Warming without particles: by radiation
Physics Narrative for 11-14
Radiation is quite different from both convection and conduction. Consider the case of warming by the Sun on a bright summer's day. As you feel the warmth of the Sun on your back, energy is shifted.
The radiation pathway does not rely on particles to allow warming to occur, but relies on electromagnetic radiation. We can be sure that warming is possible without particles just by remembering that there is a vacuum between the Earth and the Sun. In other words, there are no particles available to allow the warming to occur. The energy is shifted from the Sun to the Earth via electromagnetic radiation, and in this particular case the radiation acting is mainly infrared.
Electromagnetic radiation is best thought of as a stream of photons, or small packets of energy. Everything
glows by emitting these photons – even the whole universe, with the afterglow of the Big Bang. Both the character and the number of photons emitted changes as the object gets warmer. If it gets hot enough, you can see the glow as it becomes
red hot (or even
white hot). In fact, you can often feel the stream of photons from an object at a lower temperature, even when you cannot see them.
Hold the back of your hand about 15 centimetre above a kitchen ring as it warms up. You can feel the warmth before you see the red glow. You feel infrared photons arriving at the back of your hand. A much more dangerous experiment would be to place your hand under the grill (we most certainly are not recommending this). Then you could be sure that moving warm particles plays little part (air is a very poor conductor) in the warming process since your hand would be below the heating element.
Because everything glows, emitting electromagnetic radiation, it follows that cooler objects also emit a stream of photons. So if two objects are placed close to each other in a vacuum (no energy shifted by the heating by particles pathway), each will warm the other by radiation. However the net flow of photons is from the hotter to the cooler, so the temperature of the cooler one rises and the temperature of the warmer one falls.