Relativity on a Human Scale

Neat!!! Relativity on a Human Scale : Uncertain Principles

The real technical advance is in a recent paper in Physical Review Letters (available for free via the Time and Frequency Publications Database, because government research isn't subject to copyright): they have made improvements to their atomic clock based on the spacing of two energy levels in a single aluminum ion (described in a Science paper, also available free at the Time and Frequency Publications Database), so it is now accurate to something like one part in 1017. They now have two "clocks" (you can argue about whether they really count as official clocks in the current configuration) based on the same ion that they can compare to unprecedented precision.

This allows a couple of really cool tricks that haven't been published anywhere (yet), but which were the subject of the DAMOP talk. These clocks are now good enough to observe relativistic effects on a human scale: they see time dilation from motion of the ion at speeds of 10m/s or less, and the gravitational redshift caused by raising one of the two clocks a foot above the other.

Einstein's theory of relativity predicts a number of things about the behavior of clocks that strike most people as surprising. In particular, a stationary observer looking at a moving clock will see that clock "ticking" at a slower rate than an identical clock at rest (and a moving observer will see the same effect for the stationary clock). This has been demonstrated with big old jet airliners, and is included in the time corrections to make GPS work, but the shift is minuscule for motion at ordinary human speeds-- a clock moving at 10 m/s would tick slower than a stationary clock by about 10-16s every second, which is way too small to see (about 10s difference over the age of the universe).
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These aren't earth-shattering results, and won't transform anybody's understanding of how the universe works. They do, however, provide a beautiful demonstration that relativity is real even in situations where the speeds and distances involved are on a human scale-- you don't need to be moving at half the speed of light in the vicinity of a black hole, provided your clocks are good enough.