I wonder what were the main experiments that led people to develop the concept of wave function collapse? (I think I am correct in including the Born Rule within the general umbrella of the collapse paradigm.) Are there any instances where cases once thought to be examples of collapse have since been explained as the normal time-evolution of the wave function?

EDIT: I'm going to have to make an objection to Ron Maimon's very excellent answer about particle tracks as evidence of collapse. I've been waiting for someone to suggest what I personally have always considered the prototype of the wave function collapse, namely the appearance of flecks of silver on a photographic plate when exposed to the light of a distant star. This has the essential elements of collapse in a way that ordinary photographic exposures do not. The mere appearance of dots on a photographic plate does not signal the collapse of anything: it is readily explainable as a consequence of the rate of silver-bromide reduction being proportional to light intensity. It is only when the intensity becomes so very low that the time taken to accumulate enough energy for a single conversion becomes unreasonable that we must consider the explanation of wave function collapse.

The tracks in the cloud chamber do not demonstrate this phenomenon since the energy needed for the creation of the tracks is already available in the supersaturated gas. It is not necessary for the incoming particle to supply energy for the creation of the track, so there is no need to collapse its wave function. The straightness of the tracks is explained by Mott as an ordinary consequence of time-evolution of the wave function. There is no experimental proof that a single "particle" cannot be responsible for multiple tracks in the cloud chamber, because the tracks are not tagged according to which particle created them.

  • $\begingroup$ No, the Born rule produces probabilities, which are then open to any number of interpretations. Statistical, Many Worlds, you name it. The Born rule certainly does not put you into a collapse interpretation. If you go the history of Physics route, there are quite different rules of engagement than if you do Physics. The Questions you ask here are too leading for the History of Physics paradigm, at least as I see it. $\endgroup$ May 22 '11 at 12:22
  • $\begingroup$ @Peter Morgan It's not that important to me which way you go with it, whether you collapse the wave function or you go with Many Worlds: the experiments we are trying to explain are still the same experiments. I could have just as well asked "what were the phenomena that led people to come up with Many Worlds, or the Born rule...they're still the same phenomena aren't they? $\endgroup$ May 22 '11 at 13:24
  • $\begingroup$ @Marty, They may be the same phenomena, but the models, descriptions, and explanations that we use can be very different. There was a strongly empiricist flavor to Physics in the early days of QM, which loosely can be characterized as "don't ask for explanations", just look for effective models and descriptions of experiments, and I think you're missing an appreciation of that in your Question. There has been something of a revival of the demand for explanation, since the 1950s, perhaps, but there is no well accepted account of precisely how one should balance that with empiricism. V.Delicate! $\endgroup$ May 22 '11 at 14:12
  • $\begingroup$ The observation of the quantum Zeno effect makes wave function collapse difficult to deny. $\endgroup$ May 22 '11 at 19:00
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    $\begingroup$ In regard to your last question: every instance of what used to be thought as wavefunction collapse has since been explained as normal time-evolution. That is because wavefunction collapse itself is just approximate description of an actual real process of decoherence (in the limit of instantenous and complete decoherence). But the first part of the question about historical experiments is still interesting. $\endgroup$
    – Marek
    May 22 '11 at 21:47

Short answer: there is a review paper written only five years ago

Experimental motivation and empirical consistency in minimal no-collapse quantum mechanics, Maximilian Schlosshauer, Ann. Phys. 321, 112-149 (2006)

Here is a cite from outlook: “We have analyzed three important experimental domains —namely, SQUIDs, molecular diffraction, and Bose-Einstein condensation [...] These experiments have provided powerful examples for the validity of unitary Schroedinger dynamics and the superposition principle on increasingly large length scales.”

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    $\begingroup$ I'm sorry to say I don't find this answer helpful. $\endgroup$ May 22 '11 at 18:53
  • $\begingroup$ @Marty Green: Why? $\endgroup$ May 23 '11 at 10:07
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    $\begingroup$ I don't mean to be difficult, but since you ask: 1. I don't have access to Annalen; 2. I asked for specific examples of wave function collapse and I can't see any in your answer; 3. your examples don't seem to fit the category of cases that led people to develop the concept; 4. I don't really understand the examples referred to in the paper. On the positive side, you do seem to be arguing in favor of time evolution vs collapse, but it's not helpful because you don't explain a specific mechanism in any case. $\endgroup$ May 23 '11 at 12:59
  • $\begingroup$ @Marty: Free online version of the paper is here arxiv.org/abs/quant-ph/0506199 but afraid it won't make you happy. My main reason for the cite was to show that there is quite solid view that there are no any experimental evidence of collapse at all. Yet I am not demanding that in future some experiments may not to prove collapse. $\endgroup$ May 23 '11 at 14:14
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    $\begingroup$ @Alex Thanks Alex for dealing with objections 1 and 4. Interesting paper. But re objection 2: these are not examples of wave function collapse: they are examples where collapse doesn't occur. They are nice examples but they're not what I asked for. I'm going to put up a new question where I ask for a specific mechanism to explain the flecks of silver on a photographic plate. I hope you'll check it out. $\endgroup$ May 23 '11 at 16:02

My distinct impression is that wave function collapse is due to Dirac, and that it was not very directly based on experiment. I am addressing the question historically, as it was asked. Weyl's book although an attempt to axiomatise QM, and written based on a seminar where he, I think, had the help of Debye and Schrodinger, does not have this concept nor pose it as an axiom. Dirac's book, published a little later but based on lectures he had been giving around the world for more than a year, did.

Dirac explains in his book very clearly that on which he is basing his assertion, the wave collapse, and it isn't experiment, it is « The Principle of Physical Continuity.» Since in polarisation experiments the observed fact is that only an entire photon is observed on the other side of the polariser, and never part of one, he deduces logically, using the principle of continuity, that the photon must be in the eigenstate corresponding to that eigenvalue. My copy of his book is not to hand so I can't give an exact quote...or can I?

  • $\begingroup$ This is correct as far as Dirac goes, but the "principle of physical continuity" is just the statement that if you measure the spin, and measure again, you don't get a different answer. but I pretty sure that Heisenberg knew collapse earlier, along with Bohr and Born. When Heisenberg studied the problem of particle tracks, ionizing atoms one after another, he showed that the multi-dimensional $\endgroup$
    – Ron Maimon
    Jan 17 '12 at 6:01

I believe the notion of collapse of the wavefunction is most explicitly derived from the resolution of the 1929 Mott paradox: http://en.wikipedia.org/wiki/Mott_problem .

The Mott problem considers an electron in a spherically symmetric wave, washing over a bunch of atoms. This electron will ionize the atoms, but not in a spherically symmetric way! We know that we will see the electron's trajectory through the atoms by looking at the ionization trail, as in a bubble chamber picture.

The Mott Heisenberg analysis showed that a spherical S-wave for a high energy electron travelling through a bunch of atoms indeed does lead the atoms to ionize along certain tracks, but only if you consider the full wavefunction of the atoms and the electron. The entanglement of the two means that once one atom is ionized, the next atom will be ionized on this Everett branch in the same general direction, although with some statistical spread.

The Mott/Heisenberg analysis makes it clear that the ionization "collapses" the wavefunction of the electron. This is then turned into a general principle, whereby any interaction which gives classical information which can be irreversibly amplified up for us to see leads to collapse of the wavefunction.

The many-worlds interpretation comes later, but the spark of the mathematical ideas (although not the philosophical leaps, nor the information theory aspects analyzed by Everett under Wheeler) are mainly contained in the Mott Heisenberg analysis.

But collapse was also evident from the formulation of the atom-radiation field theory in the late 1920s, where photons are real things, and energy conservation is maintained nonetheless. I don't know the exact history, and it might be correct to attribute this to Bohr, Heisenberg, Born, Dirac, or maybe even Pauli.

  • $\begingroup$ Thank you for the excellent response, Ron. The comment field is too short for me to respond to it so I have placed my response as an edit to my question. $\endgroup$ Jan 18 '12 at 3:19
  • $\begingroup$ @ron I’ve read the 1929 paper, think it’s still the best primer on how to think about QM. Question I still have though is how this aligns with Renninger’s negative result measurements? How does the lack of interaction modify the wavefunction in an irreversible way? $\endgroup$
    – JPattarini
    Oct 21 '19 at 2:49

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