Alain Aspect is a French physicist. In the early 1980s, with collaborators in France, he performed the crucial "Bell test experiments" that showed that Albert Einstein, Boris Podolsky & Nathan Rosen's reductio ad absurdum of quantum mechanics, namely that it implied 'ghostly action at a distance', did in fact appear to be realised when two particles were separated by an arbitrarily large distance. A correlation between their wave functions remained, as they were once part of the same wave function that was not disturbed before one of the child particles was measured.

If quantum theory is correct, the determination of an axis direction for polarisation measurement of one particle, forcing the wave functon to 'collapse' onto that axis, will influence the measurement of its twin even this is on a distant star. This influence occurs despite the experimenters concerned not knowing which axes have been chosen by their distant colleagues.

Aspect's experiments were considered to provide overwhelming support to the thesis that Bell's inequalities are violated. However, his results were not completely conclusive, since there were so-called loopholes that allowed for alternative explanations that comply with local realism. See local hidden variable theory.