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Lets say I build the a time machine and I travel to the past. Suddenly I meet Isaac Newton and by mistake(or not ) I kill him...
What would happen??? We know that without him its impossible to build the time machine, fly to other planets and everything else... so what would happen? Would I just dissapear? Would the machine dissapear? Will it be the end of the Universe?
(I know it may sound as a question for the purgaroty, but I really want to know, I am seriously asking this as a physics question)

Books have been written on this subject. The short answer is that nobody knows.

Or if they do know, they aint tellin'.

The issue is indeed tricky.

Normally we describe the evolution of any events by means of a so-called "world-line", consisting of the path that an event traces in 3+1=4 dimensional space-time. Note, time acts as a coordinate here and traces out the path since it continuously progresses.


The question whether it is possible to travel to the past can be cast with this approach into a more precise question:

+++++++++++++++++
can worldlines cross??
+++++++++++++++++


It seems such crossings are not necessarily incompatible with causality ...


Let me stop here, since as of this point things are much less under control.

Bye Fridger

I tend to subscribe to the idea that time travel alter history and would lead to the creation of a new, divergent universe,

and so you cannot travel to your own past.
The moment you set foot in the past you create a new future which is different from the one you left.

My time travel rant is here;
http://www.orionsarm.com/intro/WhyNoTimeTravel.html

Serious physicians/mathematicians have speculated that if you tried to do such a thing, some strange and unknown mechanism would interfer and prevent you from commiting the act. (Like in the movie "The Time Machine")

Personally, and for no particular reason, I believe timetravel of that kind is impossible.

t00fri wrote:The issue is indeed tricky.

Normally we describe the evolution of any events by means of a so-called "world-line", consisting of the path that an event traces in 3+1=4 dimensional space-time. Note, time acts as a coordinate here and traces out the path since it continuously progresses.


The question whether it is possible to travel to the past can be cast with this approach into a more precise question:

+++++++++++++++++
can worldlines cross??
+++++++++++++++++


It seems such crossings are not necessarily incompatible with causality ...


Let me stop here, since as of this point things are much less under control.

Bye Fridger

Sorry for quoting myself, but this afternoon I had no time left. Here is a "crossing worldline" illustration. Try to understand what it means!


Bye Fridger

eburacum wrote:I tend to subscribe to the idea that time travel alter history and would lead to the creation of a new, divergent universe,

and so you cannot travel to your own past.
The moment you set foot in the past you create a new future which is different from the one you left.

My time travel rant is here;
http://www.orionsarm.com/intro/WhyNoTimeTravel.html

I agree with this view point, which is compatible with the Everett interpretation of Quantum Mechanics.

There are many things we could say about time travel. According to Stephen Hawking, a time machine isn't able to travel before its own construction. So you wont be able to visit Newton to kill him. However, if a time machine was build before your own birth, you could travel to a moment in time before your own creation and kill your mother. Then a paradox.

In classical general relativity, there are many solutions which describes space-time with closed time-like curves (Kerr-Newman solution, Godel universe, and worm holes are well known examples), which means you could use them to travel in the past.

But there is a Fondamental principle in physics : consistency of Nature. There is no paradoxes in Nature, whatever you're doing. Coherence of a theory is the most important thing. Because of this, relativity theory implies THERE IS NO FREE-WILL IN ANY CLASSICAL SPACE-TIME ! So you can't decide to travel in your own past to kill your mother. Even if a time machine was possible, you can't use it as you wish, because you don't really have a free will. Let me explain this.

In classical relativity, space-time is the set of all events, in space AND in time. All events are actually lying there, "somewhere", and they all "coexist" already. Everything is already "writen" in space-time. Relativity theory, when placed in a fully classical context, is deterministic and FATALIST. You can't change any event, because all events are already "memorised" in space-time. There's no place at all for free-will. So even if time travel is possible, you can't change anything. There is no paradox at all. Only coherence of space-time count here.

But if you really believe in free will, and aparently we have access to free will in our real universe, then the space-time of relativity theory must be demultiplied in some infinite set of space-times with many variations (the Multiverse). All those space-times are deterministic and fully fatalist, but you don't know in which space-time you are living. They becomes probabilistic. Your future isn't really "writen" somewhere, because now we have an infinite set of space-times.

Actually, space-time in classical relativity theory is just a usefull concept, a tool, to describe reality. But space-time isn't "real". It's just a representation, a partial map of events. The real space-time doesn't even exist (in the sense of relativity theory). There is no future "coexisting" with the past, and the past itself is disapearing at all moments.

If you could go back in time and the divergent universe theory is true, it explains why we haven't seen any people come from the future with their time machines, if it is possible to make the machines of course. We would be travelling along the normal "unchanged" universe, while the people from the future are meeting us in the divergent universe.

Michael Kilderry

ElPelado,

My view is that the time-line would change, and maybe your parents no longer exist, but dont believe you suddenly disappear, but instead, managed to moved from your original universe to the new alternate one.

The interesting question is, does only one time-line exist? or do they all exist? If your parents died because of your actions, are they still alive in their original time-line? If so, going back in time would not change the future, only your perception of the future.

Seb

Related to this subject I often wonder the following:
If one were to go back in time, only to change a very small thing. Would this change however lead to a completely different (far) future, due to the butterfly effect, or would the course of events remain unchanged, except for a few places and moments near our little infringement.