A billiard game would be strange if the balls had no mass. They wouldn't collide but would pass through each other, ignoring obstacles. That's roughly what it looks like to imagine light as a stream of classical particles — tiny elastic bodies flying in straight lines. A new proof puts an end to that picture.
The authors built a rigorous mathematical framework: how, in general, a quantum object can turn into something familiar and classical. They didn't tie themselves to a specific instrument or medium — only to the most general requirements of reasonableness and spacetime symmetry. It turned out that for massless particles, these requirements are incompatible. Spacetime simply doesn't allow them to become classical.
This doesn't forbid light from exhibiting wave properties or generating electromagnetic fields. But a separate classical particle — a 'classical photon'—will never exist. The same conclusion applies to hypothetical gravitons: if they are massless, they cannot become classical. Thus, the quantum world reminds us once again: familiar intuition often fails here.
If a photon had even a tiny mass, it could become a classical particle. But then light from distant stars would reach us at different speeds, and we would see stars of different colors simultaneously.