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教学参考

光子是怎么消失的？趣谈光子的三种“死法”

作者：佚名  人气： 718 次  时间：2019年08月23日  星级：

 光会消失吗？这当然是毫无疑问的。

  一、转化成粒子

  光是一种能量的表现方式，光和粒子是可以相互转化的 。想当初，爆炸后0.01秒：约1000亿度，光子、电子、中微子为主，质子中子仅占10亿分之一，热平衡态，体系急剧膨胀，温度和密度不断下降。大爆炸后0.1秒后：约300亿度，中子质子比从1.0下降到0.61。大爆炸后1秒后：约100亿度，中微子向外逃逸，正负电子湮没反应出现，核力尚不足束缚中子和质子。

  大爆炸后10秒后：约30亿度，核时期，氢、氦类稳定原子核（化学元素）形成。当宇宙冷却到109开尔文以下（约100秒后），粒子转变不可能发生了。

  我们可以想象一下，在这个阶段里面，一锅浓汤一样的光子逐渐转化成粒子，形成光子和粒子的一锅新浓汤。

  二、被“稀释”掉

  宇宙大爆炸后10^11秒（10^4年），温度约为105开尔文，物质期。在宇宙早期历史中，光主宰着各能量形式。随着宇宙膨胀，电磁辐射的波长被拉长，相应光子能量也跟着减小。辐射能量密度与尺度（R）和体积（4πR3/3）的乘积成反比例减小，即按1/R^4减小，而物质的能量密度只是简单地与体积成1/R^3反比例减小。

  直到发展到今天，147亿年之后，科学家们发现了宇宙微波背景辐射（又称3K背景辐射）是一种充满整个宇宙的电磁辐射。特徵和绝对温标2.725K的黑体辐射相同。频率属与微波范围。这是热力学温度，转换成咱们口中常说的温度就是-270.15℃，就是零下270.15℃，也就是绝对零度高一些。

  这些光就是宇宙大爆炸之初那锅浓汤中的光，随着宇宙的膨胀，慢慢地被稀释的结果，早晚有一天，随着宇宙的膨胀，连这3K也保存不下来，宇宙进入热寂。

  三、被“吃"掉

  光子可以和自由电子发生相互作用，电子吸收光子的能量后能量跃迁，这样光子就没了。光子还可以跟物质发生能量交换，变成分子的动能。相当于光子被吃掉了。

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