爱因斯坦是量子理论的奠基人，但他不接受量子理论的一些推论。他说过一句很有名的话“上帝是不投骰子的”，认为物理世界的主宰是传统的机械决定论，而非概率。始终坚持量子力学是不完备的，不是最终理论。例如按照他提出的相对论，信号传递的最快速度是光速，两个相距甚远的物体是不可能在瞬间交换信息的。
以丹麦物理学家波尔为代表的 “哥本哈根学派”认为量子力学是完备且自圆其说的。“量子纠缠”(quantum entanglement)直接挑战了经典物理学的定域性描述。也就是说两个朝相反方向运动的光子，只要不受干扰，无论相距多远都可以在瞬间传递信息。但是爱因斯坦为此提出了著名的“隐变量理论”，将此问题归于人们对某种尚未发现的“隐变量”的无知。他认为，那两个彼此背离而行的光子，一定是具有某种内在的机制，决定了它们之后的行为，只不过人们不知道罢了。也许牛郎和织女在每年分手的时候都要约定好，在日后的某个时刻彼此互相思念，只是这样的约定还没有被王母娘娘发现而已。
简单地说，如果爱因斯坦是对的，那么我们依然可以按照传统的科学理念去理解世界。如果相信哥本哈根学派是对的，那么我们得承认，真实的世界和我们通过感官所接触、所看到的世界在本质上是完全不同的。这个问题超越了科学范畴，上升为一个哲学问题。
在60年代，欧洲核子研究组织(CERN)发表了著名的论文，将这一看似抽象的哲学问题简化为一个清晰的数学不等式，称为“贝尔不等式”。贝尔不等式不成立的话，那么爱因斯坦就是错的，隐变量理论也就不正确。由于贝尔不等式可以用实验观测，争论不再形而上，科学家们埋头于于贝尔不等式的实验观测。
在过去的50年里，人们提出了不计其数的实验方案，但每一个方案都有漏洞，无法在逻辑上直接证明。上周（8／24／15）由荷兰物理学家Ronald Hanson领导的科研团队，终于实现了没有漏洞的实验观测，无懈可击地直接证明了贝尔不等式不成立。
Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km
For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed that in any local realist theory the correlations between distant measurements satisfy an inequality and, moreover, that this inequality can be violated according to quantum theory. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell's inequality. We employ an event-ready scheme that enables the generation of high-fidelity entanglement between distant electron spins. Efficient spin readout avoids the fair sampling assumption (detection loophole), while the use of fast random basis selection and readout combined with a spatial separation of 1.3 km ensure the required locality conditions. We perform 245 trials testing the CHSH-Bell inequalityS≤2 and findS=2.42±0.20 . A null hypothesis test yields a probability ofp=0.039 that a local-realist model for space-like separated sites produces data with a violation at least as large as observed, even when allowing for memory in the devices. This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification.
