Historical research guide · 0383 · 1990
Bell’s theorem: Local realism versus quantum mechanics
M. Ferrero · T. W. Marshall · E. Santos
In one minute
Miguel Ferrero of Oviedo, Trevor Marshall of Manchester and Emilio Santos of Cantabria wrote this for the American Journal of Physics, a teaching journal, so it is built to be followed line by line rather than admired. They first define realism and locality as physical principles in their own right, then derive from those two assumptions the expression Bell proposed for calculating correlations, and from that expression the incompatibility theorem: local realism and quantum mechanics cannot both be right about every prediction. The second half turns to the laboratory. Photomultipliers and polarizing filters are not perfectly efficient, so an experiment only ever samples the pairs it happens to detect. To get from what such an experiment measures to a verdict on local realism, the authors show, you must add a subsidiary assumption about how the detectors behave on the pairs they miss. Their conclusion is that those assumptions are not logically necessary — so in 1990 local realism stood unrefuted by experiment.
Connection
Chapter 1 is the evidence ladder, and this is one of the sharpest lessons on it: what an experiment establishes depends on what you assume the instruments are doing. Chapter 2 needs that lesson because the whole stochastic-electrodynamics programme — the vacuum as a real random field — lives or dies on how the photon-detection step is read.
From the source
Realism and locality are defined as separate, fundamental physical principles, and Bell’s expression for calculating correlations is derived from them — so the inequality that follows is a consequence of those two stated assumptions rather than a piece of quantum theory.Abstract, sentences 1 and 2; American Journal of Physics 58, 683 (1990)
From that expression the authors prove the theorem of incompatibility between local realism and quantum mechanics: the two cannot agree on every prediction, so an experiment able to distinguish them is possible in principle.Abstract, sentence 3
In the experiments performed by Aspect and others, photomultipliers and polarizing filters are not perfectly efficient, so ruling out local realism requires subsidiary assumptions about how those instruments operate on the pairs they do not register.Abstract, sentence 4
Those subsidiary assumptions are not logically necessary; therefore, on the evidence available in 1990, local realism had not yet been refuted empirically. This is the argument that keeps a real-vacuum, local-realistic programme such as stochastic optics live rather than closed.Abstract, final sentence
What to watch: the paper names its own decisive measurement — a Bell test whose detector efficiency is high enough that no subsidiary assumption is needed. Detection-efficient, locality-enforcing experiments were carried out in 2015 at Delft, NIST and Vienna, and the argument in this programme has since moved to the assumptions those tests still carry, contextuality among them.Abstract, sentences 4 and 5, read against the companion analysis in Physical Review A 39, 6271 (1989)
The way in
https://doi.org/10.1119/1.16400How to cite it
M. Ferrero, T. W. Marshall, E. Santos (1990) Bell’s theorem: Local realism versus quantum mechanics doi:10.1119/1.16400