物理學 1933

正電子

卡爾·D·安德森

一條彎曲的宇宙線徑跡揭示了反物質：一種和電子一樣輕、卻帶正電的粒子。

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In depth · the introduction

一條發光的徑跡，彎錯了方向——這便是最初的跡象：對每一種物質，都存在一種鏡像般的反物質。

核心想法

1932 年，卡爾·安德森正在為宇宙線——從太空傾瀉而下的粒子——拍照，記錄牠們劃過雲室的瞬間。雲室是一箱蒸氣，一個帶電粒子穿過時，會留下一條細細的液滴痕跡。一塊磁鐵把每條痕跡彎成弧線，而牠彎向哪一邊，就告訴了他這個粒子帶什麼電。

有一條痕跡，彎的方向正是一個正電荷該有的樣子——可牠又細又緩，彎得像一個輕飄飄的電子，而不像當時已知的唯一正粒子、那沉重的質子。安德森發現了新東西：一個有著電子那般微小質量、卻帶著相反的正電荷的粒子。他把牠叫作正電子。這是人類見到的第一塊反物質。

它是如何誕生的

四年前，理論家保羅·狄拉克為電子寫下了一條方程，而它——讓所有人都困惑地——竟也描述了一個與電子一模一樣、卻帶正電的粒子。包括狄拉克本人在內，多數物理學家起初都懷疑，這只是一個數學上的贗象，而非真實存在之物。

安德森，加州理工密立根實驗室裡一位年輕的實驗者，當時根本不是在追狄拉克的想法——他只是在研究宇宙線。讓結論一錘定音的竅門，是橫放在雲室裡的一塊鉛板：粒子穿過牠會損失能量、此後彎得更緊，而這就揭示了牠朝哪個方向行進。由此他才能確定，那道彎意味著一個輕粒子帶著正電荷。數月之內，英國的帕特里克·布萊克特與朱塞佩·奧恰利尼證實了這個粒子，並把牠牢牢繫到了狄拉克的預言上。安德森於 1936 年獲諾貝爾獎。

它為何重要

正電子表明，反物質是真實的，而非代數的怪癖——大自然以物質與反物質的鏡像成對出現。它證實了狄拉克把量子論與相對論聯姻所講述的，是關於這個世界的真話；它也開啟了「靠讀取徑跡來發現粒子」的時代。它還拋出了一個我們至今無法回答的謎：如果物質與反物質是等量生成的，宇宙為何幾乎全是物質？

一個可以想像的畫面

想像你把一顆顆球滾過一塊總會把牠們往側旁推一把的磁鐵。重的球幾乎不偏；輕的球急轉彎；而帶相反電荷的球，則完全朝另一邊拐。安德森看到的，是一條急轉彎（所以是輕的）、卻拐向「錯誤」方向（所以是相反電荷）的徑跡。一個輕球朝錯誤方向拐——牠不可能是任何已知的粒子，只能是一個新的：電子的鏡像孿生兄弟。

它的位置

狄拉克 1928 年的方程搭好了舞臺；安德森 1932 年的徑跡，正是它的證實——一對「理論與實驗」，恰如貫穿物理學其餘部分的那種「預言與證明」。從這裡，線索通向 1955 年的反質子，通向「反物質與物質相遇即在一道閃光中湮滅」的認識，也通向今天 CERN 那些把反氫原子「裝進瓶子」的實驗——牠們要問的是：物質與牠的鏡像，是否真的遵循同樣的法則。

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