However, in 1964 Northern Irish physicist John Bell found (opens in new tab) Einstein's argument broke down if you carried out a more complicated combination of different measurements on the two particles.īell showed that if the two observers randomly and independently choose between measuring one or another property of their particles, like position or velocity, the average results cannot be explained in any theory where both position and velocity were pre-existing local properties. This had a natural explanation, they argued, if both properties existed before being measured, contrary to Bohr's interpretation. Or the observer could choose to predict the velocity instead. When the same property (say, position or velocity) is measured on both entangled particles, the result will be random - but there will be a correlation between the results from each particle.įor example, an observer measuring the position of the first particle could perfectly predict the result of measuring the position of the distant one, without even touching it.

The article considered a pair of distant particles in a special state now known as an 'entangled' state. Read more: Einstein vs quantum mechanics. In a 1935 article with fellow theorists Boris Podolsky and Nathan Rosen, he argued there must be more to reality than what quantum mechanics could describe.

No less a figure than Albert Einstein found this idea untenable. In many cases, quantum theory doesn't give definite answers to questions such as 'where is this particle right now?' Instead, it only provides probabilities for where the particle might be found when it is observed.įor Niels Bohr, one of the founders of the theory a century ago, that's not because we lack information, but because physical properties like 'position' don't actually exist until they are measured.Īnd what's more, because some properties of a particle can't be perfectly observed simultaneously - such as position and velocity - they can't be real simultaneously.

Observation theory