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Physicists are skeptical because the standard general-relativistic wormhole needs a throat kept open by stress-energy that violates ordinary energy conditions. Quantum fields can produce limited negative energy, but quantum-energy inequalities restrict its size and duration so severely that the familiar macroscopic design becomes extraordinarily difficult. A 2022 semiclassical analysis adds that standard spherical wormhole geometries cannot arise as static limits of the dynamical solutions it studies. These are powerful, model-dependent obstacles—not a proof that every conceivable wormhole in every theory is forbidden.
The short answer: the throat needs physics ordinary matter does not provide
A traversable wormhole is not merely a tunnel-shaped drawing in curved spacetime. It must satisfy two conditions at once: a traveler must be able to pass through without encountering an event horizon, and the throat must flare outward rather than pinch shut. In the familiar Morris–Thorne-style solutions of general relativity, that flare-out condition requires a stress-energy tensor that violates a classical energy condition.
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The usual shorthand is violation of the null energy condition (NEC). For every lightlike direction, the NEC requires the stress-energy tensor to obey Tμνkμkν ≥ 0. Ordinary matter normally satisfies this condition. At a wormhole throat, the geometry demands the opposite sign in at least some directions. In plain language, the source must behave unlike familiar positive-energy matter in the critical region.
That does not mean a wormhole necessarily requires a warehouse of “negative mass.” The precise statement concerns the local stress-energy and the energy conditions it violates. Pressure, energy density and quantum-field correlations all enter the requirement; translating the result into a single exotic substance can be misleading.
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Why negative energy is not an unlimited engineering material
Quantum theory permits it in restricted situations
Quantum field theory does allow energy densities that are negative relative to a chosen reference in limited circumstances. Effects such as squeezed quantum states and boundary-induced vacuum effects demonstrate that the classical intuition “energy is always positive everywhere” is too simple. This is the reason wormhole discussions cannot stop at the classical NEC objection.
But quantum theory does not provide arbitrary negative energy on demand. Quantum energy inequalities (QEIs) place a trade-off between the magnitude of a negative energy pulse and the time or distance over which an observer can experience it. A larger negative value must be shorter-lived or more tightly confined, and attempts to extend it in time are compensated by stricter bounds.
What Ford and Roman found for static wormholes
In a 1996 Physical Review D analysis, L. H. Ford and Thomas A. Roman applied a quantum inequality to static traversable wormhole geometries. Their calculation used a minimally coupled, quantized, massless scalar field, so its conclusions belong to that field model and to the geometries they examined.
The result is a severe scale problem. A wormhole with a macroscopic throat can satisfy the inequality only if the negative-energy region is an extremely thin band compared with the throat and the surrounding curvature scales. If that geometric separation is not imposed, the allowed throat is driven toward the Planck scale, where a classical spacetime description is expected to fail. Their paper concludes that the constraints “would seem to make the existence of macroscopic traversable wormholes very improbable.” The phrase “would seem” matters: this is a model-based probability judgment, not a theorem covering every quantum field and every geometry.
The implication is practical as well as mathematical. A design would need a precisely shaped stress-energy profile, with the exotic component confined to a narrow zone while the throat itself remained large enough for passage. Quantum inequalities make that profile harder to sustain than the popular image of simply adding negative energy to a tunnel.
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A semiclassical obstruction to familiar spherical geometries
A separate argument asks whether the standard static metrics can be the long-time endpoints of a physically evolving system. Daniel R. Terno’s paper “Inaccessibility of traversable wormholes,” published in Physical Review D on 17 August 2022, studies dynamical solutions of the semiclassical Einstein equations. These equations combine a classical spacetime geometry with the expectation value of a quantum stress-energy tensor.
Terno argues that the usual spherically symmetric Ellis–Morris–Thorne and Simpson–Visser metrics cannot occur as static limits of the dynamical semiclassical solutions considered. The alternative static limits analyzed in the paper are either nontraversable or violate a quantum energy inequality. His stated conclusion is: “As a result, spherically symmetric wormholes cannot exist in semiclassical gravity.”
The scope is essential. The claim concerns spherically symmetric wormholes within semiclassical gravity and the assumptions used in that analysis. It does not establish that every nonspherical construction, every modified theory of gravity or every quantum-gravity spacetime is impossible. It does, however, remove a commonly discussed class of textbook geometries from the set of straightforward semiclassical solutions.
Another constraint: the averaged null energy condition
Some theorems use an averaged, rather than point-by-point, version of the null energy condition. The achronal averaged null energy condition (ANEC) integrates the relevant null energy along a complete, non-self-intersecting light path. Violating it can permit the focusing properties needed for a wormhole, but maintaining such a violation consistently with quantum fields is difficult.
A 2026 Scientific Reports article describes achronal ANEC as a powerful obstruction to wormholes and time machines in self-consistent semiclassical spacetimes. It discusses hypothetical quantum-foam mechanisms at Planck scales, while emphasizing the unresolved problem of enlarging and maintaining those structures. Quantum foam is therefore a speculative possibility, not an established method for producing a human-sized passage.
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Why physicists still avoid saying “all wormholes are impossible”
“Impossible” can refer to several different claims: forbidden by classical general relativity, inconsistent with semiclassical quantum fields, absent from our universe, or impossible to build with any technology. The evidence is strongest for particular combinations of geometry, matter model and gravitational regime—not for every logically imaginable theory.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesHolographic wormholes: traversable, but in a special boundary setup
In 2017, Ping Gao, Daniel Louis Jafferis and Aron C. Wall described a controlled construction involving an eternal BTZ black hole in a holographic setting. An interaction coupling the two boundaries produces negative averaged null energy. The resulting gravitational backreaction opens a temporary traversable window through what would otherwise be an Einstein–Rosen bridge.
The construction is not an isolated, asymptotically flat tunnel that engineers could place between two stars. It relies on a specially prepared coupled-boundary system and the correspondence between a gravitational description and a quantum theory on the boundary. The authors explicitly state: “However, it cannot be used to violate causality.” Traversability in this model therefore does not imply unrestricted faster-than-light travel or a practical shortcut through ordinary space.
Randall–Sundrum proposals beyond the Standard Model
Juan Maldacena and Alexey Milekhin’s 2021 paper, “Humanly traversable wormholes,” identifies solutions that can be traversed by humans in some Randall–Sundrum theories beyond the Standard Model. This is an important theoretical qualification because it shows that adding particular extra-dimensional ingredients can change the energy-condition accounting.
Those solutions depend on physics not established in experiments. They are mathematical possibilities inside an extended theory, not evidence that our universe contains a naturally occurring wormhole or that a laboratory could construct one. Their existence also does not remove the quantum-inequality and stability questions that arise when one asks how a real object would be created, maintained and connected to our surroundings.
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What the 2022 quantum-processor experiment actually did
A Nature paper published on 1 December 2022 implemented selected traversable-wormhole dynamics in a sparsified Sachdev–Ye–Kitaev (SYK) model. The experiment used a nine-qubit circuit with 164 two-qubit gates on Google’s Sycamore quantum processor. The circuit reproduced information dynamics associated with a holographic traversable-wormhole mechanism.
No tunnel opened in spacetime, no mass crossed a wormhole throat and no laboratory portal was created. The experiment tested a quantum-information model whose mathematical behavior has a gravitational interpretation in a particular theoretical framework. Calling it a simulated or encoded wormhole is accurate; calling it a physical passage through space is not.
Compare the claims on the right axes
| Question | What the cited work establishes | What it does not establish |
|---|---|---|
| Classical throat requirement | Standard Morris–Thorne-style throats require stress-energy violating a classical energy condition, commonly the NEC. | That no modified theory or nonstandard matter model can ever support a throat. |
| Quantum negative energy | Quantum fields can produce negative energy, but QEIs restrict its magnitude and duration; Ford and Roman found severe scale constraints in their analyzed static cases. | An unrestricted supply of negative energy for construction. |
| Semiclassical spherical metrics | Terno’s 2022 analysis finds the standard Ellis–Morris–Thorne and Simpson–Visser metrics cannot be static limits of the dynamical solutions considered. | A universal prohibition on nonspherical or non-semiclassical wormholes. |
| Holographic construction | A coupled-boundary interaction can make an Einstein–Rosen bridge temporarily traversable without enabling causality violation. | An engineered, asymptotically flat travel tunnel. |
| Quantum processor | A nine-qubit, 164 two-qubit-gate circuit reproduced selected SYK-model dynamics. | A macroscopic wormhole or physical transit through spacetime. |
What a credible macroscopic wormhole proposal would have to answer
Any proposal that claims more than a formal metric should address several separate tests:
- Source: Identify the actual stress-energy that creates the flare-out and state which energy condition it violates.
- Quantum consistency: Demonstrate compliance with the relevant quantum energy inequalities, including the magnitude, duration and spatial thickness of the negative-energy region.
- Dynamics: Show that the object can form from an evolving solution rather than appearing only as an imposed static metric.
- Stability: Track perturbations, backreaction and the response of the quantum fields that support the throat.
- Traversability: Keep horizons, singularities and destructive tidal forces away from the intended route.
- Embedding: Explain how the construction fits into a realistic cosmological or asymptotically flat environment rather than an idealized boundary system.
- Scale: Distinguish a Planck-scale structure from a throat large enough for a spacecraft or person.
Meeting one item does not automatically meet the others. A model can be mathematically traversable while failing to be dynamically realizable, humanly safe or compatible with known particle physics.
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Common misconceptions corrected
“General relativity forbids wormholes.”
General relativity admits wormhole metrics on paper. The difficulty is supplying physically acceptable stress-energy and obtaining a stable, traversable solution. The classical equations alone do not settle the matter.
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“Negative energy means negative mass.”
Negative local energy density in a quantum state is not the same thing as a bulk object with negative inertial or gravitational mass. Wormhole analyses concern the full stress-energy tensor, including pressures and correlations.
“A quantum computer created a wormhole.”
The quantum-processor experiment implemented a small quantum circuit that models selected information dynamics. It did not alter laboratory spacetime or create a passage.
“A theoretical counterexample proves wormholes are feasible.”
Holographic and Randall–Sundrum constructions show that blanket statements are too strong. They remain dependent on special boundary conditions or extensions of known physics, and they do not demonstrate an object that exists in our universe.
So, are traversable wormholes impossible?
The best-supported answer is narrower and more informative than yes or no. Known semiclassical physics makes the familiar macroscopic wormhole extraordinarily difficult: the throat requires exotic stress-energy, quantum inequalities sharply limit the negative energy that quantum fields can supply, and Terno’s analysis rules out standard spherical metrics as static semiclassical limits under its stated assumptions.
At the same time, specialized holographic and beyond-Standard-Model theories contain mathematical traversable solutions, and quantum processors can reproduce related information dynamics. Those results prevent a universal impossibility theorem, but they do not provide a buildable interstellar tunnel. “May be impossible” is therefore a statement about the severe constraints on realistic, human-scale wormholes—not a claim that every conceivable spacetime geometry has been ruled out.
Further reading
For broad historical background, Kip Thorne’s Black Holes & Time Warps: Einstein’s Outrageous Legacy (W. W. Norton, paperback ISBN 9780393312768) explains wormholes and time machines for general readers. It is useful context, but it predates the recent quantum-inequality, holographic and semiclassical developments discussed here.
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