Loopholes
Though the series of increasingly sophisticated Bell test experiments has convinced the physics community in general that local realism is untenable, it remains true that the outcome of every single experiment done so far that violates a Bell inequality can still theoretically be explained by local realism, by exploiting the detection loophole and/or the locality loophole.. To avoid the locality loophole, the experimenter has to arrange that the particles travel far apart before being measured, and that the measurement process is rapid and moreover successful. The most serious loophole is the detection loophole, which means that particles are not always detected in both wings of the experiment. It is possible to "engineer" quantum correlations (the experimental result) by letting detection be dependent on a combination of local hidden variables and detector setting. Experimenters have repeatedly stated that loophole-free tests can be expected in the near future (García-Patrón, 2004). On the other hand, some researchers point out that it is a logical possibility that quantum physics itself prevents a loophole-free test from ever being implemented (Gill, 2003, ; Santos, 2005, ).
Read more about this topic: Bell Test Experiments