Time. Is. Not. A. Thing.

For serious, dudes.

Check this. So for a long time now, we've thought space and time were this weird thing called space-time, a nifty little shorthand way of saying "light cones, spatial relations, and causality are intrinsically intertwined."

I call shenanigans!

I say to you - show me an objective second. Explain to me what a minute is, from an objective view. Oh right, you can't. Because our temporal reference frames are just that - referential. Informed by what's around us. Subjective.

The idea of time actually being a thing - even if it is wrapped in the sugary goodness of space (another concept I have issues with, but that's a whole other story, and involves the idea of the universe expanding at some exponent of c being kinda-sorta absurd to me) - seems ludicrous.

Details!

Okay, so. Let's say that right now, time stops for an arbitrarily long amount of "time." And then now, it starts up again. How do you know how much time passed? You can't. For all we know, time is like a twitchy teen hopped up on amphetamines, stopping and starting all the freakin' time.

Our perception of events is relative to all kinds of crazy things. It's based on how fast things are going, where we are, where the event happens, all that fun jazz. Light cones and such. You can do some crazy stuff with light-cones: you can set up scenarios that are hypothetically possible in our current understanding of the universe, wherein to one dude, two events are simultaneous; while to another event A precedes event B; and to a third, event B precedes event A.

"How can that happen?", you say incredulously, scratching your chin. "A must come before B, that's just sensical. It doesn't make sense for me to get a response to a message I haven't sent yet."

And there I agree with you - the causal chain is, in fact, a thing. To heck with Hume and all his causality-hating shenanigans, the causal chain is a for-serious thing, and that's Latin for necessary truthiness (synthetic a priori for the win, dudes and dudettes - but that's a discussion for another time [haha!]).

Okay, now we're getting into heady stuff, so I'ma cut the jokes and snark for a bit, for clarity's sake.

Here's the deal. Envision the universe as a machine, vaguely deterministic with a few variables thrown in being determined at run-time. At any given "moment," you can take a snapshot of the universe. Such a snapshot is a single link on the causal chain. It has a cause (the snapshot just before it on the chain), and an effect (the snapshot just after it).

This does not involve space in any way, shape, or form.

There's all kinds of thoughts that have to add up to get to this, like the idea that the universe can be modeled as a really complex program that "knows" all of its variables and objects it contains and their variables and so on and so forth. Basically, things like "spooky action at a distance" can happen because distance is irrelevant, the universe knows what's going on and it's all good, it can inform each particle of what the other is doing because it can access them.

But anyway.

Time, right. Time isn't a thing. There are moments, which have causes and effects. Our perception is just that - perception, and has no bearing on what's going on. We can perceive effects before causes, and it doesn't matter, because objectively, the cause comes before the effect.

Visual aid... thing of the causal chain like a string. Each infinitely small slice of the string (yay, calculus) is a moment. Now, make a loop in the string, so that it crosses over itself at a single point.

Here is, in my mind, what that represents. Let us call the moment immediately prior to the crossing point for the loop moment Alpha, and the moment immediately after the crossing point moment Omega.

There is, for all intents and purposes, no "time" between Alpha and Omega. But there is a long string of events that happen between them, which no longer have any physical presence. But they're there, because what we did in-between is send a message to our past self in moment Alpha, which then led not to moment Somewhere-In-The-Loop, but to Omega, due to influence from the looped portion of the causal chain.

...that example is kind of muddled. Let's try another one.

Let's say you build a machine that allows you to travel back in time. So you hop in your machine, rev the engine, and hop back to the strange world of 1892. What just happened, in this model?

There is a snapshot of the world that exists - actually, quite a lot - that correspond to that year. So say you pick one. So now you have a snapshot that you are going to travel to.

In terms of the causal chain, you are taking the moment that corresponds to the moment you are traveling to, copying it, and positing it as the next moment in the chain. You replace the effect of your current moment with the moment you copied from earlier in the chain. The moment is then modified to account for your existence, and - aside from that - continues on as it did.

The world you came from - the "present" - still exists in the causal chain (if previous events continue to exist, anyway; I'm still fuzzy on the idea of the universe having memory of its previous states). If you want to come back to 2010, you just do the same thing - you copy-paste the moment you left from and posit it as the next moment in the causal chain.

The whole notion that you can "travel in time" is ridiculous. Time's not a thing to travel in. Time is not a thing at all. But the causal chain is a thing, and snapshots along it are things, even if they aren't physically-accessible to us at the moment. The system - the universe - remembers past states, because we can look at the present state, observe what state it leads to, make deductions about universal rules, and work backwards from present state to previous states. That information is available, and still exists, if only in awkward and rather compressed form.

The question, "If time travel is possible, where are all the time travelers?", is stupid. It is indicative of a dogmatic approach to the concept of time. Time isn't a thing, people. Time travelers don't show up now or in the past because the "past" they travel to is later in the causal chain, because they've posited earlier snapshots in the chain as the next link after theirs on the chain.

...think of it like a linked list, alright? Each node is a snapshot, which points to the next node in sequence. But you can have nodes that point to nodes that aren't in absolute sequence.

So while normally node #382093-A points to node #382093-B, you could theoretically make it instead point to node #129710-F.

Now let's envision a flag being carried along the linked list. All this flag says is, "I am the present state of the universe." It gets passed along from moment to moment, following the path set by each node's pointer.

Meanwhile, elsewhere in the machinery of the universe, the universe is constantly checking various event flags on what's going on. In moment #382093-A - whose current pointer heads to #382093-B - there is a flag raised: "Time-travel shenanigans. Change this node's pointer to 129710-F, please."

Now, the universe doesn't check ahead or behind for flags, it's doing all these calculations at run-time. So even if that flag popped up hundreds of nodes ago, it doesn't care that 382093-A has it until it gets to it (which is indicated by the "I am present state" flag carried by the nodes themselves).

What this is getting at is that time travel hasn't happened yet (unless it has... but whatever). Basically, time travel has to wait for that moment to have held the "I am the present state" flag in order for it to be a thing.

To make things even more interesting - because they're obviously not interesting enough yet - I would posit that the nature of the universe is such that future events don't exist yet. As I've mentioned earlier in this post, envision the universe as a machine doing all of these calculations at run-time: it needs the data to perform the calculations to determine the contents of the next moment. The forward-pointer of the moment with the "I am the present state" flag points to null until the calculations are performed. The moment then exists, and the universe moves forward, along with the flag being flipped in the appropriate places.

However! If you "travel backwards" in time (remember the whole copy-pasta posit the past as the future thing), the "future" still exists up to the moment you traveled into the "past." Those moments are essential to your traveling - they are part of the causal chain that led to you time-traveling in the first place - and thus are still accessible. Yes, you might be in 1892, but you're in a version of 1892 that has a causal dependence on 2010. 2010 is now in the past. So is the "real" 1892, for that matter; you're just in a copy of it, which would hopefully contain you (otherwise, if you just posit the "real" 1892 as the next moment, your net effect on the causal chain is... nada).

One day, I'm going to have to do a post dealing with the construction of a reasonable temporal grammar, so that we can avoid this silly nonsense of using air-quotes for "past" and "future."