Science Fiction and Fact

1803
The Double-Slit Experiment
By the early 19th century, most physicists agree with Newton's well-established theory of light, which says that light takes the form of particles—what we now call photons. But English scientist Thomas Young isn't convinced, and in 1803, he designs an experiment to test the status quo. Young aims a beam of light at a barrier that has two slits. If light is made of particles, he reasons, those particles should travel in a straight line through the slits, projecting two distinct lines of light on the screen beyond the barrier. Instead, Young sees a series of dark and bright lines on the screen, a pattern that could only be produced by waves of light interfering with each other. And yet other experiments, both before and after Young's, convincingly show the particle nature of light. Physicists are left with the unsettling conclusion that light—and, as they later find, electrons (matter)—has a dual nature: Sometimes it takes the form of particles and sometimes the form of waves.

1923
Men Like Gods
Roman authors were the first to write alternate histories—tales of what the world might have been had historical battles ended differently. But stories about alternate worlds that exist simultaneously with our own came much later. H.G. Wells (pictured here) becomes the first to write a science fiction novel that uses a concept similar to Everett's theory of parallel worlds. In Men Like Gods, several English motorists inadvertently drive through an invisible barrier and cross into a parallel world. This alternate world's inhabitants, whom the travelers call "Utopians," explain that their universe split from ours 3,000 years before, and that their world and ours are only two of a vast number of parallel worlds.

1926
Schrödinger's Wave Mechanics
Physicists are still puzzled by that strange now-a-particle, now-a-wave nature of matter. But in 1926, Austrian physicist Erwin Schrödinger derives an equation capable of explaining the so-called "waveform" nature of matter. Through his equation, Schrödinger describes how matter exists as a probability wave. For example, an electron is a waveform that exists throughout all of space, but there's a higher probability of finding it in some places than in others. It's hard to believe, but the math works. Using the Schrödinger Equation, physicists can explain many atomic-scale mysteries—for instance, how electrons stay in orbit around the nucleus of an atom.

1927
The Copenhagen Interpretation
Schrödinger's equation describes the subatomic world, but the world that we know seems to follow the rules of classical—not quantum—mechanics. How can this be? Danish physicist Niels Bohr (shown here in polka-dotted tie, years later) provides a possible answer in a 1927 lecture on his theory of "complimentarity," which forms the basis for what will later be known as the Copenhagen Interpretation. By this time, researchers are aware of another surprising result of the double-slit experiment (when conducted with electrons). If, in the course of the experiment, the researchers try to track the electron as it passes through the slits, the electron starts acting like a particle, not a wave—the interference pattern on the screen disappears. The Copenhagen Interpretation holds that the very act of measuring (or observing) the electron's position causes something called "waveform collapse." In other words, observed matter acts like a particle.

1934
"Sidewise in Time"
While physicists contemplate waveform collapse, author William Fitzgerald Jenkins writes of parallel worlds under the pen name Murray Leinster. In the story "Sidewise in Time," a mathematics professor named Minott predicts that patches of parallel universes—worlds where human history unfolded differently—will start to tear through into our reality. When that actually happens, Minott leads a band of students on an adventure across the parallel worlds, hoping to locate and rule a world that lacks sophisticated technology. "Sidewise in Time" isn't the first story to feature parallel worlds, but it does bring the concept to a pulp science fiction audience when the story is published in Astounding Stories in 1934.

1935
Schrödinger's Cat
Back in physics, Erwin Schrödinger takes issue with the Copenhagen Interpretation. To illustrate his concern, Schrödinger devises a clever thought experiment in 1935: A cat sits in a sealed chamber that contains a flask of hydrocyanic acid, a poison that will kill the cat if the flask is broken. A hammer mechanism is rigged to fall and break the flask if a Geiger counter detects the decay of a single radioactive atom that is within the box. When a radioactive atom decays, it emits a special type of particle. According to the Copenhagen Interpretation, this particle exists as a waveform, thus in all possible states, until it is measured or observed. So, until someone looks into the chamber, the radioactive atom has both decayed and not decayed, and the cat is both dead and alive at the same time. This example, Schrödinger reasonably remarks, is "quite ridiculous."

1941
"The Garden of Forking Paths"
In 1941, Argentine poet Jorge Luis Borges writes a short story called, in English, "The Garden of Forking Paths." Borges' work focuses on a story-within-a-story: The protagonist, Yu Tsun, meets a man who shows him a text written by Tsun's ancestor. The text, also titled "The Garden of Forking Paths," is full of seeming contradictions. In one chapter the hero is dead, in the next he is alive, and no explanation is given for the change. At the conclusion of the story Tsun makes a startling realization. The text isn't inconsistent. It is intended to express a "web of time"—a metaverse of parallel worlds that contain every possible reality. When Tsun commits murder and faces execution for his crime, he remains unconcerned, because he realizes that his crime and approaching death are only one of many parallel chronologies, which all contain different outcomes.

1953
Wonder Woman's Invisible Twin
In 1953, Wonder Woman, Issue 59, features the DC Comics hero's first adventure in a parallel universe. After a strange encounter with invisible foes, Wonder Woman is thrown off a cliff, and as she falls, her golden lasso is struck by lightning. Suddenly, she finds herself in a parallel world where she meets her double, Tara Terruna. Wonder Woman teams up with Terruna to fight the evil Duke Dazam before returning to her own universe. This issue marks the first of many parallel-world adventures in comic books and cartoons.

1956
The Many Worlds Interpretation
Hugh Everett completes his Ph.D. thesis in 1956. It is titled "'Relative State' Formulation of Quantum Mechanics" but becomes generally known as the Many Worlds Interpretation or the theory of parallel worlds. Everett's theory provides one solution to the paradox inherent in the Copenhagen Interpretation, as seen in Schrödinger's thought experiment. The Copenhagen Interpretation relies on the idea of a waveform collapse, but Everett suggests something entirely different—a waveform "branching." Although we only see one outcome—the cat is either alive or dead—there are actually multiple parallel worlds that collectively contain all of the outcomes that we don't see.

1967
Mirror Mirror
"Captain's log, stardate unknown. We are trapped in a savage parallel universe, from which we must escape within four hours or I will face a death sentence at Mr. Spock's hands." In Season 2, Episode 33 of Star Trek, Captain James T. Kirk and his landing party accidentally travel into a parallel world while "beaming up" during an ion storm. In the parallel universe, they encounter barbaric "doubles" of the crew of the starship Enterprise. Like most fictional tales of parallel worlds, this story breaks one of the cardinal rules of Everett's Many Worlds Interpretation: Parallel worlds must never interact. But hey, this is science fiction, where a little poetic license goes a long way.

2005
Parallel Worlds Collide
Eventually, some theorists begin to argue that parallel worlds can and do interact. In his 2005 book Parallel Worlds, theoretical physicist and string theorist Michio Kaku suggests that when parallel worlds—which, according to string theory, exist in parallel dimensional membranes—bump into one another, the impact sets off an astronomical Big Bang that generates a universe-worth of elements in the fiery aftermath. It's possible, Kaku writes, that our own universe started in just this way. Is he right? As with all scientific theories, including Everett's innovative foray into parallel worlds half a century ago, only time—and a lot more work—may tell. In the meantime, science fiction, running in parallel, will no doubt break new ground of its own.