The sun's warmth crosses millions of miles of empty space to create a summer day; a campfire roasts marshmallows from several feet away. Scientists acquire understood the mathematics behind this ghostly transfer of heat since the late 19th century. But that math breaks down at very close quarters—within,for example, nano-scale electronics and solar electricity cells (with components separated or spanning billionths of a meter) where heat transfer is critical.
Source: phys.org