1. May 2, 2013

### Viru.universe

Hey guyies, can i get some really interesting and crazy paradoxes, except the grandfather one, thank you

2. May 2, 2013

### Bandersnatch

At some point in human history the solution to Y is developed. The chrononaut takes it back in time and uses it to solve Y in the past, preventing the development of the solution.

Y might be a disease, mortality, global warming, poverty, whatnot.

3. May 3, 2013

### brenan

With the possible exception of if you meet yourself. I'm not sure if the entire human body
is regrown over time or just parts of it (maybe someone here can say?)

If the entire body is regrown over time I guess there is no paradox.
If it isn't - that may constitute one?

4. May 3, 2013

### ModusPwnd

Is there a difference between physically impossible and just impossible? I dont understand what "physically" has to do with it... ?

5. May 3, 2013

### skeptic2

My favorite is the fire alarm (pop quiz) paradox. An announcement is made that there will be a surprise fire alarm next week but the exact day will be a surprise. Obviously it can't be on Friday because if it hasn't been on any of the previous days it wouldn't be a surprise on Friday. That means it can't be on Thursday either because if it can't be on Friday and it hasn't been on Monday Tuesday or Wednesday, it wouldn't be a surprise on Thursday. The same reasoning is repeated for Wednesday, Tuesday and Monday.

6. May 3, 2013

### Viru.universe

@skeptic2,
wow thats a nice one!
But on thursday, the students dont know whether the test will be today or tomorrow, that is they are not 100% confirmed, so if the test in on thursday, it'll be a surprise right?

7. May 3, 2013

### Fredrik

Staff Emeritus
My favorite is from Bill & Ted's excellent adventure. Their time machine had the strange property that the return trip has to be the same "length" (in time) as the trip. So if they go back 100 years, spend 5 minutes there, and then go "home", they will end up in the place they left, 5 minutes after they left it.

Early in the movie, Bill mentioned that the key to his father's car had been missing for a week, and that the father was sure that Bill had taken it.

(The quotes below aren't actual quotes; I'm paraphrasing).

Here's the paradox: Near the end of the movie, Bill and Ted need to go to school to take a test. To get there in time, they need to take the car. But the keys are missing. So Ted suggests "let's go back in time a week and take the keys". Bill answers "we don't have time" (because they have to leave immediately to not be late). So Ted says "OK, so how about this. We take the car to school and take the test. Then we go back in time a week, take the keys, and put them...under that rock". Bill agrees, so they walk up to the rock, lift it, and grab the keys from under it. As they're walking to the car, Bill says "Your dad was right. It was you who took the keys".

There are also some great paradoxes in the Doctor Who episode "Blink". It's a great episode, and you can see it even if you haven't seen any others. There's more than one paradox in the episode. The best one is the video conversation between The Doctor (David Tennant) and Sally Sparrow (Carey Mulligan).

8. May 3, 2013

### BobG

Pool ball is sent into the corner pocket. The corner pocket is the entrance to a time machine. The exit from the time machine is the side pocket.

The time machine only sends the pool ball far enough back in time that it hits the pool ball headed towards the corner pocket.

If neither the original pool ball nor the time traveled pool ball enter the corner pocket after the collision, then there is no pool ball to be ejected from the side pocket, the collision can't occur, and the original pool ball is never deflected. In which case it enters the corner pocket ..... You have an unstable reality that can't exist unless it doesn't exist.

If the time travel pool ball deflects the original pool ball, but not enough for it to not go in the corner pocket, then you have a stable reality. No problem.

If the time travel pool ball deflects the original pool away from the pocket, but then enters the pocket itself, you also have a stable reality. Except now you have a loop created entirely by the time travel pool ball.

The last was supposed to be the concept of the movie "Looper", but it was badly done. In other words, by the end, you had an unstable reality that couldn't possibly exist unless it didn't exist, etc.

9. May 4, 2013

### Bandersnatch

It was much better realised in Chris Smith's "Triangle"(2009).

10. May 6, 2013

### brenan

A quite underrated movie. I loved that one.

11. May 7, 2013

### FlexGunship

I'll throw my two cents in:

Read Heinlein's short story called "--All You Zombies--" (http://en.wikipedia.org/wiki/"—All_You_Zombies—"). It probably sets the world record for best time travel story ever. I never quite figured it all out, but I'm pretty confident that every character in the story is the same person. If I recall, it's only 10-20 pages long. Read it. It's worth it.

Here's the full story:
http://faculty.uca.edu/rnovy/Heinlein--All%20you%20zombies.htm [Broken]

Last edited by a moderator: May 6, 2017
12. May 7, 2013

### skeptic2

In ninth grade I read Heinlein's Time For The Stars and I recommended it to a friend. The book was about the twin paradox. He read it and used it for a book report for English. The English teacher knew nothing of relativity and gave him an F because the story didn't make any sense.

Last edited: May 7, 2013
13. May 7, 2013

### Viru.universe

oh ya! I know this one
its the craziest story i ever read

Last edited by a moderator: May 6, 2017
14. May 8, 2013

### BobG

Actually, this would still be a paradox. From the time traveling pool ball's perspective, it would spend an eternity being involved in pool ball collisions. Eventually, the accumulated stress will shatter it. Then what?

It would be an even bigger paradox for a human in this time loop. They would soon die of dehydration and starvation and their body would eventually decay. Even if that problem were overcome, they'd age.

From the point of view of a person not in the loop, you can't say what they'd see.

15. May 8, 2013

### skeptic2

There may be a problem with this scenario too. If the time travel pool ball deflects the original pool ball, it too will be deflected slightly. It will enter the corner pocket and emerge from the side pocket with a slightly altered trajectory and momentum. After a number of loops it will either miss the original ball or miss the corner pocket.

16. May 8, 2013

### Fredrik

Staff Emeritus
I like the version that goes like this: You shoot a pool ball towards the middle of the short edge of the table. Now two things can happen.

1. Nothing interesting.
2. An older version of the pool ball emerges from the side pocket and hits the side of the younger ball, deflecting its path into the corner pocket.

17. May 9, 2013

### brenan

This is the grandfather paradox again.

18. May 9, 2013

### Jobrag

Also by Heinlein "By His Bootstraps" another short story

Last edited by a moderator: May 6, 2017
19. May 9, 2013

### FlexGunship

I've always felt that Heinlein's treatment of time travel was always the most intellectually rigorous. He never proposed simultaneous alternate histories (a la Back to the Future) but instead always stuck to the idea of "one dimension of time."

As a result, there were never any "paradoxes" per se; just very interesting causal loops.

Essentially, Heinlein's take on the grandfather paradox is this: "You already know that you haven't killed your grandfather. You might not know why or how you've failed, but you know that you have."

In the case of Heinlein's Time Enough for Love Lazarus Long became the strange man whom his mother loved and then went off to war and died. He wasn't his own father, but he was a part of his own childhood memories.

20. May 9, 2013

### skeptic2

With regard to option number 2, and with backwards time travel in general, there always is a causality violation.