Which method of heat transfer requires no medium between the source and the object it warms?

1. Conduction requires two objects to be in direct physical contact. 2. Convection requires the movement of a fluid, which is itself a medium. 3. Radiation is described as the only method of heat transfer where no medium is required, since electromagnetic radiation can cross empty space. 4. Diffusion is not one of the three heat transfer methods described here at all. 5. So radiation is correct. _Source: OpenStax Physics (CC BY 4.0), Ch 11 "Thermal Energy, Heat, and Work", section 11.2 Heat, Specific Heat, and Heat Transfer_