The equation E = pc is valid (here E is energy, P is momentum and c is speed of light in vacuum) :

Yes, the equation E = pc is valid in the realm of physics. This equation is a simplification of one of the energy-momentum relations in special relativity, which is used when a particle has zero rest mass, such as a photon.

Here's a step-by-step explanation:

1. The full energy-momentum relation in special relativity is E² = p²c² + m²c⁴, where E is energy, p is momentum, c is the speed of light in a vacuum, and m is the rest mass of the particle.

2. For particles with zero rest mass (m=0), such as photons, the equation simplifies to E² = p²c².

3. Taking the square root of both sides gives two possible equations: E = pc and E = -pc.

4. However, energy is always a positive quantity, so we discard the second equation.

5. Therefore, for particles with zero rest mass, the equation E = pc is valid.