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A fifth dimension could change what gravity is in the underlying theory without making gravity look different in everyday life. In some models, a five-dimensional description can produce familiar four-dimensional gravity—and, in the classic Kaluza–Klein construction, electromagnetism as well. In others, our observable universe is a four-dimensional surface in a five-dimensional bulk. These are theoretical frameworks, not evidence that an extra dimension has been found.
What does “five-dimensional” mean?
Ordinary spacetime has three spatial dimensions and one time dimension. A five-dimensional model usually adds one more spatial dimension, but the label alone does not say what that dimension is like. It might be compact, meaning curled up at a scale that is difficult to observe, or part of a larger geometry in which our familiar spacetime is treated as a surface within a higher-dimensional bulk. Those choices lead to different predictions.
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That distinction matters because “gravity in five dimensions” is not one unique theory. The effects we might observe depend on how the extra dimension is shaped, how fields behave in it, and how the theory connects to the four-dimensional physics already measured. The Particle Data Group’s 2025 review surveys the range of extra-dimensional approaches: Extra Dimensions.
How could a fifth dimension change the explanation of gravity?
Kaluza–Klein: gravity and electromagnetism from a higher-dimensional setup
In Kaluza–Klein theory, a five-dimensional gravitational description is reduced to an effective four-dimensional one. Under the classic construction’s assumptions, the resulting four-dimensional fields include both gravity and electromagnetism. The extra coordinate is compact in standard formulations, and the details of the reduction determine what fields and effects appear in four dimensions.
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This is a theoretical unification: it shows how gravity and electromagnetism can arise together from a higher-dimensional framework. It does not demonstrate that the extra dimension exists in nature. A concise review of extra-dimensional gravity and possible consequences is available from Ignatios Antoniadis: Gravity modifications from extra dimensions.
Warped braneworlds: familiar gravity on a four-dimensional surface
In a braneworld model, our observable spacetime is represented as a four-dimensional brane embedded in a five-dimensional bulk. The geometry can be warped, and gravity can be localized so that observers on the brane recover familiar four-dimensional behavior. The extra dimension therefore need not produce an obvious failure of Newtonian gravity or general relativity at everyday scales.
Randall and Sundrum wrote that in their model “four-dimensional Newtonian and general relativistic gravity is reproduced to more than adequate precision.” That result illustrates how a higher-dimensional theory can match known gravity within its applicable regime; it is not evidence that the bulk dimension has been observed. Their paper, “An Alternative to Compactification,” appeared in 1999: Physical Review Letters.
| Question | Kaluza–Klein compactification | Warped braneworld example |
|---|---|---|
| Basic setup | A higher-dimensional theory is reduced to an effective four-dimensional description; standard formulations compactify the extra coordinate. | A four-dimensional brane sits in a five-dimensional bulk with warped geometry. |
| How gravity appears | The reduced theory contains a four-dimensional gravitational sector and, under the classic construction’s assumptions, an electromagnetic sector. | Gravity can be localized on the brane, reproducing familiar four-dimensional gravity, with possible corrections or higher-dimensional behavior in some regimes. |
| Possible experimental clues | Kaluza–Klein states and effects associated with the extra dimension’s scale. | Graviton excitations or emission, and deviations determined by the model’s geometry and parameters. |
| Key qualification | Predictions depend on the compactification and the assumptions about fields. | Recovering four-dimensional gravity at known scales does not establish that a bulk dimension exists. |
These are representative frameworks, not an exhaustive list of five-dimensional theories.
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Extra-dimensional models can imply effects that differ from ordinary four-dimensional expectations. Proposed signatures include:
- Kaluza–Klein resonances: heavier states associated with fields that can propagate in an extra dimension. A collider might look for such a state as a resonance in its decay products.
- Missing energy: if a graviton produced in a collision escapes into the bulk, it would not be detected directly. The event could instead show an imbalance in measured energy and momentum.
- Changes in gravity at short distances: some models predict departures from the familiar gravitational force at sufficiently small distances.
These are proposed ways to test models, not confirmed observations. A candidate signal would also need to be distinguished from other physics explanations. CERN describes collider searches for heavier Kaluza–Klein states and missing energy from gravitons escaping into a bulk: Extra dimensions, gravitons, and tiny black holes. Antoniadis’s review describes proposed signals including “the production of Kaluza–Klein resonances, graviton emission in the bulk of extra dimensions, and a radical change of gravitational forces in the submillimeter range.”
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What has a collider search established?
In a 2018 analysis, the CMS Collaboration searched for high-mass diphoton events in proton–proton collision data collected in 2016 at 13 TeV, corresponding to 35.9 fb−1. For a specified Randall–Sundrum model, CMS reported a 95% confidence lower mass limit of 2.3–4.6 TeV for the first Kaluza–Klein graviton excitation across coupling parameters from 0.01 to 0.2. The range is specific to that model, parameter range, search channel, and dataset; CMS did not report a discovery in that result. See the CMS analysis.
A limit of this kind says that the searched-for particle, within the stated assumptions, must be heavier than the excluded range. It does not rule out every compactification or braneworld, because other models can make different predictions for particle properties and observable effects.
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Would everyday gravity feel different?
Not necessarily. A five-dimensional model can be constructed to reproduce the four-dimensional gravity observed on our brane or in an effective four-dimensional theory. Any departure might occur only in a regime the model makes accessible to experiment, such as through new particles at a collider or a change in gravitational behavior at short distances. Whether such effects occur, and where to look for them, depends on the model.
What a fifth dimension would—and would not—mean
A fifth dimension would offer a different underlying account of gravity, potentially linking it to other fields or changing how gravity behaves beyond familiar scales. But the existence of mathematically consistent models, and experimental limits on one specific version, are not the same as detecting an extra dimension. The central distinction is between what a model predicts and what an experiment has observed.
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