3) is supposed to be "does the set of all sets which do not contain themselves contain itself?" It's a variation on the barber's paradox: a single male barber in a town shaves only those men who do not shave themselves. Who shaves the barber?

5) Give the child to someone else to return--then someone else returned it.

11) X is the "black hole number" for addition (like 0 is to multiplication and 1 to exponentation), which theoretically (IIRC) should exist. EDIT: Infinity.

12) isn't a paradox, it's a loop. Just like while(true); is.

20) neither, they are both frozen until one looks away, which never happens.

21) why not? It already happened.

Other Time Travel Ones) IMO, the Predestination Paradox is right. You can't change the past, it already happened. Thus you can't change the future either: it will be already. You can only cause it. Therefore my future self can't kill my past self--my past self already survived, and that can only be caused, not changed. Something would stop me, because it already did. That's how I think of it, anyway.

Also, here is a good one
To get across the room, you have to get to a halfway point
but to get to that halfway point, you have to get to the halfway point of the halfway point
but before you get to that halfway point, you have to get to the halfway point of the halfway point of the halfway point
So in theory, you cannot move, because to get to one point, you first have to make is to the halfway point.

Albino Pokey, these are Paradoxes/ weird concepts/ stuff that makes you think, so I guess 12 works

Assuming that the detail of the universe is infinite, that is. Id est, there's a length you can't get smaller than.

Gah, noes. My teacher started talking about 4th-dimensional hypercubes the other day and how you can unfold them into three dimensions kinda like how you can unfold a cube into two dimensions. My mind has been blown enough for one week.

But about the chicken vs egg thing - it'd be extremely unlikely for the egg to come first without something caring for it since eggs have to be kept at the right temperature and be rotated at least twice a day in order to hatch.

You haven't done hypercubes yet? But yeah, they're pretty interesting.

About time travel, some people believe that because of the butterfly effect, we should never venture backwards in time. Personally, I think nothing will change, since it has already happened. In case anything was changed, assuming chaos theory is correct, it could have a great impact and parallel worlds could be created that way. Paradoxes can't happen. Paradoxes won't happen. If a paradox was to happen at any point in time, then no point in time would make sense according to our definition of it, but we defined it, so there can't have been a paradox ever, at any point in time.

In theory.

:D HA HA HA HA!!!

Kirdneh, you do realize what you said in #10...

WATER A TREE?!?!?

Does anyone else get this lol type?...

That's not a paradox, number ten.

so, because of reverse engineering, it is impossible to change the past or future becuase if you try to, say, shoot your grand father, something will block it, like he remembers he has to go somewhere and the bullet misses? like in HHG2TG?

Y'all are talking about the Predestination Paradox and that you can't change the past by travelling back in time because it already happened. But to be honest, that really depends on how time travel works, if it ever will. Sure, at first it seems most logical that you can't kill yourself because something will stop you, but that's also pretty illogical.
For example, if you were to cause something that never really happened like, say, setting off a bomb in your home, then I don't think it's possible (or at least likely) that something will stop you. You could send the bomb back in time by itself, and make sure that it goes off in your house, one second after it arrived, then there is little that can be done to stop it. Then, because you died that early, you were never there to send back the bomb, then you didn't actually die, so you did send back the bomb, and then you have fully functional and universe-destroying paradox.

Something a little less drastic though: let's say you had a secret place in your childhood. Nobody else ever went there, nobody else ever knew about it, and you know that because it up until this day nobody else has ever found it. Now, suppose you went back in time, went to that secret place, and put a rock under it (if it was outside you could dig a hole or something). Now, your past self knows nothing about this, and nobody else does, so time flows on as normal. However, you go back to the present, and then check under your secret place, and there's the rock you put there.
I know this still complies with your rules, but suppose that you were to check for the rock before you went back in time to put it there, and it wasn't there. If it was there, then you just wouldn't do it in the first place, because the whole point of this experiment was to change time. You'd put something there that wasn't there, like another rock.

Basically, my point is this: it's pretty much impossible for something to prevent you changing time simply because what you plan to do (such as killing yourself or burying a stone) didn't happen. Sure, that happens in movies or books, but that's because the authors want to prevent a paradox. Something conveniently happens that stops the main guy changing the course of time, but that wouldn't happen in real life.

Think about it.

well, we know for a fact no paradoxes have been created as we are here alive, and not being exploded 4-th dimensionally.
but maybe another time traveler stops him, but they don't see TT B, only TT A as it is about him. TT B is someone who prevents the paradox. if someone didn't stop him then, well, we wouldn't be here debating.

so, what do you people think of Schrödinger's cat?

puggsoy wrote:

Y'all are talking about the Predestination Paradox and that you can't change the past by travelling back in time because it already happened. But to be honest, that really depends on how time travel works, if it ever will. Sure, at first it seems most logical that you can't kill yourself because something will stop you, but that's also pretty illogical...
[SNIP]
Think about it.

But you couldn't send the bomb back, because you already know that the bomb wasn't sent back. Somehow it didn't explode, because...it didn't.

H2G2G has a good example of this, and the third Harry Potter book has a pretty clearly explained example as well. After going back in time, Harry couldn't go talk to his past self, because he hadn't, and he knows that. But he is able to interact with his past self by summoning the stag, because that already had happened.

Of course, until someone invents a time machine, there's no way to know for sure.

So let's get working on that!

EDIT: Also, wasn't Schrödinger's Cat meant to show how the theory didn't work? As in, look at how ridiculous this gets when you apply it to real world objects instead of particles?

This is exactly what I'm saying. Books are all nice and dandy, being logically correct and whatnot. Seriously, I don't see what stopped Harry going out and talking to himself other than Hermione. And, if he really wanted to, he could have ignored her and gone and done it anyway.

And "somehow it didn't explode, because...it didn't" doesn't really convince me that much. I know that's what your theory shows, but it's not really an explanation of how it's true.

Seriously guys, somebody invent a time machine before I go over to AP's house and slap him

What I'm trying to say is that it is already determined. I mean, if I asked you right now, you could say that you're certain that your house didn't collapse yesterday. And the reason is simply that it didn't.

C'mon, let's get this time machine working! Then I'll try to go back and release World of Goo as my own game in 2004. Watch if the copyright statement at the bottom of thsi page changes to say "World of Goo (c) Albino Pokey"

But what is forcing it to be determined? Is the entire timeline of the universe predetermined? If so, then you're right, but that also means that the future is decided and free will isn't completely true. If not, there should be nothing stopping me changing the past.

Alternatively, time travel could work differently. I'm thinking that, in addition to actually travelling through this universe's proper timeline, it's possible that time travel could result in 1) us simply seeing the past, Pensieve-style, and 2) us travelling into a parallel universe's timeline, allowing us to interact with past (maybe future?) events, but it doesn't effect the present (i.e. our timeline).
I think the former option would be most preferred, observing the past is the most important function to time travel, if it's at all possible. The latter option would be OK, but more dangerous (e.g. it would be life-threatening to observe dinosaurs or wars).

Like the quantum many-worlds theory? We could just "jump" between branches if we changed the past, and our timeline is unaffected?

Yup. That would be cool, but as I said, dangerous. Just seeing the past would be much better, IMO.

Another thing: who are we to say that we are the original dimension? are maybe we a variant of another dimension?

Don't you guys understand chaos theory? If Bob (a fictional person I just made up) were to travel back in time to, say, the year 2000, he would be upsetting the already-determined universe on a molecular level. The problem with this is, since you can't change something that has already happened, it's a paradox. Paradoxes can't happen and won't happen. That's their definition. We come up with them for fun, because we know there's no way they can be true. If Bob were to place a bomb in his workplace at that time so he and all his colleagues would die (he is a very traumatized person) it wouldn't happen, because it didn't.

I don't think you quite understand AP when he says that. The truth is, even though you'd want to go back and create a paradox, it won't happen as something has already stopped it in the past, your past. Due to chaos theory, we have probably already lapped several branches in the multiverse, and going back in time won't change anything. It's a predetermined event, you traveling back in time, so you can't go avoiding it.

Say that Bob, when he was a child, received a gift from his future self. In the present, he knows he must eventually give that gift to himself in the past, and even though he would try to avoid it, it has already happened and it will happen. The idea that you can will yourself to create a paradox (like you're trying to say, Puggsoy) is complete and utter ********. You can't use willpower to overcome a force that is predetermined. Predetermined means it can't be avoided. Even if you try to avoid it, you will soon realize that trying to avoid it is what made it happen in the first place.

It's a little difficult to grasp, but when you do, it makes you marvel at another wonder the universe has brought forth.

note: i realized if you time travel then you would end up in space because the earth moved in that time-jump. (Doc: "you must think 4th dimensionally!" Me: "i am. i don't want to be zapped into space")

I love how people argue so much about time travel and nobody has a clue how to do so. "Yay, let's pass the black hole's event horizon into a parallel universe!" Oh, snap, the definition of an event horizon is something you can never cross. Derppity derp. It's all well and cool and all that, but also impossible. I'll take a bit of advice from Sherlock Holmes and not care whether the Earth goes around the moon or not.

we have a moon?!?

I see what you're trying to say, Red. It's just that the only way I see that that would be true is if the universe and everything that happened and will have was predetermined, before time began. Honestly I think it's really unlikely that that's so, and thus I don't think it works.

Consider this: as a child, I met myself from the future. I know it's me (not a fraud), because it told me something that only I knew, because I had only thought it in my mind. Nobody but myself could know it, ever. Then, later on, but before I reach the age that my future self was, I kill myself intentionally. Jump off a building, whatever. Maybe stab myself through the heart. Blow myself up with a bomb. I dunno why, maybe I have some strange will to cause a paradox or something. And thus I don't go back in time.

Seriously, I don't see how your theory could still work in such a case. Your sentence "Even if you try to avoid it, you will soon realize that trying to avoid it is what made it happen in the first place" doesn't overrule this situation. Killing myself can't cause me to travel back in time later on.

Don't get me started on that. I've got a whole other section of mind for argument on that sentence. That was the "new" Sherlock anyway. The original Sherlock Holmes was way cooler, he marvelled at how strange real life is in comparison to fiction, don't tell me that's more important than caring that the moon goes around the Earth.

unless, when you kill yourself, time fixes itself, and you never met yourself when you when you where a kid. Therefore, you would have no reason to kill yourself, and the event never happened.
does time fix itself?
also, does the universe only paradoxicalize itself when it is known to be a paradox? like the cat is only dead or alive when you see it? if we don't realize we made a paradox, and no one does, are you safe until you think about it?

hard to tell if this topic is a philosophical or scientifical debate

Purely philosophical. We have no facts on time travel (or time itself, for that matter), only theories, so it's all speculation.

@MOM4Evr
I think a forwards time machine wouldn't be too hard, considering relativity and whatnot. If you were to say, vibrate at the speed of light (and this is assuming our noses won't become older than our feet) for 2 years on your own time, space around you would be 50 years further, so while you only aged 2 years, everything and everyone around you has lived for a much longer period of time than you have. Although I have no clue how we would go back to our own time afterwards.

@Puggsoy
But that's a paradox. It can't happen. I know you want to go and say that you can just stop yourself from doing it, but if that were true, you just couldn't have been able to hear about it in the first place.

For future examples, let's avoid talking about humans and keep ourselves to the quantum level.

Also about everything being predetermined, that is false. There is always a level of quantum randomness that exists, despite what happens in time. Some say this is what causes parallel universes to exist, because there is always another way for something to happen.

That would work, assuming that we could survive that heat without sublimating.

Still safer than a black hole, though.