In this week's "Ask a Physicist" we're going to give some design specs to the practical time traveler.

Astute readers of the "Ask a Physicist" column may have noticed that I've had a time obsession over the last few weeks, leading up to a big time travel extravaganza. A number of you took the bait that I put out last week and asked me (via mail, twitter, and facebook) about time machines. Abe Rudo put it perhaps most succinctly:

What's standing between us and time travel? Is it true that we'll only really be able to move forward in time (as a result of relativistic travel, for example), or is backwards travel just as feasible once the mechanism is discovered?

This is a controversial subject, so let me get a couple of things out of the way right from the start:

- I'm not going to get all squishy and philosophical on you, at least not here. You guys know about your grandfather paradoxes and whatnot, so you should feel free to debate them in the comments section.
- I'm going to be a bit of a Debbie Downer here. I'll you from the outset that time travel into the past might be impossible from the standpoint of physics. The upside, though, is that we don't actually
*know*that it's impossible. - I'm only going to talk about general relativity time machines. While they may turn out to be impossible, they're
*less*impossible than non-general relativity time machines. What I mean is this: general relativity is currently our best, most widely accepted, most convincingly proven theory to describe time. And it seems to have solutions that allow for time machines. You may have your own crazy-ass theory about time that falls outside GR, and you're entitled to it, but that doesn't mean it's likely to be true. - I'll get my obligatory TARDIS, phone booth, or Delorean references out of the way early. For "realistic" time travel, you're almost certainly going to need a spacecraft, and perhaps one going a sizable fraction of the speed of light.

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Now that I've got my scolding out of the way we can get down to business. General relativity makes a number of big predictions, including the idea that mass warps space-time. If you missed my column on falling into a black hole, the effect of this is that time runs slower near a massive body than far away. You could build a time machine, of a sort, by hanging out near the edge of a black hole, allowing the universe to age must faster than you, and then returning home... to the future. But since you were going to wind up in the future if you just waited long enough, this is kind of stretching the definition of time travel.

What we really want to do is go into the past. If GR allows us to bend time, will it let us fold it back on itself?

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*Sculpture by Alan Rorie*

**Time Machine Designs**

There have been a ton of time machines designs, even if we only talk about the ones allowed by GR. However, some of them involve having the entire universe rotate (designed by Kurt Godel in 1949) or infinitely long rotating cylinders (designed by Frank Tipler in 1974) don't work because the universe isn't rotating and it doesn't appear to be filled with infinitely long rotating cylinders. However, there are a few others that we can't rule out so simply.

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__Wormholes__

You're reading io9, so I probably don't need to review the wormhole basics. But humor me anyway. A wormhole has two "mouths" connected by a "throat." Each of the mouths is a sort of three-dimensional funnel, and gravitationally, they look a bit like a black hole from a distance. You go in one mouth of a wormhole, and you come out the other end somewhere else in space. The cool part of this is that the length of the throat doesn't have anything to do with how far you travel through space, so in principle, these could be used to travel enormous distances in very short order, which is why sci-fi writers love them.

For our purposes, one of the most important features of wormholes is that if two people sitting near the opposite mouths observer each other through the wormhole, their clocks will appear to stay synchronized, even if you pick up one of them and fly them around at close to speed of light.

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I've talked about this sort of thing before, but remember how it's *supposed* to work. If you have twins, one of whom just sits around while the other flies around the universe at close to the speed of light, then when you bring them back together again, the traveling twin will have aged considerably less than the stay-at-home twin.

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We can do the same thing with the mouths of the wormhole. You fly around one mouth, and just as with the "twin paradox" the traveling side has aged less than the stay-at-home side. You can use this time difference to travel a fixed amount of time either into the future or into the past. Here is a little movie I made to illustrate this device when giving talks on time travel.

But there are problems.

For one thing, wormholes may not exist on any scale, and it's really not obvious how we'd make them if they're not already lying around. There is speculation, of course. On the very smallest scales (about 10^{-35} meters or so), the fabric of space-time may look a lot like a "quantum foam." In that case, tiny wormholes may pop into and out of existence on very short timescales. But this is far from certain. We don't really know exactly *what* happens on those scales.

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Secondly, even if microscopic wormholes exist, we don't have any real idea how we'd inflate them to human size.

But supposing we had a wormhole big enough to fly a spaceship through, what then? In early wormhole models, like the one Einstein and Rosen came up with 75 years ago, the answer was heartbreak. The Einstein-Rosen bridge had the problem of collapsing immediately before even a single photon could make it through. That was probably okay with Einstein, though. Even before this fatal flaw was discovered, he realized that wormholes could be used as time machines, and he did not see this as a good thing.

In the 1980's, Kip Thorne and his students came up with a better, more stable, wormhole model. Even if a) microscopic wormholes exist, and b) we could (somehow) figure out how to inflate them to scales big enough to fit a person through, c) we still have the problem of keeping them open. Thorne proposed "exotic energy."

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I know some of you have a problem with Dark Energy. But exotic energy is even worse. Dark energy is a substance with a tension (negative pressure) equal to its energy density. It's what's supposed to be accelerating the universe, and it's exactly the sort of thing measured in the "Casimir Effect," but our theoretical estimate is far higher (by a factor of 10^{100}) than what is actually observed cosmologically.

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What makes exotic energy worse is that it needs an even higher tension than dark energy — which is exactly the sort of thing that's supposed to be forbidden. (The reason is a bit convoluted, but the short explanation is that it would be possible to have a moving observer for whom the exotic energy density would be negative). It's been argued by some that in the environments around black holes and wormholes, exotic energy might be *locally* possible, but that's not certain.

In other words, wormholes, and thus wormhole time machines, may well be impossible.

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__Cosmic Strings__

There are other options, and one of my favorites involves cosmic strings. Cosmic strings are another one of those sci-fi staples. The basic idea is that they are incredibly dense, infinitely long, and incredibly thin structures. They also have the rather unfortunately property of perhaps not existing.

In the 1990's Rich Gott designed time travel based on a pair of cosmic strings. They can be used for a sort of one-time use time machine. Essentially, you take one string and move it in one direction at close to the speed of light, take the other string and move it in the opposite direction, and then fly around the entire getup. Here, my co-author Jeff Blomquist made a handy schematic:

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If you've done everything correctly, you should arrive back at the beginning before you left. Of course, even if cosmic strings exist (again, a big "if") you have to *really* want to make the trip, since each time machine is a single use contraption.

**What do all of these have in common?**

Rather than make a bunch of general pronouncements about how time machines *must* work, let me point out a few features that these models have in common.

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- You can't go back before the time machine was built. These models (and the Godel and Tipler time machines, as well) are all based on the idea of solving Einstein's field equations of GR. Since the solution must exist at both the entrance and exit ramp, you can't ever go back to before the time machine (or
*a*time machine) was built. This potentially answers Steven Hawking's famous question of where all the time travelers are. - The timeline is self-consistent. General relativistic solutions have to be self-consistent, which is what generally causes the brain-hurt. It's hard to figure out how to deal with a single-history version of time travel when you're dealing with big complex things with free will like us or (to a lesser extent) the borg. However, Igor Novikov showed in the 1980's that microscopic, time-traveling quantum systems
*must*be self-consistent or their probabilities go to zero. Since we're made up of big collections of quantum systems, presumably, the same rules hold for us. Feel free to debate about free will versus determinism in time travel... now. - They're based on physical phenomena that may or may not exist or be possible. That may be how chronology protection (aka "no time machines") comes into play. Sorry.

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I know some of you are going to get all excited because I haven't talked about Everett's "Many Worlds" version of quantum mechanics. That's true. Simply put, we're talking about GR time machines and the Many Worlds Interpretation of quantum mechanics doesn't have anything to say about time in GR. Of course, without parallel universes, we're left with the real possibility of paradox, and that's too damn bad.

*Dave Goldberg** is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on twitter, facebook or our blog.) He is an Associate Professor of Physics at Drexel University. Feel free to send email to askaphysicist@io9.com with any questions about the universe.*

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