雪花究竟是不是對(duì)稱的

Snowflake Symmetry

雪花究竟是不是對(duì)稱的

Q. Why are snowflakes usually perfectly symmetrical? How does one arm “know” how the other arms are growing?

問：為什么雪花通常完全對(duì)稱?其中一瓣怎么“知道”其他幾瓣在怎么成長(zhǎng)呢?

A. “The growth of a snowflake is sensitive to its immediate environment, especially the temperature and humidity,” said Kenneth G. Libbrecht, chairman of the physics department at the California Institute of Technology, who maintains a website explaining snow crystals. “It might take a half-hour for the crystal to reach its final size, and during that time it could travel a mile or more, experiencing ever-changing conditions.

答：“雪花在形成過程中對(duì)周圍環(huán)境非常敏感，特別是溫度和濕度，”加州理工學(xué)院物理系主任肯尼思·G·利布雷希特(Kenneth G. Libbrecht)說(shuō)。他開設(shè)了一個(gè)網(wǎng)站，專門解釋雪晶。“雪晶大概需要半個(gè)小時(shí)才能最終形成，在那段時(shí)間里，它能飛行一英里多，經(jīng)歷不斷變化的環(huán)境。”

“If you look at an individual arm, its shape reflects the exact history of its travels,” he said. “But the six arms all travel together, so they all experience the same history. Thus they appear to grow in synchrony, even though no arm ‘knows’ how the others are growing.”

“如果你只看單獨(dú)的一瓣，它的形狀反映著自己的旅行歷史，”他說(shuō)，“但是這六瓣一起旅行，所以它們經(jīng)歷的歷史是一樣的。因此，它們似乎是同步成長(zhǎng)，雖然哪一瓣也不知道另外幾瓣在怎樣成長(zhǎng)。”

Dr. Libbrecht compared the process to going outside on a cold day, wearing a heavy coat and seeing everyone else wearing one as well, without foreknowledge or consultation.

利布雷希特把這個(gè)過程比作冷天穿厚外套出門，你會(huì)看到其他每個(gè)人也都穿著厚外套，但是沒人能預(yù)見未來(lái)，也沒人相互商量。

“The flip side of this story is that no two snowflakes follow exactly the same path in the turbulent atmosphere, and this is why no two snowflakes are exactly alike,” he said.

“這個(gè)故事的另一面是，在混亂的環(huán)境中，沒有兩片雪花的成長(zhǎng)環(huán)境是完全相同的，所以不存在一模一樣的兩片雪花，”他說(shuō)。

“Finally, snowflakes are in fact not usually perfectly symmetrical,” he said, “although snowflake photographers like to select the ones that are.” Large, beautifully symmetrical crystals are actually quite rare, he said, but “as with people, the most attractive snowflakes tend to get the most attention.”

“最后要說(shuō)的一點(diǎn)是，實(shí)際上雪花并不總是完全對(duì)稱，”他說(shuō)，“不過，攝影者喜歡選擇對(duì)稱的雪花。”他說(shuō)，美麗、對(duì)稱的大雪晶實(shí)際上非常罕見，但是“最好看的雪花總是最引人注目。”