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Apr-27-09
 | | TheAlchemist: <alexmagnus> A very interesting post. I was once arguing with some friends about time travel over a few beers (of course :-)), and my thought was similar to your 1st point, i.e. if it is possible, then it's already happening and whatever could have been influenced has already been influenced, if that makes sense. Of course there was the point of what if you go back and try to kill your parents, but I think that if "you" in the future decide to do it, then you have actually already tried it and failed (you could theoretically succeed after your birth, but then you'd have a memory of your parents being killed sometime). So, yes, it's kind of like everything is predetermined, i.e. the whole timeline of our universe exists already (simultaneously) from beginning to end. This doesn't mean there isn't free will, but it's that you can not go back and undo something you (or anyone else) have already done. Now, I'm not really versed in physics, so I don't know if my logic is flawed and consistent with the all laws of physics, even if only in theory, so I will gladly accept any criticism and/or further explanations. And less seriously, what if people like Nostradamus were actually time travelers :-) |
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Apr-27-09
 | | alexmagnus: With scenarios 1-3 described by me it is impossible to go back further than own birth (in cases 1-2 maybe own embryonal state). In cases 2-4 changing the past doesn't affect the "original" future (except that the time traveller disappears) - but it affects the future in the parallel time. I.e., in case 2, each time travel is creating a parallel time. With cases 3-4 it's problematic because a "new myself" (in case 2 a 13-year-old, in case 4 adult) appears out of nowhere. But as I said, those last two cases are fiction reserved for the case of causality holding not always. By the way, case 1 also allows travel to the future - if everything is predetermined then so is any future moment. During the travel one simply jumps through all the needed causes in a moment (i.e. they happen instantly) - and ends up in the resulting future. Other cases obviously forbid a travel to the future. Unless we make one more assumption, namely that in each new parallel time the events are predetermined. To change them, one would need to create another parallel time (imagine the movie "Butterfly effect" with the main character travelling not only to "parallel" presents, but also to parallel futures). |
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| Apr-27-09 | | zarg: <Well, the causality principle itself is kind of an axiom.> Yes
<Travel into the future does, but not into the past.> ? If not going backward in time, is an example of <effect> happening before the <cause>, I don't know what is. <Scenario 1) the only one which doesn't break causality: I simply re-experience everything I experienced as a 13-year-old> cause : you was born
effect: you "live"
you live without being born... thus break the <causality principle>. :) |
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| Apr-27-09 | | zarg: <alex: As for your first post, I meant constancy <and> maximaity.> Ehhh no, at least my Prof. in classical mechanics lectured the original derivation of SR by Einstein, which was based on speed of light being a constant/invariant. I know there exists alternative derivations, including the one given by Landau based on locality (where c is max) and I think there even is one derivation based upon causality. However, point is, the original derivation of SR had no "maximality" requirement/assumption/principle/axiom as I remember it. The original derivation also had another "axiom" (Principle of Relativity), which is that laws of nature can be written on covariant form. So SR, boils down to tensor calculus, where the physical laws are stated independent w.r.t. non-accelerated frames of reference. Example of such SR tensor math can be seen here:
http://en.wikipedia.org/wiki/Formul... beautiful stuff. |
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Apr-27-09
| | alexmagnus: <cause : you was born
effect: you "live"
you live without being born... thus break the <causality principle>. Not in case 1. In case 1 I live because of all causes which happened before the point in time I travel to. Only in cases 3-4 I "live without beig born". |
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| Apr-27-09 | | zarg: <alex>
I don't get that at all! :)
If you go back <before> you have been born. You haven't been born <yet>. Is that obvious? |
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Apr-27-09
| | alexmagnus: <zarg> Didn't you understand the first two cases? You simply go back to the time you were. You don't get a new body or new mind - you simply re-live that time again, <with the body and mind and at the place <as you were back then>> (in the first case you may not change events, in the second one you may). See also my second post, where I said that in first three cases it's impossible to go back into time before own birth. |
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Apr-27-09
| | alexmagnus: Just for clarification: in case 2 you may not defne the events you want to change before the travel - you will forget them all during the travel. A change may happen only "accidentally" (or will happen anyway if even weak determinism is wrong). |
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Apr-27-09
| | TheAlchemist: <alexmagnus> If I may, just one question, why couldn't you go to before the time you were born in case 1? I think I didn't get "your way" entirely. As I understand it and presented it in my previous post, the "future you" (i.e. the time traveling one) could go as far back as he wants, he just couldn't influence anything that would affect his existence up to the point in the future where he went back in time. So it would be possible for "two" of "you" to exist at the same time, but the "future one" couldn't influence the "present one" in any new way, or he would remember it from his past. I.e. if the "future one" wanted to kill the "present one", he couldn't and any attempt would be remembered as an assault or whatever from his past. I hope I made myself at least a little bit clear. |
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Apr-27-09
| | alexmagnus: <If I may, just one question, why couldn't you go to before the time you were born in case 1?> Because in case 1 you appear in the past with the body, mind and at the place as you had them at the moment you travel to. So you cannot travel to the time you had no body (though you may end up on different places as particles - those which "in the future" became the spermatozoid of your father and the ovum of your mother the union of which eventually became you). |
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Apr-27-09
| | TheAlchemist: <alexmagnus> OK, thanks. |
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Apr-27-09
| | alexmagnus: < it would be possible for "two" of "you" to exist at the same time, but the "future one" couldn't influence the "present one" in any new way, or he would remember it from his past.> That would be case 5 :). And it is impossible because the existence of two yourselves already means change of events. |
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| Apr-27-09 | | zarg: <alex>
If I don't get something regarding physics, that is a very bad sign. :) If you don't see that causality principle get broken by time travel backwards, I'm simply lost. FIRST cause THEN effect
if time travel backwards, the effect arrive <before> cause, thus the principle is broken per def. |
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Apr-27-09
| | alexmagnus: So travelling to the past <is> confirm with the causality. But it would have no practial use (f.x. sending messags to the past is impossible if causality holds; also, in case 2, you get a different future <but> in a different (parallel) world while your "original" world simply goes on without you). Case 2, if possible, would be quite popular though - who wouldn't want to live his life again? Even though he will remember nothing from th previous life... |
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Apr-27-09
| | alexmagnus: <if time travel backwards, the effect arrive <before> cause, thus the principle is broken per def.> You you travel backwards, you simply "cut off" all effects which came after the point you travel to. The cause of you appearing in the past is not you travel (the effect of which is your disappearance in the present) but the same as why you appeared in the past before. You don't reverse cause and effect, you simply cut the chain of causes and effects at some point and start you life from some past effect (in case 1, living through the same causes and effects again - in case 2 being ableto create new causes and effects - and not only you but everybody.). Travel to the future contrasicts causality - because your apparance there has no cause at all (unless everythng is predeterminded and you can "jump through all causes in a moment". |
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Apr-27-09
| | alexmagnus: Also, remember the parallel time assumption in case 2. You then technically don't arrive in the past but in the parallel time which looks exactly like the past. |
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Apr-27-09
| | TheAlchemist: <alexmagnus> My case was totally different, you're right. What I meant was that if time travel is possible, time travelers are already among us doing their business. I don't know how to put it, but it's like the whole time line of our universe is already determined, it's sort of "already happened". I'm sorry if my examples are all about killing people, but it's probably the best and most drastic I can think of, so again, in this version you couldn't in any way kill your parents before you were born, because then the "future you" (the one time traveling) wouldn't exist anymore, but you could do it after you were born and in this case the "future you" would have a memory of his parents being murdered (which means the "future you" was "meant" to do it). But I guess this probably doesn't hold, right? |
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Apr-27-09
| | alexmagnus: <The Alchemist> I didn't mention your case exactly because of this ind of paradoxa. Case 4 is similar but there you <can> change events - and your age goes on as if you didn't travel in time. |
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Apr-27-09
| | alexmagnus: Also: it is (according to tody's physics) impossible to reverse cause and effect but that doesn't yet mean that it is impossible to produce <causes> whose effects will be the reverse of past effects. |
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| Apr-27-09 | | zarg: <You you travel backwards, you simply "cut off" all effects which came after the point you travel to.> Huh? That isn't time travel. Time travel backwards means taking a physical state as is, e.g. no memory loss and transporting it to the future. You talk about <time reversal>, like rewinding a movie?? I need a drink!! |
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Apr-27-09
| | alexmagnus: Well, cases 1 and 2 are time reversal, yes. In case 2 you can change the causality chain after the point in time you arive at, in case 1 you cannot. |
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Apr-27-09
| | alexmagnus: IMO time reversal is also a "subgroup" of a time travel. In both cases you end up in the past. |
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Apr-27-09
| | alexmagnus: As far as I understand, the only law which "grants" the time being irreversible is the second law of thermodynamics (which by the way, implies the causality too). So it's one single assumption,namely the ever-increasing entropy, which makes neither reversal of time nor time travel possible.
But is there any way to prove entropy increases? To me, entropy is a somewhat messy concept anyway - to unclear what is to concider "more chaotic". |
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Apr-27-09
| | alexmagnus: But the existence of parallel times would resolve the problem of both even if we assume the law to be always true. The problem is, the existence of parallel time is impossible to verify - once someone reaches one, he will have no way to connect himself with our time and tell us he is in the parallel time now... |
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| Apr-28-09 | | zarg: Second law of thermodynamics, isn't a fundamental physical law, it's statistical law and it can get broken locally. <But is there any way to prove entropy increases?> No, <arrow of time> having one direction, is an observation you (and others) have made, and as I have explained before, we can't <prove> stuff like that. We can only reject it, by observing arrow of time in the different direction. However, we could try to derive 2. law of thermodynamics from QM, but I'm not aware of such a "proof". When it comes to <time reversal>, quantum field theory has a CPT theorem, which say that the physics is preserved under inversions of charge (C), parity (P) and time (T). I think there exists a CP-violation (and thus a T-symmetry violation) related to weak interactions. This is different than breaking the 2. law of thermodynamics, but I'm not entirely up to date on this, as I thought T-symmetry was needed to avoid negative energy levels... CPT-symmetry follows from quantum field theory and are hence a build in feature of dynamics of nature as we know it, so <time reversal> is massively different from <time-travel>. Hence, it appears to me that your scenario 1 & 2, violate <arrow of time>, plus quantum field theory (ref. CP-violation). |
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