> So basically you thwack all the cells with something that pushes their lifecycles towards a similar schedule, and then later-on they tend to all still be on the same cycle?

Yeah, that much is pretty well understood. http://journals.plos.org/plosgenetics/article?id=10.1371/jou...

> Isn't that like saying two pendulums in the same house have a "collective memory" of an earthquake?

The important point isn't that there's memory (which seems pretty easy to explain), it's that it's collective. Using your analogy, it would be like finding that two pendulums in the same house have memory of an earthquake, but if there's only one pendulum then there's no memory.

This result might be regulated by quorum sensing (https://en.wikipedia.org/wiki/Quorum_sensing). Like the pendulums are talking to each other.

I think the point your parent comment is making is that "collective memory" implies more than just a common cause causing synchronization. If, as you say, "the pendulums are talking to each other" that would fit my definition of "collective memory", but if the pendulums stay synched just because they started at the same time, that doesn't seem like collective memory.

Totally aside, but funnily enough two pendulums can synchronize through weak vibrations, e.g. pendulum clocks hanging on the same wall. Observed by Huygens in 17th century:

http://www.nature.com/news/2002/020221/full/news020218-16.ht...

A cool thing to do is to hang two pendula next to each other and couple them via, say, a pencil (loop each pendulum's string once around the pencil, so the pencil is horizontal and is able to transfer energy from one to the other). Set one in motion. Eventually wonder where the rest of the day went :)

> So basically you thwack all the cells with something that pushes their lifecycles towards a similar schedule, and then later-on they tend to all still be on the same cycle?

Funny enough servers will do that as well. They will self synchronize and exhibit a thundering herd effect. They run the same software, power cuts our, or they get DoSed, maybe they restart, retry at the same schedule and so on.

Happened just last year! About exactly one year after heart bleed, all kind of websites, big and small, had their ssl certs expired! Apparently a lot of people forget to update the cert schedule