What triggers decoherence?

  • 36 Replies
  • 8865 Views

0 Members and 1 Guest are viewing this topic.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
What triggers decoherence?
« on: 13/05/2014 16:26:14 »
Decoherence

The Schrodinger’s Cat thought experiment is, unless you are a supporter of Eugene Wigner, generally solved by citing Decoherence.  The reasoning goes something like this: Instead of arguing that the intervention of a conscious observer causes the wave function of the quon to collapse; the interaction of the quon with its environment brings about Decoherence.  Typically, this involves interaction with other quons and brings about a situation similar to the older idea of a collapsing wave function.  No conscious observer is necessary for this, so the cat is not put into a state of superposition.  However, cats the world over cannot heave a collective sigh of relief, as Brian Clegg suggests they might (Dice World), because once in this box they will be either alive or dead at the end of the process.

Decoherence, it seems, can solve the cat problem, but does it raise another question of its own?  It would seem to.  Quons have been around for billions of years, presumably interacting with other quons in their environment.  Why, then, has decoherence not caused the wave function collapse of every quon in the Universe, long ago?

NB. Quon = quantum particle. http://en.wikipedia.org/wiki/Quon     

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #1 on: 13/05/2014 19:04:06 »
I don't think anyone has a satisfactory answer to this, but I'm of the opinion that the universe is fundamentally quantum, so the cat state hasn't collapsed.  If the cat interacts with a quon, you just have two states: quon in state 1 + cat alive and quon in state 2 + cat dead (for example).  This is still a fully quantum system and can be in a superposition of two states.

If you push that further, it eliminates the observer problem to an extent because if you observe the cat but don't interact with me, I can treat you as being part of this quon-cat-Bill state which is in a superposition of alive and dead cats.  Once you tell me about what you see, I'm now a part of that state as well.

Of course, this just raises the question of why we never experience being in two states at once, which is probably a question about what consciousness means...

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #2 on: 13/05/2014 21:20:32 »
Quote
I don't think anyone has a satisfactory answer to this

Does that refer to “What triggers decoherence”, or “Why hasn’t every quon collapsed?

Quote
so the cat state hasn't collapsed.
 

Does this mean that you see the cat as being in a superposition?

Quote
If the cat interacts with a quon, you just have two states: quon in state 1 + cat alive and quon in state 2 + cat dead (for example).

If the cat interacts with the quon, would that not result in a dead or alive, rather than a dead and alive situation?

Quote
…. if you observe the cat….

If I observe the cat, would that not bring about decoherence?

I suspect that I’m missing something fundamental here, so patience, please.

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #3 on: 14/05/2014 01:03:00 »
If you observe the cat, you'd be in a superposition of "Bill sees cat alive" and "Bill sees cat dead."  Until I took a look at you to figure out which, you'd be in a quantum state.  When I looked at you, an observer looking at me would see me in a superposition state.

That's one way of looking at it, but it raises the question of consciousness again and why we won't ever observe ourselves as being in a superposition even if others might observe it.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #4 on: 14/05/2014 03:01:50 »
I think I must be further along the wrong track than I realised.  My understanding was that if I, or anyone else looked in the box the wave function would collapse, or decoherence would come about, and the cat would be dead or alive, whoever looked at it. 

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #5 on: 14/05/2014 12:42:39 »
Part of the observer problem is this: we know if we have a cat in a superposition of alive and dead, we can hit it with a quon that will interact with it and take on one form if the cat is alive and another if its dead.  If this quon is not an "observer" we'd typically say the total state is now "cat+quon" and that the whole thing is in a superposition of "alive cat + quon state 1" + "dead cat + quon state 2" and that wouldn't collapse until we observer it.

But we don't know why, fundamentally, an "observer" should be any different than a quon interaction.  Fundamentally, we don't know if observation and interaction are different.  Is it correct to say that we collapse a wavefunction whereas the quon becomes part of the quantum state?  Or do we, like the quon, become part of the quantum state?  If the latter is true, we have to explain why we never experience being in a superposition, which likely requires explaining consciousness scientifically.  That's a whole can of worms.  So let's go back to the idea that we somehow collapse the wavefunction.

Now, take the cat + a single quon example above.  If we measure the quon, not the cat, we can still force the cat's state to collapse (since the cat and quon are now entangled in one state).  If you take literally (and most physicists do) that it is interaction, not conscious observation, that causes the wavefunction to collapse, we don't even have to be looking at the quon.  If we happen to bump into it, we can cause the state to collapse.  Now, replace quon with a region of the universe.  If we have contact with that region, even if we're not actively trying to measure anything, we can cause the cat to collapse, meaning that by the time we get near it, it has already assumed the alive or dead state.

If it's not clear, my own personal take is that we aren't special and that we become entangled with this whole quantum system.  I lean that way because science has continually demolished ideas based upon putting ourselves in a special place in the universe, so I suspect the same will be true here.  It is in no way a settled issue.

*

Offline cheryl j

  • Neilep Level Member
  • ******
  • 1461
    • View Profile
Re: What triggers decoherence?
« Reply #6 on: 14/05/2014 14:31:07 »
Or do we, like the quon, become part of the quantum state?  If the latter is true, we have to explain why we never experience being in a superposition, which likely requires explaining consciousness scientifically.

What are you expecting the experience of being in a superposition to be like? What could you notice about it that would be any different?

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #7 on: 14/05/2014 16:10:11 »
Or do we, like the quon, become part of the quantum state?  If the latter is true, we have to explain why we never experience being in a superposition, which likely requires explaining consciousness scientifically.

What are you expecting the experience of being in a superposition to be like? What could you notice about it that would be any different?

I'd rather not speculate on that here, since it'd be veering off topic and into new theories.

What I will say is that as quantum mechanics stands now, if all we do is extend the idea of a quon to ourselves, we will end up in a superposition naturally.  We either have to make ourselves special ([it]e.g.[/it] consciousness somehow makes us non-quantum) or the laws of quantum mechanics have to break down as things get larger ([it]e.g.[/it] some have speculated that gravity may cause quantum mechanical effects to not extend to large scales).

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #8 on: 14/05/2014 18:23:55 »
I suspect I may not have expressed my original question very well, so I’ll give it another shot.

1.  An unobserved quon is in an indeterminate state of superposition.  When it is observed, measured or interacts with anything in its environment it assumes definite characteristics.  This change is thermodynamically irreversible.

2.  If assertion 1 is correct, once a quon has decohered, there is no going back.  Photons may appear to be a special case, but I believe that is not so. 

3.  Decoherence can occur when one quon interacts with another.  No intelligent observation or measurement is needed.

4.  Given that 1-3 are correct; why have not all the quons in the Universe already succumbed to decoherence?   Have they?
     Are all quons that are in a state of quantum superposition artificially generated in experiments etc?

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #9 on: 15/05/2014 02:36:14 »
I suspect I may not have expressed my original question very well, so I’ll give it another shot.

1.  An unobserved quon is in an indeterminate state of superposition.  When it is observed, measured or interacts with anything in its environment it assumes definite characteristics.  This change is thermodynamically irreversible.


There's a problem in the premise:

1. That is incorrect in the specific case that the "environment" consists of a second quon.  In this case, you now have 2 entangled quons in a superposition of 2 states.  This superposition is in essence a single quon (a single quantum state).
2. If the environment consists of N quons and we can model this as a bunch of individual quons, then we'll be in a superposition of N quons.

The real problem comes in that we know that wave collapse happens in the sense that we only observe a single state if we look at that quon.  So somehow we as observers aren't behaving like a quon.  The problem is that it is not obvious where to draw the line between observer that can cause collapse into a single state and interaction with another quon that just sets up a more complex superposition state.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #10 on: 15/05/2014 15:07:43 »
Thanks JP.  I think I’m getting there slowly.  My trouble is that I have difficulty letting go of something until I feel I understand it; and I know that QM does not lend itself to understanding, especially by lay people.



*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #11 on: 15/05/2014 16:04:28 »
True.  It is probably best to build QM from the ground up if you want to understand it rather than jumping in to the really interesting, but cutting-edge and complex ideas like decoherence. 

I would say the starting point is to just consider the interaction of 2 quons.  Let's say quon 1 starts in a superposition of two states, 1A and 1B (the 1 signifies it is quon 1 and the A/B signifies one of two possible states). 

Now quon 1 interacts with quon 2.  In reality, interactions can be complicated, but in our thought experiment, let's say they interact in such a way that quon 2 goes into the same A or B state as quon 1.  To describe this, we now have a state that is a superposition of 2 states: 1A+2A and 1B+2B.  This is entanglement, by the way--if I measure particle 1 to be in a state, I know that particle 2 is in a matching state.

Assuming all particles behave like these quons, after interaction with N quons, we'll have a superposition of 2 states:
1A+2A+3A+4A+...+NA and 1B+2B+3B+4B+...NB
This is still fundamentally 2 quantum states in a superposition.

Before we go further, does that make sense?  We can discuss observers and wavefunction collapse next, but rather than make a long post, I figure the best way is to take one fundamental point at a time.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #12 on: 15/05/2014 23:46:20 »
Quote
This is still fundamentally 2 quantum states in a superposition.

This is because there are only two states, irrespective of how many quons are involved?

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #13 on: 16/05/2014 01:31:34 »
Quote
This is still fundamentally 2 quantum states in a superposition.

This is because there are only two states, irrespective of how many quons are involved?

No, it's because I chose a simple example.  :p

In general, you can get lots of states depending on the states of the 2 quons and how they interact.

*

Offline cheryl j

  • Neilep Level Member
  • ******
  • 1461
    • View Profile
Re: What triggers decoherence?
« Reply #14 on: 16/05/2014 07:58:56 »
4.  Given that 1-3 are correct; why have not all the quons in the Universe already succumbed to decoherence?   Have they?
     Are all quons that are in a state of quantum superposition artificially generated in experiments etc?


Wouldn't new quantum events just continue to happen? I'm not sure I understand why they would become all used up or never happen again because of decoherence. Would it be theoretically impossible to fire the same particle next week through the double slit?


*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #15 on: 16/05/2014 20:04:49 »
Quote
No, it's because I chose a simple example.  :p

My question referred to your simple example, I do admit it was a bit naive. :)

I'm going to try to find time over the next few days to put together the information from this thread, and a few other places, to see how much sense I can make of it.  In the words of the famous quote: "I'll be back."

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #16 on: 16/05/2014 23:28:10 »
So much for that plan of action!


JP, I think I’m not going to get very far with this unless/until I sort out some bits from your early posts.  You say:

“If the cat interacts with a quon, you just have two states: quon in state 1 + cat alive and quon in state 2 + cat dead (for example).  This is still a fully quantum system and can be in a superposition of two states.”

Just to be clear; you are saying that the cat is in a superposition of alive and dead before it is observed?   If so, is this the generally accepted interpretation?


*

Offline evan_au

  • Neilep Level Member
  • ******
  • 4258
    • View Profile
Re: What triggers decoherence?
« Reply #17 on: 16/05/2014 23:28:31 »
In my simplistic view:
  • On a macroscopic scale, as soon as the cat sees its own tail, the cat will take on the state of "cat is alive".
  • On a microscopic scale, as soon as a blood cell bumps into the wall of a blood vessel, the cat will be alive.
  • On a quantum scale, as soon as a photon bounces off the cat, the cat will take on some state (dead or alive, assuming the transition from one state to the other is infinitely fast...). 
Interactions which do not cause the quantum state to decohere are the exception. Particles which interact and end up with the extra particle entangled with the first in a controlled manner are an exception, especially in a human-friendly environment around 300C. That's what makes quantum computers so difficult to develop, even near Absolute Zero.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #18 on: 18/05/2014 19:30:43 »
I tried pulling the threads together with some apparent success, but I find I am wanting to ask questions that are based on a probable lack of secure foundations; so, JP, I’m coming back to your suggestion of foundation building, if that's still OK with you.

Quote
I would say the starting point is to just consider the interaction of 2 quons.  Let's say quon 1 starts in a superposition of two states, 1A and 1B (the 1 signifies it is quon 1 and the A/B signifies one of two possible states).

I’m OK with that. Step one towards becoming an expert. :D

Quote
Now quon 1 interacts with quon 2.  In reality, interactions can be complicated, but in our thought experiment, let's say they interact in such a way that quon 2 goes into the same A or B state as quon 1.  To describe this, we now have a state that is a superposition of 2 states: 1A+2A and 1B+2B.  This is entanglement, by the way--if I measure particle 1 to be in a state, I know that particle 2 is in a matching state.

Discounting possible complications, I’m with this.  I’m not going to ask why this interaction might happen, rather than another, because I guess that would come later.

Quote
  Assuming all particles behave like these quons, after interaction with N quons, we'll have a superposition of 2 states:1A+2A+3A+4A+...+NA and 1B+2B+3B+4B+...NB

I understand this as saying that after any number of identical interactions, we would have two quantum states, with all the involved particles in a quantum superposition of both states.  Is that right? 

 

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #19 on: 18/05/2014 20:48:56 »
No problem, Bill.  I think too often threads in this forum try to jump into the deep end of the pool without building foundations.  Some topics are virtually impossible to really come to grips with without foundations and I think this is one of them. 

On your questions:
1) I know you didn't ask about why this particular interaction happens, but the idea is that the interaction here is just a simple thought experiment, so we chose an idealized interaction that's simple to understand. 

2) Yes, after any number of interactions, we still have 2 states which are built up from the original 2 states.  So even though the particle has interacted with many other particles, it is still in a quantum superposition of two states. 

The point of this is to show that it is not simply "interacting with a lot of other particles" that causes collapse. 

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #20 on: 19/05/2014 17:25:18 »
Quote
The point of this is to show that it is not simply "interacting with a lot of other particles" that causes collapse.

So far, so good.  What would you recommend as the next step?

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #21 on: 19/05/2014 19:26:54 »
There's a couple ways to go.  In one sense, we've answered the question in this thread of whether decoherence explains wavefunction collapse (it doesn't).  How so? 

We have to make 1 big assumption: every interaction is fundamentally quantum in nature.  This means that these interactions are also unitary, which is a technical term that implies that information is preserved in this interaction.  I.e. if the initial quon is in superposition and it interacts with a second quon, the system consisting of both quons contains that superposition information.  It may not be as simple as the case above, because the second quon will likely bring in its own superposition, but we haven't lost the superposition, i.e. the quantum-ness of the first quon. 

Once we assume this, then every interaction no matter how complex, can be broken down into these information-preserving quantum interactions.  So if we draw a big box around everything our first quon has interacted with, we haven't lost information about the quantum-ness of it and can in theory recover its superposition. 

What decoherence says is that to get full information about the quantum state of my quon, I need to have full information about the current quon and everything it's interacted with.  Typically, this is impossible for all but very isolated systems just due to the number of particles you'd need to keep track of.  So in practice, even if we can characterize a portion of the system very well, we can't fully recover information about our initial quon: some portion of that information is tied up with particles that we simply don't have the technical ability to measure.  The important point is that in theory the information is still there and nothing has collapsed.  It's just that in practice we'll never be able to design a system good enough to capture this information. 

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #22 on: 19/05/2014 22:19:50 »
 
Quote
if the initial quon is in superposition and it interacts with a second quon
If the initial quon is not in superposition, would this mean that the interaction was not “quantum in nature”, or would the fact of the second quon being in superposition ensure the quantum nature of the reaction? 

Quote
.  So if we draw a big box around everything our first quon has interacted with, we haven't lost information about the quantum-ness of it……

Presumably this applies to the second quon as well.

Quote
…… and can in theory recover its superposition.

Just checking on the word “recover”.   I assume it is “we” who do the recovering, as in learning about; not that the quon had lost its quantum-ness, and needs to get it back.

Quote
What decoherence says is that to get full information about the quantum state of my quon, I need to have full information about the current quon and everything it's interacted with.

This is where I may go off the rails.  What I am getting from this is that decoherence does not come about as a result of quantum reactions between quons, but is a function of the extraction of information from a quantum system.

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #23 on: 20/05/2014 03:54:03 »
Quote
if the initial quon is in superposition and it interacts with a second quon
If the initial quon is not in superposition, would this mean that the interaction was not “quantum in nature”, or would the fact of the second quon being in superposition ensure the quantum nature of the reaction? 
This requires a bit of quantum mechanics to nail in detail, but every particle is in a superposition if you measure the right variable.  What I am assuming is that quantum mechanics underlies the behavior of quons.

Quote
Quote
.  So if we draw a big box around everything our first quon has interacted with, we haven't lost information about the quantum-ness of it……

Presumably this applies to the second quon as well.
Yes, basically whatever quantum information goes in has to come out.  You can't destroy information in quantum processes.

Quote
Quote
…… and can in theory recover its superposition.

Just checking on the word “recover”.   I assume it is “we” who do the recovering, as in learning about; not that the quon had lost its quantum-ness, and needs to get it back.
A better statement would be that all the information put into the box is still in there.  Nothing has been irrevocably lost.  In a handwaving way, although your original quon might be a mess once it's interacted with all the other quons in the box, information about its initial state is still preserved in there somehow.

Quote
Quote
What decoherence says is that to get full information about the quantum state of my quon, I need to have full information about the current quon and everything it's interacted with.

This is where I may go off the rails.  What I am getting from this is that decoherence does not come about as a result of quantum reactions between quons, but is a function of the extraction of information from a quantum system.

Decoherence is based on a quantum version of statistical mechanics.  Although information about my first quon is in there, it may be jumbled up with all the other quons in such a way that I can't recover it unless I get the information from all the quons. 

This isn't all that far off from classical statistical mechanics.  If I have a box of a billion classical particles (think little billiard balls rolling about rapidly) and I add to that box a single billiard ball with unknown velocity and I let them bounce around for a while, can I figure out the unknown velocity by looking in the box?  Technically, yes, if I measure all the particles.  But practically, no, since it is impractical to try to do that.  Our initial billiard ball may still be identifiable and has all the properties: mass, velocity, position, of a billiard ball, but we can't figure out its initial state unless we measure all billion other billiard balls it's interacted with.

Quantum mechanically, it gets a bit more complicated, but its a similar idea.  Our initial quon is still quantum mechanical.  It still can behave like a wave or a particle and have all those nice, confusing properties of a quon.  But to get information out about its initial state back out, we need to know detailed information about everything its interacted with and this is practically impossible in many cases.
 
One place decoherence is very useful is in explaining why quantum computers are so hard to get working (they've been '10 years off' for several decades.)  If we put a qubit in to be stored, we want to be able to maintain information about its quantum state.  But if it interacts with anything else, that information gets tangled up with the other particles it has interacted with.  Very quickly, unless it's held in extremely careful isolation, we lose useful information about its state to the environment and the environment leaks useless (to us) information into the qubit.  It doesn't collapse--it's still a quantum particle--but it has been corrupted by the environment.  If we really wanted to use that information, we'd have to look at everything it's interacted with to back out the information.  Things get even harder when you have to move qubits around and get them to interact to do useful processing with them. 

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #24 on: 20/05/2014 21:30:16 »
Quote
What I am assuming is that quantum mechanics underlies the behavior of quons.

I imagine it would be difficult to do anything in QM without this assumption.

Quote
although your original quon might be a mess once it's interacted with all the other quons in the box, information about its initial state is still preserved in there somehow.

Would this reflect the difference between waveform collapse and decoherence? 

Quote
Decoherence is based on a quantum version of statistical mechanics........
 

I know I've said this before, but if you have not already written a pop sci book, you should; and if you have, I want it!

The reason I included the “Schrödingcat” in the OP is that most of the pop sci explanations I have seen have been attempts to describe why the cat would not be in superposition.  How far off track would it be to ask if the cat might not be in superposition, but the individual quons of which the cat is composed might be?

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #25 on: 21/05/2014 12:50:21 »
Quote
What I am assuming is that quantum mechanics underlies the behavior of quons.

I imagine it would be difficult to do anything in QM without this assumption.
There are legitimate physicists questioning this assumption.  In particular, they question whether there is some transition when you collect enough mass which causes quantum mechanics to break down.  Roger Penrose is one of the major proponents of this idea.  I'd say it is definitely one of the more out-there ideas in modern physics, but it is still an interesting idea.


Quote
Quote
although your original quon might be a mess once it's interacted with all the other quons in the box, information about its initial state is still preserved in there somehow.

Would this reflect the difference between waveform collapse and decoherence? 
Wavefunction collapse means that some process causes a part of the state to vanish irrecovably.  Decoherence means that while your particle might appear to now be in only one state, its second state will be somehow stored in the environment.  So in theory, the information's still there, even if it's impractical to access it.

There's another interesting theorem in quantum mechanics related to this: the no cloning theorem.  What this says is that if you want to make a perfect copy of quon 1, you can't do it by measuring it and creating a second quon.  The reason is that measurement will collapse its state.  So if it is X% in state A and Y% in state B, and you take a measurement and see state A, you've lost information about the percentages, so you can't make a perfect copy.   Decoherence may put the quon into state A, but the percentages are somehow stored in the environment.

Quote
Quote
Decoherence is based on a quantum version of statistical mechanics........
 

I know I've said this before, but if you have not already written a pop sci book, you should; and if you have, I want it!

The reason I included the “Schrödingcat” in the OP is that most of the pop sci explanations I have seen have been attempts to describe why the cat would not be in superposition.  How far off track would it be to ask if the cat might not be in superposition, but the individual quons of which the cat is composed might be?


Thank you.  I'll send you a free copy if I ever do write such a book.  ;)

If the quons making up the cat are in superpositions, the cat itself will be, assuming that the laws of quantum mechanics don't break down at cat-scale.   

Where decoherence is often misrepresented as explaining collapse is that if the cat interacts with its environment, it may be the case that the cat assumes the "alive" state.  But the quantum information about the superposition of alive and dead is still in the environment. 

In collapse, the cat would suddenly assume the "alive" state and the information about the superposition would be gone. 

So decoherence can push the question about collapse off to the environment rather than the quon itself.  But at some point, you'll have to come back and address the fundamental question of whether the environment collapses or not.

My own guess is that collapse is probably the wrong way to think about things and that even we as observers are part of the environment.  If we observer a cat, we end up in a superposition.  But this has a major problem that it can't explain why we don't feel like we're in a superposition.  As far as I know, we can't answer this until we can relate our conscious experience to the laws of physics, which is way beyond current science.  This line of thought is studied by some physicists, but it is also fairly out-there as far as theories go.  If nothing else, it appears to be far out from current testability.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12188
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: What triggers decoherence?
« Reply #26 on: 22/05/2014 19:54:18 »
There is also an idea of thoughts having its origin as a quantum phenomena, if I remember right? As if we in some weird manner became quantum computers, but as our 'questions' fail to be perfectly formulated? Also a philosophical aspect isn't it? Why the world is arranged so that you only get that perfect answer if your question is perfectly formulated. You might think that it is the same macroscopically, but practically speaking we get along just fine without those 'perfect questions' it seems? Alternatively that it is a form of decoherence that produce consciousness, or maybe both rather? A interesting thread this one.
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #27 on: 23/05/2014 15:18:41 »
Quote from: JP
Wavefunction collapse means that some process causes a part of the state to vanish irrecovably.  Decoherence means that while your particle might appear to now be in only one state, its second state will be somehow stored in the environment.  So in theory, the information's still there, even if it's impractical to access it.

That makes sense to me, but still leaves one point to be cleared up. 

In current scientific wisdom, are wavefunction collapse and decoherence considered as two separate things, either of which might happen; or is wavefunction collapse and outdated idea that has largely been replaced by decoherence?

Quote
There's another interesting theorem in quantum mechanics related to this: the no cloning theorem.  What this says is that if you want to make a perfect copy of quon 1, you can't do it by measuring it and creating a second quon.  The reason is that measurement will collapse its state.  So if it is X% in state A and Y% in state B, and you take a measurement and see state A, you've lost information about the percentages, so you can't make a perfect copy.   Decoherence may put the quon into state A, but the percentages are somehow stored in the environment.

That’s fantastic; an explanation of the no cloning theorem that even I can follow! 

Quote
If we observer a cat, we end up in a superposition.  But this has a major problem that it can't explain why we don't feel like we're in a superposition.

So, as I sit “observing” my computer screen, I am in quantum superposition with my computer, but cannot be aware of that??

I look forward to my “free copy”, but at my age I have to suggest you don’t wait too long before making a start.  :)

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #28 on: 23/05/2014 16:42:06 »
Quote from: JP
Wavefunction collapse means that some process causes a part of the state to vanish irrecovably.  Decoherence means that while your particle might appear to now be in only one state, its second state will be somehow stored in the environment.  So in theory, the information's still there, even if it's impractical to access it.

That makes sense to me, but still leaves one point to be cleared up. 

In current scientific wisdom, are wavefunction collapse and decoherence considered as two separate things, either of which might happen; or is wavefunction collapse and outdated idea that has largely been replaced by decoherence?

It's not my particular field of expertise, but wavefunction collapse is a feature of one particular interpretation of quantum mechanics.  It is probably not something to get hung up on, since the same mathematics has alternative explanations.  This is one place where I prefer the many-worlds interpretation.  Nothing collapses, it's just that the world we are experiencing gets only one version of the quantum state, [it]i.e.[/it] when we open the box in our universe, the cat is dead.  There's a separate universe that branched off from ours when we opened the box in which it is alive and the two universes can't talk to each other.  It's easier to see in this interpretation how the two-state-ness of the cat prior to opening the box has been lost: there was a potential for us to go into either universe initially, but once we "choose" a path, we're stuck there.

Decoherence is a separate phenomenon that says that although the cat may be dead, it could pass this two-state-ness into something else so this hasn't been lost. 

On a global scale, if you could write down the quantum state of the entire universe, collapse would cause it to change irreversibly whereas decoherence would not.

Quote
Quote
If we observer a cat, we end up in a superposition.  But this has a major problem that it can't explain why we don't feel like we're in a superposition.

So, as I sit “observing” my computer screen, I am in quantum superposition with my computer, but cannot be aware of that??
This is something neither I, nor anyone really has the answer to because we have almost no idea how our conscious experience can be explained scientifically.

If all objects are quantum at their core, then you'd expect that when we observe anything it is fundamentally quon interactions and we should be able to describe ourselves as in some sort of quon superposition.  Why, after all, should we be special and not subject to the same laws as all other objects in the universe?  The problem is that we don't experience this in any obvious way.  Could decoherence explain this?  Maybe, but it's hard to say anything without having any idea how conscious experience works from a scientific standpoint.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #29 on: 23/05/2014 22:32:45 »
Quote from: yor_on
A interesting thread this one.

There's certainly a lot of information in it, and a lot to think about.  I have this feeling that the OP of a thread like this should, at some point before it dies quietly, write a brief account of what he/she has gained from it.  I intend having a shot at that, but given that I have some dozen pages of closely typed notes to work through, and an extra busy period coming up, it may not be very soon.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #30 on: 26/05/2014 13:55:53 »
Before even attempting a summary I would like to present a refined version of the Schrödinger thought experiment.

Everything is as in the original, except that the box has room for food, water and a litter tray.  The mechanism that releases the poison also stops a clock.  The cat is fitted with a heart monitor, also linked to a clock.  The experiment is set up for 48 hours. 

At the end of the time the box is opened, and the cat is found to be dead.  The clock shows that the poison was released at hour 3.

Does the heart monitor show that the cat died at hour 3, or at hour 48?

If the cat was in a live/dead superposition, would the live version have eaten or visited the tray?

I realise that these questions are based on classical/intuitive thinking, but they are the sort of questions people ask.



*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #31 on: 26/05/2014 16:30:17 »
Ok, the proper way to treat that problem is to write out the possible states of the system and then ask which is consistent with a measurement of "dead cat and clock stopped at hour 3."  When you measure and "collapse" a wavefunction, you leave the system in a superposition of all its states that are consistent with that measurement. 

So back to this specific case, if the heart rate monitor is interacting with the cat all along, it will show that it stopped at hour 3.  The cat would not have visited the tray. 

What a rigorous quantum treatment will tell you is that the instant before you opened the box, the cat was in state A and B simultaneously with

A) Dead cat, clock stopped at hour 3, heart rate monitor showing dead at hour 3, no food eaten after hour 3.
B) Alive cat, clock not stopped, heart rate monitor showing living cat, food eaten all the way up to box being opened.

In the Copenhagen interpretation of quantum mechanics, opening the box forces the system to choose one of these states.  The big question in this interpretation is what makes the "observer" more special than other, quantum interactions, which don't force collapse.

It's a little less (to me at least) confusing in the many-worlds interpretation which says that in one universe, you have state A and in one you have state B.  Opening the box entangles you (the observer) with the cat in such a way that in one universe you are observing an alive cat and in another you are observing a dead cat (moreover, these two universes can't talk to each other).  The big question here is why your consciousness inhabits only one of these universes. 

The important point to note is that for now, there is no accepted way to distinguish between these two interpretations of the mathematics of quantum mechanics.  This may get resolved when we know more about how consciousness works or it may not.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #32 on: 27/05/2014 14:36:19 »
Thanks again, JP.

We seem to be moving towards the multiverse, which probably needs a new thread.  I try to keep an open mind on the multiverse, but have lots of questions/misgivings.  I think I'll try to pull together the decoherence information before getting into anything else.

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #33 on: 27/05/2014 14:58:29 »
The multiverse may or may not need a new thread, depending how seriously you take it.  If you think of all the interpretations as just ways of trying to make sense of the mathematics of quantum mechanics, then you can pick or choose which makes the math the clearest for you.  If you take one particular interpretation seriously as physical reality, then you've gone beyond science and into the realm of philosophy in my opinion, since all are equally well supported by both theory and experiment.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #34 on: 27/05/2014 18:04:08 »
Quote from: JP
If you think of all the interpretations as just ways of trying to make sense of the mathematics of quantum mechanics, then you can pick or choose which makes the math the clearest for you.

Strange, perhaps, that I had reached that conclusion with string theory, but not the multiverse. 
This must raise the question as to whether any scientists (apart, possibly, from David Deutsch) actually believes in the physical reality of any sort of multiverse.   

Quote
  ....since all are equally well supported by both theory and experiment.

I assume you mean that there are experiments the results of which are consistent with the multiverse theories; rather than that there are experiments that could establish the existence of other universes.

OK, I accept that that is a silly question, in view of your comment about the "realm of philosophy", but it highlights the possible naivety of us hitch-hikers. 

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Re: What triggers decoherence?
« Reply #35 on: 27/05/2014 19:28:10 »
We could dive into philosophy of science now, but it suffices to say that physics is about modeling nature, generally through mathematics.  Usually there is an obviously superior interpretation of this mathematics, typically because that interpretation is the simplest or most intuitive.  In the case of quantum mechanics, the what the mathematics says is so bizarre and counterintuitive that there are several interpretations for it, each of which has a good case for being the "best" interpretation.  Until we have a real reason to pick one over another, it's up in the air. 

Of course, you'll find ardent supporters for each one, but the consensus is that all are worthwhile interpretations.

As for who takes many-worlds seriously: I tend to lean that way, but I'm open to all the interpretations.  I simply find the least bizarre interpretaion to be that there are many universes, each evolving independently and deterministically than that the wavefunction collapses or that there are pilot waves.  The problem gets pushed onto how we get stuck in one particular universe, but that's less troubling to me at least than the idea of wavefunction collapse.

Having said that, I tend to pick and choose interpretations based on what I find least confusing for the problem at hand.

*

Offline Bill S

  • Neilep Level Member
  • ******
  • 1862
    • View Profile
Re: What triggers decoherence?
« Reply #36 on: 27/05/2014 21:09:27 »
This is a first shot at a summary of my understanding of the answer to the twofold original question.

What triggers decoherence?

Why, after almost 14 billion years, have not all the quons in the Universe decohered?

It seems that decoherence is triggered when a quon interacts with its environment.  This may mean interacting with just one other quon, or with complex system.

The question as to why there are plenty of quons in the Universe that are still in superposition when quons have been interacting for billions of years is perhaps rooted in a confusion between wave function collapse and decoherence. 

Wave function collapse involves a process which is irreversible under the second law of thermodynamics, and entails the permanent loss of “quantumness”.  Once collapsed, the object involved can be totally described in terms of classical physics.   

Decoherence, on the other hand, is a less cut-and-dried concept in that, although the process that triggers it may be thermodynamically irreversible, decoherence itself is not so easy to pin down.  It appears to be much more observer specific.  If, for example, I observe a quon, I will not see it in a superposition.  As far as my observation is concerned, the wave function has collapsed, but in a broader context, the quon retains its “quantumness” and may be in states of superposition that I am unable to observe.  If I stop observing the quon; then observe it again, decoherence will happen again, and I will observe something of which the wave function has just apparently collapsed.

Decoherence brings about an apparent wave function collapse only in the frame of reference of the observer.