In a world hurtling headlong toward social and economic ruin, scientist Hank Merry finally manages to create a working matter teleportation machine, the transmatter, to be used for the transportation of raw materials from other planets to Earth. Except that the machine isn't finished, and nobody knows how it is actually working. What at first seemed to be mankind's salvation turns out to be its worst nightmare as huge chunks of the Earth and its major cities begin to disappear into nothing without a single trace left behind. Whatever the transmatter machine is doing it is making someone, somewhere, very angry.
Enter Robert Benedict, a young man trained from birth to safely cross the "threshold" and navigate the mysterious "Other Side", a neighboring universe with such bizarre geometry that several of the people who discovered it died from shock after accidentally crossing over. As it turns out, the Other Side is being mangled and torn apart by the objects and air molecules being forced through it by the malfunctioning transmatter, and it falls to Robert to find a way initiate real contact with a people he cannot see, hear, or even properly speak to, before their desperation to find and stop the transmatter forces them to destroy the entire Earth.
Written by science-fiction author Alan E. Nourse and originally released in 1951 as two short stories "High Threshold" and "The Universe Between", the story explores themes of communication between people of radically different cultures and backgrounds, the unknown complexities of the universe, resource depletion and conservation, overpopulation, and how humanity can never truly know all there is about the nature of reality.
The Universe Between contains examples of:
- Acid-Trip Dimension: For those capable of staying there long enough without panicking, the Other Side can offer a wide variety of sensory distortions and crossovers, such as tasting the brightness of light, seeing music, feeling colors, and hearing touch sensations.
- Alien Geometries: The primary reason the Other Side is so disturbing to untrained individuals. The 4-D geometry is so incomprehensible to minds familiar with only 3D space that inexperienced people crossing the threshold must either be blindfolded or enter with their eyes closed. Those who don't suffer an immediate panic reaction that can be fatal if they are unable to find their way out quickly enough, and those who do are still left shaken and disoriented by the sudden sensation of falling into a bottomless abyss.
- Another Dimension: The entire story is about the discovery of an alternate universe, and Robert's struggle to cross the seemingly impregnable linguistic, semantic, cultural, and psychological barriers blocking contact with the people there in order to solve a whole host of severe social and economic issues, including finding out why they've suddenly started launching extremely destructive attacks against his universe.
- Awakening the Sleeping Giant: The people of the Other Side knew about Robert's universe long before his knew about theirs, but they never made contact or interfered until the transmatter incident, whereupon they took advantage of their position as the Universe Between to open massive holes into his universe, sucking in colossal tracts of land and even whole chunks of major cities in a desperate attempt to find the transmatter before it ripped their space to shreds. By the time Robert finally made contact, they were on the brink of initiating massive, indiscriminate, planet-destroying land grabs in order to save themselves.
- Crapsack World: Before Robert managed to strike a deal with the Thresholders to use the distance-compression of their space for the transport of people and materials, his Earth was rapidly approaching this state. Terribly overpopulated, minable iron running out, petroleum reserves running out, fissionable fuels running out, economy headed toward total collapse.
- Dimensional Traveler: Robert Benedict and Sharnan. Robert's mother Gail can tolerate the Other Side for a few minutes at a time, but only Robert and Sharnan can cross over at will and stay for extended periods due to their specialized, early training. After making proper contact with the Thresholders, the humanity of Robert's universe all become dimensional travelers by virtue of using the automated Threshold Chambers to make intra-solar and interstellar journeys, albeit blindfolded and strapped into a chair.
- Extra-Dimensional Shortcut: Tremendous distances can be covered by crossing into the Other Side. Interstellar journeys can be completed in an instant, which the people in Robert's universe use as a convenient method of travel and materials transport.
- Hyperspace Is a Scary Place: While there is nothing intrinsically frightening or dangerous about the Other Side, the visual incomprehensibility of the higher-dimensional geometry and the physical sensations it induces (likened to having one's body fragmented into impossible geometric configurations while spiraling out of control into an utterly dark and silent vortex into nothing) can cause such severe mental shock to the unprepared that most of them go into cardiac arrest and die, and the ones who don't are severely traumatized. The Other Side itself is in fact a completely normal three-dimensional universe, but exists in such a way that people crossing over to it cannot perceive it directly, but are forced to experience it through the filter of a higher, fourth dimension. For these reasons, purposefully crossing the threshold into the Other Side requires either constant exposure from a young age or some other form of specialized training.
- More than Three Dimensions: Sort of. It was originally thought that the Other Side was a universe with four dimensions of space and one dimension of time, but after Robert makes a psychic connection with Sharnan he is able to use her senses perceive her universe as it actually is; a perfectly normal universe with three dimensions of space and one dimension of time. The reason for the distortion normally seen is because the Universe Between's dimensional orientation is such that people crossing over cannot directly perceive it, but can only view it through the distorting lens of a higher, fourth spatial dimension. Direct perception is possible, but causes extreme mental stress even in those who are otherwise highly adept at dealing with the Other Side.
- Plot Twist: Only on the very last page of the book is it revealed that our universe is the Universe Between, and Robert's universe is the alternate (but still quite similar) reality.
- Portal Crossroad World: The Universe Between's paper-thin dimensional walls allow it very easy access to a practically infinite number of alternate universes and dimensions, including some that are probably best left unexplored.
- Telepathy: It's unclear if their abilities are natural, technological, or some kind of artifact caused by dimensional distortion, but people from the Other Side are capable of directly exchanging thoughts, emotions and even sensory information with people from Robert's universe. After years of trying to decipher the fleeting bits of thought and feeling that managed to pass through the nigh-unbreakable communication barrier separating their universes, a flaw in the threshold routing system forces the Thresholders to make full contact with Robert's mind. Telepathy of any kind is not natural for his version of humanity, and even when using Sharnan's mind as a buffer the process is still extremely dangerous, painful and exhausting for him. It gets easier with repeated attempts, but that first connection carries with it a very real threat of madness or even death.
- Teleportation Sickness: The physical sensation of crossing to the Other Side is so jarring and disorienting that even people who use it daily for travel often emerge from their transport chamber feeling sick, shaky and sometimes covered with sweat despite the fact that the trip lasts less than a second.
- The Multiverse: Robert's universe and the Universe Between are only two of an infinite number of universes with radically different and often extremely hazardous natural laws, including an anti-matter universe and a universe with a reversed second law of thermodynamics. Some universes have thinner "walls" than others, allowing them to poke holes into other universes, or to have other universes poke holes into theirs.
- Thin Dimensional Barrier: The reason people are able to cross to and from the Universe Between by simply moving in the correct direction. This direction is invisible to almost everyone, but those who have crossed over with their eyes open will always see it everywhere from that point on. It is also the reason why their universe started coming apart at the seams when the transmatter machine couldn't be shut down.