In Robert Zemeckis’ Back to the Future, the filmmakers used a clever visual device to illustrate the so-called “grandfather paradox.” Back to the Future, as its name suggests, is a film about time travel. What happens if you go back in time and alter events in such a way that you accidentally kill your own grandfather — or, in this case, keep your parents from getting together before they have children? In effect, you have committed suicide by ending your family line before you are born.  In the film, the teenaged protagonist, Marty McFly, played by Michael P. Fox, time-travels back 30 years to the period when his parents met in high school, and he discovers to his horror that his future mother, then a teenager, is developing a crush on him. If his mom never gets it on with his nerdy dad to be, Marty will cease to exist. To drive home the point, the filmmakers show a snapshot of Marty and his two siblings standing in front of a wishing well as they start to fade away. Presumably Marty himself will soon fade into nonexistence as well if he can’t find a way to get his parents together, so he can gracefully step out of their pic-ture until it’s time for him to be born back into it the old-fashioned way.

The grandfather paradox is hard to explain in a few words, but you can demonstrate it with a fading photograph and a couple of lines of dialog. Back to the Future is a feature film, after all, not a science documentary on Nova. The mantra is “show, don’t tell.” In the movie, Marty McFly saves his future father from being hit by a car and becomes the object of his future mother’s amorous intentions. The snapshot of Marty and his two older siblings that he had been carrying around in this wallet starts to fade, beginning with his brother, who is the first-born. A younger version of Doc Brown, the mad scientist who later invents the time machine that will transport Marty back to 1955, realizes that Marty has already interfered with his parents’ courtship, which is why his brother is disappearing from the picture. “Your sister will follow,” he tells Marty, “and unless you can repair the damage, you’ll be next.”

Time travel has been a staple of science fiction since H.G. Wells wrote The Time Machine in 1905. However, the protagonist of Wells’ novel travelled into the future, not the past, thereby sidestepping the complications that are bound to ensue if you go monkeying around with your own family tree. Speculations about exactly what might ensue began appearing in the letters columns of pulp science fiction magazines in the late 1920s. The first story to explore the topic was Nat Schnachner’s “Ancestral Voices,” published in Astounding Tales in 1933. In Schnachner’s tale, an inventor named Emmet Penny-packer lands his time machine in the Roman city of Aquileia just as Attila the Hun’s hordes break through the city walls. In the ensuing melee, Pennypacker kills a Hun who turns out to be his own great-grandfather many times over. History is instantly reshuffled, and Pennypacker is also history, along with some 50,000 fellow descendants who disappear without a trace. Now imagine if Zemeckis had tried to make a movie out of that story in order to explore the theme of the grandfather paradox. First, there would be the expense of Attila the Hun’s hordes laying siege to Aquileia. Then, how do you show Pennybacker and 50,000 fellow descendants disappearing without trace?

In Back to the Future, Zemeckis manages to convey essentially the same idea with a single fading photograph of three siblings posing in front of a wishing well. A photograph is a perfect metaphor in this circumstance because it is inherently about time. In order to take a picture in the first place, you have to time the exposure. The image itself stops time and preserves the moment forever. This may not be so apparent if you are just posting a selfie on the Internet. But with each passing year it becomes more apparent that the moment frozen in the photograph has become a window into another time. As time goes on, the snapshot itself may begin to fade in the sunlight, quite apart from any monkeying around with the family tree.

Spoiler alert: In the end, Marty succeeds in getting his parents’ romance back on track, and the snapshot with his siblings is restored to its original condition. But everything doesn’t end exactly where it began. With help from the young Doc Brown, Marty gets the time machine powered up again, and he makes his way back to his point of departure in 1985. End of story? No, the world Marty left is not the world he returns to.

He discovers there are unintended consequences to his untimely interventions in 1955. For one thing, his nerdy father has gained confidence from his unknowing encounter with his future son, and every indication is that he has had a much more successful life. But this raises another thorny question related to the grandfather paradox. Marty may not have been erased from history, but what about the father as he remembered him growing up? And what about everybody else whose lives had been changed in large and small ways as a result of Mary’s untimely interventions? Do their original selves continue on unchanged in some parallel universe — not the least the Marty who had grown up before his time-traveling adventure transformed everything? Are there now two Martys, too sets of parents, two sets of siblings — indeed, two 1985s?

These and other potential complications have prompted many physicists to dismiss time travel as a possibility, at least the prospect of revisiting the past. It is true that time travel is theoretically achievable, according to Einstein’s general theory of relativity. Einstein’s friend and colleague, Kurt Gödel, worked out the math, which demonstrated that four-dimensional spacetime could in some circumstances loop back on itself. However, you still need to contend with the second law of thermodynamics, which has the arrow of time pointing from past to future, never the other way around. And all that is before you try to grapple with what happens when you inadvertently keep your parents from getting together and then have to scramble to play matchmaker. And what happens if you fail and your own existence is erased? In such a case you presumably would never travel back to 1955 and your parents’ romance would proceed unimpeded. Try to sort that one out!

Time travel is predicated on the assumption that past and future are places you can get to from the present. Each moment exists somewhere in perpetuity. According to the more familiar linear conception of time, each moment is like a single frame of a movie spooling through a projector. However, Einstein’s old mathematics professor, Hermann Minkowski proposed that time and space did not exist as separate dimensions but rather formed a seamless four-dimensional universe he called spacetime in which past, present and future exist eternally and concurrently. Minkowski reasoned that there had never been a "place other than at a time and a time other than at a place.”

Minkowski’s block universe, as it is sometimes called, solves the grandfather paradox, since each moment of spacetime is baked in and can’t be altered. So even if you could hop from one point of spacetime to another, there is no possibility that you could step out of your time machine and do anything to threaten your own existence or otherwise screw up the course of history. This, however, begs the question of how you can go time-traveling if you aren’t actually there in any tangible sense. There is perhaps a simpler explanation for why the grandfather paradox could never twist time into a cosmic pretzel. The possibility of time travel depends on the assumption that the past is tucked away somewhere after it has happened, and the future is already waiting in the wings for its turn on stage. But what if time has no duration, and exists only in the present moment? According to this understanding, past and future have no tangible existence beyond memory and expectation. Things still change, of course, but only within the framework of an ever-changing present.

Alone among the attributes of the natural world, time has no physical characteristics. It is not directly detectable by any of the five senses. Whether we think of time running like film through a projector or as four-dimensional spacetime, we all have a sense of duration, of something moving. But apart from actual physical changes in the world around us, we’d be hard pressed to say what it is that passes when we refer to the passage of time. We can see what is happening right now, and we remember what happened before now. Those memories are like stepping-stones in the stream of time — except that there is no stream, only memories with nothing in between.

We use spatial metaphors to talking about time, since we can’t point to the thing itself. When we say the arrow of time moves in one direction only, what is actually moving? The only arrows of time we can say for certain really move are the hands of a clock, and they may be measuring nothing more than their own ticking.

In Back to the Future, Doc Brown’s time machine is installed in a souped-up DeLorean sports car powered by plutonium. When the DeLorean accelerates to exactly 88 mph, a gizmo called a “flux capacitor” kicks in, and with a blinding flash the vehicle makes the leap into another dimension. On a test run late at night at the mall, nothing is left behind but flaming tire tracks and a vanity license plate reading OUTATIME that has come loose when the DeLorean dematerializes. In the original script by Zemeckis and Bob Gale, the vanity license plate has a different message that hints at an altogether different understanding of what is happening. It reads: NO TIME.