loophole-free Bell test
A loophole-free Bell test is an experiment that violates a Bell inequality while simultaneously closing the major loopholes that earlier tests had to leave open. Over the decades, sceptics could in principle cling to local realism by pointing to two gaps: the detection loophole, where too few particles are caught and the sampled ones might be unrepresentative, and the locality loophole, where measurement settings might somehow influence the distant side. A truly convincing test had to shut both at once.
This was achieved in 2015 by several groups. One landmark experiment in Delft entangled electron spins in diamonds more than a kilometre apart, using a clever scheme that guaranteed nearly every trial was recorded, closing the detection loophole, while the large separation closed the locality loophole. Other teams using photons reported loophole-free violations the same year, converging on the same verdict from different directions.
Together these experiments are about as close to a final word as physics offers on the matter. They confirm, with the major escape routes sealed, that nature genuinely violates Bell inequalities and that no local hidden-variable theory can describe the world. A few exotic assumptions, such as superdeterminism or the freedom to choose settings, remain logically untouchable by any experiment, but the everyday, reasonable form of local realism is now firmly and honestly ruled out.
Closing both major loopholes at once, the 2015 tests sealed the case against local hidden variables.
Even loophole-free tests cannot rule out superdeterminism — the idea that the measurement choices are themselves predetermined together with the outcomes. Most physicists regard that escape as deeply unattractive, but it remains experimentally untestable.