the scientific picture of the world. Causality is the way we the link between two successive events. Synchronicity

Nancy Delaney Cartwright

Recall van Fraassen's general scheme. Scientific models can depict both observable and unobservable processes. Science itself is supposed to tell us what is and what is not observable. We are to accept the theory if it gets right all that is observable; the rest can be of no matter to us, van Fraassen has suggested, because we could have no evidence about whether the model gets it right or not. We may believe in the model's picture of the unobservable world but that belief is based on faith not evidence, and should be recognised as such. Most of the stuff of modem physics, then, turns out to be in the unobservable part of the model: photons, quarks, gravity-waves, the electromagnetic field, forces, I suppose and even space-time itself. Where are causes? that is the question I want to discuss. We can begin with causal laws. They are surely for van Fraassen not in the model itself any more than any other kinds of laws. They enter, rather, as a constraint on the kinds of structures we will allow our models to have. In a full model for the entire history of the world, or more realistically, in a small model for a given scientific experiment, event-kinds will recur with some frequency. These frequencies are represented by probabilities-though note that the probabilities are not in the models either. If we insist that our scientific theory in a given domain be causal, that will restrict the form of the probability functions which are allowed and hence, in some way, constrain the actual models, which have finite frequencies in them only. Some patterns of correlation will be allowed and some will be prohibited. Consider van Fraassen's discussion of the Einstein-Podalsky-Rosen (EPR) paradox in the follow-up to Laws and Symmetry, Quantum Mechanics: An Empiricist View.' There van Fraassen takes a particular relationship between probabilities-calledfactoriz-

Henry Margenau

CK Hung

## Abstract Textbook quantum mechanics defines measurement using ideas that are not themselves basic physics: "observer," "macroscopic apparatus," "irreversible record." We take this definition apart and find that it bundles two different things: the physical event of an interaction, and the statistical fact that the event's result becomes hard to erase. We separate them. A measurement, in its minimal form, is a causal link: when event A and event B interact, a fact is established between them — B's state relative to A. This happens at every interaction, at every scale, with no apparatus required. What laboratories call "a measurement" is the same kind of event, after its result has spread — fanned out — into so many further causal links that undoing it is no longer practical. This reading gives the quantum eraser a clean explanation: an outcome is erasable exactly as long as its causal fan-out stays narrow. It also clarifies what decoherence theory does and does not do: decoherence is the physics of fan-out, and it does that job well; the question it was criticized for not answering (why one outcome, with Born-rule probability) belongs to a different layer. Nothing in the existing formalism changes. What changes is the definition — and with it, the need for a separate "collapse mechanism" disappears. Because the basic unit here is a causal link, the whole picture can be built directly on discrete causal structures, which we identify as the natural next step. Keywords: measurement problem, decoherence, causal link, quantum eraser, relational quantum mechanics

Hans Reinchenbach

Robert S. Cohen, John J. Stachel

Huibo Hong, Runai Quan, Xiao Xiang +5 more

As a new approach to realizing high-precision time synchronization between remote time scales, quantum two-way time transfer via laboratory fiber link has shown significant enhancement of the transfer stability to several tens of femtoseconds. To verify its great potential in practical systems, the field test in long-haul installed fiber optic infrastructure is required to be demonstrated. In this article, we implement the two-way quantum time transfer over a 103 km urban fiber link. A time transfer stability of 3.67 ps at 10 s and 0.28 ps at 40 000 s has been achieved, despite the large attenuation of 38 dB leading to fewer than 40 correlated events per second. This achievement marks the first successful step of quantum two-way time transfer in the task of high-precision long-distance field transfer systems.