Wormholes

What really is a wormhole?

In general relativity, gravity is not a force that acts on a stationary stage: it is the curvature of space-time itself. Some mathematical solutions contain two regions connected by a throat. The popular image of the tunnel is useful, but it should not be confused with a conduit dug into outer space: the shortcut is geometry.

The Einstein–Rosen bridge associated with an ideal black hole is not traversable. Morris and Thorne's models studied what conditions a gorge would have to meet for a person to be able to cross it without encountering a horizon or destructive tidal forces. The result made the traversable wormhole a rigorous tool for asking where the possibilities of relativity end.

From space shortcut to time machine

Let's assume two connected mouths. If one of them makes a trip at relativistic speed and returns, or remains near an intense gravitational field, it will accumulate less proper time than the other. The internal connection can preserve the phase shift. Entering through one mouth and leaving through the other would allow us to reach an earlier moment with respect to the outside. The machine would not travel to a date prior to its creation: the phase shift would begin when the mouths were synchronized and separated.

The decisive obstacle

The throat tends to collapse. In standard general relativity, holding it requires violating energy conditions, in particular the averaged zero energy condition. This distribution is called exotic matter: it is not antimatter or an unknown mineral, but an energy pattern with unusual gravitational properties. Quantum theory produces local negative densities, but we do not know how to accumulate them on the necessary scale and form.

What would have to be resolved

• Create or find a suitable topology.
• Stabilize the throat with energy compatible with quantum inequalities.
• Limit radiation, feedback and tidal forces.
• Prevent the formation of closed temporal curves from destroying the configuration.

That's why wormholes are valuable even if they can never be built: they connect exotic matter, chronological protection and quantum gravity in a single experimentally honest question.

Sources to deepen

Physical Review Letters: exotic energy and traversable holes · APS Physics: Modern Wormhole Models