Fifth dimension: what physics, spirituality and science fiction say

Few phrases carry as many meanings as "fifth dimension." A physicist uses it to talk about a tiny spatial axis, curled up on itself. A practitioner of New Age spirituality speaks of a state of love and unity reached by raising one's consciousness. A film director turns it into a labyrinth of bookshelves where a father watches his daughter's past. These are three different subjects, and confusing them is the most common mistake in this conversation.

It is worth touring all three territories, in order, and marking where what science has established ends and where theory, belief or imagination begins.

From the point to spacetime

A point has no extension: it is dimension zero. A line has length, a plane has width, and the space we inhabit has height or depth. In mathematical terms, dimension is the number of independent coordinates needed to locate a point.

The fourth dimension came with Einstein and Hermann Minkowski, who in 1908 proposed treating space and time as a single continuum. According to Minkowski, space and time taken separately were doomed to fade into mere shadows. The faster a body moves, the more its length appears to shrink and the more its clocks run slow, so that the speed of light stays constant. The lesson is that dimensions can be of different natures and still influence one another.

The fifth dimension in physics

The first serious proposal is Theodor Kaluza's, in 1919: rewrite general relativity in five dimensions. In that version, the equations of electromagnetism fell spontaneously out of those of gravity. In 1926, Oskar Klein explained why we do not notice the extra dimension: it would be spatial, but curled into an infinitesimal circle, far smaller than an atom. The theory did not explain, for instance, the electron's mass, but it left behind two lasting ideas. Extra dimensions can exist hidden, and different forces can be aspects of a single one in larger dimensions.

String theory picked up the reasoning. In it, particles would be vibrating strings, and mathematical consistency demands additional dimensions: 25 spatial ones in the bosonic version, 9 in superstrings, 10 in M-theory, which also allows for objects called branes. Two mechanisms would explain their disappearance. Either they are compactified into tiny shapes, such as Calabi-Yau manifolds, or our universe is a brane floating in a larger space, where only gravity would circulate. That would be an explanation for gravity's weakness compared with the other forces.

Mathematics, for its part, works in any number of dimensions without needing to visualize them. It uses analogy, as in Flatland, projection, as in the shadows of a hypercube, and topology.

The search in particle accelerators

If the dimensions exist, they should leave traces. The Large Hadron Collider, at CERN, searches for energy that disappears, as if gravitons were escaping into another dimension, for particles heavier than the known ones, and even for micro black holes. None of these signals has shown up. The ATLAS and CMS experiments have only set limits. For micro black holes, for example, ATLAS has excluded masses below about 7 TeV. Tabletop tests of gravity at short distances also rule out large dimensions, above roughly 0.08 millimeter.

The reasons for the silence are well known. Extra dimensions may be too small for the energies available, may interact weakly with matter, or may have a curved geometry that makes them hard to read. Besides, the expected signals could also be explained by other, still-unknown phenomena. Strings and M-theory remain hypotheses without experimental verification.

The fifth dimension as a state of consciousness

Outside physics, the term took on another meaning. In New Age currents, the fifth dimension, or 5D, would be a higher state of consciousness and vibration, marked by unconditional love, forgiveness, unity and the absence of fear. The third dimension, the ordinary world, would be tied to ego, separation and linear time. The fourth would be a bridge, one of spiritual awakening.

The passage is called ascension and is not a physical displacement. Adherents link it to meditation, prayer, gratitude and trauma healing. Among the traits attributed to life in 5D are the rapid manifestation of desires, telepathic communication and expanded intuitive abilities. Esoteric traditions also speak of a "mind of light," in which everything is seen as perfect, and link it to energy centers of the subtle body, such as the throat chakra.

These are beliefs and reports from those who practice them. None of these claims has been tested, and none bears any relation to the dimensions of physical theories. The coincidence of vocabulary, though, is telling: in both fields, the idea is that the reality we experience depends on our apparatus of perception.

In books, film and song

Fiction has explored the idea for more than a century. In Flatland (1884), Edwin Abbott tells the story of a Square who discovers a Sphere, come from a three-dimensional world, and is imprisoned for trying to spread the news. In A Wrinkle in Time (1962), by Madeleine L'Engle, the tesseract is a shortcut through space. Philip Pullman, in His Dark Materials, imagines parallel worlds linked by rifts, and Cixin Liu, in Death's End, takes characters into a space of four spatial dimensions, where one can see inside objects.

In film, Interstellar (2014), by Christopher Nolan, shows a tesseract built by future humans, in which time becomes space. Contact (1997) uses wormholes, and Source Code (2011), parallel realities. On TV, the opening narration of The Twilight Zone spoke of a "fifth dimension," and series such as The OA, Sliders and Stranger Things deal with alternate worlds. Even the vocal group The 5th Dimension, active at the turn from the 1960s into the 1970s, made the list with the song "Aquarius," which evoked the dawning of a new age.

What can be said

What is known for certain fits in little space. The fourth dimension, time, is established. Extra spatial dimensions are a respectable hypothesis of theoretical physics, with no evidence so far. The spiritual fifth dimension is a metaphor for a state of being. Fiction mixes all three freely.

What unites the three uses is the unease that reality might be larger than the senses can reach. It is a question about the limits of knowledge, and no accelerator, no meditation and no film has settled it.