The so-called “collapse” of the wave function in quantum theory is often illustrated by the wave/particle duality. When a photon propagates through a double-slit apparatus, it behaves like a wave. Yet, if it is observed, the non-local wave is collapsed into a single localized particle. However, both theory and experiment show that this is not a clear-cut either/or distinction, as it is misleadingly presented in traditional discussions of the double slit experiment. The interference pattern is not simply there or not, but is gradually deteriorated as more information about which slit the particle went through can be extracted from the photon measurement. This suggests that, in general, there is never any discontinuous or sudden collapse of the wavefunction. All that is ever happening is that we’re pushing information around with measurement interactions in a completely continuous (unitary) way.
Not only is collapse of the wave function totally unverifiable and nonphysical, but another big problem with collapse is that it is in blatant violation of the Schrödinger equation! Any other scientific hypothesis that both violates known laws of physics and is not verifiable would normally be immediately rejected as pseudo-science. Why, then, has the notion of collapse stuck? Perhaps because one consequence of rejecting collapse would seem to be that it would lead us inevitably to the many worlds interpretation. Strange as the many worlds interpretation may be, however, it does have the virtue of being consistent with the laws of physics, at least as we know them so far.
The many worlds interpretation is often rejected as outrageous because it seems to imply that all the separate “worlds” have some actual existence, just like ours. But, it’s more like none of the “worlds” have actual existence, including ours. To make an analogy with the theory of relativity, it’s not like there are many actual velocities of the earth in space, each existing as its own separate actualized “world.” Rather, it’s that the earth has no actual objectively existing velocity at all. Velocity only has meaning relative to a reference frame, and reality does not have any privileged reference frame. We happen to observe things in the reference frame of the Earth where that velocity is zero. If we were on the Moon, things would be different. Is there really some mystery here? How is this so different from quantum theory? The original “relative state” formulation of quantum theory seems to be in line with this view, and calling it a “many worlds” theory is just as misleading as calling relativity theory a “many worlds” theory. It’s just “many reference frames” and one world. One might complain that the “one world” is a strange one, but that’s no less true in relativity theory where nothing has any objective mass, length, time, etc. The only objective realities are the four-dimensional invariants. These are almost as weird as coherent superpositions.
It is good to remember that physical theories in general are abstractions, describing a reality that is beyond our direct experience. We experience our immediate sensations of sight, sound, etc., and never directly experience the abstractions of “atoms” or “fields” which are only indirectly inferred from experience. (The same is actually true of a “chair” or “rock” as well.) These may be useful abstractions, but we never actually experience them directly, and can never know if they really exist the way we think. In fact, we don’t really know that they exist at all. We could be a brain in a vat or having a lucid dream right now. Science tries to balance the belief in some objective reality with the fact that we can never know the thing in itself. As Heisenberg wrote,
We have to remember that what we observe is not nature in itself but nature exposed to our method of questioning.
It is actually more radical than Heisenberg suggests. Consider again the double-slit experiment. A simple photon which “measures” which slit the particle went through does not actually collapse the wave function to be localized in just one region of space. It merely entangles itself with the system. Provided no decoherence has taken place so that the coherence of the original system is not washed out in many degrees of freedom of the measurement system, then there is no sense in which an irreversible measurement interaction has taken place. So one is still free to decide what will ultimately be measured. Because there has not been any interaction with a particular well-defined measurement apparatus (by which I mean a device that involves decoherence) the attributes of the system are likewise still undefined.
The above situation with regard to a quantum system is analogous to not having defined any particular well-defined reference frame in relativity. If I do not specify a reference frame for an observation of a monolith floating in space, then it has no definite well-defined value for various properties such as velocity, mass and length. Once the reference frame is specified, however, then one can meaningfully talk about definite values for these quantities. Similarly, once one specifies a particular measurement apparatus (that involves decoherence), then one can say there is a well-defined meaning to talking about certain properties. The coherence is lost and there is no practical possibility to erase that measurement choice after the interaction with the measurement apparatus and choose instead to measure a complementary observable. And all observers will agree on what is measured.
In connection with this, Pauli has this interesting statement:
Just as in the theory of relativity a group of mathematical transformations connects all possible coordinate systems, so in quantum mechanics a group of mathematical transformations connects the possible experimental arrangements.
And Bohr writes:
In neither case [of quantum theory or relativity theory] does the appropriate widening of our conceptual framework imply any appeal to the observing subject, which would hinder unambiguous communication of experience. In relativistic argumentation, such objectivity is secured by due regard to the dependence of the phenomena on the reference frame of the observer, while in complementary description all subjectivity is avoided by proper attention to the circumstances required for the well-defined use of elementary physical concepts.
Admittedly, the analogy with relativity only goes so far. In the case of relativity, the choice of reference frame is sufficient to provide a unique and definite value for physical attributes. In quantum systems, on the other hand, although the interaction with a particular decohering measurement apparatus gives a particular observable well-defined meaning, it still does not result in a definite value (i.e., the wavefunction is not collapsed). The analogy with relativity, it seems, is a similarity between the choice of reference frame and the choice of a particular decohering measurement apparatus. These choices are sufficient to give well-defined meaning to certain physical quantities. The difference seems to be that in quantum theory, even though the quantities may have well-defined meaning, they still have not been actualized. For example, once the atom has interacted with the Geiger counter and poison bottle, it makes sense to say that Schrödiner’s cat is either alive or dead (there is no longer any coherence that would allow one to perform a measurement of a complementary observable to the alive/dead observable).
The actualization of a particular value could be described in terms of the many worlds interpretation as the choice of which world “you” get identified with. In relativity, though, one can actually imagine something analogous, but we don’t regard it as a mystery for some reason: The description of the world according to relativity does not specify which moment in spacetime we should be experiencing as “here and now”. So, what determines which point in Minkowski space is “actualized” in our experience as here and now? Why should we experience this here and now rather than some other? This question seems quite similar to the question of why we experience ourselves in one of the many worlds as opposed to some other. What “collapses” us into a particular here and now? Clearly, there is no such collapse, just as there is no collapse in quantum theory. The theory is an abstraction from the here and now. If we get confused and think that we really live in the abstraction, then we become perplexed at how the specific here and now is mysteriously “collapsed” from all the possibilities in the general, abstract world we’ve dreamed up.
There is also an interesting similarity between the role of decoherence, which effectively cuts us off from ever detecting any of the worlds that have decohered from ours, and space-like separation in relativity. There are spacelike separated regions of spacetime that can not have any interaction or communication with us. So, what justification is there for saying that they exist at all? They can never be observed or verified to exist. Is this really any different than the other branches of the universal wave function that we can no longer detect because of decoherence?
Lorrin Barth
26 May 2009
What I’ve never heard anyone ask is, “What is a particle?” Are photons and electrons made out of some sort of elemental dust?
If electron dust does not exist, if the collapse of wave function does not occur, then all you are left with is waves, waves that are described completely by the mathematics of QM.
Why is there no good explanation for the measurement problem after all of these years. It is because the collapse of the wave function does not occur.
Remove collapse of the wave function and you don’t need interpretations, the moon is there whether you look or not, a new universe is not created every time a photon delivers its energy to the screen and all this goofy stuff books are written about is gone.
Occam’s Razor is needed here.
KatGoesMiow
22 October 2009
Is this ‘collapse of a wave function’ experimental evidence that can confirm the condition of Simulatineity for Special Relativity?
Im young and so easily lost.
integralscience
22 October 2009
As far as I know, there is no experimental evidence that the collapse of the wave function provides a physical basis for simultaneity. The nonlocal correlations of quantum mechanics can not be used to send signals faster than the speed of light. Although their measurements are correlated, one observer can not determine when the other observer made their measurement.
Ed
13 September 2011
The Schrodinger (phi) isn’t a law. it is a phenomenological base equation that gives a statistical probability of particle position. And if I plot the phi function in Poincare space, the resulting suface area represents 95% probability that an electron is contained somewhere within.
Having gone to stanford, you should have known that! Unless they don’t teach it?
So, in actuallity, the collapse of the wave function represents the momentary isolation of any particle to its relative coordinate, and NOT a violation of any law
K :)
6 November 2011
We are being controlled by a giant analog-digital computer that is running simulations in a big “meta-verse” computer, rather than splitting off new universes. The analog part also makes states for our real universe evolve, but then events are selected by a random generator that constitutes the digital part, and values are selected from possibilities. The registers are cleared after selections and the results are posted to the real universe. The posting process is restricted to real numbers by manipulating the complex numbers. Of course it runs in meta-time so it has no trouble spanning all of space and time in our universe. 🙂
Kaller
7 November 2011
The collapse of the wavefunction is the same thing as the production of results according to a statistical distribution, but without the micro-causes you get with classical distributions.
Its a many to one process. It can even be undone, by “reviving” the original state. (The collapse seems to violate entropy (many to one) but it doesn’t because that is dictated by what happens to the energy states.)
It can also happen that the many is distributed across space and time. Photons in your slit experiment belong to such a distributed state. Even a partial effect is still an effect. A partial or gradual miracle is still a miracle. Particles in the Bell experiment belong to such a distributed state, even a gradual, partial correlation that is then undone again, is not a unitary change, it is the selection of a localised state that relates to the original distributed state.
Your slit photons are not unitary billiard balls. They are bouncing of the holes and going backwards, and they are also not arriving strictly randomly anyway, and as well as spatial corelations you have temporal ones – there are also correlations between arrival times called “photon bunching” even without the spatial correlations.
You can’t use symmetric correlations to pass signals or energy or information (expending entropy) which is what you need to establish reference frames.
Victor Grauer
20 November 2011
The wave function collapses upon interaction with the experimental apparatus. No observer is required. No Many-Worlds model is necessary. Quantum weirdness is confined to the atomic and subatomic domain and does not “bubble up” into the world of ordinary reality. When large numbers of quanta interrelate their individual “quantum weirdnesses” cancel each other out. Nothing to do with “consciousness.” Nothing mystical. Thus, as Bohr so wisely insisted: “There is no quantum world.”
integralscience
16 January 2012
Dividing the microworld of “quantum weirdness” from the macroworld of “ordinary reality” is a common, pragmatic approach that works “for all practical purposes” (FAPP) as John Bell has said, but there is unfortunately no clear line to be drawn between these two domains. What one decides to call the measurement apparatus and what one decides the call the system under measurement is not dictated by nature. The measurement apparatus itself can be described as part of the quantum system, in which case it evolves into a superposition of measurement result states, and not a single collapsed measurement. Moreover, superfluids and superconductors are counter-examples to the position that the weirdness of quantum coherence is somehow limited to the microscopic domain. So, the problem is not so easily brushed aside by appeal to a distinction between quantum and classical domains. There is just one physical world, and it is quantum-mechanical through-and-through.
Barry Mascarenhas
28 April 2012
Are superfluids superfluous?
Steven Lehar
4 June 2012
The apparent “collapse of the wave function” is merely an artifact of the fact that ALL detectors work on a resonance principle — elevating an electron from a lower to a higher orbital is a discrete event that absorbs exactly one quantum of energy from the passing wave, which is detected as a sudden “blip” in one location, but the wave that triggered it was an analog spatially extended field, from which exactly one quantum of energy was subtracted by the absorption event. This is true for silicon photodetectors, and for the rhodopsin molecular detection in the human retina.
Edward J Shipsey
27 July 2012
The question what is a particle? The best description of a particle is the equation which it obeys – no more no less!
Stephen
4 June 2013
The double-slit experiment is an example of the QM worldview and the GR worldview overlapping. For scientists to speak in terms of the “collapse of the wave function” is really just a figure of speech. It is their way (lacking the real explanation) of dealing with the apparently observed shift from quantum weirdness to classical modalities.
Photons, atoms, even entire molecules (they’ve all been made to work), shoot a series of them at a pair of slits and you’ll see them behave like nonlocalized “waves” under some conditions, and like localized “billiard balls” under other conditions. Many variations of the double-slit experiment have been done, and it has been demonstrated by some they’re always simultaneously particles and waves. But if you disturb one slit, such that it begins to act like a spherical wave instead of a cylindrical wave, those have no phase relation to each other and do not interfere. If you disturb both, they are apparently disturbed in different ways, and still will not interfere with each other. In all cases, a wave function is still present, but if you don’t observe it’s effects, it’s not improper to at first assume it doesn’t exist until proven otherwise. Nevertheless, the underlying mechanism that requires, say, a photon, to be both localized and nonlocalized at the same time has never yet been satisfactorily explained.
Just because a theory makes good predictions doesn’t make it correct. IMHO, the incompatibilities between QM and GR evidence the fact that there is a fundamentally different picture of reality required to unite QM and GR under one roof, and that might mean large portions of both of them may be “right” for entirely the wrong reasons. I think the standard model is like an old pair of shoes: they’re comfortable, even useful, but still, they trip you up.
Peter Faletra
12 December 2013
I doubt if Max Born would agree the wave function collapse is unreal…
“I personally like to regard a probability wave as a real thing, certainly as more than a tool for mathematical calculations. … how could we rely on probability predictions if we do not refer to something real and objective?” (Max Born on Quantum Theory)
gene
10 January 2016
I believe the two slit experiment to be terribly misleading. To start with, it is stated that when the subjects are emitted before entering the slits they are in their particle form. WHY. Wouldn’t it more sensible to conjecture that they are in wave form and thus eliminate the need to explain how a particle becomes a wave after passing through the two slits. Next , When a detector is used to determine which slit a photon went through, the electro magnetic waves produced by the current in the detector might create a decoherance effect on the incoming wave to convert it to a particle form (photon) causing the collapse of the wave function. When the investigator then pulls the plug on the detector and gets a return of the interference pattern he is completely mystified and blames it on the brain of the observer or on two worlds Would it be too much to conjecture that the collapse of a light wave leads to the formation of photons?. Where have I gone wrong? It just seems too easy.
integralscience
29 January 2016
The source is treated as emitting particles because they are known to be localized in time and space: it is known where they originate (from the source) and when (at the time of emission). Waves are not localized in that way. Regarding detection at the slits, which determines “which way” information, depending on the nature of the experiment, that information can be erased or not, giving rise to interference or not. You may be interested in learning about the delayed choice quantum eraser experiment.
drl gene
3 February 2016
O.K. Would like to break a few eggs and see if we can come up with a new omelet. Breaking egg1. particle and wave do not exist together at the same time. Bohr seemed to imply this.Breaking eggs 2. When a bound electron or atom become free from their bound state by applying a packet of energy equal to their work function, their free state comes off as a wave.Breaking eggs 3. De Broglie stated that all matter has particle and wave form. Macro particles have extremely small waves which cannot be seen. O.k. try this. the reason we cannot seen the change of macromatter to wave form is because the amount of energy necessary to overcome the work functions would be enormous.Breaking eggs 4.When the detector is turned on it produces an electromagnetic field that somehow reduces the waves energy enough to revert the wave back to its bound particle state. Don’t know if your time and space objection still apply when we attribute the waves origin to a bound particle.Also there seems to be a lot of dissention over the validity of erasure results. Have shot my load. Many many thanks to be able to participate in the forum even at an elementary level. What you guys are doing is great for scientific learning.