Late last year, an experiment carried out by scientists at the Delft University of Technology in the Netherlands appeared to demonstrate that one object can affect another from afar without any physical interaction between the two. The finding confirmed an idea so extraordinary that, nearly a century ago, Albert Einstein had rejected it with the dismissive phrase “spooky action at a distance.” In quantum theory this phenomenon is known as “entanglement,” and many physicists now regard it as the most profound and important characteristic of the physical world at the smallest scales, which quantum theory describes.
Quantum entanglement is a deeply counterintuitive idea, which seems to contradict human experience of the physical world at the most essential level. In the everyday (“classical”) physical realm, objects affect one another via some kind of contact. The tennis ball flies from the racquet when struck, and when it hits the window the glass will smash. Sure, “invisible forces” seem to act across space—magnetic and electrical attraction and repulsion, say. But in quantum theory these interactions arise from the passage of a particle—a photon of light—between the two interacting bodies. Meanwhile, Einstein showed that the Sun’s gravity corresponds to a distortion of space, to which distant objects such as Earth respond. It’s generally believed that in a quantum theory of gravity (which doesn’t yet exist), this picture will prove to be equivalent to the exchange of “gravity particles” or gravitons between the Sun and Earth.
But quantum entanglement bothered Einstein because it suggested that one particle could affect another even when there was no conceivable physical interaction between them. It didn’t matter if those particles were light years apart—measuring a property of one particle would, according to quantum theory, instantly affect the properties of the other. How could that be?
Entanglement isn’t just another of the strange features of the quantum world, alongside the idea that quantum particles are also waves and that they can be in two places at once, and so on. Rather, entanglement is arguably the central mystery of quantum theory. Pretty much everything that seems odd about the theory is encapsulated in entanglement, an idea that now stands at the very limits of our ability to understand the physical world.
And it is not just an abstruse theoretical diversion. Experimental physicists and engineers are taking an increasing interest in ever more complex examples…