This is a review of the book Quantum Reality: Beyond the New Physics, by Nick Herbert. Herbert is also the author of Faster Than Light and Elemental Mind.
The book is about interpretations of quantum mechanics. Herbert makes it very clear that this is where physics and philosophy meet and mingle, to the chagrin of many physicists. He is especially concerned with Bell's Theorem, its verification in the Clauser's and Aspect's experiments, and its moral: "Reality is non-local."
Along the way, Herbert gives a brief introduction to wave mechanics (including the first clear explanation of spherical harmonics I ever saw – math free!) and gives much interesting history of quantum mechanics. Here is the table of contents:
Preface
1 The Quest for Reality
2 Physicists Losing Their Grip
3 Quantum Theory Takes Charge
4 Facing the Quantum Facts
5 Wave Motion: The Sound of Music
6 Meet the Champ: Quantum Theory Itself
7 Describing the Indescribable: The Quantum Interpretation Question
8 "And Then A Miracle Occurs": The Quantum Measurement Problem
9 Four Quantum Realities
10 Quantum Realities: Four More
11 The Einstein-Podosky-Rosen Paradox
12 Bell's Interconnectedness Theorem
13 The Future of Quantum Reality
Appendix I Additional Readings on Quantum Mechanics
Appendix II Quantum Number
Index
I heartily recommend the book. Herbert writes very clearly, with a light, engaging style. He is what the French call an animateur, one able to clarify subtle or complex matters. Without quoting lengthy passages from Quantum Reality, I will try to convey some of his clarity.
There are a great many interpretations of quantum mechanics (QM for short). However, they can be grouped into classes. Herbert gives a list of eight. He presents it as a sampler, but it looks very complete to me. These are not, by the way, eight mutually exclusive views. There are, rather, eight opinions or issues within the whole interpretation discussion. Some are compatible with one another; others are not.
QM describes the results you will get if you perform experiments at a
sufficiently fine resolution. That's it. The waves and quantum-numbers and
such are just mathematical conveniences, like a coordinate grid. Dynamical
properties like position and momentum have no independent existence, but
depend on the experiment you choose, the way mass and velocity depend on your
frame of reference in relativity. This is called the "Copenhagen
Interpretation" because it originated with Niels Bohr and his associates at
his Copenhagen institute.
Most Copenhagenists regard any physical change in the classical realm of physics as an "observation." Thus a spot on a photographic film is an "observation" whether or not anyone ever looks at the film. Besides Niels Bohr, Werner Heisenberg and John Wheeler support the Copenhagen Interpretation.
A standard criticism of this theory is the sharp division it puts between the microcosm of quantum and the macrocosm of everyday life, when it is, after all, a single world.
(The Copenhagen Interpretation is the generally accepted interpretation
in the physics community, though I strongly suspect that this
acceptance is nominal; most physicists I have met dislike metaphysics,
don't like to think about these questions, and go with Bohr because
Bohr kept on talking longer than his opponent, Einstein, who sounded
too mystical for most physicists' tastes.)
The observation paradoxes of QM show that dividing the world into observer and
observed is mistaken. The object-subject boundary and all other boundaries
are illusory. David Bohm support this position.
This interpretation was coined by Hugh Everett in 1957 and recently popularized by Paul Davies.
A standard criticism of this theory is that myriad swarms of undetectable
parallel worlds seem a high price to pay for interpreting QM.
Just as the world of relativity obeys a geometry foreign to our everyday
notions, the world of QM obeys a foreign logic. This is an intriguing notion,
but no one seems to have done much with it. The idea was developed by John
von Neumann and Garrett Birkhoff.
That is, objects that have an "ordinary" objective existence. This body of opinion pretty much coincides with the "hidden variable" theories of QM. It claims that QM is not complete, while Bohr claimed that QM is complete, now and forever. Neo-realism supporters include Albert Einstein, Erwin Schroedinger, Louis de Broglie, and David Bohm. Any theories about "pilot waves" and the recent "decoherence" interpretations are neo-realist.
A standard criticism of neo-realist/hidden-variable theories is that they all
use use faster-than-light effects. In 1955, John von Neumann showed that any
such theory had to use FTL effects to produce the results as QM.
This is very like #2, except they insist that observations are things that happen in minds, not in instruments. They use the famous example of Schrödinger's Cat (which Schrödinger coined to show how absurd the idea of observer-created reality is) to show that there is no good theoretical reason to limit quantum ambiguities to the microcosm. "We now know that the moon is demonstrably not there when nobody looks," said N. David Mermin, summing up the position. Radical Copenhagenism is supported by John von Neumann, Eugene Wigner, and John Wheeler.
The standard criticism of this theory is that it gives an unnervingly central
position to consciousness.
This is Heisenberg's personal speculation, above and beyond the Copenhagen Interpretation. He pictures a vast and nebulous realm of possibilities, the "potentia," from which occasional members get elected to the status of actual events. The election is done randomly, by act of observation, within the limits of QM. The difference between Heisenberg's potentia and ordinary possibilities is that potentia combine with one another according to the laws of wave mechanics. Heisenberg's potentia strongly resemble Richard Feynman's spectra of possible histories, over which he performs summations, though Feynman never claimed his were more than a calculating device.
At first glance, it seems to me that 1+2 (moderate Copenhagen), 1+7 (radical Copenhagen), 4 (many worlds), and 6 (neo-realism) are four mutually incompatible positions. 3 (holism) and 5 (quantum logic) can be combined with any of the first four; 8 (potentia) can be combined with 1+2 or 1+7, and perhaps with 6 after slight modification.
In the 1930s, Bohr and Einstein had a long intellectual duel over quantum mechanics. Einstein maintained that it could not be taken as a complete account of the physical world. Bohr maintained that QM was complete. (This always struck me as a terribly dogmatic and unscientific attitude; how can you be sure that any theory is a complete description? But Bohr felt sure.)
In Physical Review, v47, p777, Einstein, Boris Podolsky, and Nathan Rosen published "Can Quantum-mechanical Description of Physical Reality Be Considered Complete?" (I have heard, but have not verified, that Einstein merely advised Podolsky and Rosen on their thought experiment and was surprised but pleased to find himself listed as an author.)
Some atoms, when excited, give off pairs of twin photons. Each of these photons is completely unpolarized. That is, if you put a polaroid filter in its path, it has a 50% chance of passing through and a 50% chance of being absorbed
The intriguing thing is that they pass or stop in symmetrical pairs, so long as both filters are held at the same angle. If a left-bound photon reaches the filter first and passes through, we know ahead of time that its right-bound twin, born of the same atom, also passes. If one photon is absorbed, so will be its distant twin.
Quantum mechanics predicts that the photons pass or stop randomly, and this checks out experimentally. QM also predicts that the twin photons should both pass or both stop for any given case. What QM does not explain is how one photon "knows" that its twin has hit something and been stopped or passed.
Mathematically, once the first photon passes or stops, you can write a new wave function for the second one, guaranteeing it will do the same. But why should you have to write a new wave function? According to Bohr, the old wave function said everything that could be said about the second photon. Einstein argued that this correlated behavior showed the photons had some structure the QM did not describe, therefore QM was incomplete.
The alternative was that the first photon's actions had some effect on the second photon. Neither Einstein nor Bohr liked this idea; since the two photons are headed in opposite directions at the speed of light, such effects would have to be transmitted faster than light. Neither side liked this idea.
In 1955, John von Neumann showed that "hidden variable" theories must all use non-local effects to give the same predictions as quantum mechanics. Since most physicists would rather eat their Ph.D. theses than admit non-local effects or forswear QM, this seemed a telling blow to hidden-variable theories and their kin, which Herbert names "neo-realist."
In 1964, John S. Bell published "On the Einstein-Podolsky-Rosen Paradox" in Physics, v1 p 195. In this, Bell proved a theorem showing that all interpretations of quantum mechanics must use non-local effects.
Go back to the set-up with the twin-photon source shining unpolarized photons through polaroid filters. Only now, don't make both filters vertical. Instead, let one filter tilt relative to the other.
The further the filters get out of alignment, the less the photons pass or stop in matching pairs.
When one filter is at right angles to the other, the rate of matching falls to zero – if the left photon passes, the right one stops, and vice versa.
How fast does the rate of matching fall off as you increase the angle between filters? Bell demonstrated that, for any theory limited to local effects, the rate of matching must lie within M ≤ f(θ), where M is the rate of matching and f is a function depending on the angle θ between the filters.
Quantum mechanics, however, predicts that, over part of the range of θ, the matching rate is greater than M. So either Bell's theorem is wrong, quantum mechanics is wrong, or the world is non-local. Bell's theorem has proved remarkably water-tight despite considerable effort. It isn't very long or complex, so there is not much room for error.
Some physicists were actually hoping that the QM predictions would turn out to be wrong – that they would "have to" (be able to) introduce corrective additions to take into account the limitations of locality.
When Einstein, Podolsky, and Rosen made up their experiment in 1935, it was a "thought experiment" – no one could carry it out in practice. However, in 1978, John F. Clauser did carry it out at Berkeley. Results supported QM and non-locality. There was one ray of hope left for localists – Clauser's experiments were carried out at slow speed; light-speed changes in the wave function could still cross the apparatus before observations were made.
In 1982, Alain Aspect conducted a similar experiment in Paris, using high-speed switches to move the light beams from one path to another, testing polarization 10 billion times every second, eliminating light-speed effects. The results still came in the same. Herbert's conclusion: The world contains non-local effects.
What does "non-local" mean? The effect pays no attention to space at all; the locations of its targets have no relevance for it, hence "non-local." Given the tight relation between space and time in special relativity (a theory as well-tested as quantum), non-local effects are also presumably non-temporal as well; it could act backward in time.
According to Herbert, a non-local effect acts instantaneously, apparently without crossing the intervening space, and without any diminishment with distance. "Instantaneously" because the twin photons can pass through the filters simultaneously and still be correlated. "Without crossing space" because the prediction does not imply any particle or field moving from one filter to the other. "Without diminishment" because the filters can be inches or light-centuries apart and the matching rate is unaffected.
Herbert also points out that, even should quantum mechanics fail and join the crystalline spheres, phlogiston, and the luminiferous ether in the ranks of obsolete theories, reality will still be non-local. The experimental results are still there and are still incompatible with local versions of QM or local anything.
Given that the world includes non-local effects, what does this mean for the eight propositions in quantum mechanics listed by Herbert and summarized at the beginning of this page?
It does not give grounds for rejected any of them, alas. However, it puts constraints on how you develop any of them.
1: Copenhagen, Part I – There is no deep reality.
2: Copenhagen, Part II – Observation creates reality.
The standard criticism of this theory is the sharp division it puts between the microcosm of quantum and the macrocosm of everyday life, when it is, after all, a single world. To this, Bell's Theorem adds the constraint that, when the world reacts to the touch of an observation, it reacts non-locally. The darkening of a spot on a photographic plate here may cause the collapse of a wave function in the Andromeda galaxy.
1: Copenhagen, Part I – There is no deep reality.
7: Radical Copenhagen – Consciousness creates reality.
The standard criticism of this theory is that it gives an unnervingly central position to consciousness. To this, Bell's Theorem adds that the power of consciousness is non-local. A glance at a photographic plate here may cause the collapse of a wave function in the Andromeda galaxy.
3: Holistic – Reality is an undivided wholeness.
Bell's Theorem is quite friendly to this position. It supplies a possible mechanism by which all the parts of reality can be bound into that wholeness.
4: Many Worlds – Reality is a growing array of parallel worlds.
Bell's Theorem makes hardly any difference to this position. It was already non-local. A choice between one outcome or another here on Earth splits the whole universe, so suddenly there are parallel versions of every remotest galaxy.
5: Quantum Logic – Reality obeys an unfamiliar, non-Boolean logic
Well, this one already advertised itself as strange. It only need make sure that the strangeness includes non-locality.
6: Neo-realism – The world is made of "ordinary" objects.
These objects are "ordinary" in the sense that they are there whether anyone looks at them or not. The standard objection to this theory was that it required faster-than-light effects, but non-local effects are necessarily faster than light, so non-locality removes the standard objection to this interpretation.
8: Potentia – The world consists of two classes, potential and actual.
Bell's Theorem tells us something about the potentia, if they exist; they are non-local, capable of stretching themselves across the universe, so to speak, and rippling back and forth through time.
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