"decoherence" - the myth

vanesch

Ok, try me

MWI is my preferred *interpretation* of quantum mechanics. But you will be disappointed: I'm not claiming that nature IS that way. 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.

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.

Coldcall

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

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.

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

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

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

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

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

Removal of the interferences is quite far from the explanation of the collapse.

What does CI stand for? Collapse Interpretation?

peter0302

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.

Well, I meant Copenhagen Interpretation, but Collapse Interpretation isn't a bad label for it.

jostpuur

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.

vanesch

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

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

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

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

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

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

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.