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The wavefunction is a real physical object after all, say researchers.
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Mathematical device or physical fact? The elusive nature of the quantum wavefunction may be pinned down at last.
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At the heart of the weirdness for which the field of quantum mechanics is famous is the wavefunction, a powerful but mysterious entity that is used to determine the probabilities that quantum particles will have certain properties. Now, a preprint posted onlineon 14 November1 reopens the question of what the wavefunction represents — with an answer that could rock quantum theory to its core. Whereas many physicists have generally interpreted the wavefunction as a statistical tool that reflects our ignorance of the particles being measured, the authors of the latest paper argue that, instead, it is physically real.
“I don't like to sound hyperbolic, but I think the word 'seismic' is likely to apply to this paper,” says Antony Valentini, a theoretical physicist specializing in quantum foundations at Clemson University in South Carolina.
Valentini believes that this result may be the most important general theorem relating to the foundations of quantum mechanics since Bell’s theorem, the 1964 result in which Northern Irish physicist John Stewart Bell proved that if quantum mechanics describes real entities, it has to include mysterious “action at a distance”.
Action at a distance occurs when pairs of quantum particles interact in such a way that they become entangled. But the new paper, by a trio of physicists led by Matthew Pusey at Imperial College London, presents a theorem showing that if a quantum wavefunction were purely a statistical tool, then even quantum states that are unconnected across space and time would be able to communicate with each other. As that seems very unlikely to be true, the researchers conclude that the wavefunction must be physically real after all.
David Wallace, a philosopher of physics at the University of Oxford, UK, says that the theorem is the most important result in the foundations of quantum mechanics that he has seen in his 15-year professional career. “This strips away obscurity and shows you can’t have an interpretation of a quantum state as probabilistic,” he says.
Historical debate
The debate over how to understand the wavefunction goes back to the 1920s. In the ‘Copenhagen interpretation’ pioneered by Danish physicist Niels Bohr, the wavefunction was considered a computational tool: it gave correct results when used to calculate the probability of particles having various properties, but physicists were encouraged not to look for a deeper explanation of what the wavefunction is.
Albert Einstein also favoured a statistical interpretation of the wavefunction, although he thought that there had to be some other as-yet-unknown underlying reality. But others, such as Austrian physicist Erwin Schrödinger, considered the wavefunction, at least initially, to be a real physical object.
The Copenhagen interpretation later fell out of popularity, but the idea that the wavefunction reflects what we can know about the world, rather than physical reality, has come back into vogue in the past 15 years with the rise of quantum information theory, Valentini says.
Rudolph and his colleagues may put a stop to that trend. Their theorem effectively says that individual quantum systems must “know” exactly what state they have been prepared in, or the results of measurements on them would lead to results at odds with quantum mechanics. They declined to comment while their preprint is undergoing the journal-submission process, but say in their paper that their finding is similar to the notion that an individual coin being flipped in a biased way — for example, so that it comes up 'heads' six out of ten times — has the intrinsic, physical property of being biased, in contrast to the idea that the bias is simply a statistical property of many coin-flip outcomes.
Quantum information
Robert Spekkens, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, who has favoured a statistical interpretation of the wavefunction, says that Pusey's theorem is correct and a “fantastic” result, but that he disagrees about what conclusion should be drawn from it. He favours an interpretation in which all quantum states, including non-entangled ones, are related after all.
Spekkens adds that he does expect the theorem to have broader consequences for physics, as have Bell’s and other fundamental theorems. No one foresaw in 1964 that Bell’s theorem would sow the seeds for quantum information theory and quantum cryptography — both of which rely on phenomena that aren’t possible in classical physics. Spekkens thinks this theorem may ultimately have a similar impact. “It’s very important and beautiful in its simplicity,” he says.
- Nature
- doi:10.1038/nature.2011.9392
Comments
Report this comment | #30564
How in God's good name can the wavefront not be real if it is influencing where the particle is? If the chances of finding the particle at a given location are as described by a wave function, this is enough to make it phyically real in my opinion.
As for Bohr's view that the wavefront is collapsed by observation: That to me has always been laughable. The wavefront is collapsed by an interaction with another object, with the chances of such an interaction occuring being governed by said wave function. and the wave function of the other object. If the particle moves on past our experiment, the issue is only that we did not observe it, and not the its wave function cannot be collapsed later.
Report this comment | #30594
In my book "Quantum-Classical Correspondence: Dynamical Quantization and the Classical Limit" (Springer, 2004) one can read by way of conclusion: "According to dynamical quantization, quantum physics can be formulated without appealing in any way to a dualistic framework of matter (wave vs. particle). Consequently, the trajectory concept exists as an intrinsic or ontological attribute for both quantum and classical particles. An electron around an atomic nucleus, for example, actually follows a certain quantum trajectory, since the density matrix does possess the same ontlogical status as the classical Wigner function. This characteristic is not destroyed by our quantization procedure." (A. O. Bolivar)
Further, on page 149 I assert the following: "The Schrödinger function plays a secondary role in the theoretical framework of quantum physics. In general, open systems are not described in terms of it. the description in terms of the von Neumann function is fundamental due to its logical priority and its physical generality". (A. O. Bolivar)
Report this comment | #30596
I read the preprint a few days ago . My conculsion Everett was right and realism rules.
Report this comment | #30597
Hallelujah.
It has been a long time coming.
Away with the Platonic smoke and mirrors, and back to physical reality.
RLO
http://www3.amherst.edu/~rloldershaw
Fractal Cosmology
Report this comment | #30650
In an alternative model of the elementary particles, the particles consist of looping EM waves. In such a model the wavefunction would certainly be physically real.
The model is described on: the neoclassical atom
Report this comment | #30651
In an alternative model of the elementary particles, the particles consist of looping EM waves. In such a model the wavefunction would certainly be physically real.
The model is described on: the neoclassical atom
Report this comment | #30652
What is really surprising about the preprint?
If I understood it correctly it can be summarized as:
1) if you know the Wave-Function you know all physical properties
2) if you know ALL physical properties you also know the WF
3) the measurement process (collapse of wave-function) is a real physical process which selects one of the possible outcome
Again, what is surprising about it?
Report this comment | #30663
Report this comment | #30669
Nearly 35 years ago a discrete fractal paradigm applicable to all scales of nature, referred to as Discrete Scale Relativity when the discrete self-similarity is exact, asserted that Schrodinger's wavefunction is a physical plasma of subquantum scale particles that are exactly self-similar to the familiar subatomic particles, but smaller by a factor of 10e17 and less massive by a factor of 10e56.
The exact physical properties of the subquantum particles comprising atomic scale wavfunctions can be calculated via the discrete scale invariant scaling equations of Discrete Scale Relativity.
RLO
Report this comment | #30670
It's usually a good idea to actually read the preprint before taking to the comments section and endorsing the work of someone else .
If you're too lazy to read the preprint, I have summarized its essence for you:
The authors begin by making two claims,
1) If a quantum state reflects an underlying objective physical reality, then a list of values of physical properties (i.e., commuting observables) should be sufficient to uniquely determine a quantum state.
2) If a quantum state does not reflect an underlying objective physical reality but is merely a mathematical tool for obtaining probabilities concerning the outcome of an experiment, then a list of values of physical properties cannot uniquely determine the quantum state. There may be several distinct states statistically compatible with a given list of values of physical properties.
The authors then want to determine whether (1) or (2) is the correct by considering a thought experiment. Suppose you prepare two different quantum states, both of which are compatible with a given list of values of physical properties. Assuming that the outcome of a measurement is determined by the measurement apparatus and the physical properties of the quantum system alone, then the authors CLAIM that you can arrange for a measurement where the apparatus returns results inconsistent with the predictions of quantum mechanics, resulting in a paradox. They then conclude the paradox constitutes proof of the quantum state (or wavefunction) corresponding to and underlying objective physical reality.
The problem here is that the results of the thought experiment are, in fact, in accordance with the laws of quantum mechanics. The authors are apparently a bit rusty with the concept of orthogonality in the context of quantum state vectors and basis functions, and consequently they have misinterpreted the outcome of their imagined experiment. Were they to be familiar with such concepts they would have realized that there is no paradox. Just read the paper and actually go over the math. I don't know what more to say. It's embarrassingly obvious...
Report this comment | #30671
Avidly have I read Wallace's work in pursuit of expanding my physics ken; no other books in my library are as treasured as his "Brief Collisions with Hideous Particles" and "Consider the Neutrino". It's truer now than ever to say that he was ahead of his time.