Is it possible to identify an impossibility?

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Impossibilities pose themselves dependent on context.

If you allow for parallel universes, maybe the grandfather paradox doesn't exist at all, because the me of universe A goes and kills my grandfather in Universe B, therefore I do not exist in B. Parallel Universes are objectionable for a multitude of other reasons: if they don't interact with us, how would you determine their existence; if they interact with us, the parallel idea hardly applies. They also pose issues in the meaning of measurement and of physical law, and you'd need abundant use of the anthropic principle.

Outside the context of Parallel Universes, the grandfather paradox is not unsolvable: you just have to postulate that Nature only allows selfconsistent worldlines. This a very neat idea, but without a machine to go back and meet my grandfather when he was young, it is possible it has no physical content....Show more

Think of it this way: If it is not possible to identify an impossibility, then you have just identified an impossibility.

Quantum mechanics does not support "parallel universes" as much as it implies the existence of all possibilities existing simultaneously in a infinitude of multiverses. They need not be parallel....Show more

I'm not exactly sure what your question is. I can take a stab at it and if you clarify, I'll edit my answer later to fit better:

The notion of parallel universes in quantum mechanics (as opposed to those postulated more recently by m-brane theory) is called Everett's view, after it's chief advocate. Wavefunctions describe superpositions, that is to say the set of possible states that each particle can be in at any given time. The particle cannot be said to be in any one state, it is in a superposition of being in all states at once. When the particle becomes entangled with the rest of the world, however, the superposition disappears and only one possibility emerges. The standard view, best described by Roland Omnes in his book Quantum Philosophy, says that only one possibility emerges. Everett's view says that each possibility emerges and the universe splits into different possible universes, one in which each possibility occurs. There are problems with this view in how to resolve situations where one possibility is more likely than another. Are there infinitely many split universes in proportional quantities? Is one universe in some sense larger than another? These problems are probably not insurmountable, but they certainly pose a good challenge to the view.

The grandfather paradox, best argued by the philosopher David Lewis and portrayed poorly in the recent film adaptation of The Time Machine, goes as follows: If I had a time machine, could I go back in time and kill my grandfather? If you assume the answer is yes, then the result would be to nullify my own birth. As a consequence, I would never have existed, and if I never existed, I wouldn't have killed my grandfather. Therefore, even if time travel is possible, it must still be somehow impossible to kill my grandfather. What then would stop me? That's where David Lewis took a wrong turn and argued that you would necessarily miss if you tried to shoot him, or some other artificial chance must stop you.

Before discussing the parallel universes solution to this paradox, you need to understand time machines: If time machines are possible, and they almost certainly are not, there is only one way they could work not currently ruled out be well established theories that have been proven correct (I will not argue the degree to which we can say they have been proven here, but I have done so in the past). First, you would have to create a small wormhole, something quantum mechanics suggests is possible. A wormhole is a tunnel through spacetime. You send something in one end it immediately emerges from the other end. Next, you would have to inflate and stabilize the wormhole, something which may very well be impossible, last I heard there were no models in existence on which anything at all, even light, could enter a wormhole without collapsing it. Finally, you would have to temporally separate the two ends of the wormhole using the twin paradox method. If you accelerate one and move it at incredibly high speeds, very near the speed of light, when it returns to rest on earth, it will have travelled through time less than the one that remained stationary. Then anything entering the stationary end would emerge from the end that moved in the past. The effect is very small. It would take a very long time and an enromous amount of energy to build a separation significant enough for one to travel back to before one's birth and kill ones grandfather, but theoretically, nobody has yet proven it impossible. (I think it probably is impossible, but it hasn't been proven yet.)

Now consider a quantum description of the possibilities for the earlier end of the time machine. It exists in a state in which many things could emerge from it. The set of things which can emerge from it is determined by the current state of the universe. Only those things which could possibly exist in the time and place in the future where the other end of the time machine exists could possibly emerge, but that is still an enormous range of possibilities. In Everett's view, then, the Universe is constantly splitting into the various possible universes that could happen depending on what could emerge. There is a possible world in which you emerge from the time machine and kill your grandfather and a possible world in which you don't, and instead you are born. Both would in fact happen. This solution works nobody can travel through time to anyplace that doesn't have a portal to the future and the existence of the portal itself causes a split for all possible things that could emerge from it.

One could make a similar argument on the conventional view, in which the future does not exist, but it is more difficult and I will not attempt to do so here.

Now here's where I run into a problem with your question. I'm still not exactly sure what you're asking. One can identify an impossibility. It is some description of the universe that is not a possible evolution of the wavefunction. There are other answers, but I'm hesitant to go into detail without knowing what you mean. This answer really has very little to do with Everett's View or the Grandfather Paradox....Show more

After much thought, I have decided that I have no answer to this question. And I thought that was impossible!...Show more

A logical contradiction would be an impossibility...Show more