There is no such thing as 'collapse of the wavefunction' - Feynman

[my_wan]

I can't ascribe to the idea that QM is not physics. The empirical data is king of all questions, and QM handles that fine. I do suspect (hope) that we are missing some basic connection that the formalism doesn't include, but that doesn't make it not physics.

I do find the "collapse of the wavefunction" hard to take seriously. Not the least of which because it's not that hard for another observer to be forced to consider me entirely in a superposition of states. Feynman emphasized the "final amplitude for the event" being a summation of a long series of amplitudes. Statistical ensembles are another construct where a large number of 'possible' states are superimposed as if a single state. Classically only one of these possible states represents the actual state of affairs, but in QM we are unable to reduce it to an actual state, like dice that takes a particular route to a particular outcome. So even if we take the wavefunction to be physically valid, it must still be formally smeared over a large ensemble of possible states without detailed knowledge of micro-states.

The fact that, in QM, possible states interfere with actual outcomes perhaps indicates some level of reality of a wavefunction. But that does not mean that the ensemble, which superimposes all possibilities over one another, is the actual state, any more than dice take all possible routes to an outcome. Only that for the dice, at the micro-level, there exist a waveform representation, rather than classical parts. There's still something quiet weird if the world we know is a persistent projection from something resembling Hilbert space, even if our model of it is a superposition of ensembles for which only a singular subset of these possibilities is real.

 

[JDługosz]

Years ago, I supposed and hoped that experience with trying to make "quantum computer" technology would shed light on the issue. Trying to keep an entangled system from collapsing is pretty much exploring the boundary conditions.

The underlying explanation appears to be "http://en.wikipedia.org/wiki/Decoherence" ". Basically, you see the superposition only if you make a measurement of the entire entangled system. If you measure part of it, you see a pure state. Once the particle interacts with "the environment" it rapidly diffuses to billions of atoms in a complex way, yet you only measure one or two particles.

Also, notice the experiments of a "quantum eraser" will restore the interference pattern.

The details of decoherence may be hard to grasp, but the idea is clear: you must measure the whole system. It's the same thing that gives you entanglement in the first place.

With a slight lay knowledge of QM, you can see roughly that the tensor product space contains vectors for both entangled parts, and you are measuring one and throwing away the other. The solution to the product space gives you a correlation between the two entangled particles, but does not state which specific state either particle is in. You cannot separate it into two separate particles with superposed states. So, the "rules" are that if you measure one particle you get a pure state. Saying it that way doesn't solve the measurement problem, but states it. But it is not mysterious: it states exactly when you see a collapsed state, and how that is contextual, and why collapse spreads like an epidemic.

 

[andrewr]

lol.

I am pretty sure the meaning of "collapse of the wavefunction" includes something that even Feynman doesn't dispute -- and that is that the probability of finding an item in a location it *wasn
t* found in (eg: after the experiment is over) is no longer predicted accurately by the probability amplitude previously calculated. That is, the probability field is no longer associated with that item.

A probability interpretation indicates where something *can* be found (possibly) -- but the point of using the word "collapse" is that something that existed before (a prediction of probability) is no longer valid.

looking at previous posts, isn't it obvious that Feynman's interpretation can be made indefensible simply by taking an overly-literalist interpretation of his statement ??
eg: If the probability field is not even associated with the photon, then it obviously can't predict anything about the photon. So, clearly, since Feynman means something by his words...

associated:
1. To join as a partner, friend, or companion.

He can only mean what he says as a matter of degree -- not as an absolute. Clearly, the wavefunction is a companion of the path of the photon, and thus loosely associated with the photon's journey (even a super-set partially overlaps any individual case.)

Funny, even the QM book I just bought from the college bookstore uses the phrase... "Wavefunction collapse" ...
Of course the author is from our competitor weed college, oops... I mean REED college REED college...

 

[jVincent]

The collapse of the wave function is real, Just like the collapse of the probability in a coin toss experiment is real. People just get confused about what it means, and start talking about it like it's some magical operation.

Consider the coin toss. At one instance the probability of getting heads or tails is 50/50 and the very next instance, there is a 100% probability that the coin ended up heads. It's magic, it weird and it's pretty much boring if you don't try to confuse people about what you mean.
What Feynman is talking about is that people confuse the wave function (a tool to predict probability) with the actual photon, and that the student should keep the distinction in mind.

A coin isn't in a state of 50/50 before we toss it, but this is the state we need if we want to describe it.

 

[Dmitry67]

Collapse can't be real, as is shown in the Wgner's friend experiment.

 

[Phrak]

I'll have to paraphrase something written by Feynman, as I can't find nor recall the source. It goes something like "If you try to understand it, it will drive you crazy." I know the word 'crazy' was included.

This worked for him. Don't try to see behind the curtain, but fill in some gaps, and call it QED.

 

[Ken Natton]

I’m not so sure if there is much point in answering your assessment Phrak, the thing that I am certain of is that Feynman doesn’t need me to defend him. However, I do find your assessment somewhat cursory and maybe not fully aware of the history – I’m thinking particularly of the period after the war at the time of the Shelter Island and Pocono conferences. I don’t know if you’re familiar with it Phrak, but there is a website called ‘Web of Stories’ that includes lots of fascinating interviews with some of the prominent people themselves. I have not found anything of Feynman there (although there are plenty of Feynman clips on YouTube), but one of the people I have found most fascinating to listen to on Web of Stories is Freeman Dyson. He is the man that demonstrated to everyone else that Schwinger and Feynman were getting similar results because they were really doing the same thing by different routes, founded on the very different personalities of the two men. In the series of Freeman Dyson videos, check out no 58 ‘Richard Feynman and his work’ but more particularly the series from no. 69 'Summer school in Michigan; Schwinger’s talks' to no. 76 'Linking the ideas of Feynman, Schwinger and Tomanaga. '

Certainly, Feynman tended to live on his instincts and fly by the seat of his pants. Dyson mentions how Feynman knew nothing of Quantum Field Theory and didn’t want to know. He could get to the same point by a much easier route. But far from not looking behind the curtain, Feynman was the arch peeker behind the curtain specifically to short-cut the systematic approach, and he did a great deal more than ‘fill in some gaps’.

 

[Phrak]

Ken Natton. Yes, 'filling in the gaps' was a poor turn of phrase.

The subject here is wavefunction collapse. Feynman chose not to be distracted by this question. This is the curtain I refer to.

"I think it is safe to say that no one understands Quantum Mechanics. (Richard Feynman)
One does not, by knowing all the physical laws as we know them today, immediately obtain an understanding of anything much.
[...]
The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that." Richard Feynman, Quantum Mechanics"