Question about Brian Greene's wormhole illustration in Fabirc of the Cosmos

[Caeru]

Question about Brian Greene's wormhole illustration in "Fabirc of the Cosmos"

I'm currently watching this show on NOVA narrated by Brian Greene called "The Fabric of the Cosmos". In the first episode, he talks about space and the fact that it has properties. In the second episode, he talks about time and how the rate at which time passes is affected by motion. He uses some brilliant illustrations to describe each topic, but there's one thing that seemed a bit off on his explanation.

First of all, he debunked this illusion that time is a constant flow. He said it's more like a series of snapshots that occupy specific points in spacetime. And if you were to plot all events on the spacetime grid (conveniently shaped like a loaf of bread in his illustration), you could cut a slice down the width of the loaf. All events along that line make up what is called a "now-slice", because it contains everything that is happening "now" (at that moment in time) experienced by each observer in all locations. This was followed by an illustration of general relativity that I don't feel the need to explain.

He goes on to talk about wormholes, saying that they connect two points in spacetime - two different "nows". But in his illustration, he walks through the worm hole and meets himself. And then talks about The Grandfather paradox, and the fact that we don't see people popping in from different times. However, if you were to step through a wormhole and meet yourself at a given time, wouldn't that create a new point in spacetime? Because, the time-traveling you wasn't in the past before you time-traveled. You created that new point in spacetime the moment you stepped through the wormhole.

Instead, when you step through the wormhole, wouldn't you find yourself in a location you were in sometime in the past, doing something you did sometime in the past, experiencing the same sensory input and knowledge that you had in the past? Would you even remember that you had stepped through a wormhole? You'd be re-living a "now" from the past... so your experience would have to be exactly that of the past. Right? That way, there's no paradox to deal with. And new points in spacetime aren't created based on your future decision to take a course of action.

Though, I don't see how it's possible for two points in spacetime to connect.

 

[yoron]

I like Brian Greene, but I don't buy his belief in time being a two way street. I know it is useful theoretically, and have given birth to a lot of ideas about what SpaceTime can do, possibly, but I don't accept time travel, at least not macroscopically. I think Penrose tried to define it as being possible on a small scale, particles, but then he spoke about an asymmetry to time at that level, as I remember it. I don't know if it is a symmetry or asymmetry, but I liked his approach with 'scales' defining it. Physics today discuss the difference between Quantum logic and the relativity we see macroscopically as a result of 'decoherence', which is another way of defining 'scales' to me.
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Also I wonder about what the no-cloning theorem has to say about it being two 'me' simultaneously? Take a look here No-cloning theorem and time travel.

 

[Fredrik]

However, if you were to step through a wormhole and meet yourself at a given time, wouldn't that create a new point in spacetime? Because, the time-traveling you wasn't in the past before you time-traveled.

If it wasn't, then the wormhole was never a path to an earlier time on your own world line. It was nothing more than a path to a region of spacetime where, apparently, there's another copy of you.

 

[bobc2]

I'm currently watching this show on NOVA narrated by Brian Greene called "The Fabric of the Cosmos". In the first episode, he talks about space and the fact that it has properties. In the second episode, he talks about time and how the rate at which time passes is affected by motion. He uses some brilliant illustrations to describe each topic, but there's one thing that seemed a bit off on his explanation.

First of all, he debunked this illusion that time is a constant flow. He said it's more like a series of snapshots that occupy specific points in spacetime. And if you were to plot all events on the spacetime grid (conveniently shaped like a loaf of bread in his illustration), you could cut a slice down the width of the loaf. All events along that line make up what is called a "now-slice", because it contains everything that is happening "now" (at that moment in time) experienced by each observer in all locations. This was followed by an illustration of general relativity that I don't feel the need to explain.

He goes on to talk about wormholes, saying that they connect two points in spacetime - two different "nows". But in his illustration, he walks through the worm hole and meets himself. And then talks about The Grandfather paradox, and the fact that we don't see people popping in from different times. However, if you were to step through a wormhole and meet yourself at a given time, wouldn't that create a new point in spacetime? Because, the time-traveling you wasn't in the past before you time-traveled. You created that new point in spacetime the moment you stepped through the wormhole.

Instead, when you step through the wormhole, wouldn't you find yourself in a location you were in sometime in the past, doing something you did sometime in the past, experiencing the same sensory input and knowledge that you had in the past? Would you even remember that you had stepped through a wormhole? You'd be re-living a "now" from the past... so your experience would have to be exactly that of the past. Right? That way, there's no paradox to deal with. And new points in spacetime aren't created based on your future decision to take a course of action.

Though, I don't see how it's possible for two points in spacetime to connect.

I'll bet that Brian Greene is imagining a 4-dimensional universe, one that is all there at once. And all of the material objects are there as well as 4-dimensional objects. I think his loaf of bread is all there at once, a static 4-dimensional world. So, his slices at different angles represent different 3-D cross-sections of the 4-D universe. The world lines are all there at once, they are not continuously extended as time passes.

Time may pass or flow, but the material structure is all there. Therefore, the only sense in which an observer moves along his world line at the speed of light relates to psychological phenomena. In this view it is usually some aspect of consciousness that is doing the moving along the world line.

The sketches below depict a candidate universe having a big bang and a big crunch (not a currently popular universe--but it makes the point for this discussion).

There are three universe examples: 1) A worldline, a, loops back on itself through a worm hole, connecting up with its earlier world line, 2) a worldline, b, goes through a worm hole into the future, 3) a world line, c, loops back to an earlier time but continues on into the future as a separate world line, 4) world line, d, enters from another universe through a worm hole.

So, the world lines are all there all of the time in this static, frozen universe. Now, you can put in a consciousness moving along world lines at the speed of light and draw your own conclusions about what the observers experience in a "real world." Or, the other view of consciousness (required to avoid zombies) is that consciousness is present all along the world line, so that 3-dimensional psychological experiences persist simultaneously and continuously over the entire world line (this seemed to be Einstein's preference).

 

[yoron]

Two consciousnesses meeting then Bob? Or one?

How about defining each 4-dimensionality from a local perspective? And assuming that each consciousness only have one 4-dimensional path existing, uniquely defined from each one?

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This one assumes a arrow of time existing though, defined locally as 'invariant'. Giving you a constant uniform 'motion' into the future, defined from lights propagation, locally becoming the arrows 'invariant clock'.

 

[bobc2]

Two consciousnesses meeting then Bob? Or one?

How about defining each 4-dimensionality from a local perspective? And assuming that each consciousness only have one 4-dimensional path existing, uniquely defined from each one?

Hi yoron, You always have interesting ideas on this stuff. I'm kind of anticipating what Brian Greene is thinking, and I may be presuming too much on his behalf. But, in this kind of 4-dimensional model all of the world lines are just there. They don't evolve in time--they are there all at once. And if you adapt the consciousness present all at once over every entire world line, then there is not meeting of anything. Everything is just all there.

If the world lines are separate world lines, there would be a consciousness associated with each separate world line. Once world lines are joined, there would be a concsiousness associated with that world line. But what the consciousness senses presumably depends on the physical 4-dimensional structure of the 4-dimensional bundle of neurons. The consciousness that returns will, after joining the original world line track, relive the original earlier life.

There seems to kind of fractal of some sort in that the structural content in the "outside 3-D world" observed at a given instant is copied on a small scale along the neuron filament bundle (to the extent that the mind perceives geometric images of the external world). The neuron structure imaging the external world seems to determine what consciousness experiences.

If you were to adapt a 3-D consciousness model in which the consciousness moves along a world line at the speed of light, the two consciousnesses would not meet, because the proper times along the world lines are different. Of course when the consciousness returns to the earlier time, it will only sense whatever the neuron structure presents. The consciousness will just relive the earlier life scenario without ever being aware that his life is repeating.

 

[yoron]

Heh :) Thanks for that one Bob, I like this kind of ideas, but I'm sort of old fashioned. I imagine us to see the world through a constant 'c'. I have no problem defining it as 'four-dimensional' though, I very much trust Einstein's intuitions and definitions on that matter. I'm not even sure if I'm that far from his ideas in my definitions, to me it just seems as if I'm turning my head at some slight angle to his description of a 'whole' SpaceTime to make it work as I think.

If you accept 'c' you have the 'curtain of light' connecting us, That's our 'whole SpaceTime' if you accept that definition (and 'gravity') . Then 'c' and our local clock should be one and the same, and you get an easy definition of what a 'time dilation' is. You also will find it working for defining a Lorentz transformation as it per definition describes everything (including the 'gravity' you should notice, depending on relative motion and mass) from a local perspective.

In such a description we could be said to have the 'multiverses' here, in our 'universe', alive and thriving. There are some things that becomes strange, if you think of Lorentz contractions from a ''whole perspective', accepting them as 'real'. Gravity is one, what happens with it as distances contract?

If you define it as I do then that's no problem. Every 'SpaceTime' is a local definition, joined by radiation, and the distances you see is unique for your definition of it. And taken as whole perspective, making conceptual comparisons of 'frames of reference' a 'global distance' in that manner is just a conceptual construct. You could say that they might not even exist, as I suspect (but that one is pure speculation). And using 'c' as a clock, and then defining 'particles' as descriptions of the relations circumstancing them, defining them, the universe becomes a definition of relations, always defined from your local point of view.

Brian Greene seems to be wondering over 'many worlds scenarios' now, but where he looks outside I would like to define them as already 'inside' :) Heh. And I think that the reason Einstein never thought of it this way was just that he became too preoccupied with QM, and possibly also felt that he in the Lorentz transformations had the final proof for 'one' universe. The Lorentz transformations is as I see it a direct result over the invariance of 'c', making it possible to transform my frame of reference to yours. But that is, to me, also our 'curtain of light', filling our eyes and defining our world. You seem to think of that in a similar fashion?
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What is most important for looking at it my way, is to stop thinking of 'c' as a 'speed'. Think of it as a 'clock' instead, there exist only a 'source' and a 'sink' for a photon, or any radiation in any experiment, even though we can use waves instead it's so much simpler to define it as 'photons' for seeing my point here.

Source and sink, and then the 'rules of the game' defining all other things, as its 'propagation'.

 

[WSFWeb]

Just a heads up that Brian has answered this question on the World Science Festival blog:

http://worldsciencefestival.com/blog/ask_brian_greene_wormholes_time_travel_and_causality"