54 research outputs found
Retrocausality at no extra cost
One obstacle faced by proposals of retrocausal influences in quantum mechanics is the perceived high conceptual cost of making such a proposal. I assemble here a metaphysical picture consistent with the possibility of retrocausality and not precluded by the known physical structure of our reality. I conclude that given the right mix of some reasonable metaphysical and epistemological ingredients there is no conceptual cost to such a picture
Does Time-Symmetry Imply Retrocausality? How the Quantum World Says "Maybe"
It has often been suggested that retrocausality offers a solution to some of
the puzzles of quantum mechanics: e.g., that it allows a Lorentz-invariant
explanation of Bell correlations, and other manifestations of quantum
nonlocality, without action-at-a-distance. Some writers have argued that
time-symmetry counts in favour of such a view, in the sense that retrocausality
would be a natural consequence of a truly time-symmetric theory of the quantum
world. Critics object that there is complete time-symmetry in classical
physics, and yet no apparent retrocausality. Why should the quantum world be
any different? This note throws some new light on these matters. I call
attention to a respect in which quantum mechanics is different, under some
assumptions about quantum ontology. Under these assumptions, the combination of
time-symmetry without retrocausality is unavailable in quantum mechanics, for
reasons intimately connected with the differences between classical and quantum
physics (especially the role of discreteness in the latter). Not all
interpretations of quantum mechanics share these assumptions, however, and in
those that do not, time-symmetry does not entail retrocausality.Comment: 22 pages, 6 figures; significant revision
New Slant on the EPR-Bell Experiment
The best case for thinking that quantum mechanics is nonlocal rests on Bellâs Theorem, and later results of the same kind. However, the correlations characteristic of EPR-Bell (EPRB) experiments also arise in familiar cases elsewhere in QM, where the two measurements involved are timelike rather than spacelike separated; and in which the correlations are usually assumed to have a local causal explanation, requiring no action-at-a-distance. It is interesting to ask how this is possible, in the light of Bellâs Theorem. We investigate this question, and present two options. Either (i) the new cases are nonlocal, too, in which case action-at-a-distance is more widespread in QM than has previously been appreciated (and does not depend on entanglement, as usually construed); or (ii) the means of avoiding action-at-a-distance in the new cases extends in a natural way to EPRB, removing action-at-a-distance in these cases, too. There is a third option, viz., that the new cases are strongly disanalogous to EPRB. But this option requires an argument, so far missing, that the physical world breaks the symmetries which otherwise support the analogy. In the absence of such an argument, the orthodox combination of views â action-at-a-distance in EPRB, but local causality in its timelike analogue â is less well established than it is usually assumed to be
New Slant on the EPR-Bell Experiment
The best case for thinking that quantum mechanics is nonlocal rests on Bell's
Theorem, and later results of the same kind. However, the correlations
characteristic of EPR-Bell (EPRB) experiments also arise in familiar cases
elsewhere in QM, where the two measurements involved are timelike rather than
spacelike separated; and in which the correlations are usually assumed to have
a local causal explanation, requiring no action-at-a-distance. It is
interesting to ask how this is possible, in the light of Bell's Theorem. We
investigate this question, and present two options. Either (i) the new cases
are nonlocal, too, in which case action-at-a-distance is more widespread in QM
than has previously been appreciated (and does not depend on entanglement, as
usually construed); or (ii) the means of avoiding action-at-a-distance in the
new cases extends in a natural way to EPRB, removing action-at-a-distance in
these cases, too. There is a third option, viz., that the new cases are
strongly disanalogous to EPRB. But this option requires an argument, so far
missing, that the physical world breaks the symmetries which otherwise support
the analogy. In the absence of such an argument, the orthodox combination of
views -- action-at-a-distance in EPRB, but local causality in its timelike
analogue -- is less well established than it is usually assumed to be.Comment: 26 pages, 7 figures; extensively revised for resubmissio
The Quantum Eraser Paradox
The Delayed-Choice Quantum Eraser experiment is commonly interpreted as
implying that in quantum mechanics a choice made at one time can influence an
earlier event. We here suggest an extension of the experiment that results in a
paradox when interpreted using a local realist interpretation combined with
backward causation ("retrocausality"). We argue that resolving the paradox
requires giving up the idea that, in quantum mechanics, a choice can influence
the past, and that it instead requires a violation of Statistical Independence
without retrocausality. We speculate what the outcome of the experiment would
be.Comment: 8 pages, 6 figures. v2: minor fixes, references adde
Quantum causal models, faithfulness and retrocausality
Wood and Spekkens (2015) argue that any causal model explaining the EPRB
correlations and satisfying no-signalling must also violate the assumption that
the model faithfully reproduces the statistical dependences and
independences---a so-called "fine-tuning" of the causal parameters; this
includes, in particular, retrocausal explanations of the EPRB correlations. I
consider this analysis with a view to enumerating the possible responses an
advocate of retrocausal explanations might propose. I focus on the response of
N\"{a}ger (2015), who argues that the central ideas of causal explanations can
be saved if one accepts the possibility of a stable fine-tuning of the causal
parameters. I argue that, in light of this view, a violation of faithfulness
does not necessarily rule out retrocausal explanations of the EPRB
correlations, although it certainly constrains such explanations. I conclude by
considering some possible consequences of this type of response for retrocausal
explanations
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