Time travel paradoxes

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timonan
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Time travel paradoxes

Postby timonan » Mon Nov 24, 2008 12:37 am UTC

Ok, so the obvious one is what would happen if you went back in time and killed your younger self. How could you do it successfully and continue to be the one who went back to do it? There's the related Back to the Future paradox...disrupting the process of your parents' meeting and threatening your own existence that way (though I doubt you would start fading away as it became more and more unlikely like Marty McFly). What if you invented the time machine, then went back and convinced your younger self to take up poetry instead of physics? Would your time machine simply disappear and leave you in the past? Would you instantly flip back to the present? Would you disappear because you can't logically exist in that state? My brain is already hurting.

I'm interested to see who can come up with the most creative time travel paradox...

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Re: Time travel paradoxes

Postby _MC_ » Mon Nov 24, 2008 5:10 am UTC

I am of the opinion that if there are two possibilities, one which creates a paradox and one which does not, the latter must be true. In this case, I would have to take the possibility of time travel paradoxes as evidence that it is impossible to ever get to a point where we would test the paradox theories. I find it most likely that time travel simply can't exist, but another alternative which I believe was either proposed or popularized by Hawking, claims that if you could go back in time, you wouldn't be able to make a change which would cause a paradox.

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Re: Time travel paradoxes

Postby thoughtfully » Mon Nov 24, 2008 5:36 am UTC

There are a number of ways to resolve the paradox without having the Universe manually twerking your free will, which is such a poorly specified concept that it ought to be taken out and shot anyway.

Many Universes/Histories: a new universe comes into existence for each possible outcome of a quantum event.

Consistent Histories: all possible histories exist at once (ok, that's meaningless outside of time. be very careful with language when discussing time!) but the inconsistent timelines destructively interfere and vanish.

The past has already happened. The time traveler only has one personal past; if (s)he goes back in time and meddles around, that's already happened. This is consistent with Consistent Histories.

Novikov (a very smart physicist) had some useful things to say on the matter. Be wary of any naive or totally imaginary biases you may have picked up from science fiction.

Also, this has been beaten to death in other threads, use the search function before starting a new topic.
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Re: Time travel paradoxes

Postby danpilon54 » Mon Nov 24, 2008 5:41 am UTC

thoughtfully wrote:The past has already happened. The time traveler only has one personal past; if (s)he goes back in time and meddles around, that's already happened. This is consistent with Consistent Histories.


This is my favorite and personal belief. Why make up the idea of many universes and all that crap when you can just say the past is the past.
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Re: Time travel paradoxes

Postby phlip » Mon Nov 24, 2008 6:26 am UTC

Novikov self-consistency also gives rise to much more interesting paradoxes than naive time travel does... "If I go back in time and kill my father, but then I'm not born, so noone could've gone back in time to etc" are interesting, but can get boring quickly... "if I go back in time and kill a man who I thought was my father, but he turns out to not be my father, and my mother only met my real father because I killed that other guy" is self consistent, but much more brain-bending (in that, if I didn't go back in time, I wouldn't've been born... so I wouldn't've gone back in time... which is also self-consistent).

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void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}
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Re: Time travel paradoxes

Postby Tac-Tics » Mon Nov 24, 2008 3:38 pm UTC

It's the flux capacitor that allows you to simply fade away instead of destroy history.

The problem smells a lot like a fixpoint problem in math. If you have a program:

Code: Select all

def pointless():
    more_pointless();

def more_pointless():
    pointless();


Calling pointless will lead to a process which never ends. It just keeps flipping violently between two states. You might wonder if this actually happens when you kill your past self, but I'm fairly certain there would be no way to ever discover this. In the above code, there would be no alterations you could make to one function short of breaking the loop to determine that your code was in fact looping between two states without having access to data shared between states (having knowledge that exists outside the timeline).

Most likely, time travel isn't possible. Any scientist who tells you otherwise is just trying to get you to buy his book.

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Re: Time travel paradoxes

Postby Robert'); DROP TABLE *; » Mon Nov 24, 2008 4:45 pm UTC

Tac-Tics wrote:The problem smells a lot like a fixpoint problem in math. If you have a program:

Code: Select all

def pointless():
    more_pointless();

def more_pointless():
    pointless();

Calling pointless will lead to a process which never ends. It just keeps flipping violently between two states. You might wonder if this actually happens when you kill your past self, but I'm fairly certain there would be no way to ever discover this. In the above code, there would be no alterations you could make to one function short of breaking the loop to determine that your code was in fact looping between two states without having access to data shared between states (having knowledge that exists outside the timeline).

Code: Select all

RuntimeError: maximum recursion depth exceeded


I quite like the idea of the self-consistent universe, because it eliminates all of the kill-yourself-before-you-travel paradoxes. Although, that doesn't mean it can't get incredibly confusing.
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Re: Time travel paradoxes

Postby Game_boy » Mon Nov 24, 2008 7:56 pm UTC

Tac-Tics wrote:
Calling pointless will lead to a process which never ends. It just keeps flipping violently between two states.


Yes. I've never seen the problem with the infinite recursion in the "grandfather paradox", but people stop and the end of the story and say "...and that's why it's impossible" when they haven't shown that at all. It's not a contradiction like they think it is.
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Re: Time travel paradoxes

Postby mysticRight » Mon Nov 24, 2008 10:10 pm UTC

A paradox never occurs. All that "has already happened" and all that "will happen" already exists. As three-dimensional beings, we can only see three-dimensional cross sections of the fourth dimension (time). If one were to go back in time and "change" something, nothing would have acutally been changed because it already exists as is, however on a different branch of time than the one pervious traveled and experienced. Going back and killing yourself or convincing yourself to take up poetry will only allow your physical self passage to the alternate branch as you move through time and space from this point.

This video explains my ideas:
http://www.tenthdimension.com/medialinks.php

Of course, this is just one idea. It may be that killing yourself on convincing yourself to take up poetry actually isn't possible at all, because going back in time will create a time loop, where now there is no clear definition of the past and the future. Both the past and the future now effect the eternal present and based on physical laws it would not be possible to alter these events.
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Re: Time travel paradoxes

Postby Tass » Tue Nov 25, 2008 6:42 am UTC

Wow, that video started out okay, but it rapidly descended into pseudoscience.

The four dimensions can be "folded" without need for a fifth.

"If we imagine the seven dimensions as a point, then draw a line from this point..." wtf?

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Re: Time travel paradoxes

Postby The-Rabid-Monkey » Tue Nov 25, 2008 9:38 am UTC

I've discussed this often enough with my Dad.
Personally, being a realist I must say, I have no fucking clue. There are a couple of things I have theorised though.
The idea of going back in time and killing yourself would create an instant paradox I think. This would be because you kill yourself, therefore you wouldn't have been alive to make the time machine to go back in time and kill yourself, which would mean that you were alive to make the time machine, etc.
There is also the chance that you might just fail constantly, and that is why you are even alive to go back and do it. So you try to shoot yourself, but get caught by the cops and locked up, meaning you never get a chance to shoot your prior self.
If you went back in time and talked to yourself, you may have driven yourself mad meaning that your whole perspective is fucked anyway.
I also worked something like this out:
To go backwards in time you must go slower than time itself and to go forwards you need to go faster than time. Basically to forwards you personally go slower and slower and slower, this means that everything zips past you, it's not exactly going forward in time, but it's as good as I can think without you accidentally jumping past the end of the universe. The idea is you make it so that one second of your life = two+ seconds for everyone else, then, you have moved forward in time. As for the idea of getting somewhere into the future and no time having actually have passed for you personally, I really don't know.
Going backwards you go faster and faster, you move so fast that you get there before you actually left. Problem with this is that even if you move at a google^google^google^google^google times light speed, you're still taking time to get there, even if it is such a small fraction that it doesn't register, you still take time. So I suppose with this, you move so fast that everything around you stops, completely, and starts going backwards. I feel though that doing this would take more energy than the universe contains, which would fuck shit up.
Basically there's just an infinite amount of if's and possibilities to make an informed decision on what would happen.

Now back onto best time paradox. Go back in time to 5 seconds ago, presuming that you can step back in time like you would step through a door, and then watch as several million of you all step through that time door all at exactly the same time flood the universe, or perhaps that single spot, shit, epic splicing.
Then if you wanted, you could step backwards in time another 5 seconds, all infinite of you and really cause some damage. Now we have an infinite amount of you flooding this particular piece of time, it's time to break out that knife and start stabbing as many as possible. Just for shits n giggles.
A paradox like this could be instantly solved by stepping forward in time again, so long as you didn't all insta-die from an infinite amount of you stepping backwards in time all at once. Just step forwards more than the amount of time you went back, into the future, so like, 1 minute, which would mean that for a full minute you would not exist in that time.
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Re: Time travel paradoxes

Postby Ziggy » Wed Nov 26, 2008 12:33 am UTC

Assuming time travel is possible, it seems obvious to me that the classic time-travel paradoxes are all impossible. However, I absolutely hate the "universe protects itself" idea fro the movie The Time Machine, and the split timeline hypothesis or anything involving quantum mechanics seems like unparsimonious speculation. All we need to do is look at the definitions of the words involved. The "past" is everything that has already happened. Therefor you can't travel to the past and change it because anything you do in the past has already happened. From your perspective, it may seem like the universe is thwarting you, but no such thing is happening. You can't go back and kill your grandfather because your grandfather didn't die. However, other, I think more interesting paradoxes, crop up. Let's say that when I was a boy, my grandfather was absolutely horrible to me. He was verbally abusive and would beat me for no reason. As a result I suffer from depression my entire life. I eventually create a time machine, and since I hate both my grandfather and my life, I decide to go back and attempt suicide by Grandfather paradox. I arrive in the past and track down my grandfather. Filled with rage, I kidnap and torture him, and in the process reveal to him who I am. However, before I actually kill him, I am tracked down by the police and flee in my time machine back to my own time. Back in the past my grandfather is questioned by the police, who dismiss him as a nut when he tells them that his grandson kidnapped him and fled in a time machine. He eventually becomes a bitter old man, and when the grandson is born, he takes revenge by treating him cruelly. Now, the whole timeline is consistent, so there is no immediately apparent time-ripping paradox. However, it should be obvious that something is not quite right. Causality has looped, with me torturing my grandfather because he beat me because I tortured him and on and on ad infinitum.
Another note, there would be no "avoiding" the future either, since the future is everything that is going to happen. this is really just the same problem, so I won't go into any depth on it.

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Re: Time travel paradoxes

Postby eternauta3k » Thu Nov 27, 2008 1:43 am UTC

The-Rabid-Monkey wrote:Now back onto best time paradox. Go back in time to 5 seconds ago, presuming that you can step back in time like you would step through a door, and then watch as several million of you all step through that time door all at exactly the same time flood the universe, or perhaps that single spot, shit, epic splicing.
I don't get it. When you look back, you're looking at a moment different from the moment of arrival. You would only see people appearing if you stood still at the instant the door takes you to. Right?
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Re: Time travel paradoxes

Postby Quixotess » Thu Nov 27, 2008 2:12 am UTC

phlip wrote:Have a read through By His Bootstraps and All You Zombies. Anyone who claims to understand them on the first time through is a liar.

Ooh! We just studied All You Zombies in philosophy class. Lawd, I think I understood it, but, um, yeah. Ouch my brain.

Another thing we talked about was the movie Somewhere in Time. This had an interesting very-weird-thing-with-time-travel in it:

When Richard was a young man in 1972, one day an old woman came up to him and put an antique watch in his hands, begging him to "come back to me." This moment made an impression on Richard, and he held on to the watch for eight years. In 1980 he stayed at a hotel and saw an old photograph of a young woman which captivated him. He manages to date the photograph to 1912 and identify the woman as a well-known stage actress and as the woman who gave him the watch - she had died the next day, and he recognizes her from the picture in her obituary. Around the same time, he learned about time travel from his old college professor (the movie's idea of time travel was extremely hokey - you're supposed to dress up in an outfit of the time period you wish to travel too, and remove all objects postdating that time period from your sight, and then do some sort of hypnotization of yourself.) Anyway, so he goes back to 1912 in pursuit of the young woman, and he finds her, and they fall in love very quickly yadda yadda. One evening he's emptying out his pockets. He takes out the watch, and she wants to see it because it's very expensive. Then he finds a 1980 penny in one of his pockets. This jolts him out of 1912 and back into 1980. He's totally devastated, he can't get back, he becomes depressed and dies.

So. See the problem?

Spoiler:
Where did the watch come from?
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Re: Time travel paradoxes

Postby tantalum » Thu Nov 27, 2008 3:29 pm UTC

I think that when you say you "travel to the past" another copy of yourself need not be created.

Imagine your life as a really long, immersive video system where you get to experience the five senses in real time and have the illusion of being able to control the video (This is kinda assuming determinism, but just go along for now). If this video scenario were indeed the case, then "going back in time" would be completely self consistent: it would just be rewinding the video. No extra copies are created, you reexperience the past, and [maybe] you change the future. In a sense, the only thing that gets sent back in time is your "consciousness", whatever that is.

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Re: Time travel paradoxes

Postby gmalivuk » Thu Nov 27, 2008 3:37 pm UTC

eternauta3k wrote:
The-Rabid-Monkey wrote:Now back onto best time paradox. Go back in time to 5 seconds ago, presuming that you can step back in time like you would step through a door, and then watch as several million of you all step through that time door all at exactly the same time flood the universe, or perhaps that single spot, shit, epic splicing.
I don't get it.

Yeah, there's no reason why this would happen. Presumably there'd just be two of you for 5 seconds, and then the you who was "already" there in the past would step through the time door, leaving the you who had "already" stepped through that door alone again.

The-Rabid-Monkey wrote:So I suppose with this, you move so fast that everything around you stops, completely, and starts going backwards.

I'm not sure why you think this could happen, either. You move really slow, and time passes quickly for you, putting you in the future. You move faster, and time slows down for you, sure. But there's no reason to think it'd ever stop and start going backwards. Why can't it just approach 0?

(What does time look like to tachyons?)
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Re: Time travel paradoxes

Postby YoungStudent » Thu Nov 27, 2008 4:40 pm UTC

Hawking, claims that if you could go back in time, you wouldn't be able to make a change which would cause a paradox.

Dont you guys get it? If you go back in time, you can do everything...but you cant go back into future...thats teh simpliest...


If you go back in time, you AND time-machine are in NEW reality...old one (future) is past. If you kill your Father, hes dead...but you will still be kept in that reality...you can never return to reality you came from...if future-traveling would be going at the speed of light...new reality forms.

More i think all of this, i find that past-traveling just cant be possiple...but who the hell knows...ill believe my "theory" forever.
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Re: Time travel paradoxes

Postby rho » Thu Nov 27, 2008 7:17 pm UTC

I'll stick with:

Douglas Adams wrote: One of the major problems encountered in time travel is not that of becoming your own father or mother. There is no problem in becoming your own father or mother that a broad-minded and well-adjusted family can't cope with. There is no problem with changing the course of history—the course of history does not change because it all fits together like a jigsaw. All the important changes have happened before the things they were supposed to change and it all sorts itself out in the end.

The major problem is simply one of grammar, and the main work to consult in this matter is Dr. Dan Streetmentioner's Time Traveler's Handbook of 1001 Tense Formations. It will tell you, for instance, how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. The event will be descibed differently according to whether you are talking about it from the standpoint of your own natural time, from a time in the further future, or a time in the further past and is futher complicated by the possibility of conducting conversations while you are actually traveling from one time to another with the intention of becoming your own mother or father.

Most readers get as far as the Future Semiconditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up; and in fact in later editions of the book all pages beyond this point have been left blank to save on printing costs.

The Hitchhiker's Guide to the Galaxy skips lightly over this tangle of academic abstraction, pausing only to note that the term "Future Perfect" has been abandoned since it was discovered not to be.


See, it all sorts itself out in the end.
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Re: Time travel paradoxes

Postby tatertom » Mon Dec 01, 2008 8:59 am UTC

I don't think time travel is possible, as time itself is not a real dimension in and of itself. Time [this thread is about theory, right?] is an adjusted measure of distance, similar to how weight is an adjusted measure of gravity. A day is the measure of one spot on the equator traveling the circumference of the earth exactly once. A second is a division of that. Atomic clocks measure in a similar fashion, with smaller bodies. This makes time relative.

If you take the items related to make an increment of time and put them in a different environment, where the two items interact differently, then you have changed time, such as how perception of time changes as you approach the speed of light. Speaking of which, take note of what a light-year is. A distance measurement, based on our solar system's three visible dimensions. So is a light-year different in another solar system, then? That would change the speed of light. Which is based on time. Which is based on distance.

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Re: Time travel paradoxes

Postby gmalivuk » Mon Dec 01, 2008 2:02 pm UTC

You can't talk coherently about change without invoking time, though. What do you mean, "traveling" the circumference of the earth?

(See the "Time: a true dimension" thread for more discussion of this.)
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Re: Time travel paradoxes

Postby The-Rabid-Monkey » Mon Dec 01, 2008 2:16 pm UTC

eternauta3k wrote:
The-Rabid-Monkey wrote:Now back onto best time paradox. Go back in time to 5 seconds ago, presuming that you can step back in time like you would step through a door, and then watch as several million of you all step through that time door all at exactly the same time flood the universe, or perhaps that single spot, shit, epic splicing.
I don't get it. When you look back, you're looking at a moment different from the moment of arrival. You would only see people appearing if you stood still at the instant the door takes you to. Right?

What I'm saying here basically is this. All occurrences of you at that instance, set their time machines for one particular instance, thus arriving there at exactly the same time. This is going to be hard to explain, but here goes...

Now, this works on the whole principle that you can have multiples of yourself in one place at the same time, due to the fact that you're going back into the past, where you still are. So if you head back ten seconds (assuming lack of infinite amounts of you arriving at same time) then there are two of you at that time, you, and your ten second past self, who will then head off into the past after ten seconds, just as you did, assuming you didn't alter time, and this leaves only one of you.
You are also to assume that time is a string that goes for an infinite amount of time each way, so you can move from one place on this "string" to another.

Now...
Say that I set my time machine (which just happens to be a door frame) for exactly 02:33:00:000 and stepped through it. Now since we are on a massively long string, a billionth of a billionth second later (relative to you, before you stepped back) another instance of you sets their time door frame to 02:33:00:000 and steps through it. This happens for every occurrence of you that arrives at the time where you actually take the step back, which is an infinite amount of occurrences of you. So you all leave at different times, relative to you, but you all leave at say exactly 02:35:00:000. Since you are traveling to an exact time, you all arrive at that exact same time.

Also, there is the note that if you travel from the spot you are currently in, to the same spot, one minute, you will end up in space. As the Earth isn't in that spot just yet.

Also, note on flooding the universe, this is incorrect. This instead happens...
Since all infinite of you arrive at exactly the same time, in exactly the same place, in exactly the same way, you're all going to be occupying the exact same space. This is going to result in one particular area of the universe having a mass of an infinite weight. Which means that you just effectively destroyed the universe in some super black hole.
Shit... if my theory is true, time travel could be possible, but would destroy the universe.
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Re: Time travel paradoxes

Postby mysticRight » Mon Dec 01, 2008 4:03 pm UTC

tatertom wrote:I don't think time travel is possible, as time itself is not a real dimension in and of itself. Time [this thread is about theory, right?] is an adjusted measure of distance, similar to how weight is an adjusted measure of gravity. A day is the measure of one spot on the equator traveling the circumference of the earth exactly once. A second is a division of that. Atomic clocks measure in a similar fashion, with smaller bodies. This makes time relative.


You can't use this as an arguement debunking time as a dimension. Yes, the way we measure time (and distance) is somewhat of a fabrication, and simply a creation of man. But the dimension of time is duration...one moment to the next. How do two objects occupy in the same point in three-dimensional space? When they exist at this point at different moments in time. As gmalivuk said: You can't talk coherently about change without invoking time.

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Re: Time travel paradoxes

Postby Kayube » Mon Dec 01, 2008 4:56 pm UTC

My problem with time travel is it would violate conservation of matter- say you did go back in time and kill your grandfather and managed to keep existing after that. Then, you didn't come from anywhere- you just popped into existence with the time machine one arbitrary day and killed someone for reasons known only to you. Since in the new timeline you were never born, you can't be taken from the future anymore, so you've just introduced completely new matter to the universe...

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Re: Time travel paradoxes

Postby gmalivuk » Mon Dec 01, 2008 6:42 pm UTC

But perhaps conservation of matter only *seems* to be a "law" because so far we've never observed anything like time travel that violates it. (After all, the original formulation of the law has already had to change once, when it was realized that matter and energy can be converted between each other.)
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Re: Time travel paradoxes

Postby BlackSails » Mon Dec 01, 2008 8:54 pm UTC

But perhaps conservation of matter only *seems* to be a "law" because so far we've never observed anything like time travel that violates it. (After all, the original formulation of the law has already had to change once, when it was realized that matter and energy can be converted between each other.)


If conservation of mass/energy isnt a law, then the laws of physics are not constant with respect to time. Which makes discussion of just about anything sort of moot.

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Re: Time travel paradoxes

Postby EdgarJPublius » Mon Dec 01, 2008 9:13 pm UTC

Kayube wrote:My problem with time travel is it would violate conservation of matter- say you did go back in time and kill your grandfather and managed to keep existing after that. Then, you didn't come from anywhere- you just popped into existence with the time machine one arbitrary day and killed someone for reasons known only to you. Since in the new timeline you were never born, you can't be taken from the future anymore, so you've just introduced completely new matter to the universe...



any sort of paradox protection (or even Novikov self-consistent paradoxes) prevents this from being a problem.

Alternatively, as per Mass-Energy equivalency, it may require an amount of energy equal to the mass created by a paradox to create a paradox.
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Re: Time travel paradoxes

Postby gmalivuk » Mon Dec 01, 2008 11:24 pm UTC

BlackSails wrote:If conservation of mass/energy isnt a law, then the laws of physics are not constant with respect to time.

How do you figure?
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Re: Time travel paradoxes

Postby BlackSails » Tue Dec 02, 2008 1:48 am UTC

gmalivuk wrote:
BlackSails wrote:If conservation of mass/energy isnt a law, then the laws of physics are not constant with respect to time.

How do you figure?


Noether's theorem says that for every symmetry in physical laws, there is a conserved quantity. In the case of time-symmetry, the conserved quantity is mass/energy. So if energy is not conserved, then the laws of physics are not symmetric wrt time.

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Re: Time travel paradoxes

Postby Monty40xi » Tue Dec 02, 2008 5:58 am UTC

I tend to view time as Flatland's 3rd dimension. The Flatland universe would look to use like a sculpture of many columns, and any given horizontal slice of the sculpture would correspond to one moment of time in Flatland. Handwave that to a 4-dimensional version, and presto. Only one timeline could exist, but within that timeline there's still the possibility for things to appear to move backwards or skip time frames.

Rigorous? No, but I like it, and I'll hang onto it until time travel experiments contradict it.
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Re: Time travel paradoxes

Postby ConMan » Wed Dec 03, 2008 2:57 am UTC

BlackSails wrote:
gmalivuk wrote:
BlackSails wrote:If conservation of mass/energy isnt a law, then the laws of physics are not constant with respect to time.

How do you figure?


Noether's theorem says that for every symmetry in physical laws, there is a conserved quantity. In the case of time-symmetry, the conserved quantity is mass/energy. So if energy is not conserved, then the laws of physics are not symmetric wrt time.


General Relativity is already known to violate Conservation of Mass-Energy - a photon traveling in an expanding metric gets redshifted and the disappearing energy doesn't seem to go anywhere. If you have a GR-based time machine, at least (e.g. a wormhole), then you'd have to examine the symmetry across the whole manifold which would include the path back through the wormhole anyway, which as far as I can tell would satisfy local CoME, at least.


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Where did the watch come from?


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I don't have as much a problem with where it came from as with how it can manage to exist for 60 years without deteriorating - since you can follow it through a 60-year loop of time and it has to end up exactly the same at the end of said loop. Unless the entire watch gets replaced through repairs over that 60 years, which also negates the "where did it come from" problem but does raise a slight Theseus' ship issue.
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Re: Time travel paradoxes

Postby meat.paste » Wed Dec 03, 2008 3:33 pm UTC

gmalivuk wrote:(What does time look like to tachyons?)


Interesting question. It's making me think about what time looks like to a photon and why is the speed of light in vacuum ~3x10^8 m/s?

Now my brain is spinning, which is typical when trying to understand time travelling into the past.
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Re: Time travel paradoxes

Postby BlackSails » Wed Dec 03, 2008 6:09 pm UTC

meat.paste wrote:
gmalivuk wrote:(What does time look like to tachyons?)

why is the speed of light in vacuum ~3x10^8 m/s?


Because c has a defined value, not an experimental one. :roll:

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Re: Time travel paradoxes

Postby Izawwlgood » Wed Dec 03, 2008 6:21 pm UTC

I read it in a Stephen Baxter book, but I'm not sure if he's the thunker of the idea:

You've got a pool table, but instead of pockets, you've got time portals. Two such are set up so that if something enters pocket A, it will emerge from pocket B at the opposite trajectory but, say 3 seconds in the past.

So say you fling a ball at pocket A. It rolls along the table, enters the pocket, and emerges from pocket B. Lets assume it's two corner pockets. Emerging from pocket B, the ball rolls along and whoops! it collides with a ball thats rolling towards pocket A, knocking it aside, preventing it from entering the pocket.

Hmmm. Silly grandfather paradoxes.

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Re: Time travel paradoxes

Postby gmalivuk » Wed Dec 03, 2008 8:24 pm UTC

meat.paste wrote:
gmalivuk wrote:(What does time look like to tachyons?)

Interesting question. It's making me think about what time looks like to a photon

A photon wouldn't "experience" any time at all, I suspect, because it travels along a null geodesic.
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Re: Time travel paradoxes

Postby ThinkerEmeritus » Wed Dec 03, 2008 9:10 pm UTC

I once read an adaptation of Fermi's paradox to time travel and unfortunately cannot remember where it was. I also can't find it on Google, and it hasn't been mentioned in this thread, so it must not be well known and may be worth repeating. Fermi remarked about extraterrestrials, "Where are they?" In other words, they have had millions of years to find us, why haven't we seen them. The same comment applies to time travel. If someone, anywhere, discovers a way to travel backwards in time (and get back), all of previous time is open to them, and they can travel anywhen. Why haven't we spotted them? In particular, why is one of them not now (Dec.3, 2008 at 4:02 PM: whoever you are, you should after all these years be able to figure out where from my posts) knocking on my back door to say "Here I am." Of course, if time travel is possible only back to the first time that a time machine was manufactured, there is no paradox, but otherwise I find this question to be the most compelling time-travel paradox of them all.

Nonetheless, coming sometime soon to a forum near you, a completely tongue-in-cheek description of the principles of a gadget that would permit travel backwards in time. Expensive to operate and currently completely impractical, but violating no law of physics known to me. Tune up your rebuttal machines.
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Re: Time travel paradoxes

Postby Izawwlgood » Wed Dec 03, 2008 9:41 pm UTC

I would counter with "Because you aren't important enough to merit a hello on my next vacation to the primitives reservation"?
Or whats to say major historical figures AREN'T time travelers who decided to shape the course of history? (Aka, Orson Scott Card's Pastwatch).
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Re: Time travel paradoxes

Postby ThinkerEmeritus » Thu Dec 04, 2008 1:39 am UTC

It is difficult to hold a conversation about time travel without having any idea how the time travel would be accomplished. To attack this problem, I will propose general principles for eventually building a time travel machine.

[Tongue-in-cheek mode on] Physics actually contains a mechanism for travelling backwards in time, as first pointed out by Feynman. It does not allow the sci-fi version of jumping to a particular time, but it does allow reversing the direction you are moving in time. So you could in principle go backwards in time, but you would move at the same rate that you would otherwise move forward. Clearly this will limit the willingness of volunteers to remain in motion, but that helps explain the absence of observed time travellers.

If the energy of a particle is reversed in sign, it automatically moves backward in time. This reversal doesn't happen spontaneously, but it can be arranged if you can dispose of the large amount of energy that results. The particle moving backward in time is called an antiparticle, and examples are well-known. Once we have adequate knowledge of the human body, it should be possible to move all the particles in the body together into corresponding negative-energy states, and that body will procede to go back in time. I assume that since the human body generates only classical information it is essentially a classical collection, and if this is not true building the machine will be a great deal harder if not impossible. Moreover, the volunteer must be protected from collisions with all positive-energy matter, which would otherwise fall into the negative-energy states and corrode the volunteer. So if the volunteer is to accomplish anything useful, he/she [hereafter he for brevity] must be returned to a forward-moving body at some time in the past. Hence useful time-travel cannot extend to times before the invention of the time machine, which further explains why time travellers are not yet seen. All the mechanical laws of physics are essentially the same when time is reversed, so there should be no anomalies during the trip. It is not possible to predict how thermodynamics will work during the time travel, but we will assume that since we know the condition of the body at the later time and not at the earlier time, thermodynamics will also reverse for the time-travelling body.

To a body travelling forward in time, like us, the time-traveller will appear to be a body made of antimatter travelling with us in a positive-time direction. The return to forward time-travel will appear to be a person-antiperson pair creation event. I have not explored the possiblities very carefully, but I think that this fact will prevent any serious paradoxes.

So all that is left is for some reader to accumulate the knowledge necessary to build such a machine. In the meantime we should be able to determine in principle if the process is safe, i.e. if there are indeed no possible paradoxes.

If anyone thinks that the machine has already been made secretly, and that we should try to find out about it, I suggest a careful examination of the biographies of all twins.

Have fun. In particular, is the volunteer moving backward still the same person or just a copy? If not the same person, is participating in the experiment, which necessarily involves the disappearance of the original, forward-moving version, a case of suicide? [/tongue-in-cheek mode]

EDIT: I have spent some time thinking about the billiard-ball paradox upthread with this mechanism for time travel. I haven't spotted a rule that would forbid the paradox. Interesting. Could it be that the mechanism works only in quantum mechanics?
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Re: Time travel paradoxes

Postby meat.paste » Fri Dec 05, 2008 9:04 pm UTC

gmalivuk wrote:
meat.paste wrote:
gmalivuk wrote:(What does time look like to tachyons?)

Interesting question. It's making me think about what time looks like to a photon

A photon wouldn't "experience" any time at all, I suspect, because it travels along a null geodesic.


I was thinking that might be true, but it was a feeling more than a knowledgeable guess. I wonder if the fundamental structure of space-time is what gives the appearance that light travels at c. In other words, is the speed of light in a vacuum a consequence of having no mass particles traveling on a null geodesic? If we had a different low level space time definition, would the speed of light be different? Do the various string/brane/loop/planck length theories predict the speed of light?
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Re: Time travel paradoxes

Postby ddxxdd » Sun Dec 07, 2008 1:22 am UTC

ThinkerEmeritus, I strongly agree with your approach, but I disagree with you on a few things.

If I turned all my protons into anti-protons, all my neutrons into anti-neutrons, and somehow created some form of "anti-physics" that made all the laws of physics inside my body go backwards, then I wouldn't go back in time. I'd just turn into a toddler.

However, if I modified this "anti-physics" machine to make THE REST OF THE UNIVERSE (or at least this planet) go backwards in time, then I would've successfully travelled back in time.

This form of time travelling creates no paradoxes. "Universal Time" doesn't go backwards, only "The-rest-of-the-universe's-time" goes backwards, while your time remains in tact. There will be no copies of you in the "past", and you can kill your grandfather all you like.

Now if only someone else on these fora could come up with other physical methods of time travel, where you can meet your younger self...
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Re: Time travel paradoxes

Postby Zak » Sun Dec 07, 2008 1:35 am UTC

This story about time travel is really great.

Part 1
Part 2
3
4

I like the idea.
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