This is something I wrote a while ago for another thread about whether particles could “really” have well-defined simultaneous values of things like position and momentum or whether the “fuzziness” is truly fundamental:
Although people usually focus on indeterminacy when discussing the mysteries of quantum mechanics, the real mystery is that QM poses a lot of problems for a realist view of reality, i.e. one where the properties of the world exist independently of our measurement. This is most obvious in the EPR experiments where properties of entangled particles are measured at different locations. What we find is that there are regular correlations between measurements made on particle A and measurements made on particle B which are inexplicable if we picture the particles as classical objects with definite properties that cannot communicate faster than light–this is what Einstein called “spooky action at a distance.”
It is sometimes imagined that the uncertainty principle, which prevents us from knowing simultaneously the value of two noncommuting variables (like position and momentum), is just a limitation on measurement ; maybe the particle has a definite position and momentum at any given time, but each time we try to measure the position it changes the particle’s momentum in a random way, and each time we measure the particle’s position it offsets the momentum. However, the EPR experiment shows it is much worse than that. The correlations between entangled particles are such that they cannot be explained by any picture of the world in which the particles have definite values for each noncommuting variable at every time, unless the particles can somehow communicate instantaneously so as soon as you measure one the other “knows” which property you measured and adjusts its own properties. This is the result known as “Bell’s Theorem,” which says that no local theory of hidden variables can explain the results of the EPR experiment.
In Huw Price’s Time’s Arrow and Archimedes’ Point he offers a little story to help us see what’s so strange about the EPR results:
quote:
By modern standards the criminal code of Ypiaria [pronounced, of course, “E-P-aria”] allowed its police force excessive powers of arrest and interrogation. Random detention and questioning were accepted weapons in the fight against serious crime. This is not to say the police had an entirely free hand, however. On the contrary, there were strict constraints on the questions the police could address to anyone detained in this way. One question only could be asked, to be chosen at random from a list of three: (1) Are you a murderer? (2) Are you a thief? (3) Have you committed adultery? Detainees who answered “yes” to the chosen question were punished accordingly, while those who answered “no” were immediately released. (Lying seems to have been frowned on, but no doubt was not unknown.)
To ensure that these guidelines were strictly adhered to, records were required to be kept of every such interrogation. Some of these records have survived, and therein lies our present concern. The records came to be analyzed by the psychologist Alexander Graham Doppelganger, known for his work on long distance communication. Doppelganger realized that among the many millions of cases in the surviving records there were likely to be some in which the Ypiarian police had interrogated both members of a pir of twins. He was interested in whether in such cases any correlation could be observed between the answers given by each twin.
As we now know, Doppelganger’s interest was richly rewarded. He uncovered the two striking and seemingly incompatible correlations now known collectively as Doppelganger’s Twin Paradox . He found that
(8.1) When each member of a pair of twins was asked the same question, both always gave the same answer;
and that
(8.2) When each member of a pair of twins was asked a different question, they gave the same answer on close to 25 percent of such occasions.
It may not be immediately apparent that these results are in any way incompatible. But Doppelganger reasoned as follows: 8.1 means that whatever it is that disposes Ypiarians to answer Y or N to each of the three possible questions 1, 2, and 3, it is a disposition that twins always have in common. For example, if YYN signifies the property of being disposed to answer Y to questions 1 and 2 and N to question 3, then correlation 8.1 implies that if one twin is YYN then so is his or her sibling. Similarly for the seven other possible such states: in all, for the eight possible permutations of two possible answers to three possible questions. (The possibilities are the two homogeneous states YYY and NNN, and the six inhomogeneous states YYN, YNY, NYY, YNN, NYN, and NNY.)
Turning now to 8.2, Doppelganger saw that there were six ways to pose a different question to each pair of twins: the possibilities we may represent by 1:2, 2:1, 1:3, 3:1, 2:3, and 3:2. (1:3 signifies that the first twin is asked question 1 and the second twin question 3, for example.) How many of these possibilities would produce the same answer from both twins? Clearly it depends on the twins’ shared dispositions. If both twins are YYN, for example, then 1:2 and 2:1 will produce the same response (in this case, Y) and the other four possibilities will produce different responses. So if YYN twins were questioned at random, we should expect the same response from each in about 33 percent of all cases. And for homogeneous states, of course, all six posible question pairs produce the same result: YYY twins will always answer Y and NNN twins will always answer N.
Hence, Doppelganger realized, we should expect a certain minimum correlation in these different question cases. We cannot tell how many pairs of Ypiarian twins were in each of the eight possible states, but we can say that whatever their distribution, confessions should correlate with confessions and denials with denials in at least 33 percent of the different question interrogations. For the figure should be 33 percent if all the twins are in inhomogeneous states, and higher if some are in homogeneous states. And yet, as 8.2 describes, the records show a much lower figure.
Doppelganger initially suspected that this difference might be a mere statistical fluctuation. As newly examined cases continued to confirm the same pattern, however, he realized that the chances of such a variation were infinitesimal. His next thought was therefore that the Ypiarian twins must generally have known what question the other was being asked, and determined their answer partly on this basis. He saw that it would be easy to explain 8.2 if the nature of one’s twin’s question could influence one’s own answer. Indeed, it would be easy to make a total anticorrelation in the different question cases be compatible with 8.1—with total correlation in the same question cases.
Doppelganger investigated this possibility with some care. He found, however, that twins were always interrogated separately and in isolation. As required, their chosen questions were selected at random, and only after they had been separated from one another. There therefore seemed no way in which twins could conspire to produce the results described in 8.1 and 8.2. Moreover, there seemed a compelling physical reason to discount the view that the question asked of one twin might influence the answers given by another. This was that the separation of such interrogations was usually spacelike in the sense of special relativity; in other words, neither interrogation occurred in either the past or the future light cone of the other. (It is not that the Ypiarian police force was given to space travel, but that light traveled more slowly in those days. The speed of a modern carrier pigeon is the best current estimate.) Hence according to the principle of the relativity of simultaneity, there was no determinate sense in which one interrogation took place before the other.
This is the problem posed by the EPR experiment in a nutshell, but instead of twins we are talking about entangled particles and instead of answers to questions we are talking about measurements of the particles’ “spin” along their 3 axes (there is an uncertainty relation between these spins). As the great physicist Richard Feynman said, “Nobody understands quantum mechanics…do not keep saying to yourself, if you can possibly avoid it, ‘But how can it be like that?’ because you will go ‘down the drain’ into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.” The weirdness of QM, and the difficulty with imagining “how can it be like than” in a way consistent with the view that reality exists before we observe it, extends to other famous experiments and thought-experiments, like the Double-slit experiment (from this page which introduces a lot of important QM concepts, including Bell’s theorem) and the Schroedinger’s Cat thought-experiment. (is the cat ‘really’ alive or dead before it is measured?) But Bell’s theorem and the EPR experiment show most clearly what the basic problem here is for a realist.
A number of different “interpretations” of quantum-mechanical weirdness have emerged over the years, with none yielding any new physical predictions (and thus being experimentally indistinguishable) but each offering a different way to conceptualize what’s “really” going on in these sorts of experiments. Here is a page which gives some good links on these various interpretations, and I’ll attempt my own summary here:
- The Copenhagen Interpretation
Basically, the Copenhagen interpretation says that we shouldn’t worry about how what’s really going on in the first place—science can only deal with correlating and predicting the results of various measurements, but it can’t tell us anything about what goes on when we’re not looking. This is basically a logical positivist perspective, and it was preferred by Bohr.
- “Objective Collapse” interpretation
Here the wave-particle duality is taken literally—the world exists as a wavelike potential when it’s not being observed, but somehow measurements periodically “collapse” the wavefunction into a definite state. Some versions of this suppose that it’s consciousness that does the collapsing, others suppose that an entangled system collapses once it reaches a certain limit in mass. Unlike the other interpretations, these might actually be expected to yield different predictions than orthodox QM—so far, there’s no evidence for anything like this though.
- The Bohm-de Broglie interpretation
Bell’s theorem shows that no local hidden variable theory can explain the results of the EPR experiment, but that leaves open the possibility of a nonlocal hidden variables theory where particles can communicate faster than light. This is the route taken by Bohm and de Broglie’s interpretation. In the Ypiarian story, this would be like the twins having a psychic link which allows one to know what question the other was asked, and adjust his own answer accordingly.
- Transactional interpretation
The EPR experiment can also be explained if you assume the future can affect the past, so that the particle’s original properties are affected by the measurements that will be made on them later, once they are separated. In the Ypiarian story, this would mean that the twin’s choices to commit or not commit various crimes would be affected by which questions they would be asked much later when they’re interrogated. This isn’t as strange as it sounds, since all the laws of physics we currently know of are time-symmetric (they look the same forwards as they do backwards) and apparently the apparent “arrow of time” emerges solely from statistical mechanics, perhaps because the universe started off in a very low-entropy state. Huw Price’s book Time’s Arrow and Archimedes’ Point , which I quoted from above, deals with this problem, and he favors a version of this interpretation.
- The Many-Worlds interpretation.
This interpretation takes the mathematical formalism of QM literally and proposes that the wavefunction is all there is. This means that when I measure the state of a particle that’s in superposition, instead of “collapsing” it into a definite state, I just become entangled with it and enter into a superposition myself; basically, I “split” into two versions of myself, one of whom observes one state and another of whom observes another. In popular accounts this is sometimes explained in terms of the entire universe splitting into parallel histories all the time, but it’s a bit more subtle than that, since different “worlds” can interfere with each other and cannot be viewed as totally "parallel,’ although thermodynamics may explain the appearance of splitting through a phenomenon called decoherence. For technical reasons this interpretation preserves locality (see question 12 of the Everett FAQ ), and it’s also 100% deterministic to boot (although it suggests an odd kind of subjective indeterminacy in which my first-person experience randomly chooses which split copy to become—hence a variation of this interpretation is the many-minds interpretation which deals with this issue a little more explicitly). This interpretation is seen as being the most theoretically elegant one by a number of physicists, and it seems that it is sometimes implicitly assumed in quantum cosmology, although physicists are often agnostic about whether other worlds/histories are actually “real.” Many-worlds could also make sense of quantum computation, which some physicists believe can be understood in terms of the quantum computer performing different computations in different worlds and then combining the results through interference.
So, those are the various interpretations…as I said, the main problem is that none of them really gives any new testable predictions, which is a bit unsatisfying. There’s some good reason to think that a theory of quantum gravity would transform our understanding of QM somewhat, so perhaps such a theory will depend on a modified version of one of these interpretations that is testable in some way. In any case, Bell’s inequality shows definitively than no classical, realist picture of the world can explain the EPR results, so whatever the truth turns out be, it’s guaranteed to violate our cherished assumptions in one way or another (faster-than light signalling, the future affecting the past, parallel universes…take your pick!)
