Time travel?

[K^2]

You're taking branching too literally. It's not discrete. And you traveling back doesn't cause anything in the many worlds. Nothing causes anything in many worlds. It is event-free until you introduce a perspective of the observer, at which point you only have a single time-line. That of the observer.

Also, all time-traveling schemes require you to be present in every point in between, whether you're going back or forward. So if you don't count that as time travel, then there is no time travel.

[eddiew]

I would argue that most people's definition of time travel is that you disappear from 'now' and appear 'then', a la the Tardis. Admittedly it is just a matter of definition, but that's what Average Joe expects it to mean, and anything else is re-purposing the term

As for branching, that was my understanding based on some reading - admittedly a few years ago - of a few multiverse and quantum theory books. From what I'd gathered, it's theoretically possible to push something into reverse-time (and that this is one proffered explanation for the nature of antimatter), but the multiverse dodges the paradox issue by giving you no way to navigate 'downstream' back to where you were. You can go upstream (backwards in time), because your particular universe only has one direct line of ancestors, but you have innumerable descendants and no way of choosing which one you move into. The classic grandfather paradox isn't a paradox, because you didn't kill your grandfather - you killed the grandfather of a different version of yourself. Yours is alive and well in his own universe, and will proceed to spawn you to go back and kill his otherself, preventing your otherself being born.

At least, that was my interpretation. It's been a while and I may be rusty now

[pg01202]

The classic grandfather paradox isn't a paradox, because you didn't kill your grandfather - you killed the grandfather of a different version of yourself. Yours is alive and well in his own universe, and will proceed to spawn you to go back and kill his otherself, preventing your otherself being born.

It's not even a paradox in the monouniverse. Killing your grandfather (preventing your grandparents from meeting, let's not be bloodythirsty) will only affect linear cause and effect. Since reverse time travel requires you to have a different frame of reference from the time you came from, you break any cause and effect locally. So all you end up doing is orphaning yourself.

Again, and not a reflection on previous posts, people make the mistake of thinking physical laws have some kind of physical manifestation rather than simply being a description of the limits of what the universe is capable of on its own. Time travel is not explained very well in any theory of relativity because they all require linearity. They can't explain two frames of references that are not in sight of one another (e.g. both on Earth but one in 2015 and the other in 1860).

Edited February 18, 2015 by pg01202

[K^2]

I would argue that most people's definition of time travel is that you disappear from 'now' and appear 'then', a la the Tardis. Admittedly it is just a matter of definition, but that's what Average Joe expects it to mean, and anything else is re-purposing the term

Even TARDIS, as fictional as it is, moves through the time streams, rather than just pop out of one place and into another. Timey-wimey stuff, etc.

As for branching, that was my understanding based on some reading - admittedly a few years ago - of a few multiverse and quantum theory books. From what I'd gathered, it's theoretically possible to push something into reverse-time (and that this is one proffered explanation for the nature of antimatter), but the multiverse dodges the paradox issue by giving you no way to navigate 'downstream' back to where you were. You can go upstream (backwards in time), because your particular universe only has one direct line of ancestors, but you have innumerable descendants and no way of choosing which one you move into. The classic grandfather paradox isn't a paradox, because you didn't kill your grandfather - you killed the grandfather of a different version of yourself. Yours is alive and well in his own universe, and will proceed to spawn you to go back and kill his otherself, preventing your otherself being born.

At least, that was my interpretation. It's been a while and I may be rusty now

Antiparticles have nothing to do with time travel. Their backwards propagation is purely a matter of reversed wave vector. The time evolution of anti-matter is exactly the same as of ordinary matter. In order to travel backwards in time, you need to have time flow actually reversed. It's very similar to difference between phase velocity and group velocity. If you wrap your head around that, you'll have the basic idea.

As for actual ways to send something backwards in time, there are a bunch of ways from General Relativity. There could be purely Stat Mech methods of doing so as well, and you will need QM to dump the extra entropy. Either way, though, what you're looking for is backwards evolution of the system. To an observer, it will appear as if the rest of the universe is running in reverse.

Many Worlds does account for any paradoxes that can arise. But so does Copenhagen Interpretation. Just in very different ways. The branching itself isn't strictly necessary. It's all about the fact that you're dealing with fields, rather than what you normally think of as particles.

Time travel is not explained very well in any theory of relativity because they all require linearity. They can't explain two frames of references that are not in sight of one another (e.g. both on Earth but one in 2015 and the other in 1860).

You have a very poor understanding of relativity, if you think that. You really shouldn't be making claims based on your own misunderstanding of physics and try to present it as fact.

Edited February 18, 2015 by K^2

[pg01202]

You have a very poor understanding of relativity, if you think that. You really shouldn't be making claims based on your own misunderstanding of physics and try to present it as fact.

Care to elaborate? Bearing in mind that I'm talking about conservation of information, not the ability to time travel.

Edit: Thought I'd give an example. If we're standing in a park and I throw a pebble at you (apologies) and then travel 1 minute back in time when you're not standing in the same place and throw that same pebble in the same way, relativity cannot explain our relative motions.

Edited February 18, 2015 by pg01202

[tuckjohn]

3000 years is a big change from a hundred years.

You shouldn't used the timeline of the earth... but rather the emigration of modern humans from africa + the end of the ice age and other climactic factors to "synchronize" things... plus there were interactions among groups, and ideas can spread far further than people (at least back then).

That is exactly the problem- there was no way for the different groups of people to communicate with each other, and thus historians are puzzled. The (slightly crazy) theory that originated from the book series that I mentioned in my previous post: There was one relatively advanced city (Atlantis in the book) that got destroyed by a natural disaster and most of the citizens emigrated from there and ended up all over the globe, where they attempted to create their own civilizations/cities.

Taking advantage of time dilation isn't really time travel, because you don't disappear then reappear in the future; you're always present somewhere in the universe, just travelling really fast or near a really big gravity well.

This is where you have to define "time travel", because technically we are moving "forward" in time.

Also on the topic of possible ways to travel back in time is that you have a cylinder where it is spinning (really, really, really fast, so fast that the face at near the speed of light), then you would take a spaceship and then fly against the "flow" of spacetime created by the rotating matter, thus going backward in time. The is one (slight) problem with this: the cylinder would have to be infinitely tall.

EDIT: I just thought of a way around this- take the spinning cylinder and bend it so that it forms a torus, and keep it spinning. Yes, that would have a problem with the inside being shorter then the outside, but I imagine some kind of piston-setup to allow the contracting and expanding of "edges" of the torus.

Edited February 18, 2015 by tuckjohn

[KerikBalm]

That is exactly the problem- there was no way for the different groups of people to communicate with each other, and thus historians are puzzled.

#1) yes there was

#2) there is no puzzle, because 3,000 years is plenty of time for an idea to spread back then, or for it to be independently derived after a global climite change conducive to the development of agriculture.

Also on the topic of possible ways to travel back in time is that you have a cylinder where it is spinning (really, really, really fast, so fast that the face at near the speed of light), then you would take a spaceship and then fly against the "flow" of spacetime created by the rotating matter, thus going backward in time. The is one (slight) problem with this: the cylinder would have to be infinitely tall.

EDIT: I just thought of a way around this- take the spinning cylinder and bend it so that it forms a torus, and keep it spinning. Yes, that would have a problem with the inside being shorter then the outside, but I imagine some kind of piston-setup to allow the contracting and expanding of "edges" of the torus.

Where to begin... this couldn't possibly work, and it appears you do not understand the time dialation effect of relativistic velocity

Edited February 18, 2015 by KerikBalm

[perk]

Where to begin... this couldn't possibly work, and it appears you do not understand the time dialation effect of relativistic velocity

He is not talking about time dialation effect of relativistic velocity as far as i understand his writing, he is talking about a lense thirring effect, of a rotating mass.. the problem is he needs to create at least an apparent horizon for that to work and then we are squarely back in "wormhole to an earlier space time event"-territory

[K^2]

Care to elaborate? Bearing in mind that I'm talking about conservation of information, not the ability to time travel.

Relativity precisely deals with abstraction of coordinate systems. To the point where time displacement in one coordinate system is a space displacement in another. The example you give is no stranger in relativity than discussing what happens if the pebble is thrown from different locations. Moreover, General Relativity does not require the coordinate system to be universal. It is a local concept.

The only restriction General Relativity places is that locally, your chosen coordinate system has Minkowski Signature. So that while the choice of time coordinate is arbitrary, you can chose precisely one direction as time and precisely three others as spacial coordinates. The rest is almost completely arbitrary.

Once you introduce the restriction of traveling along time-like trajectories, which arises from field theory, time travel in General Relativity is reduced to a problem of finding Closed Time-like Curves (CTCs). So not only is Relativity adequate for describing time travel, it gives us precise tools for analyzing requirements and consequences of time travel.

[pg01202]

You're going off one interpretation of relativity, one that current particle physics renders impossible to actually achieve. So I'd be careful with that, your analysis is perfectly valid but not necessarily true. If we knew how to describe conservation in time travel accurately this thread wouldn't exist.

[perk]

pg01202 can you elaborate? are you discussing any particular causality problem stemming from incorporating quantum field theory into gravity, or are you just throwing around words in the hopes something sticks?

[pg01202]

No. Given that no-one has managed to incorporate QFT into general relativity (as I said in my last post), it's a bit pointless to discuss the macroscopic effects since no-one has any idea what form they will take. Once you try to incorporate quantum mechanics into practical applications it entirely depends on what theory you're basing your ideas on. Beyond the scope of SR and GR, the approaches to time travel remain entirely hypothetical because they're still based on hypothetical physics.

So the ideas that I'm kicking around are, like yours, simply based on the hypothetical interpretations that I find most convincing.

[perk]

Please don't insult your readership with this silly eggdance.

Time travel is not explained very well in any theory of relativity because they all require linearity. They can't explain two frames of references that are not in sight of one another

This is factually false at the "require linearity" point. Building your whole "guesswork" on a utterly wrong understanding of GR will never lead you to any actual physics but instead science fiction, fantasy or woo.

[pg01202]

Well then feel free to give it a go. How does GR describe the relative motions of an object that occupies the same space a different times?

[perk]

This question makes no sense and is not related to any physical quantity. The object is represented by its worldline, the tangential vector to that worldline is its velocity. The change of that velocity vector field along the worldline is governed by its acceleration vectorfield. The difference of 2 velocity vectors from two different tangentspaces at two different events on the worldline has no meaning.

[pg01202]

That's my point. You seem to be stuck on talking about one method of time travel which doesn't require it, so from your perspective it's not a necessary problem, but I wasn't talking about CTCs since they're not the only method of time travel allowed by general relativity. Seriously, this is a thread about time travel. At the moment it is science fantasy because no method is even close to being physically plausible, so how about getting off your high horse and allowing other perspectives to join in the completely theoretical conversation?

Edited February 19, 2015 by pg01202

[perk]

but I wasn't talking about CTCs since they're not the only method of time travel allowed by general relativity

May i ask where you read that, so that we can find out how you missunderstood it?

CTCs are not a method of time travel, they are a result. When you have a worldline, that fullfills the criteria for "time travel". i.e. it intersects its own past lightcone, then there is a pertubation of that worldline to a CTC... the technicalities are of course more involved, when you for instance enter the past lightcone but stay on the lightlike boundary you only get an almost closed timelike curve etc...

[pg01202]

That's like saying an orbit isn't a method of circumnavigating the globe, rather a result. A CTC is a path through spacetime just as much as a geosynchronous orbit. In other words, travelling through spacetime to a previous lightcone is the technique used to achieve the curve, the result of which is arriving at that earlier time.

[perk]

You dont seem to understand what happens when such a worldline (that timetravels) exists. You dont create that worldline by "timetraveling", in our background that looks pretty much like FLRW there needs to be a topological artifact that you encounter on your worldline that sort of "loops" you back. As soon as you enter the past lightcone of your worldline you can perturb your worldline to create a CTC, but not every timetravel is a CTC. The "technique" used is the topological artifact, something that differs from the smooth, homogenous and isotropic FLRW, by quite a large amount. (Wormhole)

[K^2]

You're going off one interpretation of relativity, one that current particle physics renders impossible to actually achieve. So I'd be careful with that, your analysis is perfectly valid but not necessarily true. If we knew how to describe conservation in time travel accurately this thread wouldn't exist.

This thread wouldn't exist if everyone involved actually understood relativity and field theory in curved space time. That is obviously not the case.

I repeat, you have no idea what relativity is actually about, as you keep demonstrating with your posts. Don't project that onto everyone else.

[Bill Phil]

All time travel, if time travel is possible, is already a part of history.

[Owen5595]

Time travel into the past, its called Wormholes. Anyways, the reason we don't see effects like the Grandfather Paradox is becuase when you time travel to the past, it takes you to something called a parallel universe