# B Decoherence and the Dust Particle

1. Jan 28, 2016

### Feeble Wonk

The concept of environmental decoherence has been a perpetual thorn in my brain despite an embarrassing amount of time contemplating the idea. At this point, I'm not even sure if I understand what it is purported to resolve. In many ways, it seems to me to be primarily a theory about logical set limitations rather than a mechanism by which a particular quantum state is determined. Sadly, my mathematical inadequacies continue to pose an obstruction to discussing this in terms that you physicists are comfortable with. So, I'd like to try this via an example of decoherence that I've seen cited multiple times in the past.

The precise location of a cosmic dust particle in space is said to be determined, through decoherence, by interacting with just a few photons. First, I'd like to clarify a couple of points that I'm curious about. In what sort of time frame do the involved photon interactions need to occur to achieve decoherence induced localization?

2. Jan 28, 2016

### Staff: Mentor

It purports to answer the question "Why do we not encounter macroscopic superpositions such as the neither dead nor alive cat of Schrodinger's thought experiment?" and the answer is "They decay so quickly that we don't have a chance to observe them".

Something on the order of $10^{-6}$ seconds - that order of magnitude estimate comes from Brian Greene, and no matter what you think of his pop-sci stuff he does know how to do the calculations. That is a very extreme case, as interstellar space is about as empty as empty can be. The same dust particle in room-temperature air will decohere maybe $10^{30}$ times more quickly.

3. Jan 28, 2016

### Staff: Mentor

They do not need to occur within some timeframe. A single photon can be sufficient for decoherence, so the timeframe of decoherence is given by the density of photons around the dust particle.

4. Jan 28, 2016

### Feeble Wonk

I'm sorry. I didn't make my question clear. IF the dust particle localization requires the cumulative interaction with multiple photons, then does it matter if a significant time period elapses BETWEEN these interactions. If a single interaction is sufficient, as mfb suggested, then this question isn't pertinent.

5. Jan 29, 2016

### Demystifier

A "measurement" with a single particle (or only a few particles) is not irreversible. An important property of decoherence due to interaction with many particles is irreversibility.

6. Jan 29, 2016

### Staff: Mentor

Those many particles are in the dust particle, if the interaction with light is sufficiently incoherent (e. g. visible light or UV light hitting it and exciting some molecule).

7. Jan 29, 2016

### Feeble Wonk

But, wouldn't the excitation of a single photon be largely limited to a very small percentage of the total dust particle mass? Would that make a difference in the "degree of irreversibility" of localization?

8. Jan 29, 2016

### zonde

It's similar with me.
My problems are related to statement of the problem. What we should expect if decoherence does not happens? As I see the only way how we can find out when decoherence has not yet happened is by observing interference (say in double-slit setup).
Another uneasiness with the statement of the problem is that coherence is meaningfully defined for ensemble of particles and not for single particle. And yet I find explanations describing single dust particle.

9. Jan 29, 2016

### Feeble Wonk

This leads into my next question, which relates to my feeling that the theory is more about logical set limitation than being a mechanism of state reduction.
If, for the sake of discussion, multiple photon interactions are required to localize a cosmic dust particle in a region of otherwise vacuous space, doesn't the uncertainty regarding the potential paths of the involved photons come into play somehow? I realize that this might sound like a trivial (or even ridiculous) concern, but it speaks to the wholistic ensemble nature of theory. To me anyway, it seems like the theory is simply saying that IF the quantum state is such that a photon's path passes through this point in space, AND IF another photon passes through almost exactly the same space at approximately the same time, AND IF yet another photon from a different direction also passes through that same spot, then that point in the relative space time is localized (essentially by triangulation). And IF, further, the quantum state is such that a dust particle is at that point of convergence, then the quantum state must be reduced to those potential states that reflect that dust particle's position accordingly.
It doesn't feel as if the decoherence "caused" the localization. It seems more like a logistical limitation of the quantum state ensemble set that would be possible IF that set of circumstances were manifest.

10. Jan 29, 2016

### Staff: Mentor

That is the point. And the excitation depends on the detailed state of the dust particle. Different states react differently to the photon.

11. Jan 29, 2016

### Staff: Mentor

Last edited by a moderator: May 7, 2017
12. Jan 29, 2016

### Staff: Mentor

You need to understand the role of Hamiltonians in QM. The Hamiltonian has position parameters, the quantum objects do not have actual positions prior to decoherence (it emerges from that process), hence 'And IF, further, the quantum state is such that a dust particle is at that point of convergence' makes no sense.

Thanks
Bill

13. Feb 1, 2016

### Feeble Wonk

Apologies. My post was clumsily phrased. I'm hoping that you might be able to help me through this.
When you say that "quantum objects do not have actual positions prior to decoherence", do you mean that the dust particle in question has a "less than definite" location in the region of space-time vacuum that we are considering? Or, are saying that the region of space-time has no "locational" information at all, prior to decoherence?

I suppose it might be helpful if I tried to explain my (undoubtedly flawed) mental image of the process. I'm picturing a region of space-time in which the "historical" quantum state has already determined the existence of our dust particle via some previous physical event/interaction. However, the "precise" location of the dust particle is undetermined. It's this mental image that motivated my earlier question about the significance of time relative to the sequence of localizing interactions.

Last edited: Feb 1, 2016
14. Feb 1, 2016

### Staff: Mentor

I am saying, as I have said many many times in my posts, the QM formalism is silent on such things.

Thanks
Bill

15. Feb 2, 2016

### Feeble Wonk

Sorry Bill. I know this sort of discussion is distasteful for you. I'm aware of your ignorance ensemble inclinations, and I'm aware that the formalism is silent as to what is "really" going on during the measurement/observation/interaction event.

Also, while the mathematics is sadly beyond my understanding, I fully concede that the process of state decoherence limits potential quantum states in ways that prevent classical superposition from being observed. What I'm not clear on is why decoherence is thought to be the "mechanism of state reduction", triggering the manifestation of a proper mixture from an improper mixture (assuming that proper mixtures can actually be thought of as having independent existence). And, as is always a challenge, I'm trying to understand this concept in terms of what it implies about the nature of physical existence on an ontological level. I'm aware that that bleeds into a philosophical discussion, but I'm trying to tread carefully in an effort to avoid that.

Last edited: Feb 2, 2016
16. Feb 2, 2016

### Staff: Mentor

There's no particular reason why you should be clear on that.
Decoherence tells us why quantum mechanics, as a theory about measurement results, correctly predicts that we do not observe macroscopic superpositions. It's a bigger leap than many are willing to take to accept it as the "mechanism of state reduction".

17. Feb 3, 2016

### jlcd

Feeble. Decoherence is not mechanism of state reduction.. they are separate.. but we tend to joint them together because they occur together. Isn't this your understanding too? My thoughts are correct.. ain't it? Try asking atyy too because he is the highest authority on decoherence...

18. Feb 3, 2016

### Feeble Wonk

There are many outstanding authorities on PF, which is why I come here for answers, though I all too often struggle to understand those answers. But yes, I would definitely highly value atyy's input on virtually any subject.
The way I envision it, the decoherence essentially defines what the reduced quantum state "might" be... or at least, what it CAN'T be. So, I guess that would explain why they "occur together", as you said. But, my understanding of the phenomenon is severely limited due to my mathematical inadequacies.
I am still very curious about the questions I've asked above. Sadly, I realize that it's difficult for the PF physicists to give me the answers I'm looking for because I need the explanation to use actual language rather than formulas, and I'm afraid that this concept is one of those that doesn't translate easily into words.

Last edited: Feb 3, 2016
19. Feb 3, 2016

### jlcd

I already spent years trying to understand it including the math. Improper mixture to proper mixture is either collapse or many worlds or bohemian trajectories. So what is your specific question.. so I'd know what are the basic questions asked by laymen (or advanced laymen). What specifically are your questions. Pls. enumerate. I should be able to answer them. If I can't. I'll ask them and know the answer myself.

20. Feb 4, 2016

### jlcd

By "mechanism of state reduction" does it include decoherence causing mixed states to have each branch as separate worlds or does "mechanism of state reduction" simply meant collapse? What is the standard usage of it? Anyone?

I'm interested in your question what is its ontological sense... and relating to thinking what you were saying about dust particle localization.