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話しとしては易しいが本質は難しい｡
理科系の人は知っておくべき基礎知識。 念のため、日本語字幕でも確認した方がよい。

易　05分　145wpm　2015/01/08 新出　　

字幕：上の動画は開始後 で英語On/Off｡ 日本語選択､文字ｻｲｽﾞはｵﾌﾟｼｮﾝから｡
.　　　　　　　 （YouTubeは 日本語On/Off。 英語選択｡）

★下記英文はブラウザ クロムのマウスオーバー辞書が使えます｡

What do an ancient Greek philosopher and a 19th century Quaker（ｸｴｰｶ―教徒） have in common with Nobel Prize-winning scientists? Although they are separated over 2,400 years of history, each of them contributed to answering the eternal question: what is stuff（物質）made of?

It was around 440 BCE that Democritus（ﾃﾞﾓｸﾘﾄｽ） first proposed that everything in the world was made up of tiny particles surrounded by empty space. And he even speculated that they vary in size and shape depending on the substance they compose. He called these particles "atomos," Greek for indivisible（分割できない）.
　　（BCE： before the Common Era 西暦紀元前、「キリスト前」を避けた現代の記法）

His ideas were opposed by the more popular philosophers of his day. Aristotle（ｱﾘｽﾄﾃﾚｽ）, for instance, disagreed completely, stating instead that matter was made of four elements: earth, wind, water and fire, and most later scientists followed suit.
　　（Aristotle： 発音注意 'æristὰtl　日本語ではアリストテレス 384–322 BCE )

Atoms would remain all but forgotten until 1808, when a Quaker teacher named John Dalton（ﾄﾞﾙﾄﾝ）sought to challenge Aristotelian theory. Whereas Democritus's atomism had been purely theoretical, Dalton showed that common substances always broke down into the same elements in the same proportions.

He concluded that the various compounds were combinations of atoms of different elements, each of a particular size and mass that could neither be created nor destroyed. Though he received many honors for his work, as a Quaker, Dalton lived modestly until the end of his days.

Atomic theory was now accepted by the scientific community, but the next major advancement would not come until nearly a century later with the physicist
J.J. Thompson's（JJ・ﾄﾝﾌﾟｿﾝ）1897 discovery of the electron. In what we might call the chocolate chip cookie（ﾁｮｺﾁｯﾌﾟｸｯｷｰ）model of the atom, he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons.

Thompson won a Nobel Prize in 1906 for his electron discovery, but his model of the atom didn't stick around long. This was because he happened to have some pretty smart students, including a certain Ernest Rutherford（ﾗｻﾞﾌｫｰﾄﾞ）, who would become known as the father of the nuclear age.

While studying the effects of X-rays on gases, Rutherford decided to investigate atoms more closely by shooting small, positively charged alpha particles（α粒子）at a sheet of gold foil. Under Thompson's model, the atom's thinly dispersed positive charge would not be enough to deflect the particles in any one place. The effect would have been like a bunch of tennis balls punching through a thin paper screen.

But while most of the particles did pass through, some bounced right back, suggesting that the foil was more like a thick net with a very large mesh. Rutherford concluded that atoms consisted largely of empty space with just a few electrons, while most of the mass was concentrated in the center, which he termed the nucleus（原子核）. The alpha particles passed through the gaps but bounced back from the dense, positively charged nucleus.

But the atomic theory wasn't complete just yet. In 1913, another of Thompson's students by the name of Niels Bohr（ﾎﾞｰｱ）expanded on Rutherford's nuclear model. Drawing on earlier work by Max Planck（ﾌﾟﾗﾝｸ） and Albert Einstein（ｱｲﾝｼｭﾀｲﾝ） he stipulated that electrons orbit the nucleus at fixed energies and distances, able to jump from one level to another, but not to exist in the space between.

Bohr's planetary model（惑星モデル）took center stage, but soon, it too encountered some complications. Experiments had shown that rather than simply being discrete particles, electrons simultaneously behaved like waves, not being confined to a particular point in space.

And in formulating his famous uncertainty principle（不確定性原理）,
Werner Heisenberg（ﾊｲｾﾞﾝﾍﾞﾙｸﾞ） showed it was impossible to determine both the exact position and speed of electrons as they moved around an atom. The idea that electrons cannot be pinpointed but exist within a range of possible locations（確率的存在）gave rise to the current quantum model of the atom, a fascinating theory with a whole new set of complexities whose implications（影響）have yet to be fully grasped.
　　（implication： ふつうは「暗示、意味」｡　複数形では 「将来の影響」としても使う｡）
　　（不確定性原理は観察者効果と混同されるが､量子の二重性＝粒子・波 による特性）

Even though our understanding of atoms keeps changing, the basic fact of atoms remains, so let's celebrate the triumph of atomic theory with some fireworks. As electrons circling an atom shift between energy levels, they absorb or release energy in the form of specific wavelengths of light, resulting in all the marvelous colors we see. And we can imagine Democritus watching from somewhere, satisfied that over two millennia（千年） later, he turned out to have been right all along.