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Rochester Physicist's Quantum-'Uncollapse' Hypothesis Verified

by University of Rochester

Thanks to SPS for the link.

http://www.rochester.edu/news/show.php?id=3220

Rochester Physicist's Quantum-"Uncollapse" Hypothesis Verified

In 2006, Andrew Jordan, professor of physics and astronomy at the University of Rochester, together with Alexander Korotkov at the University of California, Riverside, spelled out how to exploit a quantum quirk to accomplish a feat long thought impossible, and this week a research team at the University of California at Santa Barbara has tested the theory, proving it correct.

Quantum particles behave in ways that from our everyday experience seem utterly impossible. For instance, quantum particles have wave-like properties and can exist in many places at once. Why the objects we see around us every day—in what physicists call the "classical" world—don't behave this way despite being made of these very same strange quantum particles is a deep question in modern physics.

Most scientists have believed that the instant a quantum object was measured it would "collapse" from being in all the locations it could be, to just one location like a classical object. Jordan proposed that it would be possible to weakly measure the particle continuously, partially collapsing the quantum state, and then "unmeasure" it, causing the particle to revert back to its original quantum form, before it collapsed.

Jordan's hypothesis suggests that the line between the quantum and classical worlds is not as sharply defined as had been long thought, but that it is rather a gray area that takes time to cross.

In the latest issue of Nature News, Postdoctoral Fellow Nadav Katz explains how his team put the idea to the test and found that, indeed, he is able to take a "weak" measurement of a quantum particle, which triggered a partial collapse. Katz then "undid the damage we'd done," altering certain properties of the particle and performing the same weak measurement again. The particle was returned to its original quantum state just as if no measurement had ever been taken.

Because theorists had believed since 1926 that a measurement of a quantum particle inevitably forced a collapse, it was said that in a way, measurements created reality as we understand it. Katz, however, says being able to reverse the collapse "tells us that we really can't assume that measurements create reality because it is possible to erase the effects of a measurement and start again."

Jordan is currently continuing to probe the differences between the quantum and everyday world. He's focusing on nanophysics, which addresses fundamental physical problems that occur on the mesoscopic level.

Posted: Thursday, August 7, 2008 | Permalink

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1. Comment #225749 by pipc on August 7, 2008 at 10:47 am

The summary certainly sounds like an extraordinary claim. I hope they have extraordinary evidence to go with it?

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2. Comment #225755 by doubtingfoo on August 7, 2008 at 10:54 am

How do they know it is in a quantum state if they can't measure it? *BRAIN EXPLODES*

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3. Comment #225762 by phil rimmer on August 7, 2008 at 11:02 am

Does the weak measurement contain any useful information? Does it continue to be useful about the restored state when the weak measurement is ....er....given back??

I think we need the Physics A-team on this. Steve Z, Oystein? Help!

EDIT If you lifted the lid on Schroedinger's cat just enough to tell that this time it was no longer breathing, if you closed it again and promised to forget the fact, might it be alive when you opened it again?

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4. Comment #225764 by morgantj on August 7, 2008 at 11:03 am

I find quantum physics quite intriguing.

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5. Comment #225767 by decius on August 7, 2008 at 11:07 am

Phil,

I think no one can help, here, without reading the original paper.

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6. Comment #225771 by phil rimmer on August 7, 2008 at 11:10 am

Comment #225767 by decius

Reading the original paper probably wouldn't do me any good anyway...:(

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7. Comment #225772 by zbob on August 7, 2008 at 11:10 am

If this hypothesis can be verified, would it not have a dramatic effect on the Stockholm Interpretation and Everett's Many Worlds Interpretation of quantum mechanics? Could this hypothesis reinvigorate David Bohm's hidden variables interpretation?

I hope some of the physicists who visit this website will comment.

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8. Comment #225788 by mordacious1 on August 7, 2008 at 11:30 am

"Because theorists had believed since 1926 that a measurement of a quantum particle inevitably forced a collapse, it was said that in a way, measurements created reality as we understand it. Katz, however, says being able to reverse the collapse "tells us that we really can't assume that measurements create reality because it is possible to erase the effects of a measurement and start again."

I don't think we can assume that measurements DON'T create reality, either, not based on this article at least.

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9. Comment #225789 by Oystein Elgaroy on August 7, 2008 at 11:31 am

I must admit I've never heard of a "weak measurement" before. I will try to read the paper, but won't be surprised if I don't understand it.

Comment #225772 by zbob

the Stockholm interpretation

I am sure you will have a bunch of angry Danes at your doorstep very soon. But maybe you can soften their hearts by mentioning the "Copenhagen syndrome" in a future post.

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10. Comment #225791 by Diacanu on August 7, 2008 at 11:32 am

All I care about is can this be used for molecular re-integration in a transporter?

I intend to be Seth Brundle without the fly fusion.

All these quantum egghads are going to invent the pieces for me, and I'm going to put them together, and get the fame.

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11. Comment #225793 by Nathanial_BB on August 7, 2008 at 11:34 am

How the hell does one 'unmeasure' something that has already been measured???????? even if it is only partially....

And I thought QP was odd when I studied it at Uni... I need a drink.

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12. Comment #225794 by Luthien on August 7, 2008 at 11:35 am

I read about this experiment last year in New Scientist. Here is a quick snippet from the magazine (I have them all in a rack in my livingroom):

"To sneek a peek at the qbit's state midway through collapse, the researchers induce a steadily increasing voltage across a wire ring. This is like teasing the qbit into "thinking" about tunnelling by making it easier to cross the barrier. Then at a certain threshold they drop the voltage back down again. It is equivalent to opening the box and then quickly closing the lid. Because quantum processes take a finite time, lowering the energy barrier then raising it again acts as a "weak" form of measurement. If we don't see the qubit tunnel it means that there is some finite probability that the qubit is in the lower enery state. In ohter words, we have gained information about a quantum system without destroying the delicate superposition. The more times we risk leaving the barrier down without the qubit tunnelling, the more certain we are of it's low energy state. Now it is time to undo any harm we have inflicted in the process. To do this, the physisists fire another kind of microwave pulse, known as a pi-pulse, at the qubit. This inverts the quantum states of the qubit: the higher energy level is now the lower level, and visa versa. The voltage is then ramped up and dropped again. If the qubit doesn't tunnel this time, it becomes more likely that it is in what is now the lower energy level. Where the first weak measurement pushed the superposition one way, the second pushes it by the same amount the other way, which means we end up right where we started." Cover Story in New Scientist, 12th May 2007 (issue 2603)

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13. Comment #225799 by zbob on August 7, 2008 at 11:41 am

9. Comment #225789 by Oystein Elgaroy

Oops, sorry about that all you Danes out there. I, of course, did intend to refer to the Copenhagen Interpretation. (And I promise to not refer to the syndrome whereby the hostage shows signs of loyalty to the hostage-taker as the Copenhagen Syndrome)

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14. Comment #225819 by phil rimmer on August 7, 2008 at 12:03 pm

Great, Luthien. Thanks. I think I even understood it. So yes, the weak measurement increases your certainty of the energy state of the qbit.

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15. Comment #225822 by phil rimmer on August 7, 2008 at 12:05 pm

Comment #225793 by Nathanial_BB

How the hell does one 'unmeasure' something that has already been measured?

Now I'm an expert on this I can explain.

A measurement is "taken"

An unmeasurement is "given back"

There, easy.

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16. Comment #225857 by niyne on August 7, 2008 at 12:46 pm

So once you measure a quantum object (it is an object, right?), it changes to a classical state? Or is a quantum state something that overlays a classical state? Is it sortof demoted or promoted? It sounds more like, when you measure it, it loses its quantum state; does this mean it might leave this classical world? Does it go away to another world? If so, how are you sure you're bringing the same one back? Or does it stay here when you measure it?

Or am I too literal-minded? :(

At any rate, please let me know when this means we get holodecks and transporters. Thanks.

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17. Comment #225868 by Nathanial_BB on August 7, 2008 at 12:57 pm

Comment #225822 by phil rimmer

(slaps forehead with palm)

Of course - why didn't I think of that!!

For the wavefunction to behave like it 'ort ta
a measurement has to be made which is sort ta'
but if the measurement is 'weak'
we can pretend we didn't peek
which all leads us back to nought ta'

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18. Comment #225871 by Donald on August 7, 2008 at 1:00 pm

I think it should be pointed out that these "macro" quantum states, such as qbit rings, phonons, other qbits proposed for quantum computers etc, are not at the same physical level as the quantum states of "fundamental" particles, such as quarks, electrons, photons etc.

These "macro" quantum states are known to result from the coherent behaviour of large numbers of other particles in the substrate. But they seem to obey the same mathematical formulas for behaviour as the "fundamental" particles obey.

Many physicists suspect that our current "fundamental" particles are not fundamental, but that eventually will be found to be the emergent behaviour of some substrate of which we at present have no idea.

In the meantime, discoveries from the "fundamental" quantum world seem to be inspiring an ever-growing number of analogous discoveries in the "macro" quantum world.

In the case of the "macro" quantum world it is easier to see that "weak" measurements could exist and be reversed because a "weak" measurment could plausibly affect only some of the large number of substrate items contributing to the "macro" quantum state.

In the case of the "fundamental" quantum world it would be new physics. (However, just conceivably it might not be all that new. Long wave radio transmissions (think: Marconi, crystal receivers, etc) are picked up by aerials much smaller than the wave length. I have never seen a really convincing explanation of how an electron in the aerial wire absorbs a photon the length of football field in order to contribute to a voltage in the wire. Perhaps that is a "weak" measurement?)

More intriguingly, the two worlds seem to have no sharp boundary. Superconductivity is explained as a result of the pairing of electrons in a quantum behaviour that results from the arrangement of large numbers of substrate atoms. Here the quantum behaviour of "fundamental" particles (electrons) is modified by emergent quantum behaviour at a different physical level (atomic lattice).

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19. Comment #225872 by J Mac on August 7, 2008 at 1:01 pm

Does it go away to another world? If so, how are you sure you're bringing the same one back?

In that part you are being too "literal minded." Saying whether a fundamental particle is the "same one" or a "different one" as before is just meaningless. So meaningless in fact that there is one perspective whereby ALL electrons in the universe can be represented by one enormously complex wave function; they all are essentially different views of the same electron.

That interpretation is a bit out there, but it does touch on the idea that electrons (and other such particles) are not "individuals" in that you could tell two of them apart.

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20. Comment #225875 by HumanisticJones on August 7, 2008 at 1:06 pm

Does this validate the Copenhaggen Interpretation and invalidate the Many Worlds Interpretation? It seems that if the Many Worlds version were true, then unmeasuring would result in the collapse of the split off universe (and raising the question of whether that universe has a quantum possibility that the unmeasuring didn't happen thus not collapsing it). This feels like it would fit better with the Copenhaggen version in that instead of collapsing a whole quantum universe, the wave form simply reverts back to the wave. Although that leaves us with seeing that the cat in the box is dead, then unseeing it and having the cat return to a state of dead&&alive.

Gah... quantum theory always strains the neurons to think about (except in those quantum states where it doesn't).

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21. Comment #225882 by J Mac on August 7, 2008 at 1:13 pm

Hmm, quantum theory has always been a hobby of mine, though I certainly don't have much of a grasp of it. It seems I should read up on the Copenhaggen Interpretation, as my understanding of it doesn't mesh with what I'm hearing here.

I thought the Copenhaggen interpretation was (somewhat simply): "Shut up and do the math, don't worry about what doesn't make sense."

All the other interpretations, such as many worlds, objective collapse, pilot wave, etc, were all ways of worrying about what didn't make sense.

If anyone smarter than me has input I'd love to hear it.

Gah... quantum theory always strains the neurons to think about (except in those quantum states where it doesn't).

I second that.

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22. Comment #225884 by phil rimmer on August 7, 2008 at 1:15 pm

Comment #225868 by Nathanial_BB

Sincere apologies I didn't realize it was you, Professor!

I am awed.

Bril!

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23. Comment #225890 by J Mac on August 7, 2008 at 1:19 pm

I suppose I shouldn't try to understand QM. As long as I don't try to understand or take any tests on it... ie, as long as there aren't any measurements of my understanding, could it not be fair to say that my understanding of QM is a superposition of various states. Surely in one of those states I have a complete understanding of it.

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24. Comment #225892 by Spinoza on August 7, 2008 at 1:21 pm

I will be the happiest person on the planet when Bohmian mechanics is vindicated.

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25. Comment #225895 by J Mac on August 7, 2008 at 1:25 pm

I will be the happiest person on the planet when Bohmian mechanics is vindicated.

Diddo. I've never quite understood the aversion to non-locality.

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26. Comment #225896 by Sargeist on August 7, 2008 at 1:28 pm

It is inexcusable for any news story about a scientific development not to include a citation or link to the actual paper.

Because of this, and because it took me a little bit of effort Googling to find the references, here are some useful papers:

Original 2006 Korotkov and Jordan paper:

http://www.ee.ucr.edu/~korotkov/papers/PRL-97-166805-2006.pdf

New Katz paper:

http://arxiv.org/abs/0806.3547

Both free access.

Mark.

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27. Comment #225930 by SomeDanGuy on August 7, 2008 at 2:03 pm

It's great having a collection of smart people around in various disciplines to further explain things like this. I'm always intrigued by quantum physics, but I don't have the background knowledge to appreciate it on my own. Thanks for the help, local physicists!

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28. Comment #226010 by J Mac on August 7, 2008 at 3:27 pm

It's great having a collection of smart people around in various disciplines to further explain things like this.

I agree with the sentiment. However if anyone (today) says they really can explain quantum physics you'd be best off keeping an eye on your daughter and a hand on your wallet, cause they're up to something.

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29. Comment #226021 by shad0w on August 7, 2008 at 3:43 pm

Thanks for the paper Links Sargeist. Very interesting read indeed!

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30. Comment #226063 by stephensmith on August 7, 2008 at 4:45 pm

I understand almost nothing of any of this. Just please tell me it means that Schrödinger's Kitty Cat doesn't have to die!

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31. Comment #226071 by J Mac on August 7, 2008 at 4:59 pm

It means if it did die it could come back to life ....

Other Comments by J Mac

32. Comment #226094 by Elwood Herring on August 7, 2008 at 5:32 pm

It's like this: you start off with four types of people, the first set contains people who understand QT, the second set don't, the third set only think they understand, and the final set think they don't understand it. Now all four of these types exhibit a certain amount of uncertainty, even (by necessity) the ones who think they are certain. So you must relabel these sets as 1) people who think they are certain they know, 2) people who believe they are certain they don't know, then 3) and 4) who are uncertain about knowing and not knowing, respectively. The problems arise when some new data is discovered concerning QT, such as the article here. This has the effect of creating much more uncertainty across the board, realigning the boundaries between those who are certain they are certain, and those that are not, etc. In effect, this creates a certain amount of superposition of uncertainties about whether to accept the new data, and whether they can be certain about it. There will inevitably be some people who will reject this new information outright, but there will more likely be a greater proportion who will be uncertain about it, creating a feedback loop of uncertainty and therefore separating the previous superposition back to more or less the original state. With me so far? The upshot of all this is that the theory of QT will always be intrinsically uncertain, being by its very nature contrary to common sense. The more experiments are done, and the more facts are learned, the less certain anyone can be that it is understandable at all. The only way to understand QT is actually to not do any research into it at all, and also to destroy all data so far collected into the entire phenomenon, then there can be no superposition of individuals in various states of uncertainty about it. In effect, the only way we can ever understand QT is to not understand it at all. That's the only way we can ever be absolutely certain.

...I think...

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33. Comment #226101 by J Mac on August 7, 2008 at 5:44 pm

Elwood Herring's Uncertainty Principle.

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34. Comment #226108 by Ed-words on August 7, 2008 at 5:52 pm

I don't know what the h--- they're

talking about, but they're the experts,

and they deserve our deepest gratitude

for placing reason over faith.

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35. Comment #226111 by Elwood Herring on August 7, 2008 at 5:55 pm

Well to be sure, the one thing I'm certain of is that I'm not certain of anything!

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36. Comment #226123 by J Mac on August 7, 2008 at 6:10 pm

The one thing I'm uncertain about is whether or not I am uncertain about anything!

:o)

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37. Comment #226309 by Stu on August 8, 2008 at 12:23 am

Can you say 'Heisenberg Compensator'?

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38. Comment #226325 by simonchase on August 8, 2008 at 1:44 am

Lucky I'm hungover so this makes perfect sense to me right now

*collapses*

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39. Comment #226333 by Steve Zara on August 8, 2008 at 1:56 am

Comment #225875 by HumanisticJones

Does this validate the Copenhaggen Interpretation and invalidate the Many Worlds Interpretation? It seems that if the Many Worlds version were true, then unmeasuring would result in the collapse of the split off universe

I am afraid it does not distinguish. In terms of Many Worlds unmeasuring would not collapse anything - it would just mean that a certain division into different worlds "heals".

Other Comments by Steve Zara

40. Comment #226339 by Quetzalcoatl on August 8, 2008 at 1:58 am

In the latest issue of Nature News, Postdoctoral Fellow Nadav Katz explains how his team put the idea to the test and found that, indeed, he is able to take a "weak" measurement of a quantum particle, which triggered a partial collapse. Katz then "undid the damage we'd done," altering certain properties of the particle and performing the same weak measurement again. The particle was returned to its original quantum state just as if no measurement had ever been taken.

Annoyingly, it doesn't say precisely what a "weak" measurement is, nor spell out precisely how it differs from a "strong" one.

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41. Comment #226343 by Luthien on August 8, 2008 at 2:01 am

Annoyingly, it doesn't say precisely what a "weak" measurement is, nor spell out precisely how it differs from a "strong" one.

Read my post above for the description! :)

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42. Comment #226346 by Oystein Elgaroy on August 8, 2008 at 2:09 am

Comment #226332 by Brian English

I don't see Swinburne's problem. To my mind, Feynman's path integral formulation of quantum mechanics allows me to think of light as particles in all situations. The experimental setup will determine which classical paths will give the dominating contributions to the integral and this in turn determines whether the results will be wave-like or particle-like.

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43. Comment #226347 by Quetzalcoatl on August 8, 2008 at 2:09 am

Luthien-

thanks. Now I just have to try and understand it!

- Furrows brow, concentrates -

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44. Comment #226363 by Sargeist on August 8, 2008 at 2:34 am

Oystein,

I loved learning about Feynman path integrals. :)
I also loved reading his book "QED", which made me realise that there is no actual difference between "reflection" and "interference".

When I was teaching physics I used to tell my students that electrons, photons etc "behaved" as if they were waves sometimes and like particles at other times, but if you were to ask "is an electron a wave or a particle?" then the answer, as far as I saw it, would be: "it is an electron."

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45. Comment #226374 by Oystein Elgaroy on August 8, 2008 at 2:57 am

Brian,

so you have been reading "The coherence of theism"? I just finished it myself and will read "The existence of God" next. I don't agree with Swinburne's objection to positrons being electrons traveling backwards in time. My understanding of his argument is that he uses the arrow of time which only applies for macroscopic systems. The fundamental equations of physics are symmetric in time, so his argument doesn't apply to single electrons/positrons.

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46. Comment #226375 by Steve Zara on August 8, 2008 at 3:00 am

Comment #226367 by Brian English

The thing is, though, that photons do do very "wavey" things, like being polarised, and they do very "particley" things like knocking electrons out of metals.

The way to treat something as both wave and particle is to use the Bohm "interpretation":

http://en.wikipedia.org/wiki/Bohm_interpretation

Other Comments by Steve Zara

47. Comment #226382 by Steve Zara on August 8, 2008 at 3:10 am

Comment #226380 by Brian English

My point doesn't really require the in depth discussion it just asks do we say photons are equally waves and particles or on a deeper level waves (or particles)?

I haven't a clue :)

Other Comments by Steve Zara

48. Comment #226386 by Quetzalcoatl on August 8, 2008 at 3:12 am

Brian-

I would be inclined to say "equally waves and particles" since they have the characteristics of both. Even though "wave" and "particle" are just our attempts at describing the underlying reality (which might well be different), you have to have some way of getting the point across.

But I know very little about this- just my opinion.

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49. Comment #226392 by Oystein Elgaroy on August 8, 2008 at 3:22 am

Comment #226379 by Brian English

Interesting point. And if Swinburne hasn't got better arguments up his sleeve I don't think he has justified the kind of causation he needs.

Comment #226375 by Steve Zara

So you are a Bohmian? I am no expert on this, but I thought that the Bohm version of quantum theory was extremely difficult to extend to quantum field theory. Do you know of any recent developments?

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50. Comment #226397 by Quetzalcoatl on August 8, 2008 at 3:31 am

Pythagoras-

Quantum theory is completely consistent with the many worlds theory. Worrying about how and when waves collapse into particles is silly. It is simple to understand experimental results in terms of the many worlds interpretation.

It may be consistent, but that doesn't mean that the many world theory is necessarily correct. Studying collapsing waves is a good way of casting light on the whole area- it shouldn't be dismissed.

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