The phrase refers to any informal method of transmitting information or rumors swiftly and unofficially by word of mouth or other means, such as runners, drum codes, or smoke signals. "The bush" refers to extremely rural, even primitive, conditions.
It originated in Australia as slang for any chain of communication by which criminals, known as bushrangers, let one another know about the movements of the official police.
The narrator uses the phrase to refer to the rumor and gossip that surrounds Tomas after his essay on Oedipus and Communist collaborators is published, for which he eventually loses his job. The narrator mentions "bush telegraph" again a little later (section 8 of Part Five, page 197), after Tomas has become a window washer, and the word quickly gets around that he's really a doctor.
Although the basic shape of the story is correct, recently published biographies differ in the particulars. Beethoven by Barry Cooper (NY: Oxford University Press, 2000) and Beethoven: The Music and the Life by Lewis Lockwood (NY: Norton, 2003) both state that Ignaz Dembscher was an amateur musician who organized chamber music sessions at his home in Vienna. He wanted to play the String Quartet in B-Flat Major, opus 30, at one of these gatherings, but he had not attended the concert premiere of the work in March 1825. When Beethoven’s secretary Karl Holz relayed the request, the composer said Dembscher would have to pay the fifty-florin subscription price he would have been charged at the concert. Holz reported that Dembscher’s joking response had been “Muss es sein?”
So perhaps Beethoven never had a verbal exchange with Dembscher face-to-face, but the latter’s question caught the composer’s fancy, and sparked him to compose the joking canon (“It must be, yes, take out your wallet!”), and a year later, the more portentous quartet.
“In Kant’s language,” the narrator of Unbearable Lightness remarks, “even ‘Good Morning,’ suitably pronounced, can take the shape of a metaphysical thesis.” The narrator then shows that the passage of time converted a minor, almost joking scrap of conversation that inspired a Beethoven composition (“Es muss sein!”) into something weighty and fateful.
Passing references to 20-century historical figures. Adolf Hitler (1889-1945) was chancellor of Nazi Germany during the latter years of the Depression and the Second World War. Albert Einstein (1879-1955) was a German theoretical physicist and philosopher responsible for the General and Special Theories of Relativity, and the father of modern physics.
Leonid Brezhnev (1906-1982) led the Soviet Union as First Secretary of the Communist Party during the Cold War -- including the period when Prague was invaded by the Soviet Union and Unbearable Lightness takes place. (Brezhnev is shown here during his visit to President Nixon in the U.S., June 19, 1973.)
Aleksandr Solzhenitsyn (1918-2008) was the Russian author of such novels as One Day in the Life of Ivan Denisovich and The First Circle, and the three-volume history The Gulag Archipelago.