[Shirosaki]

This is my favourite paradox, and this is the link to it.

I had to give a presentation in science on a paradox and I had a think about this one, so I'd like to pop the idea to you all.

What do you think?

[Tsuyu]

Which is why you can't kill Hitler.

[Firis]

That hurt my brain But it was fun and amusing...

[Tsuyu]

Originally Posted by Firis

That hurt my brain But it was fun and amusing...

The basic gist of it is this:

You can not alter the past, for it has already transpired.

Say you decide to create a time machine and go back and kill Hitler before he becomes the ruler of Germany, thus saving a lot of people.

However, if you go kill Hitler and prevent World War II you would never have gotten the idea to build the time machine in the first place!

So therefor it is impossible to go back in time and kill Hitler - because he has already lived and acted out his part in World War II.

[Skotekal]

Damn, so what your saying is the future isn't real, and can't be written, but the past is written and unchangable? Who thought of time anyways... Jerks... Damn you smart people!

[Tsuyu]

The future is very real and changeable. It hasn't transpired yet.

You can't alter the past because the "future"(our "present") you live in relies on it already having transpired the way you know it.

So basically the past can't be change because it has already happened.

[JohnDoe]

That paradox wouldn't make a good argument to say that time travel is impossible unless it worked under the assumption that anyone who would build a time machine intends to go back and kill his own ancestry, which I would think would be impossible, not even to mention how I feel about backwards time travel.

Originally Posted by Tsuyu

So basically the past can't be change because it has already happened.

It depends on what you choose to believe. I do no believing in this realm, nor want to, but I have come across a handful of theories.

If I had to choose any of these theories, I would prefer what I consider the absolute time-line, which operates under what I believe is called the self-consistency principle: If one were to go back in time, one could not do anything that would have stopped him from going back in time in the first place, otherwise he would not have gone back in time. And, if one were to go back in time, the time in which he arrived, in the past, has already happened.

Example:

I go back in time to the year 1800 to meet the first of the JohnDoe ancestry. I could not kill him, otherwise I would not have been able to go back in time in the first place. Even if the thought had occured to me, it has already happened and I have not killed him, so I do not kill him. If I should try to kill him, I would have to fail, not by choice, but by the law of possibility. I can't kill him because it would then have been impossible for me to go back in time to kill him.

While I'm visiting my ancestor who I cannot kill in 1800 from 2009, everything that I "do", I have already done, in that in the year 1800, my ancestor was visited by me, several centuries before I was born, and everything I did, I will do, as Tsuyu explained, the past is written and cannot be changed.

In short, two things:

1.) To say that some aspects of time travel are impossible because they cause a paradox is not to say that time travel itself is impossible, but damnitall if I'll ever believe that time travel is possible.

2.) Regarding backwards time travel in the absolute time-line, everything that has happened due to the result of backwards time travel will happen in the time of depature to the past.

Oh, and to those who would believe in changing the past, branching of a time-line or parallel time-lines, you guys suck and I care not what happened in Back to the Future Part II.

[Tsuyu]

It depends if you adhere to the "grandfather paradox" reasoning, or the Startrek-eqsue reasoning of multiple time lines existing.

In example; if you change something in the past, that past continues along it's own time line and your own is unchanged when you return to the present.

[Arseface]

I like the Grandfather paradox theory of time, or whatever. It's way more fun to try and work out.

[Shirosaki]

I meant the one about Jane's family tree but if that has started a discussion then okay

I believe you cannot go back into time before the time machine is switched on, because there is no recorded time in which to travel back to.

If you made a time machine in a room with one other scientist, as you get in and push a button your other self who has vanished will reappear as your normal self disappears; does that make sense? The time machine will not be able to travel to any time that it has not been in existence. Furthermore, it'll land in the same spot or somewhere random, or in space. Who knows?

[Arseface]

Originally Posted by Urahara

I meant the one about Jane's family tree but if that has started a discussion then okay

Thats the one I meant.

Originally Posted by Urahara

I believe you cannot go back into time before the time machine is switched on, because there is no recorded time in which to travel back to.

I dont understand...

Originally Posted by Urahara

If you made a time machine in a room with one other scientist, as you get in and push a button your other self who has vanished will reappear as your normal self disappears; does that make sense?

Not if you travel forward in time (after you have gone back) to a point before you entered the machine.

Originally Posted by Urahara

The time machine will not be able to travel to any time that it has not been in existence. Furthermore, it'll land in the same spot or somewhere random, or in space. Who knows?

That would be cheating. And presumably it would land in the same spot it left, or somewhere where you tell it.

[Archon'sRage]

I always knew that whenever i saw a movie that they changed the past it's impossible since it has already happened and the person who altered their fate is techinacally already done so the person who changed his/her fate never existed.

Also if you go in the past and meet yourself there will be an infinity number of you's because if you go back in time and meet yourself you will die but the you you met will grow up and do the same thing and go back in time and meet himself that will do the same thing again.

So basiclly timetraveling shows and movies should have never existed since what they did already happened and all we see is the world that would have been if he hadn't meddled with the past. Say you killed Hitler, Anne Frank would still be alive and nobody will remeber how hitler was really killed and if you killed him before WWII then It would have never happened and nobody would remember. If you go back in time and kill the first humans you would have never existed and nothing will happen of the future. This might be confusing but I have known this for many years now after thinking. If you didn't understand this then just think about it and you should understand.

[Tsuyu]

Watch the Futurama movie Bender's Big Score.

Seriously, it is all there.

[Arseface]

Originally Posted by Tsuyu

Watch the Futurama movie Bender's Big Score.

Seriously, it is all there.

Thats a good movie

[Archon'sRage]

the only time traveling episode that worked was the simpsons one wehn homer killed his grandfather and became his own.