I believe he is referring here to several developments, all of which took place in mid- to late 1950, when Gravity theory really took off.
At that time John Archibald Wheeler proposed that gravity to be investigated in two, concurrent directions of unquantized general relativity and quantized general relativity, which sparked a very interesting debate, where
a wormhole in space-time threaded by electric flux
is said to be possible, for example.
Further pursuits by Robert Henry Dicke and others in gravitational theories and experiments gave rise to scalar-tensor gravity theory, tied in turn into String Theory (please note extreme oversimplification; I'm just highlighting the flow)
However, we're not there yet. Enter 1986 and Abhay Ashtekar with his reformulation of Einstein's General Relativity theory and thus, after a short while, loop quantum gravity, an alternative to String Theory "explanation" of gravity emerges.
It is quite important because it basically postulates that celestial bodies are all interconnected on a quantum level, with gravity being another dimension. So the effect of that would be a possibility of a quantum tunnel along the "gravitational ley-lines". Of course, again a caveat: oversimplification of the theory and it's concepts, as well as pointing out the fact that use of "ley-lines" is mine, inspired by Ringo.
So, 1986 is not quite 1950s, but then again, had it not been for Wheeler's split, LQG may not grace us with it's existence at all.
A side note.
Ringo is quite good with all kinds of science stuff, mainly because he has friends he can ask about his ideas, and he can ask questions very nicely. But since it's second book in the series, I'd assume that he was not in the habit of asking questions yet, just using bits of knowledge picked up here and there...
My thinking is based on the peculiar construction: "quirk of quantum theory". There are many "quirk of quantum theory" that have been recently (or not) discovered, and every time it's used it's not a scientific, but literary construction, as the description happens to describe for example behaviour of subatomic particles in vacuum, Hawking radiation not related to black holes, magnetic field measurements etc. All of them are linked to quantum theory, but aside that have little in common.
But that's just my opinion and may not be correct, since I stumbled onto his Looking Glass series, where... let's just say I appreciate his quantum physics jokes there quite a bit. Talking about mad science turned not only popular, but funny.