A Beginner’s Guide to Time Travel

WARNING: Every now and again, people attempting to understand this topic find that their heads explode instead…not that there’s really that much difference, but I’m not cleaning up the mess.

Deja Vu All Over Again

Time travel seems to confound people. They get caught in the web of paradox—events happening that seem to contradict themselves. It’s important to do two things in order to try and understand the impossible: 1) Accepting it for what it is and not dismissing it out of hand; 2) Not trying to believe that it’s real—all these are, really, are thought experiments.

Time Travelers

The most important thing to remember is that we are all time travelers. We are so adept at it that we rarely notice. All of our lives we travel forward in time at a rate of one second per second—at least it appears that way to an outside observer. Our personal perception of the rate of the passage of time might change (a year, to a child, is FOR-EV-ER, but to an adult it is often but a wink of an eye), but the direction arrows stays resolutely at forward.

Suffice it to say, we are all stuck at that dot in the middle, just ever so slightly to the past side of the past/future demarcation due to latency of perception. We call this instant “the present”. So far, not too difficult to grasp. This is good, as it really serves as the foundation of all that is to come. As long as we remember that the past is the stuff that happened before the present, and that the future is something that comes after the present, then we’ll have made a good start.

Still, even something this simple, this basic, has been mined for a long time into turns of phrase that have the flavor of time travel: “Tomorrow never comes”, “Tomorrow is just a future yesterday”, “No time like the present”, and so on.

Getting Loopy

Perhaps the simplest time travel to the past construct is the immutable time loop:

This is exactly what it seems to be: a section of time that is repeated a never-ending number of times. What happens is this:

There’s this person named “PAT” who starts out at A, moves blithely onto B, goes to D where PAT travels back in time C to time B, moves onward again to time D, and then proceeds to the future time E. Between B and D, there are two versions of PAT: the PAT that hasn’t time traveled (PAT-1) and the PAT who has (PAT-2). This is very important to remember: they are not the same person. This cannot be emphasized enough. To understand time travel is to understand that PAT-1 and PAT-2 are different. PAT-2 has traveled through time to the past. PAT-2 is therefore that much older and more experienced than PAT-1. In this most simple of time loops, NOTHING that PAT-2 does can change any of the events between B and D. Those events are immutable. They happen the same all the time, every time, forever.

And now we face the first of what will be many paradoxes.

Causality

Here’s the problem that has been the cause of most of the head explosions visiting those who try to understand time travel. The idea of there needing to be a cause for an effect to occur. Let’s say that in order for PAT-1 to time travel, PAT-2 has to have done something that allows that. The question arises: what about the first time, when PAT-1 traveled back to become PAT-2 originally? How could that happen?

Here’s the easiest explanation: it happens because it always has happened and always will. The loop is never formed from a first event, it is always there.

I know some of you are going, “Huh?” right about now. Here’s the thing. With this concept, all of the threads (or maybe just the one thread) of time burst into being whole and complete at the time it was created. Which leads us to:

Mutability vs Immutability (or Free-will vs Predestination)

The primary paradoxes in time travel, and what makes causality such a problem for many, is that pesky “free-will” thing. Many, if not most, people who think even casually about this want to believe that we are masters of our own fates—that there is free-will. Much of time travel is predicated on the notion that time isn’t so flexible. In this case, no matter how you try to split hairs, it inevitably results in having all of time’s events being fundamentally unchangeable…at least to an outside observer of the time threads.

I think what messes a lot of people up with this is the notion that a being can know all that ever has happened and all the ever will happen, and yet still fancy people having free-will. These are mutually exclusive. Knowing the outcomes of every instance on all time threads implies that there is no free-will. Free-will implies a lack of total omniscience. Yes, yes, there are a lot of attempts to split hairs with this, but all eventually devolve into events being fixed, albeit in what can be a very convoluted and brain-detonating sort of way.

Forking the Thread

Where things start really getting fun is when an action causes a thread to fork (this also creates the illusion of free-will). This typically happens in order to avoid a causality paradox…but some of you might want to reach for the NSAID of your choice.

Here, our intrepid time traveler, PAT, shoots back in time to B; but, instead of continuing on to D as before, PAT’s experiences now head toward a different future, F. What happens to the time lines can go in a few directions:

• D continues on toward future E, just without PAT in it because of the time travel
• The future D->E exists simultaneously as future B->F (just because PAT hopped off doesn’t mean that everyone else did, too).
• The reason the travel loops C back to B instead of simply forking away at D is because all of the past up to PAT arriving at B is identical (hence time travel instead of simply making a choice)

This scenario has some very profound implications. Even though PAT arrives back in time on the same thread at time B, there is a different future. Anything that PAT does has no effect on future D->E because PAT isn’t there. In fact, any attempt by PAT to alter events on the B->D->E timeline will have no effect on how that thread plays out because PAT’s thread changed to B->D->C->B->F. The reason the B->D->E thread didn’t just disappear is because every existential thing on that time line has its own destiny to complete.

It just this sort of forking that makes things very VERY messy. If we simply take the first only-forward-in-time example, the fact is that it is forever forking anyway.

Here we show that at each arrow, PAT has a choice, the answer to either choice will result in PAT heading off into different futures. At A, PAT must decide whether or not to wake up. If so, then PAT will head down the future that begins with B, otherwise PAT’s future begins at C. (As a mind-blowing exercise, the reader is encouraged to imagine a tree like the one we have for PAT… not just for every individual on Earth, but for every thing in existence throughout the universe…and then how those threads interact with one another.)

Only Fate

A tricky aspect of time travel is that the existential thing doing the traveling, in this case PAT, is still stuck in a relativistic state, i.e. from PAT’s perspective, going ever forward in time at a rate of one second per second regardless of what the external state of time happens to be. Let me illustrate it this way:

The tree on the left shows PAT’s various future paths; the tree on the right shows PAT’s view of how time progresses. It looks sort of loopy, but in reality is nothing more than:

Yes, despite all the distractions, from PAT’s relative point of view, it’s all still just a straight path from past to future, though to the outside world it might appear that PAT is not only going back in time, but changing future events. The tragedy of this scenario is that PAT can change nothing. As a result, all those gray arrows on the right tree don’t even have to exist as PAT never inhabits them. Seems that destiny is a one-way street.

The Temporal Butterfly Effect

Many people are aware of the hypothesis that the flapping of a butterfly’s wing in China might magnify over time to become a hurricane in the Caribbean. This is used as an extreme example within chaos theory to demonstrate how one small effect can, over time, magnify itself to an effect totally out of proportion to the causative event. Wait…cause…effect…that seems so familiar…

Generally we aren’t so concerned with how stuck a time traveler is within their own experience with time. Most often we want to consider what sorts of effects that a time traveler can have when they visit the past. Basically this boils down to three categories: 1) the time traveler can’t change anything; 2) the time traveler can change some things but not others; 3) the time traveler can make profound changes in how this timeline careens toward the future.

Option 1 is self-explanatory and is a closed loop case, and as such doesn’t warrant a lot of explanation. Option 3 gives the time traveler carte blanche to muck around with history, damn the consequences. This is so free-wheeling that whatever happens in the future is pure guesswork (maybe). But option 2 provides us with some interesting scenarios.

You see, option 2 lives in the chaotic world of the Temporal Butterfly Effect (TBE). This is a time travel world where magic can happen, but often does not. The TBE, like its chaos-derived hurricane starter, can have very insignificant events ripple through time to have profound effects: for want of a nail, the kingdom was lost.

More likely, the larger scope of things remains intact. Whatever details get changed, the temporal ship more or less rights itself so that the important temporal points are reached. This doesn’t necessarily mean that you CAN’T change the future, only that the future tends to have its own way. I suppose one of the more classic examples of this is having someone go back in time to kill Hitler as a child. In spite of this, World War II and the Holocaust happen because someone else rises in his place.

The Mutability Conundrum

“But,” I can hear some of the more devilish around you thinking, “what if there are lots of time travelers, all jumping around backwards and forwards in time, just messing up everything?”

First off…I’m not even going to try to diagram that. Second…when all is said and done, it really doesn’t change anything. In the end, once you do the necessary simplification (yes, just like in math class), like I showed above, you’re just left with one timeline that goes from the start of time to the end of time. The end turns out exactly how it always had turned out and always will…because it did.

It’s at this point that I’d need to start talking about parallel universes and the like (and whether or not you can muck around with them), but I’ll leave that for another post.

Discontinuous Existence

The simple fact is that time travel is mostly illusionary, and is only apparent to those actually executing it. For those who don’t happen to be doing more than going along for the ride, the situation can be a bit more confusing.

Here, we revisit our simplified temporal tree. Instead of following PAT, let’s follow SUMNER, a non time-traveler who is effected by PAT’s little hobby. Like PAT, SUMNER’s progress to the future is A->B->E->B->D->I…, but unlike PAT, SUMNER’s experience in the B->E->B loop is erased and reset as if nothing had happened and no time had passed…effectively making SUMNER’s experience be A->B->D->I….  If that’s the case, then what are we to make of SUMNER’s B->E->B existence? Did it happen?

The short answer is that from SUMNER’s experience, it did up until the time that PAT time traveled, and then it didn’t. Whoa! What? The best way of actually thinking about this is that once the trip backward has been made, SUMNER’s B->E->B experience is effectively just a very VERY VERY real dream of PAT’s…who now of course is PAT-2, since PAT-1 is following the same simplified path as SUMNER.

See…easy.  I don’t know why people think this stuff is hard or confusing :-D.

In a Nutshell

To those not doing the actual time traveling, time remains linear and unaffected. To those engaging in time travel, they form causality loops that give the illusion of possibly changing future events, but in the end they are just as bound to the unchanging thread of AllTime as those not doing the jumping.

Now…jumping around time into the threads of parallel/alternate universes, that’s a whole different box of Schrödinger’s cats.