物理学 1933

正电子

卡尔·D·安德森

一条弯曲的宇宙线径迹揭示了反物质：一种和电子一样轻、却带正电的粒子。

Choose your version

In depth · the introduction

一条发光的径迹，弯错了方向——这便是最初的迹象：对每一种物质，都存在一种镜像般的反物质。

核心想法

1932 年，卡尔·安德森正在为宇宙线——从太空倾泻而下的粒子——拍照，记录它们划过云室的瞬间。云室是一箱蒸气，一个带电粒子穿过时，会留下一条细细的液滴痕迹。一块磁铁把每条痕迹弯成弧线，而它弯向哪一边，就告诉了他这个粒子带什么电。

有一条痕迹，弯的方向正是一个正电荷该有的样子——可它又细又缓，弯得像一个轻飘飘的电子，而不像当时已知的唯一正粒子、那沉重的质子。安德森发现了新东西：一个有着电子那般微小质量、却带着相反的正电荷的粒子。他把它叫作正电子。这是人类见到的第一块反物质。

它是如何诞生的

四年前，理论家保罗·狄拉克为电子写下了一条方程，而它——让所有人都困惑地——竟也描述了一个与电子一模一样、却带正电的粒子。包括狄拉克本人在内，多数物理学家起初都怀疑，这只是一个数学上的赝象，而非真实存在之物。

安德森，加州理工密立根实验室里一位年轻的实验者，当时根本不是在追狄拉克的想法——他只是在研究宇宙线。让结论一锤定音的窍门，是横放在云室里的一块铅板：粒子穿过它会损失能量、此后弯得更紧，而这就揭示了它朝哪个方向行进。由此他才能确定，那道弯意味着一个轻粒子带着正电荷。数月之内，英国的帕特里克·布莱克特与朱塞佩·奥恰利尼证实了这个粒子，并把它牢牢系到了狄拉克的预言上。安德森于 1936 年获诺贝尔奖。

它为何重要

正电子表明，反物质是真实的，而非代数的怪癖——大自然以物质与反物质的镜像成对出现。它证实了狄拉克把量子论与相对论联姻所讲述的，是关于这个世界的真话；它也开启了「靠读取径迹来发现粒子」的时代。它还抛出了一个我们至今无法回答的谜：如果物质与反物质是等量生成的，宇宙为何几乎全是物质？

一个可以想象的画面

想象你把一颗颗球滚过一块总会把它们往侧旁推一把的磁铁。重的球几乎不偏；轻的球急转弯；而带相反电荷的球，则完全朝另一边拐。安德森看到的，是一条急转弯（所以是轻的）、却拐向「错误」方向（所以是相反电荷）的径迹。一个轻球朝错误方向拐——它不可能是任何已知的粒子，只能是一个新的：电子的镜像孪生兄弟。

它的位置

狄拉克 1928 年的方程搭好了舞台；安德森 1932 年的径迹，正是它的证实——一对「理论与实验」，恰如贯穿物理学其余部分的那种「预言与证明」。从这里，线索通向 1955 年的反质子，通向「反物质与物质相遇即在一道闪光中湮灭」的认识，也通向今天 CERN 那些把反氢原子「装进瓶子」的实验——它们要问的是：物质与它的镜像，是否真的遵循同样的法则。

The original document

Original source text

C. D. Anderson · Physical Review 43 (1933): 491–494 · received February 28, 1933

Abstract

Out of a group of 1300 photographs of cosmic-ray tracks in a vertical Wilson chamber 15 tracks were of positive particles which could not have a mass as great as that of the proton. From an examination of the energy-loss and ionization produced it is concluded that the charge is less than twice, and is probably exactly equal to, that of the proton. If these particles carry unit positive charge the curvatures and ionizations produced require the mass to be less than twenty times the electron mass. These particles will be called positrons. Because they occur in groups associated with other tracks it is concluded that they must be secondary particles ejected from atomic nuclei.

The decisive photograph

The paper's evidence centres on a single track photographed on August 2, 1932. A particle crosses a 6 mm lead plate set across the chamber, which sits in a magnetic field of about 15,000 gauss. Above the plate the track is more sharply curved than below it: the particle had less energy there (about 23 MeV, against 63 MeV below), having lost energy passing through the lead. Because the curvature is tighter on the upper side, the particle must have been travelling upward — and the direction in which it bends, in a known field, then fixes the sign of its charge as positive.

Ruling out the proton

A positive particle of that curvature might have been a proton, but Anderson argues from the density of ionization along the track and from the range a proton of such momentum would have had that the mass is far too small. If the charge is one electronic unit, the measurements bound the mass below twenty times that of the electron — comparable to the electron itself.

These particles will be called positrons.

[ … ]

Interpretation

Anderson reports the positrons appearing in groups alongside other tracks and reads them as secondary particles ejected from nuclei. The full paper gives the curvature and ionization measurements, the reproductions of the cloud-chamber photographs, and the argument against the proton; it is four pages and is available in full at the source below. The deeper meaning — that the positron is the anti-electron predicted by Dirac, produced together with an electron when a photon converts near a nucleus — was made explicit shortly afterward by Blackett and Occhialini.

Norman Bridge Laboratory of Physics, California Institute of Technology · 1933