There was a 1960s musical group that performed under the moniker ‌The Fifth Dimension‌, but this is only one association with the 5th dimension. The other perspective, posited by Swedish physicist Oskar Klein, concerns a dimension unseen by humans where the forces of gravity and electromagnetism unite to create a simple but graceful theory of the fundamental forces. In science, dimensionality can mean many things, and today, scientists use 10 dimensions and string theory to explain where gravity and light from the electromagnetic spectrum meet.

When we think of moving from one dimension to two dimensions to the third dimension, we are transitioning between dimensions of space, that is length, area, and volume. However, fields of scientific study are much more general with their terms of dimensions.

There are many more measurements to observe in the world: temperature, time, density, speed, energy, and so much more. How do we represent these additional elements when they might vary or depend on volume or distance or one another? We add more dimensions!

These extra dimensions can allow for an additional 4th dimension or even some five-dimensional analysis of a situation where we can consider the temperature density or the energy change across the volume of a sphere or the path of a particle. These layers of dimensionality allow for greater and more detailed description of the physical reality around us.

To get a handle on these additional dimensions, we start with Albert Einstein’s special theory of relativity. Einstein proposed that the laws of physics are consistent for non-accelerating observers, no matter where in space they are, as absolute frames of reference do not exist. Einstein's theory stated that an entity’s velocity, or its momentum, is only measurable in relation to something else, and – as a way to proportionally relate all of these frames – the speed of light is a constant in a vacuum, regardless of the observer measuring it and the speed at which that observer travels. These two relationships are the postulates of special relativity.

In order to develop the mathematical tools and scientific framework to understand these changing frames, Einstein and other physicists developed the idea of four-dimensional spacetime. This framework involved the three dimensions of space (that human beings experience) in addition to a fourth dimension of time.

These measurements are represented with something called the spacetime four-vector:

When we consider other influences (like gravity), relationships become more complicated. Gravity can directly affect time and space for various reference frames, and through these concepts, Einstein further developed the theory of general relativity.

Einstein’s theory of relativity essentially suggested that space-time becomes warped, felt as gravity, by large objects like the Earth. He posited the measurement of gravitational waves and the possibility of black holes, though he spent his later years trying to disprove the idea of black holes, which scientists finally confirmed as real in 1971, decades after Einstein’s death. But 100 years after he first published his theory of relativity, scientists also confirmed the existence of gravitational waves in September 2015, when scientists from the Laser Interferometer Gravitational-Wave Observatory first detected and measured gravitational waves that rippled through space when two black holes joined.

The next search that follows this monumental measurement revolves around the quest for new particles. Quantum physics and electromagnetic waves rely heavily on a relationship between energy, waves, and particles. In an attempt to unite quantum mechanics and gravity, scientists look to confirm the existence of the graviton (a theoretical particle that would carry the influence of gravity).

There are other approaches to explain these unseen relationships. Because light, or energy, in Einstein’s theory comes from the interactions of the electromagnetic force, scientists have searched for over 100 years for ways to unite energy and light from electromagnetic forces with the other three forces (i.e. the strong & weak nuclear forces and gravity). Two theories, independently developed and proposed by German mathematician Theodor Kaluza and Swedish physicist Oskar Klein suggested the possibility of a fifth dimension where electromagnetism and gravity unify; together this is known as the Kaluza-Klein Theory.

Klein came up with the idea that the fifth dimension could be invisible to the human eye, as it is minuscule and curls up on itself like a pill bug rolls up under threat. Einstein and his assistants, Valentine Bargmann and Peter Bergmann, during the early 1930s and 1940s tried unsuccessfully to tie the fourth dimension in Einstein’s theory to an extra physical dimension, the fifth, to incorporate electromagnetism, but it became even more difficult as the “dimensional consciousness” of our perception is limited. We have a hard time conceptualizing higher dimensions outside of a purely mathematical representation.

Scientists still do not agree on how many dimensions truly exist. Some say six, some say 10, and others say ad infinitum or into infinity. Superstring theory posits that absolutely everything in this universe is a manifestation of a single object – a minuscule string. The way it vibrates determines whether it’s a photon or an electron, and everything is part of a single unified concept. Because not enough deviations can account for all the particles and forces in the universe, string theory requires at least six additional dimensions in addition to the known four. These dimensions come in two types: those that you can see and those that are tiny and curled up, like Klein originally posited, existing on a microscopic level.