Question on the Double-slit experiment

mangaroosh wrote: »

Hey guys, I just have a quick question on the double slit experiment. I've read about it a few times and have seen the "Dr.Quantum" video a few times, but there was just one thing that I'm not clear on. It pertains to the use of photons to verify which slit the "buckyballs" (is it?) go through.

I know that when photons are fired at the Buckyballs, the interference pattern, which is present when they are fired through two slits disappears, and they behave as particles which have passed through a single slit.

There are a couple of questions in one here - apologies if they are pretty basic!

I was just wondering is it the wave motion of the photon that impacts on the waves of the buckyballs, or is it the particles of each that collide?

Can the wave of the photon travel out of phase with the wave of the buckyball, except from where they collided, and cancel it out, or is it only waves of the same kind that can enhance or cancel each other out?

The typical example of the double-slit experiment involves firing electrons one at a time at a double-slit apparatus. When there are no detectors on the slits, each electron propagates through both slits, creating an interference pattern. When a detector is placed on one of the slits, the detector becomes entangled with the electrons and the interference pattern is destroyed because of this entanglement. In other words, because the detector couples to the system of electrons, decoherence occurs and the distributions of electrons behind the slit become "additive" or "what common sense would expect". This "spooky" correlation cannot be explained with classical physics. If you could fire a "classical" electron at the apparatus, it would propagate through one slit or the other, but not both. And placing a detector on one slit would not change the pattern of "classical" electrons that travelled through the other slit.

[edit]-Forgot to actually answer your questions.

1)To answer your first question, we first have to get rid of the notion of an electron being composed of both a wave and a particle. Instead, the electron is a quantum-mechanical system, described with a wave function. Two electrons would also be a quantum-mechanical system described with a wavefunction. A detector, or you, or I, is also technically a quantum mechanical system described by a wavefunction, but since we consist of many particles, our wave-function description is not coherent, and we get every-day "classical" behaviour. A detector is also made of many particles, and when an electron quantum system interacts with the detector quantum system the combine or "entangle" to form one system and the electron quantum system is no longer coherent. This causes the appearance of the "collapse" of the electron system wavefunction.

2)Similarly to the above, the detector, whether it operates using photons or some other mechanism, will entangle with the electron system. It's not interference in the sense of cancelling out. The wiki article on entanglement is good.

http://en.wikipedia.org/wiki/Quantum_entanglement