[jacquesm]

There is some analogy here with bubble memory (though that moved around physical material rather than just set up a wave).

I wonder what the minimum spacing of such waves would be, how this would compare with current estimates on the upper bounds of density for silicon and if material fatigue factors into this somehow. I can't imagine such a system to be very stable mechanically.

[jcr]

I'm really not sure, and I'd rather not speculate blindly. I've read about the basics of Mercury Line Memory in the past, and have some books that describe it, but I don't know the hard details.

BTW, Mercury Line Memory was not the first form of memory used in programmable computers. The "oldest" or "first" is debatable, but the earliest were switches/relays and vacuum tubes (AFAIK). I'd bet jgrahamc would know...

[Serow225]

Torsion wire memory was really nifty!