Decoherence over long distances - thought expt

[Karl Coryat]

Suppose 10,000 years from now, we perform the ultimate macroscopic superposition experiment: We're somehow able to prepare an entire star, complete with a large orbiting planet, as a pure quantum state. It is surrounded by a black shell, of radius 1 light-day, which is maintained at a temperature of 100mK. All other measures are put into place to ensure that the system remains a closed system.

Suppose then we go up to the shell, open a tiny porthole, and observe the (now open) system with a telescope. Would the decoherence of the star/planet (in our reference frame) be rapid, resulting in the observation of a classical situation immediately upon opening the porthole? If so, wouldn't this create the appearance of a violation of locality? Or, would it take the system a day or more to "know" that it is open to an outside environment, and if so, what do you suppose that would look like through the porthole?

I'd be interested in hearing how an answer might be arrived at mathematically (in terms of diagonalization etc.). Thank you.

 

[Karl Coryat]

For those interested: I e-mailed this to Dieter Zeh, and he was kind enough to respond. Here is what he said:

It is not necessary to think of an "environment" as being outside the system under consideration. If you are observing just one or a few variables (as you usually do - they may be collective variables), and these are entangled with other (uncontrollable) internal degrees of freedom, you cannot observe any superpositions of quasi-classical states.

However, a purely internal environment is usually sufficient but not realistic. For example, the shell in your thought experiment may be even colder than 100 mK - it would be sufficient for decoherence if it were an absorber at absolute zero. So you would instead have to assume an exactly reflecting shell, and this is what is in fact done in experiments with laser radiation in a cavity (Haroche et al.). There you can indirectly observe the slow decoherence of "mesoscopic Schroedinger cats" (superpositions of different quasi-classical fields). If you would now use your "telescope" to directly observe the fields, this would again destroy the superposition. So in your experiment just one photon scattered off the star or planet is enough - regardless of whether it were observed, absorbed somewhere else, or still floating around in the cavity or the universe.


Dieter Zeh

I read this as saying: If the shell were a perfect reflector, the pure quantum state of the star/planet hypothetically could be maintained; but as soon as the porthole is opened, the superposition is destroyed -- and quickly, from the observer's perspective. Is that accurate? Is there any experimental or mathematical support that decoherence should be observed to occur right at the boundary with the environment, even if there is a great distance between the environment and the object being observed?