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Michael S. Morris, Kip S. Thorne, Ulvi Yurtsever | Physical Review Letters | (1988)
Primary Link Pubmed DOI Openalex

Key Takeaways

Plain English Takeaway

If it were possible to build a stable tunnel through space, it could also be used to travel back in time, which might break the usual rules about cause and effect.

Study Aim

The paper aims to explore whether the laws of physics allow for the creation and maintenance of a wormhole (a tunnel-like shortcut through space) that could be used for interstellar travel. The authors also investigate if such a wormhole could be turned into a time machine, potentially allowing events to happen out of order and challenging the principle of causality (the idea that causes always come before effects). Simply put: The paper asks if making a stable space tunnel is possible, and if so, whether it could let people travel through time.

Study Design

The authors use theoretical analysis grounded in general relativity and quantum field theory. They discuss the requirements for a traversable wormhole, focusing on the need to violate the weak energy condition (WEC), which is a rule stating that energy density must always be positive for any observer. The paper examines whether quantum effects or advanced technology could allow for the existence of matter that breaks this rule, and what consequences this would have for the structure of spacetime and the possibility of time travel. Simply put: The study uses math and physics theory to see if the rules of the universe allow for space tunnels and time travel.

Findings

The research demonstrates that if an advanced civilization could create and keep open a traversable wormhole, it could be transformed into a time machine, making it possible to violate causality. The authors argue that the existence of such wormholes depends on deep and unresolved questions in physics, such as whether quantum field theory enforces an averaged version of the weak energy condition. They highlight that creating and maintaining a wormhole would require exotic matter with negative energy density, which is not known to exist in large amounts. The paper suggests that the possibility of time machines raises profound challenges for our understanding of the universe and may require new physics to resolve. Simply put: The study finds that if space tunnels can exist, they could let people travel back in time, but making them work would need strange kinds of matter we don't know how to create.

Abstract

It is argued that, if the laws of physics permit an advanced civilization to create and maintain a wormhole in space for interstellar travel, then that wormhole can be converted into a time machine with which causality might be violatable. Whether wormholes can be created and maintained entails deep, ill-understood issues about cosmic censorship, quantum gravity, and quantum field theory, including the question of whether field theory enforces an averaged version of the weak energy condition.

Referenced In

StarTalk

Season 17, Episode 31: The Casimir Effect (and Wormhole Maintenance)

Hey StarTalkers! Season 17, episode 31 was another grab-bag Cosmic Queries edition, with Neil and Chuck going through a bunch of reader questions about everything “from wine to wormholes.” The second-to-last question was a really wild one:

Cosmic Queries – From Wine to Wormholes - StarTalk Radio

(from 48:00)

Neil and Chuck are floored, and I was too. So what is this person even talking about? Is Neil right about the Casimir effect only being attractive?

The Casimir Effect Explained

Hendrik Casimir’s original paper explains the effect fully in mathematical terms, but you can get a good idea of what’s going on without the math.

Casimir was studying van der Waals forces – an attractive force between neutral molecules – and was struck by a question. What if there were two mirrors facing each other in a vacuum instead of molecules?

In the same way quantum fluctuations create virtual particles in seemingly empty space, electromagnetic vacuum fluctuations would happen between these mirrors (and outside of them, too).

In free space, the whole spectrum of wavelengths can exist. But if the mirrors were close enough together, specific waves would be amplified while others were cancelled out. This would leave only waves where a whole number of half-wavelengths fit in the gap (see below).

These fields contain energy, but there is an imbalance. Outside, we have all wavelengths, but inside, only selected ones. This creates a difference in radiation pressure, pushing the plates microscopically closer together. This is the Casimir effect.

Neil’s Slight Mistake: Casimir Forces Can Separate Plates

Neil makes a slight error when he said that it’s a purely attractive force. Under some circumstances, with carefully-chosen materials, you can also make repulsive Casimir forces. It’s not normal but it is possible.

Where the Question Comes From

The mind-bending question likely has its origins in a 1988 paper co-authored by Kip Thorne, which has an interesting section on “wormhole maintenance.” In it, the authors wonder how a highly advanced civilization might be able to keep a wormhole they created open, and suggest that they may use perfectly conducting spherical plates to produce a repulsive Casimir effect to kind of pry it open.  

Is it possible? Even the authors of the paper didn’t know. Given the challenge in even testing Casimir’s idea on Earth, it seems practical concerns would stop them even if theory doesn’t.

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