Delay Line Memory is one of the most delightfully cyberpunk parts of computing history. You can go and read the relevant Wikipedia article (and I encourage you to do so) but in short, the idea is that you’re reading and writing a fixed-length string of bits into an analog recirculating medium – that is, representing your data as a modulated wave within a material, not as discrete ones and zeros. Naturally there’s a limit to the speed at which you can read or write (a function of the properties of the material you’re using), and you need to wait for the bit you want to travel ‘around’ again before you can read or write it. It’s sequential rather than random access.
The earliest forms of delay line memory come from radar technology, where delay lines were used to compare fresh versus historical signals and filter out non-moving objects. You’d achieve this by feeding your incoming radar signal into the delay line, then inverting it and comparing it to the next cycle of fresh data. Ta-da, everything that’s identical to the previous string of bits is substracted, leaving only ‘interesting’ things on your radar screen. We’re already in cool destructive interference territory and we haven’t even got to computer memory yet! This tech moved from war efforts to computers (mostly) via the efforts of J. Presper Eckert, who helped build ENIAC(!) and had beef with Von Neumann(!!).
By necessity, delay line memory needs to match the clock cycle of the computers it’s connected to (broadly true of modern RAM, as well). Not that many materials have the right properties to sustain a string of bits at the right cadence. A solution that migrated from radar tech to computing was to use a big ol’ barrel of mercury, which also had to be maintained at a specific temperature (40ºC) for optimum performance. In case you weren’t aware, mercury is extraordinarily toxic and bio-available. Cool tech, Eckert!
Imagine making this pitch today: “Hey, I’ve got this great idea for computer memory – it’s a quite hot tub of deadly and readily absorbable poison that holds a couple of kilobytes. Can I sign you up for our angel round?”
Later on, after folks decided that unpleasantly warm and noxious liquid metal wasn’t an ideal data storage mechanism, the tech evolved. They tried magnetorestriction (storing data as a torsional wave in a wire coil), or using bucket-brigade-type devices (like the CCDs in digital cameras!) where a stream of bits as represented by electrical charges would move stepwise all the way down to the end of the chain of semiconductors, and could then be recirculated. Neat!
Obviously this tech has long since been superseded by memory that isn’t limited by sequential access, the RAM we know and love today. What I find delihtfully fascinating about this concept is that it could apply to any medium where you can set up a modulating wave. Any recirculating loop can be a delay line storage device. This is science fiction territory!
As we all know [gestures broadly] the Apollo 11 mission left mirrors on the Moon’s surface so Earth-based astronomers could bounce lasers off it. You can still do this yourself if you know where to point your laser. I can’t track down the provenance of this idea, but if you modulated the frequency of just such a laser to encode your bit stream, you could use the distance between the Earth and the Moon as a form of delay line memory. Sure, you’ve got a 765,000 kilometre round-trip latency hit and atrocious bit loss due to atmospheric scattering, but still – with some error-checking encoding and a powerful enough laser, no sweat!
You might use a very well sound-insulated room with a microphone and a speaker in it, and recirculate a string of data as noise into the room. If you were feeling saucy (and had access to a bunch of high-power theoretical physics equipment) you could potentially drop your bits into the vibrations of a time crystal.
I wanted to write about this not because it has any practical application today (and also I don’t have any lasers kicking around) but because the lore of computing is far more Star Trek -adjacent than you can imagine. Digging into how we arrived at our current state of anything gets me fired up about the possibility space inherent in the things we build. Tech, or design, or art, or writing, or whatever – is a bigger canvas for weird and amazing ideas than you’d think.