# The grandfather time paradox



## Mighty mouse (Nov 23, 2008)

The old 'grandfather paradox' suggests time travel is not possible as it would permit you to go back in time and kill your grandfather.
Researchers at MIT are considering computers that spend 100 years solving a problem then send the solution back in time.
It involves a type of space-time that enables time traveling with closed time-like curves,  a bit like a piece of paper folded over on itself, so that opposite ends touch and create a shortcut.
The possibility is however that nature must somehow enforce causality to stop the grandfather paradox by making the probability that you won’t be born in that universe in the first place high so it will not take place.
How Time-Traveling Could Affect Quantum Computing


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## PTeppic (Nov 23, 2008)

Now my head hurts...


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## sloweye (Nov 23, 2008)

Ah, the envelope effect.

*head begins to shake,spin and eventualy explode*


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## Mighty mouse (Nov 23, 2008)

Does it mean we might not be getting Groundhog day the sequel after all?


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## The Ace (Nov 23, 2008)

We can hope, MM, we can hope.


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## Nik (Nov 23, 2008)

So, after the quantum processor has salad-tossed its qubits for a considerable time, the answer comes back....

"42"


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## TheEndIsNigh (Nov 24, 2008)

Oh dear the time travel problem again.

So let's suppose that somehow you could travel back in time say one minute.

The problem is that in that one minute the cosmos has not been hanging about. In fact it's been quite busy. Even if you consider 'local events the Earth has rotated so your position has moved 16 miles. The planet has moved through space about 10,000 miles and the sun has nipped a fair distance along the spiral path of the galaxy.

Now where will you end up. Either the whole of creation just jumped back one minute (that will be magic not science) or your sitting in space watching the the Earth whiz along without you (well till your lungs burst).


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## Scifi fan (Nov 24, 2008)

Good point, TEIN, but presumably the time machines will take the galactic spin into account. Or, perhaps, the time machine zips you through the Earth a hundred years per second, and, in that time, the gravity keeps you attached to the planet.


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## TheEndIsNigh (Nov 24, 2008)

If don't return to the correct point in space then you would have travelled in time but not to the time/universe before because everything in that time had your mass somewhere physically different.


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## Drachir (Nov 24, 2008)

Hey, you are taking all of the fun out of time travel fiction.  Next you will be posting the proof for the non-existence of vampires.


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## TheEndIsNigh (Nov 24, 2008)

There's nothing funny about time travel Drachir it's a deadly serious business. 

Imagine if it were possible. You would have all to be constantly watching your back for snotty little bu**ers that had come back to do for you.

There would be videos of the actual crucification* with the catholic church vying for copyright/film rights (and all other historic events) 

Adam and eve would have psychoanalysts, sex therapist, priests and the like all sneaking about the garden arguing that the garden was given to them so why not the tree of knowledge. God knows where that would end.

Television would be totally different. Why watch Coronation St. when we could all watch Sodom and Gomorrah 24/7. It would be a nightmare I tell you a nightmare .




*(always liked that phrase especially the fiction part)


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## Mighty mouse (Nov 24, 2008)

> Oh dear the time travel problem again.
> 
> So let's suppose that somehow you could travel back in time say one minute.
> 
> ...


The curved paper model they are referring to is in Einsteinian space time, they are not suggesting 'moving' in isolated time - that would be impossible!


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## TheEndIsNigh (Nov 24, 2008)

If I go back in time then everthing should be as it was - I wont be there, folded or not, since I wasn't there the last time. I will be somewhere else entirely. 

If I go back in time and everything is not as it was then forget about your grandfather worry about yesterday.

This is classic paradox stuff usually used to disprove mathmatical theories since time began. You know the one if for thing to be true then 2 must equal 3 then thing is false.

If your suggesting that the time paradox can somehow be overcome then we will need to start all over again with does 

1+1=2 

and work our way up from there.

However given the accepted practice of if there is an impossible paradox then the theory is defective I'll stay on "the time travel is impossible side of the fence".


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## PTeppic (Nov 24, 2008)

TheEndIsNigh said:


> Adam and eve would have psychoanalysts, sex therapist, priests and the like all sneaking about the garden arguing that the garden was given to them so why not the tree of knowledge. God knows where that would end.



And don't get me started on Health & Safety... Some do-gooder would be bound to go back. Adam and Eve would have to wear work-wear suitable for gardening and tending all the animals, and if the apples from THAT tree are so easily eatable, then it'll definitely need a fence. And probably... a sign!!


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## TheEndIsNigh (Nov 24, 2008)

Pteppic: How right you are. 

And then who do you suppose they could prosecute for  an unsafe working environment. 

Not to mention prosecutions under the dangerous pets act for keeping poisonous snakes without the proper licence or suitable enclosures.


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## Ursa major (Nov 24, 2008)

It would be the end of hisstory as we know it, (very) basically.


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## Karn Maeshalanadae (Nov 24, 2008)

Yes, perhaps, but when people talk about time travel, it's always travel to the past.

Technically I think time travel to the future could be possible. All you'd need is ridiculously high speeds and a large body of mass to slingshot around. Just exactly what would happen, hypothetically, if a space ship spun around earth at, say, warp six for three or four days?


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## Ursa major (Nov 24, 2008)

We're all travelling to the future; and at the far-too-fast speed of 1 second per second. (Some of us are going to travel further than others - although I expect TEIN will point out that a lot of us will be travelling to exactly the same point.)


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## TheEndIsNigh (Nov 24, 2008)

Not quite Ursa - Some of us may not make it.


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## Karn Maeshalanadae (Nov 25, 2008)

Ursa major said:


> We're all travelling to the future; and at the far-too-fast speed of 1 second per second. (Some of us are going to travel further than others - although I expect TEIN will point out that a lot of us will be travelling to exactly the same point.)


 

You know what I mean, Ursa. I mean like, travelling two, three, four thousand years in an instant.


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## Drachir (Nov 25, 2008)

TheEndIsNigh said:


> There's nothing funny about time travel Drachir it's a deadly serious business.
> 
> There would be videos of the actual crucification* with the catholic church vying for copyright/film rights (and all other historic events)
> 
> ...


 
Actually I suspect that if time travel were developed it would be the end of all religions based on supposedly real historical events once everyone could see what really happened.


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## Dave (Nov 28, 2008)

I think they have just proved time travel is possible:





> In the Shakespeare example, there's no actual logical inconsistency,” Aaronson explained. “You go back in time and dictate the plays to him, therefore he writes the plays, therefore the plays come down to you, therefore you're able to go back in time and dictate the plays to him, etc. Everything is consistent – there's no paradox! Or rather, the only ‘paradox’ is one of computational complexity: a difficult task seems to have been performed (namely, Shakespeare's plays being written), but without anyone ever doing the actual creative work of writing those plays.


There is evidence that Shakespeare did not write down his plays himself. It wasn't usual to write down plays, they were a performance art, and actors were often illiterate. They learnt their lines by heart, and often ad libbed. Other plays that have survived from that period are copied by actors or the audience, not the author, and often published at a much later date than they were written. Scholars still argue about the exact order in which Shakespeare wrote his plays, and he was an actor himself, not particularly well educated or schooled. 

This explains everything if a time-traveller went back and handed him the scripts.


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## Whitestar (Dec 7, 2008)

If you were to ask me if time travel to the past was impossible about ten years ago, I would have agreed with you. However, quantum physicists already know that time travel is possible at the microscopic scale. In fact, the quantum eraser experiment is already proof of that. But the question is, can macroscopic objects such as you or I can time travel? However, this is not in the realm of the quantum eraser experiment.  Anti-particles are already traveling into their past, and their histories are known. Basically from our view point, the moment of their creation is their destruction and their destruction from our view point is their creation. In order to really experiment with this we would need to find ancient particles whose creation/destruction occurred long in our past, but anti-matter does not last long due to the inbalance between matter and anti-matter we observe.

In the case of the "Twin Paradox" it's only a paradox due to our lack of knowledge on the mechanism of how time and space interact with the past. Quantum physics has already taught us to ignore what appears to be paradoxical and just accept it. The real solution may just be to accept a new form of logic and some theorists have already subscribed into quantum gravity. That being said, there are a few possbilities for time traveling into the past.

One possibility would probably involve the classical grandfather paradox. A time traveler travels back in time to prevent his parents from meeting and succeeds. What would actually happen? Now, before I answer that question, let me first point out about those pesky paradoxes. For those of you who are concerned about the possibility of erasing your own existence (and the entire universe), well don't be. Look at this way, if a time traveler goes back in time to prevent from humans evolving into intelligent beings and erases history, none of us would even know about it! *We would be gone and the evolution of humans would have never happened! * So, what's to worry about, eh? LOL. 

Okay, it's time (pun intended) now for me answer the grandfather paradox question. What would happen to the time traveler if he successfully prevents his parents from meeting? Since the conservation of energy is based on the symmetry of time, time travel would have to preserve this and the conservation of energy. Hence, whenever a time traveler succeeds in preventing his birth, his existence in the present would not literally disappear like Marty McFly in Back To The Future, but would continue to exist unaffected. Essentially, the time traveler would be the man with no past, or very likely the time traveler's matter would rearranged or reformed, or displaced to another form or item, but he would never cease to exist due to the laws of energy conservation. It would be a type of a reset button, where the universe reforms to accomodate the new timeline.

The second possibility involves the time traveler traveling back into his past and succeeds in preventing his birth from happening, then time will branch out and lead into a parallel universe, providing that the parallel universes are self looping (close casual loop) and are already there. The idea of a time traveler being responsible for creating parallel universes is a direct violation of the laws of energy conservation, therefore, they would have to have been there at the same time when our universe was created during the time of the big bang, forming many regions of separate universes, creating each universe/timeline with particles that already existed. In this case, a time traveller will just get shunted to one that already matched the future he just created. It would basically give the impression that these parallel universes were created by the time traveler, however, all it will do is allow him to exist in what appears to be the universe he came from and an altered timeline. But instead, it would be a pocket dimension where the events he altered occured.

Noted theoretical and quantum physicist Jon Cramer has discovered with his quantum eraser experiment that time traveling into the past allows us to change it, but the present always remains unchanged. The minute we look at the particle's present, it is fixed. So, there is no need for concern about erasing your own existence in the present because it has already been recorded into history when it was observed, hence, you're safe. But here is the twist: even though you can erase or prevent your own birth in the past and still exist in the present, you can erase your past simply by staying in the present without actually time traveling at all!  

The reason why we're convinced that time travel into the past is not possible is because of our limited perception of time, specifically, linear time, at least that is how it appears on the surface but according to the quantum eraser experiment, time is circular not linear. So, don't be surprise if one day you receive a knock on your door from someone who claims to be your great-great-great grandchild!


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## TheEndIsNigh (Dec 7, 2008)

> Whitestar


 
I'm sorry. but if you go back in the past and stop your existence from happening then by definition you don't exist. 

If you do exist, then you either failed to prevent your existence happening and you are either not the person you thought you were or you are asleep in some padded cell having one hell of a nightmare.


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## Whitestar (Dec 7, 2008)

TheEndIsNigh said:


> I'm sorry. but if you go back in the past and stop your existence from happening then by definition you don't exist.
> 
> If you do exist, then you either failed to prevent your existence happening and you are either not the person you thought you were or you are asleep in some padded cell having one hell of a nightmare.



Well, as I stated before if you cease to exist then you wouldn't even know about it! LOL. Anyway, you would not literally disappear out of existance because it would violate the laws of energy conservation and since energy is based on the symmetry of time, time travel would have to obey this too. If a time traveler successfully prevents his parents from meeting the most likely scenerio would be that nothing would happen to him or his matter would reform to accomodate the changes in the timeline.   

But you don't have to take my word for it, check out the following link:

http://en.wikipedia.org/wiki/Conservation_of_energy

look at Noether Theorem:
_The conservation of energy is a common feature in many physical theories. It is understood as a consequence of Noether's theorem, which states every symmetry of a physical theory has an associated conserved quantity; if the theory's symmetry is time invariance then the conserved quantity is called "energy". In other words, if the theory is invariant under the continuous symmetry of time translation then its energy (which is canonical conjugate quantity to time) is conserved. Conversely, theories which are not invariant under shifts in time (for example, systems with time dependent potential energy) do not exhibit conservation of energy -- unless we consider them to exchange energy with another, external system so that the theory of the enlarged system becomes time invariant again. Since any time-varying theory can be embedded within a time-invariant meta-theory energy conservation can always be recovered by a suitable re-definition of what energy is. Thus conservation of energy for finite systems is valid in all modern physical theories, such as special and general relativity and quantum theory (including QED).
_

Here's another good link:

http://www.eftaylor.com/pub/symmetry.html

As it stands, no event has been observed that violates time invarance (symmetry, at least not outside the margin of the uncertainty principle).  Basically this give us the reason behind the conservation of energy and thermodynamics. 

Also, the quantum eraser experiment has proven that time traveling into the past is possible at the microscopic scale with particles. But it remains to be seen if macroscopic objects such as you and I can time travel. So far, the experiment has resulted in changing the past of a particle, but here's the twist: the present state of the particle alway remains fixed. Which means that you can change the past, but the present is never altered or compromised. Sounds weird and contradictory but it proves that fact is stranger than fiction.


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## Vladd67 (Dec 7, 2008)

TheEndIsNigh said:


> Oh dear the time travel problem again.
> 
> So let's suppose that somehow you could travel back in time say one minute.
> 
> ...


I remember a story in the comic Starlord and then in 2000AD about a mutant bounty hunter who among his many weapons had time bombs which moved its victims forward or back in time where they reappeared in space the planet no longer being in the same spot.
Strontium Dog - Wikipedia, the free encyclopedia


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## Pyan (Dec 7, 2008)

> In the Shakespeare example, there's no actual logical inconsistency,” Aaronson explained. “You go back in time and dictate the plays to him, therefore he writes the plays, therefore the plays come down to you, therefore you're able to go back in time and dictate the plays to him, etc. Everything is consistent – there's no paradox! Or rather, the only ‘paradox’ is one of computational complexity: a difficult task seems to have been performed (namely, Shakespeare's plays being written), but without anyone ever doing the actual creative work of writing those plays.



For a really funny take on this idea, try The Technicolor Time Machine by Harry Harrison...

(Spoilers...)


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## Jimmy Magnusson (Dec 11, 2008)

TheEndIsNigh said:


> Oh dear the time travel problem again.
> 
> So let's suppose that somehow you could travel back in time say one minute.
> 
> ...



Wouldn't it rather be the reverse? If you _only _went back in time the planet wouldn't have arrived at your position yet...


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## TheEndIsNigh (Dec 12, 2008)

See what you mean. It could go either way though on reflection I'm tending to agree wth you.

Either way, you're still in a pickle.

Welcome to the Chrons by the way.


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## ManTimeForgot (Dec 21, 2008)

Even assuming that blue-shifted radiation didn't fry what-ever you tried to send back in time beyond recognition there are still other variables that would need considering before you would much up the time-stream.

Notions like temporal momentum and parallel universes would tend to prevent your actions from affecting your time-line, and presumably other future possessees of time-travel will stop you from mucking with their time-line; assuming that you are sufficiently distinguished in terms of quantum values to even enter their time-line to begin with.


If you were to enter your own time-line (for what-ever reason, or how?), then you would be presented with a butterfly effect that even assuming you had quantum computing would take a prodigious amount of processing in order to fully understand/process.  Why would you risk mucking up millions of years of history on the assumption that making "Change X" will have beneficial results?  If "Change X" could be shown to be beneficial, then presumably time-travelers have already made it, and you would consider it to have been the natural course of history.  But if "Change X" could not be shown to be either beneficial or detrimental due to a lack of qualification one way or another, then presumably time-travelers in the future would adopt a code of behavior comparable to the "Temporal Prime Directive."  It's pure hubris to assume that all changes _you_ make will be beneficial, and so I doubt anyone else is going to just let you make changes on a whim, or even after careful study.

MTF


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