Or as Stephen King posits in his new science fiction thriller "11/22/63," there's theory number three: history is flexible. Your backward travels can warp the course of future events (as long as you don't create a paradox, like challenging yourself to a duel).

King wonders what would happen if you time-trekked back to 1963 and killed the assassin before he got to President Kennedy. Would changing that watershed moment have prevented the country’s military escalation in Vietnam, saved the lives of RFK and MLK, yadda, yadda yadda -- in short, prevented many of the latter half of the 20th century’s ills? Those questions frame the basic premise of King’s book.

Assassinations and rifts in the space-time continuum are not foreign concepts to America’s King of Pulp. In “The Dark Tower’’ series, magical doors link far-flung worlds. In “The Dead Zone,’’ the clairvoyant protagonist shoots the president to avert nuclear Armageddon. Here, to kick-start the plot, King builds a wormhole in the pantry of a diner in Lisbon Falls, Maine. Like an express train, the time tunnel connects two destinations in history: the present and Sept. 9, 1958. Al, the diner’s tetchy proprietor, has been there and back a few times, mainly to buy hamburger at 54 cents a pound so he might sell 2011 burgers for $1.19. “Turns out I’m no longer tied to the economy the way other people are,’’ he jokes. Then Al finds a higher cause: Surveil Lee Harvey Oswald, determine whether he is the lone gunman, and take him out.

King ups the stakes with his own twists. Every visit back in time, no mat ter how long, takes only two minutes in the present. While in the past, travelers age normally. To accomplish his mission Al would need to go back to 1958 and stay five years. But his lung cancer would prevent him from lasting until 1963. The solution? Recruit Jake Epping, a 35-year-old high school English teacher, divorced, no children, and our first-person narrator. Jake takes up the quest, chucking his cellphone - “Keeping it would be like walking around with an unexploded bomb’’ - to live full time 53 years ago, pseudonymously as George Amberson. Jake/George soon discovers history is resistant to change, in direct proportion to the size of the event he wants to bend. “Obdurate’’ is the refrain. But the past can also be redeemed. If Jake kills Oswald and returns to 2011 to find the world ain’t better, a journey back restores history. “Every trip is the first trip,’’ Al says. “Because every trip down the rabbit-hole’s a reset.’’

The historical novel is already a well-established literary time machine, and King, who was 16 when JFK was shot, has done his homework, setting his characters on plausible collision courses with actual people and lovingly populating his “Land of Ago’’ with period details: drive-ins, pop songs, pep clubs, and finned convertibles. King balances his nostalgia on the cusp of tumult, just before this more naive world would be homogenized by television and strip malls and its smaller mind would wake up to racial injustice and military quagmire. As the author said in a recent interview, “11/22/63 was our 9/11.’’

[Read the rest of the review here]

Ethan Gilsdorf is the author of “Fantasy Freaks and Gaming Geeks: An Epic Quest for Reality Among Role Players, Online Gamers, and Other Dwellers of Imaginary Realms.’’ He can be reached atwww.ethangilsdorf.com.

[this first appeared in the Boston Globe]