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I'm looking to use time travel as a big element in my world. I want there to be a somewhat reasonable explanation for it though, so it doesn't turn into some magical wooble-booglly tech stuff. (credibility is an amazing way to engage a reader)

I've been reading A Brief History of Time, by Stephen Hawking. I read that like Gravity, there are other "Major" forces in the universe, such as Electro-magnetism. I also read somewhere that gravity can bend time (time would flow slower near heavy objects).

While I have no doubt these effects are negligible compared to a human life span, I was wondering if forces other than Gravity would also be able to bend time. And if so, would that mean time is basically another spatial dimension (like X, Y, Z) which we just can't perceive as such?

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The most rigorous depiction of a time travel device that I am aware of is the "Tipler Cylinder" (sometimes called the "T" machine).

The principle is rather simple, twisting or warping space-time in such a fashion that a light cone begins to point to "otherwhen", allowing access to different times (or places). While simple in concept, the mechanism to do so would require an infinitely long cylinder of neutronium spun to almost the speed of light in order to deliver the required torque to space-time. My local contractor hung up when I asked for a quote....

Light cone

The concept of a light cone explained. As you travel forward in time, you can see or remember events inside the cone (bounded by the speed of light. Anything outside the cone is invisible and inaccessible by definition since any signal would have to move faster than the speed of light to get to you).

T Machine

This diagram provides a relatively clear explanation of how the Tipler Cylinder is supposed to work. The technical term for what is happening is "frame dragging", and in general relativity, even the modest amount of bending of space-time by an object like the Earth should demonstrate this effect, although it would be so small that measurements would probably be at the limit of accuracy for most modern equipment.

It is possible that finite objects like rapidly rotating neutron stars or black holes, or finite T Machines made by some super science could induce enough frame dragging in order to be useful as time machines, although I have never seen any proof of this.

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    $\begingroup$ Next time, try asking for an infinitely thick cylinder, then cut it as needed after delivery. You can also offer to return the excess material thus removed. $\endgroup$ – a CVn Jun 16 '16 at 12:44
  • $\begingroup$ The 10% restocking fee for returns makes that course of action somewhat impractical.....;-) $\endgroup$ – Thucydides Jun 16 '16 at 17:42
  • $\begingroup$ "My local contractor hung up when I asked for a quote...." [+] $\endgroup$ – Pelinore Mar 2 at 4:47
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Yes time is our fourth dimension. We are living in a four dimensional spacetime with three space dimensions and one time dimension. There are two ways to alter time: Time dilation caused by motion with very high speed and by gravity. One can say: "Space tells matter how to move and matter tells space how to curve". I don't know if time can be altered by one of the other three forces.

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  • $\begingroup$ Alright, thank you! But, something else I read a while ago, (or saw on Vsauce, unsure), but I thought it wasnt possible to "name" time the fourth dimention, since there is no clear order? $\endgroup$ – Berry M. Jun 16 '16 at 7:21
  • $\begingroup$ What do you mean with "clear order"? Einstein stated that time and space is really intertwined. And theoretically you could move backwards and forwards in time as like as someone can move in space. Time is no longer linear or absolute; it is relative. $\endgroup$ – BobbyPi Jun 16 '16 at 12:44
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    $\begingroup$ @BerryM. As for an order: What is the second dimension (as opposed to the first or third)? I doubt that anyone can give a generally applicable answer. However, it's generally said that spacetime is comprised of three spatial dimensions and one temporal dimension, and that the spatial dimensions come first. $\endgroup$ – a CVn Jun 16 '16 at 12:47
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    $\begingroup$ @BobbyPi: I believe that the OP (or the thing he read) means that it isn't appropriate to call time the fourth dimension in the same sense that it isn't appropriate to call Ringo Starr the fourth Beatle — as MichaelK says, there's no unambiguous identification of which dimensions are the first, second and third.  It would be just as valid to say that time is the first dimension, and the spatial dimensions are the second, third and fourth.  Or, in fact, that there is no first, second, third or fourth dimension, but just a four-dimensional thing that can be tilted — the axes are arbitrary. $\endgroup$ – Peregrine Rook Jun 16 '16 at 18:33
  • $\begingroup$ Not only are the axes arbitrary, but they are dependent on your own velocity. In four-dimensional Minkowski space, the four directions are essentially symmetrical, and you are always traveling in some direction (a combination of the four directions) at the speed of light (so that the your velocity vector projected on all four direction vectors will have a total magnitude of 'c' at all times). Altering your own spatial velocity will alter the direction that time "points in" for you. (E.g. moving faster in the "x" direction means moving slower in the "t" direction.) This is special relativity. $\endgroup$ – Some Guy Mar 2 at 10:41
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I'll preface with "I'm not a physicist", and fiddling with time and space is something that gives work to actual physicists who are smarter than all of us collectively.

Classically, space and time form 4 dimensions. A simple manifestation of time is movement, as movement can be expressed as variation of space over time. The way I understand it, gravity deforms space and time, but it doesn't alter the direction of time, which would make going back impossible this way.

If you are interested in travelling to the future, there's a loophole of sorts. You could use time dilation when travelling close to speed of light to effectively travel into the future (in actuality, time would simply move much slower for you than it would on Earth, hence a second for you might translate in centuries in the outside world). It's a one way trip.

Our current understanding is simply that time doesn't work that way. You can't skip forward or backward, you're stuck with going with the flow, and although said flow may be doing funky stuff at high speed or under high gravity it still goes in the one direction. Then again, there are a lot of things we think we understand about the universe, then we find out we didn't.

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It's all about having a good grasp of your Time and Relative Dimension in Space (if only there were a good acronym for that, like TARDIS or something).

Apparent time travel to the future of a physical location in space (faster than 1s/s we all manage by just sitting around) seems permitted under current physics at great energy expenditure costs (all you need to do is shift your $\vec{u}$ vector to be mostly in the physical dimensions rather than the time dimension relative to your starting inertial frame and back again while not dying horribly in the process), so that should be fine and easy, as long as you have galactic levels of energy lying about ready to be used, which you do, I'm sure.

Time travel to the past, also known as FTL signalling, involves a complete breakdown of causality and appears strictly, inexorably, ineluctably, unforgivingly, inescapably forbidden by our understanding of physics. Which must mean, of course, that our understanding of physics in incomplete, because, as we all know, past-oriented time travel is too cool not to exist. Paradoxes be damned, who does not want to try and seduce their own 19-year old grandma, after all? Don't answer that.

So let's assume an abundance of multiverses with thin walls. This means that when you go "back" (using the old negative mass-lined wormhole generator that looks deceptively like a blue police box on the street corner) you're really invading an alternate universe. No wonder the young version of grandma thought you're out of this world. You really, really are.

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  • $\begingroup$ Actually " a complete breakdown of causality and appears strictly, inexorably, ineluctably, unforgivingly, inescapably forbidden by our understanding of physics" isn't strictly so as the really puzzling thing is so many physical laws are time symmetric, with only a few quirky exceptions. This suggests time travel should be normal, but it's not. What's so puzzling is that time only goes in one direction. That "negative mass-lined wormhole" should be an exotic matter-lined wormhole. The exotic matter exerts a negative pressure to keep the wormhole open. $\endgroup$ – a4android Jul 10 '16 at 12:47
  • $\begingroup$ @a4android The only law that prevents time-symmetry is the second law of thermodynamics. But yet we do not fully grasp its origins, as in quantum physics, we see quantum tunneling (might or might not decrease entropy in some cases). But we never see entropy "tunneling", and warm water will never separate into hot water and ice cubes. $\endgroup$ – Bloc97 Oct 25 '16 at 23:51
  • $\begingroup$ @Bloc97 Not so. That only deals with the thermodynamic arrow of time. Particle physics, electrodynamics, cosmology, Newton's laws of motion are time symmetric. It's an old text, but it's an excellent introduction to the multiple arrows of time. T. Gold (ed) THE NATURE OF TIME (Cornell University, 1967). This was the easiest one to find at short notice. $\endgroup$ – a4android Oct 26 '16 at 3:04
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Time travel to the future is no problem, all you have to do is go very very fast (like close to the speed of light) - this is in the theory of Special Relativity. Time slows down for you therefore you are moving into the future at a rate greater than 1 second per second relative to everyone else, and so effectively time travelling to the future.

Time Travel to the past, though? I'll leave that as an exercise for the reader...

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For accuracy, have a look into closed time-like curves. The answer to "accurate" time travel may simply be that "it is already happening that way and will have always already have happened that way and had always been about to already have happened that way"; there was no before and after, only a self-consistent wave-function.

But where's the fun in that? If you want fun time travel, I'm in agreement with Separatrix's comment: don't go into the details at all, just take it as given. How many old sci-fi movies/shows/stories look silly when they try to explain the details? You can avoid that by describing what it's doing, not how it's doing it; it never once bothered me that I didn't know how a tricorder worked when watching Star Trek.

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  • $\begingroup$ But some were driven to distraction to make it work, on another planet no less, 50 years later. $\endgroup$ – JDługosz Jun 16 '16 at 2:05
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Ultimately the reader will have to accept time travel, because if you could give a perfectly reasonable explanation, we could already time travel in real life. Ideally those stories add a new key element to physics that was unknown to us (or it was just theoretical), and that does not affect anything else, thus not changing physics and the world more from how we know it, which makes it very easy to keep a consistent, logical narrative.

What I find more interesting is giving the invention of time travel out of the hands of human. Either Aliens somehow (who cares, it was the aliens) invented it, or humans from the far far future did (at least we invented it somehow far far away from our future).

You could maybe extend these to a point where no one reasonably understands why they work. The reason for them to invent time travel maybe was to populate the past, because their future present home planet is dying. Unfortunately in all that hassle advanced knowledge was lost and they only ended up with the working technology.

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One simple (to explain) method of time travel is to split off a copy of the universe, identical except that time moves more slowly there than in the original universe. So a year might pass here at our rate of time, while in the alternate universe, only a day has passed. Imagine if you had split off the alternate universe Jan 1, 2000. It would now be Jan 17, 2000 there. Want to travel into the past? Just point your alternate-universe portal device to the slow universe, and step into the year 2000!

This type of time travel eliminates all the usual time-travel paradoxes. Travel back in time and kill yourself? You won't disappear. Get back home, and you are still here.

Since we are talking fiction, you are not limited to the boring physics-centered time travel methods. You just need a way to step between universes, and a way to trigger a universe cleave. Both take very little energy.

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    $\begingroup$ (1) But, if you have independent universes, then there is no clear reason to expect parallel development.  The butterfly effect gets to rear its ugly head in a big way.  You could step into the other universe in the year 2000 and find that you didn’t even exist there (then?) — and that Al Gore had just been elected president.  (2) What’s the point in having time travel if you can’t change the present in your timeline?  That’s no fun.  The scenario you describe is equivalent to having a holodeck create a simulation sandbox that looks like the past but isn’t real. $\endgroup$ – Peregrine Rook Jun 16 '16 at 18:51
  • $\begingroup$ When I wrote the above, I nearly said, “The butterfly effect gets to rear its ugly head, big time,” but I didn’t intend to make that pun.  (Like Orson Welles, I will serve no pun before its time.)     :-)     ⁠ $\endgroup$ – Peregrine Rook Jun 16 '16 at 18:52
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    $\begingroup$ If you split the parallel universe off of your universe ("main branch") on Jan 1 2000, then you most definitely would be in that alternate universe. Because it's a copy. $\endgroup$ – James T Francis Jun 16 '16 at 21:26
  • $\begingroup$ OK, yes, you would exist in the history of that universe, but, even if you go to January 17, you’ve allowed sixteen days in which you could have died. That’s what I meant when I said you might not exist there then. $\endgroup$ – Peregrine Rook Jun 20 '16 at 1:25
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    $\begingroup$ Yes, that makes sense. A lot can happen in a week! $\endgroup$ – James T Francis Jun 23 '16 at 13:43
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An alternative way of looking at time travel.

Temporal symmetry causes us to travel back and forth through time constantly. Unfortunately, your body and mind is also caught up in that shift like the rest of the universe, so you don't remember it and don't change anything.

To achieve 'time travel' then, we must find a way to isolate the matter and energy of a person from that normal flow. Most methods discussed use wormholes or massive rotating bodies to isolate an area of space in which that matter and energy exist.

If you are willing to allow a form of teleportation then the energy requirements for time travel reduce significantly since you would only need to isolate a narrow beam path for the energy that does not even need to support life during the passage.

Reassembally on the other end becomes the problem at this stage. Perhaps you are limited to traveling only to times which had a functional teleportation chamber. Or, perhaps you have devised a method of self reassembling energy into matter. In which case, awesome.

This then leaves us with the problem of targeting your destination. If you are relying on the natural ebb and flow of temporal symmetry, well, we've never had any tools that can measure this, we just know that mathematically it should happen. If you manage to build an energy isolation tunnel as I suggest, then you finally have the first steps for a tool to measure that flow. Which means that you, as the author, get to decide when, and when too, your characters can travel based on that ebb and flow, as well as how accurately they can measure and predict it.

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