In established physics there is no observed fifth dimension: experiments describe our universe with three dimensions of space and one of time. When physicists talk about a fifth dimension, they almost always mean a hypothetical extra dimension of space, first proposed by Theodor Kaluza in 1921. It is not a second kind of time, and it is not a “realm of possibilities.”
Searches for extra dimensions have so far found nothing. Gravity still follows Newton’s inverse-square law down to separations of 52 micrometres (0.052 mm), and the Large Hadron Collider (LHC) has seen no sign of gravity leaking into extra dimensions.
Key facts about the fifth dimension
- Time as a dimension: Hermann Minkowski’s 1908 lecture “Space and Time” combined space and time into four-dimensional spacetime.
- First fifth-dimension theory: Theodor Kaluza (1921) used a fifth dimension to unify gravity and electromagnetism; Oskar Klein (1926) proposed that it is curled into a tiny circle.
- String theory: superstring theories are formulated in 10 dimensions; M-theory adds an eleventh.
- Short-range gravity test: the inverse-square law holds down to 52 micrometres (Lee et al., 2020).
- Status: no extra dimension has been detected. All remain theoretical proposals.
What is a dimension in physics?
The number of dimensions is the minimum number of independent coordinates needed to specify a point. A position on a line needs one number, a position on a map needs two, and a position in a room needs three. To pin down an event, something that happens at a particular place and moment, you need four: three for space and one for time.
That is the sense in which physics has dimensions. Each one is a direction or quantity along which something can be measured, not a separate world.
Is time really the fourth dimension?
Time is a dimension of spacetime, but calling it “the fourth” is a convention of listing order, not a ranking. In 1908 the mathematician Hermann Minkowski showed that Albert Einstein’s special theory of relativity could be written as geometry in a four-dimensional “world” of events. In his lecture, separate space and separate time fade into shadows, and only their union remains independent.
Time is not simply another direction of space, though. In the geometry of relativity it enters with a different mathematical sign from the space coordinates. That structure underlies relativity’s prediction that observers moving relative to each other measure different lengths and durations. In general relativity, gravity also changes how fast clocks tick, as explained in our article on whether gravity slows down time.
What did Kaluza and Klein propose about a fifth dimension?
In 1921 Theodor Kaluza published a paper, communicated to the Prussian Academy of Sciences by Einstein, that extended general relativity to five dimensions. In that framework, gravity and electromagnetism appear as parts of a single geometry. Kaluza did not explain why we never notice the fifth dimension; he simply assumed that nothing in nature varies along it.
In 1926 the Swedish physicist Oskar Klein gave that assumption a physical reason. He proposed that the fifth dimension is curled up into a circle so small, roughly the Planck length of about 10⁻³³ centimetres, that it cannot be seen. CERN uses a helpful comparison: a tightrope walker can move only along a cable, while ants on the same cable can also walk around it. A dimension curled up that small would be hidden from view.
Kaluza–Klein theory did not become accepted physics. According to the MacTutor history of Klein, most physicists moved on, and the idea was not explored seriously again for nearly half a century.
Why does string theory need 10 or 11 dimensions?
String theory replaces point-like particles with tiny vibrating strings. Superstring theories are formulated in 10 spacetime dimensions, six more than we observe. In 1995 Edward Witten showed that 11-dimensional supergravity arises as a low-energy limit of one 10-dimensional superstring theory, an insight that led to the proposed M-theory in 11 dimensions.
In these theories the extra dimensions are usually compactified, curled up much as Klein suggested. String theory is a mathematical framework that has not yet produced a confirmed experimental prediction. For more, see how string theory explains gravity.
Could an extra dimension be large?
Possibly, which is why experiments can test the idea. In 1998 Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali proposed that two or more extra dimensions might be as large as a millimetre. Ordinary particles would be confined to our four-dimensional spacetime, while gravity could spread into the extra space. That would explain why gravity is so much weaker than the other forces, a puzzle known as the hierarchy problem.
In 1999 Lisa Randall and Raman Sundrum proposed an alternative with a single extra dimension. Spacetime is strongly curved, or “warped,” along that dimension. Both models predict effects that experiments can look for: gravity behaving differently at short distances, and particles or energy apparently vanishing in high-energy collisions.
How do scientists test for extra dimensions?
If gravity leaked into large extra dimensions, its strength would rise faster than the inverse-square law at short range. A precision laboratory experiment published in 2020 by J. G. Lee and colleagues measured gravity between test masses separated by 52 micrometres to 3.0 millimetres. Newtonian gravity fit the data. Any new gravity-strength force must act over ranges shorter than 38.6 micrometres (95% confidence).
At the LHC, a graviton escaping into extra dimensions would carry away energy, leaving collisions with a jet of particles and a large momentum imbalance. A CMS search using 13 TeV collision data found no excess over the Standard Model background. For the Arkani-Hamed–Dimopoulos–Dvali model, it excluded a fundamental gravity scale below about 10.7 to 5.2 teraelectronvolts, depending on whether there are two or seven extra dimensions.
| Framework | Total dimensions | What the extra dimension(s) do | Status (October 2026) |
|---|---|---|---|
| Everyday space | 3 (space only) | None | Observed |
| Relativity: Minkowski spacetime (1908) | 4 (3 space + 1 time) | None; time joins space as a coordinate | Established and extensively tested |
| Kaluza (1921) and Klein (1926) | 5 | One tiny curled-up dimension unifying gravity and electromagnetism | Historical proposal; not detected |
| Arkani-Hamed, Dimopoulos and Dvali (1998) | 4 + 2 or more | Large extra dimensions that dilute gravity | Constrained by gravity tests and LHC; not detected |
| Randall and Sundrum (1999) | 5 | One warped dimension explaining gravity’s weakness | Not detected |
| Superstring theory / M-theory | 10 / 11 | Six or seven compact dimensions | Theoretical; no experimental confirmation |
Source note: Minkowski (1908), Kaluza (1921), Klein (MacTutor biography), Arkani-Hamed et al. (1998), Randall and Sundrum (1999), Witten (1995), Lee et al. (2020), CMS Collaboration (2021).
What about the “fifth dimension” in popular culture?
Books, videos and spiritual writing often describe the fifth dimension as a space of possibilities, alternate timelines or higher consciousness. Those are philosophical or fictional ideas. They are not the definition used in physics, and no experiment supports them. Related ideas such as parallel universes do appear in some theories, but as speculative extensions, as our explainer on string theory and the multiverse discusses, not as a measured fifth coordinate.
What scientists don’t know yet
Physicists do not know whether extra dimensions exist. Experiments have ruled out some large, simple versions. Dimensions curled up near the Planck length would be far beyond the reach of any current experiment, so their absence cannot be proved either. The honest answer to “what is the fifth dimension?” is that it remains a mathematical possibility. It is not a discovered feature of the universe.
Related reading
Update note: This article was substantially revised on October 12, 2026. The revision gives the physics definition of a fifth dimension first and labels the “possible worlds” framing as non-scientific. It credits Oskar Klein with the curled-up dimension and corrects the definition of a dimension. It also replaces an uncredited popular-science analogy and adds current experimental limits from short-range gravity tests and the LHC. The page’s multiple H1 headings have been replaced with a single, consistent heading structure. It was drafted with AI assistance and checked against the sources listed below.
Sources
- H. Minkowski, Space and Time (lecture, Cologne, 21 September 1908; English translation, Wikisource)
- Th. Kaluza, On the Unification Problem in Physics (1921; English translation, Int. J. Mod. Phys. D, 2018)
- MacTutor History of Mathematics, University of St Andrews, Oskar Klein
- Overduin and Wesson, Kaluza-Klein Gravity, Physics Reports 283 (1997)
- E. Witten, String Theory Dynamics in Various Dimensions, Nuclear Physics B 443 (1995)
- Arkani-Hamed, Dimopoulos and Dvali, The Hierarchy Problem and New Dimensions at a Millimeter, Physics Letters B 429 (1998)
- Randall and Sundrum, A Large Mass Hierarchy from a Small Extra Dimension, Physical Review Letters 83 (1999)
- Lee, Adelberger, Cook, Fleischer and Heckel, New Test of the Gravitational 1/r² Law at Separations down to 52 μm, Physical Review Letters 124 (2020)
- CMS Collaboration, Search for new particles in events with energetic jets and large missing transverse momentum (EXO-20-004), JHEP 11 (2021) 153
- CERN, Extra dimensions, gravitons, and tiny black holes
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