级我们称之为量子。打个比方，有一辆车在高速上行驶。
As you increase your speed, you would normally continuously go from, say, 20 miles/hour up to 70 miles/hour.
当你加速时，正常情况下你会从每小时32公里逐渐加速到110公里。
Now, if you had a quantum atomic car, you wouldn't accelerate in a linear fashion.
现在，如果你有一辆原子车，你就不会一码一码地线性加速。
Instead, you would instantaneously jump, or transition, from one speed to the next.
而是瞬间从低速跳变到高速，不需要任何过渡。
For an atom, when a transition occurs from one energy level to another,
对于一个原子在两个能级之间发生一次跃迁，
quantum mechanics says that the energy difference is equal to a characteristic frequency, multiplied by a constant,
量子力学认为，能级之间的能量差是根据跃迁时的特征频率乘以一个常数得出的，
where the change in energy is equal to a number, called Planck's constant, times the frequency.
也就是说能量的变化，等于一个常数，也就是普朗克常量，乘以跃迁时的特征频率。
That characteristic frequency is what we need to make our clock.
利用这个特征频率，我们可以制造原子钟。
GPS satellites rely on cesium and rubidiumatoms as frequency standards.
GPS卫星以铯和铷原子的共振频率为频率标准。
In the case of cesium 133, the characteristic clock frequencyis 9,192,631,770 Hz.
铯133，它的特征频率为91亿9263万1770 赫兹。
That's 9 billion cycles per second. That's a really fast clock.
也就是每秒走90亿圈。这个钟真是超级无敌快。
No matter how skilled a clockmaker may be,
无论多么出色的钟表匠，
every pendulum, wind-up mechanism and quartz crystal resonates at a slightly different frequency.
每一组钟摆、齿轮和石英晶体三者之间的共振频率总会有细微的差别。
However, every cesium 133 atom in the universe oscillates at the same exact frequency.
但是，宇宙中的每一个铯133原子都遵循着同样精准的共振频率。
So thanks to the atomic clock, we get a time reading accurateto within 1 billionth of a second,
幸亏有了原子钟，我们可以精确到一秒的十亿分之一，
and a very precise measurementof the distance from that satellite.
由此就可以得出与卫星之间的准确距离。
Let's ignore the fact that you're almostdefinitely on Earth.
我们假设你正好位于地球表面。
We now know that you're at a fixed distance from the satellite.
已知你与卫星之间的距离不变。
In other words, you're somewhere on the surface of a sphere centered around the satellite.
换句话说，你就在以卫星为中心的一个球体表面的某个地方。
Measure your distance from a second satellite and you get another overlapping sphere.
再根据第二个卫星来测量距离，就可以得到另外一个有重叠的球体。
Keep doing that, and with just four measurements, and a little correction using Einstein's theory of relativity,
不断累加，四个卫星测量得到的四个球体重叠于一点后，再根据爱因斯坦的相对论进行细微的修正，
you can pinpoint your location to exactlyone point in space.
你就可以精准地知道自己的位置。
So that's all it takes: a multibillion-dollar network of satellites,
精确定位就是这么实现的：一个用数十亿美元构建起来的卫星网络，
oscillating cesium atoms, quantum mechanics, relativity, a smartphone, and you. No problem.
振动的铯原子，量子力学，相对论，一部智能手机，还有你。小菜一碟！