**By Penthar:**

The schemes for time travel from established science tend to fall into a few categories:

Special relativity is the theory with all that jazz about traveling near the speed of light. Changes in masses, lengths, and most importantly here, time.

This is what you might call a one way ticket to the future; traveling near the speed of light slows time down for you relative to somebody not along for the trip. It’s the Planet of the Apes scenario (made a bit more clear in the Boulle book than in the Heston movie but eh).

Backwards travel doesn’t seem to come up, unless you get into the speculative world of tachyons, which would travel faster than light (and we have a few threads around here dedicated to them). But those aren’t thought to really exist and even if they did, it doesn’t seem they’d let us travel back through time.

**General relativity**

This is essentially a more thorough version of special relativity, that actually amounts to a theory of gravitation. This is the one that talks about mass/energy curving spacetime. Much more info on that is available in What causes gravity? , Thoughts/Theories on Gravity, and particularly Space Displacement by Matter as Related to Gravity

“Since arrangements of mass/energy shape both space and time in this theory, it becomes possible to manipulate spacetime in such a way that you get what’re called closed timelike curves. To take something I said a while back in another thread:

There are certain solutions that contain curves that would allow some observer to travel into his future but arrive in his past. Normally you follow a path through spacetime that leads from past to future–these special paths, however, loop back and intersect themselves in the past. In GR these timeloops are called closed-timelike curves.

The problem is that to get spacetime to act like that you need to figure out what configurations of mass and energy will do the trick. Goedel found the first one a half century ago–in a non-expanding rotating universe it’s possible to travel around and follow one of these time loopy paths (the trick lies in the fact that if you send off a beam of light in such a universe it will seem to do a kind of U-turn as the universe revolved around it–so you can take a kind of shortcut and head it off).

Others ways to produce these closed timelike curves involve some of the weird arrangements of matter you’ve probably heard of if you’ve ever looked into time travel–rotating, infinitely long cylinders or Gott’s idea of two cosmic strings (very dense, very thin, very long massive filaments out in space–theoretical toys as of now) flying past each other both create geometries that allow timeloops to exist. Playing with wormholes can also lead to CTCs.

Some argue that such solutions to Einstein’s equations aren’t physical and that they’re only mathematical gags–they simply can’t arise physically. Others argue that nature conspires against such timeloops and that they’re immediately destroyed if they should ever arise (that’s Stephen Hawking’s Chronology Protection Conjecture)–so we (and apparently causality) are safe.

But many physicists take them very seriously and believe they may very well exist (or at least are able to exist). J.R. Gott (the guy with the cosmic strings above) has suggested that a closed timelike curve could explain where the universe came from–namely, itself. There are papers out of the Institute for Advanced Study suggesting that CTCs in computers could make solving difficult problems a lot quicker and a lot easier.”

General relativity also allows another kind of “one-way ticket to the future” time dilation that doesn’t involve zipping around at near-light speeds. This variety is called gravitational time dilation and relies on the fact that clocks tick more slowly in gravitational wells–clocks at the base of a very tall tower tick ever so slightly more slowly than clocks in the top-floor penthouse. The stronger the gravitational field, the more pronounced the effect. If you take a bunch of mass and construct a kind of shell around yourself or take a little trip down near the (very hazardous) surface of a neutron star, you could wait a while then climb out to find time outside your gravitational well was going faster and you’re “in the future.”

There are other senarios, like this one (from an old paper on this subject called Constructing Time Machines):

*The spacetime describing a gravitational shock wave exhibits the unusual property that, depending on their impact parameter, geodesics which cross the shock may experience a discontinuous jump backwards in time. In this section, we investigate whether this phenomenon can be exploited to construct a time machine.*

Unfortunately, after analyzing what happens if two such shock waves head straight for each other, the author concludes that answer to that one is “no.” But even though that one doesn’t look like it works out for would-be time travelers, the fact that there are such possibilities at all should be encouraging.

Other oddities of spacetime geometry that general relativity might allow to exist could also function as time machines. Wormholes would be links between different spots in space and, conceivably, time. A Caltech physicist named Kip Thorne and a couple grad students wrote a paper a number of years ago analyzing how a wormhole could conceivably be not only constructed but turned into a time machine. So there’s that.

**Quantum Mechanics**

This is another area of physics time-travel enthusiasts sometimes look to; this is the way things operate on the very smallest of scales.

In (I believe) the second installment of our This Week in Science threads in the science forum, the first story was a New Scientist article concerning work that seemed to indicate that (while allowing travel backwards through time) quantum mechanics doesn’t allow paradoxes. Convenient. :

**“No paradox for time travellers”**

“THE laws of physics seem to permit time travel, and with it, paradoxical situations such as the possibility that people could go back in time to prevent their own birth. But it turns out that such paradoxes may be ruled out by the weirdness inherent in laws of quantum physics.

Some solutions to the equations of Einstein’s general theory of relativity lead to situations in which space-time curves back on itself, theoretically allowing travellers to loop back in time and meet younger versions of themselves. Because such time travel sets up paradoxes, many researchers suspect that some physical constraints must make time travel impossible. Now, physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria have shown that the most basic features of quantum theory may ensure that time travellers could never alter the past, even if they are able to go back in time.

The constraint arises from a quantum object’s ability to behave like a wave. Quantum objects split their existence into multiple component waves, each following a distinct path through space-time. Ultimately, an object is usually most likely to end up in places where its component waves recombine, or “interfere”, constructively, with the peaks and troughs of the waves lined up, say. The object is unlikely to be in places where the components interfere destructively, and cancel each other out.

Quantum theory allows time travel because nothing prevents the waves from going back in time. When Greenberger and Svozil analysed what happens when these component waves flow into the past, they found that the paradoxes implied by Einstein’s equations never arise. Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place (www.arxiv.org/quant-ph/0506027). “If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you,” says Greenberger. “

“This is a very nice idea,” says physicist Avshalom Elitzur of the Weizmann Institute in Rehovot, Israel, who also suggests that further work in the area could help to clarify the nature of time itself. “Time is a very mysterious thing.”

In fact, in my thread Just one of those mysteries… I slipped into discussing a theory cooked up six decades ago by the physicists John Wheeler and Richard Feynman that used waves going back through time to explain why electrons feel a force when they try to accelerate: ;;

*Wheeler and Feynman used the fact that the equations of electromagnetism are symmetric in time, working backwards as well as forwards. They allow not only the so-called retarded waves of radiation that we’re used to (that arrive somewhere after they left) but also “advanced” waves (that arive somewhere before they were emitted””that is, travel backwards through time). In their model, the electron accelerates and emits radiation that goes forward in time (ignoring advanced waves) and eventually causes another particle (called the absorber) to accelerate and, in turn, emit retarded and advanced waves. The advanced waves are going backwards through time so they arrive back at the original electron at the exact instant it accelerated in the first place, providing the radiation reaction force that makes the electron resist acceleration.*

That particular thread was on inertia (the resistance of masses to accelerations) so the idea that inertia itself might be due to similar time traveling waves was floated. Anyway, the point is that the quantum world is no stranger to weirdness with time.

**Subilluminatus replied**

That’s a man-sized chunk of reading there but at a glance it looks like alot of this speaks to what I was referring to earlier about relative time dilation as opposed to actual movement through time. And how the theories that do propose genuine time travel seem to always be based on cold calculations that, so far as I can see, real world factors would render entirely irrelevant.

Without having really dug into your links yet, are you personally of the belief that genuine time travel is possible, or…? By “genuine time travel” I mean actually travelling back or forward in time. For example, travelling back from today to the year 2000

**Penthar replies**:

There are a few things to note here. Physics isn’t done yet. There currently exist two very powerful theories (general relativity and quantum mechanics) that cover different domains and facets of the physical universe. Unifying them into one theory would yield a new one which would have to itself be probed to see what time travel schemes it does or doesn’t allow.

But until such a unified theory exists, you can only look at the current (very good, by the way) individual theories that exist now to see what they allow. And one thing general relativity unquestionably allows is “travel to the future” via time dilation. However unsatisfying a mode of time travel that is to you it is the only one that’s been observed experimentally and indeed is a central component of the theory.

Further, taking a step into the slightly more speculative, Sergei Krasnikov has played around and come up with a theoretical kind of “tube” a relativistic spacecraft could unfurl behind it as it travels that would allow the ship to return home shortly after it left, regardless of how far it went or how long it was gone. Not quite time travel as you’re considering it but interesting anyway.

With that in mind you should realize that until this kind of thing (aside from time dilation) can be demonstrated experimentally every scheme for time travel will necessarily be “cold calculations”–that is the nature of theoretical work. I’m not sure what you mean by saying real world factors would make, say, closed timelike curves irrelevent.

I imagine you either mean (1) they can’t actually occur in the real world, in the spirit of the Chronology Protection Conjecture mentioned above or (2) it would be very hard to ever create them. If it’s the latter I hate to break it to you but time travel (if it ever becomes technically feasible) won’t be easy.

You may indeed need to do something ridiculous like getting two rotating black holes together or whatnot. If that’s the kind of thing you mean by real world limitation then you should brace yourself because it doesn’t seem like time travel (regardless of what the original articles says) is going to be possible for a long while.

My post above was just to give a very brief overview of what is allowed theoretically today. If you want to know how time travel could work with real (known) physics, that’s it. The stuff in my post above (along with a few variations on the themes) are pretty much all that exists. Note that the UConn professor from the original post in this thread is talking about generating a closed timelike curve. His original paper on this stuff 3 years ago was about how solutions to the Einstein field equations containing closed timelike curves can be found when considering the gravitational field of a cylinder of light.

As for whether I believe time travel is possible, I would lean toward yes. Hell, I don’t think it’s implausible that you generate a little bit of time travel every time you lift up a heavy textbook (though that goes into something a little different than Time Machine-style time travel).

With enormous thanks to Penthar and all involved in this thread called Professor Predicts Human Time Travel This Century