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The idea that certain things, events or people can "cause" other things to happen plays a huge role in human life.
某些事情、事件或人可以“导致”其他事情发生，这一观点在人类生活中扮演着重要角色。
We constantly desire to know "why" things happen, in science, love, sports, philosophy, and so on.
我们总知道这些事情‘为什么’会发生在科学、爱情、体育、哲学等等领域。
But because the underlying laws of physics don't care about the direction of time,
但是因为物理学的基本定律并不关心时间的方向，
cause and effect don't have the same meaning at a fundamental level. It's not that anything goes.
所以因果关系在基本层面上并没有相同的意义。并不是什么事都发生了。
The basic constituents of the universe - the particles and forces of modern physics -
宇宙的基本组成部分——现代物理学中的粒子和力——
behave in predictable ways according to the laws of nature.
按照自然规律以可预测的方式运行。
In principle, you can just as easily know their past paths as their future ones.
原则上，知道它们过去和未来的轨迹一样容易。
The current momentum and position of a particle determine its movement forward in the next second,
粒子当前的动量和位置决定了它在下一秒的运动，
but they also determine how it was moving in the previous second.
但它们也决定了前一秒的运动。
Neither is really a "cause" preceding an "effect", there's just a pattern that particles follow.
两者都不是‘因’接着‘果’，这只是粒子遵循的模式。
Kind of like how the integer after 42 is 43, and the integer before it is 41,
有点像是整数42后边的是43，而这个整数前边的是41，
but 42 doesn't "cause" 41 or 43 – there's just a pattern traced out by those numbers.
但是42并不是41或43的‘因’——这不过是这些数字遵循的一套规律而已。
At a fundamental, microscopic level, all we can say is that there are patterns between events.
在基本的、微观的层面上，我们所能说的是事件之间有模式。
The macroscopic, human-scale concepts of cause and effect only emerge
因果这种宏观、人类范围的概念仅在
when you have larger collections of particles, like humans!
有一大群粒子的时候才会出现，如人类！
As we know, time does have a direction for larger-scale systems,
我们知道，在大范围系统中，时间确实是有方向，
and we can indeed talk about a spark causing oxygen and hydrogen to turn into water and an explosion.
并且我们确实也可以谈论一场导致氧气和氢气变成水和爆炸的火花。
"Spark plus oxygen plus hydrogen" and then "water plus explosion"
“火花加氧气加氢气”，然后“水加爆炸”
is a sequence the universe follows, and it only happens in one direction.
是宇宙遵循的一个顺序，它只在一个方向上发生。
You never see a reverse explosion where water spontaneously splits into oxygen and hydrogen gas
你永远不会看到反过来的爆炸——水自发地分裂成氧气和氢气
and then at the very end emits a little spark.
然后在最后发出一点火花。
One way of thinking about causes is that the tiny spark has great "leverage"over the future.
思考原因的一种方式是，这小小的火花对未来具有巨大的“影响力”。
If you hadn't lit the spark, we wouldn't have seen a giant explosion.
如果不是你点燃了火星，我们就不会看到巨大的爆炸。
It doesn't work the other way: removing or changing a tiny part of the giant explosion
但反过来却不是这样的：稍微改变或移除大爆炸的一小部分
doesn't imply that there wasn't a preceding spark.
并不代表先前就没有火花。
When a small change to the present implies a big change to the future,
当现在的一个小改变导致未来出现大改变时，
the small thing we're changing is generally thought of as a "cause". Leverage can also go the other way.
我们正在改变的小事情通常被认为是一个“因”。反过来也说得通。
Take this new pencil: the wood it's made of contains trace amounts of radioactive carbon-14,
拿这个新铅笔为例：这种木材中含有微量的放射性碳14，
created by nuclear bomb testing. If the pencil didn't contain that carbon-14,
是由核弹试验产生的。如果铅笔中不含碳14
that would imply that no nuclear bombs had been detonated in the last 80 years;
那么这意味着，在过去的80年里，没有任何核弹被引爆；
while if you removed a pencil-sized amount of one of the atomic bombs, this pencil would still be basically the same.
然而，如果你移除核弹里相当于铅笔大小的一部分，这支铅笔基本不会有什么变化。
In this case, the fact that the pencil has lots of carbon 14 means that it has a lot of "leverage" over the past.
在这种情况中，铅笔中含有很多碳14意味着它对过去发生的事情很重要。
Instead of calling the carbon 14 in this pencil a "cause" of the earlier detonation of hundreds of nuclear bombs,
我们不把这之前比中的碳14称为先前引爆的数百只核弹的‘因’，
we call it a "record" of the bombs.
我们称它为爆炸的‘记录’。
In general, when a small change to the present would imply a big change to the past,
总之，当现在的一个小改变意味着过去的大改变时，
the small thing we're changing is thought of as a "record" or a "memory".
我们正在改变的小事就会被认为是一种‘记录’或一段‘记忆’。
So the distinctions between "cause" and "effect", "records" and "predictions",
所以‘因’和‘果’，‘记录’和‘预测’之间的区别，
aren't fundamental to underlying physics – they only really make sense on the large scale, because of the direction of time.
并不在于基础的物理原理上——它们只有在大范围内才有意义，因为时间有方向。
