A copper wire hit by a laser is like a kitchen where the chef turns on the stove and watches the stopwatch at the same time. Copper atoms start to dance, losing the order of the crystal lattice, while electrons, like waiters in the dining room, instantly feel the change. But how much disorder do they need to notice? It turns out, just the very beginning of melting is enough.

Scientists fired single laser pulses of varying intensity at thin copper films. A special setup measured how terahertz radiation, something between a radio wave and infrared light, passed through the metal in the first picoseconds after the flash, which are trillionths of a second. From this data, they calculated conductivity: copper's ability to carry electric current. Before melting, electrons constantly bumped into boundaries between microscopic grains of the metal, like runners on a track with hurdles. But when copper began to melt precisely at these boundaries, the hurdles disappeared, and the current briefly became stronger.

Computer simulations confirmed that atoms and electrons here cannot act independently. Melting is not just the crystal thawing; it's an instant rewiring of the paths for electric current. The laser pulse that was supposed to destroy everything momentarily makes the metal more conductive. That same chef, by turning on the stove, inadvertently opens a shortcut through the kitchen for the waiters.

Now it's clear: ultrafast optical measurements can show the exact moment melting begins. This matters for laser material processing and for understanding how matter behaves under extreme conditions.