> If it becomes 'easier' to compress as you squash it, I assume there is some level of resistance before they attempt to squash it, right? Something measurable

> If it becomes 'easier' to compress as you squash it, I assume there is some level of resistance before they attempt to squash it, right? Something measurable right? Yes, there is a measurable force resisting compression, called radiation pressure. (You could reasonably call it photon pressure or light pressure, meaning the same thing.) https://en.wikipedia.org/wiki/Radiation_pressure https://en.wikipedia.org/wiki/Radiation_pressure This is the force used in the idea of a light sail. https://en.wikipedia.org/wiki/Solar_sail https://en.wikipedia.org/wiki/Solar_sail It's also used in optical tweezers. https://en.wikipedia.org/wiki/Optical_tweezers https://en.wikipedia.org/wiki/Optical_tweezers