Noticing today's

RocketBoom on the

Ontological Paradox made me start musing about one of the

examples, specifically:

On his 30th birthday, a man who wishes to build a time machine is visited by a future version of himself. This future self explains to him that he should not worry about designing the time machine, as he has done it in the future. The man receives the schematics from his future self and starts building the time machine. Time passes until he finally completes the time machine. He then uses it to travel back in time to his 30th birthday, where he gives the schematics to his past self, closing the loop. Of course, the schematics must have come from somewhere.

There might be a simple way around the problem of where the information comes from if you accept the

"many-worlds" interpretation of quantum mechanics:

- Let M be the multiverse, or the set of all universes.
- Let u be a universe in M where the man successfully develops the time machine on his own.
- All he has to do is find any other universe u' in M such that the current time in u' is equal to a previous time in u (i.e. 30 years ago).
- If, technically, he travels across universes from u to u' (as opposed to really traveling in time in his own universe) then the information (the plans) really come from u, not u', so there is no spontaneous generation of information.
- The fact that the plans generate a "time" machine which allows the loop to "close" is completely accidental since "time" travel would in reality continually travel to another universe (e.g. once in u', he would need to find a u'', etc.)
- Also, because he is traveling between different universes (as opposed to traveling backwards in time) he doesn't break any known laws of physics.

There are some problems with this way of thinking about things though...

If there are universes "in the past" and there are no limits to how far we can go "back", it implies that there is always at least one universe right now that is undergoing the very first instant of the big bang. I don't think "many worlds" allows such a juxtaposition. It's probably not a given that a universe

u' can always be found.

You might say that this doesn't matter, because he's only going back 30 years... however, once the loop is "closed", it might easily execute more than n = (the age of the universe/30) times... if you imagine it takes n universes (

u_{1}...u_{n}) to perpetuate this cycle, then the last universe

u_{n} in the chain must have been undergoing the first instant of the big-bang when he left his original universe

u.

"why do we seem to only know about one universe"

There has always been another weird problem with the multiverse interpretation... supposedly, different event probabilities create different universes in which each probability actually happened. If that's true, then

why do we seem to only know about one universe -- i.e. why does our consciousness seem always to travel with this particular branching path and not one of the others?

Then I realized this isn't such a problem after all: we might only remember this universe because at any given moment our memories are the sum total of our experience along only the path we are in. Our other "selves" would likely have different memories, but all similarly self-constrained to their paths.

There's no way to "peek around the quantum veil" as it were.