A photon is a spin-1 Boson, usually considered a pointlike particle. A photon moved at a fixed velocity called the 'speed of light.' This specific velocity is denoted by the letter 'c' which originally comes from the latin word, 'Celeritas,' which means ''swiftness.'' In SI units, the speed of light in a vacuum is 299,792,458 meters per second (1,079,252,848.8 km/h).
Why does the speed of light move at lightspeed?
The reason has to do with the permittivity and permeability of spacetime which can help define the speed of light. We may write this as
(√με)^(-1) = c
That is, 1 divided by the square root of the product of permittivity ε and permeability μ (which are inherent properties of the spacetime continuum) yields the speed of light again, given by c.
Moving on, a photon has momentum given by the equation
E = pc
The energy-momentum relationship is given as
E² = (Mc²)² + (pc)²
Since the rest mass of a photon is zero, this reduces to
E = pc
(We can ignore square root signs since there is no such thing as a negative momentum)
It is interesting to note, that at the limit of a particle at rest we have
E = Mc²
However, particles are never truly at rest in nature. The reason has to do with the Uncertainty Principle. If a particle did come to complete rest, this would violate the Uncertainty Principle which briefly states that the momentum and the position of a particle may never be completely known simultaneously - there is always an element of uncertainty in your system. If a particle was at complete rest, the particle would have a tremendously accurate position making it's momentum highly uncertain. Therefore, a particle always retains a little bit of momentum, so effectively E = Mc² is a good approximation and nothing more. I say nothing more, but in reality there is deep significance to relating energy to mass with the coefficient of the speed of light squared, but that is irrelevant for this post. Keep in mind, a photon is never at rest because or become at rest, as we have explained it has no rest mass.
The photon is the 'quantum of light' - that is, the sunshine you see on a fresh spring morning is made up of these tiny particles we call the photon. Photons are very frequent in nature - cosmologically-speaking, they make up the background radiation of spacetime. The background radiation is believed to be, the radiation remnant of the Big Bang. The Big Bang marks the beginning of space, matter and energy and even time itself according to theory.
A photon does not decay spontaneously in spacetime. A photon decays when interacting with another particle. One possible decay mode is that a pair of photons can decay into an electron and a positron. A positron is the antiparticle of the electron. An interesting phase transition occurs when an electron and a positron come together, they reduce back into gamma energy - those photons which first created it. The reason why this happens is because of charge conservation. An interesting thing to consider is that there are no anti-photons in nature - this is not reserved for photons alone, but is an interesting facet for a number of particles in this universe. The reason why the photon does not have an antiparticle is because the photon is it's own antiparticle.
Because a photon has momentum, it can also be related to the wave vector k
p = ħk
Where ħ is the angular momentum or also known as the quantum action which has dimensions of momentum multiplied by distance or energy multiplied by time.
As has been discussed, the photon does not have a rest mass. If it did have a mass, it would have a mass on the scale of around 10^(-51)kg which is tremendously small. The fact a photon has zero mass is in fact a wonderful prediction, both of experimental and mathematical gauge theory. Interestingly however, a photon does behave like it has a mass in a superconductor - the reason why is because the electromagnetic force inside a superconductor becomes short range and so behaves like it has a rest mass.
Particles, like an electron emit photons when being accelerated. This is predicted by the bizarre-looking Larmor formula
P = ⅔ (e²/c³)a²
Where P denotes power (units of energy over time), a is the acceleration and e is the electric charge. Power is the rate in which energy is transferable. This process for an electron can be thought of as a type of electromagnetic inertia.
It is also said that a photon does not experience time because time is stretched to infinity for the reference frame of the photon. Essentially, a photon which does not move in time would not move in space as well if one was to take relativity seriously... however there is a way out of this problem. The solution is by saying the photon does not have a frame of reference.
To finish up, the photon today remains as one of the most understood particles in the standard model. It has taken years of understanding no less, since the day Max Planck predicted their existence from his work on black body radiation whereas our modern understanding of the photon may be fully attributed to Albert Einstein.