What is a photon?
A photon is a particle of light. Or, more specifically, it is a particle that represents light. A photon will carry energy.
Can a photon travel at the speed of light?
Since a photon is a particle of light, this means that it does not need to accelerate to light speed, as it is already travelling at the speed of light when it is created. A photon does not rest and then reach the speed of light at a certain length of time, or even instantly. A photon is always travelling at the speed of light, from the moment of creation.
We can easily imagine a photon as a solid ball of mass. This would explain why we seem to think that a photon should accelerate to the speed of light instead of already travelling at the speed of light from the moment of existence.
But if we imagine a snowball being created, it will have to be thrown in order to gain speed, this could be how you imagine a photon, but is actually not the case.
So, what kind of particle is a photon?
A photon, what we have described to be a particle of light, is not a traditional particle as you may know particles. Instead, it is something called a ‘quantum object’, it is partly wave and partly particle.
The term quantum object is used to describe an object that has dynamics that classical physics cannot describe.
A photon acts like a wave when it is initially created, this is ideal since waves have nothing stopping them from reaching a certain speed from the moment they are created.
Imagine throwing a pebble into a pond and seeing the water waves ripple away. The ripples do not get bigger as they more pebbles are thrown in the same spot, the waves are immediately travelling at the nominal speed they are when they are created.
More about waves
The way that waves are created is that a deformation in the material or field medium will cause it to snap back to the state of equilibrium and then go over this equilibrium.
This will happen over and over in both directions until the equilibrium is reached. The material or field medium is simply the object/medium that the waves are created on or in. Water would be the medium of ripple waves.
The way that the waves oscillate back and forth causing the medium in the surrounding are to also be affected by the waves and to join in with the wave, hence the ripple effect.
This means that the speed of the wave depends on the medium’s ability to snap back in relation to the wave. Throwing in a larger pebble will only make the waves taller, not faster.
This is because the when the medium that the wave is in is constant across a certain distance, the wave speed will also be constant across this specific region.
As well as this, in an area of constant medium, a wave cannot accelerate, and is created at the exact wave speed for that region.
Does this just go for quantum objects?
No, this behavior does not just apply to quantum objects and waves. It actually is the same for all waves: from the waves of the ocean to the waves on a piano string.
So, this is not a behavior that is exclusive to photons, since other particles that are not quantum can act as waves and have the same characteristics.
How does a photon travel at the speed of light from creation?
It is commonly believed that a photon travels at the speed of light simply because it is a massless particle. Although it is true that a massless particle can travel at the speed of light, and that a photon is a massless particle, this is actually not the reason that it travels at the speed of light from the moment of creation.
The reason it travels at the speed of light from the moment it is created is because it is a wave. There are other quantum particles that do have mass and are also created at their optimum speed without the need to accelerate.
An example of this is an electron. Since an electron is a quantum object and has mass, it is still partly a wave, so will act in the same way as a photon.
Conclusion
A photon does not accelerate to the speed of light because it is created already travelling at the speed of light. This is because it is a quantum object, making it partly wave and partly particle. The light waves that it is part of will always be travelling at the speed of light from the moment of creation.