Getting him to leave his hometown of Karlsruhe takes some persuading; there and in his tiny Berlin apartment, he writes one uncompromising work after another for the likes of violinist Anne-Sophie Mutter and conductor Riccardo Chailly.
And then, like Moby-Dick, there he is - large, bushy-haired, invitingly affable - beaming and shaking hands a few seats down from you in the Berlin concert hall. Such was the case when the Philadelphia Orchestra played his work while on tour a few years ago. Was that really Rihm helping a complete stranger find a lost cellphone?
The Great White Rihm has a warmth of personality even the smartest musicians don't know is there.
"You know, I thought I sensed some humor in his music," said British violinist Tasmin Little while working on Rihm's Dritte Musik for Violin and Orchestra. And she was right.
But getting Rihm to talk about himself and Astralis is a labyrinthian process. No phone calls. No e-mail. Only questions submitted (in English) by fax and returned by fax, on handwritten pages - in German.
"I envy someone living in a different century like that," admits Donald Nally, who directs the Crossing in Astralis, and who has named his entire season after the piece.
What seems like stubborn eccentricity is, in fact, time management.
"I never learned to drive because - please believe me - I simply didn't have the time to take the test," Rihm wrote. "And e-mail doesn't work for me because I never learned to type. It would take hours to write, 'Many thanks for your invitation. Unfortunately, I can't come.'
"I use a fax and mobile phone. That's at least something! Isn't it? The modern world can breathe easy. I'm taking part."
He has plenty to show for restricting communication - a catalogue of 400–plus works, and they tend to be dense. His industry appears to be escalating. "I hardly ever give lectures any more for this simple reason: To write a lecture, I have to spend at least as much time on it as it would take me to compose something. So, how does a composer live? He works on being able to work."
The 2010 Salzburg Festival got a particularly rich earful of Rihm with his opera Dionysos, written over 15 years and just out on Euroarts DVD. Characteristically brainy, the opera is based on Friedrich Nietzsche texts, the main character being "N," who goes from one life challenge to the next and ends up lamenting the dismemberment of his alter ego - while clutching severed body parts.
No wonder Rihm's music is considered difficult.
"My alleged reputation as a challenging composer almost amuses me," he wrote. "In Europe . . . my compositions are far too beautiful and audience-friendly. Those are all just wrinkles in the face of the zeitgeist. Apparently, the many-faceted reactions to my work are the reasons why my music comes up again and again."
The United States has been slow to pick up on Rihm's dreamy, abstract, introspective works. Nally included Astralis in his Christmastime concert in part because it is based on the famous Novalis poem about the balance of joy and sorrow in the universe.
Rihm's musical setting also is said to contemplate the destruction caused by World War II, with a sense of unfolding discovery arising from mental images of atoms constantly vibrating and transforming. The prolonged softness that expresses that - the piece is sung as softly and slowly as possible - can challenge concentration for singers and listeners.
"I myself have sung in a choir for many years. I am aware . . . that this is the most difficult thing to do," Rihm wrote. "But it's the only way to achieve the floating or uncertain nature of things. No one can decide to write an intentionally difficult piece. In the same way, no one intentionally writes a good or bad piece.
"Things become what they are."
Casting his net wide with compositional techniques means his music scrupulously avoids conventional polarities - happy, sad, etc. - and never tells you what to think of it. "For listeners, I have only one piece of advice: Listen. Nothing more."
Nonetheless, he is at the center of ongoing discussions about the virtues of tonality and atonality, and is seen as an enemy of the former and champion of the latter.
"But no!" he protests. "For heaven's sake! Tonality is something immensely vital and beautiful. Only the term 'strictly tonal' bothers me," he wrote. "It's like saying just because I like to drink good wine that I . . . would not drink water, beer, juice . . . ."
Out of necessity, he ignores the world's perceptions of him.
"For a while, that upset me," he admitted, "until I noticed that I function as a sort of mirror for secret or obvious opinions, prejudices, wishes, longings, needs, confirmations, etc.
"Mass reactions such as applause or booing . . . are momentary and leave no trace."
Which frees him to go on to the next piece.
8 p.m. at St. Paul's Episcopal Church, 22 E. Chestnut Hill Ave.