Paul Halpern

is a University of the Sciences physics professor and the author of "Einstein's Dice and Schrödinger's Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics"

Simone Zelitch

teaches at Community College of Philadelphia and is the author of "Judenstaat," an alternative history about a Jewish state established in Germany after World War II

The results of November's election took many of us by surprise, and challenged our ideas of inevitability. Certainly, many of Donald Trump's supporters see his unexpected victory as fresh proof that the mainstream media are dominated by elitists. The pundits should have seen it coming. What about Brexit, and the wave of populism spreading across Europe? What about the clear anger of the Rust Belt and the rural communities between the coasts?

As time passes, the factors that led to a Trump victory are far more likely to seem obvious, and Hillary Clinton's defeat a fait accompli. This narrative will find its way into the frozen history found in high school textbooks.

However, there are those who continue to insist that Trump is not their president. After all, countless factors could have changed the election's outcome. Trump's electoral majority came down to some tens of thousands of voters in three pivotal states: Pennsylvania, Michigan, and Wisconsin. What if turnout had been higher in urban areas? What if Vice President Biden had entered the race? If Bernie Sanders had been the Democratic nominee, would we be about to inaugurate our first Jewish socialist president? To some, these counterfactuals feel just as likely as the actual result of the election. Will they wake up and discover it's all a dream and find themselves in a more logical world?

In short, we're living in an alternative-history moment. It's no surprise that Amazon scored the highest ratings in its history when it streamed an adapted Philip K. Dick novel, The Man in the High Castle, envisioning a world where Germany and Japan won World War II. Dick wrote the novel in 1962 after reading a book, Bring the Jubilee by Ward Moore, that imagines what would have happened if the Confederacy had won the Civil War.

Many alternative histories focus on those two pivotal, recognizable eras when a single moment - a battle or a critical assassination - could have changed the direction of history. Yet sometimes - as now - that moment takes the form of a presidential election. Sinclair Lewis' 1935 novel, It Can't Happen Here, imagines right-wing populist President Buzz Windrip, champion of the "forgotten man." In Philip Roth's 2005 novel, The Plot Against America, actual aviator and fascist-sympathizer Charles Lindbergh is elected president in 1940, and the United States is poised to go to war with England.

Ray Bradbury's classic alternative-history story, "A Sound of Thunder," introduces a different twist: Could history be altered through time travel? When a member of a dinosaur safari, who has ventured millions of years into the past, panics and steps on a butterfly, a chain reaction to the insect's destruction results in numerous changes to the present-day Earth of the tale. Variations in the way certain English words are spelled and an altered outcome to a pivotal presidential election offer stark evidence that the voyage through time disrupted reality's original timeline, setting it on a new course.

Physicists are familiar with the "butterfly effect" via chaos theory. According to that field of study, hypothetically the flapping of a butterfly's wings over one continent might snowball over time into a change in the weather in another. That's because certain scientific equations are so sensitive to initial conditions that a slight change might lead to a wholly different outcome. If backward time travel were possible, therefore, a minute change in the past might indeed lead to a significant alteration of the present, as Bradbury envisioned in his story.

Could each alteration lead to a forking timeline and a parallel universe? Contemporary physics offers various hypothetical schemes for the branching of reality. Among these is Hugh Everett's unconventional description of what happens when researchers measure particle properties. Known as the "Many Worlds Interpretation" of quantum mechanics, it purports that whenever observers take quantum measurements that result in several possible outcomes, reality bifurcates and the observers themselves split into numerous replicas. Each replica observer records one of the possibilities, never knowing that the other observers are witnessing different results in their own versions of the world. If observers split like amoebas, their measurements no longer collapse multiple possibilities down to one, as mainstream quantum theory suggests.

If the universe is, in fact, a multiverse, how should we respond? Do we close ourselves inside what feels safe and familiar, or can we become more open to infinite possibilities? If we accept that reality itself is fragile and variable, that understanding could lead to empathy, and break down the echo chamber that, some might believe, causes the divisions in our country.

Paths lead in infinite directions, and we must be open to unexpected versions of the future. Most of all, we can no longer say, "That'll never happen."