Send the measurement device through a double slit

[greypilgrim]

Hi.

Double slit experiments are being performed successfully with increasingly large molecules. Some physicists (e.g. Anton Zeilinger) believe it might work with viruses as well. Assuming it works with a system that qualifies as a measurement device (be it a virus or something else complex enough), this looks like a Wigner's friend kind of situation. Would this rule out some interpretations of QM?

 

[jfizzix]

This is a very interesting question, to say the least:)

If an object is sophisticated enough that it can store information about which slit it passes through, the act or capability of acquiring this information requires an interaction with the slit, which is essentially the same as if a measurement device was placed on the slit. The interaction itself destroys the interference pattern.

Large objects should be able to display two-slit interference, so long as which-slit information cannot be encoded into the object.

 

[greypilgrim]

Some further thoughts:

• What if the information is erased later, as in a delayed choice experiment?
• What if the information is not erased, but there's no possible way for outside observers to check it?
• What if there is a way, but the outside observer only decides not to check the information?

 

[DrClaude]

Some further thoughts:

• What if the information is erased later, as in a delayed choice experiment?
• What if the information is not erased, but there's no possible way for outside observers to check it?
• What if there is a way, but the outside observer only decides not to check the information?

In one experiment with C₇₀, they showed that the emission of thermal photons destroys the interference pattern. Outside observers are not necessary. Any apparatus big enough to record which-way information would most certainly decohere very quickly.

 

[Lord Jestocost]

As stated by Časlav Brukner in "Elegance and Enigma, The Quantum Interviews" (edited by Maximilian Schlosshauer):

"... any increase of partial information about the particle’s path will always mean a corresponding loss in visibility of the interference pattern, and vice versa. Most importantly, it is not relevant whether we read out that information. All that is necessary is for the information to be present somewhere in the universe."

 

[DrChinese]

Some further thoughts:

• What if the information is not erased, but there's no possible way for outside observers to check it?
• What if there is a way, but the outside observer only decides not to check the information?

It matters whether information is passed to the environment that could, in principle, allow which path to be determined. We know interference disappears if that occurs, even without anyone looking at it.