Einstein's equations allow wormholes - shortcuts through spacetime connecting distant points. But to keep one open, you'd need "exotic matter" with negative energy. Whether such matter can exist is unknown, whether it should exist is debated.
MikeRun, CC BY-SA 4.0, via Wikimedia Commons
A bridge too far
The idea of a shortcut through spacetime is based on Albert Einstein's theory of general relativity. In 1935, Einstein and physicist Nathan Rosen explored a mathematical solution to the field equations that connected two different points in spacetime. This structure, formally called an Einstein-Rosen bridge, is the earliest scientific model of what we now call a wormhole. These original wormholes, however, were not traversable. They would form and then collapse so quickly that not even a beam of light would have time to make it from one end to the other. Any object attempting to pass through would be destroyed by the intense gravitational forces as the bridge pinched off. The tunnel would be a one-way trip to oblivion, not an interstellar highway.
The exotic matter prescription
The modern concept of a stable, traversable wormhole emerged from the work of physicists Kip Thorne and his graduate student Mike Morris in 1988. They were originally investigating a plausible method of faster-than-light travel for Carl Sagan's novel Contact. They determined that to hold a wormhole open and prevent its collapse, one would need a substance with unusual properties. This material, called "exotic matter," must possess a negative energy density.
Normal matter, from planets to people, has positive energy density, which creates attractive gravity. Exotic matter would do the opposite; it would be gravitationally repulsive, pushing spacetime apart. This outward pressure is necessary to counteract the wormhole's natural tendency to collapse in on itself. Such matter would violate what physicists call the null energy condition, a rule stating that the energy density of a region of space can't be negative. While no macroscopic amount of exotic matter has ever been observed, a real-world quantum mechanical phenomenon called the Casimir effect shows it is possible. When two uncharged conductive plates are placed very close together in a vacuum, the space between them has a lower energy density than the space outside, creating a measurable attractive force. This demonstrates that regions of negative energy density, relative to the surrounding vacuum, can exist.
The paradox problem
Even if exotic matter could be manufactured, building a stable wormhole presents further theoretical challenges. The amount of negative energy required would be immense. To hold open a wormhole with a "throat" just one meter in diameter would require an amount of negative energy equivalent to the mass of the planet Jupiter.
A stable wormhole could also function as a time machine. As theorized by Morris, Thorne, and Ulvi Yurtsever in 1988, one could accelerate one mouth of the wormhole to near the speed of light and then return it. Due to time dilation, less time would have passed for the moving mouth than for the stationary one, creating a pathway into the past. This allows for causality violations, such as the grandfather paradox. To address this, Stephen Hawking proposed the "chronology protection conjecture" in 1992. This idea suggests that the laws of physics, perhaps through quantum effects, would prevent the formation of such time loops, possibly by causing a feedback loop of vacuum energy that would destroy any wormhole before it could be used to violate causality.
💡Fun Facts
The first wormhole models, known as Einstein-Rosen bridges, would collapse too quickly for even a single photon to pass through.
The Casimir effect, a real quantum phenomenon first predicted in 1948, creates a tiny region of negative energy density.
A stable wormhole could theoretically become a time machine if one of its mouths were accelerated to near-light speeds.
Stephen Hawking coined the term "Chronology Protection Agency" to describe the metaphorical aspects of physics that he conjectured would prevent time travel paradoxes.
