万物简史 第77期:事物的测定(25)

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装配完毕以后，米歇尔的仪器看上去很像是一台18世纪的鹦鹉螺牌举重练习机。它由重物、砝码、摆锤、轴和扭转钢丝组成。

At the heart of the machine were two 350-pound lead balls, which were suspended beside two smaller spheres. The idea was to measure the gravitational deflection of the smaller spheres by the larger ones, which would allow the first measurement of the elusive⁴ force known as the gravitational constant, and from which the weight (strictly speaking, the mass) of the Earth could be deduced.

仪器的核心是两个635千克重的铅球，悬在两个较小球体的两侧。装配这台设备的目的是要测量两个大球给小球造成的引力偏差。这将使首次测量一种难以捉摸的力--所谓的引力常数--成为可能，并由此推测地球的重量（严格来说是质量）。

To a physicist⁵, mass and weight are two quite different things. Your mass stays the same wherever you go, but your weight varies depending on how far you are from the center of some other massive object like a planet. Travel to the Moon and you will be much lighter⁶ but no less massive. On Earth, for all practical purposes, mass and weight are the same and so the terms can be treated as synonymous. at least outside the classroom.

对于物理学家而言，质量和重量是两个截然不同的概念。无论在哪儿，物体的质量总是不变的，但是物体重量大小的变化取决于其与另一个如行星那样的巨大物体的中心距离。在月球上我们会变得很轻，但是我们的质量却不变。在地球上，为了实用起见，质量和重量数值上是相等的，因此可以用相近的办法处理，至少在教室外可以这么做。

Because gravity holds planets in orbit and makes falling objects land with a bang, we tend to think of it as a powerful force, but it is not really. It is only powerful in a kind of collective sense, when one massive object, like the Sun, holds on to another massive object, like the Earth. At an elemental level gravity is extraordinarily⁷ unrobust. Each time you pick up a book from a table or a dime⁸ from the floor you effortlessly overcome the combined gravitational exertion⁹ of an entire planet. What Cavendish was trying to do was measure gravity at this extremely featherweight level.

引力使行星保持在轨道上，使物体砰然坠落，因此很容易被认为是一种强大的力，其实不然。它只是在整体意义上强大：一个巨大的物体，比如太阳，牵住另一个巨大的物体，比如地球。在基础的层面上，引力极小。每次你从桌子上拿起一本书，或从地板上拾起一枚硬币，你毫不费劲就克服了整个行星施加的引力。卡文迪许想要做的，就是在极轻的层面上测量引力。

 

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