Time travel snavel

Time travel stories are funny.

The base of a time travel or of a time-cape story is , almost always, that time travel allow to change the past.

I have yet to see a time travel history where the protagonist is an historian looking for info about an obscure period, and some form of "change the things" does never come up (0). 

But it is not actually possible - and that, even if time travel itself was at all possible.

 

If the universe has only one temporal line, it is valid the Novikov principle.

The only time travels possible are the ones that re-enforce the stability of the time-line, as forking is not possible (it would require another time dimension that, simply, is not there).

As an IT guy, I tend to see as an issue of macroscopic quantum mechanics and information...

Trying to change the past makes it, really, a quantum superposition of the original time-line and the new one, the contribution of each one determined by the probability of  the action to succeed.

As a result, when the action goes near 50% of success probability... the information upon which the action has been originally planned had a reliability of 50%.

In other words, the more you try to change the past, the less you know what it was to be changed or why.

Nowm one thing must be clear... which usually is not clear at all.

When tackling time travels, one should remember that he is really using a meta-time to narrate its effects, i.e. a fifth (6th, 7th, 8th etc..) dimension, temporally orthogonal to the space and time dimension(s) of the universe he describes.

The universe  should be seen, then,  as a film reel or - even better - a painting - something static, with no dynamic of its own.

The painting is a better metaphor, because its internal dynamic is actually al a product of its observer, which is situated outside of it.

In a single timeline universe, the action of a time traveller produce very much the effect of an author "pentimento" (1) in a painting... two versions of the painting, usually in a limited area, superposed.

If the transparency of the pentimento  (:= probability of success of the time traveller's action) is 50%, it is impossible to tell which was which.

Which was the action of the traveller, and which the past he wanted to change. 

If the universe allows for multiple time-lines, vice-versa, it just means that what we call time is only one of  a set of non-spatial dimensions, that there is at least a time dimension orthogonal to the one we are used to perceive.

The universe is akin then not a classic painting... it is a stack of paintings, each one slightly different from the one up and below it.

Or, to be a bit more subtle and precise, it is a painting done with the veiling technique. Applied over and over again, each possible variation laid on top of the other with a infinitely shallow layer of paint, till the  resulting painting becomes as thick as it is wide and tall (2).

Every possible outcome of any non mechanically determined action - be it from a time traveller, a man deciding which side to enter his bed, an ant taking a route with a leave, an electron traversing a diffraction grid or a radioactive atom decaying - is a different 4D universe(3).

Is, not becomes nor create. Is.

A time traveller may go back in time and "change" something, then stay in the time-line  and see the product of his actions and have the illusion that he changed history but, as I said, it is an illusion.

What changed, the very moment he moved from his position in the time flow, was the universe in which he resided.

In his original universe, nothing changed at all - the companions of the time traveller, that are waiting to see their universe change around them, or some other SF idiocy, are going to die of old age damning the traveller's name, while nothing happens.

Travel to the past may not be impossible, but trying to change it surely is.

On the other hand, it is maybe possible to go there and have it to follow the story as it is known - the bootstrap paradox. If, and only if, the universe is a mono-time-line one.

 But that is another story...


(0)  To be honest, I read a very good story based on time travel, once, "Universal War One"  by Denis Bajram - if I had to re-read only a comic, this year, I'd say that is the one. You'd probably will have guessed: it uses a bootstrap paradox, and has a character concluding "Time is stronger than what you think. It already is the by-product of all the time travels happened in it. Time cannot be changed by anybody."

(1)Pentimento: in oil painting, it happens relatively often that the artist changes some feature, and paints over it.  However, oil colours can be partially transparent, and it is relatively simple, in many cases, to guess at the shape of what was changed, from the "shadows" appearing below the final pictorial surface.  

(2) Omitting the -light prefix from light year, which in a way doesn't matter, it can be said that the visible universe is 4 dimensional cube with a spatial [3D] area that has a side of some 90 billion years and a temporal thickness of just 13.9 billions years. It appears the painter hasn't ended yet... 

As for the thickness of each veiling, IF space is indeed quantized over the length/time of Planck, that should apply also to the extra, otherwise not-perceptible time dimension where the parallel universes are laid.  A time of Planck is something x 10^-44  , no the number of strata in the painting would be something x 10^62, a number followed by 62 zeros.

(3) Separated parallel universes are, again, probably a simplification - it implies a perceptible discontinuity between one and another. Most probably, a meta-universe would be some kind of continuous mesh, with no hard boundaries between a parallel and the other, the difficulty in going from one to another probably connected to the entropy - information content - differences between the considered 4d space. 4D spaces whose information content grew equivalent, could as well merge back... and fork again.OK, now my head officially hurts...