12/2/2023 0 Comments Caesium 133 atom 1 second![]() So I’ve got a caesium atom, and I want to calibrate my stopwatch. The second is defined as “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”. You can always be sure that a clock calibrated to such a standard will be as good or better than all other clocks. OK, so it can’t be measured precisely, but having a definition that you can approach and which will always be the same duration for the rest of the life of the universe has some merit. So if you want a really accurate clock, the best you could do is measure the number of seconds in the year right now and try and adjust for changes in the Earth’s orbit.īut a cesium atom at absolute zero will never, ever change it’s physical properties. But not just any year - a particular year - because the Earth is, after all, slowly hurtling towards the sun.īut it’s hard to measure the duration of a year in the past. So then the second was redefined as being 1/31,556,925.9747 of a year. ![]() What’s a day? One rotation of the earth.īut some rotations of the earth are longer than others. OK, how long is that? There are 24 of them in a day. It used to be sufficient to say that seconds are 1/60 of a minute. Previous definitions of the second were less precise. It’s useful because the standard is based on an objectively quantifiable feature of the universe which will never change. You can’t actually measure it, but you can get close.
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