G117-B15A is believed to be the most stable optical clock in the universe.

via SyFy

There are atomic clocks that also lose time on much longer timescales than our white dwarf clock, but they vibrate at such high rates that when you divide their actual period by the change in that period (a standard way to measure stability) they actually are less stable. For example, one kind of atomic clock measures vibrations that have a period of 2.5 femtoseconds (2.5 x 10−15 seconds), which is incredibly fast, especially compared to the 215-second period of the white dwarf. Because of that short period, the timescale of their decay is much higher — that clock would lose one whole cycle of 2.5 femtoseconds every 20 minutes or so, whereas the white dwarf takes over a billion years to double its cycle time to 430 seconds.

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