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Ask Brian Greene: Wormholes, Time Travel, and Causality

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bg bwBrian Greene continues the WSF Live Forum all month long. Each day, he’ll answer one of your questions for this ongoing series that delves into the fundamental nature of space, time, and reality as we may or may not know it.

If you traveled through a wormhole to an earlier time, would your presence create a “new” version of that earlier time or would it be the “original” version of that time?
— via physicsforums.com

First, bear in mind that while the notion of a wormhole finds a natural place in the mathematics of Einstein’s general relativity, there is no experimental or observational evidence that wormholes exist. Moreover, even if they did exist, it is unclear whether one could generate a macroscopic wormhole and keep it open, allowing a human to safely travel through it. So, you should consider the whole collection of ideas that includes wormholes and time travel as highly speculative.

OK. Let’s speculate. If you did have a wormhole—a tunnel from one location in space to another—hen it’s pretty clear how to turn it into a time machine: move the openings of the wormhole relative to one another (so that time at the two openings elapses at different rates), or let the two wormhole openings experience a different gravitational potential (keep one opening near the edge of a black hole, the other opening far from the black hole), as this too causes time at the two openings to elapse at a different rate. Either of these procedures will cause the two wormhole openings to no longer be synchronized, to no longer occupy the same moment in time. Passing through the wormhole would thus not only take you from one place to another, but also from one time to another.

Now to the question: If any of this is really possible (again, hugely speculative), and if you move through a wormhole and find yourself back, say, on January 1, 1900, then your being there is an immutable property of that moment in time. There is one and only one version of January 1, 1900 (ignoring more exotic speculations involving the many worlds approach to quantum mechanics) and your presence would be one of its defining features. By passing through the wormhole today and going back to that earlier time, you would be fulfilling your ironclad destiny to appear at that earlier moment. If you think of all of space throughout all of time as a giant block (in NOVA’s Fabric of the Cosmos, episode 2, we schematically illustrate this), then the wormhole would provide a tunnel with one opening here today, and the other opening somewhere else (say on earth) but reaching back along the block to January 1, 1900. One interesting point: If you think about it for a moment, either of a wormhole’s openings can never appear at an earlier time than the first moment that the wormhole was created. Remember, to create a wormhole time machine, we cause one wormhole opening or the other to pass through time more slowly than the other (as above, using a black hole for example) but each opening is still going forward in time. So based on the wormhole’s history, there is generally a limit to how far back in time it would allow you to travel.

But the bottom line is that your presence back in time would be part of the “original” version of that moment–as that moment has but one incarnation.

If you want to see any of this spelled out in more detail, borrow a copy of the Fabric of the Cosmos book from your local library and check out chapter 15, where I go through the above in a more step by step manner.

See other questions from this series

Brian Greene is co-founder of the World Science Festival and professor of physics and mathematics at Columbia University. His books include The Elegant Universe, The Fabric of the Cosmos, and The Hidden Reality. NOVA’s miniseries “The Fabric of the Cosmos” airs Wednesday nights on PBS.

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