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Decoherence - the myth

by Coldcall
Tags: decoherence, myth
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Coldcall
#19
Jul24-08, 07:04 AM
P: 275
Vanesch,

"I'm claiming that MWI is a good way to *picture* the way the quantum formalism is working. I keep open the possibility that against all odds, quantum mechanics might be a "true" description of nature, but I don't really think that we can say so until we have a better understanding about the interplay of gravity and quantum mechanics"

Yes i would not argue with any of that - sorry to dissapoint

You are distinguishing a way to "picture" qm and you have your favoured way to see things. I also understand why MWI appears a good interpretation because it allows all probabilities to actually exist instead of them all dissapearing into thin air once a measurement or decoherence occurs. I also think MWI seems more reasonable from the Schrodinger's cat paradox, if its a genuine paradox. However as i've said before i think Schrodinger's cat is not the paradox it appears to be and the cat will collapse the wave function before any observer looks in the box.

By the way has a (non-lethal) Schrodinger's cat experiment ever actually been done? Surely its pretty simple to test using a water gun or something that wont kill the cat?

"Let me go in your direction. I think that solipsism is then even better. I think that there is actually only ONE important observer in nature, and that's me. I think that before I was born, the classical world didn't exist, and I think that after I'm dead, it won't exist anymore. All the other human beings, animals and rocks are not true conscious beings, some only act behaviorally that way. I'm the universal wavefunction collapser. Not you. Just me. Now you again."

I think thats going too far. I do actually believe that we are all capable of collapsing wave functions, but on a scalable system which means that animals can only collapse a wave function of which they can percieve because of their limited sensory devices. So humans can work with atomic matter because we can set up the double slit. A cat could not because they dont have the consciousness or self-awareness, nor the sensory devices to force a photon to behave with particle/wave duality.
vanesch
#20
Jul24-08, 09:37 AM
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Quote Quote by Coldcall View Post
However as i've said before i think Schrodinger's cat is not the paradox it appears to be and the cat will collapse the wave function before any observer looks in the box.
The Schroedinger cat paradox is a paradox for the quantum formalism - but surely not "for real". The Schroedinger cat paradox illustrates that IF we accept a unitary time evolution all the time, and IF we accept that everything is described by quantum mechanics, then it follows that the "cat state" must have a live and a dead component. It is a dramatic way to illustrate what it really means to have unitary time evolution and the superposition principle: it means you cannot get rid of the extra terms the way you do using a projection.

In other words, the Schroedinger cat paradox pushes your nose on a FORMAL problem: it is impossible for a complicated physical process, described nevertheless by unitary dynamics, to bring about genuine state projection.

No problem, you say. Make time evolution slightly non-unitary (non-linear). There are people who do this. However, we now run into a second difficulty. If we modify the time evolution in such a way as to make projection a physical process, then it becomes impossible to make that Lorentz-invariant. In other words, a physical process that implements a true projection will require an absolute ether frame. This is nothing else but Bell's theorem (with a few additional assumptions, such as no "gods book" correlations, and no superdeterminism).

And here we have a REAL paradox, at least in the idealized gedanken experiment of EPR.

If we can build a real EPR experiment, we will have data that will - independently of any quantum theory - give us a genuine paradoxial situation, unless we give up on relativity. If we insist on keeping the spirit of relativity (we live on a 4-dim spacetime manifold, with no preferred slicing) then true EPR data (as predicted by quantum theory) present a real headache. Funnily, unitary quantum theory can describe it perfectly, and respect relativity.

So, if we keep relativity, and if we keep strict unitarity, then we have the formal problem of the Schroedinger's cat.
If we give up on unitarity and introduce genuine collapse, then, through EPR, we have a problem with relativity. Even without quantum mechanics, the pure data of the EPR paradox would collide with relativity, were it not that unitary quantum theory can explain it and is compatible with relativity.

In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid.

By the way has a (non-lethal) Schrodinger's cat experiment ever actually been done? Surely its pretty simple to test using a water gun or something that wont kill the cat?
What would be the observable effect ? You would open the box, and observe in 50% of the cases, a wet, angry cat, and in 50% of the cases a purring, dry cat.
Coldcall
#21
Jul24-08, 10:44 AM
P: 275
Vanesch,

Very interesting post thank you.

"In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid."

Exactly. I dont accept the cat is alive and dead at the same time. In fact, i dont accept the experimental set up would constitute a quantum state in superposition until a human observer opens the box.

The wave function collapse would occur somehwere in the chain between the atom and the cat.
peter0302
#22
Jul24-08, 10:54 AM
P: 869
Maybe the reason it's so popular is that it's the only thing that attempts to provide an objective and logically self-consistent definition of wavefunction collapse?
Coldcall
#23
Jul24-08, 11:39 AM
P: 275
Quote Quote by peter0302 View Post
Maybe the reason it's so popular is that it's the only thing that attempts to provide an objective and logically self-consistent definition of wavefunction collapse?
Yes it nicely reduces the phenomemon into an objective process. I think "ensemble/statistical interpretation" fits into that category as well except it kind of side steps any philosophical assumptions whatsoever.

We have interpretations to suit all needs..step right up folks
peter0302
#24
Jul24-08, 01:10 PM
P: 869
Right right. I mean, clearly, to me, decoherence solves the worst problem with CI, which is the ambiguity and subjectivity of the terms measurement, observe, collapse, etc. We've got an objective explanation for why classical objects don't display interference, Schrodinger's cat, etc, that requires no new assumptions (like pilot waves or extra worlds), though is compatible with either one or many world scenarios. That makes it a very powerful concept.

Does it solve the measurement problem? Define "solve." If solve means moot otherwise valid objections to QM, then it does that. If solve means "prove this is the way it is", well no theory can do that.
jostpuur
#25
Jul24-08, 02:55 PM
P: 2,061
Quote Quote by peter0302 View Post
Maybe the reason it's so popular is that it's the only thing that attempts to provide an objective and logically self-consistent definition of wavefunction collapse?
Removal of the interferences is quite far from the explanation of the collapse.

Quote Quote by peter0302 View Post
Right right. I mean, clearly, to me, decoherence solves the worst problem with CI, which is the ambiguity and subjectivity of the terms measurement, observe, collapse, etc.
What does CI stand for? Collapse Interpretation?
peter0302
#26
Jul24-08, 04:07 PM
P: 869
Quote Quote by jostpuur View Post
Removal of the interferences is quite far from the explanation of the collapse.
I disagree. Decoherence shows that the branches of the wave function have less and less, and eventually no chance of future interaction as the quantum system becomes more entangled with the environment. While what happens to these other branches cannot be known (i.e. MWI or single-world), "wave function collapse," i.e., our obtaining a single definite value for an observeable, is fully explained. We just don't know why we get the one we get.

What does CI stand for? Collapse Interpretation?
Well, I meant Copenhagen Interpretation, but Collapse Interpretation isn't a bad label for it.
jostpuur
#27
Jul24-08, 06:06 PM
P: 2,061
Quote Quote by peter0302 View Post
Decoherence shows that the branches of the wave function have less and less, and eventually no chance of future interaction as the quantum system becomes more entangled with the environment.
I agree so far.

While what happens to these other branches cannot be known
What "other" branches? It sounds like you are implicitly assuming, that one branch becomes more special than others, under decoherence. Otherwise it wouldn't make sense to talk about something happening to the other branches.

If some particle is in superposition of several possible eigenstates of some observable quantity, the particle's entanglement with the macroscopic environment is not going to make one eigenstate more special than others. The particle maintains non-zero amplitudes for being on all initial eigenstates. Only the interferences vanish, due to the entanglement with macroscopic environment.
vanesch
#28
Jul25-08, 12:00 AM
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Quote Quote by jostpuur View Post
I agree so far.



What "other" branches? It sounds like you are implicitly assuming, that one branch becomes more special than others, under decoherence. Otherwise it wouldn't make sense to talk about something happening to the other branches.

If some particle is in superposition of several possible eigenstates of some observable quantity, the particle's entanglement with the macroscopic environment is not going to make one eigenstate more special than others. The particle maintains non-zero amplitudes for being on all initial eigenstates. Only the interferences vanish, due to the entanglement with macroscopic environment.
I guess peter calls "other branches" those that do not correspond to the observed outcome (by you?).

What decoherence helps to explain is that we don't see livecat+deadcat interference, or some spooky cat state which is in between. Whatever branches will appear, will be "consistent dead cat" and "consistent live cat" branches which, each by themselves, look entirely classical.

What decoherence doesn't explain, is why we are only aware of one of these branches, and, indeed, what happens to those we aren't aware of.
Do they "disappear in a puff of logic" (that's Copenhagen's stance) at a certain point (which is called the Heisenberg cut, and which must be part of whatever is called a measurement) ?
Do they continue their existence, but "we" aren't part of it (but "they" are) ? That's MWI's stance.
Is QM simply a statistical description of some underlying process - in other words, is the quantum wave function simply some description of our knowledge about a system, and not a physical description of the system itself ? That would then explain the "collapse" as just an "update of our information" ? But it begs then the question of what IS a physical description.
Is the wavefunction just part of the physical state, and is an extra physical quantity present which indicates which branch of the wavefunction is the "right" one ? That's Bohm, or any other hidden-variable theory.

That's still the remaining interpretational difficulty. Decoherence didn't solve that. Decoherence removed unnecessary objections to the continued existence of the different branches, namely the "obvious objection" of seeing a cat in a superposed state of dead and live.
In other words, decoherence indicated us that we probably won't be able to find out experimentally whether the branches "really continue to exist" or whether "they collapse", as experimentally, they will give identical, classical results.
jms5631
#29
Jul25-08, 12:16 PM
P: 55
Quote Quote by peter0302 View Post
Right right. I mean, clearly, to me, decoherence solves the worst problem with CI, which is the ambiguity and subjectivity of the terms measurement, observe, collapse, etc. We've got an objective explanation for why classical objects don't display interference,
I think peter really captured the curx of the arugument here, in capturing the real value of decoherence. Though certainly decoherence likely does not solve the measurement problem by itself (though there a fair number who believe it may) it removes one of the most disconcerting aspects of QM, ultimate subjectivity. Decoherence seems to be highly effective at restoring an objective world without the need of an observer, and articulating the mechanism which helps bridge the ambiguous quantum world, with the familiar classical one. It needs further refinement yet, but many in the physics community believe that it will be an important part of any eventual solution to the measurement problem. I personally find it unsettling the some tend to try to minimalize its importance in closing the chasm between two different realms of nature.
Maaneli
#30
Jul25-08, 02:24 PM
P: 520
Quote Quote by Coldcall View Post
Maaneli,

I love the way Bell wrote - a real poet type phycist! He would have made learning physics a pleasure with the way he attacked the BIG issues.

I was kind of thinking about this yesterday about my cat. For instance, its odd that cats cannot see themselves in a mirror. They clearly see something but their perception cannot stretch to the concept that they may be looking at themselves.

What if each level of organism can only observe certain physical realities? Hence from a qm perspective they can only collapse certain wave functions which are accessible to them. If this is the case then one can imagine that before humans appeared the world may have looked very different. One could use that same scaling to go all the way back to the first primitive organism.




Hey Coldcall,

I appreciate and respect your comments on this post about the overselling of decoherence as a myth to the measurement problem and the way you defended yourself against ZapperZ.

But, I'm not sure if you know that the above quote by Bell is actually intended to ridicule the CI projection postulates. Bell himself was a stauch advocate of hidden variable (or more precisely "beable") formulations of QT like the de Broglie-Bohm pilot wave theory and GRW spontaneous collapse - neither of which require the existence of human observers to invoke projection postulates.

By the way, I believe your idea of using the consciousness of animals to cause collapse might have also been along the same lines as the ideas of Eugene Wigner on the measurement problem. I think he also believed that consciousness causes collapse. However, I would like to point out some obvious questions and difficulties with the view you propose:

When there were no living organisms around on earth (~4.5 billion years ago) there would be nothing to collapse the wavefunctions of the universe; do you think that means that the universe was in a coherent superposition state? Also if living organisms did cause collapse, what would be the range of their ability to collapse the wavefunctions of nonliving matter? Would organisms on earth be able to collapse the wavefunction of matter 100 light years away or only here on earth? How would you quantify such an ability?

Even if you said there may be other life on other planets, even they did not exist a certain time back such as 11 billion years ago. So was the wavefunction of the universe undergoing a coherent and unitary state vector evolution? You may know that any of these views contradicts the evidence from cosmology which says nothing of the sort.

Anyway, just some things to consider.
vanesch
#31
Jul26-08, 04:49 AM
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Quote Quote by Coldcall View Post

"In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid."

Exactly. I dont accept the cat is alive and dead at the same time.
But could you accept two worlds, one in which there is a live cat, and one in which there is a dead cat, you living in one of them ?
vanesch
#32
Jul26-08, 05:18 AM
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Quote Quote by Maaneli View Post
So was the wavefunction of the universe undergoing a coherent and unitary state vector evolution? You may know that any of these views contradicts the evidence from cosmology which says nothing of the sort.

Anyway, just some things to consider.
First of all, I would say that the application of quantum theory to cosmology is certainly as of now ill-defined, as we don't know how to deal with gravity in quantum theory. But apart from that, what do you think would be the observable consequences of coherent unitary state vector evolution during the early universe ? In what way would that imply anything different than the classical evolution one would have otherwise ?
Coldcall
#33
Jul26-08, 05:32 AM
P: 275
Maaneli,

"But, I'm not sure if you know that the above quote by Bell is actually intended to ridicule the CI projection postulates. Bell himself was a stauch advocate of hidden variable (or more precisely "beable") formulations of QT like the de Broglie-Bohm pilot wave theory and GRW spontaneous collapse - neither of which require the existence of human observers to invoke projection postulates."

Yes I'm aware of his views on Bohm and i don't agree with him there, and I think the be-ables is again just another "treatment" of the problem. However he himself admitted the demise of locality, and most phycists would say hidden variables are not viable. Ironic because he falsified his own argument for hidden variables.

But all this does not stop me respecting him and his deep views on qm :)

"By the way, I believe your idea of using the consciousness of animals to cause collapse might have also been along the same lines as the ideas of Eugene Wigner on the measurement problem. I think he also believed that consciousness causes collapse."

Yes though I dont know know whether "observership" consitutes "consciousness" or vice versa. I do think its related to biology in some causal fashion. And considering we now know that quantum effects occur in biology the argument that it does not occur in our brains is unsupportable. Max Tegmark argued this in 2000 and new evidence fasilfies that argument against quantum "consciousness". That does not prove its happening but its taken away another reason for it not to be happening.

"When there were no living organisms around on earth (~4.5 billion years ago) there would be nothing to collapse the wavefunctions of the universe; do you think that means that the universe was in a coherent superposition state? Also if living organisms did cause collapse, what would be the range of their ability to collapse the wavefunctions of nonliving matter? Would organisms on earth be able to collapse the wavefunction of matter 100 light years away or only here on earth? How would you quantify such an ability?"

Good questions :) But this is not my theory; it was first proposed by Wheeler in PAP. First of all, if Quantum mechanics is really fundamental to the universe then a universal wave function scenario would be quite natural. In fact Hawking & Harte have used the same idea but their conclusions dont involve the biological scope achieved by Wheeler.

As i understand the theory; In a wave of universe probabilities the one which evolves subjectively to produce the first self-evolved biological system collapses in a retro-causal manner. The microbe would be able to sense something, so the universe at that moment would be as defined or developed as was necessary for that microbe to have evolved in the probablity wave. Consistency is maintained. So in essence that first little microbe or nanobe or whatever it was held open the reality wave - our universe. I dont think the microbe has to have the "collapsing range" in the way you are looking at it. If QM, through entanglement or whatever function insists on consistency then the microbe could have very far reaching retro-causal effects.

"Even if you said there may be other life on other planets, even they did not exist a certain time back such as 11 billion years ago. So was the wavefunction of the universe undergoing a coherent and unitary state vector evolution? You may know that any of these views contradicts the evidence from cosmology which says nothing of the sort."

I'm not quite sure what you mean in the last part but I think the state of the universe was more or less fixed by the evolution of the first microbe, again because of consistent histories. However I'm not saying later observations did not cause further retro-causal history to occur. In fact we know the universe has changed in certain profound ways during its development, dark energy, acceleration etc..

If you can point out some sort of cosmological evidence that rules out a retro-causal universe wave form i'd be interested in reading it. Wheeler's Delayed choice showed retro-causality is real. I believe the quantum eraser is kind of similar.

Just to add: Why i like this theory so much is because it

1) Accepts the observer as causal agent as every qm experiment has demonstrated.
2) Solves the biocentric tuning problem - anthropic coincidences
3) Solves the reason why it appears to be a fluke anything self-organised at all, because in a wave of universal probability, the HUP kicks in and the practically impossible becomes possible.
4) Gives a part for consciousness/awareness as opposed to zombie like machine biology.

Atleast Quantum physics actually answers how the universe could have got started and produced biology against all the odds.
vanesch
#34
Jul26-08, 08:07 AM
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Quote Quote by Coldcall View Post
I'm not quite sure what you mean in the last part but I think the state of the universe was more or less fixed by the evolution of the first microbe, again because of consistent histories.
You now have a problem here: that cosmic first microbe (as a material structure) only appeared in certain terms in the wavefunction, at a certain time. In fact, there are terms of the wavefunction which had other microbe-states way way way before microbes appeared in "our" universe. They were just in branches with smaller complex coefficients. So how come that *they* didn't collapse the wavefunction, but our "latecomer" microbes did ?

See, we have:

|wavefunction_at_time_0> = |quarks>|photons>...

evolution of the wavefuntion, still no microbes:

|wavefunction_at_time_t1> = |dusty universe> + |universe with stars1> + |universe with stars2> + ... + |universe with stars3234> + ...|universe with one big black hole>

evolution of wavefunction:

|wavefunction_at_time_t2> = |dusty universe> + |universe with stars 1 and a unicorn> + |universe with stars 2 and no microbes> + ... |universe with stars3234> + ...|universe with one big black hole>

evolution even further:

|wavefunction_at_time_t3> = |dusty universe> + |universe with stars 1 and a unicorn and some flying bananas> + |universe with stars 2 and our microbes> + ... |universe with stars3234 and very intelligent giant ants> + ...|universe with one big black hole>

evolution still further:

|wavefunction_at_time_t3> = |dusty universe> + |universe with stars 1 and a unicorn and some flying bananas> + |universe with stars 2 and humans and all that> + |universe with stars 2 which looks like ours, but in which the dinosaurs weren't destroyed> + ... |universe with stars3234 and very intelligent giant ants who killed themselves> + ...|universe with one big black hole>

So why did the unicorn at time t2 not collapse the wavefunction, but why did our microbes do so at t3 ?

If you reply by saying that our microbes were the first, then that's definitely not true. There WAS an immensely small amplitude to have a universe with a unicorn at almost any time in the sense that the amplitude for the particles to be "by concidence" in the right configuration to make a unicorn pop out of dust at any time (even right now) wasn't strictly 0. These amplitudes are so small that they represent almost zero odds. But they are not 0.
Coldcall
#35
Jul26-08, 09:52 AM
P: 275
Vanesch,

"You now have a problem here: that cosmic first microbe (as a material structure) only appeared in certain terms in the wavefunction, at a certain time. In fact, there are terms of the wavefunction which had other microbe-states way way way before microbes appeared in "our" universe. They were just in branches with smaller complex coefficients. So how come that *they* didn't collapse the wavefunction, but our "latecomer" microbes did ?"

i think i understand what you are asking. Not sure i can answer 100% but I'll give it a try

The way I see Wheeler's PAP is that because its based on an observer-centric version of qm only once a biology has occured which is capable of meeting the requirments needed for observer status does the universal wave function collapse. So the first universe to create a microbe capable of being an observer is the one that collapses, so there cannot have been any previous microbe-states otherwise they would have come into existence instead. However that doesnt rule out that sometimes, or maybe more often than not, the microbe evolves in the superposition, the wave function collapses into a real universe and then the microbe cant reproduce and dies. hence with no observer the universe disappears down the memory hole. Like a recycling bin.

"If you reply by saying that our microbes were the first, then that's definitely not true. There WAS an immensely small amplitude to have a universe with at almost any time in the sense that the amplitude for the particles to be "by concidence" in the right configuration to make a unicorn pop out of dust at any time (even right now) wasn't strictly 0. These amplitudes are so small that they represent almost zero odds. But they are not 0."

But the odds that a universal wave function evolves a viable Unicorn-inhabited universe before the one with microbes makes it almost impossible. Lets face it, the unicorn-centric fine tuning which would need to go into the wave function for such an outcome is highly unlikely. Our microbe universe, even though it is wildly coincidental as it is, is a darn sight more probable than a universe which spontaneously evolves unicorns as its first valid observer.
vanesch
#36
Jul26-08, 10:20 AM
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Quote Quote by Coldcall View Post
But the odds that a universal wave function evolves a viable Unicorn-inhabited universe before the one with microbes makes it almost impossible. Lets face it, the unicorn-centric fine tuning which would need to go into the wave function for such an outcome is highly unlikely. Our microbe universe, even though it is wildly coincidental as it is, is a darn sight more probable than a universe which spontaneously evolves unicorns as its first valid observer.
You seem to forget that in a wavefunction, if there is unitary evolution, there can be AT THE SAME TIME a component that will allow a microbe to emerge 3 billion years ago, and a very tiny component with a unicorn in it *right away*. There are no "odds" in a unitary evolving wavefunction: we simply have different terms of the wavefunction. So the statement that "our microbe universe is more probable than a unicorn universe" simply means that the coefficients of their predecessors in the wavefunction have different coefficients. But you should agree with me that it is still more probable that NO life evolves in a universe, or that a different form of life evolves in a universe. So "our" universe, when "our first microbe" came out, wasn't so very very more probable either. If, in our universe, the first microbe evolved after about 10 billion years, then you should admit that the odds that somewhere else another microbe evolved after just 9 billion years isn't so remotely impossible - the odds must be comparable, which means that their precursor terms in the universal wavefunction had comparable amplitudes. Early unicorns had precursors with smaller amplitudes, but they were nevertheless there.

But now you are telling me that in order for a precursor term in the wavefunction to develop a living thing, and hence to collapse out all other precursor terms, that term must have a respectable amplitude. Small terms, even though they have evolved highly sophisticated living creatures, are not capable of collapse. That saves us from the early unicorn. It can nevertheless make disappear other precursor terms, with even larger or comparable amplitudes, as long as they didn't evolve enough living material at that moment, although they would have evolved microbes, but they were just a bit late.

Now still more difficult. What is "earlier" and what is "later" depends on the frame of reference. Imagine two precursor universe terms in a neck-to-neck race in the unitary evolution to make the first microbe (and hence to collapse away the competition). Let's call them "term15" and "term208". Suppose that they make their first microbe on a spacelike interval, and on planets in different motions (remember that each precursor term now describes an entire universe: the planet on which the microbe that appear in term15 appears, doesn't even have to exist in term208. The microbe in term15 is a totally different creature than the microbe in term208, different genetic code, etc... Now, from the frame of reference of the microbe in term15, it appears to be the "first". It is of course the only one in term15. But if it maps the universe described in term208 into its own spacetime, then it is "earlier" than the event that corresponds to the microbe creation in term208, at least in its own frame of reference.
And now we look at term208. In this term, there's also a microbe created. When it goes through the same exercise as did the microbe in term15, then it comes to the conclusion that, in ITS frame of reference, it came first ! It is of course the only microbe in its entire universe described by term208, but if it maps the event in the universe of term15 into ITS spacetime, then in ITS frame of reference, it came first.

So, who wins ? What universe is now going to disappear, what microbe will never have come into existence because its precursor term of its universe was collapsed away by the awareness of the other ?


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