Wormholes and Time Travel
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 09 / Article 06
What if space had a shortcut?
Wormholes let physicists ask how spacetime might connect distant places—and whether those connections could create loops in time. The mathematics is rich. Turning such a geometry into a real, traversable object is an unresolved question.
A solution is the beginning of a question.
Imagine reaching a distant destination by taking a shorter route through spacetime. A wormhole is one way to explore that possibility mathematically.
There are several tests between writing down a geometry and finding one in nature: what supports it, how it forms, whether it survives disturbances, and what observations would reveal it. This article follows those tests while keeping measured time dilation separate from hypothetical journeys into the past.
What is a wormhole?
A wormhole is a hypothetical spacetime geometry connecting regions by an additional route. Its entrances are commonly called mouths; the narrow connecting region is the throat. Depending on the model, the mouths can lead to different regions of one universe or to separate exterior regions.[1]
Folded-paper pictures can help suggest a shortcut, but their extra drawing dimension is a visualization tool. The physical question concerns distances and causal paths within spacetime. A wormhole need not be a tube that bends through a surrounding space we could otherwise enter.[2]
Which wormholes could a traveler cross?
The Einstein–Rosen bridge
In 1935, Einstein and Rosen introduced a bridge construction while investigating a description of particles in general relativity. Their goal was not a design for interstellar transport.[3]
The bridge in the maximally extended eternal Schwarzschild geometry connects two exterior regions mathematically. Yet neither a massive traveler nor a photon can pass from one exterior to the other. Its nontraversability follows from the causal structure, even without someone trying to enter it.[4]
The Morris–Thorne question
Morris and Thorne instead asked what a geometry would need for a person to cross. In their familiar static, spherical models, the throat must stay open, the route must avoid an event horizon, and tidal stresses and travel times must be suitable for the traveler.[5]
Writing a geometry with those features leaves another task: identifying matter and fields that can support it. A static solution also does not, by itself, prove that the system could be formed or would remain stable after a disturbance.[5]
Why does negative energy enter the discussion?
Einstein’s equations relate spacetime curvature to energy, momentum, pressure, and other stresses. For the standard Morris–Thorne throat in Einstein gravity, the geometry’s opening requires a violation of the null energy condition near the throat.[5]
The null energy condition, in compact form
This must hold for every null vector k, representing a lightlike direction. The weak energy condition instead requires nonnegative energy density for every timelike observer. They are related assumptions about stress-energy, not interchangeable definitions.[6]
Negative pressure alone is insufficient. For a perfect fluid, the null energy condition is ρ + p ≥ 0, with ρ expressed as energy density. A cosmological constant has p = −ρ and saturates this condition. Wormhole stresses are generally directional, so their radial pressure matters.[6]
What quantum physics permits—and constrains
Quantum fields can have negative local renormalized energy density, defined relative to an appropriate reference state. This does not provide a freely collectable substance with unlimited negative energy. Quantum energy inequalities constrain suitable averages, with the precise bounds depending on the field and physical setting.[7]
The Casimir force between closely spaced surfaces has been measured. That observation tests boundary-dependent quantum-field effects; it does not demonstrate material capable of holding open a wormhole.[8]
Ordinary antimatter also provides no shortcut to the requirement: antiparticles are positive-energy excitations, just as ordinary particles are. Opposite electric charge does not mean negative energy.[9]
Ford and Roman applied quantum bounds to static spherical wormholes and found severe restrictions on throat and negative-energy-region sizes. Their analysis is a strong constraint within specified semiclassical assumptions, rather than a theorem excluding every conceivable wormhole model.[10]
What does “time travel” mean in physics?
Reaching someone else’s future
Two clocks following different paths through spacetime can accumulate different amounts of proper time. A traveler may return having aged less than people who stayed behind. Relativistic clock differences are experimentally established, including comparisons using precise atomic clocks.[11]
In this sense, the traveler reaches a later stage of other people’s lives while experiencing less elapsed time. Both clocks still advance along their own paths; this process does not return the traveler to an earlier event.[11]
Returning to an event in your own past
A closed timelike curve, or CTC, is a future-directed timelike path that closes on itself in spacetime. Going around a planet returns you to a place at a later time. A CTC returns to the same spacetime event, making a causal loop possible.[12]
Some mathematical solutions contain such curves. Gödel’s rotating cosmological model is a famous example; its properties do not describe our observed universe. Finding a solution of the equations does not establish that nature can form that spacetime from realistic conditions.[13]
How could two mouths produce a time loop?
In the Morris–Thorne–Yurtsever thought experiment, relative motion can make the mouths accumulate different proper times while their wormhole connection persists. Once they are brought into a suitable arrangement, passing through can link events with different times on external clocks.[2]
This construction assumes that the wormhole and its supporting fields survive the procedure. It is a conditional argument about a geometry, not an established engineering method.[2]
The offset exceeds the transit correction plus the exterior journey. Returning to B early leaves time to wait and close the loop.[14]
A time shift alone does not guarantee a CTC. It must be large enough, relative to the available return route, to let the complete journey close.[2]
Would nature prevent a time machine?
Paradoxes ask whether a history is consistent
The grandfather paradox imagines an action that removes the conditions for that same action to occur. Classical studies of CTCs can instead ask whether self-consistent histories exist. Some toy models have consistent solutions, sometimes more than one. This does not prove that time machines exist or that travelers could freely rewrite a recorded past.[15]
Hawking’s chronology protection conjecture
Hawking proposed that physical laws may prevent closed timelike curves from forming. Quantum fields could respond strongly as a chronology-violating region approaches formation, altering the geometry through backreaction. These arguments motivate chronology protection; they are not a universal proof against every possible causal loop.[16]
Kay, Radzikowski, and Wald proved a more specific obstruction: for their class of horizons and quantum fields, standard quantum-state regularity fails at certain horizon points. The usual renormalized stress-energy construction then breaks down there. This does not establish infinite energy everywhere, or determine what a complete quantum-gravity theory would do.[17]
What does topological censorship rule out?
Under assumptions including asymptotic flatness, global hyperbolicity, and an averaged null energy condition, topological censorship restricts causal access to nontrivial spacetime topology. Changing those assumptions changes what the theorem establishes. It is distinct from both chronology protection and cosmic censorship, which concern different questions.[18]
Why do wormholes appear in entanglement research?
ER = EPR is a conjectured connection
Maldacena and Susskind proposed a relationship between Einstein–Rosen bridges and quantum entanglement. Certain entangled black-hole states have geometric descriptions involving bridges. The broader ER = EPR proposal asks how far that relationship extends; it does not turn every entangled pair into a usable tunnel.[19]
Traversability in controlled holographic models
Gao, Jafferis, and Wall showed that coupling the boundaries of a particular anti-de Sitter model can produce negative averaged null energy and make its bridge traversable. The construction includes an explicit interaction between the two sides and respects causality.[20]
Such models are valuable because gravity and quantum information can be studied in complementary descriptions. Their boundary conditions and interactions are part of the physics; they do not automatically supply a method for opening a shortcut between distant locations in our universe.[20]
What the quantum-processor experiment did
In 2022, researchers implemented a simplified quantum model on a nine-qubit circuit and studied teleportation dynamics they interpreted through a gravitational description. The experiment explored the proposed relationship between quantum dynamics and wormhole physics.[21]
Caltech explicitly explained that the experiment did not create an actual spacetime wormhole. It was a controlled quantum system, not a portal through which an object could travel.[22]
Follow-up researchers challenged how strongly the simplified model supported the gravitational interpretation; the original team defended the experiment’s scope. That debate concerns the interpretation of the model, not an observed tunnel opening in the laboratory.[23], [24]
How could astronomers search for a wormhole?
Researchers calculate how hypothetical geometries would bend light or affect radiation from nearby matter. In some models, an image can closely resemble a black-hole image when the source and observer are on the same side. Light arriving through the throat from the other side can produce different patterns.[25]
These are model predictions. A useful test must specify the geometry, the light source, and the observing conditions. An unusual bright ring by itself would not identify a wormhole.[25]
Gravitational waves offer another route. Certain compact, horizonless models can imitate the early ringdown of a black hole, while differing at later times. A claimed difference would need careful comparison with ordinary astrophysical explanations, detector effects, and the assumptions behind each model.[26]
No confirmed observation has established a traversable spacetime wormhole. Studying a geometry’s possible signal, or implementing related equations on a processor, is a different achievement from detecting the object in nature.[22]
What would turn a model into a physical proposal?
A credible proposal must connect several pieces: a spacetime geometry, a consistent source of stress-energy, a way the configuration could arise, and a description of how it responds to disturbances. Allowing an additional kind of field or changing the gravitational theory changes the problem, and introduces further predictions to test.
Even a traversable mathematical solution need not be comfortable for a traveler. Tidal forces, radiation, throat dimensions, and the time available for passage all matter. These are separate questions from whether the route exists at all.[5]
Wormholes remain useful to physics because they force precise questions about geometry, energy, and information. Time-loop models do the same for causality: they reveal which assumptions make a history predictable and where familiar descriptions stop being reliable.
Science fiction can choose a stable gateway as its starting premise. Physics asks what would support that premise—and what evidence would let us recognize it in the universe.
Sources and further reading
Original relativity and quantum-field research, laboratory reports, and model-based observational studies. Sources checked in September 2026; all wormhole illustrations are hypothetical.
- Max Planck Institute for Gravitational Physics — WormholeIntroduces hypothetical mouths, shortcuts, and time shifts while distinguishing models from established physical objects.
- Morris, Thorne and Yurtsever (1988) — Wormholes, Time Machines, and the Weak Energy ConditionA conditional construction turns differential mouth aging into a time offset through an assumed traversable wormhole.
- Einstein and Rosen (1935) — The Particle Problem in the General Theory of RelativityThe original bridge was proposed as a geometric model of particles that avoided field singularities.
- Collas and Klein (2011 preprint) — Embeddings and Time Evolution of the Schwarzschild WormholeThe maximally extended Schwarzschild bridge cannot be crossed between its exterior regions, even by light.
- Morris and Thorne (1988) — Wormholes in Spacetime and Their Use for Interstellar TravelA static traversable geometry requires a regular throat, no horizon, and acceptable passage conditions.
- Kontou and Sanders (2020) — Energy conditions in general relativity and quantum field theoryDefines the null and weak energy conditions and their perfect-fluid forms, including the role of pressure.
- Fewster (2012) — Lectures on quantum energy inequalitiesExplains quantum negative energy, reference-state subtraction, and bounds on suitably averaged energy densities.
- Lamoreaux (1997) — Demonstration of the Casimir Force in the 0.6 to 6 micrometre RangeReports a torsion-pendulum measurement of the Casimir force between closely spaced conducting surfaces.
- David Tong — Quantum Field Theory, Chapter 5: Quantizing the Dirac FieldShows particles and antiparticles as positive-energy excitations above the vacuum in the quantized Dirac field.
- Ford and Roman (1996) — Quantum field theory constrains traversable wormhole geometriesApplies quantum energy inequalities to static spherical wormholes and finds severe restrictions on their length scales.
- Chou et al. (2010) — Optical Clocks and RelativityOptical clocks measured the unequal elapsed times predicted from motion and gravitational potential differences.
- Francisco S. N. Lobo (2010) — Closed Timelike Curves and Causality ViolationA closed timelike curve returns to its starting spacetime event while remaining locally future-directed.
- Ellis (2000) — Editor’s Note on Gödel’s Cosmological SolutionsGödel’s rotating, non-expanding cosmology contains closed timelike curves but does not describe the observed expanding universe.
- Kip S. Thorne (1992) — Closed Timelike CurvesWormhole mouth identifications can form closed causal routes in a conditional spacetime construction.
- Friedman et al. (1990) — Cauchy Problem in Spacetimes with Closed Timelike CurvesSelf-consistency requires local events to fit a global solution; it does not establish branching timelines.
- Hawking (1992) — Chronology protection conjectureProposes quantum backreaction as an obstacle to creating closed timelike curves.
- Kay, Radzikowski and Wald (1997) — Quantum Field Theory on Spacetimes with a Compactly Generated Cauchy HorizonProves failure of standard quantum-field regularity at base points of a compactly generated Cauchy horizon.
- Friedman, Schleich and Witt (1993; corrected preprint 1995) — Topological CensorshipRestricts causal access to nontrivial topology under asymptotic flatness, global hyperbolicity and averaged null energy assumptions.
- Maldacena and Susskind (2013) — Cool Horizons for Entangled Black HolesProposes the ER = EPR connection between entanglement and bridge geometries.
- Gao, Jafferis and Wall (2017) — Traversable Wormholes via a Double Trace DeformationA boundary interaction produces traversability in a controlled anti-de Sitter model without violating causality.
- Jafferis et al. (2022) — Traversable Wormhole Dynamics on a Quantum ProcessorA nine-qubit experiment implements a simplified quantum model and interprets its teleportation dynamics through holography.
- Caltech (2022) — Physicists Observe Wormhole Dynamics Using a Quantum ComputerThe experiment’s institution explicitly explains that no actual spacetime wormhole was created.
- Kobrin, Schuster and Yao (2023) — Comment on Traversable Wormhole Dynamics on a Quantum ProcessorQuestions whether the simplified model uniquely demonstrates the claimed gravitational dynamics.
- Jafferis et al. (2023) — Response to the Comment on Wormhole DynamicsThe experiment’s authors defend their interpretation and clarify the scope of the model.
- Chen et al. (2024) — Observational Signatures of Traversable WormholesModel images can resemble black holes or differ when light reaches observers through the throat.
- Cardoso, Franzin and Pani (2016) — Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?Compact horizonless models can imitate early black-hole ringdown, motivating careful tests of later-time behavior.
The Nature of Space and Time
- Special Relativity: Time Dilation and Length Contraction
- General Relativity: Gravity as Curved Spacetime
- Quantum Mechanics: Wave-Particle Duality
- Quantum Field Theory and the Standard Model
- Black Holes and Event Horizons
- Wormholes and Time Travel · You are here
- Dark Matter: Hidden Mass
- Dark Energy: Accelerating Expansion
- Gravitational Waves
- Toward a Unified Theory