The Anthropomorphized Time Traveling Paradox.

[JPBenowitz]

The most famous time traveling paradox is the Grandfather Paradox, where the traveler goes back in time to kill his own grandfather and thus prevents his own birth--a paradox. An adaptation to this paradox is Hawking's Mad Scientist Paradox where a mad scientist opens up a wormhole one minute into the past and assassinates himself. Since this is a paradox Hawking concludes that the wormhole cannot exist. The rest of this thread is my attempt at solving this paradox via a thought experiment while allowing such a wormhole or equivalent time traveling mechanism to work without causing a paradox.

Possibility 1. Closed Time-Like Curves behave in such a way that causing a paradox is entirely a statistical phenomenon analogous to the 2nd Law of Thermodynamics. Let's suppose that we cannot go back in time a single minute or even a hundred years. Suppose we can only go back so far as to the probability of creating a paradox is statistically equivalent to that of the 2nd Law of Thermodynamics. As an example, at this moment let's say we can travel back 5 billion years, before the Earth even formed. It would be impossible to compute which events would actually cause a paradox and to accidentally cause one would be equivalent to reducing the total entropy of the universe.

 

[Simon Bridge]

What you are saying is that there may be some sort of protection of history involved, where you can only travel back in time in such a way that you cannot produce a paradox?

Your mechanism is off ... there is always some plausible way to negate the time traveler.
i.e. 5 billion years ago, the time traveller could let off WMDs and so drastically alter the course of evolution ...
i.e. trillions of years ago ... the traveller could leave a bomb cunningly designed to go off at the first signs of life happening etc.
... also see "butterfly effect".

Another approach to protection of history is to realize that everything you do in the past has already happened - so it's a kind of fate. You feel you have free will in the past but you don't - you only have free will in the future: but when is that?

A more popular approach is to invoke multiple universes - and the time traveler just kills one version of his grandfather... the one in whose timeline he [the grandfather] was killed by a time-traveller from an alternate future [you]. The time machine only travels along the resulting timeline.

It's a staple of science fiction:
"Times without number" John Brunner
"Meddler"" Philip K Dick
"Flight of the Horse" Larry Niven
... most relevant to your ideas.

The main problem is not so much the possiblity of negating yourself but what it does to the law of causality.

What hawking and the others are doing is not telling you to avoid paradoxes, but using the fact that a paradox can be set up to illustrate that you cannot logically keep cause-and-effect and have time travel: one of them has to go.

To protect causality, time travel cannot happen ... the alternative is to modify causality.
Movies usually do this by setting up a preferred or special history where causality works normally and everyone else only has causality most of the time.

But you have not solved the paradox because the paradox is about logic and causality not the probability of accidentally killing your grandfather.

 

[JPBenowitz]

What you are saying is that there may be some sort of protection of history involved, where you can only travel back in time in such a way that you cannot produce a paradox?

Your mechanism is off ... there is always some plausible way to negate the time traveler.
i.e. 5 billion years ago, the time traveller could let off WMDs and so drastically alter the course of evolution ...
i.e. trillions of years ago ... the traveller could leave a bomb cunningly designed to go off at the first signs of life happening etc.
... also see "butterfly effect".

Another approach to protection of history is to realize that everything you do in the past has already happened - so it's a kind of fate. You feel you have free will in the past but you don't - you only have free will in the future: but when is that?

A more popular approach is to invoke multiple universes - and the time traveler just kills one version of his grandfather... the one in whose timeline he [the grandfather] was killed by a time-traveller from an alternate future [you]. The time machine only travels along the resulting timeline.

It's a staple of science fiction:
"Times without number" John Brunner
"Meddler"" Philip K Dick
"Flight of the Horse" Larry Niven
... most relevant to your ideas.

The main problem is not so much the possiblity of negating yourself but what it does to the law of causality.

What hawking and the others are doing is not telling you to avoid paradoxes, but using the fact that a paradox can be set up to illustrate that you cannot logically keep cause-and-effect and have time travel: one of them has to go.

To protect causality, time travel cannot happen ... the alternative is to modify causality.
Movies usually do this by setting up a preferred or special history where causality works normally and everyone else only has causality most of the time.

Couldn't we just invoke Novikov's Self-Consistency Principle?

 

[Simon Bridge]

Couldn't we just invoke Novikov's Self-Consistency Principle?

Think it through - does that allow time travel without modifying causality?

It's basically a "protection of history" clause, just like the one described in post #1.
The trick is to figure out how to go back in time without causing a paradox of some/any kind.
You should find that this principle means that the probability of a time machine being invented is zero.

 

[Vanadium 50]

Please reread the PF Rules section on overly speculative posts, everybody.