Snowflake Symmetry
Q&AJanuary 24, 2015
Victoria Roberts
Q. Why are snowflakes usually perfectly symmetrical? How does one arm “know” how the other arms are growing?
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 websiteexplaining 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.
“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.”
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.
“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.
“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.”
question@nytimes.com
雪花究竟是不是對稱的
科學2015年01月24日
Victoria Roberts
問：為什麼雪花通常完全對稱？其中一瓣怎麼「知道」其他幾瓣在怎麼成長呢？
答：「雪花在形成過程中對周圍環境非常敏感，特別是溫度和濕度，」加州理工學院物理系主任肯尼思·G·利布雷希特(Kenneth G. Libbrecht)說。他開設了一個網站，專門解釋雪晶。「雪晶大概需要半個小時才能最終形成，在那段時間裡，它能飛行一英里多，經歷不斷變化的環境。」
「如果你只看單獨的一瓣，它的形狀反映着自己的旅行歷史，」他說，「但是這六瓣一起旅行，所以它們經歷的歷史是一樣的。因此，它們似乎是同步成長，雖然哪一瓣也不知道另外幾瓣在怎樣成長。」
利布雷希特把這個過程比作冷天穿厚外套出門，你會看到其他每個人也都穿着厚外套，但是沒人能預見未來，也沒人相互商量。
「這個故事的另一面是，在混亂的環境中，沒有兩片雪花的成長環境是完全相同的，所以不存在一模一樣的兩片雪花，」他說。
「最後要說的一點是，實際上雪花並不總是完全對稱，」他說，「不過，攝影者喜歡選擇對稱的雪花。」他說，美麗、對稱的大雪晶實際上非常罕見，但是「最好看的雪花總是最引人注目。」
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