想一想，要是把时间倒转来看，发射光子的过程会变成什么样子呢？
Consider what the emission process looks like when viewed reversed in time.
但是偶尔这偶极场会转换极性——南极和北极倒转，这个过程似乎需要几千年的时间。
But occasionally this dipole field switches polarity north and south reverses, and this process seems to take a few thousand years.
推测我们会采取什么样不同的方法是容易的，但时间不会倒转。
It would be easy to speculate about what we could have done differently, but that wouldn't turn back the clock.
推测我们会采取什么样不同的方法是容易的，但时间不会倒转。
It would be easy to speculate about what we could have done differently, but that wouldn't turn back the clock.
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