Can a photon stand still? Or must it always be moving? And if it must be moving, must it always travel at light speed? After all, the atoms of my body move at different speeds.
Can’t “stand still” and must move at speed of light. The atoms of your body have “rest mass”. Photon has no “rest mass”. It stops, it ceases to exist.
Where does the energy go when it ceases to exist? Does it break down into quarks?
It can in principle, but far more often it’s just transferred to another particle in the form of kinetic and/or potential energy. For example, when a photon hits an atom, it can sometimes give its energy to one of the electrons, promoting it to a more energetic orbit around the atom.
It’s also impossible, it turns out, for a photon to spontaneously turn into other particles. (Basically, it can’t be done without violating either conservation of energy or conservation of momentum.) A photon has to interact with a second particle that siphons off a bit of its energy or momentum in order decay into multiple other particles and still balance the books.
Photons disappear vin a number of ways – they can be absorbed by an atom or molecule, kicking them into a higher state of excitation. It can be absorbed by a crystal lattice, adding to its overall energy (going into phonon modes, and thence into heat). It can be absorbed by an electromagnetic field.
In each case, the photon energy goes into the system that absorbed it, raising its energy by the same amount.
Photons may be emitted by such systems, as well, in which case the system energy is lowered by that amount. Thus, spontaneous emission by an atom or molecule will result in a lower energy atom or molecule, plus a photon.
No need for esoteric entities to account for the energy – it’s simply transferred within the system.
Photons move at the speed of light, but light will move more slowly in a dielectric medium. This is viewable as an interference or cooperative phenomenon, but it’s true that the speed of light is fastest in vacuum and slower elsewhere. Extreme cases of slowing are often due to light absorption and re-emission, rather than the above mechanisms.
Depending on the level of detail you look at, even the non-extreme cases of light slowing down in a medium can be viewed as absorption and re-emission.
Given the ability to create the perfect dielectric medium, how much could it slow down a photon?
This was my understanding. Photons always move at c. The light itself can be slowed down to almost nothing, but at the microscopic scale, the photons are just running into things at c and then being re-emitted a short time later.
Fun fact: It takes 8 minutes for light from the surface of the sun to reach the earth, but that energy was created in the center of the sun. It took those photons 300,000 years to reach the surface, despite the distance from the center to the surface of the sun being much smaller than the distance to Earth.