有一个科学术语叫做半衰期（half life）。例如，铀238的半衰期约为45亿年，意思就是，每过45亿年，一个U-238原子核会有一半的概率衰变。有一个著名的射击游戏叫做《半条命》，其实这个游戏的英文名是half life，本意就是“半衰期”，所谓“半条命”实际上是望文生义的误译。
为了帮助大家理解这个概念，美国韦尔斯利学院（Wellesley College）的化学教授Mala Radhakrishnan博士写了一首诗《我们半衰期的日子》（Days of Our Half Lives）。
Days of our half lives
My dearest love, I'm writing you to tell you all that I've been through.
I've changed my whole identity,
But loved, I cannot pretend to be.
When I was uranium 238,
You were on my case to start losing weight.
你对我的体重一直挑剔。【注释：on sb's case : 不停地指责】
For five billion years I hoped and I prayed,
And finally I had an alpha decay.
Two protons, two neutrons went right out the door,
And now I was thorium 234.
But my nucleus was still unfit for your eyes,
Not positive enough for its large size.
But this time my half-life was really not very long,
Because my will to change was quite strong.
It took just a month, not even a millennium,
To beta decay into protactinium.
But you still rejected me right off the bat,
但你马上拒绝了我：【注释：right off the bat : （美国俚语）立刻，马上】
Protactinium—who's heard of that?
So, beta decay, I did much more to become uranium 234.
Myself again, but a new isotope,
You still weren't satisfied, but I still had hope.
Three alpha decays, it was hard, but I stayed on
Through thorium, through radium, and then radon.
I thought I would finally please you,
My mass was a healthy 222.
But you said, although I like your mass,
I do not want to be with a noble gas.
You had a point, I wasn't reactive,
So in order to please you, I stayed proactive.
A few days later I found you and said,
Two more alpha decays and now I am lead.
You shook your head,
You were not too keen on my mass number of 214.
I had a bad experience with that mass before,
An unstable astatine walked right out the door.
So in order to change, I went away,
But all I could do was just beta decay.
My hopes and my dreams started to go under,
Because beta decay doesn't change a mass number.
To bismuth, then polonium, I hoped and I beckoned,
My half-life was 164 microseconds.
And then finally I alpha decayed,
And then I was lead with the prize worthy mass of 210.
I've got to admit I was getting quite tired,
My patience with you had nearly expired.
You were more demanding than any I dated,
And much of my energy had already been liberated.
But you still weren't happy, but you had a fix,
I really like the number of 206.
So I waited for years until the day,
Which began with another beta decay.
And then one more, and finally in the end
I alpha-ed to lead 206, my friend.
To change any further I wouldn't be able,
No longer active, but happily stable.
It took me billions of years to do,
And look how I changed and all just for you.
And wait, what did you say?
“You've gotten so old,
that I'd rather be with a young lass of gold”
Well, I gave up.
We're through, my pumpkin.
我们结束了，亲爱的。【注释：pumpkin : （口语）亲爱的】
Shouldn't all my effort be counting for something?
Well, you won't be able to rule me anymore,
Because I'm leaving you, not for one atom, but four.
That's right. When you were away defusing,
I met some chlorines that I found quite amusing.
So we're going to form PbCl4,
And you won't be hearing from me anymore.
See, over the years I've grown quite wise,
I've learned that love is about compromise.
You still have half of your half-lives to live,
So now you go out there, it's your turn to give.