Seems to me we are already doing that. You can walk on a path going forward in time passing many things along the way, until we reach a point where we turn around and retrace our steps going back in space, yet still going forward in time. In our journey back along the path we will pass and bifurcate all the items we saw along our first part of our journey.

keithprosser2 - 13 November 2011 10:14 PM

I don’t have a very sophisticated idea of ‘spacetime’, so this conjecture is undoubtedly naive.

I imagine spacetime as a sort of vertical column of clear plastic, about 6"x6” square in section (representing a 2d projection of 3d space) but potentially infinitely high to represent time.  A timeline is a ‘wavy pipe’, something like a colored bendy drinking straw, embedded in the plastic.  The present (or any other time) would be a plane cutting through the column.

I am not sure if my timeline (for my future) is “already there” embedded in the plastic block and I move inexorably along it or if my timeline grows as time passes.  If it already there then everything would be pre-determined, so I prefer the latter.

Right, you’ve put your finger on the issue. Here’s the problem though: if the block grows with time, it won’t just grow for you, it should grow along a plane at right angles to the timeline for the whole universe at once, for all simultaneous events, as ‘the present’ continues constructing itself into the past. OK?

But we know from Einstein that simultaneity is relative. So what is in the past for you will be in the present or future for someone else, simply in virtue of their motion relative to you. There is no single, objective “present” for the entire universe. That plane at right angles to the timeline is dependent upon one’s reference frame, which in turn is dependent upon one’s accelerated motion. (Or on one’s proximity to gravitational fields, which is the same thing).

So in Einstein there is no way to make sense of a single, objective present moment for the universe.

keithprosser2 - 13 November 2011 10:14 PM

In either case, if I were to travel back in time I would create a totally separate time line for myself.  I suppose that the ‘old’ Keithproser2 timeline would still be embedded in the plastic but a totally new KP2 timeline would appear (and start to grow) at some point in the plastic block.
 
If I subsequently interact with an object, that object’s timeline would ‘bifurcate’, in the sense that its existing timeline would remain ‘fixed’ in the plastic, but a new ‘branch’ would be created.

So in effect there would be two sets of timelines - the ones that were “already there” (ie the timelines that led up to me making the trip) and a set of newly produced timelines (produced directly and indirectly by the action of the time-machine) that constitute a (potentially) different reality. 

As I see it, it wouldn’t matter if I killed my grand-father in the new reality - he would still have existence in the original reality.
So I can’t affect history by going back in time - I can only create a different parallel history.  So if you sent me back in time to kill Hitler, WW2 wouldn’t happen for me in my reality but it would still happen for you.

I don’t know how the two realities are kept apart, but if the multiple universe hypothesis has any basis in reality then perhaps that is not an insuperable problem.

If that is as clear as mud, I wouldn’t be at all surprised.

I think this is quite clear, and it is one of the standard options for making sense of (a hypothetical notion of) time travel. When you “travel into the past” you actually aren’t traveling into your own past, but into the past of a very similar you in a very similar parallel universe. When you kill your grandfather, you aren’t actually killing the person who was in your direct timeline, but someone very similar, in a slightly different universe.

The problem with all this is that although it’s neat philosophy and neat for writing strange science fiction stories, we have no reason to believe any of it is true or possible.

I think I must have been addressing a possible solution to the apparent paradoxes of time travel rather than it’s possibility.  A species of time travel is possible, I think.  If you are accelerated your time slows down, so if a capsule was shot into space, accelerated to near light speed and finally returned to earth the occupants local calendar might say 2012 but earth-bound calendars on their could be, say, 2013. or even 9013.  In that way you could travel to any future time - 2000 or a million years ahead say - well within the ‘three-score-and-ten’ years of a human lifetime.  But as you can only travel one-way, you could never return to tell us what the future holds in store.
Returning to my metaphor of the clear plastic column, it is obvious I would make a poor sort of god because I can’t think in more then 3d.  My metaphor transcribes the time dimension into a spatial one, so I have to lose one of the spatial dimensions and use x,y in the column’s cross section to stand for the x,y,z of actual points in space. 

The plastic column idea also relies on imagining an observer external to the universe - it gives a (one dimension light) impression of how a god might see the universe.  I well understand the issue of simultaneity, but after reading Doug’s post I wonder if it applies.  Certainly observers within the universe - within the plastic column - will not agree as to what events occur ‘at the same’ time, does that apply to a god-like observer outside the universe? 

I know that the idea of an observer external to the universe is a tad metaphysical to begin with, but thinking about it might shed some light on what sort of inferences or extrapolations from the metaphor might apply or not apply to the real universe…  but it’s probably not relevant to the topic in hand anyway, so I’ll think about it on my own while the rest of you carry on without my digressions…

keithprosser2 - 14 November 2011 06:56 AM

I think I must have been addressing a possible solution to the apparent paradoxes of time travel rather than it’s possibility.  A species of time travel is possible, I think.  If you are accelerated your time slows down, so if a capsule was shot into space, accelerated to near light speed and finally returned to earth the occupants local calendar might say 2012 but earth-bound calendars on their could be, say, 2013. or even 9013.  In that way you could travel to any future time - 2000 or a million years ahead say - well within the ‘three-score-and-ten’ years of a human lifetime.  But as you can only travel one-way, you could never return to tell us what the future holds in store.

Right. This is completely uncontroversial.

keithprosser2 - 14 November 2011 06:56 AM

Returning to my metaphor of the clear plastic column, it is obvious I would make a poor sort of god because I can’t think in more then 3d.  My metaphor transcribes the time dimension into a spatial one, so I have to lose one of the spatial dimensions and use x,y in the column’s cross section to stand for the x,y,z of actual points in space. 

The plastic column idea also relies on imagining an observer external to the universe - it gives a (one dimension light) impression of how a god might see the universe.  I well understand the issue of simultaneity, but after reading Doug’s post I wonder if it applies.  Certainly observers within the universe - within the plastic column - will not agree as to what events occur ‘at the same’ time, does that apply to a god-like observer outside the universe? 

As I understand it, all there is is a four-dimensional manifold, and it can be cut across by a ‘present’ an infinite number of ways. There is no privileged ‘now’ or ‘at the same time’. Or to put it another way, what’s true about Einstein is that his equations don’t just describe how things seem, but rather how things are. It doesn’t just seem that one gets shorter in the direction of acceleration, or that one’s clock ticks slower, or that different events become simultaneous for you, it is that these things happen.

keithprosser2 - 14 November 2011 06:56 AM

I know that the idea of an observer external to the universe is a tad metaphysical to begin with, but thinking about it might shed some light on what sort of inferences or extrapolations from the metaphor might apply or not apply to the real universe…  but it’s probably not relevant to the topic in hand anyway, so I’ll think about it on my own while the rest of you carry on without my digressions…

I don’t think there’s anything wrong with doing a thought-experiment like you suggest. It’s just that (at least as far as I understand Einstein) it wouldn’t help get you an objective, simultaneous present for the whole universe. Or at least it wouldn’t do so in a way that would make the other, different ‘presents’ just as real.

It doesn’t just seem that one gets shorter in the direction of acceleration, or that one’s clock ticks slower, or that different events become simultaneous for you, it is that these things happen.

A nit: one’s clock is measured to tick slower by an observer in a different frame of reference. It is measured to tick at normal speed in one’s own frame of reference. No single frame of reference is privileged; there is no true or fundamental frame of reference (although it’s theoretically possible to devise an ‘objective’ frame of reference based on minimizing the relative momenta of all the masses in the universe.)

Chris Crawford - 14 November 2011 09:50 AM

It doesn’t just seem that one gets shorter in the direction of acceleration, or that one’s clock ticks slower, or that different events become simultaneous for you, it is that these things happen.

A nit: one’s clock is measured to tick slower by an observer in a different frame of reference. It is measured to tick at normal speed in one’s own frame of reference.

Yeah, absolutely. I’ve been making that clear before but sometimes I leave it out. Your internal clock slows down for other people not in your reference frame who are not accelerated. For you, your clock ticks as normal.

This also means that Einsteinian time travel into the future wouldn’t give you a life that would appear longer to you. You’d live the same four-score-and-ten years, but everything else would just be very sped up around you.