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Quantum nonlocality revealed: Why do quantum particles seem to violate the speed of light limit?
The profound characteristics of quantum mechanics and the related non-locality issues have long been the focus of discussion among physicists and philosophers. Conventional ideas of physics are challenged as we try to understand the mysteries of the quantum world, as quantum particles behave in ways that appear to violate the limits of the speed of light. How did this come about? This is exactly the question that the Bell Test attempted to answer.
Since 2015, all Bell tests have shown that the assumption of local hidden variables is inconsistent with the behavior of physical systems.
Background and significance of Bell experiment
The Bell experiment, named after John Stuart Bell, was designed to test the relationship between quantum mechanics and Albert Einstein's theory of local realism. The local realist position holds that the behavior of particles must be explained by certain unobserved local variables, which are called "hidden variables." However, this view was challenged with the introduction of Bell's inequality.
Quantum entanglement and the EPR paradox
Quantum entanglement is the core concept of the Bell experiment. In 1935, Einstein and his colleagues proposed the famous EPR paradox, stating that the predictions of quantum mechanics seemed to imply that information could be transferred instantaneously between particles, which would violate the law of causality. This means that the interactions between quantum particles are not just driven by some local hidden variable, but may be non-local.
If some information is known, then according to Heisenberg's uncertainty principle, there is other information that cannot be known.
Experimental verification of Bell's inequality
Experiments on Bell's inequality involve measurements on two or more entangled particles. Experimental designs typically involve observing a particle, such as a photon, and selecting its properties (such as its polarization) to measure. If the experimental results violate Bell's inequality, the hypothesis of local hidden variables can be ruled out. All Bell test results to date support the predictions of quantum physics rather than local hidden variable theory.
The Bell experiment that made history
Since the 1970s, physicists have begun conducting various Bell experiments. Some important experiments include:
- In 1972, Stuart J. Friedman and John Crowther performed the first experiment to observe a violation of Bell's inequality.
- In 1982, Alain Aspert and his team performed the famous Bell test in France, which was the first experiment in which the measurement settings were randomly selected during the flight of photons.
- In 2015, the experiment of Hensen et al. successfully closed both the detection loophole and the locality loophole, which provided stronger experimental support for the violation of Bell's inequality.
The rise of quantum information theory
Due to the violation of Bell's inequality, scientists realized that the unique properties brought by quantum entanglement laid the foundation for the prosperity of quantum information theory. This new field of physics focuses on potential applications in quantum computing and quantum communications, particularly quantum cryptography. Quantum cryptography uses the properties of quantum systems to develop secure communication methods, which is undoubtedly an important application of quantum mechanics.
Looking to the future
With the advancement of experimental technology, physicists' understanding of the quantum world continues to deepen, and more complex Bell experiments are also ongoing. Not only do they verify the theoretical predictions of quantum mechanics, they also make us re-examine the nature of reality. In this universe full of uncertainty, can we find some form of certainty?
