On 21 January 2014 14:18, Pierz <[email protected]> wrote: > I am putting this out in order to clarify my understanding - hopefully the > MWI experts out there can help me out here. A while back I asked whether > the past can be undefined at a quantum level the way the future is. I asked > this because I recall (somewhat vaguely unfortunately) reading or hearing > something from Stephen Hawking in which he appeared to argue that at a > certain very early point in the universe, there is no longer a single > history, but quantum uncertainty comes into play, with important > implications. Anyway, the response to my question indicated that most > people still assume that history represents a single, well-defined line > through the multiverse (I'm assuming MWI here, even though I know it's not > the dominant theory/interpretation). >
If not it is becoming so. I thought in an recent (informal) poll at some scientific meeting most physicists now said they subscribed to the MWI being the most likely interpretation? Can't remember the details though. > > I have been thinking about this and it occurs to me that firstly, the > single history is only partially true. Since quantum interference patterns > occur in MWI due to interference between universes, which can only occur if > universes can merge again after splitting, then at least at this level, the > past is not well defined. If a universe merges back with another from which > it had temporarily diverged, then an observer within that universe cannot > say which path he followed to get there. She followed all possible paths. > Of course those divergent universes were only trivially different, or else > decoherence would have made the merging impossible. But of course in any > real universe, there will be a vast number of such "nanohistories", because > of the immense number of quantum interactions where merging occurs. So at > this very short time/space scale level at least, it is impossible to define > a single history. Correct? > Imho, that is correct. The reason universes tend to diverge more than they merge would be that the multiverse is far from thermodynamic equilibrium this close to the Big Bang. > > However at a macroscopic scale, it appears difficult for history to be > intrinsically ambiguous. In other words the network of "nodes" of the > multiverse is like a tree not a net. There may be tiny branches that rejoin > one another at the smallest scale, but the limbs of the tree cannot merge > back together. I can always define a single route back to the trunk, though > if I go further up the tree, I will be forced to decide repeatedly which > way to go. This branching is defined by time, so doesn't this effectively > give an "arrow of time"? Yet the laws of physics are not supposed to be > directional in time except through aggregation of effects as entropy. Are > these two "arrows" related? How? > > See above. I didn't realise I was answering your later question when I wrote that! To expand slightly... My opinion is that branching exceeds merging for the same reason that there is a thermodynamic arrow of time. To see this, imagine a universe at thermodynamic equilibrium. All processes can play out equally in either time direction in such a universe (every googolplex years a Boltzman brain pops up for a split second - but its time sense could go either way along the time axis, they're now equally (un)likely). There is no reason why the quantum processes involved in the MWI would not be similarly balanced once there was no thermodynamic arrow of time. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
