66,000 yottayears is a 6.6 followed by 28 zeroes. It looks like this.66,000,000,000,000,000,000,000,000,000
This is the expected lifespan of a single electron, according to new measurements described in this week's Physical Review Letters from physicists at Italy's Borexino experiment. It's roughly five-quintillion times the current age of the universe, as Physics World notes.
This is about what should be expected given the Standard Model of Physics, which is our current, albeit incomplete, best description of the universe's fundamental units of matter and force. Electrons aren't expected to decay because there are no other, less massive particles for them to decay into that would allow for a transfer of the electron's charge. We might imagine it decaying into an electron neutrino and a photon, both fundamental particles of lower/no mass and both particles without charge, but this would break all kinds of rules, particularly the one that forbids the creation or destruction of electric charge.
Not that that's strictly a bad thing. We know the Standard Model is incomplete, after all, as it fails to account for dark matter, dark energy, gravity, and the relationship between antimatter and matter, just to start. This incompleteness is why we hunt for unlikely occurrences like electron decays in the first place and why experiments like Borexino exist.
"Despite the present undisputed validity of [the law of charge conservation], experimental tests of charge conservation remain a way to search for physics beyond the Standard Model, and deserve to be investigated with the highest possible sensitivity," the paper notes. "An experimental search for the hypothetical charge non-conserving decay of the electron, which is the lightest known charged particle, into a neutrino and a photon is reported in this paper. No presently viable theory predicts such a decay, and a large charge violation is excluded by the absence of macroscopic effects in matter."
Borexino is primarily a solar neutrino detector. It's underground and consists of a giant sphere some 18 meters in diameter featuring a shell of petroleum-based liquid that has the neat property of lighting up every time an electron is knocked loose from one its constituent atoms by an incoming neutrino. 2,000 photomultipliers are tasked with amplifying and registering these tiny flashes.
The trick is to account for the detector's busy background chaos of photons from other sources and to just grab photon "events" of around 256 kilo-electron-volts, an energy corresponding to half the electron rest mass. This would indicate an electron decay and, indeed, New Physics.
But, after chewing through 408 days of Borexino data, the researchers behind the current study found no decay events. With this period of data collection and using an instrument of such high sensitivity, it's now possible to say that an electron will not decay before 66,000 yottayears. It might decay after that, but that will require more precise measurements for longer and longer periods of measurement. For now, however, we can say with some assurance that electrons just don't decay.
An open-access version of the Borexino study can be viewed/downloaded at arXiv.