Einstein's general equation derived from the relativistic definitions of the energy and momentum of a particle:
• E²=p²c²+m²c⁴.
Where E is the total energy of the particle, p is the momentum of the particle, c is the speed of light, and m is the mass of the particle.
When a particle is at rest i.e. p=0 then this equation reduces down to:
• E=mc²; (p=0)
But When a particle with no mass i.e. m=0, the equation reduces to:
• E=pc; (m=0)
Since photons have no mass and therefore get all of their energy from their momentum, so they must follow: E=pc; If an object has momentum without mass is only possible if this is a wave. A wave transports momentum via its waving motion and not by physically transporting an object with mass.
Moreover, if a particle has no mass and is at rest then its total energy is zero:
• E=0; (m=0) & (p = 0)
Conclusion a particle with no mass and is at rest is nothing at all since it has no energy.
Therefore, if an object with no mass is to physically exist, it can never be at rest but must always travel at the universal speed limit c; such is the case with light. Light consisting of photons and photons don't have rest mass but momentum and energy. Momentum is the directional property of an object in motion that describes its ability to influence another object upon impact.
In addition to that, if the object travels less than the universal speed limit c, say, at some speed v then we can always choose a reference frame travelling along with the object so that the object will be at rest in this reference frame.
