Grandfather Paradox & Self Consistency Principle: Are They Different?

[BadgerBadger92]

TL;DR Summary

Are these two different phrases for the same thing?

https://www.popularmechanics.com/science/a41106690/grandfather-paradox-time-travel/

I’ve been studying the self consistency principle, being the idea in time travel that history (the past) must be conserved. When I read about the grandfather paradox it sounded like the same thing. Are these two different?

 

[DaveC426913]

Yes. They're mutually exclusive.

If you have one, you don't have the other.

The SCP is a(n invented) rule that attempts to ensure the GP is impossible.

 

[BadgerBadger92]

Yes. They're mutually exclusive.

If you have one, you don't have the other.

The SCP is a(n invented) rule that attempts to ensure the GP is impossible.

Excuse me, I thought the Grandfather Paradoxwas the idea you can’t go back in time to kill your grandfather. Got it backwards

 

[DaveC426913]

The GP posits that you could - which is what creates the paradox.

 

[BadgerBadger92]

The GP posits that you could - which is what creates the paradox.

Thanks for clearing it up!

 

[PAllen]

Just thought I would add there are time travel paradoxes that the Novikov self consistency principle does not prevent. Basically, Novikov considers spacetime as whole, and states there is one state of every event. Time travel is possible, but the single state of everything avoids many paradoxes. Thus, it is perfectly ok for grandfather to meet time traveling grandson who claims to be from the future. But there would not be some alternate grandfather to whom this never happened. Instead, this just is part of grandfather's history, and grandson might have heard about this before they left.

Note that some interpretations Novikov principle imply no significant material body can follow a CTC (but they could follow an open time loop instead). These argue that, e.g. a decaying lump of uranium following an actual CTC creates a Novikov problem because when the CTC closes, you have changed the past because the number of decayed atoms must suddenly change (with any nontrivial material body, its exact collection of atoms and states is continually changing - it is just easier to argue with uranium decay). With a 'near CTC', there is no problem you have e.g. a future state of the uranium body coming out of a wormhole next to its earlier state. Both then age from there, and at some point the 'younger one' is sent back to through the wormhole to be come the older one. Note, the distant past and distant future have only one state of the body.

However, Novikov does not prevent so called information paradoxes. My favorite is that a time traveler visits Shakespeare with a copy of "Julius Caesar", while Shakespeare has not yet written this play. She gives him the copy she brought back. Shakespeare likes it, transcribes it, burns the copy. There is no Novikov violation in this sequence - this is just the way it "always was". But the play has no author whatsoever!

 

[PeterDonis]

My favorite is that a time traveler visits Shakespeare with a copy of "Julius Caesar", while Shakespeare has not yet written this play. She gives him the copy she brought back. Shakespeare likes it, transcribes it, burns the copy. There is no Novikov violation in this sequence - this is just the way it "always was". But the play has no author whatsoever!

The classic treatment of this type of scenario in the SF literature is Heinlein's "By His Bootstraps".

 

[DrGreg]

My favorite is that a time traveler visits Shakespeare with a copy of "Julius Caesar", while Shakespeare has not yet written this play. She gives him the copy she brought back. Shakespeare likes it, transcribes it, burns the copy. There is no Novikov violation in this sequence - this is just the way it "always was". But the play has no author whatsoever!

In the British sci-fi sitcom Red Dwarf, one of the lead characters turns out to be his own father (due to time travel). Trying to work out what that would mean for his DNA inheritance is boggling my mind.

 

[PAllen]

In the British sci-fi sitcom Red Dwarf, one of the lead characters turns out to be his own father (due to time travel). Trying to work out what that would mean for his DNA inheritance is boggling my mind.

That's cute, and I do think it could be arranged/argued not to produce a Novikov inconsistency.

The only way I know of to prevent this class of paradox is to ban "information CTCs". All of these cases have information flowing in a CTC. In my mind, this, for all practical purposes, forbids time travel (which I think is true of our universe). To prevent informational CTC, you are led in steps to saying that a time traveler cannot interact in any way with the past, which effectively means they are not really there. The first step is to conclude that information can't flow from the traveler to the past. But there are no known physical interactions where this will not occur.

 

[jbriggs444]

The classic treatment of this type of scenario in the SF literature is Heinlein's "By His Bootstraps".

Or "All You Zombies" also by Heinlein where the protagonist is his/her own father. And mother.

 

[PeterDonis]

Or "All You Zombies" also by Heinlein where the protagonist is his/her own father. And mother.

Yes, but IIRC there isn't a book whose information content is a "circle" in time, as described in post #8. In "By His Bootstraps", there is.

 

[BadgerBadger92]

Or "All You Zombies" also by Heinlein where the protagonist is his/her own father. And mother.

Kind of sounds like “The Terminator” where the soldier Kyle was John Conner’s father all along

 

[DrGreg]

In the British sci-fi sitcom Red Dwarf, one of the lead characters turns out to be his own father (due to time travel). Trying to work out what that would mean for his DNA inheritance is boggling my mind.

That's cute, and I do think it could be arranged/argued not to produce a Novikov inconsistency.

For what it's worth, having given this some further thought, I agree there's no logical contradiction, but there would be some DNA passing round a loop without any outside source, similar to the Shakespearean example mentioned earlier.

• His maternal DNA, including his X chromosome, would be inherited from his mother and her ancestors in the usual way
• About half of his paternal DNA (from his paternal grandmother = his mother) would have to be identical to the correponding segments of his maternal DNA
• The other half his paternal DNA (from his paternal grandfather = his father = himself), including all of his Y chromosome, would be inherited from himself and would therefore be passing round a loop without any outside source

So, logically consistent, but highly unsatisfactory to have DNA with no source.

 

[Hillbillychemist]

Summary: Are these two different phrases for the same thing?

https://www.popularmechanics.com/science/a41106690/grandfather-paradox-time-travel/

I’ve been studying the self consistency principle, being the idea in time travel that history (the past) must be conserved. When I read about the grandfather paradox it sounded like the same thing. Are these two different?

For your consideration:
"The Time Traveler" - J. Brydon Stewart
Tony built a time machine
and traveled back in time
From Thursday night at eight-fifteen
To Monday night at nine.
Excited by his breakthrough,
in temporal transportation,
He sadly didn't factor in
One major complication.
You see, we are not standing still
We're hurtling around,
in orbit of the sun, one hundred
times the speed of sound.
So, when poor Tony flipped the switch,
with glee upon his face,
He found himself, three days ago,
alone in outer space.

 

[Vanadium 50]

All together now...

I'm my own grandpa
I'm my own grandpa
It sounds funny I know
But it really is so
I'm my own grandpa

 

[Mister T]

I’ve been studying the self consistency principle, being the idea in time travel that history (the past) must be conserved. When I read about the grandfather paradox it sounded like the same thing. Are these two different?

I think the grandfather paradox is an example that illustrates the self consistency principle. The former being a bit of concrete reasoning, the latter abstract reasoning.

 

[DaveC426913]

I think the grandfather paradox is an example that illustrates the self consistency principle. The former being a bit of concrete reasoning, the latter abstract reasoning.

I've always rankled at the idea of the SCP. I don't see any logic in it except as a form of wishful thinking.

"Well if we don't have a way to avoid the grandfather paradox, then we can't have time travel. And that sucks. So let's just say the timeline "heals itself", or whatever".

It's a form of magic. How it works is irrelevant; only that it works in our favour.

 

[jbriggs444]

I've always rankled at the idea of the SCP. I don't see any logic in it except as a form of wishful thinking.

If one wishes to consider a universe in which one can have conventional time travel then the Grandfather paradox forces one to a small set of alternatives. One can ignore the paradox and craft the story anyway. One can accept the model of a single block universe and the SCP. Or one can invoke some flavor of branching multiverse.

[One of] Niven's resolution(s) is that the simplest (and, therefore, most probable) sort of universe where the SCP is adhered to is one in which time travel is never invented. Accordingly, time travel will most likely never be invented in our universe.

 

[DaveC426913]