How two quantum systems interact ?

[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.