Time Travel? Don't Forget to Pack Your Wormhole


Challenges Ahead

There are several theories for how the laws of physics might work to prevent time travel through wormholes.

"Not only do we assume (time travel into the past) will not be possible in our lifetime, but we assume that the laws of physics, when fully understood, will rule it out entirely," said Robert Owen, an astrophysicist at Oberlin College in Ohio who specializes in black holes and gravitation theory.

According to scientists' current understanding, keeping a wormhole stable enough to traverse requires large amounts of exotic matter, a substance that is still very poorly understood.

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General relativity can't account for exotic matter -- according to general relativity, exotic matter can't exist. But exotic matter does exist. That's where quantum theory comes in. Like general relativity, quantum theory is a system for explaining the universe, kind of like a lens through which scientists observe the universe.

However, exotic matter has only been observed in very small amounts -- not nearly enough to hold open a wormhole. Physicists would have to find a way to generate and harness large amounts of exotic matter if they hope to achieve this quasi-faster-than-light travel and, by extension, time travel.

Furthermore, other physicists have used quantum mechanics to posit that trying to travel through a wormhole would create something called a quantum back reaction.

In a quantum back reaction, the act of turning a wormhole into a time machine would cause a massive buildup of energy, ultimately destroying the wormhole just before it could be used as a time machine.

However, the mathematical model used to calculate quantum back reaction only takes into account one dimension of space-time.

"I am confident that, since (general relativity) theory has not failed yet, that its predictions for time machines, warp drives and wormholes remain valid and testable, regardless of what quantum theory has to say about those subjects," Davis added.

This illustrates one of the key problems in theories of time travel: physicists have to ground their arguments in either general relativity or quantum theory, both of which are incomplete and unable to encompass the entirety of our complex, mysterious universe.

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Before they can figure out time travel, physicists need to find a way to reconcile general relativity and quantum theory into a quantum theory of gravity. That theory will then serve as the basis for further study of time travel.

Therefore, Owen argues that it's impossible to be certain of whether time travel is possible yet. "The wormhole-based time-machine idea takes into account general relativity, but it leaves out quantum mechanics," Owen added. "But including quantum mechanics in the calculations seems to show us that the time machine couldn't actually work the way we hope."

Davis, however, believes scientists have discovered all they can about time machines from theory alone, and calls on physicists to focus first on faster-than-light travel.

"Until someone makes a wormhole or a warp drive, there's no use getting hyped up about a time machine," Davis told LiveScience.

Accomplishing this will require a universally accepted quantum gravity theory -- an immense challenge -- so don't go booking those time-travel plans just yet.

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