Not exactly. The collection of particles, even if you detect them one at a time, displays an interference pattern. However, if you make an effort to detect which slit each particle goes through, you see that each one behaves as a particle, like you would expect, and the interference pattern vanishes.

And this is true whether you detect which slit it passes through before or after its point of impact has been detected.

That's what's weird. A particle can be particle-like or wave-like, but not both, and the thing that determines which it is is whether or not you will look.

It's not about humans looking at it. It's about collapsing the wave function. The only to "look" at a particle is to make it interact with something (trigger some chain reaction that ultimately sends a signal to one of your senses, e.g. a photo multiplier). The moment it interacts with that thing, the wave function collapses.

This part is not mysterious.

The mysterious part is, what in the world could be the thing that acts as a wave function and then collapse to a single point when it interacts with something else? What underlying reality does this hint at?

You would think this, but the quantum erasure experiment appears to demonstrate that a photon which has been marked so as to determine its path and then subsequently randomly unmarked so as to irretrievably destroy that information once again displays interference. The delayed choice quantum eraser appears to demonstrate that this decision to either record or destroy path information may be delayed until after the reference photon has arrived at the detector, with corresponding absence or presence of interference.

It's not just about interaction causing collapse, or, if it is, a particle which has been interacted with and collapsed may be subsequently uninteracted with and uncollapse back into a wave.

http://en.wikipedia.org/wiki/Quantum_eraser http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

That part is not mysterious either - the collapse simply doesn't occur.

Say you have a photon in a superposition of state A and state B - think position. If it is in state A it hits a detector, if it is in state B it does not. If the detector is hit, it displays 'HIT' on its screen - otherwise it does not. If 'HIT' is displayed, the researcher thinks photon was in state A, otherwise the researcher thinks it is in state B.

The result of the experiment, without any wavefunction collapse, is a superposition of two states of the entire system:

State 1: Photon is in A, Detector was Hit, Researcher thinks photon is in A

+

State 2: Photon is in state B, detector was not hit, researcher thinks photon is in B.

As you can see, the fact that the researcher never sees a photon in a superposition of states a+b (he doesn't ever see the detector both lit up and not) is explained without any wavefunction collapse.

> Say you have a photon in a superposition of state A and state B - think position.

Why are you assuming the particle has got to have a "position"?

The fact that a quantum particle can interfere with itself would be experimental evidence that it doesn't in fact have a definite position.

Looking however means shooting particles at something to see what bounces off, so it must necessarily affect the system.

This is what I was wondering about: how does a particle ‘know’ it is being observed? Something has to interact with it physically, and once affected its physical properties change. Is that right?

I’m a total layman here but so fascinated with all this. Trying to get a better understanding.

Particles don't know anything. The way we see anything, even with our own eyes, is to analyze reflected photons. You can't see something without hitting it with something first and analyzing what bounces back. Obviously, if you're bouncing photons off something, you're going to affect its state. Usually this effect is negligible, but in the case of trying to watch individual particles, well, it's no longer negligible, it affects the experiment.

The problem is people tend to assume observation is passive, they think seeing something does't affect it; this simply isn't true at all. Eyes only work because the sun is continually spraying and bouncing photons off everything. Sight isn't passive at all, you need a photon source spraying photons everywhere, like a flash light.

What's strange about the doulbe slit experiment isn't that watching forces the particles to act like particles, that's the expected behavior all the time, the same as a single slit. What's strange is that when you don't watch it, you get the interference pattern indicating a wave that isn't acting like a particle. That's not at all expected and indicates the particle is going through both slits and interfering with itself. I'm a total layman as well, but that's my take on it.

That is very wrong. A particle doesn't 'know' it is being observed. However, when it interacts with other particles, the new state of both particles depends on the old states of both particles.

For a better explanation than I could possibly write here, see http://lesswrong.com/lw/r5/the_quantum_physics_sequence/.

I get that particles don’t know anything, that’s why I put it in quotes. But then again, how do we know they don’t know anything? Could elementary particles possess a form of consciousness?

For any reasonable definition of consciousness, no.