Transcript discussion Vaidman

July 20 15.45
The time-symmetric QW and the MWI
Speaker: Lev Vaidman
Floor speakers (in order of appearance):
Bacciagaluppi
Wallace
Lehner

73

[questions only]

Bacciagaluppi
1. [extended question as had prepared to be commentator] Apart from your speculation at the end you were saying maybe we should opt for the trivial?..two-states?..for the universe and your argument was otherwise we would get effects from the ABL?..rule. Now, I’m just wondering about a slightly different way of seeing things which - I won’t insist on it but it may be something to have a look at. So if we take a different psi-final [going to board] and have a psi-initial which are different, there is a natural way of trying to define a many-worlds interpretation would be following …Gell-Mann, Kent and Hartle by setting up a two-vector decoherence condition so, real part on a trace of psi-f, v-alpha n of t-n….[continuing description of expression being written on board]..this thing different - equal to zero for different histories. Now, by the way, Gell-Mann and Hartle show that if the initial and final state are pure this is a very restrictive condition so they suggest to consider mixed states. But I’m also wondering if one shouldn’t try the following. Especially in a many-worlds interpretation in which probabilities should be understood along the lines of Deutsch and Wallace as guiding the acts of agents that are branching - is it the case that we should expect in general an agent to have access both to psi-initial and psi-final? Should we expect the probabilities to be well-defined according to this rule, defined by a psi-initial and psi-final or shouldn’t we rather expect that we might have different kinds of agents with different arrows of time and then have something like: it’s only the marginals of the probabilities that should be defined for different kinds of agents and we don’t really worry if we don’t have decoherence with respect to two states. So, the idea would be that if we are initial observers we average over the final state and vice versa. Something like that; just two small comments, of course it would be difficult to study this in complete generality but maybe in bi-partite systems we’ve got Alice and Bob and Alice and Bob only have a classical communcation channel and they have…..directions of time…well-known by Barnum et al, the most general probability measure for setups of this kind are more general than the quantum states and that may be related to the possibility of different arrows of time and anyway I think this would be a good setting to study it.

Vaidman
2. I was trying to tailor - looking at a simple example with one beam-splitter this boundary condition in the future which are different from just …..And I saw how many disasters they caused. So, it’s really not simple to get a single time error and still have something non-trivial different in the future. Maybe I should mention one example which is not proper for this conference because it’s really to make a single world out of this theory, and this is Ahronov’s conjecture that there is…………..quantum states, a very special one. It’s really this one [indicating on board]: we go to the end of the universe, we go to a particular branch and we take one of it and we put it backwards in time. The picture will not be like this, there’ll be many red things going on all over but the overlap, he believes, and fits the time is not too long, we really single-out one trajectory, our world, and by fiat we’ll have right probabilities, everything is correct, and I think it’s better than collapse, I don’t think it’s better than the many-worlds interpretation because it’s very special, I take a very special final condition. So, unless you take a mixture, for a mixture everything goes, you take a mixture from here, a mixture from there, everything correct comes together and it does not……our experience. Maybe it’s a little too much because it takes everything, not only all many-worlds, multiple worlds, you can split them in different ways; in a mixture you can do whatever, like teleportation. You can teleport one state it’s free to several?..worlds, you can teleport another state and it makes…..in many other worlds.

[new question]

Wallace
3. A quick comment. You gave good reasons for not wanting two vectors for the whole universe……if I’ve got this right, the difference between a forward and backward-evolving vector is the forward-evolved vector we evolve from the measurement forward and then shift?..the backwards is the other way. And also the whole universe………measurements……in this context, in the case of the whole universe we’ve got no measurement so the initial state evolves forwards to the end of time and the second?..state evolves back to the beginning of time and of course the Schrodinger equation is time-symmetric, the Schrodinger equation doesn’t come with a little arrow built in, so actually, if we had two states of the whole universe we really just have two states both evolving through time in whatever direction together and there………something redundant about what’s going on there. So I think the naturalness of making this forward/backward-evolving thing does rely on these……unusual?..steps.

Vaidman
4. First, it’s kind of tricky. The measurements are inside, everything is unitary so measurement is just macroscopic objects coming to microscopic objects and separating microscopic…..distances or states. So, in fact I first put this argument that I have to put another source and then I said my probabilities are wrong. I’m not very strong about this probability wrong, because probably if put some different final states it will kind of wash out and I will not know now, the probability might come out correct because we have this macro?scopic object in each branch, this wave function is the same so it’s not completely clear but anyway it’s very difficult to tailor consistently the…middle?.state, what about the error of time. So it’s not simple; the only thing which I know it works it’s a complete mixture or a single branch probably.

[new question]

Lehner
5. An interesting way I think you can think about that is in terms of Everett’s relative state picture, that the backwards vector in time is just a relative state of subsequent observation. That there’s nothing sort of unnatural about ascribing - there’s nothing that tells us in the Everett story which of the two relative states we really should pick for the description of anything that happens in between, so you also get that symmetry. In Everett’s case, what takes care of the….that there is a sort of inherent time asymmetry in the description of the observer. I mean the observer is supposedly this Turing machine and so has some inherent time asymmetry. Do you see that as a plausible way of looking at that or do you think that’s something that we’d better avoid?

Vaidman
6. I think first that there’s no conceptual difference between the Everett way and this way. Everett just wanted to consider one observer and that’s what his world?.proved?. Every macroscopic object he is in contact with should be in a well-defined state……….......superposition. So I think you’re correct, it’s easier and more natural just to discuss this vector……quantum states which are close to observers. There are many events far away, it’s just a waste of time to think about them. So I think………..even better, more natural when you consider just local things, you just consider quantum systems which influence you and it’s relative state, of course the same story. Given the experiment, you will consider your forward and backward states for micro-particles.