London ? Princess Diana would have been 50 years old today, perhaps the only certainty about what might have been in a life abruptly cut short by a 1997 car crash in Paris, with a new boyfriend, two months past her 36th birthday.

Officially, there are no plans for marking the birth anniversary; Prince William, Diana’s elder son, will be in Canada on Friday with his wife Catherine on their first big international tour as husband and wife.

But the “what if?” questions abound as the world looks back on Diana’s life and legacy.

Andrew Morton, the British journalist who was Diana’s confidant and collaborator on an explosive book about the marriage turmoil that led to a royal divorce, believes she might now be living in the United States.

“A lot more billionaires live in America than in Britain,” Morton said.

“And she probably would have snagged a guy with all the toys; you know, the guy with the private jet, the boat and the house in the Hamptons. Maybe started a second family. She always wanted a baby girl, and that was an ambition that she held very dear.”

A new life in America is exactly the future imagined by British writer Monica Ali in her new novel “Untold Story.” It depicts a princess closely modeled on Diana who fakes her own death, changes her name and rebuilds her life in a small American town — until the paparazzi who dogged her past threaten catch up with her.

The enduring fascination Diana commands in the United States was in evidence on the latest cover of Newsweek, which showed a computer-generated image of Diana at 50, strolling next to William’s bride.

The image, which came under widespread criticism for being “creepy,” was for an article by Newsweek editor-in-chief Tina Brown titled “If She Were Here Now.”

However irresistible that question is to some, Diana’s former secretary Patrick Jephson says the speculation is “entertaining perhaps, but hardly useful.”

“The first Mrs. Wales might by now be solving conflicts, banishing poverty, feeding the world’s hungry or even breeding spaniels in happy rural obscurity. Alas, we will never know,” Jephson said in a commentary for The Daily Telegraph newspaper.

“Instead we have an even greater enigma. Why is it that 14 years after her death she continues to figure so large in popular imagination?”

It’s a power that can be measured in dollars — $800,000 in the case of a black dress in which Diana danced with John Travolta in 1985, sold at auction in Toronto last month, or $34,000 for a letter to her nanny, auctioned in 2008.

Inevitably, there’s a Facebook page, where one post says “neither time nor reason, will change the way we feel.”

A website, high50.com, offers some speculation about what a 50-year-old Diana would be doing.

Contributors speculated that she might be living in New York, getting botoxed, happily married or happily divorced (again), undergoing therapy, working hard for charities.

“She was a very down-to-earth kind of woman and I think she’d have embraced 50 with a certain amount of resolution. I think she’d have found it quite amusing that she’d gotten that far,” said Bruce Oldfield, one of the British designers who worked for Diana. One of his dresses for her sold for $163,000 in the auction.

“I think she’d live in New York,” Oldfield said.