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This year marks the 30th anniversary of Robert Zemeckis' classic time travel movie Back to the Future. The popular film follows a long tradition of fictional explorations of the possibility of human travel through time. Coincidentally, around the same time that Marty McFly first hopped into Doc Brown’s DeLorean time machine in 1985 and launched himself thirty years backwards into the world of his adolescent parents, a cognitive neuroscientist named Endel Tulving was in the process of discovering a non-fictional means of journeying through time—“chronesthesia,” or mental time travel. Chronesthesia is a fundamental and perhaps exclusive human trait that allows us to mentally revisit our past, and use our experiences there to mentally project ourselves into various hypothetical futures. An adaptive trait, arguably responsible for the rise of human civilization, chronesthesia is an innate capacity of virtually all human brains, so as fantastical as the idea of time travel seems, we human beings have been doing it for as long as we’ve been human beings.
As much fun as it is to contemplate hopping in a DeLorean and ending up thirty years in the past or future, the Doc Brown model of time travel is a far less accurate analogue to the way chronesthesia works than is the quantum mechanical model employed by Michael Crichton in his 1999 novel Timeline. In Back to the Future, Marty is transported bodily to a specific moment in the historical linear past. When Crichton’s time travelers embark on their journey to the Middle Ages, however, they do not travel to the year 1357 as it actually played out in the timeline of history of which their own present is the culmination, but an alternate 1357 in a parallel universe that is almost—but not completely—identical to the one from which they departed. And the travelers themselves are not bodily transported to this alternate 1357, but converted into information that is reassembled into alternate versions of them that are almost—but not completely--identical to the selves that departed from that timeline
As arcane as the quantum model of time travel might appear to anyone without a decent background in theoretical physics, it is strikingly similar to the way mental time travel works. Our mental travels to the past and future involve episodic memory, or memory of autobiographical events (as opposed to semantic memory, which is memory of discrete facts). Contrary to the way it feels in our everyday experience, episodic memory is constructive, rather than reproductive. In other words, our memory of a given event is not a literal playback of it, but a brand new mental experience constructed out of memory traces stored in various locations in the brain. Our memories of past events are, in essence, alternate realities similar—but not identical—to the reality as we actually lived it.
The down side of constructive memory is its factual unreliability (equivalent to Timeline’s “transcription errors”), which can be inconvenient if you’re giving eyewitness testimony in a bank robbery trial, for example, or arguing with a work colleague about which one of you came up with a ground-breaking idea first. That inconvenience is a small price to pay, however, for the up side of constructive memory, which is its nearly infinite capacity for creativity. Because we construct our memories anew each time we recall them, we can tailor them to fit our present needs and purposes, whatever those needs and purposes might happen to be. Depending upon the elements we highlight in our construction, and the emotional values associated with those elements, an episode from our autobiographical past can provide us with practical instruction, pleasurable nostalgia, or even corrective regret. In the other direction, we can manipulate memories of the past to construct hypothetical futures for ourselves, trying them on for size by mentally “living” them through a sort of projective virtual reality.
Whether we wish to move forward in time, or backwards, understanding the constructive nature of the time machine between our ears can make us all savvier and more intentional time travelers.
