Grandfather Paradox: Debunking the Myth of Time Travel and Altering the Past

[Godswitch]

The Grandfather Paradox

Something that's been on my mind for sometime...

If you were to travel back in time and kill your grandfather you would no longer exist...I believe this is false and What I'm saying is:

If you travel back in time, you have removed yourself from your original timeline, and are viewing your grandfather as an observer.

You kill your grandfather...Doing this should mean you cease to exist, but because you have removed yourself from your original timeline and time before your point of travel will have no effect and you are free to carry on from that point onwards as you have created another timeline in parallel to your original timeline.


So what I am saying is killing your grandfather is possible and will have no effect on the time traveller as he has removed himself from the timeline his grandfather will go on to create. You will still exist but in another life you will not exist, this is the point I believe the Paradox has not made clear.

You could argue you kill your grandfather and you do not make the the time machine, but as you have already created an alternate timeline you will have already left.


In simpler terms: If you had 10 minutes of video and that was your life and you erased 6 minutes from the middle you would have 2 minutes on each side and on those 2, 2 minute portions you still exist...The 6 minutes are the point, you kill your grandfather to the point you travel back in time...

 

[Dale]

Hi Godswitch, welcome to PF. Once someone makes a time machine it will be fairly easy to test. Until then it doesn't really matter.

 

[JesseM]

Among physicists who consider the possibility of backwards time travel in GR, the most popular resolution seems to be Novikov self-consistency principle which just rules out the possibility a time traveler could "change" anything (though she could influence events in the past), though a physicist named David Deutsch also suggested that in the many-worlds interpretation of quantum mechanics a time traveler might end up in a different branch of history than the one they started from (other physicists have disputed that this is plausible even in a many-worlds context, see here for a discussion). None have proposed the idea of a single history which can be "written over" as you seem to be suggesting with the videotape analogy, the idea doesn't really make sense unless you suggest some sort of "meta-time" dimension in which the timeline can "change". Anyway, probably the most likely answer is that when a theory of quantum gravity is found it will rule out the time travel solutions that appear in relativity, an idea known as the chronology protection conjecture.

 

[bcrowell]

There is a good discussion of this kind of thing in the final chapter of Thorne, Black Holes and Timewarps. Also there's a book called Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time, by Gott.

 

[bcrowell]

Some online info:

summary:
http://plato.stanford.edu/entries/time-travel-phys/

Novikov self-consistency:
http://authors.library.caltech.edu/3737/
http://authors.library.caltech.edu/6469/

defense in depth:
http://arxiv.org/abs/hep-th/9202090

loopholes in arguments based on energy conditions:
http://philsci-archive.pitt.edu/archive/00004240/

 

[PAllen]

To me, the much more interesting type of paradox is the 'self generating information paradox' which is not prohibited by Novikov, which is why I favor some variant of chronology protection.

My fravorite example: A Shakespeare scholar travels back in time to see the first production of Julius Caesar, to see if it is different from the modern canon. They find Shakespeare struggling with a new play, unable to decide how to proceed. They give him a copy of the play as received in modern times. Shakespeare loves it, produces it, etc. Per Novikov, all is fine - except that Julius Caesar has no origin, no author.

 

[JesseM]

To me, the much more interesting type of paradox is the 'self generating information paradox' which is not prohibited by Novikov, which is why I favor some variant of chronology protection.

My fravorite example: A Shakespeare scholar travels back in time to see the first production of Julius Caesar, to see if it is different from the modern canon. They find Shakespeare struggling with a new play, unable to decide how to proceed. They give him a copy of the play as received in modern times. Shakespeare loves it, produces it, etc. Per Novikov, all is fine - except that Julius Caesar has no origin, no author.

I wonder, even in a logically possible universe where time travel was possible, maybe it would be possible to show that such information loops would be very improbable? Physicists have suggested that one could use a quantum path integral approach to define a probability distribution on various self-consistent evolutions (some initial conditions in simple models involving time-traveling billiard balls can have an infinite set of self-consistent evolutions), so maybe if you analyzed some simple model where a computer could pass information back to itself, you could show that if there are self-consistent evolutions where any information passed back has a low amount of "order" in some sense (defined in terms of Kolmogorov complexity or something), these evolutions are much more probable than ones where ordered information is created "for free". Maybe this wouldn't work, but would be an interesting problem for someone to try to analyze...

 

[Godswitch]

To me, the much more interesting type of paradox is the 'self generating information paradox' which is not prohibited by Novikov, which is why I favor some variant of chronology protection.

My fravorite example: A Shakespeare scholar travels back in time to see the first production of Julius Caesar, to see if it is different from the modern canon. They find Shakespeare struggling with a new play, unable to decide how to proceed. They give him a copy of the play as received in modern times. Shakespeare loves it, produces it, etc. Per Novikov, all is fine - except that Julius Caesar has no origin, no author.

I don't think that holds 100% true either...You could say from some perspective or point of view the play has no Author or Origin...And you could also say ultimately Shakespeare is the Author of the play, regardless of the fact he copied his own work!

Would you agree?

I'm holding with the fact that if you travel through time in turn you are moving to and thus creating a new timeline previous to your original timeline which will carry on at a rate relative to you and by that I mean even though you go back through time the time period you left does not stand still, it will carry on, it is only to the traveller does the future stand still.

This is to say you could travel back in time and kill oneself and you will still exist.

 

[PAllen]

I don't think that holds 100% true either...You could say from some perspective or point of view the play has no Author or Origin...And you could also say ultimately Shakespeare is the Author of the play, regardless of the fact he copied his own work!

Would you agree?

I'm holding with the fact that if you travel through time in turn you are moving to and thus creating a new timeline previous to your original timeline which will carry on at a rate relative to you and by that I mean even though you go back through time the time period you left does not stand still, it will carry on, it is only to the traveller does the future stand still.

This is to say you could travel back in time and kill oneself and you will still exist.

Well, that's a common Sci Fi model, some have attempted to make it work in QM many worlds, but it is definitely not consistent with GR. The Novikov criterion specifically applies to a scenario where there is exactly one time line, and exactly one thing happens at a given spacetime coordinate. In that scenario, the self generating information paradox is allowed and can only be interpreted as no origin/authorship for the play.