How do photons carry heat thru space? What is the property of photons that enables them to do this? Do photons always carry heat?
A photon is a quantum concept, and heat is a classical concept. You can't mix them like this. The topic title is better since it references the classical concept of light, not photons. Light has no temperature and does not carry heat. It has frame dependent energy and that energy, if absorbed by some material, might heat the material, as evidenced by Earth warming up during daylight hours.
Light is simply energy. A hot object can give off light at all sorts of frequencies (it is IR in some reference frames and not others), cooling the emitting object. That light can warm an object that absorbs the light, so in effect transferring heat even though it isn't heat enroute. An example is a microwave oven that effectively shines a light that is designed to be absorbed by food and not by other stuff. It transfers energy (which isn't particularly thermal energy) to light which then transfers the energy to the food which manifests as heat. This isn't IR light, at least not in the frame of the oven. In this case, light.
As Halc says, light is a quantized form of electromagnetic energy. (An electric field coupled with a magnetic field, whose cycles are 90 degrees out of phase.* They propagate together through space.) It has momentum. When it hits something that can absorb the energy at that quantized frequency**, it absorbs that packet of energy, which is converted into a number of other types of energy. A very common conversion is into the bonds of molecules (like little springs, molecular bonds wobble to-and-fro, up-and-down, side-to-side). This wobbling of molecular bonds is commonly detected as a warming of the material.*** . However, there are myriad other forms of energy it might be converted to. eg. It might kick electrons into higher orbitals (excitation, which is the source for fluorescence). TLDR: light or photons do not "carry" heat; they carry momentum, which is simply a form of energy. And one form of energy can be converted to another - such as heat. * Please Register or Log in to view the hidden image! ** not all frequencies of energy are absorbed by all materials, e.g. visible light passes through glass. That's the "quantized" part of light. There are no molecular bonds in glass that can be absorbed by the wavelengths of visible light. But ultraviolet light is, which is why glass can protect you from sun burn. *** It may be helpful to note that atoms and molecules do not have temperatures. Temperature is a property of the average kinetic energy of a bulk material.
"Heat" is technically defined to be a transfer of energy between two objects or systems due to a temperature difference between them. Photons can transfer energy from one place to another. Absorbed energy can cause an object to heat up. Infrared light can be described as electromagnetic waves or as bundles of photons. Classical electromagnetic waves can transfer energy from one place to another, and so can photons, so either way they can cause heating when they are absorbed by something (and cooling when they are emitted from something). In the classical picture, oscillating electric and magnetic fields. In the quantum picture, photons are excitations of the electromagnetic field.
