I have a kind of followup question (which, if I understand correctly, was touched on a bit in the article). If time is a tangible dimension, is it possible that the "passage of time" is an illusion? I remember the past and because of causality, the events are ordered. Time seems to flow from the past to the future and it never flows from the future to the past. This seems obvious to us, but I've always wondered why time doesn't flow backwards.
Just to take a silly example, what if all the "points in time" just exist (and are ordered -- I don't propose to break causality)? They don't flow at all. From my perspective, at every "point" along the time axis, I can recall the past and it will be ordered as if it "happened", but each point could be independent (though constant). If I could remember "forward" through time, then this would be obvious, but since I can only remember "backward" through time, at every point it appears as if I have progressed through time.
I suppose the interesting thing is that causality is uni-directional. Things can only happen in a certain order in the past. But this is not true of the future. Even if I have perfect knowledge of the present, there are some things I can not predict about the future (quantum mechanics FTW). I wonder why that is (because we are flowing through time? Ha ha!)
Sorry for the diversion, but if someone that is better educated than me could shed some light on the matter, I'd be grateful.
> what if all the "points in time" just exist... They don't flow at all.
This sounds similar to the "block universe", where the universe is a 4D "block" with a certain height, width, depth and duration; where change over time (e.g. an object fading from black to white) is just like change over space (e.g. an object with a colour gradient)
> since I can only remember "backward" through time, at every point it appears as if I have progressed through time.
I've seen a related idea come up in discussions of "Boltzmann brains" (can't find a reference ATM): if the 'arrow of time' is due to increasing entropy, then regions where entropy is decreasing (e.g. an open system radiating heat) could be thought of as experiencing time backwards. What we see from the outside as, say, a collection of photons entering a camera lens and bleaching an existing image off the film; from the inside would be experienced backwards as the taking of a photograph.
> Even if I have perfect knowledge of the present, there are some things I can not predict about the future
That might also be true about the past! There's an unsolved problem in physics called the "black hole information paradox", which points out that if we have a bunch of mass (like a star, or a vast number of elephants floating in space) we might calculate that in the future they'll collapse down into a black hole. Yet if we have a black hole, there's no way to know what it was formed from in the past (stars, elephants, etc.). Black hole formation appears to lose information (like an AND gate), but that's strange since all known microscopic laws of physics (i.e. not the second law of thermodynamics) are reversible.
It gets even weirder. The laws of physics are time reversible as far as we know. Very basically, it means that if you froze our universe and reversed all the velocities of the elementary particles, and then restarted the simulation again, it would be as if time was running backward. In other words, the computer program that simulates our universe forward in time is the same as the computer program that simulates our universe backward in time. This makes the fact that we remember the past and not the future even weirder: if the laws of physics are identical forward or backward in time, then how come the future doesn't affect our memory like the past does?
The theory that tries to explain why we nevertheless only remember the past is called thermodynamics. It has to do with how the universe started out in a very orderly state. If you start the simulation in an orderly state it's likely to become less orderly as you run the simulation. Whether you run time forward or backward doesn't matter: in both directions it will get less orderly. According to thermodynamics that's why we remember the past and not the future. If god had started the simulation with time running backward we'd still have the same experience: we experience the past as whatever direction is toward the more orderly state.
Basically, the flow of events in time is computed recursively by applying the laws of causality, and we experience the passage of time through continual iterative changes applied to the portion of the spacetime event causality tree that represents us.
So, your intuition about time being a illusion is "correct". But in such a context, most other "universal" properties are also an "illusion". Not in a philosophical way, but in a tangible, soon to be practical way. I'll try to pretend to explain, even though it might not be a good idea.
(Please keep in mind, most or all the words following are incorrect, in a way reminiscent of recounting the history of earth by saying: "A fish walked out of the water and became the emperor of China.")
We can imagine our local universe is a thing like a multidimensional boiling bubble. It recursively sub-divides itself, which makes it interesting, and also creates space-time. The start of our local universe's time is the so-called "big bang", which is when it started this wild and lovely fractal. After a little bit there was enough complexity to implement something like the physics we can observe now.
It turns out, a lovely universe can be created from just recursive subdivision. We're smart enough now that we've defined computability in terms of this, for example S K combinitors, etc. Math and physics will eventually catch on. (The astute reader will notice there are things we can imagine, that are uncomputable, math that is un-representable, and physics that won't work with this system. Which isn't really a problem yet.) Thankfully, this configuration happens to be very interesting.
But wait. How did time start? Well, um, it didn't. What we note as time is the constant, change (adding new nodes), expansion, and complexification of recursive subdivision, which happens in units of the "things" dividing, which is like the quantization that we observe in time, matter, and energy. It's "frequency" is also somewhat dependent on it's local environment, since it's "pulled on" by what attached and intertwined with, exhibiting the relativistic effects of gravity and motion.
Perhaps you've heard of the universe described as the three dimensional surface of a four dimensional sphere. But what's inside that sphere? Actually it's the past. Sitting there right now, for real, probably just as it was, fully connected. Of course we're zooming away from it a varying rates, but pretty fast, with the "force" of the universal expansion behind it.
So if the past is actually sitting right there, not that far away from us, we could just go to it right? Well, maybe. It would take a lot of "energy", and stretch the "universal surface" out of whack. But perhaps one could. But what about those paradoxes? Date your mom and your hand starts fading away? Accidentally kill your grandpa and poof out of existence, or not? Well, thankfully those paradoxes can't happen, because even if you're altering the physical past, you still lived through the past you lived through. The altered future that it creates will be "forever" behind your present. Another way to think about it is: any time you effect the past of this universe, it pushes it slightly in the direction of another dimension, in which there is a whole "copy" (or copy-on-write) of the universe with just that thing altered (so far).
This still doesn't answer why we don't bump into a lot of time travelers. Are we doing this the first time through? Or is it just really expensive, boring and useless to muck about in the past? Or are none of neighbors from the universes next door interested? This presupposes that you can re-run through the time dimension. Only at the edge are we "moving", or are new nodes being added. You would have to fork a whole other universe starting at that time slice, which is probably unfeasible unless you have "massive" extra-"universal" compute power. Is it really that important? Especially given that due to the fractal nature of nature, you can fork another sub-universe in this universe! Much easier. Or even better, just take the tiny part you want and copy and paste. Done. Or just get it right the first time. Come on now people.
Of course the future doesn't exist. Which makes it fun when we can seem to predict it using such things as Newton's old equations, and speculative execution.
Just to take a silly example, what if all the "points in time" just exist (and are ordered -- I don't propose to break causality)? They don't flow at all. From my perspective, at every "point" along the time axis, I can recall the past and it will be ordered as if it "happened", but each point could be independent (though constant). If I could remember "forward" through time, then this would be obvious, but since I can only remember "backward" through time, at every point it appears as if I have progressed through time.
I suppose the interesting thing is that causality is uni-directional. Things can only happen in a certain order in the past. But this is not true of the future. Even if I have perfect knowledge of the present, there are some things I can not predict about the future (quantum mechanics FTW). I wonder why that is (because we are flowing through time? Ha ha!)
Sorry for the diversion, but if someone that is better educated than me could shed some light on the matter, I'd be grateful.