《那些古怪又让人忧心的问题》第43期:半空的杯子(2)

The vacuum on the left is surrounded by barriers, so air molecules can't easily get in. The water, being a liquid, doesn't expand to fill the vacuum in the same way air does. However, in the vacuum of the glasses, it does start to boil, slowly shedding water vapor into the empty space.

左边那个杯子里的真空四周都被包围着，因而空气分子没法轻易进入。水是液体，不会像气体那样膨胀开来填满真空。然而，暴露在杯子围成的真空中的水确实会沸腾，慢慢地水蒸气就会进入到真空的那一部分里。

While the water on the surface in both glasses starts to boil away, in the glass on the right, the air rushing in stops it before it really gets going. The glass on the left continues to fill with a very faint mist of water vapor.

虽然左右两个杯子里的水都会沸腾，但涌入右边杯子的空气会在沸腾愈演愈烈之前就把它遏制住，而左边的杯子则会继续产生稀薄的水蒸气。

After a few hundred microseconds, the air rushing into the glass on the right fills the vacuum completely and rams into the surface of the water, sending a pressure wave through the liquid. The sides of the glass bulge slightly, but they contain the pressure and do not break. A shockwave reverberates through the water and back into the air, joining the turbulence already there.

几百微秒后，涌入右边杯子的空气彻底填满了原先真空的部分，并撞上水面，产生一道压力波。杯子的侧边轻微突出来了一点，但还是能够经受住压力不至于破碎。反弹回来的冲击波穿过杯子中的水又回到了空气中，和空气中已经存在的波动汇合起来。

The shockwave from the vacuum collapse takes about a millisecond to spread out through the other two glasses. The glass and water both flex slightly as the wave passes through them. In a few more milliseconds, it reaches the humans' ears as a loud bang.

真空消失产生的冲击波会在约1毫秒的时间内传播到另两个杯子所在的地方，当冲击波穿过杯子和水时，它们都会轻微地发生形变。又过了几毫秒，人耳将听到一声巨响。

Around this time, the glass on the left starts to visibly lift into the air.

在这个时刻前后，肉眼已经可以明显地看出左边的杯子飞了起来。

The air pressure is trying to squeeze the glass and water together. This is the force we think of as suction. The vacuum on the right didn't last long enough for the suction to lift the glass, but since air can't get into the vacuum on the left, the glass and the water begin to slide toward each other.

空气压力试图把杯子和水挤在一起，这就是我们平时所说的“吸力”。右边杯子里的真空持续时间不足以产生能够抬升杯子的吸力，而由于空气无法进入左边杯子里的真空区域，因而杯子和水开始相向滑动。

The boiling water has filled the vacuum with a very small amount of water vapor. As the space gets smaller, the buildup of water vapor slowly increases the pressure on the water's surface. Eventually, this will slow the boiling, just like higher air pressure would.

沸腾的水只向真空区域输入了很少量的水蒸气。随着水与杯子之间的空间减小，积聚起来的水蒸气慢慢开始在水表面产生压强。随着压强的升高，沸腾的速度会减慢下来。

However, the glass and water are now moving too fast for the vapor buildup to matter. Less than ten milliseconds after the clock started, they're flying toward each other at several meters per second. Without a cushion of air between them-only a few wisps of vapor-the water smacks into the bottom of the glass like a hammer.

此时水与杯子之间的相对运动速度太高，这点水蒸气产生的压强已经不足以产生什么效果了。在开始不到10毫秒的时候，水会与杯子以每秒数米的速度撞到一起，隔在它们之间的没有气垫缓冲，只剩下那少得可怜的一点水蒸气，水就像一把大锤子一样砸在杯子的底部。