So I've been thinking about time and time-travel today, thanks to watching the Star Trek film on the weekend and a conversation I had with a friend on the way to the train station last night. I tried to explain my understanding of space-time, complete with Einsteinian relativity and Lorenz dilation/contraction, and all it served to prove was that I don't have it straight in my own head!

Time-travel is an old sci-fi standby, used everywhere from Back To The Future to various incarnations of Star Trek to H. G. Well's classic "The Time Machine" (if you haven't read this one yet, avoid the movie and read the book -- seriously). Accordingly, there are many different mechanisms for time-travel -- flux capacitors, 4-dimensional hypercubes, "slinging" around a nearby star at greater than Warp 10 -- and just as many warnings, side-effects or admonishments against altering "the" timeline -- but maybe time is more resilient than that. In any case, let's have a recap on what we know about time (and space).

Time? Space? What's this 'space-time' malarkey?

Our universe (or Bubbleverse?) probably started off as a Planckoscopic[1], high-density, enormously energised region -- maybe it quantum-tunnelled into existence, or maybe it "bounced" from a previous Universe collapse (a "big crunch") ... or maybe something else altogether. It's a bit hard to tell, because before[2] the "big bang" there was neither space nor time -- and questions like "what was there instead" are fundamentally incompossible, like asking a non-synaethesiac what the colour blue tastes like.

So. Some time "after" the big bang our fundamental forces and dimensions established themselves. It looks like we've probably got 3 "unfurled" spatial dimensions -- normal "3D" living -- and only one "temporal" dimension (a.k.a. time). Apparently observed effects like gravity and electromagnetism don't work without this 3+1 dimensional arrangement, but dammit, I'm a computer programmer, not a physical mathematician.

Thing is, these 4 dimensions are not solid, fixed planes as we here in the well-below-light-speed world tend to think of them -- Einstein showed that space and time (probably[3]) curve, bend, stretch and compress in relation to each other and the speed of light. Matter matters, too -- curving space-time and (maybe) creating gravity, although I think the jury is still out about the whole gravity thing. But the stretching and compressing seems to be pretty well-understood, and has even been confirmed with experiments involving an atomic clock and a really fast jet.

Einstein and others predicted that an object moving at a sizeable fraction of the speed of light would appear fore-shortened in the direction of travel (a  Lorenz transformation) -- space has actually contracted in that direction. The effective mass of the object also increases. Even worse, time is dilated -- the object appears to "slow down" because the temporal dimension is warping. The icing on the relativity-cake (the cake is a lie!) is that for constant velocities there's no way to tell who is "moving" and who is "stationary" -- so to you, it looks like time is slowing down for your light-speed-approaching friend; for her, it looks like YOUR time is running slow.

Acceleration is even more fun, and is what was tested with the atomic clock and jet -- the extremely accurate clock placed into the jet, which then accelerated to a speed fast enough to register a temporal slow-down. Sure enough, get the clock back and it is no longer synchronised with it's control clock on the ground. So you can slow time down by accelerating really fast! (You don't personally notice the time dilation -- to you, accelerating through space, the poor little plebs on Earth appear to be leading frantic, sped-up lives.)

My quite possibly flawed idea of this is that the sum of our velocity through all four dimensions equals the speed of light. You speed up in a spatial dimension, you slow down in a temporal dimension -- so at our current relative speeds we are falling through time at almost light-speed!  (This would also mean that light in a vaccuum is travelling through space but not time -- the photon that excites the cones inside your eyeballs today is the same age as the first photon formed in the big bang.

Let's do the time-warp (again)!

Cool, so we can slow down time by accelerating close to the speed of light, but what about going backwards? If we could accelerate above the speed of light, would that mean we were going backwards through time? Conventional wisdom (er..) says yes, but it's not possible -- an object accelerating up to light-speed would develop an infinite effective mass, and space would contracted to zero, and time would stop. But accelerating an infinite mass takes infinite energy ... so it's a hard limit.

(This doesn't stop there being some surreal particles in existence that are already travelling faster than light (FTL) -- we haven't found any such "tachyons", but that hasn't stopped us including them in sci-fi!)

It kind of sounds false  to me just now -- if time is a dimension, why can't we move backwards and forwards like in the other three spatial dimensions? Maybe we don't need to travel faster than light to travel backwards, we just have to get UP to the speed of light to stop our fall through time. That, or figure out what is drawing us so quickly in one direction and create its opposite to slow us, then reverse our direction of travel.

So yeah, even if time is one-dimensional (not multidimensional like I suggested in a previous post) I think travelling backwards should only be as hard as, say, generating anti-gravity to stop us falling down a well.

[1] A microscope made entirely of plankton[1.1].
[1.1] Come on, a plankton microscope would be cool! But in this case I really mean "smaller than Planck length". Might've made that word up, yep.
[2] Yeah, concepts like "before" get a bit silly when there the temporal dimension doesn't yet[2.1] exist.
[2.1] "Yet" is also silly at this point. Probably so is "at this point", come to think of it.
[3] That's science -- nothing is ever final, certain. And we LIKE it that way!