物理學 1897

陰極射線

J. J. 湯姆森

一束發光的射線在場中彎曲——原來它是一種比任何原子都輕得多的粒子。

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空蕩蕩的玻璃管裡，一束淡淡的輝光在磁鐵的牽引下向旁邊彎去——湯姆森意識到，他正看著一塊比原子還小的物質。

核心想法

十九世紀末，物理學家能讓一束奇怪的綠色輝光，從抽空玻璃管的負電極那一端流過。他們稱之為陰極射線，並為「它究竟是什麼」爭論了幾十年：是充滿空間、那個看不見的「以太」中的一陣波瀾，還是一雹微小的帶電粒子？

湯姆森用一次測量了結了爭論。他讓射線穿過電場與磁場，看它彎了多遠，再反推出每個粒子的電荷與質量之間的比值。答案令人震驚：這些粒子約比最輕的原子——氫——還輕一千倍。原子長久以來被視為物質最小、不可再分的碎片，可它裡面竟還藏著更小的東西。今天，我們把那東西叫作電子。

它是如何誕生的

陰極射線管是一八九〇年代一個熱門的謎。以德國人為首的一派，受那些讓射線看起來像波的實驗所引，堅信它是以太中的一種擾動。英國人則偏向粒子說。一條尖銳的證據卻不利於英國人：海因里希·赫茲曾試圖用電場去偏轉射線，卻什麼都沒看到——帶電的粒子，難道不該拐彎嗎？

執掌劍橋卡文迪許實驗室的湯姆森，找到了那個破綻。赫茲的管子裡氣體還太多，把電荷帶走，抵消了電場。湯姆森發現，只要把管子抽到更好的真空，射線便乾脆地朝正極板彎去——恰如一束負電粒子所必須表現的那樣。接著他測量它在電場與磁場中各彎了多少，那極小的質量便從數字裡直接掘了出來。

它為何重要

兩千多年裡，「原子」一詞意味著「不可切割」——物質最後、最小的那一塊。湯姆森卻找到了更小的一塊，而且無論用哪種氣體、哪種金屬，都是同一種。這意味著：每一種元素的每一個原子，都共享一種共同的成分——電子。這是堅實的原子上裂的第一道縫，它通向了原子核、通向了量子物理，也通向了所有現代電子技術。不過湯姆森很謹慎：他測的是一個比值，而非粒子的質量本身——那個確切的數字，還要再過兩年、由別人之手才能鎖定。

一個可以想像的畫面

想像你把一個球滾過一台強力風扇、同時又滾下一道斜坡。它偏了多遠，能告訴你關於這個球的一些事——沉重的保齡球幾乎紋絲不動，輕盈的乒乓球則亂飛。湯姆森看不見他的粒子，卻能看到束流在電場與磁場中「偏」了多遠。那巨大的偏轉告訴他：這些粒子輕如羽毛——輕得根本不可能是整個的原子。

它的位置

湯姆森的電子，坐在經典物理與現代物理的轉軸上。在它身後，是把電與磁綁在一起的法拉第與麥克斯韋；在它身前，是一連串的級聯反應——他自己的學生盧瑟福找到原子核、玻爾把原子量子化，以及整場量子革命。本館中關於電磁、以及關於量子世界的線索，都在這裡交匯——在一隻玻璃管裡，原子的第一個部件終於被稱了重。

The original document

Original source text

問題——兩種相互競爭的看法

J. J. Thomson · Cathode Rays · Phil. Mag. 5th ser. 44 (1897): 293–316 · Cavendish Laboratory

The experiments discussed in this paper were undertaken in the hope of gaining some information as to the nature of the Cathode Rays. The most diverse opinions are held as to these rays; according to the almost unanimous opinion of German physicists they are due to some process in the aether to which—inasmuch as in a uniform magnetic field their course is circular and not rectilinear—no phenomenon hitherto observed is analogous: another view of these rays is that, so far from being wholly aetherial, they are in fact wholly material, and that they mark the paths of particles of matter charged with negative electricity.

赫茲錯過的電偏轉

It was only when the vacuum was a good one that the deflexion took place, but that the absence of deflexion is due to the conductivity of the medium is shown by what takes place when the vacuum has just arrived at the stage at which the deflexion begins.

[ … ]

If we measure the deflexion experienced by the rays when traversing a given length under a uniform electric intensity, and the deflexion of the rays when they traverse a given distance under a uniform magnetic field, we can find the values of m/e and v.

帶負電的物質粒子

As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.

一種新狀態的物質

Thus on this view we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter—that is, matter derived from different sources such as hydrogen, oxygen, &c.—is of one and the same kind; this matter being the substance from which all the chemical elements are built up.

Cavendish Laboratory · Cambridge · 1897