The paradoxa (is that correct?) of time travel have been discussed on the SDMB up to a point where there can hardly anything new be added, so I do NOT call for a discussion but rather short handy replies.
Has science found a final solution to the famous grandfather’s paradox?
This page http://philarete.home.mindspring.com/philosophy/grandfather.html
gives a couple of points of view, but many of them contradict each other. Has the problem finally got solved (eg by excluding the possibility of time travel, postulating some mysterious power that prevents my killing grandpa, or anything else), or is it still disputed?
Science works by observation (the collection of data), the formulation of hypotheses to explain the observations, and experimentation to test the hypotheses. Experimental data provides additional observations–rinse and repeat. If a hypothesis correctly predicts future observations and experimental results, so much the better.
So far as I am aware, there are no observed instances of time travel (into the past), so the whole “paradox” is moot. In the absence of evidence to the contrary, the whole discussion thus belongs in the realm of philosophical speculation.
In short, there is nothing for science to explain.
Science fiction, OTOH, has plenty of solutions. Some include:
You can’t kill your grandfather; no matter what you do, events will prevent you from doing it (e.g., “Try and Change the Past,” by Fritz Lieber)
You can kill your grandfather, but when you return to the present, nothing will be changed (Alfred Bester’s “The Men Who Murdered Mohammed”)
If you kill your grandfather, you will fade away into nothingness.
The grandmother paradox – you kill your ancestor, but end up taking his place (I forget the novel with that in it – fairly recent, and about time traveling back to San Francisco in the 60s).
There are probably dozens of other variations.
Which is correct? You pays your money and you takes your choice.
There is one thing that can be said in advance of scientific observation.
The only outcome absolutely excluded is any outcome the description of which both asserts and denies precisely the same assertible. But take care: the terms “assert” and “deny” and “precisely the same” must be interpreted without the usual ambiguities of daily language. The statement “John is alive and John is dead” is NOT excluded unless we tack on a definition such as “(where for some entity E to be ‘dead’ is for E to be other than ‘alive.’) and (where each iteration of ‘John’ references numerically-one entity over all instances).” Thus what is really excluded is best stated as “John is-and-is-not alive.”
(Observing these rules with care allows them to cover such cases as ‘Shroedinger’s Cat.’)
BTW, the plural of paradox is paradoxes…though for philosophers it’s paradise!
Or an explanation that actually comes close to matching physical models of how the universe might work - Every time there is a decision point in the universe, it splits into two new ones - one with the original decision, and one with the other. That means that there are an almost infinite nummber of universes, all of which together cover all the possible combinations of events that have ever occured.
So if you go back in the past and kill your grandfather, all you did was split a universe off and created yet another one. In fact, you created millions of universes, each new one representing every change at a quantum level made by your existance.
That makes the paradox somewhat meaningless - there are still universes where grandpa is alive, including the one you came from. There is no ‘future’, but a zillion different ‘futures’. If you go into the future in one of them where grandpa doesn’t exist, that’s really no different than travelling to one in the past where Grandpa never existed in the first place.
So how can you exist in that universe if Grandpa was killed before you were born? Simple - you are just a traveller, and it’s no different than if you travelled back in time to a universe before you were born. If you think about it, isn’t that the same paradox? How can you be there if you haven’t been born yet?
Does this violate causality? Not if we expand causality to include the whole set of universes.
There are a number of scientific ideas about what would happen, one of which is the Many Worlds model mentioned by Sam Stone. The other big contenders are the Strong Censorship Principle (time travel is impossible) and the Weak Censorship Principle (time travel is possible, but you can’t kill your grandfather anyway). I get the impression that the OP was already familiar with these ideas, and just wanted to know if there were any evidence one way or the other… To which the answer is no, there is not.
However, Stephen Hawking addresses the paradox in one of the chapters of his new book, The Universe in a Nutshell. Basically, the physics makes it look like nothing bigger than, say, an elemental particle can travel in time (other than forward like the rest of us, of course).
I’m sorry, I missed that query. Someone else on the SDMB asked me the same question. It’s very gratifying, because, you see, I wrote the sig line. It’s a CalMeacham original. I write all my sig lines. If ever do use a quote by somebody else, I’ll attribute it. Even if I have to attribute it to “Unknown”.
Regarding the original question, BTW: Robert Forward, former Hughes researcher, founder of Forward Industries, and hard SF author, doesn’t believe in the Grandfather Paradox, and challenges anyone to prove to him (in a scientific, physically defensible way) that it even exists. For his take on the idea, read his book TimeMaster.
If a hypothesis correctly predicts future observations and experimental results, so much the better.