Augmented Double-Slit Experiment

[scikidus]

I don't know enough about QM to answer these questions for myself, so I'm going to post them here.

In all of the following setups, we start with a the normal double-slit experiment assembly, along with detectors, and a computer (to store information about the detectors). In all cases, I want to know what will appear on the interference screen (which the photons are hitting).

1. The detectors are not plugged in.

2. The detectors are plugged in, the computer records the data, and the computer is destroyed.

3. The detectors are plugged in, the computer records the data, and the computer is sealed in a block of cement and hidden away.

4. The detectors are plugged in, the computer records the data, and the researchers observe the interference screen without ever checking the computer.

5. The detectors are plugged in, the computer records the data, the researchers observe the interference screen, and then check the computer.

6. The detectors are plugged in, the computer records the data, some researchers observe the interference screen without ever checking the computer, and researchers observe the interference screen, and then check the computer.

I think that's enough scenarios for now.

 

[Nelson]

I'm assuming you mean detectors at the slits and not the screen, in which case:

1: If the detectors do not actually interact with the photons/electrons (whatever you want to use) then there will be a typical interference pattern on the screen.

2-6: The interference pattern is destroyed, as the measurement by the detector causes the wavefunction of the particle to collapse at the slit. It is irrelevant whether or not someone checks the results before they check the screen, or if some people do and some don't.

 

[scikidus]

I'm assuming you mean detectors at the slits and not the screen, in which case:

1: If the detectors do not actually interact with the photons/electrons (whatever you want to use) then there will be a typical interference pattern on the screen.

2-6: The interference pattern is destroyed, as the measurement by the detector causes the wavefunction of the particle to collapse at the slit. It is irrelevant whether or not someone checks the results before they check the screen, or if some people do and some don't.

So a non-conscious entity can collapse a wave function? Can you explain that in greater deatil?

 

[Nelson]

The collapse occurs because of interaction with the outside world. This is commonly referred to as decoherence. There are some good resources online. These concepts are not by any means simple and I don't really want to gloss over them, so if you feel like reading a bit more, I would start here: http://en.wikipedia.org/wiki/Quantum_decoherence , and if you feel like some heavier reading: http://arxiv.org/ftp/quant-ph/papers/0306/0306072.pdf . The Zurek paper is great because it is not too heavy on the mathematics. Now for my summed up explanation.

To quote the wiki page decoherence is best explained as:

decoherence is the mechanism by which the classical limit emerges out of a quantum starting point and it determines the location of the quantum-classical boundary. Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way.

In this way it can be any interaction with the environment that causes the wave function to collapse, including a measuring device that never even ends up being read by humans. Decoherence occurs the moment the particle interacts with the classical environment, and it does not depend on when the measurements are read by any conscious observer. If this were the case, then the question would become what level of consciousness is significant. Does the observer need to have a full understanding of the results they are seeing or is a young child sufficient? If a child is sufficient, then why not a chimp, and if a chimp then why not a slightly less intelligent animal? And so on, and so on until we come down to the question of why isn't the interaction with the measurement device itself sufficient? You can find some stuff online about these sort of questions here: http://en.wikipedia.org/wiki/Quantum_mysticism and here: http://en.wikipedia.org/wiki/Measurement_problem . However, here we are getting into what it means to be conscious and that is more philosophy than physics.