How two quantum systems interact ?

[ronan1]

The difference is that decoherence happened in order to make you aware of the rope and thus because of the irreversibility of decoherence you can not go back at the begginning of the universe to explain the reality of the rope !

and if you considere that no decoherence occur, you cannot even talk about rope

If you suppose that no decoherence occur before and that the time you see the rope was the ONLY time decoherence (lets call it D) occur, then it was by pure chance that the rope appear and in fact everything, even you

because if you go back at the wavefunction of the universe, it evolve in so many direction that you can get anything: any world (in the many world interpretation) at the time D occur. So your rope could have been a flying elephant (pink if you want) !


It seems completely crazy but without decoherence before D the world can be anything (almost at least) at the time of D

and with multiple decoherences before D, then we need to explain why decoherence happened before.

For my orginal question, could we say that even if interaction happen all the time, they get concretized only after decoherence ?

thus making a definition for interaction in quantum realm:
interaction = way in which the whole system evolves
and not:
interaction = exchange of energy between a and b
which would be the classical definition (or the after D defintion)

Now, "way in which the system evolves" seems to not be satisfactory
we can not even describe it

Is it the only chance?

What about Bohmian mechanics or its possible future derivatives ?
What would they say about it ?
Do you kow ? would it be more easy to grasp ?

 

[vanesch]

because if you go back at the wavefunction of the universe, it evolve in so many direction that you can get anything: any world (in the many world interpretation) at the time D occur. So your rope could have been a flying elephant (pink if you want) !

Absolutely. In MWI, we consider that there's a "world" (a term in the wavefunction) with a rope, and that there's also another "world" (another term in the wavefunction) with a pink flying elephant, and there's a world where the dinosaurs didn't get extinct, and there's a world where the solar system never formed etc...

You happen to observe the one with the rope. That's simply 'the place where you are'. In the same way as you happen to be on planet earth, and not in some other remote galaxy, and that you are living in this period, and that you didn't live in Julius Caesar's time.

It seems completely crazy but without decoherence before D the world can be anything (almost at least) at the time of D

But also AFTER decoherence! Decoherence only means that the guy who sees the rope won't be bothered by the guy who sees the pink elephant and that the guy who sees the pink elephant won't be bothered by the guy who sees the rope ; that no experiment ever by one will indicate the presence or not of the other. Which, in its turn, means that you can just as well forget about the others.

Before decoherence, there were interference phenomena possible, while after decoherence they became so complicated to realize that they are essentially impossible.

For my orginal question, could we say that even if interaction happen all the time, they get concretized only after decoherence ?

Not really. You could probably say however that observations only get concretized after decoherence (most of the time, observations imply decoherence in a way).

thus making a definition for interaction in quantum realm:
interaction = way in which the whole system evolves
and not:
interaction = exchange of energy between a and b
which would be the classical definition (or the after D defintion)

Yes, that's correct: interaction determines the way the whole system evolves. However, it is based upon energy exchange which can be written down in explicit "separated system" states, which is enough to determine the time evolution (interaction) for all states.

Now, "way in which the system evolves" seems to not be satisfactory
we can not even describe it

But of course we can describe it! Why do you think we can't ? That's what the quantum formalism is for.

What about Bohmian mechanics or its possible future derivatives ?
What would they say about it ?
Do you kow ? would it be more easy to grasp ?

For people who cannot get loose of the classical Newtonian paradigm, it can help them get a picture of quantum theory that can ease their mind

 

[ronan1]

But of course we can describe it! Why do you think we can't ? That's what the quantum formalism is for.

When you ll describe it you will use object such as pink elephant and rope (or rings, or 2 electron atom) So you will need to choose one universe among many and not the whole multiverse

Thus you will not describe the whole world but an arbitrary one!


Also, decoherence happened many times before you observe for the first time, so you finally cannot live in any world but some specific ones
This make the thing wierd: it is like finnaly the world need something else to evolve: decoherence and thus the MWI is not satisfactory : it needs an explantion why do we choose one special world and why we cannot go back to a precedent decoherence to change completely the world from a rope to a pink elephant


What is the cause of these precedent decoherences ?
complicated interactions ? : not very satisfactory

Not really. You could probably say however that observations only get concretized after decoherence (most of the time, observations imply decoherence in a way).

observation getting concretized ? What do you mean ?
there is no observation before decoherence
It would make more sense that this the interactions which are in superposition (as the systems are in superposition) that get concretized (one of them).

By saying that the interaction "chosen" is concretized it means that the interaction actually takes place. It seems for me more correct that saying that observation are concretized.

For people who cannot get loose of the classical Newtonian paradigm, it can help them get a picture of quantum theory that can ease their mind

In which way ?
Do they say that the world is evolving completely deterministcaly
but that we cannot access the variables which would permit us to make precise prediction ?

 

[vanesch]

When you ll describe it you will use object such as pink elephant and rope (or rings, or 2 electron atom) So you will need to choose one universe among many and not the whole multiverse

Thus you will not describe the whole world but an arbitrary one!

Yes, you will be limited to only a part in the whole "universe" description. But that will also be the case in, say, a classical, infinitely large Newtonian universe, where you will only be able to describe a lump of the total (infinite) universe. You'll have to be satisfied with that part which is related to your "neighbourhood" (even if that extends for 15 billion light years, that's still a small spec in an infinite universe).

Also, decoherence happened many times before you observe for the first time, so you finally cannot live in any world but some specific ones

Indeed. Note that you have the same problem with classical thermodynamical irreversibility. If you have hot water, you don't know whether it was ice before or not.

This make the thing wierd: it is like finnaly the world need something else to evolve: decoherence and thus the MWI is not satisfactory : it needs an explantion why do we choose one special world and why we cannot go back to a precedent decoherence to change completely the world from a rope to a pink elephant

Well, we cannot go back in thermodynamics either. But you still seem to be attached to the idea that nothing can evolve if it is not expressed in classical terms of some sort.
What is true however, is that there needs to be something "else" that explains why we observe THIS world (the one with the rope) and not THAT world (with the elephant). But that's an old philosophical problem, called heacceity: what makes "me" "me" ? Why am I here, and not there ? Why this world and not that world ?

So the question is not strictly quantum-mechanical. Only the extravagantness of certain interpretations of quantum theory makes one think of what was tacitly taken for granted in more conventional and classical views.

What is the cause of these precedent decoherences ?
complicated interactions ? : not very satisfactory

I think this is because you think that decoherence is some special happening ; it isn't.
When two particles interact, the overall quantum state can be an entanglement of two particles. When the 2-particle system is entangled like this, you cannot have interference anymore with one single particle. We can, as such, say that one particle "decohered" with the other one. Say that two particles are a proton and an electron, and that they are conscious (who knows ). They can have two "states of awareness": being "spin up" and being "spin down" (probably related to their mood or something...).
You can have a state which is |up>|down> for instance, where the electron has one state of awareness (up) and the proton has one state of awareness (down).
But now, they interact, and they get into a state like |up>|down> + |down>|up>. Well, this means now that there are now TWO states of awareness for the electron, one where there is an "electron-awareness" that is "up" and another "electron awareness" that is "down". And there are two proton-awarenesses too. Well, the electron awareness that is "up" will live in a world where there is also a proton in a state "down", while the electron awareness that is "down" will live in a world where there is also a proton in state "up".

So in a way, the "world" of the electron has "decohered" in two worlds, one in which it is aware of being "up" and one in which it is aware of being "down", with corresponding proton states and everything.

But it's not really a decoherence here, because the state |up>|down> + |down>|up> can easily evolve into something like |up> |up> for instance. That's a thinkable quantum evolution. As such, the two "worlds" namely "updown" and "downup" will INTERFERE to make one single world "upup". |up>|down> doesn't lead its own life "irreversibly and independently of |down> |up>.

However, if you play the same game with 10 000 different particles, then there is already much less chance that |up>|down>|down>|up> ... |up> will interfere again with |up> |up> ... |down>, because most of the time, at least one of the factors will remain orthogonal. The more different systems that are composed, the less likely that it becomes that under time evolution, they will interfere. So if you have a state with 10^23 different systems (different air molecules for instance), chances that they will interfere again are very remote indeed, and will practically not happen. So the terms are now "separated for ever". This is when interaction gives rise to decoherence.

observation getting concretized ? What do you mean ?
there is no observation before decoherence
It would make more sense that this the interactions which are in superposition (as the systems are in superposition) that get concretized (one of them).

By saying that the interaction "chosen" is concretized it means that the interaction actually takes place. It seems for me more correct that saying that observation are concretized.

An interaction is not "chosen". It happens, irrespectively of whether we call it an observation or not. Observation is then just "being aware of one of the terms". And others will be aware of different outcomes, but they are simply not "you" but a copy of "you".

In which way ?
Do they say that the world is evolving completely deterministcaly
but that we cannot access the variables which would permit us to make precise prediction ?

Yes.

 

[ronan1]

An interaction is not "chosen". It happens, irrespectively of whether we call it an observation or not. Observation is then just "being aware of one of the terms". And others will be aware of different outcomes, but they are simply not "you" but a copy of "you".

You seems to be sure of the correctness of MWI, but it is not the only one interpretation, haecceity is deeper in MWI than in other interpretation

because MWI doesn't forbiden you to jump to another world !
while in a classical view, it forbiden you because it is said that there is one world.

The difference between classical thermodynamics and QM is that if you know the beginning of the universe you can predict the whole world while in QM, you can not!

I think this is because you think that decoherence is some special happening

Ok so for you it happens all the time, universes are splititng constantly, it is again MWI and it doesn't solve anything except that you can think quantum mechnicaly
with MWI you think only in QM but you have a big problem explaining why there is a classical view and why I am in this world AND WHY I CANNOT CHANGE WORLD !

Yes.

So Bohm interpretation doesn't have any problem that we mentioned !
It is true that as currently stated it don't work but there is still a possibility (apparently Bell's theorem have been refuted recentely, isn'it ?)

 

[vanesch]

You seems to be sure of the correctness of MWI, but it is not the only one interpretation, haecceity is deeper in MWI than in other interpretation

No, I'm not "sure" about the MWI interpretation. I've often said that I find it the interpretation that is closest in spirit to the actual formalism of quantum theory, and I still stand by that point. I do not consider that it is any "true" view of reality. I'm agnostic about what reality really is.

But MWI is the only framework in which decoherence makes any sense. If you have any other view on the measurement process (such as collapse), then you don't need any consideration of decoherence. Decoherence is what gives MWI a leg to stand on.

So in order to even discuss sensibly about decoherence, one has to place oneself in an MWI viewpoint. In other views, there's a deus ex machina that "ends" quantum mechanics and in one way or another makes you transit into a classical world, and there's no need (or no meaning to) decoherence.

because MWI doesn't forbiden you to jump to another world !

Do you sometimes jump into experiencing being another person ? Do you "change bodies" sometimes ? Why do you (as a subjective experience) experience *this* body and not another body ? Classical mechanics doesn't forbid you to jump into experiencing another body. The same reason that makes you "stick to your body" is probably also applicable to why you stick to your own world in MWI.

The difference between classical thermodynamics and QM is that if you know the beginning of the universe you can predict the whole world while in QM, you can not!

Well, quantum-mechanically, you CAN predict the whole world (wavefunction). You can only not predict what world a specific "you" will experience. But if you pick out a world (a term in the wavefunction) you can tell that a person living in that world will experience this and that.

Let's assume there's a classical universe, in which classical beings evolve biologically into sentient beings. You cannot predict what "you" will experience. You CAN maybe predict what the being that is sitting there under the tree is experiencing. But you cannot predict that that being will be "you". In the same way, ideally, in QM, you CAN predict what a being in a specific world (decohered term in the wavefunction) will experience. But you cannot predict that that world will be YOURS.

Ok so for you it happens all the time, universes are splititng constantly, it is again MWI and it doesn't solve anything except that you can think quantum mechnicaly
with MWI you think only in QM but you have a big problem explaining why there is a classical view and why I am in this world AND WHY I CANNOT CHANGE WORLD !

Yes. But that's not proper to quantum theory per se.

So Bohm interpretation doesn't have any problem that we mentioned !
It is true that as currently stated it don't work but there is still a possibility (apparently Bell's theorem have been refuted recentely, isn'it ?)

Bohmian mechanics works very well. Bell's theorem is not refuted - it is misunderstood by those who think they refuted it. They refuted something Bell's theorem never claimed.

Bohmian mechanics is not "harmed" by Bell's theorem as in Bohmian mechanics, you allow for superluminal causal effects explicitly: the measurement at Alice's has a direct causal influence on Bob's measurement, even if they are 2 lightyears apart. As such, Bohmian mechanics is not compatible with relativity - which is the main reason why many people don't find it inspiring to work in it. But in an ether view, you can get rid of relativity too, and that's what Bohmian mechanics does: it is a strictly pre-relativistic, Newtonian view, with sufficient "fiddle terms" that make all effects of relativity and quantum theory appear. But it works.
There where Bohmian mechanics really becomes terribly ugly, is in quantum field theory. Nevertheless, there are (after-the-fact) ways of introducing in Bohmian mechanics even effects that mimick quantum-field theory results.

The point is, that Bohmian mechanics is never an "inspiration" for further work. It can accommodate what has been discovered in quantum theory. But it works.

 

[ronan1]

Do you sometimes jump into experiencing being another person ? Do you "change bodies" sometimes ? Why do you (as a subjective experience) experience *this* body and not another body ? Classical mechanics doesn't forbid you to jump into experiencing another body. The same reason that makes you "stick to your body" is probably also applicable to why you stick to your own world in MWI.

That's right :)

Bohmian mechanics works very well. Bell's theorem is not refuted - it is misunderstood by those who think they refuted it. They refuted something Bell's theorem never claimed.

Ok, Bell theorem does not apply to Bohmian mechanics.
Is Bohmian Mechanics not a local hidden variable theory ?

The point is, that Bohmian mechanics is never an "inspiration" for further work. It can accommodate what has been discovered in quantum theory. But it works.

Why not an inspiration ?
Bohm found inspiration in it : "Wholeness and the implicate order"
It is also a good way to view Quantum mechanics, quite intuitive.

What about relational quantum mechanics ?
It says that basically decoherence is just a matter of point of view.

let put an example:

O1, O2 and S are three quantum system
O1 study S,
O2 study O1+S

according to O1,
S = a|up> +b|down> (a^2+b^2)=1

according to O2 :
O1+S = |O1 up>|up> + |O1 down>|down> (|O1 up> being the state of O1 observing S=up and same for down)

But in fact when O1 observe S
S= up or S= down
let say that O1 actually observe S=up

Now O2 observe O1+S , will he find :
O1+S = |O1 up>|up> ?

and if he observe S alone, will he find:
O1+S = |up> ?

So it is not completely relational as something seem to be there no matter who measure it: S=up
?

Thank you for the discussion, I think I get in some way a better picture of QM even if I feel like something is missing :p

maybe the question about relational quantum mechanics should be put in another thread.

 

[vanesch]

That's right :)
Ok, Bell theorem does not apply to Bohmian mechanics.
Is Bohmian Mechanics not a local hidden variable theory ?

No, it is not local, in the sense that interactions are immediate and at a distance (and don't even diminish with distance). It's pretty close to Newtonian mechanics, except that we now also have a wavefunction living in multi-dimensional space which dictates the forces, and not just the positions of the particles.

Why not an inspiration ?
Bohm found inspiration in it : "Wholeness and the implicate order"
It is also a good way to view Quantum mechanics, quite intuitive.

It is a very good way to get intuition for non-relativistic, ONE-PARTICLE spinless quantum mechanics. Spin is a horror in Bohmian mechanics (although it comes out quite nicely in the end).

What about relational quantum mechanics ?
It says that basically decoherence is just a matter of point of view.

To me, relational quantum mechanics = many worlds + solipsism.

The solipsism part then makes that we can get away with the "many" in the many worlds. But that's just my opinion. It is not shared by everybody

I would like to add something I like to stress when talking about interpretations of quantum mechanics. This is my personal opinion, but I think it helps putting things in perspective. I think that we don't have the final theory of the universe yet. If history is some lesson, before we have it, it is probably wrong to speculate about whatever properties it might have as an extrapolation of what we already have. I'm not even sure we'll ever have it, and even if we have it, whether we will know that we have it. As such, I think that hoping to know how nature really really is, is idle business. Nature might be totally different than what we might now think it is like at its most fundamental level. It might retain certain aspects we've discovered already, but it might also be totally different. As such, any "reality" we might deduce from our current theories is probably "wrong" anyways. Maybe "reality" is less weird than we deduce from our current theories, but it might also be even much more strange and weird than what are now our wildest fantasies if history is any guide.
So in a certain way, our guess about reality anyway being "wrong" at the most fundamental level, we might as well make the best guess we can that goes with our current theories - realizing the relativity of its value.

This is why I prefer MWI: it is closest to the spirit of the actual quantum formalism, and it helps one get a feeling for it. It's all I ask about a "view" or an "interpretation" of a theory: help me get some "picture", some "feeling" for it. I don't lie awake about its philosophical and moral implications, given that it is probably wrong anyways, and that I haven't gotten any better guess than that. I think it is an error to try to think up grandiose world views that adhere to our intuition and butch up the formal side of things for that. But I think it is just as well misguided to think that our current theories already touch the "final truth". This is just preliminary guesswork. And that's probably a permanent situation.