How sunlight delivers energy across empty space
Heat moves in three main ways: conduction passes energy through touching particles, convection carries it in moving fluids such as air or water, and radiation travels as electromagnetic waves. The first two need matter to pass energy along. Radiation does not: changing electric and magnetic fields can propagate through a vacuum.
The Sun’s hot material emits a broad range of electromagnetic radiation, including visible light and infrared. As that radiation travels outward, it does not warm the emptiness it crosses in the way a moving current of warm air would. The energy is transferred when something absorbs it. In a spacecraft, for example, sunlight absorbed by a surface makes its particles move more energetically, raising the surface’s temperature.
Why a vacuum does not stop heat transfer
A vacuum blocks conduction and convection because there are few or no particles to collide or flow. It does not block incoming radiation. This is why a sunlit spacecraft can heat up even while surrounded by space, and why spacecraft need ways to manage both absorbed sunlight and the heat they emit back outward as infrared radiation.
The same distinction explains how a vacuum-insulated bottle works. Its empty gap greatly limits heat carried by contact or moving air, but radiation can still cross the gap. Reflective layers reduce that remaining transfer by sending much of the radiation back. A vacuum is excellent at stopping heat that needs matter to travel; it cannot stop energy carried by light.
