Superposition of Histories

[craigi]

What is the current understanding of the possibility of the nature of macroscopic superpositon of possible histories of the universe?

Hopefully, I've worded that unambiguously, but I'll try to clarify it.

I've never seen a discussion on this and I'm not entirely sure why.

We're aware of superpositon of states at the quantum level and we're aware of being able to carefully prepare superposed macroscopic states when sufficiently isolated from their environment. When we talk about this isolation, it must be entirely probablistic, instead of being anything definitive, since wave fuctions inevitably cannot be entirely contained.

I think the natural intuitive understanding, or perhaps given to us from the Copenhagen interpretation, is that the universe progresses on a classical level with individual particles moving from definite state to definite state, when they interact in some what that we deem to be a measurement. As I understand it, under this model, we're unsure as to what actually constitutes a measurement and have made little progress either on a theoretical level or on an experimental level in understanding what the process of making a measurement actually is.

So back to my question. We're aware of the possibilities of macroscopic superposition, but can we know anything about to what extent macroscopic superpositions exist for the history of the universe? Do we have any reason to believe that superpositions must be either very small in scale or very rare? Can we presume that the universe must largely progress according to our intuitive classical understanding or could there be effects that we find difficult to accept because they don't fit this world view?

Any help on this would be great.

Thanks.

 

[StevieTNZ]

You might be interested in this paper: http://vcq.quantum.at/research/research-groups/brukner-group/news/details/386.html

Not sure if that helps in any way, however.

 

[craigi]

Thanks. That is useful and it does seem to confirm my suspicion that causality as we naturally see it is, at best, an approximation.

I suspect that what they have observed actually manifests itself on much larger scales but we're still blinkered to it by forcing a classical causality onto quantum physics.

 

[bhobba]

As I understand it, under this model, we're unsure as to what actually constitutes a measurement and have made little progress either on a theoretical level or on an experimental level in understanding what the process of making a measurement actually is.

We know when a measurement has occurred - its when decoherence has singled out a basis and its now in an improper mixed state - that's basically the modern view of measurement. For example a few stray photons from the CBR is enough to decohere a dust particle and give it a 'definite' position.

The issue is the details haven't been worked out in full generality and we have stuff like the so called factoring problem.

But most (most - but not all) physicists believe that issue has been resolved - others remain - but that one is no longer a problem.

Thanks
Bill

 

[craigi]

We know when a measurement has occurred - its when decoherence has singled out a basis and its now in an improper mixed state - that's basically the modern view of measurement. For example a few stray photons from the CBR is enough to decohere a dust particle and give it a 'definite' position.

The issue is the details haven't been worked out in full generality and we have stuff like the so called factoring problem.

But most (most - but not all) physicists believe that issue has been resolved - others remain - but that one is no longer a problem.

Thanks
Bill

It is my understanding that the idea that decoherence solves the measurement problem under a Copenhagen interpretation is just a misnomer. What it actually answers is the question of why we don't find quantum behaviour more frequently in macroscopic systems but has nothing to contribute on how or indeed whether a wavefunction actually collapses under observation.

 

[bhobba]

It is my understanding that the idea that decoherence solves the measurement problem under a Copenhagen interpretation is just a misnomer. What it actually answers is the question of why we don't find quantum behaviour more frequently in macroscopic systems but has nothing to contribute on how or indeed whether a wavefunction actually collapses under observation.

I am not going to discuss what decoherence does or does not do - it gets far too heated for my liking. Check out:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

But I wasn't talking about the measurement problem - I was talking about when you can consider a measurement has occurred - that's part of the measurement problem - but it also has other aspects. It is considered by many to solve the preferred basis problem (which is roughly what is meant by when a measurement has occurred) but does not solve the collapse issue to everyone's satisfaction nor does it solve the issue of why we have outcomes at all.

Thanks
Bill