Causality Paradox

[Bright_Star]

I've started watching Star Trek:Voyager on BBC America & one of the episodes I've watched deals with time travel. In it someone mentions the concept of a "Causality Paradox". Just what does that mean anyway?

[dalesys]

One take: " '—All You Zombies—' " by Robert A. Heinlein (R.A.H. R.A.H. R.A.H!)

[mjr]

I'm rectally sourcing this, however, based upon this link:

https://en.wikipedia.org/wiki/Causality_(physics)

it would appear to me that a "Causality Paradox" would be something happening before or after it's supposed to. And because it shouldn't happen (because it either has no cause, or the cause happens after the event, I would guess).

I think that's the paradox.

It would be like if a glass was knocked off of a table after it shatters on the floor.

[Kittish]

The classic example is going back and time and killing one of your grandparents before whichever parent they birthed is conceived. Another example would be travelling back in time and showing previous you how to build a time machine, after you'd spent years getting it to work.

[csquared]

The Causality Paradox is a loop. A causes B causes C causes A. That is the example given in All You Zombies. Another would be to go back in time and tell yourself how to build a time machine. One of the stories in the Stainless Steel Rat series also used the Causality Paradox. Villain attacks the future. Hero goes back in time to stop the villain. Villain attacks the future to stop hero from coming back in time.

What Kittish referenced is often referred to as the Consistency Paradox or the Grandfather Paradox. Taking an action that would inherently prevent you from taking that action.

[wolfie]

There's another one in the PERN universe - the story deals with the Question Song, all but one of the dragon roosts mysteriously emptying after the last Thread fall, and a VERY long span between Thread falls.

This one also appears in the anthology "Time Wars", edited by Poul Anderson.

[EricKei]

wolfie -- It's been so long since I last re-read those books. You're describing the events of Moreta's Ride (and, indirectly, some of what happened in the original sets of books), correct? ...so long that I've forgotten -- Did Lessa "time it" back to Moreta's era and effectively inspire the Question Song that led to the mass temporal migration?

[Seanette]

No. Lessa's story and Moreta's both involve time travel, but in very different time periods, and the two never get near each others' times.

[Kittish]

The Causality Paradox is a loop. A causes B causes C causes A. That is the example given in All You Zombies. Another would be to go back in time and tell yourself how to build a time machine. One of the stories in the Stainless Steel Rat series also used the Causality Paradox. Villain attacks the future. Hero goes back in time to stop the villain. Villain attacks the future to stop hero from coming back in time.

What Kittish referenced is often referred to as the Consistency Paradox or the Grandfather Paradox. Taking an action that would inherently prevent you from taking that action.

You're right, my first example is indeed an example of a Consistency Paradox.

I couldn't remember the name of the Heinlein story. It's a beautiful example, and a pretty good story, too.

[wolfie]

Did Lessa "time it" back to Moreta's era and effectively inspire the Question Song that led to the mass temporal migration?

Actually, the story in "Time Wars" is the only PERN story I've read. Lessa did go back in time to inspire the song and migration, but the only mention of Moreta was a reference to the ballad "Moreta's Ride".

[EricKei]

Thanks to ye both

Interesting you're writing the planet's name like that -- Not everyone is aware that it's an acronym (iirc: something like "Planet: Earthlike; Resources: Nominal")

[Sakka]

Another story that may fit this is The Door Into Summer by Heinlein.

The protagonist gets sent to the future and is confused at how some equipment is very intuitive to use, exactly how he would set it up if he was designing it. That the competing company, with a slightly different design was also started at the same time, by the same person as Company A.

[Ironclad Alibi]

All this talk of time travel and causality and no one has mentioned the Terminator movies? The bad guy in the first one becomes the good guy in the second one because of changes wrought by time travel.

[csquared]

Another story that may fit this is The Door Into Summer by Heinlein.

I think that fits the Continuity Paradox. The future he knows does not exist unless he goes back to the past. It does feed into the theory that our time line is fixed and time travel is predetermined.