How does your smartphone know your location? - Wilton L. Virgo

翻译人员: Vivi Fu 校对人员: KuaiKuai Gong

How does your smartphone know 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

答案藏在你头顶 19000多公里高空中的卫星上，

in an orbiting satellite that keeps time to the beat of an atomic clock

卫星利用基于量子力学的

powered by quantum mechanics.

原子钟来运行。

Phew.

呼...

Let's break that down.

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

首先，为什么知道 卫星上的时间很重要？

when location is what we're concerned about?

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

首先，你的手机要计算出

is how far it is from a satellite.

你离卫星有多远。

Each satellite constantly broadcasts radio signals

每个卫星都会间歇性地 发射出无线电波信号，

that travel from space to your phone at the speed of light.

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

然后计算出与卫星之间的距离，

using the simple formula, distance = c x time,

用这个简单的公式：距离=c*时间，

where c is the speed of light and 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.

但是，有一个问题。

Light is incredibly fast.

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,

地球上的每一个地点， 即便彼此相距很远，

would seem to be the same distance from the satellite.

看起来也都会和卫星相隔同样的距离。

So in order to calculate that distance to within a few dozen feet,

所以为了计算出相隔只有几米远的距离，

we need the best clock ever invented.

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

它们不会多走或者少走一秒钟，

even if they ran for the next 300 million years.

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 out some repetitive action

换句话说，也就是钟在固定时间内 会重复同样的动作

to mark off equivalent increments of time.

来标记相等时段的时间。

Just as a grandfather clock relies on the constant swinging

就好像老式钟表依靠重力控制的钟摆

back and forth of a pendulum under gravity,

会以固定频率的摆动来工作一样，

the tick tock of an atomic clock

原子钟的走动

is maintained by the transition between two energy levels of an atom.

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,

量子力学认为每个原子都带有能量，

but they can't take on just any arbitrary amount.

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.

这些等级我们称之为量子。

As a simple analogy, think about driving a car onto a freeway.

打个比方，有一辆车在高速上行驶。

As you increase your speed,

当你加速时，

you would normally continuously go from, say, 20 miles/hour up to 70 miles/hour.

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,

现在，如果你有一辆原子车，

you wouldn't accelerate in a linear fashion.

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

量子力学认为，

that the energy difference is equal to a characteristic frequency,

能级之间的能量差是 根据跃迁时的特征频率

multiplied by a constant,

乘以一个常数得出的，

where the change in energy is equal to a number, called Planck's constant,

也就是说能量的变化， 等于一个常数，也就是普朗克常量，

times the frequency.

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 rubidium atoms as frequency standards.

GPS卫星以铯(Cs)和铷(Rb)原子的 共振频率为频率标准。

In the case of cesium 133,

铯133(中子数为133的铯元素)，

the characteristic clock frequency is 9,192,631,770 Hz.

它的特征频率为 91亿9263万1770 赫兹。

That's 9 billion cycles per second.

也就是每秒走90亿圈。

That's a really fast clock.

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 都造不出来这样的钟，

every pendulum, wind-up mechanism

每一组钟摆、齿轮和 石英晶体三者之间的

and quartz crystal resonates at a slightly different frequency.

共振频率总会有细微的差别。

However, every cesium 133 atom in the universe

但是，宇宙中的每一个铯133原子

oscillates at the same exact frequency.

都遵循着同样精准的共振频率。

So thanks to the atomic clock,

幸亏有了原子钟，

we get a time reading accurate to within 1 billionth of a second,

我们可以精确到一秒的十亿分之一，

and a very precise measurement of 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 almost definitely 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

换句话说， 你就在以卫星为中心的一个球体

centered around the satellite.

表面的某个地方。

Measure your distance from a second satellite

再根据第二个卫星来测量距离，

and you get another overlapping sphere.

就可以得到另外一个有重叠的球体。

Keep doing that,

不断累加，

and with just four measurements,

四个卫星测量得到的 四个球体重叠于一点后，

and a little correction using Einstein's theory of relativity,

再根据爱因斯坦的 相对论进行细微的修正，

you can pinpoint your location to exactly one point in space.

你就可以精准地知道自己的位置。

So that's all it takes:

精确定位就是这么实现的：

a multibillion-dollar network of satellites,

一个用数十亿美元 构建起来的卫星网络，

oscillating cesium atoms,

振动的铯原子，

quantum mechanics,

量子力学，

relativity,

相对论，

a smartphone,

一部智能手机，

and you.

还有你。

No problem.

小菜一碟！