Yes, there is, see e.g K meson decay:

<"There are fundamental reasons for expecting that nature at a minimum has CPT symmetry–that no asymmetries will be found after reversing charge, space, and time. Therefore, CP symmetry implies T symmetry (or time-reversal invariance).

[...]

Until 1964 it was thought that the combination CP was a valid symmetry of the Universe. That year, Christenson, Cronin, Fitch and Turlay observed the decay of the long-lived neutral K meson, , to p + + p -. If CP were a good symmetry, the would have CP = -1 and could only decay to three pions, not two. Since the experiment observed the two pion decay, they showed that the symmetry CP could be violated. If CPT symmetry is to be preserved, the CP violation must be compensated by a violation of time reversal invariance. Indeed later experiments with K 0 systems showed direct T violations, in the sense that certain reaction processes involving K mesons have a different probability in the forward time direction (A + B � C + D) from that in the reverse time direction (C + D � A + B)."<<<>>>>

Time passing slower on objects moving at very high velocities doesn't change that IMO, since a snapshot of their statuses at every point of our "universal time" still exist.

Based on this assumption, Both time-Travel and time-Reversal would mean that the <entire> set of particles in the universe would have to be restored to the exact status they were at _that_ point of time. There is <no> other option, and hence there is no use talking about a person separately traveling back in time: That person's particles are part of the entire set of universe' particles and <cannot> be separated. We can only talk about the universe as a whole, and hence traveling back in time means that the entire universe changes as whole.

Even if some infinite source of power could reverse the statuses of all the particles in the universe, it would have to do it to its <own> particles as well, and I can't really see how that is possible.

Hence I don't think there is any possible way to travel back in time or reverse time. Not even Theoretically.

One day it may be possible to move fast enough experiencing slower time passing _compared_ to people who stayed on earth, and hence "travel to future" in some sense, but I seriously doubt even that.

--

Assuming for the sake of discussion that some infinite force could reverse the statuses of all the universe particles (including itself), and looking at your 4 scenarios:

Scenarios 1 & 2) There is no relation to the mature alex here, since the entire universe is reversed, hence you also lose <any> consciousness or knowledge u had following to that point of time (the particles in your brain are exactly the same as they were back when u were 13).

Since it wasn't you who travelled back in time, but the entire universe reversed, you don't "disappear" from our time: our time is simply gone when the universe status was reversed.

Whether "you" can influence events is irrelevant. The entity you call "I" (i.e. the mature alex) no longer exist. The only question asked is whether or not the universe will re-experience the exact same chain of events happened in the first time or not. Apparently the answer to this question is: No.

Scenarios 3 & 4 & 5) Here our infinite force will need to generate another object that did not exist in the previous time: a mature or a young alex. Obviously in such case the exact chain of events cannot repeat itself regardless of entropy or whatever. Again, u don't disappear from our time, since the entire universe state is reversed.

But if there was any possible way ever to travel back in time into such parallel time/universe, it would mean that there can be such evidence, now wouldn't it alex? I mean if it was possible to get there then surely there had to be an evidence for its existence, right?

So regarding:

<Since the discussion here is purely theoretical, I see no reason to omit the possibility of their existence>

If u think there will never be any possible way even to get _evidence_ for their existence, then why talk about a possible time machine that can actually _travel_ to such parallel universe?

I don't know whether you've seen the movie "The Butterfly Effect", but as we can see there, Theoretically, if such parallel time/universes exited and there was some kind of gate to switch between them, then it would have been possible to get evidence for their existence, <if> someone, somehow got to that gate. But until that happens, it's all in the area of "fairy tails" or "science fiction" (at best) for me.

I think that adding assumptions that have zero evidence and cannot be tested, into scientific discussions, do not contribute even if those discussions are purely theoretical.

In my view, a theoretical scientific discussion should be based on things that can be tested: proved or refuted. Things that we have real reasons to believe they exist.

However, there is strong experimental evidence that such a time-machine is not possible: we have <not> been invaded by hordes of tourists from the future! :)

Anyway, I will not apply the "boring physics" conjecture and rule out FTL before there is evidence of such a thing. So far, I have stated the physics point-of view when assuming <flat-space> (Special Relativity), but as we know, space-time is <curved> and SR only apply locally, away from strong gravitational fields.

Back when I studied, quantum gravity (QM + GR) was not a mature theory, and still isn't. Quantum field theory, combine "only" QM + SR, and our model isn't complete until we have gravity (curved space-time) on quantum level.

Assuming this “traversable wormhole” doesn’t connect between different universes (i.e. the fairy tail), I suppose it connects different sides of a space-time curve of some gravitational field.

I don’t see how u can go back in time just inside that space-time while the rest of the universe remains in its original state, since gravitational fields affect each other.

But even if we ignore the above, and assume such “traversable wormhole” exist (even though as far as I know, there is zero evidence for it), I still don’t understand how u can go back in time with it: Being part of a <single universe> if u somehow appear in that gravitational field in passed time, it has to mean that this gravitational field exist in our universe in past time, meaning that its particles somehow reversed to the past. Now how could that happen just by you slipping through some wormhole?

As far as I remember, his father didn't really understand what was happening to him, but the main character (eventually) did, and so he could at least prove it to himself by controlling that time switching process.

And there were some evidence actually: Changes in his brain that would require years, happening in minutes in the timeline that he "left", and his behavior in that time line while he "wasn't there".

Those kind of evidence could probably interest scientist who are into that subject, had he told them about those time switching travels..

<But if you are talking about an actual time machine that _anyone_ could use, then you would have had all the evidence that u want in such case, wouldn't you?>

That is only if everybody gets into the same time/timeline. If everybody gets into a different timeline, we have only on time machine possessor in each world.

If you really want to understand, there is no way around the math and physics of space-time. I tried to open some old books on general relativity, but gave up pretty soon... at least for me, it would require considerable efforts to dive into this topic now. Phew!

Nevertheless, there exists multiple geometries, more or less exotic, which <closed timelike curves> can be found as solutions to Einsteins Field Equations (EFE). While arrow of time leads forward locally, globally the observer may return to an event in the past. Thus violating causality, and opening Pandora’s box with time travel paradoxes.

The traversable (Morris-Thorne) wormhole, is a solution of EFE, just like black holes is. So according to the theory, a wormhole could be created by Universe itself, or we might even picturing man (or alien) made wormholes. The wormhole is constructed in such a way, that humans could pass through, while making a shortcut in space-time, it isn't FTL travel locally.

To make it into a time-machine, they slow down time at one end of the wormhole, e.g. by rotating it very fast for a period. So theoretically, if such a wormhole was created in 2000, time would "stand still" at the rotating end and move forward to 2009 at the other end.

So Alex meeting himself, is a solution of the GR, mixed with SR rotation. Alex jump time backwards by entering the 2009 end of the wormhole...

This is a possible theoretical solution of EFE. There are problems, related to exotic matter and violation of the energy conditions. In my view, such thought experiment demonstrate that we still lack the proper theoretical framework for gravitation.

Our time machine doesn’t allow us getting back to our original timeline? Assuming that it does (I wouldn’t have bought one that doesn’t..), then everybody meet on the same (original) timeline, exchanging experiences..

<But how to test it the way that everybody interested really knows it works and is then able to distribute it?> Are we discussing marketing issues now? :) Again, assuming the machine allows us to get back to the original timeline, we should have no problem here.

I really don’t think that talking about the possibility of having a time machine that travels between parallel timelines, goes hand in hand with the assumption that we will never be able to get evidence that such parallel timelines/universes exist..

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<zarg> <If you really want to understand, there is no way around the math and physics of space-time>

I don’t disagree with that of course, but no matter how deep & complex those theories are, we can always try to judge their final conclusions using strong basic assumptions, and things that we know for sure. And that is the thing I was trying to do here.

<To make it into a time-machine, they slow down time at one end of the wormhole, e.g. by rotating it very fast for a period. So theoretically, if such a wormhole was created in 2000, time would "stand still" at the rotating end and move forward to 2009 at the other end>

As much as it is mind-boggling thinking about objects experiencing time passing slower then others in our universe, due to having different energy levels, I can still accept that somehow. But going <backward> in time is a <totally different> issue IMO.

If I correctly understand what u describe here, then it is not the way I define going back in time, but more like the first case of <different> objects experiencing time passing at different rates:

One end of the wormhole experiencing slower time passing then the other end, but alex cannot be on both ends at the same time, if he slips through the “2009 end” to the “2000 end”, it means that he physically was at the “2009 end” and so he won’t meet a younger version of himself at the “2000 end”, he will only meet an environment that you could say its chronological age is yr 2000, but no object there ever experienced any <later> chronological time.

What do we know for sure??

We have a set of nonlinear second-order hyperbolic partial differential equations with initial conditions on a geometry, and some odd looking solutions.

We can't escape that.

<One end of the wormhole experiencing slower time passing then the other end, but alex cannot be on both ends at the same time, if he slips through the “2009 end” to the “2000 end”, it means that he physically was at the “2009 end” and so he won’t meet a younger version of himself at the “2000 end”, he will only meet an environment that you could say its chronological age is yr 2000, but no object there ever experienced any <later> chronological time.>

No <meta>, closed timelike curves, means you get back to same point in space, and can face an event of the past. If Alex go back, he will face the other end as it was in 2000. The old Alex, can kill the young Alex or his own grandfather...!

BTW, I don't think Hawking's objection, that we haven't seen tourist from the future, is such a strong objection really.

A global deflation effect yes, but that doesn't exclude possibility for local inflation...!

i don't understand fancy physics, so i tend to approach discussions about time travel with about the same attitude as i approach the writings of the zen-master. (more or less like fiction.)

btw, a co-worker of mine had a book by her desk about "dream symbols" - a kind of reference book where you could look up stuff from your dreams to learn what it meant. the reference part was too brief in its explanations (for each indexed word) to be any fun, but the 2-3 introductory chapters were pretty... interesting. :o)

[the dream book, by betty bethards]

<
what we call life is really a school. there are many schools or levels of consciousness in the universe, and earth is one of the most important. [...] we are infinite creative energy or love energy, and all of our hurts, disappointments and disillusions come from a failure to recognize this truth of who and what we are.

the name of the game is growth. in all our experiences we either go through or grow through them. if we just go through them, then we will repeat them again and again until we learn the lesson behind the appearances. if we grow through them, we are free to move on to the next step in the learning process. [...]

the soul is the etheric or energy body, the eternal part of our beings. [...] you have layers or energy bodies within the soul which you shed as you go to higher levels of spiritual awareness. [...] once you have learned your lessons on the earth plane, having gained balance mentally, emotionally, physically and spiritually, you then go to the celestial plane and live in a much higher energy level. [...]

you intentionally have chosen the circumstances of your present existence. before you ever incarnated you knew what the first 28 years of life had in store for you. you chose your parents, sex, race, nationality, socioeconomic conditions, astrological sign. whatever your circumstances, you set them up. [...] everything we perceive of as suffering is really a wonderful opportunity to correct past mistakes or imbalances and move toward our ultimate goal of enlightenment.

[...] we stop blaming others for our unhappiness and realize no one can limit our lives. we cannot change others, but we can change ourselves and move beyond any given situation. every relationship and every experience is nothing more than an opportunity to grow through limitations. >

as a parent, it's a good thing to know that our children's souls have chosen us, and not the other way around - and of course it's good to know that stuff i used to blame on my parents really is my own (soul's) fault... ;o)

seriously, what's most far out - from a rational viewpoint - "scientific" debate about time travel, or spiritual/supernatural dream interpretation? [the latter as opposed to the more scientific/psychoanalytical branch of the trade].

We can look at the universe as a whole, the universe has a global age (i.e. the age of the universe, that I referred to as "Universal time" in my original post here regarding the time travel issue), at each point of that time, there <exist> a snapshot of all the universe' particles and their states (even though it is impossible to actually record that snapshot). And if we are currently in time t, and object X is sent to past time (t-s), then object X resides in a universe which all its particles and their states are exactly the same as they were in time (t-s), while we still reside in a universe which all its particles and their states are exactly the same as they were in time t (minus object X maybe).

I think we can say we know this for sure or call this an "axiom", if we want to be able to have any kind of discussion about such time travelling issue.

And I don't think it contradicts your: <We have a set of nonlinear second-order hyperbolic partial differential equations with initial conditions on a geometry, and some odd looking solutions. We can't escape that.>

As I said, my <assumption> is that parallel timelines and universes don't exist (I elaborated on that issue in my later discussion with alex). By “parallel universes” I also include the possibility of having all “versions of the universe” along the timeline available for switching. Since we can't “switch” between those versions of the universe (using a “wormhole” or a “time machine” or whatever), unless they <exist> concurrently. And defining time as the 4th dimension doesn't change that IMO.

<zarg: No <meta>, closed timelike curves, means you get back to same point in space, and can face an event of the past.>

I think I lost u here <zarg>, are u saying that the <same objects> exist on both ends of the wormhole when u “slow down time” on one of its ends? That one version of alex experiences time moving slower then the other and then they meet? Isn't having the same objects at both ends, means that they don't reside in the same universe-and-time to begin with? Why would we need to slow down time on one end if that was the case?

<If Alex go back, he will face the other end as it was in 2000. The old Alex, can kill the young Alex or his own grandfather...!> Didn't u say that this causality paradox means that alex could never go back in time like that? And doesn't that mean that there is something wrong with all those wormhole time travel assumptions?

--

<frog: seriously, what's most far out - from a rational viewpoint - "scientific" debate about time travel [..]>

what garbage!!!

i never read such garbage or that nonsensical <zen-master> garbage. (only my friends on yahoo read that to me). if i read that garbage myself , i won't be able to sleep at night . thank god such garbage is not there on carlsen page - there are some rating garbage and some nice entertaining 'attacks' from different forum on carlsen page - and that is really good garbage because it helps me sleep.

bad garbage is what causes insomnia and good garbage is what cause sleep ;)

-----*----

btw, meta , the book that I recommended you will lead to some answers about time travel. i don't want to discuss that here. but through mails , perhaps, we will discuss.

-----*-----

there are things people know and then science catches up and explains it in a very nice way.

quantum physics explained many things which smart people have known since ages. and quantum physics brought another dimension of clarity to that 'knowing'.

now, i did it again and i may get myself sucked into a very long debate :)

well, at the moment that is just your belief not knowing.

belief is not that powerful but when you experience that belief to be true then it becomes knowing.

just like riding bicycle - you may have one theory how it is possible to ride bicycle and another theory how it is impossible (there is this much force of air and this dynamics , that dynamics. theoretically and scientifically and logically , it is proved that riding bicycle is impossible etc. etc.)

you can believe either theory but neither belief will be so empowering as the knowing - knowing through experience that you can ride the bicycle . after that you won't need either of theory and will never spend years of time proving (using great logical and scientific skills) which theory of the bicycle riding is correct ;)

similarly if you know (instead of believing) that you chose your parents , it will be liberating. otherwise it is a @#$%*&!#.

regarding "supernatural" phenomena, i have some "spiritual" beliefs of my own. however, i'm too humble about these things to claim that i <know> or that what i believe in makes me <smarter> than other people. and i certainly don't go around naming people <fools> (or worse) because they don't share my beliefs, my values or my interests.

until you joined the debate here, angslo, nobody was speaking about this or that view as being <garbage> either - at worst i referred to time travel and some of (the late) bethard's writings as being "fictional" and/or "far out", in my opinion. unless we refrain from arrogant labling using terms like "foolish", "garbage" and "bs", i'm pretty sure the so far civil debate in alexmagnus' forum will derail or come to a halt.

btw, do you find bethards' writings to be wise, truthful and valuable, angslo? if so, what's your take on the following, which i quoted:

<we cannot change others, but we can change ourselves and move beyond any given situation.>

froggy, you trying to get me into your favorite hobby - endless debates? ;)

well, i replied in my forum :)

----*---

btw, why you playing webmaster/moderator of carlsen page again , froggy?

alexmagnus, i'm not sure i understand this argument of yours. in particular this: "the one with the higher K is normally the weaker player - i.e., "on average", the loser."

do you mean in terms of rating points or in terms of absolute results?

if two stable players meet, one rated 2300 and one rated 2500, then certainly the record over a significant number of games, say 20, will be in favour of the higher rated player (with the lower K). or generally, in games between stable 2500 and stable 2300 players, the 2500 players clearly score (much) higher than the 2300s.

however, the question isn't which group scores higher, but which group that on average scores higher/lower than what the expectancy table prescribes (or predicts, if you like). assuming a "correct" table (or formula) for 200 point differences, one should break even on average, at least as long as the K remain the same for both groups of players.

the latter assumption, however, is what breaks down according to gm krasenkow, when he blames inflation on the different K over and below 2400. i assume his reasoning is that players pass 2400 on their way upwards with a K of 15 (by beating 2400+ players), and drops down again with a K of 10, while their "adventure" above 2400 really was only a sign of normal fluctuation in their performance, for a significant number of players. (some players obviously continue to perform at a higher level).

the good thing about this hypothesis of krasenkow, is that it can be simulated (or simply calculated) to learn something about the conditions that need to be met in order for it to become true, if possible. (one seeming assumption, that sub-2400 players break 2400 by playing 2400+ players, is only true to a very limited degree - sub-2400 players play other sub-2400 players in a great majority of their games).