Why Parallel Universes?

According to "Scientific American" There are at least 4 kinds of possible paralell universes. My personal favorite is the kind that can exsist right in our own space time. If our universe is realy infinite, (99.99 percent of universes with a microwave backround like ours should be infinte), AND There are limited number of possible confiugrations of matter inside any hubble volume (a hubble volume is the only volume of space we could ever see and interact with since objects outside of our hubble volume are accelarating away from us faster than the speed of light) then if you look out far enough in our OWN universe there is a hubble volume out there that is identical to our own! (though you could never actualy see or reach it)Well I guess there doesnt HAVE to be one identical to our own but after a certian point they DO have to start repeating themselves.Here's why using a handy metaphore:In a Math class I took where we deturmened that there HAD to be a number of people living in new york city that had an identical number of hairs on their heads. If the maximum number of hairs on the human head is the maximum aria possible for a human scalp times the maximum possible number of hairs per square inch. Then any city with more residents then the maximum possible number of hairs nessesaraly has to have a number of people with an identical number of hairs on their heads. (It turns out that new york is such a city).In the same way a space time with infinite hubble volumes will have some hubble volumes with identical configurations of matter.In fact since it appears that on a realy large scale space is uniform (big foamy bubbles of superclusters). Then you don't have to go nearly as far to find identical hubble volumes (since there are no hubble volumes of completly empty space or ones with a maximum amount of matter)Sadly we will never be able to reach these paralell identical hubble volumes. But I don't mind there is plenty to explore here in our own quant little hubble volume some 30 billion light years in diamiter.

Brian Greene has a great explanition of time travel in his latest book. "The Fabric Of the Cosmos"Basicly he says that there are 2 stances you can take on time travel that are not rife with parodox. The first stance is that when you travel back in time you merely travel to a paralell universe that is similar to you own. If you go back in time and attempt the cleche' killing of your own father, you can go ahead and kill him with no reprocussions because he is merely one of an infinie number of identical fathers. A parallel "you" will not be born in this universe but you (the one that killed your father) won't wink out of existance because you still have a father back in your own universe.This is a very unristrictve kind of time travel in that you can do whatever you want and then see the reprcussions without worrying that you will wink out of existance. Remember you can travel forward in time just by accelarating so that if you wanted to see what a world would be like without you in it ala "it's a wonderful life" all you have to do is hop in your space ship and travel at a nice relativisitc speed.The second interpretation is much more restrictive but still doesn't allow for paradox. It is my personal favorite becase it makes for good short stories.In this interpretation of time travel When you travel back in time you DON'T go to a parralell universe. There is no need for multiple universes.So what happens when you go back to kill daddy? You can't. Why not? Becase you exsist! and therefore couldn't have traveled back in time to prevent your own existance. The paradoxes come from the misconception that when you travel back in time, time "starts again" from where you arrive. But it doesn't! Everything has already happend.If in the future you wind up going back in time, you are ALREADY there! When you travel back you can't "change things" to make a "second past." That past already exsits WITH your changes. Space time is then seen like a frozen river or a loaf of bread (Greene's term). If we could look at a time traveler outside of space time we would see his/her whole trajectory through space time as one solid unmoving shape.I like this second intepretation best even if it does imply a kind of fatalistic deturminisum.

I was wrong. Our hubble volume is actually Much much larger than that.

You said,"If you were to travel back in time (in your own universe) and kill your father, not much would happen, other than your mother being alone the rest of her life. If you are already in the year 1970(or whatever) to kill your father, you exist."No, according to the second unchanging theory YOU COULD NOT KILL YOUR FATHER if you traveld back in time to before you were born. Or if you could kill him you came into exsistance through some means that didn't nessesitate your father being alive before you were born.
In the second determainistic veiw there are NO multiple versions of history you either had a father that you went back in time and killed or you don't.If the second determainstic veiw of spcaetime is correct then you only come into existance ONE way and history does NOT change even when you travel through time. If you remember your mother telling you that your father was killed by some mysterios stranger then maybe you did kill him. But One thing is certan, when you return to your own time you will find it is exsactly the same as when you left it*.*Again according to the determanistic veiw of time travel.

I thought it would be a good idea to post a second response based on the other interpretation of traveling back in time.In this theory when you travel back in time to kill your father you are realy traveling to a parallel universe, one in whitch your father is killed by you. If you kill him a person with your name will not be born and indeed your mother will be alone for the rest of her life etc (unless she finds someone else etc). But if you could find some way to travel BACK to your own original universe you father would still be alive and well.

What kind of imbalance? Are we still discussing the possibilities of time travel? Are you suggesting that if "Dr. X" traveled into a parralell universe he would disrupt it's "time line?" I don't think so, the two explinations for time travel I have discussed are ways to avoid paradoxes (is that what you mean by imbalance?).This message has been edited by The Dread Dormammu, 11-06-2004 03:43 PM

Yes I have seen that movie.
It was never explained why The jumper gained stength and power when his alternate selves died off. If that were true why don't we all become super powerful around 86 when our alternate selves are dying of old age? In a similar way How could paralell universes become "imbalanced" by travelers?