v1.0.1 / chapter 20 of 20 / 01 sep 07 / greg goebel / public domain
* In the 1930s, the puzzles of quantum physics led to the creation of the Copenhagen interpretation in hopes of providing solutions. There matters more or less rested until the 1970s, when new findings began to dislodge old thinking, resulting in an intense and perplexing debate.
* By the 1930s, the Copenhagen Interpretation had become basically dogma for most physicists, with holdouts like Einstein regarded as more or less old fossilized reactionaries. By the 1970s, as discussed in the previous section, experimental physics was beginning to catch up with thought experiments. Researchers were able to implement experiments that everyone had thought impossible, and were able to manipulate single atoms, electrons, and photons.
One of the other reasons that physicists remained committed to the Copenhagen interpretation for decades was John von Neumann's "proof" that there were no hidden variables. Although there had been objections to it at the outset, John Bell finally gave it the death blow. It turned out that von Neumann had made a disastrously false assumption in his calculations by obtaining an average value and then using that as the only valid value. Logically, this was exactly as if a supply officer for a military unit had learned the average size of the troops in the unit and then ordered a single size of uniform for all the troops -- obviously not a good idea. Bell, a good-natured Irishman not given to sharp statements, described the von Neumann "proof" as "silly". Even the amazing von Neumann had his off days.
In sum, in the last decades of the 20th century the dominance of the Copenhagen interpretation began to weaken and it is no longer regarded as dogma. As Einstein and Schroedinger were only too aware, there were problems with the Copenhagen interpretation all along anyway. Von Neumann pushed it far enough to say that nothing really exists until it is observed by a consciousness, as described by the parable of Schroedinger's Cat, and that only for starters. If Alice checks on the state of Schroedinger's Cat to see if the poor kitty is alive or dead and then reports the result to Bob, then it might be said Alice is in a superposition of states -- one Alice who thinks the kitty is dead, another who thinks the kitty is alive -- until she reports to Bob one way or another, but not both, collapsing her wavefunction.
The whole chain "infinite regress", as von Neumann put it, could be extended until it is the entire cosmos that is in a superposition of states until it is observed. However, if it has to be observed by some consciousness, was there a Universe before consciousness evolved? John Wheeler suggested that possibly the decoherence due to observation by a consciousness went back in time to the origins of the Universe. This line of thinking has been referred to as the "participatory Universe".
This sounds completely nuts, and it seems likely Wheeler was only playing with ideas or pointing out contradictions in quantum thinking. Most who knew Wheeler described him as a perfectly intelligent and straightforward person, usually tidily dressed in neat suits and very proper, very polite, more reminiscent of a banker than a physicist. Some who knew him well saw that underneath the layers on layers of politeness was a core of daring and a quirky sense of humor. In addition, Wheeler suggested in one of his papers that the observation could even be made by a machine, a relatively simple one in fact, that nobody could possibly believe had any more consciousness than a concrete block. To be sure, it takes a consciousness to be aware of the Universe, but this is blindingly obvious and it is hard to see what its relevance is to physics. If anyone ever seriously believed that consciousness was a requirement for collapse of a wavefunction, very few do now.
To complicate matters further, nobody is even close to acquiring a clear scientific definition of what consciousness really is -- and there are some who suggest it's not definable, that it's a "qualia". The color blue is a qualia, for example; although it can be defined scientifically in terms of a range of wavelengths of light, it is impossible to describe to someone who has been stone-blind from birth. It is also clear that there is more going on in our heads than we are conscious of; some suggest much more, that the conscious part is relatively unimportant in determining our behavior, a consideration that makes matters even more obscure. A few have tried to link the ambiguities of consciousness to the ambiguities of quantum physics. It is hard to say this exercise lacks merit, but it is clearly very risky to try to explain one ambiguous phenomenon in terms of another: it brings up the image of two of Schroedinger's cats eating each other's tail.
In sum, one of the weaknesses of the Copenhagen interpretation is that it avoids the issue of where quantum decoherence occurs. One of the clues to the answer to this problem is the fact that researchers working on quantum computing have found it very hard to prevent decoherence in even a very simple system of atoms. This strongly suggests that quantum uncertainty does not and cannot extend into the macroscale through an infinite regress; the idea that, say, the Moon doesn't exist when nobody's watching it no longer has many backers, if it ever really did. In another series of experiments that the founders could not have imagined, researchers are now conducting experiments on decoherence of simple systems to see if they can set bounds on the phenomenon.
* If the Copenhagen Interpretation is no longer the law of the land, what are the alternatives? There is a wide range of options and so this discussion will limit itself to the more popular ones.
As mentioned, Louis de Broglie had come up with one alternative as far back as 1927, suggesting that the Schroedinger wavefunction was a real function that guided real particles along their paths. The Copenhagen clique rejected this notion, denying the notion of hidden variables that it implied and judging the whole idea just so much excess baggage. De Broglie also had problems working out the math and getting the concept to work.
However, the idea didn't go away, though it went underground for a long time. In 1952, David Bohm published two papers that revived and extended de Broglie's original concepts. Bohm was uncomfortable with the Copenhagen interpretation; his friend Murray Gell-Mann explained: "Marxists prefer their theories to be deterministic." The de Broglie-Bohm interpretation was generally ignored for decades, partly because it had some features physicists weren't happy to accept, and partly because the Copenhagen interpretation was the law of the land. It is said that Bohm sent his papers to Niels Bohr and Bohr didn't bother to reply.
Bohm's concepts envisioned that the Schroedinger wavefunction included a form of energy not known to classical physics that was related to what he called the "quantum potential" or "pilot wave". In the two-slit experiment, the pilot wave would exist through both slits and guide real particles through the slits to obtain an interference pattern. Although the de Broglie-Bohm interpretation does state that there is a real particle following a real path, the statistical nature of the wavefunction and the Heisenberg uncertainty principle remain in effect, and only probabilities for the location of particles can be determined. Similarly, the de Broglie-Bohm view retains decoherence: attempting to observe the interaction would interfere with the pilot wave and eliminate the interference pattern, as is observed.
The sticking point of the de Broglie-Bohm theory was nonlocality. To account for quantum weirdness, disturbance of the pilot wave had to propagate through it instantaneously. Physicists were reluctant to discard Einstein's theory of special relativity, all the more so because special relativity states that what is instantaneous to a person in one state of motion, or "frame of reference", is not to a person in a different frame of motion.
Suppose Alice is flying around in a starship and Bob is back on Earth. If Alice has some magic scheme to send a message from point X to distant point Z instantaneously, then Bob will, depending on circumstances, actually see the message arrived at Z before it was sent from X. This is a violation of the law of cause and effect. Bohm proposed that there was a special "preferred" frame of reference where the instantaneous action occurred. This was such an ugly idea that it was no wonder many physicists rejected it.
David Bell didn't, thinking that Bohm had provided a consistent and accurate theory of quantum physics that incorporated hidden variables. It was the fact that von Neumann had said this couldn't be done that led Bell to demolish von Neumann's "proof"; Bohm's work would also lead Bell to come up with his test for nonlocality. Since that test has been validated, nonlocality has become widely accepted, and that means that the de Broglie-Bohm view is now an active competitor to the Copenhagen interpretation. The de Broglie-Bohm theory has been modified as well, most significantly by Bell, who discarded the pilot wave and asserted that all that was needed were the equations that defined the trajectories of particles.
As far as Bohm's assertion of a preferred frame of reference, it can be argued that it is irrelevant. Suppose Alice and Bob both have one of a pair of entangled photons. If there is a frame of reference in which Alice checks on the polarization of her photon first, she also determines the polarization of Bob's photon; but to validate this, Bob has to check on the polarization of his photon afterwards. If it appears in a different frame of reference that Bob has done his check first, then he has determined the polarization of Alice's photon when she checks on it. Think of it as a game which either one wins, depending on the frame of reference of the observer, but it doesn't matter either way.
This demonstrates that there is no cause and effect relationship between the checks performed by Alice and Bob. It also helps illustrate why quantum nonlocality can't be used as an instantaneous communications system: if it doesn't matter which one, Alice or Bob, goes first, it is impossible to believe that any information has been sent from one to the other. It's screwy, yes, but no more screwy than much else in quantum mechanics: "You know the two-slit experiment? It's like that."
* Another, fairly popular, school of physicists has, in response to the devious concept of superposition of states, have come up with a magic way of banishing it, known as the "multiverse" or "many-worlds" theory.
The basic idea was developed in 1957 by Hugh Everett III (1930:1982), then at Princeton. The basic idea is that an observation of a quantum system that has a range of possible values actually results in the manifestation of all possible states, but with each result in its own branching parallel universe. In other words, at all times an infinity of parallel universes are being born from our universe. The most prominent advocate of the many-worlds theory is David Deutsch, and he takes the idea very literally.
Many physicists are not enthusiastic. John Wheeler, who was Everett's advisor and helped him develop the idea, ultimately dumped it, claiming it was too much metaphysical excess baggage to carry around. Some found this condemnation a bit rich, considering Wheeler's writings on the participatory universe concept.
The many-worlds advocates insist that there is nothing particularly bizarre about the idea. At first hearing this comes across as something like Jedi mind tricks -- "the Force gives me power over the weak of mind" -- but after a bit of thought it starts to seem a little less unreasonable. In its current revision, the many-worlds theory has discarded the idea of a continuous branching of infinite numbers of new universes and says, somewhat more economically, that the possible results of the observation occur in an infinity of existing parallel universes. This would seem logical, at least by the bizarre standards of logic in quantum physics, in that if there is already an infinite number of parallel universes it would be redundant to keep creating new ones all the time.
In the many-worlds scheme, Schroedinger's cat is not half-alive and half-dead. It is alive in one universe and dead in another, and the act of observation places us in one of these two particular universes. In the two-slit interference experiment, a photon will go through one slit in one universe and the other in a second universe. In interfering, the photon will merge the two universes.
One of the really impressive beauties of the many-worlds scheme is that quantum nonlocality, the EPR paradox, is no particular problem. Suppose there are two entangled particles that exist before measurement in a superposition of states with possible values of, say, "+" or "-". The many-worlds scheme says that in one universe, they have the values of "+" and "-" respectively, and in a second universe, they have the values of "-" and "+" respectively. A measurement simply confirms which of the two universes the observer is actually in. Imagine drawing each universe as a cartoon on a transparent sheet, and then overlaying the two sheets; a measurement is merely a process of selecting one of the sheets.
In some ways the many-worlds interpretation is infuriatingly preposterous, in some ways it is appealingly clean and elegant, at least if it's compared to the alternatives. British-Australian physicist Paul Davies (born 1946) said very fairly it was "cheap on assumptions but expensive on universes." Deutsch has pointed out that the many-worlds theory may even be testable, at least in principle, using a specially designed measurement system that records the outcomes of the two-slit interference experiment. In one universe, it would record a photon going through one slit, in the second it would record it going through the other, and when interference merged the two universes again, the machine would be able to compare notes. How such a machine might be built is another question, but as discussed earlier, in recent years physicists have been able to take thought experiments that nobody in past decades thought would ever be more than that -- and convincingly demonstrate them, much to everyone's astonishment.
Some advocates of the many-worlds interpretation, like Murray Gell-Mann, have tweaked the semantics, calling it the "many-histories" interpretation, referring to each one of the set of possible states in any quantum system as simply a "history". This distinction has the advantage of dropping the scifi-flavored and heavy weight of infinite parallel universes for the more abstract but less cumbersome concept of different histories. It may seem to be a strictly semantic distinction, but quantum physics does involve a certain unavoidable wrestling with semantics in the first place, and "many-histories" also forms a neat link with Feynman's sum-over-histories approach to QED. In fact, once the link is made, the two simply seem like slightly different perspectives on the same idea.
* Another approach, known as "quantum logic" and promoted by von Neumann, attempts to take the rules of quantum physics, simply accept them as givens, and construct a logical system of rules out of them. This has a certain appeal at first sight, but the critics point out that, since the rules of conventional logic don't apply very well to quantum physics, the rules of quantum logic don't apply very well to classical physics or for that matter even day-to-day life. The late Heinz Pagels put it neatly, saying that it is like "inventing a new logic to maintain the Earth is flat if confronted with the evidence that it is round."
Certainly, it could be argued that each form of logic would need to be used in its own domain, but that brings back the unanswered problem of where one domain ends and the other begins, and in the end it is hard to see that quantum logic has really bought us any further understanding of the matter. It is just a reshuffling of what we already know and ends up being excess baggage, too. Quantum logic has its following, but it is a small one.
* It is hard to take the subject of quantum weirdness seriously and it is hard to know how much of it physicists actually take seriously. Many physicists find the whole notion of quantum weirdness a distraction, a waste of time. Steven Weinberg recalled a visit to a colleague at another academic institution, and asked what had happened to a particularly promising young graduate researcher there. His colleague shook his head sadly and replied: "He tried to understand quantum mechanics."
Even those who are interested are far from having anything resembling agreement on the matter. In the mid-1980s, the British Broadcasting Corporation performed radio interviews of eight prominent physicists and asked them for their thoughts on quantum weirdness. Each answered with a different read on the matter and insisted the other seven were completely wrong.
To be sure, we are free to use any model of a physical system that we like, as long as it fits the facts. Of course not all theories are created equal. A theory should make predictions beyond the known facts that can be tested, or in other words needs to have some "predictive power", and a theory should also have a certain elegance and economy, being as easy to understand and use as possible and discarding all excess baggage. In most fields of science, these constraints end up greatly narrowing the range of acceptable theories, often down to just one, but it is an indication of the difficulties and ambiguities of quantum physics that there are a range of possible and acceptable theories, in which the concepts of "elegance" and "economy" become subjective to a greater or lesser degree.
Some physicists have been willing to keep an open mind on the subject and consider the relative merits of different theories, or even use contradictory theories in different contexts. As far as the rest of us go, it is useful to keep an open mind, while, in another exercise of duality, taking it all with a grain of salt. The well-known science columnist Martin Gardner did a fairly good job of deflating some of over-the-top ideas about quantum weirdness in one of his essays:
BEGIN QUOTE:
No physicist denies that a quantum particle is a ghostly thing for which it is impossible to build consistent models using the space-time of classical Einsteinian physics ... But from the ghostliness of an electron it does not follow, at least for most physicists, that a stone or a tree is equally ghostly ...
Consider a rainbow. It is as observer-dependent as an electron. Nothing is "out there" that deserves to be called a rainbow. Each person sees a different rainbow, a rainbow that has no position in space until it is observed. In a sense, the rainbow has no reality apart from its observation. On the other hand, the bow is mind-independent in that it can be photographed. It is a pattern that rests firmly on a structure of relations between falling raindrops, light from the Sun, and an eye or camera lens.
END QUOTE
In the same way, while much might be made about wave-particle duality, imagine placing a coin on a table. It will appear heads or tails, one or the other but never both, depending on how it is placed, and nobody thinks anything strange about that. Another homely analogy is the old-style can-bottle opener, now generally obsolete in an era of easy-open lids and possibly a mystery to younger readers. It could be used to open a can or a bottle depending on which end was used, but it couldn't reasonably be used to do both at once.
As far as a something appearing to be a particle or a wave given what kind of experiment is performed on it, that isn't all that much different from other kinds of tests. For example, a classroom examination on, say, calculus, will demonstrate how much students knows about calculus, but it will say absolutely nothing about much students know about English literature.
For another interesting example, superposition of states can be thought of as much like trying to tune in two AM radio broadcast stations that have overlapping transmission bands so that both are received with equal volume at the same time -- this won't happen with FM radio, incidentally. This "superposition" will undergo "decoherence" when the radio dial is tuned to one AM station to bring it in loud and clear.
One fun analogy to quantum indeterminacy wouldn't have made sense before the computer era. Computer users are generally familiar with the concept of a "wildcard", generally the asterisk ("*") character, which can match any collection of characters to help list specific groups of files in a directory. For example, the expression "*.exe" will match all the executable program files in a directory, the expression "*.txt" will match all the text files, and "m*.txt" will match all the text files starting with "m". The wildcard in itself expresses a range of possibilities that is converted to specifics by being exercised. The idea that quantum entities are something like wildcards is not too hard to grasp, though it all it does is provide a comfortable handle on the phenomena without telling us any more than we already know.
Of course, there really is something weird going on at the quantum level, and there's no way around it. Quantum computing, however mad it sounds, has been demonstrated to at least the proof-of-concept stage, though it is still unclear if it will ever be workable in practice. Although many working physicists find the subject of quantum weirdness annoying, it's not about to go away. It's like having a door in a house into a secret room, with a combination lock that nobody ever seems to figure out and may very well not even have a legal combination. Most of the occupants of the house just shrug and forget about it, but somebody keeps coming back to it and keying in combinations again.
The point is that quantum weirdness leads to more confusion than seems to be really needed; some of the models seem to obscure matters instead of clarifying them. Are things really that difficult? Dick Feynman put that uncertainty down in terms that were pure Feynman:
BEGIN QUOTE:
We have always had a great deal of difficulty understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me.
You know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem ... It has not become obvious to me that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem.
END QUOTE
Maybe all that will happen is that one of these days we'll simply get used to it, and nobody will see anything weird about it any more.
There's also a certain logical failing in going too far beyond experimental results. Arthur Stanley Eddington once observed that a proliferation of theories was a symptom of a lack of data. As mentioned, we can choose any model we like if it fits the data and works effectively, but on one side of the coin increasingly precise and comprehensive data tends to gradually constrain the range of possible models, and on the other side of the coin a model that isn't backed up by any available data is little more than speculation. The tangle of quantum theories tends suggest a certain amount of desperation.
Certainly, people can speculate, but speculations on quantum weirdness often end up sounding like Zen jokes. Maybe it's not a question of whether God plays dice with the Universe so much as He seems to have a sense of humor. I personally believe He does.
* I usually find that documents I write end up being several times more work than I expect they will be. This one wasn't too bad. I was expecting that trying to sort out the concept of quantum weirdness might be troublesome, so that didn't come as a surprise, and the only part that tripped me up was particle physics.
I never thought of particle physics as being all that interesting and didn't want to spend too much time on it, hoping to get done with it in a single chapter. It didn't work out that way, since a discussion that could pretend to stand up required a good deal of effort, and the chapters on particle physics ended up becoming like writing a "book within a book". I did find it more interesting after I got into it, but it still seems a bit eye-glazing and it was definitely more work than I wanted to do. The scary thing was that after a while I realized: "Bob, I'm starting to understand this!"
* Sources include:
Some materials were obtained from the Microsoft Encarta encyclopedia and the Wikipedia online encyclopedia -- the Wikipedia was particularly handy for figuring out birth and death dates of even relatively obscure scientists -- and various elementary chemistry texts were consulted for the chapter on the electronic structure of atoms.
The section on neutrino searches was the last major chunk of this document that I had to put together. It was heavily dependent on the websites for the various experiments. Trying to wade through such materials can be extremely tiresome, since websites vary greatly in the quality of their writing, and scientific websites of course tend toward materials of the class found in scientific journals; even when they can be read by lay people, it may be painful to try to pick out anything of general interest in them. Fortunately, the University of Oulu in Finland had a very nice "Ultimate Neutrino Page" that made it much easier for the layperson to navigate through the shoals.
* Revision history:
v1.0.0 / 01 dec 06 / gvg
v1.0.1 / 01 sep 07 / gvg / General polishing, radiodating controversy.
The chapter on superconductivity was originally released as a stand-alone
article in 1997.