the underlying assumptions rig the debate

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That's correct, but you have to realize that "our information changes" includes the information about what constitutes the past.

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You have to realise that it DOESN'T include information that we already have - Zeilinger is quite clear about that. It doesn't even include information we're going to get in the future !g

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What do you mean by information we already have?

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Exactly what I said. According to Zeilinger, information we already have is fixed - you can't change anything that we already know.

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Did you understand my question and analysis about the path of the photon at any particular stage.

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Did you understand the quotes I gave from Zeilinger ?

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All that matters is whether the potential for knowing the photon's path is, right? So if someone can know it's path, it will indicate and have travelled in the universe of our experience on one path, right?

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But that same photon down the road if someone encounters it, and there is not the ability to determine it's path, then it will indicate in our universe a superposition and wave-like propogation, right?

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Not quite - according to Zeilinger you only get the wave-like behaviour if the path information is completely unavailable. If someone in the future is going to measure it then that is what will be seen - no matter what could be determined. You can only see the wave-like behaviour if the path information is irrevocably lost without anyone seeing it.Read the quotes I gave again.

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Please substantiate that. Where does he say "information is already fixed"? The quantum eraser shows the exact opposite, that information can be erased.

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I stated that according to Zeilinger information that we already know is fixed - and it is quite clearly stated int eh quotes that I already provided. According to Zeilinger's idea of the relationship between QM and Information Theory the Quantum State IS a representation of the information we have. The uncertainty is in the information we don't have.Zeilinger explains "collapse" in terms of gaining more information, and updatign the representation to reflect that (and thus reducing uncertainty - and in information theory, information IS reduction in uncertainty).

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Nope, not at all. You exhibit a major misunderstanding of what the quantum eraser experiment shows. It shows that indeed you can see a change in the photon's path back again; that once collapsed, the photon's path can become uncollapsed.

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You're going to have to substantiate that - say, by referring to experiments where the path information is measured and only then "erased" as opposed to experiments where it is erased without being measured.
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As I understand it, Zeilinger is saying that if the path information is measured then we will not get an interference pattern. Even with a "quantum eraser" later hiding that information from later observations of the photon (if such an arrangement is physically possible - I don't know that it is).t

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However, he does indeed think the past is formed by the present questions; the photon's path is determined by the observer's question, and so implies an actual connection between conscious observation and reality.

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I don't remember anything speciically dealign with this point. So far as I can tell mechanical observatisn are entirely good enough in Zeilinger's view.

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For example, what if the information is lost. Say the initial observers are all dead. Does the photon revert back to wave-like status in it's path?

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Zeiinger doesn't address that question in the material I've seen, so I don't know what his view is on the matter.

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In the quantum eraser experiment, it is clear that if one were to measure the photon at one point, it would have travelled along one path. I think in the context of what I am talking about, you are not seeing the significance of that. Imo, the causal effect of the photon at that point in time, whether observed or not, is likely to be the effect of the photon in a collapsed state, and history at that point in time would reflect that. Now, you can say it is indeterminate, like Wheeler, but if you do, you are also stating that perhaps a great deal of the past is indeterminate.

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We're discussing Zeilinger's views, not mine. I am not sure to what extent Zeilinger carries his ideas of indeterminacy to the macroscopic level or to ordinary history.The rest of the paragraph seems confused. It doesn't make sense to me at all.e

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Personally I disagree, but first let me point out that no experiment I know of has validated your idea here. It shouldn't be too hard to do, but it hasn't been done that I know of (maybe someone else knows of something?).

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In other words you know of no experimental evidence to substantiate your view, but nevertheless you hold that "my" view - by which you mean Zeilinger's - is wrong.Given that you don't know the actual facts of the matter don't you think that you should refrain from referring to Zeilinger's view as a "major misunderstanding" ?eEdited by PaulK, : Provide reason for edit here.W

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You should reread your quotes of him you provided in context. He referred to the whole apparatus and experimenter, I believe, when he made the comments you referred to.

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I just did. There's no reference to consciosuness in the immediate context of any of the quotes, nor is there any claim that the experimenter should be considered part of the experimental apparatus.I'll deal with the rest in my reply to your next post..Edited by PaulK, : No reason given.

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So if the same photon, based on this principle, at any point in time can be determined to travel along a single path, it will travel along that path. But if at a later point in time, it cannot be determined what path that same photon took even in principle, then it will have taken all possible paths even if in the past situation it took only one path.

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This is not Zeilinger's view. First I will deal with the logical problem. If we measure the photon's path before it meets the quantum eraser then it's path is knowable and so your argument fails.To go to Zeilinger's view I will repeat the quote: If we do not make the observation we do not have the information on th path and thus we must still represent the photon as "smeared" accross all paths. If we do make the observation then we do have the information and any future observations must be consistent with that informationThus I conclude that in Zeilinger's view the photon is only collapsed prior to meeting the eraser if we measure it's path. If we do not then - according to the statement above - it remains in the "smeared" state.Unless you have experimental evidence to the contrary it's just your opinion versus Zeilinger's.(

I suggest that you check the title of this subthread again. It is about Zeilinger's views, not mine. I support my assertions with direct quotes from Zeilinger as I should.As I understand Zeilinger's views the statements follow from his ideas on the relationship between QM and information theory. And according to Zeilinger the collapse is a consequence of gaining more information through pbservation. Do I have to keep repeating these quotes ?The link, BTW is the one you provided earlier in the thread. I do not have it handy since I downloaded the paper at the time and I refer to my local copy when necessary.

I'll keep it short and simple. I am not convinced that the past can be materially affected by future events. (Which I beleive is what Percy is referring to - you've clouded the issue enough times)..

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One problem though you keep ignoring is that information can change back again.

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You mean that Zeilinger is "ignoring" it. But can it "change back" ?
(Why do I keep having to point out that it is Zeilinger's views we are discussing here ?) [quote] Now, you have made the argument, and did so before quoting Zeilinger, that this does not change the past because the past is indeterminate, but the path when in superposition does reveal an interference pattern, right? So it has real world effects.
[/quot]And we get an interference effect BECAUSE the path is indeterminate. If the path the photon followed had any material effect then we wouldn't get the interference pattern.And I notice that you're clouding the issue again. The major point is whether the sort of changes proposed in your OP can happen.

I've given my answer.d

I mean that there is no definite path that the photon follows. THe photon is "smeared" accross all the possible paths, with the energy distributed according to the probability of those paths. That's why we see an interference pattern.i.e. we get an interference pattern because the path is indeterminate

I think I have been very clear on my position.

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Also, reviewing and thinking about Zeilinger's theory, I think he does believe that once the Bit of information is spent, even if not observed by a conscious observer, that the photon remains in the collapsed state, but I am not sure how he reconciles that with the quantum eraser experiment and think this is a logical error because along it's path in the quantum eraser experiment, the photon though unmeasured by people could have been measured and we know from earlier experiments that it would have showed a collapsed state, right?

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The reconciliation is that the bit is not "spent" until the photon's state is measured. Only when the information becomes available through measurement does the state collapse. I've quoted Zeilinger to that effect more than once in this thread.I suspect that you are wrong about the experiments - but I'll wait for you to produce the actual experiments so we can see if they are inconsistent with Zeilinger's ideas.i

I hope that we can agree that it is unlikely that Zeilinger is contradicting himself in a single paper, and that we should look for a plausible resolution.I think that the answer is in this piece I quoted earlier Now if the experimental apparatus does not include a detector which could register the path before the quantum eraser removes the information that would let us determine the path, then the path is not available from the experimental apparatus. So I don't think that "in principle" refers to what we could do if we changed the experimental apparatus - because the apparatus affects the results.I do think that the "in principle" refers more to the latter part - to the need to completely destroy the information that would let us perform the measurements - even if it requires an ideal system beyond our current technological capabilities (perhaps beyond any practical capability).So I think that you have miunderstood what Zeilinger means by "in principle""