The Measure of all Mass


The mass of the Earth is about M = 5.97219 × 1024 Kg. The mass of the Sun is M = (1.98855 ± 0.00025) × 1030 kg. The unit that we use to measure these masses is the kilogram, determined from a block of platinum and iridium, that celebrated its 125th anniversary last year. That famous block (Figure 1) resides at the International Bureau of Weights and Measures in Sèvres, near Paris. Created in 1889, it is kept in a vault which requires three keys to open (only two of which are in France). There are six official copies of the kilogram that are checked against the original at fixed intervals. There are also about a hundred daughter-blocks around the world, and those are being calibrated in France against the original every few decades.

Figure 1. The international prototype of the kilogram.

In recent years, those calibrations have revealed a disturbing discrepancy — the parent block appeared to increase in mass (compared to its daughters) by about one part in sixty billionths! The cause for this change is not entirely clear (it may have to do with substances used to clean the block), but the variation has accelerated the plans to replace the actual block of mass by a standard that depends on physical constants.

There is a huge advantage to a definition that is based on constants of nature, over one that employs a particular block of mass.

Imagine, conceptually, that you want to accurately convey to an extrasolar civilization the mass of a certain object. If your definition is based on a platinum-iridium block, you would have to physically transport the block to them. On the other hand, if it is based on physical constants and the laws of nature — which, as far as we know, are universal —> Read More