a reply to: swanne
This is correct. There is something called the Novikov self-consistency principle. The theory in its simplest form states that the probability of
creating an event that gives rise to a paradox is always zero. However, in Everett's many-worlds interpretation of quantum mechanics (the one that I
subscribe to), every probability event splits the universe. For example, let's say you've heated up matter so much that you have quark plasma. You
select an individual quark and measure its charge. There is a 33% chance that it'll be a down quark and have a -⅓ charge, and a 67% chance that
it'll be an up quark and have a +⅔ charge. According to Everett's theory, your measurement causes the superposition state (it is both
simultaneously) to decohere. When this happens, the world splits into three parallel universes. In two of them, the quark is up and has a +⅔
charge, and in one of them, it's down and has a -⅓ charge.
You can extend this to systems of larger numbers of particles which simultaneously all exist in superposition when not being measured. Trillions of
trillions of trillions of point-like particles make up your 3-dimensional self, and each has a probability distribution of being in one of a set of
given states. Depending on which state occurs, your future is changed and you go down a different road. This is always happening, all the time, and
so parallel universes are essentially created at a virtually infinite rate. There is probably a limit, as quantum physics tells us that even time
should be quantized, and so whatever that quantization is would be the rate at which splitting happens, assuming staggering is impossible.
If you were to travel back to your past, you change that past simply by being there. The air will flow around you differently, and perhaps that might
cause a butterfly to change course by 0.1 degrees, thus preventing a little girl 20 miles away from catching it. That little girl's future is
altered, and the whole thing amplifies. The longer it goes on, the greater the ripple effect is. This phenomenon is called the Butterfly Effect,
where the name implies that even a single flap of a butterfly's wings is enough to alter the entire universe, if only slightly, and change the path.
Wait long enough, and this will become disturbingly obvious.
Of course, by being there, you are guaranteed by the Butterfly Effect to modify the whole world, which includes the past version of you. This may
only be a slight modification, but it is beyond one unit of quantization. As such, you would most probably not travel back in time to the same point
from the same point in exactly the same space at exactly the same time using exactly the same machine made of exactly the same atoms. It would be a
little different. And then a different you would go back, and it would happen again, and again, and again. The cycle would be infinite and ripple
into massive changes.
However, we know that doesn't happen, because some particles time-travel slightly. Even though they do this, they don't forever alter our futures.
There is a simple solution - any time traveler can't be from our universe - he must be from a parallel one, or else this iteration of us wouldn't