DYNAMICS Engr 80
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Date Created: 09/12/15
EINSTEIN39S LEGACY VIEWPOINT The Quantum Measurement Problem A J Leggett Despite the spectacular success of quantum medianics QM over the last 80 years in explaining phenomena observed at the atomic and subatomic level the conceptual status of the theow is still a topic of lively controversy Most of the discussion centers around two famous paradoxes or as some would have it pseudoparadoxes associated respectively with the names of Einstein Podolsky and Rosen EPR and with Schrodinger s cat In this Viewpoint I will concentrate on the paradox of Schrodinger s cat or as it is often known to my mind somewhat misleadingly the quantum measurement paradox Basically the quantum measurement paradox is that most interpretations of QM at the microscopic level do not allow de nite out comes to be realized whereas at the level of our human consciousness it seems a matter of direct experience that such outcomes occur indeed it seems so difficult to imagine what it would be like for the world to be otherwise that I suspect that Immanuel Kant had he had occasion to consider the problem would have classi ed our knowledge of this state of affairs as synthetic a priori t is convenient to classify reactions to this problem into three broad classes de ned by the following three different views on the status of QM a QM is the complete truth about the physical world at all levels and describes an external reality b QM is the com lete truth 39 the sense that it will always give reliable predictions concerning e of experiments but describes no external reality c QM is not the complete truth about the world at some level between that of the atom and that of human con sciousness other non quantum mechanical principles intervene I brie y discuss each of these possibil ities in turn for a more extended discussion see 1 Let s start with option a Consider the following two questions 1 In a typical situation involving an ensemble of microscopic entities such as a Young s slits experiment with for example electrons or neutrons in which the QM description of the ensemble is by a superpo sition of amplitudes corresponding to alter native microscopic possibilities A and B eg went through slit 1 and went through slit 2 is it the case that each individual member of the ensemble either de nitely realizes alternative A or de nitely realizes alternative 2 In a thought experiment of the Schrodinger s cat type involving an ensem ble of macroscopic objects eg cats for which the formal QM description of the Department of Ph sits University of Illinois at Urbana Champaign Urbana IL 61801 USA www5ciencemagorg SCIENCE VOL 307 ensemble of relevant universes is by a superposition of amplitudes corresponding to macroscopically distinct alternative states A and B eg cat alive and cat dead is it the case that each member cat of the ensemble either de nitely realizes alter native A or de nitely realizes alternative B in the absence of inspection by a human agent I believe that a large majority of the portion of the physics community that ad vocates option a would answer no to the rst question and yes to the second 2 us ent given in favor f these answers involves the phenomenon of decoherence As a result of the latter phenom enon it is impossible to see any effects of interference between for example the living and dead states of the cat and it is argued that therefore one state or the other has been de nitely realized irrespective of wheth er we have or have not observed the partic ular cat in question As I have argued at greater length else where I believe this ar ument embodies a gross logical fallacy It confuses the question oftruth with the question of evidence At the microscopic level the adherents of view a felt mostly obliged to reject a realistic in terpretation the evidence they would cite ag 39 st it is the well known phenomenon of interference between possibilities A and B By the time we get to the macroscopic level the evidence has gone away but the QM formal ism is in no way changed thus its interpreta tion cannot have changed either 0 complete my argument at this point it would be necessary to discuss also those interpretations of QM such as the Bohm de Broglie hidden variables interpretation that answer yes to both e a ove ues tions and those such as the Everett Wheeler many universes interpretation that answer no to both Because space is limited I will just state my own view that both these interpretations amount to little more than verbal window dressing of the basic paradox and thus that no interpretation of class a is viable 3 11 FEBRUARY 2005 I next turn more brie y to option b According to the adherents of this view the whole formalism of QM amounts to nothing but a calculational recipe designed in the last resort to predict the probabilities of various directly observed macroscopic out comes is particular cat is deadalive and the symbols occurring in it such as the probability amplitudes correspond to nothing in the real world The extreme operation alism implied in this view is o en softened the observation that under many condi tions relevant to human existence the ex perimental predictions of QM are as ifquot the world had behaved classically this argument most explicit in the consistent histories or decoherent histories interpre tation However that o servation does not get around the f act that these conditions are not invariably ful lled in particular it does not exclude a priori the possibility that we may some day be able not merely to generate quantum superpositions like that of chrodinger s cat but to observe the corre sponding interference effects Personally if I could be sure that we will forever regard QM as the whole truth about the physical world I think I should grit my teeth and plump for option b Finally what of option c Indeed there have been a number of concrete proposals to modify standard QM at some level interme diate between that of the atom and that of an consciousness the currently best developed one being probably that associ ated with the names of Ghirardi Rimini Weber and Pearle All of these proposals have in common the feature that at a suf ciently macroscopic level the precise threshold depends on the speci c proposal the superpositions predicted by the formal extrapolation of the QM formalism do not occur rather some non QM mechanism in tervenes and guarantees the realization of a de nite macroscopic outcome for each par ticular member of the ensemble in question In principle once the threshold for such real ization is speci ed it would be possible to test such theories unambiguously by comparing their predictions with those of stan Q however for the test to be de nitive it is ob viously necessary that QM continues to pre dict interference effects ie that decoherence which of course is a concept only meaning ful within the QM formalism has not washed them out If one can indeed tect the char acteristic QM interference effects at a given level of macroscopicness then it is a reason 871 m 39U quot1 D gt m m n 139 O z