The source paper 'Visualizing Interstellar's Wormhole' (James, von Tunzelmann, Franklin and Thorne, 2015), which documented the wormhole geometry rendered for the film Interstellar, states that the physical feasibility of traversable wormholes remains unresolved: quantum inequalities on negative energy 'appear to place severe limits' on wormhole size and, 'after some strengthening', will likely prevent macroscopic wormholes — 'likely, but not certain' — while a higher-dimensional bulk could 'in principle' hold a wormhole open without exotic matter on our brane, about which little is known. The underlying open problem is to determine whether the known laws of physics — semiclassical gravity (general relativity plus quantum fields on curved spacetime) and concretely specified beyond-Standard-Model frameworks such as Randall–Sundrum braneworlds — permit traversable wormholes of macroscopic size. Later literature has settled delimited pieces: a no-go for static spherically symmetric wormholes in semiclassical gravity, a no-go for perturbative constructions in effective field theory with higher-derivative corrections, a microscopic Standard-Model-embeddable traversable wormhole, and a humanly traversable wormhole in a specific braneworld requiring conditions far from those of our universe. A 2024 review of wormhole restrictions from quantum energy inequalities confirms that the general macroscopic question — including quantum inequalities for self-interacting fields and on curved spacetimes, and bulk-supported wormholes compatible with our universe — remains open.

Background

A wormhole is a spacetime 'tunnel' with two mouths in distant regions joined by a throat; it is called traversable if an observer or light signal can pass from one mouth to the other in finite proper time without crossing an event horizon. General relativity permits essentially any wormhole geometry once the matter sourcing it is specified, but Morris and Thorne (1988) showed that holding open a traversable throat requires 'exotic matter': a stress-energy tensor violating the null energy condition (NEC), i.e. having negative energy density as seen in some reference frames. Quantum field theory does admit locally negative energy densities (e.g. the Casimir effect between conducting plates), which opened the possibility that semiclassical gravity — classical spacetime geometry sourced by the expectation value of a quantum stress tensor — might support wormholes. Negative energy is itself constrained: quantum inequalities, introduced in this context by Ford and Roman, bound how much negative energy an observer can measure and for how long, as an uncertainty-principle-type tradeoff between magnitude and duration. Ford and Roman (1996) applied such bounds to static traversable wormholes and concluded that the throat must be near Planck size or the geometry must involve enormous discrepancies between characteristic length scales; whether inequalities in some strengthened form forbid macroscopic traversable wormholes outright has been debated ever since (Krasnikov 2003 argued by explicit examples that they do not forbid 'spacetime shortcuts'). Related rigidity results constrain 'short' wormholes that could act as time machines — a link going back to Morris, Thorne and Yurtsever (1988): topological-censorship theorems, which forbid nontrivial spacetime topology from being visible from infinity; the averaged null energy condition (the requirement that energy density averaged along a complete null geodesic be nonnegative) imposed on achronal geodesics, i.e. geodesics no two points of which can be joined by a timelike path; and chronology protection, the expectation that the laws of physics forbid closed timelike curves. Recent constructions therefore focus on 'long' wormholes whose traversed null geodesics are achronal; such wormholes cannot violate causality but are of limited use for fast travel.

The source paper (James, von Tunzelmann, Franklin and Thorne 2015) documents the three-parameter wormhole metric — scoped out with the film's ray-tracing renderer DNGR (the Double Negative Gravitational Renderer of the film's visual-effects company) — and states the physics status quoted in the abstract, concluding that natural traversable wormholes in our universe seem 'very unlikely' and artificial ones 'pretty dim' prospects.

The modern literature adds several threads directly on the feasibility question. In holography — the AdS/CFT correspondence, which relates gravity in an asymptotically anti-de Sitter (AdS) bulk space to a quantum field theory living on its boundary — Gao, Jafferis and Wall (2017) showed that coupling the two boundaries of an eternal BTZ black hole (a black hole in three-dimensional AdS spacetime) generates negative averaged null energy whose gravitational backreaction makes the Einstein–Rosen bridge (the non-traversable wormhole interior of an eternal black hole) traversable: a wormhole held open by quantum effects, but in a holographic laboratory far from our asymptotically flat universe and requiring a nonlocal coupling between the two mouths. Maldacena, Milekhin and Popov (2018) constructed a four-dimensional traversable 'long' wormhole in Einstein–Maxwell theory plus charged massless fermions, supported by negative Casimir-like energy, which embeds in the Standard Model only if its overall size is far below the electroweak scale (the roughly 100 GeV energy scale of electroweak symmetry breaking), i.e. only at microscopic size. Maldacena and Milekhin (2021) then constructed, in the Randall–Sundrum type-II braneworld (a model in which our four-dimensional universe is a 'brane' inside a five-dimensional AdS-like bulk space), wormholes big enough for a human to traverse, with mouths resembling magnetically charged black holes and the throat held open by bulk fermions — but requiring beyond-Standard-Model matter, an ambient environment far colder than the present universe, and with no formation mechanism. On the no-go side, Terno (2022) proved that static, spherically symmetric traversable wormholes of the Morris–Thorne type (and related regular 'black-bounce' geometries, which interpolate between a black hole and a traversable wormhole as a parameter varies) cannot exist in semiclassical gravity: their would-be static limits are non-traversable and violate a quantum energy inequality, independent of the field content proposed to support them. Freivogel, Kontou and Krommydas (2022) proposed the smeared null energy condition (SNEC), a bound on the null energy averaged over a finite segment of a null geodesic — precisely a 'strengthened quantum inequality' of the kind the source anticipated — and proved semiclassical singularity theorems from it; a sharper variant, the double smeared null energy condition (DSNEC), averages over two null directions instead of one. Kontou (2024) reviews the entire interface between quantum energy inequalities and wormholes and lists what remains open. Kanai, Maeda and Yoshida (2025) proved no-go theorems against obtaining traversable wormholes perturbatively from near-extremal (close to maximally charged) black holes within effective field theory (the framework treating gravity as the low-energy limit of quantum gravity, with quantum corrections appearing as higher-derivative terms in the action), independent of the correction's form. Freivogel, Fumagalli and Tomašević (2026) showed that the transmission of low-frequency scalar probes through the Maldacena–Milekhin–Popov wormhole is parametrically suppressed on light-crossing timescales, while charged massless fermions traverse it with essentially unit probability, so the practical traversability of existing constructions depends on the probe.

Problem Statement

Determine whether the known laws of physics permit traversable wormholes — in particular traversable wormholes of macroscopic throat size, like the one visualized for Interstellar — to exist, naturally or artificially, in our universe. Per the source, the question decomposes into two linked routes, which are jointly posed:

(1) Quantum-inequality route (semiclassical gravity): establish whether quantum energy inequalities — the existing bounds on negative energy averaged along timelike or null curves, or suitably strengthened versions (e.g. the achronal averaged null energy condition, the smeared or doubled smeared null energy conditions), formulated on curved spacetimes and for general, including self-interacting, quantum fields — forbid macroscopic traversable wormholes; or, conversely, exhibit a wormhole spacetime of macroscopic size whose supporting quantum state satisfies every applicable inequality. An answer on the no-go side must cover general geometries (rotating and dynamical, not only spherically symmetric; non-perturbative, not only perturbative near-horizon constructions) and must state precisely the class it excludes; a result restricted to a subclass already established in the literature does not resolve this question.

(2) Higher-dimensional bulk route: determine whether a higher-dimensional bulk can hold open a traversable wormhole whose four-dimensional brane carries no exotic matter, within a concretely specified framework (e.g. a Randall–Sundrum-type braneworld with stated matter content) under conditions compatible with our universe — present-day ambient temperature, energy density and cosmological state, without requiring a colder or denser environment or a fine-tuned initial state — or prove that no such bulk-supported macroscopic traversable wormhole exists in any our-universe-compatible framework of this class whose bulk matter content could in principle support a wormhole (a no-go proved in a single chosen framework does not resolve this route, and the framework must not be one in which a wormhole is impossible by construction).

The question is answered affirmatively if a construction satisfying the stated conditions is exhibited on either route, and negatively only if both routes are closed by general no-go results; a result confined to a subclass of geometries, or to a construction that fails any stated condition, does not resolve that route. Because the objective is whether such wormholes can exist in our universe, the our-universe requirements — present-day ambient temperature, energy density and cosmological state, without a colder or denser environment or a fine-tuned initial state — are part of the stated conditions on both routes.

References

  1. Oliver James, Eugénie von Tunzelmann, Paul Franklin, Kip S. Thorne (2015). "Visualizing Interstellar's Wormhole." American Journal of Physics 83, 486–499. DOI: 10.1119/1.4916949; arXiv:1502.03809; https://arxiv.org/abs/1502.03809
  2. Michael S. Morris, Kip S. Thorne (1988). "Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity." American Journal of Physics 56, 395–412. DOI: 10.1119/1.15620; https://doi.org/10.1119/1.15620
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  13. Rikpratik Sengupta, Shounak Ghosh, Mehedi Kalam, Saibal Ray (2022). "Traversable wormhole on the brane with non-exotic matter: a broader view." Classical and Quantum Gravity 39, 105004. DOI: 10.1088/1361-6382/ac61ae; arXiv:2105.11785 (preprint posted under the title "Wormhole on the Brane with Ordinary Matter: The Broader View"); https://arxiv.org/abs/2105.11785
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