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But time is like a river in another way.
不过时间似流水还有另一层含义
It flows at different speeds in different places.
其在不同地点有着不同速度
And that is the key to traveling into the future.
而这正是穿梭于时间的关键所在
The idea was proposed by Albert Einstein
这一概念在100多年前
over 100 years ago.
由阿尔伯特·爱因斯坦提出
He realized that there should be places where time slows down
他认为时间在一些地方会减慢
and others where time speeds up.
而在另一些地方则会增快
He was absolutely right,
这话可是千真万确
and the prove is right above our heads, up in space.
证据就在我们头顶的那片太空中
This is the Global Positioning System, or GPS.
这是全球定位系统，又叫GPS
A network of 31 satellites
是由绕地球轨道运行的
in orbit around the earth.
31颗卫星形成的网络
The satellites make satellite navigation possible.
正是这些卫星使卫星导航变为可能
But they also reveal that time runs faster up here
但同时也表明
than it does down on earth.
在这里时间流逝得比地球上快
Inside each spacecraft is a very precise clock.
每个航天器内都有一个十分精确的时钟
But despite being so accurate,
尽管已经十分精确
they all gain around
但他们每天都大约会走快
a third of a billionth of a second every day.
十亿分之一秒的三分之一
The system has to correct for the drift.
系统必须更正这一偏差
Otherwise that tiny difference would upset the whole system,
否则这细微的差异会扰乱整个系统
causing every GPS device on earth
致使地球上所有GPS设备
to go out by about six miles a day.
每天出现约6英里的误差
You can just imagine the mayhem that would cause.
其引起的混乱可想而知
The problem doesn't lie with the clocks.
问题并不在于时钟
They run fast because time itself runs faster here than
时钟走得快是因为在这里时间本身
it does down below,
就比在地球上快
and the reason for this extraordinary effect
产生这种奇异现象的原因
is the mass of the earth.
在于地球的巨大质量
Einstein realized that matter drags on time,
爱因斯坦意识到，物质滞延时间
slows it down like the slow part of a river.
像河流的缓流部分那样令时间减速
The heavier the object, the more it drags on time,
物体越重，滞延时间越长
and this startling reality
这一惊人的事实
is what opens the door to the possibility of time travel to the future.
打开了一扇穿越未来的希望之门
I admit this is a difficult concept to understand,
我承认这个概念很难理解
so let's take a simple example.
所以让我们来看一个简单的例子
This is the Great Pyramid of Giza.
这是吉萨的大金字塔
It weighs over 40 million tons,
重达4000多万吨
and like all heavy things,
与其他所有沉重的物体一样
it's actually slowing down time.
它会减慢时间
The effect is small,
不过影响微弱
billions of times smaller than that of the earth.
只及地球影响力的几十亿分之一
But if we exaggerate it drastically,
但是如果大幅放大这种影响
You can see the principle at work.
就能看到这种原理在起作用
