Time travel / grandfather paradox

[Melllvar]

One of the more interesting articles I've seen on Physorg: Time travel experiment demonstrates how to avoid the grandfather paradox (also the abstract is available here)

Ok, so they didn't actually experiment with "time-travel" (that would be kinda hard given current technology), that's just sensational journalism. From what I can tell what they actually did was model a theory of time travel off of quantum teleportation and then performed an experiment on that, the outcome of which supports the idea that, in my own laymans terms, you can only travel in time if your journey is consistent with itself (i.e. time travel is possible but only in cases that prevent this particular paradox).

In slightly more technical terms, it seems that they theorized closed timelike curves operate like quantum channels used in teleportation, and also that closed timelike curves are post-selected for consistency. The experiment then involved entangling two qubits in a single photon, then attempting to change the state of one of the qubits. The other qubit can then only teleport if the previous qubit's state was unchanged in the experiment. In terms of the grandfather paradox it seems the teleporting qubit would model the grandfather while the one being flipped would model the grandchild. The states of the qubits are measured before and after the experiment to see if they are consistent (being inconsistent would be the equivalent of the grandchild having "killed" the grandfather).

Honestly I'm not sure what the great value of this really is, except that they've developed an "alternative quantum formulation of CTCs based on teleportation and postselection" and performed an experiment which gives similar results to what they predict. Regarding that they're quoted as saying:

Although one would need a real general relativistic CTC actually to impose final conditions, we can still simulate how such a CTC would work by setting up the initial condition, letting the system evolve, and then making a measurement. One of the possible outcomes of the measurement corresponds to the final condition that we would like to impose. Whenever that outcome occurs, then everything that has happened in the experiment up to that point is exactly the same as if the photon had gone backward in time and tried to kill its former self. So when we ‘post-select’ that outcome, the experiment is equivalent to a real CTC.

 

[Cogwulf]

When theorising about something like time travel, you can more or less just make anything up. This is just quantum masturbation.

The grandfather paradox is the most discussed aspect of time travel, but I've always thought that the most serious problem would be that if you went back in time, it would mean that there were two copies of identical atoms existing simultaneously. This would potentially create many problems related to conservation laws

 

[Melllvar]

Criticism noted, however I think work like this is interesting in that it takes such topics to a more serious level. Much of what would have once been considered metaphysics or science fiction has developed into actual science as understanding and technology improved. At the very least they did manage to get it published in Physical Review Letters.

My understanding (which isn't very much) is that closed timelike curves involve a particle (or whatever) traveling from one position in space-time and returning to their original at some point, which doesn't imply that they are coexisting along with their past selves, so it isn't necessarily a volation of conversation of mass or energy (it's more like a predestination paradox). So in more South Park-y language, it would obey Terminator rules instead of Primer rules.

One of my hypotheses is that time travel would be far more likely to be possible on a microscopic scale or over very short time frame than on an everyday level. In a sense conservation laws themselves can be violated over a 'quantum' timeframe, as seen in tunneling effects and such (i.e. the particle passes through a potential barrier greater than it's energy level, but only in situations where doing so requires it's possessing an anomalous amount of energy for an almost immeasurable amount of time [i.e. to paraphrase a former professor, it "borrows" the energy for an amount of time on a scale smaller than the uncertainty limit]). Even if some version did involve a violation of conservation laws (which I don't think CTCs do, but what do I know), it might still be possible in a similar way, where the 'violation' can only occur for an undetectable amount of time.

 

[Vrecknidj]

Can someone answer these questions for me?

The Earth is spinning at about 1,000 miles per hour and it's moving at about 60,000 miles per hour around the sun. And the Sun is moving at a much higher speed throughout the spiral arm of the Milky Way galaxy. And the galaxy is in motion.

If I were to travel back in time, why does anyone think I'd be so lucky as to land on the Earth? I mean, the Earth today is hundreds of millions of miles (at least) away from where it was 20 years ago. So, if I want to go back in time 20 years, how the Hell am I going to find the right spot so that I land on the Earth? And, even if I'm lucky enough to land in my own solar system, it's (pardon the pun) astronomically unlikely that I'll land on the Earth. And, if I land on the Earth, I'd think I have a much better chance of landing IN the Earth, or in the oceans than somewhere safely on land.

Dave

 

[Agapooka]

Stellarium is a wonderful piece of software that is able to track the movement of planets and space bodies through time. You'd have to combine time travel with teleportation in order to transport yourself to the same location on Earth at a different point in time. You'd have to be able to have control over both the time and the place that are to be the final destination.

As for the grandfather paradox; I believe that it can be solved in the context of parallel universes, but only if it possible to travel between these parallel universes. Essentially, you could travel in time, which would effectively be traveling to a parallel universe where the state of things is identical to a past situation. Your grandfather could be killed in that parallel universe, and you will never be born within that universe, but you will have been born in your universe of origin, although you will no longer be present there, because you will have traveled to a parallel universe.


Agapooka

 

[Melllvar]

Can someone answer these questions for me?

Not really, but I'll throw out some ideas anyway. First the completely unrealistic ones that I just made up off the top of my head and are better fitting to a movie-like version of time travel than anything seriously proposed by credible physicists:

1) When a mass travels back in time it maintains a similar dynamical state to the other objects it would have come in contact with along that path through time. For example, if you ride a train from point A to point B, then immediately on reaching point B you travel back in time to when you left at point A, your body experiences the opposite dynamical forces and state of motion that it experienced on the forward trip.
2) Mass and time are somehow linked up in a weird way that causes a mass traveling through time to arrive in the past at a similar location as other masses that were present at its temporal origin (this would seem hard to do since several objects might have been at the same location at the temporal origin, but are in different places at the destination time).
3) The geocentrists were actually correct, and the entire universe revolves around the Earth (or better yet, the entire Earth and universe revolve around you specifically), so during any time travel event you always arrive at the same location you left but at a different point in time. This is probably the ultimate in anthropocentrism.
4) God loves you?

More seriously:

1) I don't really have any serious answers. This argument really just discredits the movie-version time travel stuff though, where a whole person goes back in time and does stuff in the past (kills Hitler or whatever), as opposed to something slightly more realistic like "time travel" on a sub-atomic level.
2) Although, to humor the idea even more, you could just travel back a really short amount of time, and you'd still be in your approximate location.


Tangential remarks as long as I'm here (for thread semi-completeness): any type of faster than light travel or information transmission would also be equivalent to time travel, which would be the case for theoretically proposed (although I don't think seriously expected to exist) tachyons, and if entanglement was found to involve information transmission as opposed to non-locality (it's been experimentally shown that entangled particles can "react" to each other at a speed at least 100,000 times the speed of light [the limit of experimental accuracy - it's assumed to be instantaneous]). There's also the mathematical model of antimatter particles as being their non-anti counterparts moving backwards in time, which is (I think) the interpretation used in quantum field theory.


Anyway, it's fun to think about.

 

[Agapooka]

Alternatively, one might be able to time-travel, but because they would also get younger and their memories would also go back in time, they would have no way of being aware of their intention to travel back in time...

In other words, it would be like rewinding a movie: the characters have no awareness of the movie being rewinded, nor does it affect the plot in any way.

 

[Anthile]

There is another part of the grandfather paradox, I thought about recently but that I have never seen before elsewhere: It means that a grandfather and his grandchild would have the exact same DNA. That's just not possible.

 

[Jesse]

I never got the point of time travel discussion. The power to run something like that is far beyond what we could conceive of.

Weirdly I think time travel can never be created before the time machine exist. Once we create a time machine we can go forward in time and people can come back to that point. Which is why singularity will happen as soon as we invent a time machine.

 

[Melllvar]

There is another part of the grandfather paradox, I thought about recently but that I have never seen before elsewhere: It means that a grandfather and his grandchild would have the exact same DNA. That's just not possible.

Um... what? Why would they have the same DNA, and why would it not be possible (identical twins have the same DNA)?

 

[Dr. Freeman]

He is saying that you would become your own grandfather. (which would result in an infinite loop of genetic defects and a everchanging genome.)

edit:
The time travel police will probably break down the door to the place where you are "becoming your own grandfather" and shoot you, just for the sake of avoiding the paradox.

 

[SpaceYeti]

While it may be a convenient plot element, particularly when the time travel is a major part of the story. I doubt real time travel would be that way. There's an accidental, or perhaps incidental, assumption of fate in there. If you travel back in time, then from the moment you arrive the future is all in the future from that point. There is no necessity that anything happen the same way in relation to the things you changed. In fact, simply going back changes something and could potentially result in a redonkulous butterfly effect... just because you traveled to the past in the first place, even if you came right back.

We can't assume there's some sort of fatalistic force of nature at work which would force you to not have sex with your grandmother instead of your grandfather doing it, or from shooting your father, or from taking back a nuke and nuking Canada, or slitting your younger self's throat, or any other sort of calamitous event which did not happen in your history. If you took a gun into the past, and you shot your father before you were conceived, why would some unlikely series of events make it happen that you still get born? There's no reason to suppose time is fixed like that.

So if you traveled to the past and killed your grandfather, then you had sex with your grandmother in your place, there's no certainty that she'll even get pregnant. If she does, the child will not be one of your parents, but a different person. Your parent from that grandmother will not be born. That child might find your other parent and have children, but it would be very unlikely to have children even in the order your parents did, to live lives remotely similar to your parents' after they met, or anything like that. Because once you go to the past, the point that you go to is now "now". And the future unfolds from now. It's not like you're in a sub-time-line, where you change a few things but your sub-time-line is forced back into the main one. That's all sci-fi (and so is the idea of time travel as we're discussing it in the first place).

 

[Agapooka]

I believe that the question is the following:

If you were to prevent your future birth while existing in a moment that precedes that of your birth, how would you explain your origin? Of course this question stems from the ignorance of the mechanism of time travel, supposing that it is possible at all. For example, it assumes that one does not become younger when they travel into the past.

The grandfather paradox seems to be rooted in the causal relationships that we associate with the progression of time. It's not so much a question of any kind of fatalistic force, but more a question of "Ok, if it were to happen, what would have caused one's present existence?"

The entire paradox, or at least the order in which it is explained also creates an interesting timeline:

BIRTH OF SELF => TRAVEL INTO PAST => KILL GRANDFATHER

Notice how "KILL GRANDFATHER" event will always fall AFTER the "BIRTH OF SELF" event in any retelling of the story. Is there a possible way to retell the story in the actual chronological order?


Agapooka

 

[Vrecknidj]

Anyway, it's fun to think about.

I agree. And, I've thought about some of the faster-than-light business before as well. Though I currently find myself more drawn to the idea of hyperdimensionality as a solution to some of the apparent paradoxes (such as some in quantum theory) regarding what seems to be faster-than-light information exchange.

But, the time-travel problems I mentioned above are about as troubling for me when watching movies about time travel as are the problems I have with ghosts in movies. I can't really understand why ghosts would be affected by gravity, and yet, they seem to follow people around as though they were subjected to the same laws of gravity and inertia as physical things. I don't get that.

Dave

 

[Agapooka]

I think it's because the actors in the movies are subjected to the force of gravity? :P

And because they didn't bother to strive for accuracy, because they thought the idea was ridiculous, anyway?


Agapooka

 

[socialexpat]

lol sensational journalism aka sewer journalism?
Mmm further reply is not needed for me ..

 

[Latro]

Um... what? Why would they have the same DNA, and why would it not be possible (identical twins have the same DNA)?

Not identical, the same actual physical material. It would have nowhere to come from if you were never conceived.

Anyway, as I recall, didn't Hawking mathematically show some horrible thing about closed timelike curves, like that they require negative energy or something?

 

[Melllvar]

LOL, I think there are two different paradoxes being referred to here:

1) The traditional grandfather paradox, where you go back and kill your grandfather before he met your grandmother, thereby having prevented yourself from existing. In which case, you never existed to kill your grandfather, so he would have met your grandmother and you would have existed.

2) The one from Futurama, where you go back and have sex with your grandmother, thereby becoming your own grandfather.

In case of (1), I still don't see what having the same DNA would have to do with anything. How can you speculate on what DNA you would have had if the question of existence is undecidable? In case of (2), the DNA thing would seem to still make it a paradox, since homologous recombination during meiosis alone should guarantee your grandchild doesn't have the same DNA as its grandfather.

Anyway, as I recall, didn't Hawking mathematically show some horrible thing about closed timelike curves, like that they require negative energy or something?

I didn't know this but apparently you're right (not sure it applies or doesn't apply to CTCs though):

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter). .... Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."[39] .... However, this theorem does not rule out the possibility of time travel 1) by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) and 2) in regions which contain exotic matter....

 

[Zionoxis]

Here is one thing I would like to point out about teleportation. Assuming that it either breaks about your atomic makeup, to fire it at light speed so it may travel through time, it would then have to put all those atoms back together. If we had the technology to successfully put atoms together and recreate life, than there would be controversies as to whether the person was really THAT person or simply an exact copy of that person.

On top of that, our medical ability would basically cause people to live forever. Oh, you have a tumor? Ok, we will just rewrite your molecular code. Stand here, DING! I don't believe that this kind of power can ever truly be achieved by mankind as a whole. Just something to think about.

I'm sure there will be someone to qualify specifics, but you may use as high of a syntax as you like, but in the end, he and I are both equally as qualified to answer such questions. We both are not and neither is anyone else.

 

[Hadoblado]

http://en.wikipedia.org/wiki/Novikov_self-consistency_principle
Fucking love this shit!

 

[Melllvar]

Found another interesting article on the subject just now: Big Bang simulated in metamaterial shows time travel is impossible

At first, the researchers thought that, if they could build a metamaterial in which light could move in a circle (and so that its mathematical description were identical to particles moving through spacetime), then they could create CTCs.

But when further analyzing the situation, they found restrictions on how light rays could move in the model. Although certain rays could return to their starting points, they would not perceive the correct timelike dimension. In contrast, rays that do perceive this timelike dimension cannot move in circles. The researchers concluded that Nature seems to resist the creation of CTCs, and that time travel - at least in this model - is impossible.

Paper on arXiv: Modeling of time with metamaterials

 

[Jah]

It should be evident from relativity-theory that backwards time travel is impossible.

(I think)


But the slowing down and apparent stop of time should be perfectly possible. Effectively jumping forwards (if you're slowed down) in time. (re. the Twin Paradox.)

 

[Melllvar]

It should be evident from relativity-theory that backwards time travel is impossible.

How so? Most of the hypotheses for time travel put forth in physics are based on general relativity.

 

[Jah]

I know, but they assume that speeds faster than light are possible for any reason.
In the equations Time is a scalar, not a vector, which means there are only positive values. (you only measure it's magnitude. In quantum physics you'll have more complex ways of explaining why time only moves one way, but those are better explained by people more experienced than me. Watch the movie that follows: )

Regarding time "The Distinction of Past and Future":
YouTube - Richard Feynman - The Distinction of Past and Future. Part 1

 

[Melllvar]

That's only necessary for "time travel" in a flat space-time, in GR there are solutions that don't involve faster than light travel (again, see CTCs).

Those videos are talking about the time symmetry of (most of - I think the weak force is supposed to be time-asymmetrical) the laws of physics and the statistical irreversibility of thermodynamics. I'm not really sure it's relevant to disproving "time travel" as discussed in any of the articles I linked to, as it's based on classical physics anyway. Even assuming that some laws of physics are fundamentally irreversible, since the object "time traveling" would continue along a worldline away from it's starting point until it eventually returns to that point it doesn't necessarily follow that it would have to violate those time-asymmetrical laws. After all, the mere fact that objects in different reference frames experience time differently, and have different axes for time, doesn't violate the laws of physics because of any kind of time-asymmetry.

That basically what's being discussed mainly, is an object whose time-axis changes until it returns to it's starting point in time and space. If the stuff Feynman's talking about disallowed that then it would seem that relativity itself would be flawed and impossible as well, and we know it isn't.

 

[Jah]

Ok, I see where I jumped the gun.
I was thinking just a regular four-dimensional Monkowski spacetime.

But doesn't this presuppose a curved universe ?
and in that case, what about our present conclusions that the universe most likely is a flat one (see. Lawrence Krauss "Universe from nothing")

 

[Melllvar]

Well, whether CTCs can actually exist or not is another question (the last articles I posted were saying no, under the model they used). Right now they just occur as solutions to the Einstein field equations, but (afaik) there are lots of solutions to the equations that can't necessarily exist in the physical universe. Even if the universe is generally flat (and yeah I think that's how research has been pointing) it can still be curved in any local region (around mass).

I think I read once that CTCs would exist in a rotating universe, and I'm pretty sure that rotating black holes have a twisting/rotational effect on spacetime, so I'm going to hypothesize that one possibility would be in the region of a rotating black hole. Although I'm really out of my league with this paragraph, I don't really have the mathematical background to understand GR (yet), this is just based on what I've read. Also this might be prevented from being a valid origin of CTCs by the same thing Latro mentioned (see last paragraph of this article, or post #18 in this thread).

I'm not really taking a position for or against time travel at all here, I just think it's interesting and don't want to see the possibility of it discounted too early.

 

[crykillraven]

Hi all , I'm not a real scientist but I have done some thinking about Grandfather paradox and think that it might not be a limiting factor in propsed time travel to "the past" - probably I am wrong but would like to hear what others think.
1: Einstein tells us that Past Present Future all coexist
2 : If this is the case then all events "just are " and exist simultaneously
3 : If all events exist simultaneously then despite events APPEARING to progress in a sequential fashion to us - they actually DON'T and there is NO causal relationship between Past present and future .

The point Im trying to make here is , that I think Einstein is actually implying that all events " just are " and are NOT sequentially linked to each other.
If this really is the true nature of time
Then I don't see any problem going back and killing your Grandfather his grandfather etc - would make no difference ,
Because if all events DO coexist then NO event is influenced by any other so "going back " and changing one event will not change any other.

I know that this is not how we normally think of time ,Am I misinterpreting what Einstein is implying ? Not sure.