Multiple universes hypothesis
Another hypothesis is that there are an infinite number of universes, one for each possibility. Here, should the time traveller kill his grandfather, one universe would have a live grandfather, and another universe would have a dead one. Failing that, the universe would annihilate itself, for such a paradox would defy its laws.

Another theory concerning the classic grandfather paradox is that such an event would create a new universe, one in which the aforementioned deed was committed. This would not affect the committer's universe, nor the committer himself.


Can-Not Because Has-Not theory
This theory states that the present is not the forefront of time, and so we are our future selves' past. Thus, if sometime in the future a time travel device were created, someone from the future would have already brought it back to us, thus establishing itself as "already" existing in our time as a result - and likely copied and recopied. Since our present selves are still wondering about time travel, this theory states that we will never be able to build a time machine, because if we are still wondering, then no one from the future has built a time machine and brought it back with them to us, and if no one in the future has built a time machine, then we in the future will not build a time machine, and no one can ever build a time machine because no one in the future has built one. This theory, however, creates a Causality Loop.


It led me to the Many-worlds Interpretation (MWI).

[In layman's terms, this means that, in some sense, there is a very large, perhaps infinite, number of universes and that everything that could possibly happen in our universe (but doesn't) does happen in another universe.]

[The many-worlds interpretation is DeWitt's popularisation of Everett's work, who had referred to the combined observer-object system as being split by an observation, each split corresponding to the different or multiple possible outcomes of an observation. These splits generate a possible tree]


And then I found "Quantum suicide and immortality".

Explanation of the thought experiment
An experimenter sits in front of a loaded gun which is triggered or not triggered depending on the decay of some radioactive atoms.

Repeated runs of the experiment
With each run of the experiment there is a 50-50 chance that the gun will be triggered and the experimenter will die. According to the Copenhagen interpretation, the gun will eventually be triggered and the experimenter will die. If the many-worlds interpretation is correct then at each run of the experiment, the experimenter will be split into one world in which he lives and another world in which he dies. After many runs of the experiment, there will be many worlds. In the worlds where the experimenter dies, he will cease to exist.

The experimenter's point of view
However, from the point of view of the non-dead copies of the experimenter, the experiment will continue running without his ceasing to exist, because at each branch, he will only be able to observe the result in the world in which he survives, and if many-worlds is correct, the surviving copies of the experimenter will notice that he never seems to die, therefore "proving" himself to be invulnerable to the gun mechanism in question, at least from his own point of view.

Nuclear bomb
Another example is where a experimenter detonates a nuclear bomb beside himself. In almost all parallel universes, the nuclear explosion will vaporize the experimenter. However, there should be a small set of alternative universes in which the experimenter somehow survives (i.e. the set of universes which support a "miraculous" survival scenario).

Quantum immortality
The idea behind quantum immortality is that the experimenter will remain alive in, and thus remain able to experience, at least one of the universes in this set, even though these universes form a tiny subset of all possible universes. Over time, the experimenter would therefore never perceive his or her own death.


If that's the case, it bears a similarity to the half-life theory for the decay of chemical substances.

Theories about time are always interesting.

P/S: I do not own these theories. They are all from wikipedia, which is a great source for valuable information.

Labels: Thoughts