# Why does observation collapse the wave function?

In one of the first lectures on QM we are always taught about Young's experiment and how particles behave either as waves or as particles depending on whether or not they are being observed. I want to know what about observation causes this change?

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This is actually an unresolved question in QM. There are many interpretations of QM. Some attempt to define what constitutes measurement and what causes collapse. In some interpretation, wavefunctions never collapse. In some others, wavefunctions are not a good enough description for quantum systems. The canonical interpretation, Copenhagen interpretation, simply dodges this question. –  C.R. Sep 1 '12 at 12:49
I suggest you read multiple different introductions from some of the standard books: as with so many things in physics, there are different equivalent ways of looking at wave-function collapse. The actual physics of these points of view is what they have in common, and you only learn that by reading about all the ways. – For me, Feynman's description remains the best, if I had to point out a single one. –  leftaroundabout Sep 1 '12 at 22:07

In the following answer I am going to refer to the unitary evolution of a quantum state vector (basically Schrodinger's Equation which provide the rate of change with respect to time of the quantum state or wave function) as $\mathbf{U}$. I am going to refer to the state vector reduction (collapse of the wave function) as $\mathbf{R}$. It is important to note that these two processes are separate and distinct. $\mathbf{U}$ is understood well and can be modelled accurately with the equations of QM, $\mathbf{R}$ is not well understood and it is some physicist's thoughts that QM will need to be modified to incorporate this state vector reduction process.

There is much to say about the $\mathbf{R}$ process, but I will address your question directly; basically "is it consciousness that reduces the state vector/collaspes the wave function?". Among those who take this explanation seriously as a description of the physical world, there are those who would argue that - as some alternative to trusting $\mathbf{U}$ at all scale and believing in a many-world type view point - that something of the nature of this $\mathbf{R}$ process occurs whenever the consciousness of an observer becomes involved. E. Wigner once sketched a theory of this kind in Nature in the 60s. The general idea was that unconscious matter or inanimate matter, would evolve according to $\mathbf{U}$, but as soon as a conscious entity becomes physically entangled with the state, then something new comes in and actually reduces the state (some $\mathbf{R}$ process).

The posit that it is consciousness that causes this collapse is very hard to debunk, due to the very nature of this type of argument. However, if you consider the following example, it should be clear that this picture is far from complete; and that this argument for consciousness causing the $\mathbf{R}$ process is not sufficient. Consider the weather, the detailed weather patterns that occur on any planet, being dependent of chaotic processes, which much be sensitive to numerous individual quantum events. if the $\mathbf{R}$ process does not actually take place in the absence of consciousness, then no particular weather pattern could ever establish itself out of the morass of quantum-superposed alternatives. Can we really believe that the weather on these planets remain in complex-number superpositions of innumerable distinct possibilities - just some total hazy mess quite different from actual weather - until some conscious being becomes aware of it and then at that point, and only that point the superposed weather becomes actual weather? I don't think so - do you?

Personally I think we can expect some amendment to QM if this process $\mathbf{R}$ is ever going to be sufficiently explained. One candidate model to explain this reduction process is the gravitationally induced state-vector (and its decedents). There are strong reasons for suspecting that the modification of quantum theory (QT) that will be needed, if some form of $\mathbf{R}$ is to be made into a real physical process, must involve the effect of gravity in a serious way. Some of these reasons have to do with the fact that the very framework of standard QT fit uncomfortably with the curved-space-time that GR demands. Yet most physicists seem reluctant to accept that it maybe QT that needs adjustment to facilitate a successful union with GR. Roger Penrose describe a new model (based on other candidates) in his book The Shadows of the Mind (not an easy read!) that uses a quantum gravity model to explain the elusive quantum process $\mathbf{R}$ - this is well worth a read if you want a better understanding of this mysterious process and it implication on human consciousness.

I hope this helps.

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An electron, indeed any particle, is neither a particle nor a wave. Describing the electron as a particle is a mathematical model that works well in some circumstances while describing it as a wave is a different mathematical model that works well in other circumstances. When you choose to do some calculation of the electron's behaviour that treats it either as a particle or as a wave, you're not saying the electron is a particle or is a wave: you're just choosing the mathematical model that makes it easiest to do the calculation.

The next question is OK what is an electron then? At the moment our best description is that the electron is an excitation of a quantum field. Using quantum field theory allows us to calculate the behaviour of electrons whether they happen to be involved in particle-like or wave-like interactions. This doesn't mean that the electron is a quantum field, and we'll almost certainly replace quantum field theory by some even more complicated e.g. some future development of string theory.

The collapse of the wavefunction is a separate issue, and one that has generated much debtae over the years. I think the general consensus is that the collapse of the wavefunction is a manifestation of a more general process called decoherence.

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The wave-particle debate should have been put to rest eons ago. As John Rennie said, field theory is how modern physics is done. It is unfortunately, in classes students are still being taught the Bohr Model etc under "Modern Physics". Go figure. –  Antillar Maximus Sep 1 '12 at 14:22
I think that the "students debate" is about use of "strict vision of the quantum field theory" OR a "not so strict": an object (particle) not need all the infinite space to describe it, it is sufficient (see Colder picture below) a "cut of space", like a circle in torn of the center of the wave packet. Each particle have your "local field", describing it... It is like to compare a sprite with the whole-screen image, to describe a little thing. Next, ok, another debate/consensus is decoherence. –  Peter Krauss Sep 17 '12 at 11:20

# Towards a better picture of the duality

On Young's double-slit experiment the wave-particle duality (one by one photon) is more a problem of "picture of the model" than a philosophic one: see Y. Couder interpretation, by your self (!),

The quantum particle HAVE a location, there are only a limitation in choose a good pictoric model when you is constrained by "wave or particle" picture options: Couder demonstrates that a good picture, of an "intermediary wave/particle object" model, exists!

Imagine a "localizable object" that haven't a well-defined boundary, but have a well-defined distance-limit (~lambda) to interact with obstacles (other objects).

On this little video you see the objects one by one, changing (or not) the rectilinear trajectory by the "oscilating interaction" with the (double-slit) obstacle, not by the after-obstacle-screen as "observer".

There are an article online about the experiment.

PS: of course, if a observation constitutes measurement before the screen, it will be interfere on result, changing the interference pattern at the screen.

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