I grew up in the orchards of the Okanagan Valley in Canada and apples have been an important part of my life. Apples also played an important role in the work of Newton, as he reputedly described the moment he began to wonder about gravity as being the result of seeing an apple fall out of a tree. Often, an apple is used as an analogy to describe another important concept in gravitational physics. I'm talking about a specific solution to Einstein's field equations, the Schwarzschild wormhole. Of course, the name wormhole comes from the idea that worms or caterpillars who feast on the flesh of an apple can tunnel through the interior in order to create a shorter path between two points on the surface. In this analogy, the skin of the apple is our regular four-dimensional spacetime and the flesh of the apple of some higher dimension in a hyperspace. Carl Sagan famously said about wormholes, "It's just possible that you might emerge in another part of spacetime, some where else in space, some when else in time." So what exactly is a wormhole? The short answer is that a wormhole looks like a black hole. But instead of a trash compacting singularity at the center, a wormhole opens back up into a distant region of spacetime. In 1916, Ludwig Flamm was studying the Schwarzschild black hole solution to Einstein's field equations, when he discovered that a second solution was possible. This second solution described a white hole, a region in space that ejects matter from its event horizon. Flamm then lined up and joined the necks of both the black hole and the white hole and boom, the concept of a spacetime bridge was born. Today, we call this an Einstein-Rosen bridge after it was rediscovered by Einstein and Rosen in 1935. It wasn't until 1957 that the word wormhole was first used to describe a connection between two points in spacetime. This time by scientist Charles Misner and John Wheeler. Let's be very clear right off the bat here, white holes and wormholes are purely hypothetical and unlike black holes, there's no observational evidence of their existence. Mathematically speaking, wormholes can exist and not only can they tunnel through space but it's also possible for them to tunnel through time. Throughout the 60's, 70's and 80's, wormholes entered popular culture through novels like A Wrinkle In Time, The Forever War, and Carl Sagan's Contact. Even modern video games like Portal and Portal 2 employ wormholes as their central game mechanic. In fact, the mathematical development of wormhole theories seems to be heavily influenced by science fiction. When Carl Sagan was writing his science fiction novel, Contact, he approached the famous black hole physicist, Kip Thorne, and asked him how it will be possible for a human being to travel vast distances across the galaxy using a rotating black hole. Thorne suggested instead that Carl consider travelling through wormholes. When Thorne put pen to paper to start figuring out the mathematics, they discovered that wormholes are inherently unstable. Not only would the neck of a Schwarzschild wormhole be too narrow to permit the passage of human being, but the wormhole itself would close up extremely quickly making it possible to only squeeze through a bit of information before the wormhole is destroyed. However, Thorne realized the neck of the wormhole could be held open with some kind of material that would repel the wormhole's walls gravitationally. In reality, we have no idea what kind of material this would be, all of the regular material in our universe acts through gravitational attraction. If regular matter won't do the job, Thorne posited that a spherical wormhole could be kept open using a form of material with a negative energy density. In fact, when the University of Alberta's physicist Don Page was approached by Thorne, Page demonstrated that any shape of wormhole requires a negative energy density to be held open in much more elegant mathematics. Physicists call this exotic material and although there are no examples that we know of, there's nothing written in the laws of physics that prevent it from existing in our universe. The distinguishing feature between an unstable and a traversable wormhole is therefore, the presence of this exotic material. One such traversable wormhole was theorized by a scientist named Homer Ellis, who demonstrated a solution to the Einstein field equations that permit safe passage through the wormhole in either direction. Named after him, the Ellis wormhole was used as a template for the wormhole in Interstellar, which carries the crew of the Endurance from orbit around Saturn to Gargantua in a distant region of the universe. We don't touch much on the science of time travel but one of my favorite movies on the subject has to be Back to the Future. In it, Doc Brown accidentally sends Marty McFly travelling backwards in time from the year 1985 to 1955 in his time travelling DeLorean. One of my favorite fan theories for how the DeLorean works, is that the flux capacitor stores enough energy, 1.21 giga-watts, and not jiga as Doc Brown says, to create the negative energy density required to amplify a tiny wormhole. Just big enough and just long enough for the DeLorean and its passengers to squeeze through the wormhole before it closes behind them.