Speed of light

QUESTION: I know that the speed of light in vacuum is c = 30000 km/s. Also according to relativity no body can travel faster than light. But why cannot light travel faster than 'c'?
Why is 'c' a limiting value?

Answer:

We need to look at Dirac's equation for relativistic momentum.

E² = p²c² + m_o²c⁴

E is the energy of the 'particle', p is its momentum, m_o its rest mass and c the speed of light.

For the photon m_o is zero and therefore E = p.
However if a particle has zero mass it will take zero force to accelerate it and so it will instantaneously reach the maximum speed – that is c.

As you say by the theory of relativity the maximum speed for a body is c and so this is the speed that the photon will reach. The constancy of the speed of light is also a fundamental postulate of relativity and this again confirms that this maximum speed will be c.