If you know Rosanne Cash only as Johnny Cash’s daughter, then you haven’t had your heart shattered, your life changed, your spirits lifted—then dashed into the dust—by one of her dangerously beautiful songs. You haven’t sighed tragically over her doomy, painfully romantic “Sleeping in Paris” or had your emotional life caught up on “The Wheel” or found yourself alone in a darkened room with an attractive stranger listening to her breathtaking, heart-racing “Runaway Train.” You have missed one of the most gifted singer-songwriters of our time.

Her songs are intense; they stay with you like a lifelong fever. They create worlds lit by what Cash described to me as “the ebullience that comes from darkness.” She’s not a country and western singer in the tradition of her famous father. She’s American music’s theoretical physicist of love.

I’ll get to the connection between love and theoretical physics (seriously) a little later, when I come to our conversation about multiverse theory. But first, let’s get this identity thing straightened out. Cash is not a country gal, never was. She only lived in Nashville for nine years, she pointed out when we met for lunch near her apartment in the heart of New York City’s Greenwich Village. She grew up in Southern California, was a Beatlemaniac rock ’n’ roll chick in her youth, lived in Europe and has been a New Yorker for 20 years.

Her memoir tells of her struggle to escape from her father’s shadow, cutting her first album in Munich, reluctantly accepting his help when she returned to Nashville, where she married a brilliant singer-songwriter (Rodney Crowell, author of what I think is one of the greatest country songs ever, “Til I Gain Control Again”).

By the time they divorced, in 1992, she had moved to New York with her daughters and it was there she found herself personally and musically—self-discovery perhaps best expressed in her dreamy song “Seventh Avenue.”

The more she came into her own, the more comfortable she seemed living with her father’s legacy. Back when Rosanne was a SoCal Beatles and Byrds teenybopper, and a little embarrassed by country music’s retro image, her father painstakingly wrote out in pencil a list of 100 great country songs she ought to know. She put it away somewhere, but didn’t forget it.

The album she made in 2009 called The List contains 12 of the songs. There have been reports the list itself was thought to be long lost.

“I have it!” she told me.

“It’s now in a file cabinet on my third floor.” She says she plans to make another album from it sometime soon.

The culmination of her reconciliation with her father’s shadow, the most beautiful expression of their enduring love, is the haunting and unbearably sad duet she recorded with him shortly before his death, a song she wrote called “September When It Comes” (on her Rules of Travel album). Warning: See a cardiologist before you listen. Once you hear it, you will never recover as long as you live.

Or til September, her metaphor for death. There is something both enigmatic and transcendent in the verse she wrote for her father in that duet that demonstrates a master of the fusion of music and emotion:

I plan to crawl outside these walls, close my eyes and see
and fall into the heart and arms of those who wait for me
I cannot move a mountain now, I can no longer run
I cannot be who I was then, in a way, I never was.

The café she chose for lunch, in the West Village, is the very epicenter of New York’s literary haute Bohemia. It’s set among rows of elegantly genteel brownstones whose original gaslight lampposts still flicker at night. The realm of Edith Wharton, Edna St. Vincent Millay, Mary McCarthy, Djuna Barnes. Which is appropriate since Rosanne is not just a songwriter but an accomplished writer of prose, author of a much praised short story collection and a memoir, Composed, a beautifully restrained, gracefully written document.

I wanted to talk to her about songwriting. In her memoir, she mentioned a songwriting mentor named John Stewart. “He wrote this song I recorded, ‘Runaway Train,’” she tells me now. “I didn’t know him when I got the song. We liked it, but there was no bridge. So we asked him if he would write the bridge. He was well known as a songwriter, he had written ‘Daydream Believer’”—everybody’s guilty pleasure Monkees song—“and he wrote ‘Gold,’ that duet with Stevie Nicks. And he was known as a deep songwriter. So asking him to add a bridge seemed a bit forward. But he did. So then it became a big hit and I still hadn’t met him and he came to Nashville and...”

I interrupt her to ask her more about that bridge. The song is racing along at a runaway train pace in the first two verses, as the lovers express alarm at how out-of-control their feelings are becoming.

Things are accelerating with exhilarating momentum and then the bridge jams the brakes on, melodically and emotionally.

“That bridge,” I ask Cash, “it goes, ‘I’ve been here before, and now it’s with you’?”

“Yeah?” she says warily.

“I wondered about that.”

“Really?” she says. “It seems grafted on?”

“It seems like, they’re deliriously, dangerously falling for each other then suddenly, ‘Oh, I’ve done this before.’”

She laughs. “It was grafted on,” she concedes, but she thinks of it more as providing “a nice melodic release to build back up into the next verse.”

Actually it makes it a more complex song. I like Katy Perry, but Katy Perry wouldn’t have that bridge. It’s a pause for reflection: What am I, crazy? And then the next verse returns with accelerated, exacerbated vigor to the madness, only this time it’s with conscious deliberation and self-awareness—yes, I’m crazy and I don’t care—that makes giving in to the moment even more knowingly risky. In other words, it’s good to remember in the heat of the moment— when you think nothing like this has ever happened to you—that it has. And then, it’s good to forget.

Then she remembers something her mentor told her about her songwriting. John Stewart “always said, ‘Where’s the madness?’ You know, if I would try to write a perfect song. ‘Where’s the madness, Rose?’”

I ask what songs she’s writing now.

“Well, there’s one called ‘Particle and Wave.’”

 “Is the male a particle and the woman a wave?”

“Something like that...but part of it is that I have a deep love of theoretical physics.”

Whoa. That comes out of left field.

“It started 30 years ago when I became interested in astronomy. I read about light shifts and that led me to theoretical physics. Things like time and how long it takes light from stars to get here. Black holes. Where you would come out if you went into a black hole.” She tells me a beautiful story about a physics-inflected song she’s working on, about how “light only slows to shine on the other’s face.”

“I had a conversation with Brian Greene [the celebrated physicist and author]. I asked him if God was the unified field. Greene’s response, she says: ‘It depends on your definition of God.’

“Theoretical physics is like a religion to me,” Cash continues, “and I have a lot of friends who are scientists. And I can only grasp this little part over here. I have a friend Lisa Randall, she’s one of the top theoretical physicists at Harvard. She just came out with a book, Knocking on Heaven’s Door. She’s very pragmatic about all of this stuff even though she’s a theoretical physicist. But a lot of her colleagues go way off into parallel universes.”

“Multiverses?” I say (realizing only later that songs are multi-verses, in more ways than one).

“Multiverse theory” is the idea much discussed recently in theoretical physics that there may be a potentially infinite number of possible universes encompassing all possible eventualities, in which infinitesimal and grand differences play themselves out.

“It’s freeing to me,” Rosanne says. “The choices I’m making in another universe might be better, but they might be worse. I might be doing pretty well.”

She says her friend Randall is a multiverse skeptic. “She thinks it’s narcissism.”

“Why narcissism?”

“Because she doesn’t think that every choice you do or don’t make opens up a parallel universe. It’s not all centered around you.”

Then Rosanne tells this amazing story that might be very sad or very uplifting depending on which emotional universe you’re currently inhabiting.

“Do you know the band the Eels? OK, it’s not a very well-known band. Mark Everett, it’s basically him.

“His dad Hugh Everett was a theoretical physicist at Princeton, who, I don’t know if he invented the multiverse theory, but if he didn’t invent it, he refined it.” I wonder where this is going.

Mark found his father, Hugh, dead of a sudden heart attack, she continued. “He was a very distant father. So there were two children. [After] the father died, the daughter, Mark Everett’s sister, committed suicide so that she could be with her father in a parallel universe.”

“Oh my God,” was all I could muster. The sadness and dangerousness of theoretical physics. Like love songs. It’s all about attraction and separation isn’t it?

“It was horrible. So Mark Everett is the last of his family left. He went to Princeton and talked to his father’s colleagues and tried to understand the multiverse theory so he could find out who his dad was. And the BBC did a documentary about him. So I went to see them speak, these physicists and Mark. There was a Q&A with the audience and the last question, this woman asked the physicist, ‘So is heaven...when you die, do you just go to a parallel universe? Is that what heaven is?’”

“Is that what heaven is?” Song title!

“None of the physicists wanted to touch this question. They looked at each other and then finally one of them said, ‘It’s possible.’”

“How could it not be possible?” I ask, carried away by the novelty of the idea.

“Right,” she says. “But if it’s true, the you that’s in the parallel universe—is that the real you, and the one here is the specter?”

I feel myself being shifted, flung, back and forth between potential universes. Heaven. And, of course, I remind myself, hell. My gloomy side prompts me to say, “And there could also be a million suffering yous.”

“Exactly, exactly,” says Rosanne, who does, after all, write about suffering.

So here’s my theory about why she is attracted both to theoretical physics and to songs of love and suffering: Because quantum physics introduced the idea of ineradicable unpredictability into the orderly world of Newtonian physics. We know, for instance, that half the uranium atoms in a given amount will split in a certain time, uranium’s “half-life,” but there’s no way of predicting which atoms will stay together and which will split, emitting dangerous radioactivity. Einstein believed there were “hidden variables” we haven’t yet discovered; most quantum physicists disagree and think it’s an insoluble mystery. Quantum uncertainty. Like love. Who will stay together, who will split.

And unpredictability, fate, whim, emotional changeability, irrational acts and unpredictable passions, eyes happening to meet or not meet across a crowded room. Rosanne’s songs are about the maddening quantum physics of love.

I have one final question on the subject. “Do you think that love songs, in some way, created love or would love have been the same without great love songs?”

“What a question, Ron!” she exclaims in mock alarm. “The real question is, Did art create love? There is a woman who just wrote about this—I was talking to her at a dinner party—she found that songs about love existed in every culture.”

The woman turns out be Helen Fisher, a well-known anthropological writer and researcher. She represents one pole of an interesting, ongoing debate. There are those who believe romantic love is “natural” in some way, with all the torrents of jealousy, aggression and madness that go along with it. On the other side of the debate are, for instance, the authors of Sex at Dawn, who think that our closest primate ancestors were more like the bonobo chimps, who have lots of sex but little of the partnering associated in humans with love—and crimes of love too. So we should behave more like loveless bonobos, I guess. It makes for less drama. But don’t we love the drama?

We talk about the songs we heard that first made us experience love, as opposed to just sex. For her, it was the Beatles’ “I Want to Hold Your Hand.”

“It struck you, that song?” I ask.

“Oh dumb,” she replies. “Blind and dumb.”

I wonder if every culture, every human-occupied parallel universe has the kind of sad love songs she’s such a wizard at writing. I ask her about a phrase she used in her memoir for our affection for profoundly lacerating sad songs: “morbid joy.” She had cited one of the all-time country weepers. George Jones’ “He Stopped Loving Her Today”—because after a lifetime of unrequited love, he died that day.

“I can barely pronounce the name of that song without bursting into tears,” I say.

She laughs. “I know, I know, the first time you heard it and it came to that line and you realized what happened—my God!”

“But why are we attracted to ‘morbid joy’?” I ask.

“Because if it doesn’t get expressed in art and culture, then you get depressed. It has to be expressed; it’s an essential part of our nature.”

“George Jones is better than a pill?”

She laughs. “Yeah. This is why we don’t go crazy. Because we can put it out there.”

“September When It Comes” written by Rosanne Cash & John Leventhal. Rosanne Cash published by Chelcait Music (BMI), administered by Measurable Music LLC, A Notable Music Co. John Leventhal published by Lev-A-Tunes (ASCAP)