What does it mean to be ‘quantum’? A physicist explains the basics behind Einstein’s spooky actions at a distance

Imagine shining a flashlight across a dark room. You can predict exactly what the light will do: travel in a straight line from one point to another. That seems obvious, because in the world we see around us, light appears to follow a single, clear path. Quantum mechanics paints a far stranger picture. If you zoom in to the atomic scale, light does not behave as though it follows only one straight route. Instead, a particle of light explores every path available to it at once. One path may indeed be the straight line across the room. But others could involve the light bouncing off walls, curving through the space or tracing wildly improbable detours before reaching its destination. In a sense, nature keeps all these possibilities “alive” simultaneously. One outcome emerges only after the light is “observed,” or a measurement is performed. The path you observe – usually the straight line – is simply the most probable result after all the possible paths interact, making some outcomes more likely and others less likely. This idea feels almost impossible from the perspective of everyday life, yet it lies at the heart of quantum mechanics. Today, scientists like me are learning…

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