Traversable Wormholes

In 1988, Kip Thorne dared to ask if a human could survive a trip through a wormhole. The answer: only if you could line its throat with the negative mass of Jupiter — matter that has never been seen and may not exist. Since then, quantum gravity has reimagined the idea, not as an engineering problem, but as a test of whether spacetime is made of entanglement.

The case for negative energy

A traversable wormhole is not a black hole. It has no event horizon. Enter one mouth, exit another — potentially light-years away — in finite time and without being spaghettified. But general relativity demands a price: the throat must be threaded with exotic matter, defined by negative energy density. This violates the Null Energy Condition (NEC), a bedrock assumption in classical gravity.

For a 1-meter-wide throat, calculations suggest −10²⁷ kg of negative mass is required — roughly the mass of Jupiter, but negative. No known form of matter or field produces this at macroscopic scales. Quantum effects like the Casimir vacuum between plates generate negative energy, but only in amounts like 10⁻⁹ J/m³ over micrometers — too weak by more than 50 orders of magnitude.

The Ford-Roman quantum inequalities further constrain how much negative energy can be concentrated in space and time. The more you try to amplify it, the shorter its duration. This is not a gap in engineering; it may be a law: nature forbids large, sustained negative energy densities.

Where quantum gravity complicates the picture

After 2016, traversable wormholes reappeared not in astrophysics, but in quantum theory. The Gao-Jafferis-Wall (GJW) protocol showed that in a holographic universe (via AdS/CFT), coupling two entangled quantum systems could create a negative-energy pulse that briefly opens a wormhole — without bulk exotic matter. The tunnel is sustained by quantum entanglement.

In 2021, Maldacena, Milekhin, and Popov constructed a 4D wormhole solution using Casimir energy from 10²⁰ species of massless fermions — an absurd number, but theoretically valid. It proved that exotic matter need not be inserted by hand; it can emerge from quantum fields in curved spacetime.

Then in 2022, a team including Jafferis reported a “wormhole on a quantum computer.” Using Google’s Sycamore (9 qubits), they implemented a holographic teleportation protocol based on the Sachdev-Ye-Kitaev (SYK) model. The qubit’s journey matched the expected dynamics of GJW wormhole traversal.

But it was not a wormhole in spacetime. It was a quantum simulation whose dual gravitational interpretation resembles a wormhole. No spacetime was warped. No negative energy was deployed. And in 2023, Kobrin et al. argued the scrambling dynamics matched generic quantum chaos, not uniquely holographic behavior. The consensus: a brilliant demonstration of duality, but not evidence a wormhole can exist in our universe.

What's unknown

We do not know whether traversable wormholes can exist in a universe with a positive cosmological constant. Every robust construction lives in Anti-de Sitter (AdS) space — a theoretical universe with negative curvature, unlike ours. In de Sitter-like space (ours), the cosmological horizon may prevent long-lived entanglement or negative energy buildup. Proposals exist, but none are as rigorous.

We also do not know if the ER=EPR conjecture — that entangled particles are connected by microscopic wormholes — can ever be tested. It suggests spacetime emerges from entanglement, but the conjecture applies to non-traversable, Planck-scale bridges (~10⁻³⁵ m). Making one traversable requires injecting classical information at light speed — defeating the purpose of a shortcut.

Finally, stability remains open. A 2024 study by Emparan et al. showed that higher-dimensional wormhole solutions collapse into black holes under perturbation — even a single photon passing through might trigger disaster. If all traversable wormholes are unstable, they are not shortcuts; they are traps.

Why this has to do with other realms

The wormhole problem has migrated from gravity into quantum information and thermodynamics. The amount of entanglement needed to open a wormhole mirrors the complexity of quantum circuits that scramble information, a direct link to concept holographic error correction. If spacetime geometry is a code, then building a wormhole is less like civil engineering and more like cracking a cryptographic hash.

It also forces a confrontation in philosophy of physics: if a simulated wormhole behaves like a real one, does the distinction matter? This echoes debates in concept simulation hypothesis, where the fidelity of a representation determines its ontological status. When the simulation obeys the same dualities as reality, the line dissolves.

An open question

If entanglement can open a wormhole in theory, and our universe is maximally entangled at the cosmological horizon, why are there no macroscopic wormholes in the sky?

Key sources

Morris & Thorne (1988), American Journal of Physics — original traversable wormhole paper
Gao, Jafferis & Wall (2016), arXiv:1608.05687 — GJW mechanism
Jafferis et al. (2022), Nature 612, 51–58 — “wormhole on quantum processor”
Emparan et al. (2024), JHEP — stability of higher-dimensional wormholes
Maldacena & Susskind (2013), Fortschritte der Physik — ER=EPR conjecture
to verify: quantum inequalities in de Sitter space — open problem in semiclassical gravity

Abhishek