(单词翻译:单击)
Here's something you might not have thought about:
有些事情你可能没想过：
The magnetic north pole that compasses point to, near the geographic north pole,
罗盘指向的北磁极靠近地理北极，
is actually the south pole of the Earth's magnetic field.
实际上是地球磁场的南极。
That's why it attracts the north poles of magnets.
这就是它吸引磁铁北极的原因。
But that hasn't always been true.
但事实不总是这样。
Today's compasses would've pointed south about 1.1 million years ago,
如今的罗盘在110万年前指向南方，
when the geomagnetic north was also a magnetic north pole.
那时地理的北极也是北磁极。
But then, the magnetic poles switched places:
但随后，磁极改变了位置：
the magnetic south pole went north, and the magnetic north pole went south.
南磁极向北移，北磁极向南移。
They swapped again a little less than a million years ago,
它们在不到100万年前就又互换了一点儿。
and again about 780,000 years ago, giving us today's orientation.
在大约78万年前再次移了一点儿，形成了如今的方向。
Now, if you like doomsday scenarios,
现在，如果你喜欢末日场景，
you might've heard that we're due for another switch that'll make electronics useless and kill us all.
你可能听说过我们注定要再次转换磁极，这会让电子设备变得毫无用处，也会把我们都杀了。
But that's a little extreme.
有点儿极端了。
And saying that we're "due" implies that we know a lot about when and why these swaps happen, which we really don't.
要说“注定”，就意味着我们很了解互换发生的时间和原因，但事实是我们真的不了解。
The Earth's magnetic field comes from electromagnetic induction,
地球磁场源于电磁感应，
where iron moving in the outer core generates an electric current,
外地核的铁移动产生了电流，
with the same physics that power plants use to generate electricity.
发电厂利用同样的物理原理来发电。
And that electric current creates a magnetic field.
电流产生磁场。
As molten rock reaches the surface of the Earth and cools,
当熔岩到达地表并冷却时，
any iron in it tends to line up with the Earth's magnetic field.
里面的任何铁元素都与地球磁场相一致。
So the iron in solidified lava acts like a time capsule for what Earth's magnetic field looked like when it solidified.
因此，凝固熔岩所含的铁就像地球磁场凝固时的时间囊。
Some places, like where two tectonic plates have been consistently moving away from each other,
有些地方如一直相互远离的两个地壳板块，
have had molten rock coming out and solidifying for millions of years.
冒出的熔岩已经凝固了几百万年。
The farther you get from these divergent plate boundaries,
离这些不同板块的边界越远，
the older the iron-filled rocks are, and the older the trapped magnetic field is.
含铁的岩石越古老，里面的磁场越久远。
Many of these boundaries are on the ocean floor.
许多这种边界都在海底。
So scientists can measure how the Earth's magnetic field has changed over time
因此科学家们可以通过乘船把敏感的磁测量设备带到海上
by bringing sensitive magnetic measuring equipment with them on ships across the ocean.
来测量地球磁场是如何随时间变化的。
They first tried it back in the 1950s and '60s, and every measurement since has confirmed the same story:
他们在20世纪50、60年代做过第一次尝试，之后的每次测量都确认了一件事：
there has been a lot of flip-flopping.
磁场有很多次逆转。
Rocks of one age will have magnetic north pointing south, just like Earth's magnetic field is today.
某个年代的岩石拥有的磁场北磁极指向南方，就像地球如今的磁场一样。
But other rocks are backwards: magnetic north points north.
但其他的岩石相反：北磁极指向北方。
The rocks themselves and the iron trapped inside haven't flipped over or anything,
岩石本身和被困在里面的铁没有翻转或其他变化，
so the Earth's magnetic field must have reversed over time.
所以一定是地球磁场随时间逆转了。
And it wasn't just once or twice, either.
而且不是一两次。
There have been at least a hundred of these so-called magnetic field reversals.
至少有100次这种所谓的磁场逆转了。
In fact, there's evidence for reversals going all the way back before the Cambrian period, more than half a billion years ago.
事实上，5亿多年前的寒武纪之前，就有证据证明这种逆转一直存在。
Despite these and other measurements, like radioactive elements in rocks,
尽管科学家有这样那样的测量手段，如测量岩石的放射性元素，
scientists haven't found much of a pattern in the timing of magnetic field reversals.
但仍没有在磁场逆转的时间里发现很多模式。
Sometimes they happen back-to-back, geologically speaking.
有时，从地理上讲，它们是紧接的。
But other times, there are millions of years between them.
但其他时候，两次之间相隔数百万年。
That's one reason it's a little silly to say that we're "due" for another
我们说另一次“注定”发生有点儿蠢的一个原因是
just because it hasn't happened for around 780,000 years.
它已经有大约78万年没再逆转了。
The next one might be soon, or we could be at the beginning of something like the Cretaceous Normal Superchron,
下一次可能不久就发生，或者我们可能进入了像白垩纪超静磁带这样的时期，
a period of about 40 million years without a single reversal.
白垩纪超静磁带是4000万年没发生一次磁极逆转的时期。
Or a reversal might've happened more recently than we thought.
又或者逆转可能比我们想象的要晚。
Some scientists studying rocks and sediments from the Black Sea
一些科学家研究了黑海的岩石和沉积物，
recently found magnetic evidence for a pair of reversals around 41,000 years ago, with only a couple hundred years between them.
最近发现了两个磁场逆转的证据，它们发生在大约4万1千年前，相隔只有几百年的时间。
But that would be a pretty extreme pair of reversals,
但这可能是两次极端的逆转，
since we think that a single flip generally takes a few thousand years from beginning to end.
因为我们认为一次逆转从开始到结束通常要经历几千年。
Although there's a ton of variation here, too: reversals could take hundreds, or thousands, or tens of thousands of years.
尽管其中也有很多变化：逆转可能需要几百年、数千年或数万年。
During a reversal, we think that the existing magnetic field generally weakens until it's almost completely gone.
在逆转过程中，我们认为现有的磁场通常会减弱直到几乎完全消失。
And then the field gradually builds up in the opposite orientation.
然后磁场会在相反的方向逐渐形成。
Since the Earth's magnetic field helps protect us from some of the most extreme forms of interstellar and solar radiation,
因为地球磁场有助于保护我们免受一些最极端的星际和太阳辐射，
a reversal would be pretty bad news if it started tomorrow.
如果逆转从明天开始将是一个非常糟糕的消息。
Because of all the extra radiation,
因为所有额外辐射的原因，
satellites in low-Earth orbit and electronics down here on the surface might get fried over time and stop working.
低地轨道上的卫星和地面上的电子设备可能会随着时间的推移而混乱，停止工作。
If the power grids go down, that could lead to a lot of problems.
如果电网减弱，会有很多麻烦。
Not to mention, those kinds of radiation can damage cells,
更不必说，这种辐射会损害细胞，
so it might cause extra bad sunburns or increased rates of cancers in humans and other living things.
由此可能会造成额外的严重晒伤或增加人类和其他生物癌症的发病率。
But it still probably wouldn't be apocalyptic.
但它可能不是世界末日。
Now, despite all this research about when reversals happened,
现在，尽管科学家们研究了所有磁极逆转发生的资料，
scientists still don't know what causes them.
但仍不知道它的成因。
They're not sure what has enough energy to affect the Earth's core so dramatically,
他们不确定什么东西有如此充足的能量，能如此戏剧性地影响地心，
changing the way a ton of iron and molten rock are sloshing around.
改变大量的铁和熔岩的移动。
In the past, it could've been asteroids or comets smacking into Earth
在过去，原因可能是小行星或彗星撞击地球，
and sending powerful shockwaves through the planet,
在地球上发出强大的冲击波，
or triggering climate events that changed how its mass is distributed.
或者引发气候事件改变其质量分布。
Or it could have to do with big chunks of tectonic plates descending into Earth's mantle,
还有可能与地幔中大量的地壳板块有关，
or blobs of magma moving up from the deep mantle and throwing things out of balance.
或者是岩浆从地幔深处向上移动，使磁场失去平衡。
But it's hard to find evidence of exactly what's happened in Earth's core for the last billion years,
但我们很难找到确切证据，证明过去十亿年地心发生了什么。
and our computer simulations can only predict so much with incomplete datasets.
我们的电脑根据不完整的数据只能预测这么多。
So geophysicists are still at the guessing stage of science, at least for now.
所以，地质学家们目前仍处于猜测阶段。
Thanks for watching this episode of SciShow Space,
感谢您收看本期的太空科学秀，
and thanks especially to our patrons on Patreon who help make this show possible.
同时特别感谢Patreon对本期节目的大力支持。
If you want to help us keep making videos,
如果你想我们继续制作此节目，
you can go to patreon.com/scishow to learn more.
可以登录patreon.com/scishow了解更多信息。
And don't forget to go to youtube.com/scishowspace and subscribe!
也不要忘了登录youtube.com/scishowspace点击订阅呦！
