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Science

Measurements Are the Only Reality, Say Quantum Tests

James Glanz

Published Dec 1, 1995• 4 citations
Publisher

Abstract

Quantum mechanics insists that the characteristics of a particle or wave have no reality until they are measured, an idea that has long made physicists uncomfortable. New experiments suggest more strongly than ever that they will have to live with that discomfort. By studying pairs of photons that have correlated polarizations, a research group showed that the results make sense only if the polarizations do not exist apart from measurement.

DOI
10.1126/science.270.5241.1439

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Quantum mechanics insists that the characteristics of a particle or wave have no reality until they are measured, an idea that has long made physicists uncomfortable. New experiments suggest more strongly than ever that they will have to live with that discomfort. By studying pairs of photons that have correlated polarizations, a research group showed that the results make sense only if the polarizations do not exist apart from measurement.
1.1 Waves and particles in quantum mechanics In spite of the fact that the extraordinary progress of experimental techniques make us able to manipulate at will systems made of any small and well defined number of atoms, electrons and photons making therefore possible the actual performance of the gedankenexperimente that Einstein and Bohr had imagined to support their opposite views on the physical properties of the wavelike/particlelike objects (quantons) of the quantum world it does not seem that, after more than eighty years, a unanimous consensus has been reached in the physicist's community on how to understand their strange properties. Unfortunately, we cannot know whether Feynman would still insist in maintaining his famous sentence It is fair to say that nobody understands quantum mechanics. We can only discuss if, almost thirty years after his death, some progress towards this goal has been made. I believe that this is the case. I will show in fact that, by following the suggestions of Feynman himself, some clarification of the old puzzles can be achieved. This chapter therefore by no means is intended to provide an impartial review of the present status of the question but is focused on the exposure of the results of more than twenty years of research of my group in Rome, which in my opinion provide a possible way of connecting together at the same time the random nature of the events at the atomic level of reality and the completeness of their probabilistic representation by the principles of Quantum Mechanics.
Quantum physicists have made many attempts to solve the quantum measurement problem, but no solution seems to have received widespread acceptance. The time has come for a new approach. In Sense Perception and Reality: A Theory of Perceptual Relativity, Quantum Mechanics and the Observer Dependent Universe and in a new paper The End of Realism I suggest the quantum measurement problem is caused by a failure to understand that each species has its own sensory world and that when we say the wave function collapses and brings a particle into existence we mean the particle is bought into existence in the human sensory world by the combined operation of the human sensory apparatus, particle detectors and the experimental set up. This is similar to the Copenhagen Interpretation suggested by Niels Bohr and others, but the understanding that the collapse of the wave function brings a particle into existence in the human sensory world removes the need for a dividing line between the quantum world and the macro world. The same rules can apply to both worlds and the ideas stated in this paper considerably strengthen the Copenhagen Interpretation of quantum mechanics.
As children, many of us imagined our toys coming to life the moment we looked away, just like in Toy Story. Maybe they walked across the room and secretly planned adventures behind our backs. Surprisingly, physicists ask a very similar question about quantum particles: do they have definite properties before we measure them, or is reality decided at the moment of observation? Understanding the answer has led to technologies such as quantum computing, quantum teleportation, and quantum cryptography. In the quantum world, where particles such as electrons and positrons live, objects do not have definite positions or states but can exist in a superposition of possible states. Even stranger, two particles can become entangled, meaning measuring one, instantly predicts information about measuring the other, no matter how far apart they are. If our particles were Toy Story characters, Woody and Buzz, we could simply ask them questions in a courtroom trial and reveal the secret. In quantum mechanics, these questions are essentially measurements. However, Woody and Buzz can cheat in the trial, and so can our particles, by preplanning the outcome of any measurement in advance. This idea is known as a local hidden-variable theory, and for a long time, it was thought that there was no way to distinguish between such preplanned outcomes and outcomes determined only at the moment of measurement. In 1964, John Bell proposed a test, known as the Bell test, which can reveal this fact. This test can expose any hidden planned strategy. No matter how cleverly our particles coordinate, Bell’s test gives them no easy way to fake the results. His method is a statistical comparison between the results of measurements performed on a pair of particles, which consists of some inequalities that are satisfied if particles have pre-determined outcomes before they are separated, and violated for some of the quantum cases where particles live in a superposition of outcomes, and their outcomes are correlated to each other. But there is still a loophole. Remember the courtroom trial? Now imagine Woody and Buzz invoking the Fifth Amendment and refusing to answer questions. In physical terms, this means that the measurements are inefficient and detectors sometimes fail to produce a result. That missing information can mislead us if we perform our analysis based on the detected particles, a procedure known as post-selection, and this loophole is commonly referred to as the detection loophole. Many efforts have been made to close this loophole, both by increasing the detection efficiency of the detectors and by finding settings in which the minimum efficiency required for a reliable result is lower. In this thesis, we further investigated this loophole under the post-selection assumption. We considered a model in which the efficiency of each measurement depends on its outcome. We then studied how the Bell tests behave when the underlying theory of reality is either a local hidden-variable theory or a quantum-mechanical theory.