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If you went back in time and visited your granny during her ninth birthday don’t kill her! Because if you put a gun to her head and pull the trigger she could not have given birth to your mum. YOU certainly were never born…

Therefore you could not have killed anyone as you never existed. Now this means your granny couldn’t have been killed by you. She didn’t die nine years old. This permits you to be born.

If you were born could you go back and kill your granny? No, not your real granny. This general idea has been used in Back to the future. Marty nearly stops himself from being born when he prevents his parents from falling in love.

Solving The Problem
1, You simply can’t change history meaning if you go back in time you have no free will(as shown in Twelve Monkeys, Crime Traveler etc. Events will get in your way if you try to kill your granny. YOU CAN’T DO IT.

2, When you so-called change history you’re actually moving up a different branch in time into another universe. The previous universe (where you were born) still exists. When you fire the gun you’re really killing another version of your grandmother.

3, Any actions you make in the so-called past has no affects on the present. It’s a different time-line universe.

The laws true solutions don’t present real time-travel as you go into another world.

Conclusion
We can’t simply use the grandmother paradox to rule out time-travel claiming it causes logical inconsistencies. We need to look at all the possibilities.

How could we build a time machine?
The mathematics of general relativity suggest that under extreme conditions space-time might become so warped it would be possible to travel back in time. There is also the possibility of tunnels in time created by negative mass.

Very long rotating cylinders of matter-proposed by Frank Tipler

Kerr’s spinning blackholes-since most stars spin this becomes worth looking (proposed by Roy Kerr).

Cosmic strings-(as Richard Cott suggested)

Travelling faster than light – would take us back in time(as suggested by solutions to relativity). Wormholes might allow us to outpace light (if you walk through the short cut quicker than light through convential (normal) space.

Tachyons -are sub-atomic particles which always travel faster than light and therefore move back in time constantly. They have not yet been found and remain hypethical.

Contracting Universe- Time might then be running backwards but since everything else also would it’s unsuitable.

Macro-wormholes(Kip Thorne showed how we could use it as a time machine).

Suppose you could go back in time, lets say several decades and found your grandfather when he was two years old. In his house you could grab a knife and stab him to death. He doesn’t get the chance to have children with your grandmother. Therefore either your mother or father doesn’t get born. Your parents can’t give birth to you because one of them don’t exist. You could never have been born and don’t even exist. But could your grandfather have been killed by someone who doesn’t exist? He must have lived through his childhood. This would allow you to exist if this is the case. Seemingly you can go back in time to commit the murder if you are born but then you would never have been born. And so on and so on. This situation is not consistent with itself. It doesn’t make sense and can’t possibly happen.

Solutions

1, You simply can’t change the past. Time will stop you limiting your freedom while you’re in the past from your point of view. This puts the concept of freewill in serious danger especially if you tell people what’s going to happen to them in their future. If you believe when you go back in time you are from one possible future from everyone else’s point of view they can simply go up any root in time they want.

According to quantum physics Many Worlds theory there are a huge amount of universes where every possibility occurs between them all. In some you’re the opposite sex. In some you won the lottery etc.

You might be heading towards the universe you originally came from before you travelled back in time. Everything will happen the way you remember it. But all the people you meet are free to decide what they want to do and enter a different universe. Since it’s not possible for you to be there you disappear from their lives. Meeting a time traveller from your future could therefore be very strange.

2, A parallel universe might be created when you seem to change the past. Imagine if time itself was just like a tree. The different branches show different ways events could have happened. Every time we decide to do or not to do something time splits. Even if we are not aware we decided something it have affects. Quantum physics reveals a many worlds theory like this.
Conclusion
Since this parallel universe is not really your past (despite it’s first appearance) anything you do there does not affect you. You can prevent a version of yourself from being born because you are not really related to anyone there. They just look very like your family and friends. You are not home! You may be somewhere that looks like the place you live but a different universe in quantum physics is a completely different reality.

3, No matter what you do in what is really a parallel universe you will do back to your previous universe which is not affected by your previous actions. A space-time wormhole could lead you back to your original unaffected universe.

What came out of the wormhole if the ball never went into it?

Another common example of a paradox is a ball that goes through a wormhole connected with a moment in the past. Therefore it comes out of the other space-time wormhole mouth actually before it went in! Then what might happen? If the ball then hits it’s younger self out of the way of the mouth then it never goes in. But if the ball never did go in the wormhole then how can it ever come out. The existence of the older version of the ball is destroyed i.e. it never falls back in time. But then this version certainly can’t hit the younger version out of the way. So it must go into the wormhole as a collision with it’s older self is evidently the only force that could and did stop the ball from entering the wormhole. This of course is unexplainable and is logically inconsistent.

But the situation could happen differently to allow it to become self consistent.

1, What if after the ball comes out of the wormhole at an earlier time it does hit it’s previous self but this collision is what makes the ball fall into the mouth in the first place! It hits the other version into the mouth. This would imply that the past or present is affected by the future. In fact in this case the past is dependant on the future.

2, The ball might simply hit it’s other self only slightly so the direction of the ball is not altered enough to cause a paradox.

3, The ball might just miss it’s younger self.

Nature might protect time and prevent paradoxes. From theories and many stories it’s clear that paradoxes cannot happen in the real world.

In the terminator movies John had something important to tell Sarah. Thank her for her help through the hell. Tell her not to give up or he will never exist.
Who wrote the speech?
John certainly didn’t write it. He was told it since he was a kid from Sarah who could remember it. Sarah just recalls what Kyle told her in 1984. Kyle just remembered what John told him to say.
Conclusion
No one wrote it. It exists somehow but not by John, Kyle or Sarah being creative. No one had to write it because it was created by the affect the future on the past had on each other.

If I in 2000 study the history of work done in a private factory and learn about the development of a time machine! Their scientists worked from designs and plans noted in a book (never published)they used. They had no idea how to achieve such technology until they read it.
Then to experiment with temporal(time) paradoxes I am sent back to 1983. The entire building has not even been built as I arrive. Later accidentally I meet one of the scientists and talk to him. When I hear how excited he is I hand him that book which he uses future success. The answers! He is more than willing to read and use the book.

But who came up with the idea of how the time machine is built?
Not any of the scientists as they just followed instructions in that book.
Not me as all I did was hand a scientist a book. Noone actually wrote the book and no one had to work it out.

Is this allowed by physical laws?
Noone really knows…

——————
Paul Curran from Co Derry, Ireland

[This message has been edited by Paul (edited 12 January 2001).]

[This message has been edited by Paul (edited 23 January 2001).]