[Physics] Time Travel

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Do you think time-traveling into the past is possible? If yes, what kind?

Yes and I think you could kill your Grandfather [Multiple-Timelines]
27
15%
Yes but I do not think you could kill your Grandfather [Consistent-Timeline]
28
16%
Yes but you would risk a time-loop [Screw-Up Timeline]
5
3%
Yes but something entirely different.
19
11%
No, I do not think this is possible at all.
74
42%
How in hell should I know?
16
9%
If I could timetravel I would stop you from wasting my time with this stupid thread!
6
3%
 
Total votes: 175

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Shakleton
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[Physics] Time Travel

Postby Shakleton » Mon May 12, 2008 10:06 pm UTC

Hello Guys!

Due to the fact that I have watched a lot of "Star Trek - Voyager" on YouTube lately I have become more interested (at least for the moment) in the possibility or impossibility of time traveling.
Please keep down your "That's not possible at all"-Signs. I just want to get a discussion going. What are your opinions on this topic?

First of all, we are talking about time traveling to the past. We all already know about the fact that time travelling into the future is possible using a spaceship reaching a very high velocity so time dilation kicks in and we have the time passing slower for ourselves and thus are traveling into the future. ( Hooray for my first use of the word "thus". I always wanted to do that. :) )

Speaking of time traveling to the past there is one major problem: The grandfather-paradoxon

For the ones who do not already know it:
Say you traveled to the time your grandfather was a child and killed him. He could never have begeted your father. Thus (yay again) you would never have been born, would not have been able to make time travel, would not have been able to kill your grandfather would have been born, would make time travel, would kill your grandfather would not have been born would not kill your grandfather would have been born wouldkillyourgrandfatherwouldnothavebeenbornwouldnotkillyourgrandfatherwould.... I think you get the point.

Provided by Past-Special-Relativity-Physics and Science Fiction, there are three possibilities for the grandfather-paradoxon to be solved.

:arrow: If you would go back in time and kill your grandfather you would stay existent the way you are but the timeline you experienced before your travel would be erased. In the past you would stay who you are, nothing would happen to you. You just would not be born later. (Sorry for the bad grammar. My English seems not to be "Highly-Hypothetical-Time-Paradox-proof") Because you are creating a completely new past, present and future this is called the "Multiple-Timeline-Theory"

:arrow: If you would go back in time you would not be able to kill your grandfather for some reason. Maybe your gun would not work or you would have a change of heart or something completely different. This would imply that you could only do time traveling to the past if history would show that you did it in the first place. You could not alter the timeline, you would just follow it as it was, including the time travel itself. This means in a linear timeline somebody who has not been born yet would pop out of nowhere with the intention to kill his grandfather. Then he would not do it for some reason. And later, he would be born just as usual. And even later he would make a time travel to kill his grandfather. Because in this theory there is only one timeline that nobody can alter it is called the "Consistent Timeline Theory"

:arrow: If you would go back in time to kill your grandfather the events would follow the disription above. That would trap us all forever in a time-loop because no-one could ever go past the point of the time travel because at this point all history since the murder of a child that happened to be the grandfather of the time traveler would have been altered. And after that it would be altered again because nobody killed the child. And again and again and again. Nobody would get annoyed because we all would think it is the first time we are experiening that timeline. Because this is so messy I call it the "Screw-Up Timeline Theory"

If you have any other ideas of possibilites, post them below!
I'm leaving the technology for time traveling into the past open for discussion!

Now, let us begin the thread!

What are your opinions? What do you think?

Shakleton
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Re: [Physics] Time Travel

Postby Klotz » Tue May 13, 2008 12:05 am UTC

The thing about time travel is that any future time travel to the past has already happened. Since my grandfather didn't die before my father was born, nobody will ever go into the past and kill him.

The only way that time travel can exist and paradoxes can be avoided is by multiple timelines.

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Re: [Physics] Time Travel

Postby heydonms » Tue May 13, 2008 1:10 am UTC

Klotz wrote:The only way that time travel can exist and paradoxes can be avoided is by multiple timelines.


Assuming you have free will.

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Re: [Physics] Time Travel

Postby danpilon54 » Tue May 13, 2008 3:04 am UTC

I said the second choice, but I don't think it violates free will. You could try to kill your grandfather, nothing stops you from trying. But it has been proven that you do not succeed, because nobody in fact killed your grandfather. Having impossible results to potential actions does not violate free will. I could try all I want to be president and its not going to happen, but doesn't mean there is some crazy mystical force stopping me from becoming president.

Multiple timelines is just so much less likely than a continuous timeline. Since there is no evidence for either, it is best to assume a continuous timeline.
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Re: [Physics] Time Travel

Postby TheCoelacanth » Tue May 13, 2008 6:46 am UTC

Where's the "Yes, and I think you could kill your grandfather, and then become your own grandfather" option.

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Re: [Physics] Time Travel

Postby NathanielK » Tue May 13, 2008 6:47 am UTC

I chose the first option, but on second thought I think I like the second better. In either case, I think the (lack of) evidence suggests that it is not possible to travel to a time before the first time machine is built. The first person to build a working time machine, though, had better be prepared for a bunch of tourists to step out and start asking him stupid questions as soon as he turns it on.
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Re: [Physics] Time Travel

Postby ian » Tue May 13, 2008 9:34 am UTC

Otter/duck paradox would occur

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Re: [Physics] Time Travel

Postby Firnagzen » Tue May 13, 2008 11:43 am UTC

If I ever get a time machine in future, I'm going to send it back to myself... Oh, about now.

*Looks around*

Sad. No time machine! Possible reasons:
-I have a terrible memory. I'm probably going to forget to do that, because I'd be having so much fun with the time machine, anyway.
-I got abducted by the secret Time Police.
-I realized that I'd probably screw up the universe if I ever got a time machine to play with, so I didn't.
-I didn't get a time machine. :(

Also,
ian wrote:Otter/duck paradox would occur

What's the otter/duck paradox?
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Re: [Physics] Time Travel

Postby Nimz » Tue May 13, 2008 12:42 pm UTC

TheCoelacanth wrote:Where's the "Yes, and I think you could kill your grandfather, and then become your own grandfather" option.

That would be the Philip J. Fry paradox.

Also, the cool science nerds say "closed time-like curves," not "time travel." Furthermore, if you believe the many worlds interpretation of quantum mechanics, I don't see how multiple timelines in general relativity is much different. Although I don't know how free free will is if every possible outcome actually happens in one universe or another. It's not like you chose which universe you're in and are thus reading this post - the universe you're in chose you to read this post. So, if free will exists, closed time-like curves are impossible, or they are associated with such spatial dislocation that it is impossible to affect anything observable from the time and place of origin. Which would mean that if you were to get back to your point of origin, you would arrive after you left. Doesn't sound very exciting.
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Re: [Physics] Time Travel

Postby danpilon54 » Tue May 13, 2008 2:10 pm UTC

Key words:

Nimz wrote:if you believe the many worlds interpretation of quantum mechanics, I don't see how multiple timelines in general relativity is much different.


Theres no more evidence supporting multiple universes in quantum mechanics than in anything else. The fact of the matter is, if there are multiple universes, by definition we can never detect them, meaning their existence is meaningless. The subject of multiple universes will always be a philosophical one, and never a truly physical one in my opinion.
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Re: [Physics] Time Travel

Postby andyisagod » Tue May 13, 2008 2:25 pm UTC

danpilon54 wrote:Key words:

Nimz wrote:if you believe the many worlds interpretation of quantum mechanics, I don't see how multiple timelines in general relativity is much different.


Theres no more evidence supporting multiple universes in quantum mechanics than in anything else. The fact of the matter is, if there are multiple universes, by definition we can never detect them, meaning their existence is meaningless. The subject of multiple universes will always be a philosophical one, and never a truly physical one in my opinion.


Except that at the moment it seems one of the best ways of interpreting quantum mechanics. In terms of time travel it actually might help out a bit. My problem with option 2 is that it doesn't seem consistent with quantum mechanics assuming the time line is going to stay fixed you could essentially travel back in time and know the outcome of a quantum event (say an electron being measured spin up or spin down). This would imply some kind of hidden variable theory which has pretty much been ruled out or implies a paradox in quantum mechanics that if you travel back in time you can essentialy known more about a quantum system than the state function. I say the many worlds interpretation may help here because it allows the quantum effects to continue you just end up in a different future universe (which would make the time machine useless for winning the lottery!)

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Re: [Physics] Time Travel

Postby Mettra » Tue May 13, 2008 5:06 pm UTC

One problem that always comes up with time travel (into the past, of course) is the need for some kind of time-recursive mechanism. In other words, the mechanism that allows you to travel into the past needs to travel into the past itself (and therefore needs another mechanism to travel into the past etc. etc. turtles). If you find a mechanism to travel into the past (making use of the large energy near a black hole, for example), you can only travel as far back as the mechanism is still valid.

So in other words, let's say you built a machine that you can use to travel back into time. You start using it and it either 'unbuilds' itself as you make the progression, or it relies on another mechanism (some natural phenomenon, for instance) that starts to 'unbuild' itself.

Interestingly, some argue that this is why we don't have visitors from the future - we need to build a mechanism first so that they can 'travel' back to it.

-------
On a more serious note, if time travel (into the past) is possible, there would need to be some kind of many-worlds situation because, as 'andyisagod' mentioned, you would otherwise be able to know too much about states - basically you can think of this as a violation of the uncertainty principle. This seems like a rather big no-no to me, personally, so I would say, within the construct of the physics we have, that it is either impossible or there is some kind of limiting mechanism.

Slightly offtopic: the discussion of whether or not we have free will cannot, by the very nature of the topic, lead to any serious discussion, and it certainly isn't a valid argument for or against any kind of proposition to do with the discussion at hand.
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Re: [Physics] Time Travel

Postby tehmikey » Tue May 13, 2008 5:56 pm UTC

I think we would need to take the doom field into consideration. Any time created duplicate is doomed. Futurama has taught me most of what I need to know about time travel.

I am curious what happens when the time traveler gets back to the time when he made the jump. I guess the first time jumper goes back in time while the older past jumper continues on? What if you then convince the young self to 'not' travel back in time? Would there then be 2 of you forever? If I went back in time 30 seconds, I would clone myself essentially. This whole area is a bit grey.

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Re: [Physics] Time Travel

Postby nsmjohn » Tue May 13, 2008 7:12 pm UTC

Mettra wrote:One problem that always comes up with time travel (into the past, of course) is the need for some kind of time-recursive mechanism. In other words, the mechanism that allows you to travel into the past needs to travel into the past itself (and therefore needs another mechanism to travel into the past etc. etc. turtles). If you find a mechanism to travel into the past (making use of the large energy near a black hole, for example), you can only travel as far back as the mechanism is still valid.

So in other words, let's say you built a machine that you can use to travel back into time. You start using it and it either 'unbuilds' itself as you make the progression, or it relies on another mechanism (some natural phenomenon, for instance) that starts to 'unbuild' itself.

Interestingly, some argue that this is why we don't have visitors from the future - we need to build a mechanism first so that they can 'travel' back to it.


Watch "Primer".

I really don't know and have no opinion on which answer is right, but the idea of time travel interests me immensely and I always thought that if it is possible the only route to go would be the multiple timelines one. As Nimz pointed out this is invariably the multiple worlds interpretation of Quantum Mechanics. I have this long explanation in my head but I am just going to keep it to myself because it sounds strange to even me (and I tend to over explain everything).
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Re: [Physics] Time Travel

Postby Herman » Tue May 13, 2008 8:05 pm UTC

Time travel is interesting because it allows for phenomena that seem very unphysical: you could violate conservations of energy and momentum, reverse entropy, do an infinite number of computations in no time at all, and of course, kill your grandfather. And yet they tell me that general relativity allows for it. So I suspect that GR will turn out to be an approximation (valid under certain conditions) of The Deep Physics from Before the Dawn of Time. The apparent possibility of time travel happens under conditions where GR as an approximation breaks down, because the Deep Physics will preserve causality. Without causality, you might as well not have physics, because anything would be possible.

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Re: [Physics] Time Travel

Postby taby » Tue May 13, 2008 8:30 pm UTC

In case anyone's curious about this aspect of GR, this seems to be a decent review of the topics discussed here:

http://arxiv.org/abs/gr-qc/0206078

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Re: [Physics] Time Travel

Postby Vaniver » Tue May 13, 2008 10:29 pm UTC

Time travel has no place in rational theory.

(excluding, of course, moving forward in it)
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Re: [Physics] Time Travel

Postby bentheimmigrant » Wed May 14, 2008 1:02 pm UTC

Vaniver wrote:Time travel has no place in rational theory.

(excluding, of course, moving forward in it)



But if someone were to build a wormhole generator, and be able to transport one end of the wormhole close to a black hole, surely time would be traveling slower at that end than the other, and therefore passing through the wormhole is traveling back in time (to a point after the machine was made of course).

Could you then transport said wormhole end to somewhere else, and keep it's lag in time? Or do you have to stay near the black hole? Because obviously if you could bring it back with a set time difference, you'd have two linked points in time (and space) of your choice.
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Re: [Physics] Time Travel

Postby Shakleton » Wed May 14, 2008 1:58 pm UTC

Image

I personally thank that this is theoretically posiible but still way from our abilites of controlling space, matter and time. But I think one day we will be able to create Wormholes or something similar for a link in time. Taken the fact that we do not kill ourselves before that happens.
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Re: [Physics] Time Travel

Postby ATCG » Thu May 15, 2008 1:29 am UTC

bentheimmigrant wrote:. . . if someone were to build a wormhole generator, and be able to transport one end of the wormhole close to a black hole, surely time would be traveling slower at that end than the other, and therefore passing through the wormhole is traveling back in time (to a point after the machine was made of course).

Could you then transport said wormhole end to somewhere else, and keep it's lag in time? Or do you have to stay near the black hole? Because obviously if you could bring it back with a set time difference, you'd have two linked points in time (and space) of your choice.

This is actually pretty close to a proposal developed by Kip Thorne and his students at Caltech. One has to accept the feasibility of constructing and maintaining a wormhole (one calculation suggests "that the amount of negative energy [allowed by quantum mechanics under highly specialized conditions] needed to keep open a one-meter-wide wormhole is roughly equal in magnitude to the total energy produced by the sun over about 10 billion years)" - but if you can get past that, building a time machine is a relative breeze:
Brian Greene, in The Fabric of the Cosmos, p.465, wrote:One blueprint for building a time machine is now clear. Step 1: find or create a wormhole wide enough for you, or anything you want to send through time, to pass. Step 2: establish a time difference between the wormhole mouths - say, by moving one relative to the other. That's it.

Greene doesn't invoke black holes. (Why make things hard? :) ) Instead, he chooses an example in which one end of a wormhole remains on Earth while the other is loaded into an intergalactic runabout and zoomed to the Andromeda galaxy and back at just shy of lightspeed. The runabout returns to an Earth where several million years have elapsed, but if the runabout if sufficiently fast, its occupants experience the passing of mere days or maybe even hours. And the same is true of both ends of the wormhole. The wormhole is now a corridor connecting two different times separated by several million years.

BTW, while doing a little fact checking, I came across something interesting: IMDb reports that a Steven Spielberg film titled Interstellar is in production for a 2009 release. The film's story is credited to Kip Thorne.
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Re: [Physics] Time Travel

Postby Mr Pete » Thu May 15, 2008 7:05 pm UTC

Shakleton wrote: :arrow: If you would go back in time to kill your grandfather the events would follow the disription above. That would trap us all forever in a time-loop because no-one could ever go past the point of the time travel because at this point all history since the murder of a child that happened to be the grandfather of the time traveler would have been altered. And after that it would be altered again because nobody killed the child. And again and again and again. Nobody would get annoyed because we all would think it is the first time we are experiening that timeline. Because this is so messy I call it the "Screw-Up Timeline Theory"


There's an assumption that events will happen in the exact same way as it plays through, but what about probability events? (Quantum 'stuff'. Sure thats tiny, but there'll be a lot of it in the intervening years, mebbe cascading to break the loop in some way).

Thoughts like this is why I shouldnt read multiple threads at the same time :roll: .

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Re: [Physics] Time Travel

Postby Luthen » Fri May 16, 2008 12:04 pm UTC

I like to combine the Cones of Causality(my coinage) from GR, Relativity's reversibility and Heisenberg's Uncertainty Principle to form my own view of time:
  • You can travel back in time - but not outside the Cone
  • However, the past isn't fixed - it exists in a wave-form until you arrive in the past (your memories are just an illusion)
  • Similarly your past isn't fixed - if you go back, you can kill your "Grandfather" it just mean your memories of him are false
  • Due to the uncertainty principle you can't be sure of where you'll end up in the past or the future

I like even if its a cop-out.

Yes I know that that's not what Heisenberg Uncertainty Principle is but its my extension of it.
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Re: [Physics] Time Travel

Postby andyisagod » Fri May 16, 2008 2:14 pm UTC

Luthen wrote:I like to combine the Cones of Causality(my coinage) from GR, Relativity's reversibility and Heisenberg's Uncertainty Principle to form my own view of time:
  • You can travel back in time - but not outside the Cone
  • However, the past isn't fixed - it exists in a wave-form until you arrive in the past (your memories are just an illusion)
  • Similarly your past isn't fixed - if you go back, you can kill your "Grandfather" it just mean your memories of him are false
  • Due to the uncertainty principle you can't be sure of where you'll end up in the past or the future

I like even if its a cop-out.

Yes I know that that's not what Heisenberg Uncertainty Principle is but its my extension of it.


It strikes me that it would make more sense to only allow time travel to regions of space time that are space-like separated (not casually related). That would actually deal with the grandfather paradox. Since you wouldn't be able to change anything that could effect you setting off in the future. I don't really see what you gain or what reason you could have for only allowing time travel to your past light cone.

I don't understand your idea about your memories being an illusion either, assuming they are caused by the arrangement and interaction of the neurons in your brain they are a real physical concept, or rather they represent a consequence of an actual physical/chemical process that has happened. I guess what I'm trying to say is how does that past know your going to travel back there and 'observe' it, collapsing its wave function and yet not collapse the first time you go through (or the 2nd or 3rd if you keep traveling through the same piece of time). Also if your memories of your grandfather are false are you 'false' in the same sense?

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Re: [Physics] Time Travel

Postby wst » Fri May 16, 2008 3:31 pm UTC

Nimz wrote:
TheCoelacanth wrote:Where's the "Yes, and I think you could kill your grandfather, and then become your own grandfather" option.

That would be the Philip J. Fry paradox.

The nasty in the pasty/ Past Nastification...

This video is good for thinking about time travel/whatever you wish to call it...
http://youtube.com/watch?v=JkxieS-6WuA
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Re: [Physics] Time Travel

Postby Badion » Fri May 16, 2008 3:50 pm UTC

No! but if possible, shouldn't. and heres why.

If you could jump back into another time the mere interactions you have with people COULD cause the present time to change and thus COULD result in you unable to go back in time at all. In addition to this dangerous and paradoxical problem there is of course the non paradoxical problem of germ and viral interaction. It is very likely that you would carry back some disease that COULD be untreatable. Imagine the effects of AIDS coming into the human population in the late 1300's during plague times and all.
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Re: [Physics] Time Travel

Postby Xanthir » Fri May 16, 2008 6:03 pm UTC

andyisagod wrote:
Luthen wrote:I like to combine the Cones of Causality(my coinage) from GR, Relativity's reversibility and Heisenberg's Uncertainty Principle to form my own view of time:
  • You can travel back in time - but not outside the Cone
  • However, the past isn't fixed - it exists in a wave-form until you arrive in the past (your memories are just an illusion)
  • Similarly your past isn't fixed - if you go back, you can kill your "Grandfather" it just mean your memories of him are false
  • Due to the uncertainty principle you can't be sure of where you'll end up in the past or the future

I like even if its a cop-out.

Yes I know that that's not what Heisenberg Uncertainty Principle is but its my extension of it.


It strikes me that it would make more sense to only allow time travel to regions of space time that are space-like separated (not casually related). That would actually deal with the grandfather paradox. Since you wouldn't be able to change anything that could effect you setting off in the future. I don't really see what you gain or what reason you could have for only allowing time travel to your past light cone.

Travelling outside of your past light cone may involve faster-than-light travel, which is why it may be impossible.

andyisagod wrote:I don't understand your idea about your memories being an illusion either, assuming they are caused by the arrangement and interaction of the neurons in your brain they are a real physical concept, or rather they represent a consequence of an actual physical/chemical process that has happened. I guess what I'm trying to say is how does that past know your going to travel back there and 'observe' it, collapsing its wave function and yet not collapse the first time you go through (or the 2nd or 3rd if you keep traveling through the same piece of time).

What he's saying is that memories are sort of an accident of circumstance, not a fullproof record of the past. The universe *could* have unfolded in an infinite number of ways - the fact that it unfolded in a particular way in your memories is no guarantee that it will do the same again, or that it even has to do the same again.

andyisagod wrote:Also if your memories of your grandfather are false are you 'false' in the same sense?

I think a better way to put it is that your memories aren't of the grandfather you just killed. Other people in the world remember the now-dead grandfather, you remember a completely different grandfather that doesn't exist for anyone but you.

As for you being false, you *are* in a very particular way. Essentially you're not causally connected to anything in the world. There are no chain of events that led you to appear - to the people that existed previously in the world, you're a random quantum fluctuation. Thus you can't 'uncause' yourself by killing your grandfaster, preventing the invention of the time machine, ending life as we know it, or anything else like that, as nothing in the world 'caused' you at all!
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Re: [Physics] Time Travel

Postby andyisagod » Fri May 16, 2008 7:05 pm UTC

Xanthir wrote:Travelling outside of your past light cone may involve faster-than-light travel, which is why it may be impossible.


I understand that in general faster than light travel is not allowed, but if we're talking time travel then something interesting must be causing it like a worm hole which would essentially allow you to travel between the two points through the worm hole faster than light would take to propagate through normal space between the two points. In principle I'm pretty anti-time travel at all but i can't see why retaining special relativity is more important that keeping causality.However I think this is where i could be very wrong since this is quite an arbitrary choice.

Xanthir wrote:As for you being false, you *are* in a very particular way. Essentially you're not causally connected to anything in the world. There are no chain of events that led you to appear - to the people that existed previously in the world, you're a random quantum fluctuation. Thus you can't 'uncause' yourself by killing your grandfaster, preventing the invention of the time machine, ending life as we know it, or anything else like that, as nothing in the world 'caused' you at all!


I don't think i understand how you me and everyone else are not casually connected to the universe, I was definitely caused and continue to cause events in the future. Do you mean our conciseness isn't connected? if you are not causally connected then how could you have any effect on the past. If you mean that your time traveled self appearing in the past does not have a cause then i have to agree with you but i was refereeing to the current you and current memories that have not time traveled but could turn out to be false if someone travels back in time and messes with history. which was what Luthen originally posted i believe.

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Re: [Physics] Time Travel

Postby skeptical scientist » Sat May 17, 2008 2:16 am UTC

I suspect time travel is impossible. If it were, I would tend towards the consistent timelines version.

However, in fiction I'm a big fan of the principle "time travel works, and has whatever properties are needed in order to tell the story you want to tell." If you need multiple timelines, there are multiple timelimes. If you need bootstrapping* to be possible, it is. I thoroughly enjoyed the Star Trek Voyager time travel episodes. (And there were a lot of them, weren't there?) I think my favorite was Future's End.

*Bootstrapping is the process of obtaining a time machine or other technological device by duplicating one that was brought back from the future, or else duplicating one you already possess and sending it back in time to your past self. Bootstrapped devices literally invent themselves into existence. The term comes from the idea of "picking yourself up by your bootstraps" by way of a Heinlein short story.
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Re: [Physics] Time Travel

Postby Strilanc » Sat May 17, 2008 2:52 am UTC

Well, maybe it turns out that changing the past doesn't change the present, because you have a continuum of universes separated by time. If you go back in time 50 years and kill your grandfather, then you have only killed your grandfather in universe t-50. Presumably you will then alternatively eventually exist or not exist in segments of universes of length 50 (eg. You don't eventually exist in universes (t-50, t], but you do eventually exist in universes (t-100, t-50]). You might need to think about that one for a bit to see why.

Of course that situation brings up all kinds of interesting possibilities. Consider an experiment: you will send either "1" or "0" a small amount of time into the past. If you receive "0", send "1". If you receive "1", send "0". If you receive nothing, send "0". If there is a 'first' universe, this tells you whether or not you are an even or odd number of time steps from it! Of course you need to plan out all these experiments before doing the first one, or you will desync due to the differing results in different universes. Even worse: if physics isn't deterministic, the universes will already be desynced and this type of experiment will fail.

I'm not sure if you can create an experiment that will determine whether or not there is a first universe.
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Re: [Physics] Time Travel

Postby Sir_Elderberry » Sat May 17, 2008 3:22 am UTC

My theory has always been that, if you were going to make any changes at all to the timeline, then they've already been made in the past. The year 1999 has already happened. If I was going to do anything to it, I have already done it and its effects are already felt. Building a time machine and going back, I can do things, but I cannot change anything, because it would inevitably affect me, my memories, and my time machine. I would never know that anything had changed.
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Re: [Physics] Time Travel

Postby Luthen » Sat May 17, 2008 11:42 am UTC

Xanthir wrote:
andyisagod wrote:I don't understand your idea about your memories being an illusion either, assuming they are caused by the arrangement and interaction of the neurons in your brain they are a real physical concept, or rather they represent a consequence of an actual physical/chemical process that has happened. I guess what I'm trying to say is how does that past know your going to travel back there and 'observe' it, collapsing its wave function and yet not collapse the first time you go through (or the 2nd or 3rd if you keep traveling through the same piece of time).

What he's saying is that memories are sort of an accident of circumstance, not a fullproof record of the past. The universe *could* have unfolded in an infinite number of ways - the fact that it unfolded in a particular way in your memories is no guarantee that it will do the same again, or that it even has to do the same again.

That's what I meant.

Xanthir wrote:
andyisagod wrote:Also if your memories of your grandfather are false are you 'false' in the same sense?

I think a better way to put it is that your memories aren't of the grandfather you just killed. Other people in the world remember the now-dead grandfather, you remember a completely different grandfather that doesn't exist for anyone but you.

As for you being false, you *are* in a very particular way. Essentially you're not causally connected to anything in the world. There are no chain of events that led you to appear - to the people that existed previously in the world, you're a random quantum fluctuation. Thus you can't 'uncause' yourself by killing your grandfaster, preventing the invention of the time machine, ending life as we know it, or anything else like that, as nothing in the world 'caused' you at all!

If a time traveller from the future appears as a random quantum fluctuation then you can sorta play a "Get out conservation of energy free" card, except that you'd presumably quantum-tunnel back to whence you came.

Plus I want a grandfaster.
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Re: [Physics] Time Travel

Postby Vaniver » Sat May 17, 2008 4:11 pm UTC

Even if wormholes were possible, wouldn't they destroy anything that goes through them?
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Re: [Physics] Time Travel

Postby dwg » Fri May 23, 2008 11:57 am UTC

Provided by Past-Special-Relativity-Physics and Science Fiction,

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My opinion and what I think is that Einstein in his special theory was describing our experience of time, not the properties of time. In other words, like Yogi Berra, Einstein never said half the things he said.

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Re: [Physics] Time Travel

Postby Xanthir » Sun May 25, 2008 12:35 am UTC

andyisagod wrote:
Xanthir wrote:Travelling outside of your past light cone may involve faster-than-light travel, which is why it may be impossible.


I understand that in general faster than light travel is not allowed, but if we're talking time travel then something interesting must be causing it like a worm hole which would essentially allow you to travel between the two points through the worm hole faster than light would take to propagate through normal space between the two points. In principle I'm pretty anti-time travel at all but i can't see why retaining special relativity is more important that keeping causality.However I think this is where i could be very wrong since this is quite an arbitrary choice.

The typical "make a wormhole, accelerate one really fast so that time dilation actually separates the two endpoints in time" scenario still keeps both wormholes within the other's light cone. You'd need to accelerate one of the wormholes faster than light to take it out of the other's light cone.

andyisagod wrote:
Xanthir wrote:As for you being false, you *are* in a very particular way. Essentially you're not causally connected to anything in the world. There are no chain of events that led you to appear - to the people that existed previously in the world, you're a random quantum fluctuation. Thus you can't 'uncause' yourself by killing your grandfaster, preventing the invention of the time machine, ending life as we know it, or anything else like that, as nothing in the world 'caused' you at all!


I don't think i understand how you me and everyone else are not casually connected to the universe, I was definitely caused and continue to cause events in the future. Do you mean our conciseness isn't connected? if you are not causally connected then how could you have any effect on the past. If you mean that your time traveled self appearing in the past does not have a cause then i have to agree with you but i was refereeing to the current you and current memories that have not time traveled but could turn out to be false if someone travels back in time and messes with history. which was what Luthen originally posted i believe.

If you stay right where you are and someone else travels back in time, then (generally) there are two possiblities. The first is that nothing happens - by travelling back in time, the chrononaut split off into a different universe, and so his actions have no effect on you. The second is that your memories change to fit the universe. Your memories aren't separated from the world - if it turns out that Hitler won WW2 but was assassinated at the height of his power by a Thai diplomat, that's what you'll remember, because that's what actually happened in your past. (Really, your memories don't change, it's just that the 'you' that you are is different than the 'you' that you were before history was changed. You never formed those memories where Hitler lost, so nothing ever had to change.) This nonsense about people's memories changing slower than the world around them is just that - nonsense. No remotely believable form of time travel allows it; it's pure artistic license.
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Re: [Physics] Time Travel

Postby Cryopyre » Sun May 25, 2008 6:02 am UTC

Sorry, but I would say that time-dilation is no more time-travel then freezing yourself and then thawing yourself. You just make the surroundings seem to go faster, and you can't go back.
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Re: [Physics] Time Travel

Postby Mettra » Sun May 25, 2008 1:56 pm UTC

Cryopyre wrote:Sorry, but I would say that time-dilation is no more time-travel then freezing yourself and then thawing yourself. You just make the surroundings seem to go faster, and you can't go back.


You can 'travel' arbitrarily far into the future in an arbitrarily short amount of time. I don't see the difference between this and 'time travel'. It's also a hell of a lot less problematic, and we're pretty sure it can actually be done.


/edit - noticed something else

ATCG wrote:This is actually pretty close to a proposal developed by Kip Thorne and his students at Caltech. One has to accept the feasibility of constructing and maintaining a wormhole (one calculation suggests "that the amount of negative energy [allowed by quantum mechanics under highly specialized conditions] needed to keep open a one-meter-wide wormhole is roughly equal in magnitude to the total energy produced by the sun over about 10 billion years)" - but if you can get past that, building a time machine is a relative breeze:


Wouldn't putting that much energy into a relatively small volume of space create a gravitational well (or in this case, a gravitational 'hill') and interfere with the creation/stability/usability of the wormhole? The specific dimensions are important, of course, but that's an awful lot of energy to maintain a localized 1-meter effective area.
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Re: [Physics] Time Travel

Postby TheCoelacanth » Mon May 26, 2008 3:33 am UTC

skeptical scientist wrote:Bootstrapping is the process of obtaining a time machine or other technological device by duplicating one that was brought back from the future, or else duplicating one you already possess and sending it back in time to your past self. Bootstrapped devices literally invent themselves into existence. The term comes from the idea of "picking yourself up by your bootstraps" by way of a Heinlein short story.


Another Heinlein short story also uses a similar idea.

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Re: [Physics] Time Travel

Postby ATCG » Mon May 26, 2008 4:48 am UTC

Mettra wrote:
ATCG wrote:This is actually pretty close to a proposal developed by Kip Thorne and his students at Caltech. One has to accept the feasibility of constructing and maintaining a wormhole (one calculation suggests "that the amount of negative energy [allowed by quantum mechanics under highly specialized conditions] needed to keep open a one-meter-wide wormhole is roughly equal in magnitude to the total energy produced by the sun over about 10 billion years)" - but if you can get past that, building a time machine is a relative breeze:

Wouldn't putting that much energy into a relatively small volume of space create a gravitational well (or in this case, a gravitational 'hill') and interfere with the creation/stability/usability of the wormhole? The specific dimensions are important, of course, but that's an awful lot of energy to maintain a localized 1-meter effective area.

The best I can do for an answer is an expansion of the quote above, which came from Brian Greene:
Greene, in The Fabric of the Cosmos, p.467, wrote:. . . in the 1960s, [John] Wheeler and Robert Fuller showed, using the equations of general relativity, that wormholes are unstable. Their walls tend to collapse inward in a fraction of a second, which eliminates their utility for any kind of travel. More recently, though, physicists (including Thorne and [Michael] Morris, and also Matt Visser) have found a potential way around the collapse problem. If the wormhole is not empty, but instead contains material -- so-called exotic matter -- that can exert an outward push on its walls, then it might be possible to keep the wormhole open and stable. Although similar in its effect to a cosmological constant, exotic matter would generate outward-pushing repulsive gravity by virtue of having negative energy (not just the negative pressure characteristic of a cosmological constant13). Under highly specialized conditions, quantum mechanics allows for negative energy,14 but it would be a monumental challenge to generate enough exotic matter to hold a macroscopic wormhole open. (For example, Visser has calculated that the amount of negative energy needed to keep open a one-meter-wide wormhole is roughly equal in magnitude to the total energy produced by the sun over about 10 billion years.)

13 Roughly speaking, if you passed through a region containing such exotic matter at nearly the speed of light and took the average of all your measurements of the energy density you detected, the answer you'd find would be negative. Physicists say that such exotic matter violates the so-called averaged weak energy condition.
14 The simplest realization of exotic matter comes from the vacuum fluctuations of the electromagnetic field between the parallel plates in the Casimir experiment . . . Calculations show that the decrease in quantum fluctuations between the plates, relative to empty space, entails negative averaged energy density (as well as negative pressure).
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Re: [Physics] Time Travel

Postby smithjcn » Mon May 26, 2008 2:22 pm UTC

A major problem with this ongoing discussion of time travel, as with most discussions of time travel, is that thus far in the thread I've not seen any clear definition of what people mean when they use the word "time." Is it just remotely possible that the lack of a common understanding of, and agreement on, exactly what we mean by the word "time" could be an impediment to a fruitful, satisfying discussion of the topic? I'm inclined to think that this might be the case. If anyone is interested in what I believe is a constructive step toward fixing this problem you will find an essay on the topic at http://smithjcn.googlepages.com/time Although this essay does not explicitly address the issue of time travel, per se, if one accepts the arguments put forth there (and of course it's not a foregone conclusion that everyone will accept them, but I believe they're at least worth a careful look) it will follow that time travel of the sort portrayed in science fiction literature is not possible; the only sort of time travel possible, for better or for worse (according to this view of time), is the sort in which we're all currently engaged.

That's my story, and I'm sticking to it. At least until something better comes along. So on this note, I'll look forward to reading (and will welcome with open arms) your proposed alternative definitions of time. Then let's all get back to talking about time travel.

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Re: [Physics] Time Travel

Postby Ramses IV » Mon Jun 02, 2008 1:46 am UTC

I voted no, because time travel into the past is not, and will not, be possible.

However, given that I MIGHT be wrong, I think the Multiple-Timelines theory is the most likely and/or probable.
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