It should be no surprise that loose talk by scientists on tantalizing subjects like backward causality can impair the judgment of science writers working on a short deadline. A recent paper by Aharonov, Cohen, Grossman and Elitzur at Tel Aviv University, provocatively titled “Can a Future Choice Affect a Past Measurement’s Outcome?” so intoxicated Philip Ball, writing in Physics World, that a casual reader of his piece would likely conclude that the answer was “Yes! But no one quite understands how it works, and it probably has something to do with free will.” A more sober reading of the original paper’s substantive content would be
- As John Bell showed in 1964, quantum systems’ behavior cannot be explained by local hidden variable models of the usual sort, wherein each particle carries information determining the result of any measurement that might be performed on it.
- The Two State-Vector Formalism (TSFV) for quantum mechanics, although equivalent in its predictions to ordinary nonlocal quantum theory, can be viewed as a more complicated kind of local hidden variable model, one that, by depending on a final as well as an initial condition, and being local in space-time rather than space, escapes Bell’s prohibition .
This incident illustrates two unfortunate tendencies in quantum foundations research:
- Many in the field believe in their own formulation or interpretation of quantum mechanics so fervently that they give short shrift to other formulations, rather than treating them more charitably, as complementary approaches to understanding a simple but hard-to-intuit reality.
- There is a long history of trying to fit the phenomenology of entanglement into inappropriate everyday language, like Einstein’s “spooky action at a distance”.
Surely the worst mistake of this sort was Herbert’s 1981 “FLASH” proposal to use entanglement for superluminal signaling, whose refutation may have hastened the discovery of the no-cloning theorem. The Tel Aviv authors would never make such a crude mistake—indeed, far from arguing that superluminal signalling is possible, they use it as a straw man for their formalism to demolish. But unfortunately, to make their straw man look stronger before demolishing him, they use language that, like Einstein’s, obscures the crucial difference between communication and correlation. They say that the initial (weak) measurement outcomes “anticipate the experimenter’s future choice” but that doing so causes no violation of causality because the “anticipation is encrypted”. This is as wrongheaded as saying that when Alice sends Bob a classical secret key, intending to use it for one-time-pad encryption, that the key is already an encrypted anticipation of whatever message she might later send with it. Or to take a more quantum example, it’s like saying that half of any maximally entangled pair of qubits is already an encrypted anticipation of whatever quantum state might later be teleported through it.
Near the end of their paper the Tel Aviv group hits another hot button, coyly suggesting that encrypted anticipation may help explain free will, by giving humans “full freedom from both past and future constraints.” The issue of free will appeared also in Ball’s piece (following a brief but fair summary of my critique) as a quote attributing to Yakir Aharonov, the senior Tel Aviv author, the opinion that humans have free will even though God knows exactly what they will do.
The authors, and reviewer, would have served their readers better by eschewing the “concept” of encrypted anticipation and instead concentrating on how TSVF makes a local picture of quantum evolution possible. In particular they could have compared TSVF with another attempt to give orthodox quantum mechanics a fully local explanation, Deutsch and Hayden’s 1999 information flow formalism.
The issue of free will appeared also in Ballâ€™s piece (following a brief but fair summary of my critique) as a quote attributing to the Yakir Aharonov, the senior Tel Aviv author, the opinion that humans have free will even though God knows exactly what they will do.
God only knows what that means!
I think this is a shame because I actually think that retrocausality is a viable research direction, but it needs to be argued much more carefully. In my opinion, Huw Price et. al.’s analyses of retrocausality are much better argued, see
and the recent FQXi article http://fqxi.org/community/articles/display/170
In my view, the main problem with the Tel Aviv article is that the language is not very clear, so it is not at all clear what they are claiming to have shown. If the claim is simply that you can derive a kind of local hidden variable theory incorporating retrocausality from the weak values in the TSV formalism then I suppose that is OK. However, they seem to be arguing that their construction proves that there must be retrocausality and here the argument falls flat. It is based on the (unstated) assumption that weak measurements disturb the underlying ontic state by a negligible amount, but all we actually know is that they disturb the wavefunction by a negligible amount, and this is not the same thing if we are allowing a framework with hidden variables.