Why a photon cannot rest?

Answer 1

A photon pass through different media at different speed, but that speed always is the highest speed it can achieve at those circumstances.

That is because a photon's existence not depend on its structural energy to keep its structure stable but depend on its momenta energy to keep its existence in a dynamic balanced state --- just like an acrobat riding on a single wheeled bike --- he has to swing back and forth to stay on the seat. If you are structurally complex enough, for instances, got four leg like a chair, you can stand on them still; but if you only got one leg, standing still is not the way to keep you upright --- you have to swing back and forth to achieve that. Or you can speed forward all the way or spin. In the case of a photon, it does all three of these movements --- speed forward all the way and vibrate up and down and spins. That is what happening in the photon ---it doesn't have the structural complexity to keep its existence but have to depend on a combination of its momenta energy to achieve that.

The forward momentum give it the inertial energy to keep its basic structure and the vibration momentum keeps this structure dynamically stable, the spin momentum keeps the electric field and magnetic field alternatively oscillating to keep a dynamically balanced fields state.

There is a dynamically balanced equilibrium point between these momenta forces, before that point is reached, the energy distribution in the photon will only keep going one way --- flow to the lower potential direction. And that direction is the forward speed momentum which not reached the saturated point. Only at that balanced point, the energy distribution becomes two way adjustment mechanism --- if the forward speed not reached the maximum (actually it is the forward momentum not reached the maximum) at that circumstances, vibration energy will be converted into forward momentum; when an outside force give an extra forward momentum, that extra momentum energy will be converted into vibration momentum. That mechanism keeps the forward momentum constant --- note: it keeps the forward momentum constant but not the speed constant, the speed constant is the result of the constant forward momentum. That is the reason when light pass through different media have different speed --- under a constant momentum, it can only achieve different speed at different resistance. In vacuum space there is no resistance so the speed can keep c --- a constant.

All these strange behaviour of a photon is decided by its unique character feature --- it is the smallest unit of energy and mass that can stably exist. It doesn't have structural energy to keep its very minimal structure in a thermodynamically stable state but it has to depend on its momenta energy to do so. That is the reason it behaves different from other bigger things --- its thermodynamically stable state is always at a constant speed (result from its constant forward momentum) while other bigger things will achieve a thermodynamically more stable state by losing its momentum energy and slow down.