Double Bubble Slit Experiment

Most recent answer: 12/24/2012

Q:
what will happen if physicists fused the double slit experiment and the bubble chamber detector. i mean like they put the double slits in the bubble chamber and ire electron at them. What would we see?
- Bassem (age 18)
ramboland
A:
A bubble chamber is designed to leave a track showing which way a particle went. If the bubbles were tightly spaced enough to indicate which slit each electron goes through,  that's exactly what's needed to destroy the interference pattern between the transmissions through the two slits. The transmission pattern would just be the sum of the ones for the separate slits. If, on the other hand, the bubbles were sparse and the slits close together, the track might not give good information on which slit each electron went through. In that case some of the interference effect would remain.

I'd be very surprised if any two-slit interference remained in the bubble chamber, but Lee isn't so sure.

Mike W. (w. suggestions from Lee H)

(published on 12/24/2012)

Follow-Up #1: which-way information and decoherence

Q:
you mean the pattern would be destroyed on the basis that we would have know its position or is there a physical relation between the destruction of the pattern and the bubbles?
- Bassem (age 18)
ramboland
A:
The human awareness plays no special role in the process, so far as we know. What matters is that the physical situation differs, depending on which slit the object went through. To put it more formal language, let's start with a state that has some initial liquid state, |Li>, and some combination of electron states going through each slit, (|A>+|B>)/21/2.

|Li>(|A>+|B>)/21/2

What happens over time?  If the liquid is in a separate tank away from the slits, you just get

|Lf>(|WA>+|WB>)/21/2

where the waves WA and WB coming from the slits show interference.
If the slits are immersed in the liquid, the final state becomes:

(|LfA>|WA>+|LfB>|WB>)/21/2

where |LfA> and |LfB> are different liquid states for different electron paths.  Now even if the two parts of the wave overlap in space, they're part of distinct quantum states and can no longer interfere. We say that these parts of the electron's state have "decohered".

Later on, if you include the quantum state of your brain, it's part of this decoherent result, but the interference is lost already as soon as some sort of practically irreversible difference occurs for the two paths.

Mike W.

(published on 01/24/2013)