The Most Beautiful Experiment
Quantum physics informs us that a system exists in superposition — that is, in all possible states — until we observe that it is only in one specific state.
According to a 2002 poll of Physics World readers, the “most beautiful experiment” in physics is one that simply and elegantly demonstrates how observation affects quantum systems: The double slit experiment. The double slit sets aside causality, determinism, and the notion that reality is “out there” as it blurs the line between the observer and the system being observed.
In the double slit experiment, a series of single photons (light particles) are fired at a solid plate that has two slits. On the other side of the solid plate, a photographic plate is set up to record what comes through those slits.
The question: What will we see on the photographic plate?
The answer: If one neglects to observe which slit a photon passes through, it appears to interfere with itself, suggesting that it behaves as a wave by traveling through both slits at once. But, if one chooses to observe the slits, the interference pattern disappears, and each photon travels through only one of the slits.
The formation of the interference pattern requires the existence of two slits… But how can a single photon pass through two slits simultaneously? At that point, we are forced to consider each photon as a wave that travels through both slits… Or we have to think of the photon as splitting and going through each slit separately — and wondering how the photon “knows” a pair of slits is coming.
The only solution is to abandon the idea of a photon — or any other quantum system — as having a location in spacetime until it is observed.