《那些古怪又讓人憂心的問題》第44期:半空的杯子(3)
Water is very nearly incompressible, so the impact isn't spread out over time-it comes as a single sharp shock. The momentary force on the glass is immense, and it breaks.
This “water hammer” effect (which is also responsible for the “clunk” you sometimes hear in old plumbing when you turn off the faucet) can be seen in the well-known party trick of smacking the top of a glass bottle to blow out the bottom.
你在聚會上看到的猛擊玻璃瓶子頂部會把瓶底打碎的現象就是由這種“水錘”效應產生的。如果你關閉連着老式管道的水龍頭，有時也會聽到由此產生的沉悶的咚咚聲。When the bottle is struck, it's pushed suddenly downward. The liquid inside doesn't respond to the suction (air pressure) right away-much like in our scenario-and a gap briefly opens up. It's a small vacuum-a few fractions of an inch thick-but when it closes, the shock breaks the bottom of the bottle.
當瓶子被猛擊時，它會被突然往下推。瓶中的液體沒法即時對吸力（氣壓）產生響應——就像我們這裏一樣——於是瓶底會短暫地出現一個空隙。當這個1釐米厚的真空區域最終閉合後，衝擊波就會擊碎瓶底。
In our situation, the forces would be more than enough to destroy even the heaviest drinking glasses.
而在我們假設的情形下，產生的衝擊力足以擊碎最厚實的杯底。The bottom is carried downward by the water and thunks against the table. The water splashes around it, spraying droplets and glass shards in all directions.
碎掉的瓶底被水流推着砸到桌上，發出梆梆聲。杯子裏的水也會在周圍濺開來，水珠和碎玻璃飛得到處都是。Meanwhile, the detached upper portion of the glass continues to rise.
與此同時，和杯子下半部分分離的上半部分會繼續向上飛。After half a second, the observers, hearing a pop, have begun to flinch. Their heads lift involuntarily to follow the rising movement of the glass.
0.5秒後，兩個旁觀者在聽到一聲巨響後嚇得往後退了一點。他們的頭也不由自主地跟隨着飛到天上的杯子向上擡起。The glass has just enough speed to bang against the ceiling, breaking into fragments . . .
杯子的速度恰好夠一頭撞到天花板上，然後裂成碎片……. . . which, their momentum now spent, return to the table.
……隨着動量的消散，它們最終又回到了桌面上。The lesson: If the optimist says the glass is half full, and the pessimist says the glass is half empty, the physicist ducks.
這個故事告訴我們：如果樂觀主義者說杯子是半滿的，而悲觀主義者說杯子是半空的，那麼物理學家就該跑路了。1 Even a vacuum arguably isn't truly empty, but that's a question for quantum semantics.
1． 其實就算是真空，你也可以說它不是真的是空無一物，不過這已經牽扯到量子語義學了。
