The Dirac Transmitter

Wells had Cavorite;
James Blish had spindizzies.

Wells had the Time Machine;
Bob Shaw had slow glass;
Blish had the Dirac transmitter.

Wells presented each idea in just one work whereas later science fiction writers imagined future societies with multiple technological advances. Thus, the Cavorite sphere leaves Earth never to return at the end of The First Men In The Moon and the Time Machine disappears into time never to return at the end of The Time Machine but spindizzies, Dirac transmitters and anti-agathics coexist in Cities In Flight.

The Time Machine: travel to the future.
Slow glass: images from the past.
The Dirac transmitter: messages from the future.
World-line cruisers: travel backwards in time in the future! (Also in Blish.)

Blish developed the idea of the Dirac transmitter through six stages:

(i) a story co-written with Robert Lowndes;
(ii) a story submission to John Campbell;
(iii) the submission fully developed as Cities In Flight;
(iv) "Beep";
(v) "Beep" expanded as The Quincunx Of Time;
(vi) Midsummer Century, companion volume to Quincunx.

(i) In "Chaos, Co-ordinated" (1946) by James Blish and Robert Lowndes, aliens have an instantaneous communicator called the Dirac transmitter although, not having read the story, I do not understand why an alien device is named after a Terrestrial physicist.

(ii) Campbell critiqued the submission. Blish, heeding the critique, wrote the Okie series that became Cities In Flight. In the submission, the Dirac transmitter had behaved as in "Beep" (see below) but this would not have fitted with Okie culture so the transmitter reverted to being merely an instantaneous communicator (see above).

(iii) Every Dirac transmission from anywhere in the universe is instantaneously received by every Dirac device. Thus, private or secret Dirac communication is impossible.

(iv) Every Dirac transmission from any part of the four dimensional space-time continuum is received simultaneously as an almost instantaneous beep of sound and light by every Dirac device. Individual messages can be extracted and slowed down so that those who monopolise the Dirac transmitter have unique knowledge of the future. The characters, living in our future, receive several messages from different periods of their future, including one from the world-line cruiser, so, as Brian Aldiss said, this story really is about the future.

(v) In the expansion of "Beep," the characters also receive messages transmitted from societies that are the settings of two otherwise unrelated works by Blish. Thus, the reader can next read those works.

(vi) One of those works, written while "Beep" was being expanded, is Midsummer Century. In that novel, a message received thousands of years earlier in Quincunx is itself transmitted as a "beep" just in case any receiver needs to know its content.

Thus, a beep sent in Midsummer Century and received in the beep in the expansion of "Beep" is the ultimate development of the idea of the Dirac transmitter.