Calilassea, how is time travel possible? How would you account for the grandfather's paradox and many other paradoxes associated with it? Also, if it's possible, then why haven't we got visitors from the future?

quas wrote:Calilassea, how is time travel possible? How would you account for the grandfather's paradox and many other paradoxes associated with it? Also, if it's possible, then why haven't we got visitors from the future?

Oh lovely, a proper, hard question. I like those.

In order for time travel to be possible, you need to create something called a 'closed timelike curve'. This, in short, is a path through space-time that loops back on itself in the time dimension. Trouble is, closed timelike curves only appear in specific solutions to the Einstein Field Equations, usually involving extreme changes in space-time curvature, such as those found in the neighbourhood of a black hole. Indeed, one of the metrics in which CTCs occur is the Kerr Metric, which is used to model the space around a rotating black hole with zero net electrical charge (a different solution applies to black holes possessing electric charge).

In the case of the Kerr Metric, there are two regions where the equations become singular. One of these regions is the surface defined by the event horizon of the black hole. There is, however, a second singular region for this metric, one that lies outside the event horizon, and which is known as the ergosphere. Between the ergosphere and the event horizon is a strange region of space, one in which fairly exotic things happen, and one of these is frame-dragging at speeds in excess of that of light, which results in some fairly bizarre phenomena taking place. Now, the point to remember here is that thus far, CTCs are known to exist within certain classes of exact solution, including some highly artificial ones, such as the Van Stockum Dust, this latter metric also opening up the possibility of multiple futures (and pasts!) for an entity travelling along its CTCs. Whilst quite a few of these metrics are highly artificial, at the moment, they point to the fact that in general relativity, CTCs are not actually forbidden as such, merely that the space-time curvature conditions enabling CTCs to appear spontaneously within a given space-time metric are fairly exotic (though, as far as we know, still physically permissible).

Whether or not one can generate a CTC to order to facilitate time travel is one of those interesting questions. In theory, there is nothing stopping you from generating the relevant space-time curvature to achieve this, and thus opening up all sorts of cans of worms. Trouble is, the practical details of doing so appear to place extremely strict restrictions upon what is possible. For example, using a traversible wormhole as your means of time travel (or, for that matter, as your handy means of instantaneous hopping across vast interstellar distances) is fraught with problems centring upon the requirement for vast quantities of negative energy (as in the negative energy equivalent of the output of an entire galaxy of stars over a period of many millions of years). Worse still, this negative energy has to be confined to a narrow band encircling the throat of the wormhole, a band that cannot exceed 10-32 m in width. To put this in perspective, a proton is approximately 10-15 m across, so we're dealing with a region of space whose width is 17 orders of magnitude smaller than a proton. At the moment, we don't have any realistic means of confining any amount of energy, positive or negative, into such a tiny width of space in a controlled manner. Likewise, other means of launching time travel run into difficulties with respect to energy requirements and confinement requirements, all of which involve numbers that are hopelessly beyond our current reach.

This all means that at the moment, those numbers mean we're not going to start emulating Dr Who any time soon. However, the fact that the laws of physics don't appear to place an explicit prohibition on time travel is itself troubling for one or two people. It's one reason why physicists in the know think that when a proper, working theory of quantum gravity is in our hands, general relativity is going to take something of a beating with respect to such matters. At the moment, general relativity doesn't explicitly prohibit time travel, provided you can produce the requisite space-time curvature, but, thanks to a nice paper by Hawking, you always need substantial quantities of controllable negative energy in order to achieve this, and at this point, quantum thermodynamics has a lot to say on the subject.

Basically, there's a big gulf between "theoretically possible" and "realisable in practice", and in this instance, the gulf appears for the moment to be unbridgeable. What's troubling for physicists is that general relativity doesn't impose an explicit prohibition upon time travel, and appears to leave that task up to the second law of thermodynamics in its quantum formulation, which isn't a full prohibition as such because, as is well known to those of us who deal with creationist canards about the second law of thermodynamics, it's possible to have a local decrease in entropy provided there's an energy input from somewhere else generating a greater increase in entropy to compensate (as in "see that big yellow thing in the sky").

It's one reason why physicists want a working theory of quantum gravity, because they suspect that such a theory will provide a fundamental prohibition preventing us from being overrun with time tourists from the future. What form such a prohibition will take remains to be seen (if it materialises), but one thing we do know is this: general relativity is troublesome in that the conditions it imposes upon solutions of the Einstein Field Equations tend to be arcane, intimately coupled to the underlying mathematics without too much regard for the accompanying physics on occasions, and on other occasions overly prescriptive against perfectly reasonable physical solutions. It works, and it works extremely well at predicting large-scale observed phenomena, but it's regarded as being incomplete and in need of superseding for good reason.

Mick wrote:Evidence accessible to just one person, for instance is private evidence

How do you distinguish true "private evidence" from (a) falsehood, and (b) delusion?

If two people have different "private evidence" how do you determine which of them (if either) is correct?

Mick wrote:Again, all he is saying is that you can know that christianity is true despite the other evidences. Here the tesitmony of the holy spirit is a defeater for just about anything you throw at him. I dont see much wrong in this for the believer.

Picture a grand conspiracy against joe. His enemies tailor great evidence in his disfavor-they make it look like joe committed murder. Theres dna evidence, fake videos, etc. for any external and objective viewer, joe looks guilty. But what does joe think? Well, joe knows deep inside he did no such thing. He cant prove it, he cant show it, and he knows the public evidence is against him, but he stands fast in his innocence.

This is what craig is getting at. Joe doesnt believe his innocence in faith or wilfull ignorance, he rests on his inner experiences.

Listen, we know what Craig is saying, and you just keep repeating it. And it's exactly as I have portrayed it before. Craig doesn't have experiences of "not comitting murder". Craig has some weird emotional state of thinking there is "some mind out there that really loves him". Furthermore, noone is planting evidence against god anywhere, except of course that this is what Craig asserts when he starts babbling about Satan. Problem is, Craig is talking out of his ass here, because he's a priori defining evidence contrary to his inner experience as being either outright invalid and wrong, or as having been planted by someone with malicious intentions. In other words, Craig has set himself up to be INCAPABLE of accepting disproof of christianity. This is not the position of a rational person, this is the position of a nutcase, a fucking fruitcake.

With this experience in his memories, Craig is going to think the real world is what his experience was, not what the REAL WORLD AROUND HIM ACTUALLY TELLS HIM. Craig is assuming that, no matter what happens, sooner or later something is going to affirm that "the mind that really loves him" will turn out to exist anyway. This is clinical insanity, he's no different from the people who believe they're Elvis or Napoleon and you can't seem to point out what sets Craig apart from them, other than the qualities of the experience itself. But the qualities of the experience doesn't constitute evidence that the experience is real, as in actual physical fact, any more than the qualities of thinking you're Napoleon makes you so.

Double post, following on from this post.

What you have to remember here, is that the relativistic concept of space differs fundamentally from that of prior physical theories in an important and radical aspect.

The first point to bear in mind is that any representation of space involving a coordinate system, ultimately brings you face to face with the concept of a 'metric' for that space, which in effect, defines what is meant precisely when we talk about the 'distance' between two points. When this is combined with the concept of a manifold (which is a mathematically precise means of defining what we mean by a 'well behaved' space, and invokes some topological concepts), then you end up with what is known as a Riemannian manifold, which is considered to be the mathematical entity that best represents real physical space. Because a Riemannian manifold has a distance metric associated with it, this implies immediately that the manifold in question is associated with another mathematical entity called a vector space, which means that the full panoply of linear algebra applies to that manifold. More importantly, that vector space, because of the existence of the distance metric, is a normed vector space and an inner product space, which are two important concepts allowing us to specify in what sense the space in question is "well behaved". The existence of a norm for the vector space means that there exists a precise, rigorous and meaningful definition of 'length' in the space, and since a norm is always associated with an inner product of some sort, this in turn allows us to place the concept of 'right angle' (or, in higher dimensions, orthogonality) on a rigorous footing too. In truth, the inner product is the fundamental unit in a vector space, because every definable inner product leads to a norm, and every norm in turn leads to a distance metric. However, in a geometrical setting, a distance metric is considered, if you like, the means of defining the properties that a space has, because a distance metric in that setting involves relations between the coordinates of the space, and in that setting, a standard, unified concept of 'inner product' exists. Extensions to linear algebra and vector spaces arose from the fact that translating the geometrical intuitions arising from coordinate geometry to, for example, the analysis of sets of functions, proved to be wonderfully useful and unifying, and so the generalised concepts of inner product, norm and metric arising therefrom have their own separate branch of mathematical analysis. But I digress. At bottom, the key points to remember here, before proceeding, are:

[1] Every space, once a coordinate system is defined for it, has a distance metric defined for it;
[2] That distance metric is associated naturally with a norm and an inner product, making the space a particularly well-behaved form of a vector space;
[3] Such a space can be mapped onto a Euclidean space of equivalent dimensionality, and is therefore a manifold.

To make matters even more interesting, if the space in question is complete (i.e., in a precise mathematical sense, there are no "points missing" from the space), then the space is also a Hilbert space, but this is of peripheral interest here.

Once you have a coordinate representation of a space, all the powerful tools of manifold theory, vector space theory and linear algebra can be brought to bear upon that space, as can vector analysis (a different branch of mathematics from vector space theory, just to confuse matters a little!) and the extension thereof in the form of tensor analysis, which covers the behaviour of objects more complex than standard vectors that can exist in that space. Incidentally, tensor analysis has its own all-encompassing definition of 'inner product' for coordinate spaces, from which a norm and a metric naturally arise, and in addition, provide the existence of a 'metric tensor', which not only defines how distance is constructed within the space, but also how curvature is determined within the space.

Now, In the past, even as the full flowering of tensor analysis was starting to get under way in the latter half of the 19th century, and the idea of coordinate metrics as a means of defining the properties of a space were being placed upon a rigorous footing, along with all the other mathematical developments I've described above, time was treated as something entirely separate. Time was treated as an entity distinct from the metrical coordinates of a space, with an independent behaviour.

What makes relativity so radical, is that it treats time as another metrical coordinate. It integrates time and space into a greater whole, subject to all the provisions of an appropriate 'distance metric' which now has to take separation in time into account as well as separation in space. All of which, moreover, has to behave in a well-defined manner when one space-time is subject to Lorentz transformation onto another space-time. Because time is now treated as another metrical coordinate within the coordinate system, instead of being separate therefrom, all points within that coordinate system now have a time coordinate, which as a natural corollary means that every path through such a coordinate system involves 'time travel' of some sort, because that path will pass through points that have a time coordinate, and that time coordinate can potentially vary along that path.

But, here's the fun part. Because time is now coupled to space coordinates within general relativity, entities cannot be thought of as having places, without also including some sensible means of defining their moments as well. In short, relativity does away with the idea that 'where' and 'when' are distinct and independent. In relativity, the two are coupled by the metric for the space-time, and moreover, coupled in such a manner that even inertial motion affects your measurement of that 'where' and 'when', to the point that, for example, there no longer exists a concept of global simultaneity within a relativistic space-time - two events that appear to be 'simultaneous' to one set of observers may appear to be separated in time by a different set of observers, courtesy of the intricacies of that distance metric, and how that distance metric is transformed from a 'stationary' space to a 'moving' space.

What makes all of this even more hilarious, of course, is that making time another metrical coordinate imposes certain symmetries between time and space that didn't exist before. Prior to relativity, you didn't have to think about this, and you could treat time in an entirely intuitive manner, without worrying about the effect of so doing upon your treatment of space. Now, those days are over, at least in precise work, and time has to be treated as another metrical coordinate, because thus far, all our measurements of various relevant phenomena point to this being a necessity. This brings with it an unfortunate natural consequence, namely, that just as you can travel forwards and backwards in space (however 'forwards' and 'backwards' are defined), making time another metrical coordinate means you can, in theory at least, do the same with time. This is problematic for numerous reasons, even before we start to consider various paradoxes. First, our intuition (and for that matter, every clock on the planet) tells us that time is moving forwards. How can it be possible to travel back to the past?

Well, the moment you make time another metrical coordinate, this becomes a possibility, simply because you now impose certain symmetries upon time that weren't present before. The relativistic view is not that time itself is moving, rather, it's that the universe and its contents are moving in a given direction along a time coordinate axis, and what's more, your motion in terms of pure space coordinates has an impact upon how fast you move along this time axis. General relativity adds even more nastiness, from the standpoint of intuition, to the mix by letting gravity affect your motion along this time axis, so that if you're in the neighbourhood of a sufficiently strong gravitational field, your motion along the time axis is seriously slowed. In the relativistic world, clocks simply measure your movement along this time axis, which happens even when you're stationary in terms of pure space coordinates. By treating time as "another space coordinate", if you like, relativity permits travel into the past, provided that certain somewhat exotic conditions I described in the previous post are met, and this arises because time is now treated as another metrical coordinate.

Don't worry if this buggers your mind for a while, because it has that effect even upon world class mathematical physicists during their early student days. As quite a few of them will happily admit.

Mick wrote: Evidence accessible to just one person, for instance is private evidence

Experience, by a single person, inaccessible to others, is of dubious significance: it is certainly Not 'evidence' that can be verified.

A feeling you have a special friend that other people can't see or hear or detect does in no way qualify as evidence.

it is question begging to call his experience some weird emotional state, thats something in need of argument. Theres no a priori defining, its argued in his work. But to know that, youd have to read his books along with plantingas. We both know you havent done that, so why the bore?

No, no argument needed. Given the two alternatives, an example of a well documented phenomenon (delusions and hallucinations) vs an explanation that is unverifiable and lacks any evidence at all, it is a true no-brainer when it comes to determining what is overwhelmingly more likely.

Unless of course, you're infected with the religion virus.

Mick wrote:it is question begging to call his experience some weird emotional state, thats something in need of argument. Theres no a priori defining, its argued in his work. But to know that, youd have to read his books along with plantingas. We both know you havent done that, so why the bore?

Oh look, it's the appeal to a lot of apologetic screeds you still haven't read. I don't need to have Craig describe his experience at all, because all that matters is that he's having an internal experience the nature of which Craig can't demonstrate to anyone and, consequently, he's incapable of elucidating, even to himself, whether actually represents something external to his mind or whether it is just a delusion thereof. But Craig is telling us that he's TRUSTING this experience to an extend where observations from the real world that contradicts it becomes irrelevant to him. The experience is right, because... well, because Craig says so. No justification is given. It's "self-authenticating" apparently.

Xeno wrote:

Mick wrote:Evidence accessible to just one person, for instance is private evidence

How do you distinguish true "private evidence" from (a) falsehood, and (b) delusion?

If two people have different "private evidence" how do you determine which of them (if either) is correct?

Objectively?.i dont know. Craigs point doesnt concern it.

virphen wrote:No, no argument needed. Given the two alternatives, an example of a well documented phenomenon (delusions and hallucinations) vs an explanation that is unverifiable and lacks any evidence at all, it is a true no-brainer when it comes to determining what is overwhelmingly more likely.

Unless of course, you're infected with the religion virus.

there is a difference btween your knowing that craig had that experience and his knowing it. Youre trying to shoehorn this debate into one of public evidence.

RealityRules wrote:

Mick wrote: Evidence accessible to just one person, for instance is private evidence

Experience, by a single person, inaccessible to others, is of dubious significance: it is certainly Not 'evidence' that can be verified.

If by this you mean public verification, then sure. Any other trivial points to make?

Mick wrote:

virphen wrote:No, no argument needed. Given the two alternatives, an example of a well documented phenomenon (delusions and hallucinations) vs an explanation that is unverifiable and lacks any evidence at all, it is a true no-brainer when it comes to determining what is overwhelmingly more likely.

Unless of course, you're infected with the religion virus.

there is a difference btween your knowing that craig had that experience and his knowing it. Youre trying to shoehorn this debate into one of public evidence.

Self-serving bollox.

Sorry. Just calling a spade a spade.

Mick wrote:Objectively?.i dont know. Craigs point doesnt concern it.

Of course Kraig's point concerns it. He is claiming knowledge based on a source of information for which delusion isn't just an alternative explanation, but a better one, given a greater degree of parsimony. In light of what is known about the human brain's abilities in virtual-reality generation, Kraig's position doesn't remotely stand up to scrutiny.

Don't let unpleasant facts stop you from wibbling, though. At least it keeps us amused.

Im quite amused too, hack. Just like I was in the talk about double negatives when you took off with your tail between your legs.

I didn't take off, with my tail between my legs or otherwise, not least because I was and am correct on that point, but it's hardly a topic of great interest to me. and my time online is very linmited at the moment. I may come back to it at some point.

Lovely non-sequitur, though.

I have "personal evidence" that you are wrong on that point hack.

Hehe.

Of course it's possible that Craig has had this experience (in other words, that he's not lying about it), but if so he can't use it to convince others of the truth of his beliefs. So that's probably why he goes to such great lengths to come up with philosophical arguments that he feels he can use for that purpose. But he shouldn't really do that either. Sure, it says in the Bible that the believer should always be prepared to defend his/her beliefs, but it's quite clear from numerous passages that you are either called by God and therefore 'get it', or you're not and don't/can't. This goes all the way from the sayings of Jesus ('pearls before swine', 'narrow gate', 'shake dust off feet', 'hidden these things from the wise and understanding and revealed them to little children' etc.) to 1. Corinthians 1:17-31, and probably more.

I have a hard time seeing though, how this kind of revealed truth cannot be said to work for other positions as well (well, I can see how one would say it cannot if one is living within one of the 'boxes' that claim such revealed truths, but not if viewed from the outside, and more objectively).