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quddusaliquddus
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the title says it all.
dav2008 said:The monkey would fling feces at the detector screen.
Well, it's in superposition with a monkey that already has.Parlyne said:Is that before or after it types the collected works of Shakespear?
shahzk said:i don't believe what mumeishi said is true... the delayed choice quantum eraser experiment shows that even after measurement, once the "information" is erased the interference pattern re-emerges... so the awareness of the information is in fact what collapses wave functions. the monkey question is quite significant then... i wonder if there has been an experiment to find out.
Joncon said:So, you do a double slit experiment and see an interference pattern. Fine, you don't know which slit each photon passed through, all makes sense.
Now you add polarizers behind each slit, orthogonal to each other, and do the experiment again. Each photon can only pass through one slit so the interference pattern disappears. Do YOU know which slit each photon passed through? No, of course not. So why does the interference pattern disappear if you have no knowledge/awareness of the photons' paths?
In my opinion, I think it would be appropriate to say that the particle *has* traveled both paths, at least to the extent that that would be the quantum mechanical description of the state after the slits. In particular, even if there are which-way detectors sitting by both the top and bottom slit, the passage of the photon through the slits—according to strict quantum mechanics—simply entangles the path of the particle with the state of those detectors.
StevieTNZ said:The interference is still there. It is just hidden. When do you 'erasure of information', two fringes appear.
The reason there is still disagreement as to what constitutes measurement is that it makes no experimental difference according to quantum mechanics. The way QM works under the Copenhagen interpretation is that you have to split the world into two parts, the “observer” or measurement device, and the “observed” or the particles you’re measuring.questionpost said:I don't get this argument: it's already been shown that something that supposedly has no consciousness, such as a simply machine, can collapse a wave function by means of measurement the same as any other measurement.
lugita15 said:The reason there is still disagreement as to what constitutes measurement is that it makes no experimental difference according to quantum mechanics. The way QM works under the Copenhagen interpretation is that you have to split the world into two parts, the “observer” or measurement device, and the “observed” or the particles you’re measuring.
The measurement device is assumed to behave classically. The particles in the observed system are in a superposition of states described by the wave function which keeps evolving until it interacts with the classical measurement device. The question is where to draw the line. You could consider a photon to be the observed system and an atom to be the measuring device, but you can also consider the photon-and-atom system as in a superposition of states, and take a Geiger counter to be the measurement device. So there is this von-Neumann chain, going from elementary particles to Geiger counters to human beings, and we have to decide where to cut it off.
Von Neumann proved in his famous "Bible" of QM that regardless of where you cut the chain, you would get the same experimental results. But he argued that wherever you cut the chain you have things made out of particles on each side of the cut, so there’s no principled way to place the cut in the middle. So he decided that you should place the cut between the human mind and the human body, because he believed that the mind is non-physical. Hence "consciousness causes collapse" was born. Nowadays, the most popular view is decoherence, where there is no real collapse, it's just that when you have a large number of particles in the environment interacting with the system, the wave function becomes smeared out and looks like it has collapsed. So decoherence gives us a reasonable place to cut the chain, when the number of particles involved reaches a critical number so that interference effect become negligible.
questionpost said:Yeah, an atom *does* technically measure a photon if it absorbs it. You draw the line between measurement and no measurement. If it doesn't collapse it's wave function, it's not a measurement. So if you shine a photon through a prism and the prism doesn't absorb it, then what you get are two polarization angles, the atoms didn't absorb it, so it's still subject to superposition.
A ruler can measure length, does that mean it has consciousness?
StevieTNZ said:Wouldn't an atom have probabilities to whether it absorbs it or not (superposition)? Unless, of course, it has probability 1 of absorbing the photon - then there is no measurement issue there.
It's not quite so clear-cut. If the atom absorbs the photon, the atom goes into an excited state. If it doesn't absorb the photon, the atom stays unexcited. Suppose the photon can either go left or right, and to the right there's a huge atom that's guaranteed to absorb it if it goes there, so we have two possibilities:questionpost said:Yeah, an atom *does* technically measure a photon if it absorbs it. You draw the line between measurement and no measurement. If it doesn't collapse it's wave function, it's not a measurement.
lugita15 said:It's not quite so clear-cut. If the atom absorbs the photon, the atom goes into an excited state. If it doesn't absorb the photon, the atom stays unexcited. Suppose the photon can either go left or right, and to the right there's a huge atom that's guaranteed to absorb it if it goes there, so we have two possibilities:
State 1. Photon goes left/atom unexcited
State 2. Photon goes right/atom excited
So instead of thinking of the system as the photon which is in a superposition of left and right states until it's wave function is collapsed by the interaction with the atom, we can instead think of the photon-and-atom system which remains in a superposition of states 1 and 2 until some third object, say a Geiger counter, makes a measurement of the system. But of course, we can also talk about the photon-and-atom-and-Geiger-counter system, whose wavefunction remains in a superposition of states until...
I think you get my point. As Von Neumann proved, in quantum mechanics there is no experimental way to determine the dividing line between measurement device and measured system, and thus no way to find out at what stage of complexity or for what number of particles the wave function "really" collapsed. So the question of where you cut the Von Neumann chain remains. You could cut it at the "top", leading to consciousness-causes-collapse. Or you could not cut it at all and consider the whole universe to be the "observed" system, whose wave function never collapses, leading to the Many Worlds interpretation. Or you could use decoherence to find how high you can make the cut without smearing out interference effects too much.
questionpost said:If an atom absorbs a photon, the atom technically measures it. If it doesn't, it's not a measurement.
As I said, you can take the wave function of the photon-and-atom system, and this wave function will not collapse due to any interactions between the photon and the atom. It will only collapse if you use a third object to measure the system.questionpost said:If an atom absorbs a photon, the atom technically measures it. If it doesn't, it's not a measurement.
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