Not-so-Spooky Action at a Distance
It is sometimes claimed that all points throughout the universe are instantly connected. This is a reference to quantum nonlocality, which is an established fact of quantum mechanics, well-documented by Bell-type experiments. But what does nonlocality mean, and how can it be reconciled with relativity, which says that nothing, not even information, can propagate faster than the speed of light?
Einstein, Podolsky, and Rosen first raised the issue of nonlocality in a 1935 article. They proposed a thought experiment, in which quantum measurements on a spatially extended (entangled) quantum state are intrinsically random, but as soon as a measurement is made, the measurement results across the entire system are instantly determined.
To illustrate nonlocality, we consider simultaneous measurements of entangled photons. The entangled photons are emitted from a common source in opposite directions. The photon pair inherits the source’s zero quantum spin (no net polarization). If Alice and Bob measure the photons at spatially separated points A and B using parallel vertically polarized filters, and if Bob measures a vertically polarized photon, then the conservation of quantum spin means that Alice measures a horizontally polarized photon. Individual measurement results are random, with equal probabilities for a vertical or horizontal result, but the results for the pair are strictly and instantaneously anticorrelated, even when the measurements are physically separated and simultaneous.
The shaded light cones in Figure 1 show the allowable domain for causal effects emanating from simultaneous measurements at A and B within the empirical constraints of relativity and locality. The light cones represent light pulses expanding at the speed of light from points A and B. The instantaneous correlation of physically separated measurements at points A and B, outside of each other’s light cone, graphically illustrates the nonlocality of the photon pair’s correlated measurements. Einstein famously referred to nonlocal correlations as “spooky” action at a distance.
Einstein, Podolsky, and Rosen suggested a simple interpretation of the correlated results, without invoking nonlocal superluminal interactions. They suggested that measurement of one photon simply reveals a preexisting but “hidden” property of the entangled state. Hidden properties could be locally inherited from the photon pair’s common origin, and they could determine the correlated measurement outcomes, which only appear random. In this case, discovering this property simply reveals the correlated and locally determined outcome of the other photon’s measurement. No spooky action required.
However, in 1964, John Bell devised a test for hidden variables, based on the statistics of measurements using non-parallel analyzers. Numerous experiments have demonstrated that the statistics of multiple measurements violate Bell’s test. The results prove that measurement results cannot result from preexisting hidden properties, unless those hidden properties are themselves nonlocal. The results show that quantum nonlocality and relativity coexist within any objective interpretation of reality. Although quantum nonlocality cannot be used to transmit information superluminally and it does not empirically violate relativity, explaining their coexistence is one of the most vexing conceptual problems of physics.
Before we can reconcile the coexistence of quantum nonlocality and relativity, we need to introduce the Dissipative Conceptual Framework (DCF). As described in the essay Is Quantum Randomness Fundamental?, the DCF contextually defines the physical state by a perfect reversible process of measurement within the context of a system’s positive-temperature ambient surroundings. Whereas prevailing interpretations of physics regards entropy as an emergent property or a measure of an observer’s ignorance of a system’s precise state, the DCF defines entropy as a fundamental contextual property of the physical state.
Perfect measurement is reversible, but reversible measurement is not always possible. Between irreversible transitions, a system is reversibly measurable and it therefore exists as a state. This describes reversible mechanical time for states. An irreversible transition, in contrast, marks the irreversible production of entropy and an irreversible advance of thermodynamic time. During an irreversible transition, a system is not continuously and reversibly measurable, and it does not exist as a DCM state. The system exists in transition between states. The DCF recognizes system time as a complex property of state, spanning both reversible mechanical time and irreversible thermodynamic time.
If we assume that there are no hidden variables, then the measurement of an entangled photon culminates a process of irreversible transition from the photon’s common source state to photons with definite but random polarizations. Once the irreversible measurement is completed, the measurement result is classically recorded and definite, and it can be reversibly determined. At point B (Figure 1), Bob reversibly records his definite measurement result and transmits it to Alice via a signal photon polarized with the orientation that he measured. Alice simultaneously records her photon’s measurement result at point A. Since their detectors are parallel, their results are anticorrelated and she immediately knows the results of Bob’s measurement and the orientation of his signal photon. The light cone and Bob’s signal photon both reach Alice at point C. Knowing the signal photon’s orientation, Alice reversibly measures it and confirms the correlation of their results.
Alice’s observation of her measurement at A, Bob’s observation and transmission of his measurement at B, and Alice’s measurement of Bob’s signal photon at C are all reversibly conducted over mechanical time. Reversibility means no entropy production. With no change in entropy, mechanical time is not just reversible; it is also time symmetrical. With time symmetry, asserting that an initial event causes a future event and asserting that a future event causes the initial event are equally valid. This expresses the idea of retrocausality, and it exists over time-symmetrical mechanical time. The time-symmetry of recording and transmitting the measurement results creates a deterministic chain of causality and retrocausality within a single instant of thermodynamic time, represented in Figure 1 by A↔C↔B. The photons at A and B are entangled by virtue of the deterministic link connecting them. No hidden variables or spooky action is required to explain the deterministic and nonlocal correlation of measurements at A and B.
We next consider measurements using obliquely oriented polarizers. If Bob’s analyzer is rotated 45 degrees, the system’s contextual framework changes. Bob’s and Alice’s measurement results are no longer strictly correlated. Alice can no longer know Bob’s result based on her own results, and she cannot reversibly measure his signal photon. The time-symmetry link of causality and retrocausality connecting their measurement results is broken and the photons at A and B are no longer entangled. The observed statistics of measurements reflect local measurements on photons with definite anticorrelated but random polarizations. Again, no hidden variables or spooky action is required to explain the observed results.
The righthand axis of Figure 1 shows the record of Alice’s measurement events at points A and C, as measured by her reference clock. Even when the events at A, B and C are correlated within an instant of thermodynamic time, the reference clock continues to mark the irreversible passage of reference time. Alice experiences the irreversible passage of time between recording her measurement at point A and her recording of Bob’s measurement at point C. The irreversible flow of an observer’s reference time and the empirical constraints of relativity preclude superluminal exchange of information between observers across their reference time. The DCF successfully explains the mechanical details of nonlocality, and it explains how relativity and quantum nonlocality compatibly coexist without spooky action or hidden variables.
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For a full-text technical article on DCF and nonlocality, see https://www.preprints.org/manuscript/202007.0469/v4
