No show in American television history, it is safe to say, has ever put so much effort into maintaining historically appropriate ways of speaking — and no show has attracted so much scrutiny for its efforts. The three seasons that have been broadcast, set between 1960 and 1963, triggered endless arguments in online discussion forums, with entire threads devoted to potential anachronisms.

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When I spoke recently with Matthew Weiner, the creator, executive producer and head writer of “Mad Men,” he readily admitted that goofs sneak through on his show. He said he still regrets allowing the character Joan to say “The medium is the message” in the first season, four years before Marshall McLuhan introduced the dictum in print. But he defends Joan’s year-end valedictory, “1960, I am so over you,” by pointing to the Cole Porter song “So in Love” from “Kiss Me, Kate.” Scholars of semantics might disagree, seeing a nuance between Porter’s use of the adverb so, which quantifies the extent to which the character is in love, and the later Generation X-style spin on the word as an intensifier meaning “extremely” or “completely” without any comparison of relative degree.

Other lines that have struck a discordant note with quibblers include Don’s “The window for this apology is closing” and Roger’s “I know you have to be on the same page as him.” Window in its metaphorical sense (as in a window of opportunity) and on the same page evidently date to the late ’70s. In a piece in The New Republic, the linguist John McWhorter complained that Peggy’s line “I’m in a very good place right now” is actually in a bad place, historically speaking. Even interjections can come under fire. When the character Sal reacts to the abrupt end of a screening of “Bye Bye Birdie” by exclaiming “awwa!” his falling-and-rising intonation has a 21st-century tinge, according to the linguist Neal Whitman.

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To a large extent, Weiner and his staff members brought this festival of nitpickery on themselves through their own perfectionism. The show is famous for its loving attention to retro details, most notably in the set design (Weiner has been known to halt production over matters as subtle as the size of fruit in a bowl) and wardrobe (the actresses bravely suffer through the exquisite discomfort of vintage undergarments). Language naturally comes under the same microscope. To try to ensure accuracy, Weiner and his fellow writers sometimes take cues from the films and books of the era, but, as Weiner told me, those sources don’t necessarily provide the best window into genuine speech patterns. “You’re much better off if you can find a letter from your grandmother,” he said. He did acknowledge that Joan owes much of her sultry style to the writings of Helen Gurley Brown, the author of ’60s advice books like “Sex and the Single Girl” and “Sex and the Office.”

Any theory of time travel has to confront the devastating “grandfather paradox,” in which a traveler jumps back in time and kills his grandfather, which prevents his own existence, which then prevents the murder in the first place, and so on.

One model, put forth in the early 1990s by Oxford physicist David Deutsch, can allow inconsistencies between the past a traveler remembers and the past he experiences. So a person could remember killing his grandfather without ever having done it. “It has some weird features that don’t square with what we thought time travel might work out as,” Lloyd says.

In contrast, Lloyd prefers a model of time travel that explicitly forbids these inconsistencies. This version, posted at arXiv.org, is called a post-selected model. By going back and outlawing any events that would later prove paradoxical in the future, this theory gets rid of the uncomfortable idea that a time traveler could prevent his own existence. “In our version of time travel, paradoxical situations are censored,” Lloyd says.

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For instance, a bullet-maker would be inordinately more likely to produce a defective bullet if that very bullet was going to be used later to kill a time traveler’s grandfather, or the gun would misfire, or “some little quantum fluctuation has to whisk the bullet away at the last moment,” Lloyd says. In this version of time travel, the grandfather, he says, is “a tough guy to kill.”