Pettersson's music is in this way existential, not in the sense of implying that existence is meaningless, but in the sense that equates life with music in an absolute way (I am paraphrasing another remark of Thomas Sanderling). This music is a profound meditation on existence and as such, it is unlike other music.
Pettersson's life is exactly the same span as Shostakovich's, but five years later: 1911 to 1980 instead of 1906 to 1975. They were both plagued by ill health. They both suffered, but in different ways. Shostakovich lived on the edge of a precipice, never knowing from one day to the next if Stalin would give orders for him to disappear as happened to so many of his friends. But Pettersson had the equally great burden of growing up in poverty with an alcoholic, violent father.
Their music is radically different, remarkably so. Pettersson got a later start and studied with members of the avant-garde in Paris (Milhaud, Messiaen, Leibowitz), while Shostakovich had a more traditional education at the St. Petersburg Conservatory. There is more of traditional form in Shostakovich. At this point, it is hard to say much about what is going on in Pettersson's music. It is perhaps too primeval to be called "tonal" but it certainly is not atonal. It is obsessive in its concentration on certain pitches and motifs. It is captivating in its emotional power. Perhaps this music even has a moral force; Pettersson certainly thought so.
Let's end by putting two symphonies side by side. First, the Symphony No. 5 of Shostakovich, composed in 1937 when he was thirty-one years old:
That is certainly a very serious piece of music, even though it was written in an attempt to stave off criticism. Now let's listen to the Symphony No. 8 of Pettersson. There are two parts, the first about 20 minutes and the second about twenty six or seven minutes.
The symphony was written in 1969 when Pettersson was fifty-eight years old.